Keto-enol tautomerism as transformative electron/hole traps to promote charge carrier separation for record-high H2O2 photosynthesis in real world

Keto-enol tautomerism as transformative electron/hole traps to promote charge carrier separation for record-high H2O2 photosynthesis in real world | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Physical Sciences - Article Keto-enol tautomerism as transformative electron/hole traps to promote charge carrier separation for record-high H 2 O 2 photosynthesis in real world Tianyi Ma, Fang Ma, Xiaodong Sun, Liqun Ye, Chunqiu Han, Yongye Wang, and 11 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5211465/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Covalent organic frameworks (COFs) are excellent photocatalysts for hydrogen peroxide (H 2 O 2 ) photosynthesis, but are often limited by retarded charge carrier separation. Presently, donor-acceptor (D-A) type COFs are commonly used to enhance the separation of photogenerated electrons and holes at the molecular level by promoting the migration of electrons towards the acceptor and the movement of holes towards the donor. However, the significantly slower kinetics of the synchronous water oxidation and oxygen reduction reactions (WOR and ORR) often result in the accumulation of photogenerated carriers, which induces strong Coulomb forces, in turn adversely affecting the carrier separation efficiency of D-A type COFs. Herein, it is observed that keto-enol tautomerism can function as dynamic traps for both electrons and holes, alternately capturing them, while the counterpart holes and electrons participate in and are consumed during asynchronous oxidation and reduction reactions. This represents the first example of T-C type COFs (T denotes traps units; C denotes catalytic active units), which can effectively weaken the Coulomb force by reducing charge carrier accumulation, resulting in rapid charge transfer and prolonged lifetimes of free charge carriers for efficient alternating photocatalytic WOR and ORR. Our in-depth research indicates that imine COFs based on 2,4,6-trihydroxybenzaldehyde-1,3,5-tricarbaldehyde (Tp series) exhibit enhanced photocatalytic activity compared to those based on 1,3,5-benzenetricarboxaldehyde (BT series), which can be attributed to the occurrence of keto-enol tautomerization. Notably, the optimal Tp imine COF (TpBpy) displays an ultrafast rate for H 2 O 2 photosynthesis, reaching 3.79 mM h -1 , surpassing all previously reported photocatalysts. More importantly, when employed in a flow-reactor system, TpBpy also showcases exceptional effectiveness for continuous photocatalytic H 2 O 2 synthesis, achieving a solar-to-chemical conversion (SCC) efficiency of 0.038%, representing the highest performance recorded to date under natural sunlight conditions. This work offers molecular-level guidance for designing efficient photocatalysts for H 2 O 2 photosynthesis and proposes standardized criteria for obtaining reliable SCC values. Physical sciences/Chemistry/Catalysis/Photocatalysis Physical sciences/Materials science/Materials for energy and catalysis/Photocatalysis Covalent organic frameworks keto-enol tautomerism charge carrier separation hydrogen peroxide photosynthesis Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Covalent organic frameworks (COFs) represent an innovative category of photocatalytic materials that have gained significant attention for the photocatalytic production of hydrogen peroxide (H 2 O 2 ) [ 1 – 3 ] . The advantages of COFs can be summarized as follows: (1) The highly organized porous architecture of COFs offer an increased number of reactive sites while minimizing the carrier migration distance to the sub-nanometer scale, thereby improving the efficiency of photocatalytic processes; (2) The spectral absorption range of COFs can be extended to the visible and infrared regions through opportune molecular design, providing a good platform for the utilization of visible light energy and infrared heat energy; (3) Specific reaction active sites can be efficiently designed based on the characteristics of different photocatalytic reactions [ 4 – 12 ] . However, the current photocatalytic performance of COFs for H 2 O 2 photosynthesis falls significantly short of industrial requirements. The primary reason is that, although the sub-nanometer migration distance of charge carriers in COFs provides convenience for catalytic reactions, this same scale of migration distance is incapable to sufficiently weaken the Coulomb forces between photogenerated electrons and holes. Consequently, this limitation hampers the separation of charge carriers [ 13 ] . In other words, the ability of COFs is limited, which poses a considerable obstacle to the sustained separation of photogenerated carriers, thereby challenging their practical application in H 2 O 2 photosynthesis [ 14 ] . Over the past decade, researchers have extensively focused on the problem of photogenerated charge carrier separation in COFs. The most promising approach is to utilize the effective molecular design advantages of COFs to construct a series of donor acceptor (D-A) structured COFs [ 15 , 16 , 17 ] . By regulating the charge distribution difference between donor and acceptor units, a push-pull effect mechanism is induced, facilitating the migration of photogenerated electrons and holes to the acceptor and donor units, respectively [ 18 ] . However, although the D-A structure holds promise for improving the separation of photogenerated charge carriers, a significant gap remains between the rapid migration of these charge carriers and the comparatively slower kinetics of the water oxidation and oxygen reduction reactions that occur on the D and A units. The reaction rates are generally several orders of magnitude lower than the migration rates of the charge carriers. This discrepancy leads to the accumulation of photogenerated carriers, causing strong Coulombic interactions among the carriers. Such interactions hinder the separation efficiency of the photoinduced electron-hole pairs in D-A structured COFs, which ultimately limits the photoactivity of COFs for H 2 O 2 photosynthesis. Currently, this issue represents a common challenge faced by almost all the existing photocatalysts and photocatalytic reactions [ 19 ] . At present, the most effective approach to address the conflict between swift carrier migration and sluggish oxidation-reduction reactions is the addition of hole-sacrificial reagents to the photocatalytic H 2 O 2 production system. These sacrificial reagents rapidly consume the photogenerated holes present on the catalyst surface, thereby diminishing the Coulombic forces among photogenerated carriers. This process leads to an increase in the number of photogenerated electrons available for O 2 reduction, ultimately enhancing H 2 O 2 photosynthesis [ 20 , 21 ] . However, this method also possesses several significant drawbacks. Firstly, this strategy is unable to accelerate both photocatalytic H 2 O oxidation and O 2 reduction reactions concurrently, as the photogenerated holes are consumed by the sacrificial reagents [ 22 – 27 ] . Secondly, the introduction of sacrificial reagents complicates the catalytic system and adversely affects the purity of the H 2 O 2 solution. Lastly, the incorporation of these sacrificial reagents increases economic costs, and the majority of these sacrificial reagents are not environmentally friendly, imposing considerable environmental issues [ 27 – 35 ] . Therefore, to address the discrepancy between rapid carrier migration and slow oxidation-reduction reactions, it is still necessary to focus on catalyst design. Here, we draw inspiration from the sacrificial reagent scheme to create units for the interconversion of electron and hole traps within the catalyst. These units are designed to alternately capture photogenerated electrons and holes, while the resulting photogenerated holes and electrons then engage in photocatalytic oxidation and reduction reactions, respectively. This methodology effectively reduces Coulombic forces by limiting the accumulation of photogenerated electrons or holes, enabling ultrafast charge transfer and longer-lived free charge carriers. This ultimately facilitates effective alternating photocatalytic H 2 O oxidation and O 2 reduction reactions, promoting H 2 O 2 photosynthesis. Based on the above considerations, we have designed a series of Tp (2,4,6-trihydroxybenzaldehyde-1,3,5-tricarbaldehyde) imine COFs with keto-enol tautomerism, which exhibit significantly higher photocatalytic performance compared to the BT (1,3,5-benzenetricarboxaldehyde) series COFs that lack of keto-enol tautomerism. Mechanistic studies indicate that Tp-series imine COFs display enhanced photoinduced dynamic behavior, including increased photocurrent density, decreased charge transfer resistance, and enhanced surface photopotential. Taking the optimal TpBpy photocatalyst as a case study, in situ infrared spectroscopy, femtosecond time-resolved spectroscopy, and theoretical calculations have demonstrated that the Tp unit in TpBpy alternates between electron and hole traps during the keto-enol tautomerism process. When Tp is in ketone form, it acts as an electron trap, facilitating the migration of electrons towards Tp and holes towards Bpy, resulting in the oxidation of adsorbed water on Bpy to generate H 2 O 2 . When Tp is in the enol form, it acts as a hole trap, facilitating the migration of holes towards Tp and electrons towards Bpy-H + , resulting in the reduction of adsorbed oxygen on Bpy-H + to generate H 2 O 2 . Consequently, TpBpy exhibits an exceptionally rapid rate of H 2 O 2 photosynthesis, reaching 3.79 mM h − 1 , making it the most efficient catalyst for H 2 O 2 photosynthesis known to date. Based on our experimental findings, we corroborate the unique role of the electron/hole trap interconversion unit in weakening the Coulombic forces acting on charge carriers, thereby promoting ultrafast charge transfer and enhancing the photocatalytic redox process. This charge carrier separation mechanism directly correlates the outstanding photocatalytic performance of the Tp-series imine COFs with the presence of keto-enol tautomerism. Results and Discussion To affirm our hypothesis, we first analyzed and summarized over forty-nine published papers on Tp imine COFs ( Table S1 ). We found that the photocatalytic performance of all Tp imine COFs was much higher than that of BT imine COFs (Fig. 1 a). To further demonstrate the authenticity and universality of this phenomenon, we designed and synthesized eight Tp imine COFs and eight corresponding BT imine COFs, with their unit structures shown in Figure S1 . The synthesis procedures, material characterization findings, and photoelectrochemical tests of these COFs are provided in the supporting information (experimental section and Figures S2-S56 ). Then, we assessed the photocatalytic activity of these COFs for H 2 O 2 photosynthesis. As depicted in Fig. 1 b and Table S2 , the Tp series of imine COFs exhibited significantly enhanced performance in comparison to the BT series of imine COFs, with improvements ranging from several to dozens of times, regardless of the incorporation of monomers with varying lengths, diverse functional group modifications, and differing numbers of linkages (C2 and C3). Among them, TpBpy exhibited the best photocatalytic activity, achieving H 2 O 2 photosynthesis rate of 3.79 mM h − 1 . This rate is presently two orders of magnitude greater than those previously recorded for COFs across a range of bonding types. This includes the covalent triazine series [ 36 ] , sp 2 carbon-conjugated series [ 37 ] , polyimide series [ 38 ] , hydrazone-linked series [ 39 ] , imine series [ 40 – 49 ] , and other bonding categories [ 50 ] , in addition to polymers such as resins [ 51 ] and g-C 3 N 4 [ 52 ] , as illustrated in (Fig. 1 c and Table S3 ). Furthermore, TpBpy also showed much higher activity than BTBpy for photocatalytic H 2 generation, benzoamine conversion, and CO 2 reduction ( Figure S57 ). Clearly, the above photocatalytic results align with the various findings mentioned in Fig. 1 a. This further confirms our initial conjecture that studying imine COF synthesized from Tp is meaningful and also provides additional evidence that the potential keto-enol tautomerism in Tp imine COF has a significant enhancing effect on H 2 O 2 photosynthesis. To reveal the functional molecular structure of Tp imine COFs during the keto-enol tautomerism process, we focused on the imine bonds and hydroxyl groups of Tp molecules ( Figure S58a ). Firstly, we compared the hydroxyl groups with methoxy groups (BTBpy) and molecules without functional groups (OMe-Bpy). The activity of BTBpy and OMe-Bpy were much lower than that of TpBpy ( Figure S58b and Table S4 ), indicating that hydroxyl groups are important functional units for the keto-enol tautomerism process. Secondly, regarding the imine bonds (–C = N–), we compared the activity of TpBpy with S-TpBpy and CN-SP 2 samples in the absence of imine bonds. Their extremely low photocatalytic activity also revealed that imine bonds are important functional units for the keto-enol tautomerism process ( Figure S58b ). More importantly, in this research, the Fourier transform infrared spectra (FT-IR) of these Tp imine COFs ( Figures S59-S79 ) showed the presence of keto and amine bonds, but not enol and imine bonds. The FTIR findings indicate that the Tp-derived imine COFs (“E 3 K 0 ” state for TpBpy, illustrated in Figure S58a ) have undergone a thermodynamic transformation to β-ketoenamine-linked COFs (“E 0 K 3 ” state for TpBpy, illustrated in Figure S58a ). [ 53 – 56 ] Additionally, the chemical structure of TpBpy under Uv-vis irradiation was analyzed by X-ray photoelectron spectroscopy (XPS). As shown in Figure S80 , after half an hour of UV-vis light exposure, a portion of the imine structure emerged, suggesting the presence of amine-to-imine tautomerism, in line with other studies indicating that photoisomerization results in the conversion of keto-amine to enol-imine structures [ 57 ] (“E 1 K 2 ” and “E 2 K 1 ” states for TpBpy, illustrated in Figure S58a ). Hence, the superior photocatalytic performances of Tp imine COFs are reasonably attributed to the characteristic “keto-enol tautomerism”. Nonetheless, further research is necessary to delve into their operational mechanisms in photocatalysis. The ability of the samples to dissociate excitons was assessed by investigating the temperature-dependent photoluminescence (PL) spectra to determine the exciton binding energy ( E b ). The integrated PL intensity of both TpBpy and BTBpy decreased consistently as the temperature rose from 80 to 280 K (Figs. 2 a and 2 b), primarily due to thermally activated non-radiative recombination processes [ 58 , 59 , 60 ] . By using the Arrhenius equation I (T) = I 0 /(1 + Aexp(‒ E b / k B T)) [ 57 ] to fit the experimental data, the E b values of TpBpy and BTBpy were obtained (Fig. 2 c). The E b value for TpBpy was found to be 107.4 meV, significantly lower than that of BTBpy (122.9 meV), suggesting that the incorporation of Tp moieties can accelerate exciton dissociation and thus promote the generation of long-lived photogenerated charge carriers. Subsequently, the surface potentials of TpBpy and BTBpy were measured using an atomic force microscope (AFM) under both dark ( Figure S81 ) and under light irradiation (Figs. 2 d- 2 g) conditions. When exposed to light, excited electrons are generated within the bulk material, and a portion of these charges can be separated and transported to the material surface. This results in the buildup of charges on the surface, leading to a change in surface potential. As depicted in Figs. 2 e and 2 g, TpBpy exhibits a significant increase in average surface potential of 28.35 mV under light conditions, while BTBpy shows a minor increase of 5.09 mV compared to their respective dark states. The substantial variation in surface potential for TpBpy suggests that it possesses higher efficiency in charge separation and transport, allowing for a greater number of electrons and holes with longer lifetimes to drive the photoredox reactions. As expected, the transient photocurrent response of TpBpy shows a greater enhancement compared to that of BTBpy (Figs. 2 h). Additionally, the Nyquist plot of TpBpy under irradiation exhibits a smaller semicircle (the radius corresponds to the catalyst/electrolyte interface resistance, R ct .) than that of BTBpy (Figs. 2 i). Consistent with the E b and surface potential variations, both the transient photocurrent and Nyquist results indicate that Tp moieties are more effective in separating and transferring charges than BT moieties. It is worth noting that the transient photocurrent responses and Nyquist plots of COFs with the structures shown in Figures S1 and S58a were also measured, and these findings consistently suggest higher efficiencies in separating and transferring photogenerated charge carriers in the Tp imine COFs with “keto-enol tautomerism” property. To investigate the dynamic behavior of excited state charge in TpBpy and BTBpy systems, femtosecond transient absorption spectroscopy (fs-TA) was utilized to observe the processes post photoexcitation of the photocatalyst. The samples were excited with 400 nm pump pulses, and the fs-TA spectrum was recorded using a probe pulse. As shown in Figs. 3 a and 3 b, in the wavelength range of 450 ~ 515 nm, the negative signal corresponds to ground-state bleaching (GSB) since the sample can absorb these wavelengths (as shown in the UV-vis absorption spectra in Figure S32 ). Subsequently, after 0.75 ps, the negative signal transitions to a positive signal attributed to excited-state absorption (ESA). Additionally, a stimulated emission (SE) feature at 570 nm, consistent with the steady-state emission spectra ( Figure S82 ), emerges. The rise, stabilization, and decay processes of the signals at 475 nm and 570 nm, as depicted in Fig. 3 c and Table S5 , occur almost simultaneously. The kinetic fitting of the signal transition at 475 nm and 570 nm reveals a rapid rise time constant of 1.66 ps, corresponding to the transfer of excited electrons and holes via vibrational relaxation to the conduction band (CB) bottom and valence band (VB) top, as illustrated in Fig. 3 g. Immediately following photoexcitation, another ESA at 720 nm was observed. The positive ESA signal likely originates from hole transfer from electron-acceptor Bpy moieties to electron-donor BT moieties, as the 2,2′-bipyridine unit possesses stronger electron-accepting ability than the benzene ring unit in BTBpy [ 61 ] . The kinetic fitting of the signal transition at 720 nm shows a rapid kinetic with the time constant of 2.37 ps. Notably, the fitting of these kinetics reveal two transition time constants of 1.66 ~ 2.37 ps and 247 ~ 255 ps, with the fast constant associated with rapid charge transfer ( \(\:{\varvec{\tau\:}}_{\mathbf{T}}\) ) involving annihilation or exciton (and polaron) dissociation aided by transfer and trapping [ 62 , 63 , 64 ] . Once the exciton or polaron is dissociated, the separated state is long-lived. Consequently, the slow-transition constant is associated with carrier recombination ( \(\:{\varvec{\tau\:}}_{\mathbf{R}}\) ), as illustrated in Fig. 3 i [ 65 , 66 ] . In comparison to BTBpy, the negative GSB signal centered at 510 nm in TpBpy exhibits a significant enhancement (Figs. 3 d and 3 e), indicating that the electron in the ground state of TpBpy is more easily excited to the excited state. Besides, there are noticeable negative signals even after 1000 ps across the entire measuring wavelength range, suggesting the generation of abundant long-lived free charge carriers. Interestingly, TpBpy demonstrates a broadened SE ranging from 540 nm to 750 nm. The emission at 650 nm increases at a slower rate compared to the direct excited state at 510 nm. As depicted in Fig. 3 f and Table S6 , the rise at 510 nm is limited by the instrument response function (IRF) and takes approximately 80 fs, while the rise time at 720 nm takes 0.90 ps. This rise time is caused by electron transfer from Bpy moieties to Tp moieties. The rapid charge transport is achieved through the electron and hole trapping effect of Tp moieties, generated by keto-enol tautomerism, as schematically demonstrated in Fig. 3 g. Accordingly, theoretical calculations indicate that the structural isomers resulting from keto-to-enol tautomerism (E 1 K 2 and E 2 K 1 ) exhibit lower CB bottoms and higher VB tops in comparison to the structure lacking tautomerism (E 0 K 3 ). Due to the varied local environments of TpBpy, their SE exhibits inhomogeneous broadening. The kinetic fitting of the signal transition at 650 nm (Fig. 3 f) reveals a decay time constant of 3.14 ps along with an additional long-lived component. The slower decay kinetic (3.14 ps) is probably a result of the trapping process operating in reverse, as shown in Fig. 3 h. The kinetic fitting of the signal transition at 510 nm (Fig. 3 h) reveals two decay time constants of 1.68 ps and 1.08 ns. It is evident that the lifetime ( \(\:{\varvec{\tau\:}}_{\mathbf{R}}\) ) of photogenerated free charge carriers in TpBpy (1.08 ns) has been significantly extended in comparison to BTBpy (0.25 ns) [ 67 ] . Additionally, TpBpy exhibits a smaller \(\:{\varvec{\tau\:}}_{\mathbf{T}}\) (0.90 ~ 1.68 ps), signifying quicker exciton dissociation (in line with a lower exciton binding energy, E b ) and other charge transfer processes (Fig. 3 h). The enhanced charge transport and prolonged carrier lifetime of TpBpy can be attributed to keto-enol tautomerism. In situ Fourier transform infrared (in-situ FTIR) spectrometry is a valuable tool for divulging the photocatalytic mechanisms. The in-situ FTIR spectra of TpBpy and BTBpy during H 2 O 2 photosynthesis under a continuous steam-saturated O 2 flow and water vapor are depicted in Figs. 4 a- 4 d. Vibrations corresponding to C = C (1342 cm -1 ), C-H (1378 cm -1 ), and benzene ring (1441 cm -1 ) for TpBpy are clearly observed in both dark and light conditions, as shown in Figs. 4 a and 4 b. Notably, the signal intensities of vibrations related to C-OH (1402 cm -1 ), O-H (from C-OH, at 3252 cm -1 ), as well as C = C and C = N (1649 cm -1 ) [ 40 , 68 ] increase with prolonged illumination, while the signal intensities of vibrations of C = O (1678 cm -1 ) [ 69 ] and C-N (1321 cm -1 ) decrease. This indicates a gradual transformation from the keto-amine structure of TpBpy stabilized in the dark to the enol-imine structure (as depicted in Figure S58a ). Additionally, new infrared vibration signals at 791 and 1200 cm -1 are attributed to O-O bonding and an endoperoxide intermediate species, respectively. The vibrational intensities of these signals increase gradually with the duration of irradiation. Moreover, the vibrational intensities of PyH + (from 1500 to 1600 cm -1 ), C = NH + (1615 cm -1 ), and N-H + (2727 cm -1 ) also increase with prolonged illumination. In the case of BTBpy (Figs. 4 c and 4 d), vibrations corresponding to C = C (1341, 1653, and 1673 cm -1 from BT and Bpy components), C-H (1382 cm -1 ), benzene ring (1437 cm -1 ), and C = N (imine, formed by the synthesis reaction between BT and Bpy monomers) can be observed in both dark and light conditions. Unlike TpBpy, there are no infrared vibrations of C­OH, C-N, and similar vibrations, suggesting that there is no keto-amine/enol-imine photoisomerization. Upon exposure to light, the vibrations of O-O bonding (791 cm -1 ) [ 70 , 48 ] and endoperoxide intermediate species (1206 cm -1 ) [ 71 ] become visible and increase over time. Concurrently, the protonation level of N increases, as indicated by the rise in vibrational intensities of PyH + , C = NH + (1615 cm -1 ), and N-H + (2784 cm -1 ). The in-situ IR data suggests that BTBpy also follows a 2e - one-step redox process to produce H 2 O 2 , but without mediation by keto-to-enol tautomerism. Significantly, the initial levels of protonation in TpBpy and BTBpy are challenging to observe, and their degrees of protonation escalate as the reaction progresses (due to the presence of catalyst, O 2 , H 2 O, and UV-Vis light). This indicates that protonation occurs concurrently with the photocatalytic reactions and intensifies as the reaction proceeds. Compared to BTBpy, TpBpy exhibits higher intensities of intermediate species and protonation, which aligns with the observed performances of the photocatalytic reactions depicted in Fig. 1 . The in-situ IR spectra of TpBpy were measured under different conditions to investigate the factors affecting the occurrence and degree of keto-to-enol tautomerism. Condition 1 involved dispersing TpBpy in water vapor with O 2 flowing, as shown in Figs. 4 a and 4 b. Condition 2 involved dispersing TpBpy in water vapor with Ar flowing, as shown in Fig. 4 e. Condition 3 involved dispersing TpBpy in Ar flowing, as shown in Fig. 4 f. It was observed that keto-to-enol tautomerism occurs, but to a lesser extent when there is no O 2 flowing (Fig. 4 e). Additionally, there is a slight enhancement in protonation. However, in the absence of O 2 and water vapor, even after 30 minutes of photoexcitation, there were minimal keto-to-enol tautomerism or protonation signals (Fig. 4 f). These findings suggest that UV-vis light is a crucial prerequisite for keto-to-enol tautomerism, and the presence of O 2 and water vapor as reactants significantly enhances the degree of keto-to-enol tautomerism [ 72 ] . Considering the structure tautomerism, intermediate species, and the protonation of N, a keto-enol tautomerism-mediated 2e - one-step redox process leading to the generation of H 2 O 2 can be summarized as depicted in Fig. 4 g. To further investigate the impact of keto-enol tautomerism on charge separation, theoretical calculations of electronic configurations for the initial and intermediate structures presented in Fig. 4 g were performed. The thermodynamically stable β-ketoenamine (E 0 K 3 ) structure serves as the initial structure, followed by the adsorption of water molecules on the Bpy active sites. Subsequently, the photoisomerization of β -ketoenamine (keto-to-enol tautomerism) results in the formation of E 1 K 2 and E 2 K 1 , utilizing photogenerated holes to produce hydrogen peroxide and protonation of pyridine. Notably, before the photocatalytic water oxidation reaction (WOR), the charge on the Tp unit was − 0.449 |e| and − 0.461 |e|, whereas after WOR, the formation of E 1 K 2 and E 2 K 1 through keto-to-enol tautomerism resulted in Tp unit charges of -0.539 |e| and − 1.072 |e|, respectively. Correspondingly, prior to WOR, the Bpy unit side charge in the E 0 K 3 structure was + 0.434 |e| and + 0.803 |e|, while after WOR, the positive charge on the Bpy unit side increased to + 0.908 |e| and + 1.011 |e|. In conjunction with the above transient absorption analysis, it is noticed that the enol-free Tp units act as electron traps, guiding the flow of electrons towards the Tp side and accumulating them, thereby promoting the flow of holes towards the Bpy active sites, where the adsorbed water was photocatalytically oxidized to generate H 2 O 2 . Similarly, comparing the charge density difference before and after the photocatalytic oxygen reduction reaction (ORR), the charge amounts of the Tp unit in E 1 K 2 and E 2 K 1 before ORR are − 0.517 |e| and − 1.285 |e|, respectively. After the ORR, the enol-to-keto tautomerism forms E 0 K 3 , with the charge amount on the Tp unit side being − 0.449 |e|. Correspondingly, in E 1 K 2 and E 2 K 1 before ORR, the positive charge amounts on the Bpy unit side are + 0.722 |e| and + 0.936 |e|, respectively, while in E 0 K 3 after ORR, the positive charge amount on the Bpy unit side decreases to + 0.434 |e|. It can be observed that the enol-containing Tp units formed by keto-to-enol tautomerism act as hole traps, guiding and trapping the holes on the Tp side, thereby promoting the flow of electrons towards the Bpy active sites for photocatalytic reduction reactions, where the adsorbed oxygen on Bpy-H + is reduced to H 2 O 2 . Notably, during oxidation and reduction processes, the charge flow direction in donor-acceptor (D-A) type COFs is consistent and unidirectional, with electrons moving towards the acceptor and holes transferring to the donor. In contrast, in Tp-derived imine COFs like TpBpy, when in the keto form (E 0 K 3 ), electrons flow towards Tp causing oxidation on Bpy side; while in the enol-containing forms (E 1 K 2 and E 2 K 1 ), holes move towards Tp resulting in reduction on Bpy side. The ultimate goal of H 2 O 2 photosynthesis is industrial application. Therefore, the separation-free and continuous-flow reaction process should be the primary condition (Fig. 5 a), and natural sunlight excitation with real spectral range, zero energy consumption, and excellent environmental adaptability is another condition (Fig. 5 b). Here, firstly, a flow reactor capable of continuously producing products without the need for subsequent separation processes was employed to assess the performance of H 2 O 2 photosynthesis ( Figure S83 and S84 ). In this setup, TpBpy served as the photocatalytic filling material ( Figure S83 ), while a peristaltic pump regulated the flow rate of ultrapure water throughout the system. Initially, a Xenon lamp was employed as the simulated sunlight source in a laboratory environment ( Figure S84 ). As shown in Fig. 5 c, the average H 2 O 2 concentration recorded over a 20-hour period was 172 µM, and the concentration of H 2 O 2 produced by TpBpy remained relatively stable. The photocatalytic generation rate of H 2 O 2 in our custom-built flow reactor reached 1429 mM h − 1 m − 2 for TpBpy, considerably higher than the recently reported sunlight-driven synthesis rate in a flow reaction system, such as TAPT–FTPB COFs (376 mM h − 1 m − 2 ). [ 3 ] Furthermore, TpBpy demonstrated a notably improved solar-to-chemical conversion (SCC) efficiency of 0.040%, exceeding the performance of TAPT–FTPB COFs at 0.010%. [ 3 ] Subsequently, to achieve the ultimate goal of H 2 O 2 photosynthesis, natural sunlight was used to as the light source in the open-air environment ( Figure S85 ). As shown in Fig. 5 d, the H 2 O 2 concentration produced by photosynthesis and the corresponding solar-to-chemical conversion (SCC) efficiency were found to be positively and negatively correlated with natural sunlight intensity, respectively. The average photocatalytic generation rate of H 2 O 2 reached 1030 mM h − 1 m − 2 , and an SCC efficiency reached 0.038%. This consistent stability over the course of the 3-day outdoor experiment and 20-hour indoor experiment for H 2 O 2 production demonstrates the remarkable cyclic stability of TpBpy. To date, there have been no reports on the use of COFs in flow reactions for the outdoor production of H 2 O 2 . Consequently, to compare the H 2 O 2 photocatalytic efficiency of the separation-free system under natural sunlight, we investigated all the reported immobilization systems ( Table S7 ). As shown in Figure S86 and S87 , by immobilizing the TpBpy catalyst onto a glass slide (0.3 m × 0.4 m) as a coating film and subsequently submerging the catalyst-coated glass slide in stagnant ultrapure water (18.25 MΩ cm), the immobilization reactor utilizing TpBpy achieved a H 2 O 2 photosynthesis SCC of 0.029%. This exceeds all the reported performance of immobilization systems, such as COF-2CN, (0.0075%) [ 73 ] , PI-BD-TPB (0.024%) [ 74 ] , and COF-N32 (0.019%) [ 40 ] . Additionally, the SCC of dispersion systems under natural sunlight was also compared. TpBpy showed the highest SCC among all reported photocatalytic materials under similar conditions ( Figure S88 and Table S7 ). Therefore, whether in the flow phase, immobilization phase, or dispersion phase, TpBpy exhibited superior SCC performance compared to other photocatalysts under natural sunlight. It is evident that the SCC of all photocatalysts for H 2 O 2 production under natural sunlight remains far lower than the 0.10% associated with natural photosynthesis. Therefore, improving photocatalytic performance remains the foremost challenge for the industrial application of photocatalytic H 2 O 2 synthesis. Although recent reports indicate that SCC values have exceeded 1.00% under laboratory conditions, these high SCC values often result from continuous optimization of various experimental parameters. Some studies even fail to provide complete experimental details and data, leading to a misleading assessment of the maturity of photocatalytic H 2 O 2 synthesis technology. When utilizing fixed-bed reactors that mimic the structure of plant leaves and employing natural sunlight as the light source, the SCC for photocatalytic H 2 O 2 synthesis holds greater scientific significance. Therefore, we advocate for the use of SCC values obtained under natural sunlight in immobilization or flowing phase reactors as a comparative metric for evaluating the performance of photocatalytic H 2 O 2 synthesis. Conclusion In this work, it was demonstrated that the photoinduced keto-enol tautomerism can act as variable electron/hole traps, promoting carrier separation for effective H 2 O 2 photosynthesis. The keto-enol tautomerism facilitates exciton dissociation and charge transfer, resulting in shorter charge transfer duration and longer free carrier lifetime for photoredox reactions to take place. The Tp imine COFs exhibited increased photo-induced dynamic behavior, including elevated photocurrent densities, lower charge transfer resistances, and significantly improved photocatalytic activities compared to BT imine COFs with improvements ranging from several to dozens of times. Through in-situ FTIR spectra and fs-TA analysis, supported by theoretical calculations, it was observed that Tp moieties can transition between acting as electron traps and hole traps due to keto-enol tautomerism in TpBpy. In the keto form (E 0 K 3 ), electrons migrate towards Tp trap and holes towards Bpy side, leading to adsorbed water been oxidized at the Bpy catalytic active sites. Conversely, in the enol-containing forms (E 1 K 2 and E 2 K 1 ), holes move towards Tp trap and electrons towards Bpy side, resulting in adsorbed oxygen on Bpy-H + been reduced at the Bpy catalytic active sites. Based on this advanced carrier separation mechanism, TpBpy achieved an exceptionally rapid H 2 O 2 photosynthesis rate of 3.79 mM h -1 in pure water. And it also exhibits remarkable efficiency in the photocatalytic generation of H 2 O 2 in a flow-reactor system under natural sunlight, achieving a solar-to-chemical conversion efficiency of 0.038%, exceeding the performance of all previously documented photocatalysts to the best of our knowledge. Declarations Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 22476109, 2476110), the Hubei Provincial Natural Science Foundation of China (No. 2022CFA065), and the 111 Project (D20015). X. Y. K. acknowledges the support from the Lee Kuan Yew Postdoctoral Fellowship with start-up grant (024042-00001). T. 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TpBpy displayed exceptional effectiveness for continuous photocatalytic H 2 O 2 synthesis, achieving a solar-to-chemical conversion (SCC) efficiency of 0.038%, representing the highest performance recorded to date under natural sunlight conditions. Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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Ma","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA6ElEQVRIiWNgGAWjYDACZhBhAMTsDQwMCSDOAQI6eOBaeIBKExKI0QJnSYCUE6PFnp352WOeAoZo/plvn254+INBju9GAuNnHjxaeJjZzI15DBhyZ9xON7sBdJix5I0EZmn8WhjMpEFaGm6nsYG0JG64kcBAQAv7N7CW+TePgbXUA7Uw/8avhQdiy4YbbGAtCQY3Etjw23KYp0xyjoFE7sYzIIelSRjOPPOwzXIOHi3s/ce3Sbz5Y5M77/gxtps/bGzk+Y4nH77xBo8WEGDiYZCAsUEMxgYCGoBKfhBUMgpGwSgYBSMaAADJWUX74X+OcgAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-1042-8700","institution":"RMIT University","correspondingAuthor":true,"prefix":"","firstName":"Tianyi","middleName":"","lastName":"Ma","suffix":""},{"id":381614085,"identity":"af7559ae-5d7c-4e29-b4c4-05ce99fb2a57","order_by":1,"name":"Fang Ma","email":"","orcid":"","institution":"China Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Fang","middleName":"","lastName":"Ma","suffix":""},{"id":381614086,"identity":"99f3bc74-8cc3-444c-94bc-82be497fca7a","order_by":2,"name":"Xiaodong Sun","email":"","orcid":"","institution":"Liaoning University","correspondingAuthor":false,"prefix":"","firstName":"Xiaodong","middleName":"","lastName":"Sun","suffix":""},{"id":381614087,"identity":"35bc8b79-0257-48a7-8e12-096727ba151a","order_by":3,"name":"Liqun Ye","email":"","orcid":"https://orcid.org/0000-0001-6410-689X","institution":"China Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Liqun","middleName":"","lastName":"Ye","suffix":""},{"id":381614088,"identity":"cd159fa0-6cfe-4df1-8b81-5418a9f64e7d","order_by":4,"name":"Chunqiu Han","email":"","orcid":"","institution":"China Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Chunqiu","middleName":"","lastName":"Han","suffix":""},{"id":381614089,"identity":"b16f768e-3424-4639-8058-8d7ee7efac75","order_by":5,"name":"Yongye Wang","email":"","orcid":"","institution":"China Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Yongye","middleName":"","lastName":"Wang","suffix":""},{"id":381614090,"identity":"a19cab84-882c-4c9a-9d3a-88584d2d4e0b","order_by":6,"name":"Anqiang Jiang","email":"","orcid":"","institution":"China Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Anqiang","middleName":"","lastName":"Jiang","suffix":""},{"id":381614091,"identity":"d3508806-92ed-47c9-900f-616d16450861","order_by":7,"name":"Ying Zhou","email":"","orcid":"","institution":"School of Oil \u0026 Natural Gas Engineering, Southwest Petroleum University","correspondingAuthor":false,"prefix":"","firstName":"Ying","middleName":"","lastName":"Zhou","suffix":""},{"id":381614092,"identity":"d46ff17f-c2bb-43c2-820f-a0e0e88897e2","order_by":8,"name":"Guijie Liang","email":"","orcid":"https://orcid.org/0000-0003-3005-5929","institution":"Hubei University of Arts and Science","correspondingAuthor":false,"prefix":"","firstName":"Guijie","middleName":"","lastName":"Liang","suffix":""},{"id":381614093,"identity":"26b5b9be-a664-45b3-814a-d82cd2c42726","order_by":9,"name":"Huiqing Wang","email":"","orcid":"","institution":"China Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Huiqing","middleName":"","lastName":"Wang","suffix":""},{"id":381614094,"identity":"7849d20a-12af-4a81-b11a-49d8499808b2","order_by":10,"name":"Li Wang","email":"","orcid":"","institution":"China Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Li","middleName":"","lastName":"Wang","suffix":""},{"id":381614095,"identity":"3b8fce8f-2fda-40ea-8888-e8387669df0e","order_by":11,"name":"Binbin Jia","email":"","orcid":"","institution":"China Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Binbin","middleName":"","lastName":"Jia","suffix":""},{"id":381614096,"identity":"ecf57c13-f382-4439-9a81-17fd93e2a8fb","order_by":12,"name":"Yingping Huang","email":"","orcid":"","institution":"China Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Yingping","middleName":"","lastName":"Huang","suffix":""},{"id":381614097,"identity":"9813f78f-6499-4169-91cf-ecb5e7830768","order_by":13,"name":"Hongwei Huang","email":"","orcid":"https://orcid.org/0000-0003-0271-1079","institution":"China University of Geosciences Bejing","correspondingAuthor":false,"prefix":"","firstName":"Hongwei","middleName":"","lastName":"Huang","suffix":""},{"id":381614098,"identity":"eead499a-e712-43de-96b5-1f1eea828a6e","order_by":14,"name":"Xin Ying Kong","email":"","orcid":"https://orcid.org/0000-0003-3492-1388","institution":"Nanyang Technological University","correspondingAuthor":false,"prefix":"","firstName":"Xin","middleName":"Ying","lastName":"Kong","suffix":""},{"id":381614099,"identity":"94fb5764-a0e5-4c9d-82d9-7bb99db331c0","order_by":15,"name":"Hui Li","email":"","orcid":"","institution":"RMIT University","correspondingAuthor":false,"prefix":"","firstName":"Hui","middleName":"","lastName":"Li","suffix":""},{"id":381614100,"identity":"d6edba50-326d-443b-ab27-9933ba15412c","order_by":16,"name":"Niu Huang","email":"","orcid":"https://orcid.org/0000-0001-7860-9199","institution":"China Three Gorges University","correspondingAuthor":false,"prefix":"","firstName":"Niu","middleName":"","lastName":"Huang","suffix":""}],"badges":[],"createdAt":"2024-10-06 06:15:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5211465/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5211465/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":72585609,"identity":"33abdd84-22f3-4281-b353-69cfdbfe1891","added_by":"auto","created_at":"2024-12-30 06:11:22","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":282722,"visible":true,"origin":"","legend":"\u003cp\u003e(a) A diagram depicting the structural and performance differences between COFs synthesized using the Tp monomer and those synthesized using the BT monomer. (b) Photocatalytic performances of the COFs in H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e synthesis (Conditions: 298 K, Xenon lamp, λ \u0026gt; 420 nm, light intensity of 100 mW cm\u003csup\u003e-2\u003c/sup\u003e, 10 mg catalyst, and 10 mL water). (c) Comparison of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production rates for various COFs, highlighting the superior performance of TpBpy (this work) relative to other reported COFs.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5211465/v1/154406ed6df8dca1c3c937c2.png"},{"id":72584256,"identity":"08cf4664-290c-4865-b80e-a82e64ec7a11","added_by":"auto","created_at":"2024-12-30 06:03:22","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":218297,"visible":true,"origin":"","legend":"\u003cp\u003e(a-b) Temperature-dependent PL spectra with excitation wavelength at 455 nm and (c) extracted exciton binding energies of BTBpy and TpBpy. AFM images in surface potential mode for (d) BTBpy and (f) TpBpy under light illumination. Surface potential profiles along the lines in (e) and (g) for (d) BTBpy and (f) TpBpy. (h) Photocurrent response and (i) EIS spectra of BTBpy and TpBpy.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5211465/v1/a0cf54ee54b98bcdae41f4f0.png"},{"id":72584260,"identity":"4a9cff94-69aa-4df4-8bb4-26cc8cda9edd","added_by":"auto","created_at":"2024-12-30 06:03:22","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":183360,"visible":true,"origin":"","legend":"\u003cp\u003eTransient absorption (TA) information of BTBpyand TpBpy excited at 400 nm. TA spectra of (a) BTBpy and (d) TpBpy. Time slices of the TA spectra of (b) BTBpy and (e) TpBpy in water. TA kinetics of (c) BTBpy probed at 475 nm, 570 nm, and 720 nm, and (f) TpBpy probed at 510 nm and 650 nm.\u003cstrong\u003e \u003c/strong\u003eSchematic illustration of electron transitions after photoexcitation for (g) BTBpyand (h) TpBpy. (i) Charge transfer kinetic constants derived from Figures 5c and 5f.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5211465/v1/b270ae2db95e52d6c64cef81.png"},{"id":72585720,"identity":"4ee85073-e127-400d-82d8-f757969edd18","added_by":"auto","created_at":"2024-12-30 06:19:22","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":211270,"visible":true,"origin":"","legend":"\u003cp\u003eIn situ FT-IR spectra of (a-b) TpBpy in water vapor with O\u003csub\u003e2\u003c/sub\u003e gas flowing, (c-d) BTBpy in water vapor with O\u003csub\u003e2\u003c/sub\u003e gas flowing, (e) TpBpy in water vapor with Ar gas flowing, and (f) TpBpy in Ar gas flowing. (g) Charge density difference of the TpBpy catalyst during the H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis processes.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5211465/v1/999113c6652798ec689f183d.png"},{"id":72584261,"identity":"09bd39d0-5541-4ea8-8c41-991f43200c04","added_by":"auto","created_at":"2024-12-30 06:03:22","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":215811,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Development history of photocatalytic synthesis of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e for industrial applications in reactors, (b) Comparison between simulated sunlight and natural sunlight, (c) H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production performance in flow method and its corresponding SCC efficiency in laboratory environment under simulated sunlight irradiation, (d) H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production performance in flow method and its corresponding SCC efficiency in the open air under natural sunlight irradiation.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5211465/v1/87defb5ef980fee8fb27da91.png"},{"id":72587141,"identity":"fd6ba75e-d127-4dc8-b0fc-cdbbd5508aa4","added_by":"auto","created_at":"2024-12-30 06:35:23","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1575741,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5211465/v1/e75e5ab4-9440-4ba6-9de1-c5a36204e04e.pdf"},{"id":72584262,"identity":"1daf797d-3a76-48cf-85bb-811b267d0219","added_by":"auto","created_at":"2024-12-30 06:03:22","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":39891892,"visible":true,"origin":"","legend":"Supporting Information","description":"","filename":"SupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-5211465/v1/969f3b9bd442104566bdbe5c.docx"},{"id":72586953,"identity":"d838c913-0920-4026-b67a-617c27c88732","added_by":"auto","created_at":"2024-12-30 06:27:22","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":22055,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTOC\u003c/strong\u003e:\u003c/p\u003e\n\u003cp\u003eTp imine COFs displayed the higher photocatalytic activity than BT imine COF due to the presence of keto-enol tautomerization, which can be as variable electron/hole traps to promote carrier separation. TpBpy displayed exceptional effectiveness for continuous photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e synthesis, achieving a solar-to-chemical conversion (SCC) efficiency of 0.038%, representing the highest performance recorded to date under natural sunlight conditions.\u003c/p\u003e","description":"","filename":"TOC.png","url":"https://assets-eu.researchsquare.com/files/rs-5211465/v1/8ba339f25323f50858c3df44.png"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"\u003cp\u003eKeto-enol tautomerism as transformative electron/hole traps to promote charge carrier separation for record-high H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis in real world\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eCovalent organic frameworks (COFs) represent an innovative category of photocatalytic materials that have gained significant attention for the photocatalytic production of hydrogen peroxide (H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e)\u003csup\u003e[\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e. The advantages of COFs can be summarized as follows: (1) The highly organized porous architecture of COFs offer an increased number of reactive sites while minimizing the carrier migration distance to the sub-nanometer scale, thereby improving the efficiency of photocatalytic processes; (2) The spectral absorption range of COFs can be extended to the visible and infrared regions through opportune molecular design, providing a good platform for the utilization of visible light energy and infrared heat energy; (3) Specific reaction active sites can be efficiently designed based on the characteristics of different photocatalytic reactions\u003csup\u003e[\u003cspan additionalcitationids=\"CR5 CR6 CR7 CR8 CR9 CR10 CR11\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e. However, the current photocatalytic performance of COFs for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis falls significantly short of industrial requirements. The primary reason is that, although the sub-nanometer migration distance of charge carriers in COFs provides convenience for catalytic reactions, this same scale of migration distance is incapable to sufficiently weaken the Coulomb forces between photogenerated electrons and holes. Consequently, this limitation hampers the separation of charge carriers\u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. In other words, the ability of COFs is limited, which poses a considerable obstacle to the sustained separation of photogenerated carriers, thereby challenging their practical application in H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eOver the past decade, researchers have extensively focused on the problem of photogenerated charge carrier separation in COFs. The most promising approach is to utilize the effective molecular design advantages of COFs to construct a series of donor acceptor (D-A) structured COFs\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e. By regulating the charge distribution difference between donor and acceptor units, a push-pull effect mechanism is induced, facilitating the migration of photogenerated electrons and holes to the acceptor and donor units, respectively\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. However, although the D-A structure holds promise for improving the separation of photogenerated charge carriers, a significant gap remains between the rapid migration of these charge carriers and the comparatively slower kinetics of the water oxidation and oxygen reduction reactions that occur on the D and A units. The reaction rates are generally several orders of magnitude lower than the migration rates of the charge carriers. This discrepancy leads to the accumulation of photogenerated carriers, causing strong Coulombic interactions among the carriers. Such interactions hinder the separation efficiency of the photoinduced electron-hole pairs in D-A structured COFs, which ultimately limits the photoactivity of COFs for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis. Currently, this issue represents a common challenge faced by almost all the existing photocatalysts and photocatalytic reactions\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAt present, the most effective approach to address the conflict between swift carrier migration and sluggish oxidation-reduction reactions is the addition of hole-sacrificial reagents to the photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production system. These sacrificial reagents rapidly consume the photogenerated holes present on the catalyst surface, thereby diminishing the Coulombic forces among photogenerated carriers. This process leads to an increase in the number of photogenerated electrons available for O\u003csub\u003e2\u003c/sub\u003e reduction, ultimately enhancing H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e. However, this method also possesses several significant drawbacks. Firstly, this strategy is unable to accelerate both photocatalytic H\u003csub\u003e2\u003c/sub\u003eO oxidation and O\u003csub\u003e2\u003c/sub\u003e reduction reactions concurrently, as the photogenerated holes are consumed by the sacrificial reagents\u003csup\u003e[\u003cspan additionalcitationids=\"CR23 CR24 CR25 CR26\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e. Secondly, the introduction of sacrificial reagents complicates the catalytic system and adversely affects the purity of the H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e solution. Lastly, the incorporation of these sacrificial reagents increases economic costs, and the majority of these sacrificial reagents are not environmentally friendly, imposing considerable environmental issues\u003csup\u003e[\u003cspan additionalcitationids=\"CR28 CR29 CR30 CR31 CR32 CR33 CR34\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e. Therefore, to address the discrepancy between rapid carrier migration and slow oxidation-reduction reactions, it is still necessary to focus on catalyst design. Here, we draw inspiration from the sacrificial reagent scheme to create units for the interconversion of electron and hole traps within the catalyst. These units are designed to alternately capture photogenerated electrons and holes, while the resulting photogenerated holes and electrons then engage in photocatalytic oxidation and reduction reactions, respectively. This methodology effectively reduces Coulombic forces by limiting the accumulation of photogenerated electrons or holes, enabling ultrafast charge transfer and longer-lived free charge carriers. This ultimately facilitates effective alternating photocatalytic H\u003csub\u003e2\u003c/sub\u003eO oxidation and O\u003csub\u003e2\u003c/sub\u003e reduction reactions, promoting H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis.\u003c/p\u003e \u003cp\u003eBased on the above considerations, we have designed a series of Tp (2,4,6-trihydroxybenzaldehyde-1,3,5-tricarbaldehyde) imine COFs with keto-enol tautomerism, which exhibit significantly higher photocatalytic performance compared to the BT (1,3,5-benzenetricarboxaldehyde) series COFs that lack of keto-enol tautomerism. Mechanistic studies indicate that Tp-series imine COFs display enhanced photoinduced dynamic behavior, including increased photocurrent density, decreased charge transfer resistance, and enhanced surface photopotential. Taking the optimal TpBpy photocatalyst as a case study, in situ infrared spectroscopy, femtosecond time-resolved spectroscopy, and theoretical calculations have demonstrated that the Tp unit in TpBpy alternates between electron and hole traps during the keto-enol tautomerism process. When Tp is in ketone form, it acts as an electron trap, facilitating the migration of electrons towards Tp and holes towards Bpy, resulting in the oxidation of adsorbed water on Bpy to generate H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. When Tp is in the enol form, it acts as a hole trap, facilitating the migration of holes towards Tp and electrons towards Bpy-H\u003csup\u003e+\u003c/sup\u003e, resulting in the reduction of adsorbed oxygen on Bpy-H\u003csup\u003e+\u003c/sup\u003e to generate H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. Consequently, TpBpy exhibits an exceptionally rapid rate of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis, reaching 3.79 mM h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, making it the most efficient catalyst for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis known to date. Based on our experimental findings, we corroborate the unique role of the electron/hole trap interconversion unit in weakening the Coulombic forces acting on charge carriers, thereby promoting ultrafast charge transfer and enhancing the photocatalytic redox process. This charge carrier separation mechanism directly correlates the outstanding photocatalytic performance of the Tp-series imine COFs with the presence of keto-enol tautomerism.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003e \u003c/p\u003e \u003cp\u003eTo affirm our hypothesis, we first analyzed and summarized over forty-nine published papers on Tp imine COFs (\u003cb\u003eTable \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e). We found that the photocatalytic performance of all Tp imine COFs was much higher than that of BT imine COFs (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). To further demonstrate the authenticity and universality of this phenomenon, we designed and synthesized eight Tp imine COFs and eight corresponding BT imine COFs, with their unit structures shown in \u003cb\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e. The synthesis procedures, material characterization findings, and photoelectrochemical tests of these COFs are provided in the supporting information (experimental section and \u003cb\u003eFigures S2-S56\u003c/b\u003e). Then, we assessed the photocatalytic activity of these COFs for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb and \u003cb\u003eTable S2\u003c/b\u003e, the Tp series of imine COFs exhibited significantly enhanced performance in comparison to the BT series of imine COFs, with improvements ranging from several to dozens of times, regardless of the incorporation of monomers with varying lengths, diverse functional group modifications, and differing numbers of linkages (C2 and C3). Among them, TpBpy exhibited the best photocatalytic activity, achieving H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis rate of 3.79 mM h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. This rate is presently two orders of magnitude greater than those previously recorded for COFs across a range of bonding types. This includes the covalent triazine series\u003csup\u003e[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/sup\u003e, sp\u003csup\u003e2\u003c/sup\u003e carbon-conjugated series\u003csup\u003e[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/sup\u003e, polyimide series\u003csup\u003e[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/sup\u003e, hydrazone-linked series\u003csup\u003e[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/sup\u003e, imine series\u003csup\u003e[\u003cspan additionalcitationids=\"CR41 CR42 CR43 CR44 CR45 CR46 CR47 CR48\" citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]\u003c/sup\u003e, and other bonding categories\u003csup\u003e[\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]\u003c/sup\u003e, in addition to polymers such as resins\u003csup\u003e[\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]\u003c/sup\u003e and g-C\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]\u003c/sup\u003e, as illustrated in (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec and \u003cb\u003eTable S3\u003c/b\u003e). Furthermore, TpBpy also showed much higher activity than BTBpy for photocatalytic H\u003csub\u003e2\u003c/sub\u003e generation, benzoamine conversion, and CO\u003csub\u003e2\u003c/sub\u003e reduction (\u003cb\u003eFigure S57\u003c/b\u003e). Clearly, the above photocatalytic results align with the various findings mentioned in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. This further confirms our initial conjecture that studying imine COF synthesized from Tp is meaningful and also provides additional evidence that the potential keto-enol tautomerism in Tp imine COF has a significant enhancing effect on H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis.\u003c/p\u003e \u003cp\u003eTo reveal the functional molecular structure of Tp imine COFs during the keto-enol tautomerism process, we focused on the imine bonds and hydroxyl groups of Tp molecules (\u003cb\u003eFigure S58a\u003c/b\u003e). Firstly, we compared the hydroxyl groups with methoxy groups (BTBpy) and molecules without functional groups (OMe-Bpy). The activity of BTBpy and OMe-Bpy were much lower than that of TpBpy (\u003cb\u003eFigure S58b\u003c/b\u003e and \u003cb\u003eTable S4\u003c/b\u003e), indicating that hydroxyl groups are important functional units for the keto-enol tautomerism process. Secondly, regarding the imine bonds (\u0026ndash;C\u0026thinsp;=\u0026thinsp;N\u0026ndash;), we compared the activity of TpBpy with S-TpBpy and CN-SP\u003csub\u003e2\u003c/sub\u003e samples in the absence of imine bonds. Their extremely low photocatalytic activity also revealed that imine bonds are important functional units for the keto-enol tautomerism process (\u003cb\u003eFigure S58b\u003c/b\u003e). More importantly, in this research, the Fourier transform infrared spectra (FT-IR) of these Tp imine COFs (\u003cb\u003eFigures S59-S79\u003c/b\u003e) showed the presence of keto and amine bonds, but not enol and imine bonds. The FTIR findings indicate that the Tp-derived imine COFs (\u0026ldquo;E\u003csub\u003e3\u003c/sub\u003eK\u003csub\u003e0\u003c/sub\u003e\u0026rdquo; state for TpBpy, illustrated in \u003cb\u003eFigure S58a\u003c/b\u003e) have undergone a thermodynamic transformation to β-ketoenamine-linked COFs (\u0026ldquo;E\u003csub\u003e0\u003c/sub\u003eK\u003csub\u003e3\u003c/sub\u003e\u0026rdquo; state for TpBpy, illustrated in \u003cb\u003eFigure S58a\u003c/b\u003e).\u003csup\u003e[\u003cspan additionalcitationids=\"CR54 CR55\" citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]\u003c/sup\u003e Additionally, the chemical structure of TpBpy under Uv-vis irradiation was analyzed by X-ray photoelectron spectroscopy (XPS). As shown in \u003cb\u003eFigure S80\u003c/b\u003e, after half an hour of UV-vis light exposure, a portion of the imine structure emerged, suggesting the presence of amine-to-imine tautomerism, in line with other studies indicating that photoisomerization results in the conversion of keto-amine to enol-imine structures\u003csup\u003e[\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e]\u003c/sup\u003e (\u0026ldquo;E\u003csub\u003e1\u003c/sub\u003eK\u003csub\u003e2\u003c/sub\u003e\u0026rdquo; and \u0026ldquo;E\u003csub\u003e2\u003c/sub\u003eK\u003csub\u003e1\u003c/sub\u003e\u0026rdquo; states for TpBpy, illustrated in \u003cb\u003eFigure S58a\u003c/b\u003e). Hence, the superior photocatalytic performances of Tp imine COFs are reasonably attributed to the characteristic \u0026ldquo;keto-enol tautomerism\u0026rdquo;. Nonetheless, further research is necessary to delve into their operational mechanisms in photocatalysis.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe ability of the samples to dissociate excitons was assessed by investigating the temperature-dependent photoluminescence (PL) spectra to determine the exciton binding energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e). The integrated PL intensity of both TpBpy and BTBpy decreased consistently as the temperature rose from 80 to 280 K (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), primarily due to thermally activated non-radiative recombination processes\u003csup\u003e[\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e, \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e]\u003c/sup\u003e. By using the Arrhenius equation \u003cem\u003eI\u003c/em\u003e(T)\u0026thinsp;=\u0026thinsp;\u003cem\u003eI\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e/(1\u0026thinsp;+\u0026thinsp;Aexp(‒\u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e/\u003cem\u003ek\u003c/em\u003e\u003csub\u003eB\u003c/sub\u003eT))\u003csup\u003e[\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e]\u003c/sup\u003e to fit the experimental data, the \u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e values of TpBpy and BTBpy were obtained (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). The \u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e value for TpBpy was found to be 107.4 meV, significantly lower than that of BTBpy (122.9 meV), suggesting that the incorporation of Tp moieties can accelerate exciton dissociation and thus promote the generation of long-lived photogenerated charge carriers. Subsequently, the surface potentials of TpBpy and BTBpy were measured using an atomic force microscope (AFM) under both dark (\u003cb\u003eFigure S81\u003c/b\u003e) and under light irradiation (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed-\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg) conditions. When exposed to light, excited electrons are generated within the bulk material, and a portion of these charges can be separated and transported to the material surface. This results in the buildup of charges on the surface, leading to a change in surface potential. As depicted in Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg, TpBpy exhibits a significant increase in average surface potential of 28.35 mV under light conditions, while BTBpy shows a minor increase of 5.09 mV compared to their respective dark states. The substantial variation in surface potential for TpBpy suggests that it possesses higher efficiency in charge separation and transport, allowing for a greater number of electrons and holes with longer lifetimes to drive the photoredox reactions. As expected, the transient photocurrent response of TpBpy shows a greater enhancement compared to that of BTBpy (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh). Additionally, the Nyquist plot of TpBpy under irradiation exhibits a smaller semicircle (the radius corresponds to the catalyst/electrolyte interface resistance, \u003cem\u003eR\u003c/em\u003e\u003csub\u003ect\u003c/sub\u003e.) than that of BTBpy (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei). Consistent with the \u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e and surface potential variations, both the transient photocurrent and Nyquist results indicate that Tp moieties are more effective in separating and transferring charges than BT moieties. It is worth noting that the transient photocurrent responses and Nyquist plots of COFs with the structures shown in \u003cb\u003eFigures \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e and \u003cb\u003eS58a\u003c/b\u003e were also measured, and these findings consistently suggest higher efficiencies in separating and transferring photogenerated charge carriers in the Tp imine COFs with \u0026ldquo;keto-enol tautomerism\u0026rdquo; property.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo investigate the dynamic behavior of excited state charge in TpBpy and BTBpy systems, femtosecond transient absorption spectroscopy (fs-TA) was utilized to observe the processes post photoexcitation of the photocatalyst. The samples were excited with 400 nm pump pulses, and the fs-TA spectrum was recorded using a probe pulse. As shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, in the wavelength range of 450\u0026thinsp;~\u0026thinsp;515 nm, the negative signal corresponds to ground-state bleaching (GSB) since the sample can absorb these wavelengths (as shown in the UV-vis absorption spectra in \u003cb\u003eFigure S32\u003c/b\u003e). Subsequently, after 0.75 ps, the negative signal transitions to a positive signal attributed to excited-state absorption (ESA). Additionally, a stimulated emission (SE) feature at 570 nm, consistent with the steady-state emission spectra (\u003cb\u003eFigure S82\u003c/b\u003e), emerges. The rise, stabilization, and decay processes of the signals at 475 nm and 570 nm, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec and \u003cb\u003eTable S5\u003c/b\u003e, occur almost simultaneously. The kinetic fitting of the signal transition at 475 nm and 570 nm reveals a rapid rise time constant of 1.66 ps, corresponding to the transfer of excited electrons and holes via vibrational relaxation to the conduction band (CB) bottom and valence band (VB) top, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg. Immediately following photoexcitation, another ESA at 720 nm was observed. The positive ESA signal likely originates from hole transfer from electron-acceptor Bpy moieties to electron-donor BT moieties, as the 2,2\u0026prime;-bipyridine unit possesses stronger electron-accepting ability than the benzene ring unit in BTBpy\u003csup\u003e[\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e]\u003c/sup\u003e. The kinetic fitting of the signal transition at 720 nm shows a rapid kinetic with the time constant of 2.37 ps. Notably, the fitting of these kinetics reveal two transition time constants of 1.66\u0026thinsp;~\u0026thinsp;2.37 ps and 247\u0026thinsp;~\u0026thinsp;255 ps, with the fast constant associated with rapid charge transfer (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varvec{\\tau\\:}}_{\\mathbf{T}}\\)\u003c/span\u003e\u003c/span\u003e) involving annihilation or exciton (and polaron) dissociation aided by transfer and trapping\u003csup\u003e[\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e, \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e, \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e]\u003c/sup\u003e. Once the exciton or polaron is dissociated, the separated state is long-lived. Consequently, the slow-transition constant is associated with carrier recombination (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varvec{\\tau\\:}}_{\\mathbf{R}}\\)\u003c/span\u003e\u003c/span\u003e), as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ei\u003csup\u003e[\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e, \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn comparison to BTBpy, the negative GSB signal centered at 510 nm in TpBpy exhibits a significant enhancement (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee), indicating that the electron in the ground state of TpBpy is more easily excited to the excited state. Besides, there are noticeable negative signals even after 1000 ps across the entire measuring wavelength range, suggesting the generation of abundant long-lived free charge carriers. Interestingly, TpBpy demonstrates a broadened SE ranging from 540 nm to 750 nm. The emission at 650 nm increases at a slower rate compared to the direct excited state at 510 nm. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef and \u003cb\u003eTable S6\u003c/b\u003e, the rise at 510 nm is limited by the instrument response function (IRF) and takes approximately 80 fs, while the rise time at 720 nm takes 0.90 ps. This rise time is caused by electron transfer from Bpy moieties to Tp moieties. The rapid charge transport is achieved through the electron and hole trapping effect of Tp moieties, generated by keto-enol tautomerism, as schematically demonstrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg. Accordingly, theoretical calculations indicate that the structural isomers resulting from keto-to-enol tautomerism (E\u003csub\u003e1\u003c/sub\u003eK\u003csub\u003e2\u003c/sub\u003e and E\u003csub\u003e2\u003c/sub\u003eK\u003csub\u003e1\u003c/sub\u003e) exhibit lower CB bottoms and higher VB tops in comparison to the structure lacking tautomerism (E\u003csub\u003e0\u003c/sub\u003eK\u003csub\u003e3\u003c/sub\u003e). Due to the varied local environments of TpBpy, their SE exhibits inhomogeneous broadening. The kinetic fitting of the signal transition at 650 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef) reveals a decay time constant of 3.14 ps along with an additional long-lived component. The slower decay kinetic (3.14 ps) is probably a result of the trapping process operating in reverse, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh. The kinetic fitting of the signal transition at 510 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh) reveals two decay time constants of 1.68 ps and 1.08 ns. It is evident that the lifetime (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varvec{\\tau\\:}}_{\\mathbf{R}}\\)\u003c/span\u003e\u003c/span\u003e) of photogenerated free charge carriers in TpBpy (1.08 ns) has been significantly extended in comparison to BTBpy (0.25 ns)\u003csup\u003e[\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e]\u003c/sup\u003e. Additionally, TpBpy exhibits a smaller \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varvec{\\tau\\:}}_{\\mathbf{T}}\\)\u003c/span\u003e\u003c/span\u003e (0.90\u0026thinsp;~\u0026thinsp;1.68 ps), signifying quicker exciton dissociation (in line with a lower exciton binding energy, \u003cem\u003eE\u003c/em\u003e\u003csub\u003eb\u003c/sub\u003e) and other charge transfer processes (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh). The enhanced charge transport and prolonged carrier lifetime of TpBpy can be attributed to keto-enol tautomerism.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn situ Fourier transform infrared (in-situ FTIR) spectrometry is a valuable tool for divulging the photocatalytic mechanisms. The in-situ FTIR spectra of TpBpy and BTBpy during H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis under a continuous steam-saturated O\u003csub\u003e2\u003c/sub\u003e flow and water vapor are depicted in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea-\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed. Vibrations corresponding to C\u0026thinsp;=\u0026thinsp;C (1342 cm\u003csup\u003e-1\u003c/sup\u003e), C-H (1378 cm\u003csup\u003e-1\u003c/sup\u003e), and benzene ring (1441 cm\u003csup\u003e-1\u003c/sup\u003e) for TpBpy are clearly observed in both dark and light conditions, as shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb. Notably, the signal intensities of vibrations related to C-OH (1402 cm\u003csup\u003e-1\u003c/sup\u003e), O-H (from C-OH, at 3252 cm\u003csup\u003e-1\u003c/sup\u003e), as well as C\u0026thinsp;=\u0026thinsp;C and C\u0026thinsp;=\u0026thinsp;N (1649 cm\u003csup\u003e-1\u003c/sup\u003e)\u003csup\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e]\u003c/sup\u003e increase with prolonged illumination, while the signal intensities of vibrations of C\u0026thinsp;=\u0026thinsp;O (1678 cm\u003csup\u003e-1\u003c/sup\u003e)\u003csup\u003e[\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e]\u003c/sup\u003e and C-N (1321 cm\u003csup\u003e-1\u003c/sup\u003e) decrease. This indicates a gradual transformation from the keto-amine structure of TpBpy stabilized in the dark to the enol-imine structure (as depicted in \u003cb\u003eFigure S58a\u003c/b\u003e). Additionally, new infrared vibration signals at 791 and 1200 cm\u003csup\u003e-1\u003c/sup\u003e are attributed to O-O bonding and an endoperoxide intermediate species, respectively. The vibrational intensities of these signals increase gradually with the duration of irradiation. Moreover, the vibrational intensities of PyH\u003csup\u003e+\u003c/sup\u003e (from 1500 to 1600 cm\u003csup\u003e-1\u003c/sup\u003e), C\u0026thinsp;=\u0026thinsp;NH\u003csup\u003e+\u003c/sup\u003e (1615 cm\u003csup\u003e-1\u003c/sup\u003e), and N-H\u003csup\u003e+\u003c/sup\u003e (2727 cm\u003csup\u003e-1\u003c/sup\u003e) also increase with prolonged illumination.\u003c/p\u003e \u003cp\u003eIn the case of BTBpy (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed), vibrations corresponding to C\u0026thinsp;=\u0026thinsp;C (1341, 1653, and 1673 cm\u003csup\u003e-1\u003c/sup\u003e from BT and Bpy components), C-H (1382 cm\u003csup\u003e-1\u003c/sup\u003e), benzene ring (1437 cm\u003csup\u003e-1\u003c/sup\u003e), and C\u0026thinsp;=\u0026thinsp;N (imine, formed by the synthesis reaction between BT and Bpy monomers) can be observed in both dark and light conditions. Unlike TpBpy, there are no infrared vibrations of C\u0026shy;OH, C-N, and similar vibrations, suggesting that there is no keto-amine/enol-imine photoisomerization. Upon exposure to light, the vibrations of O-O bonding (791 cm\u003csup\u003e-1\u003c/sup\u003e)\u003csup\u003e[\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]\u003c/sup\u003e and endoperoxide intermediate species (1206 cm\u003csup\u003e-1\u003c/sup\u003e)\u003csup\u003e[\u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e71\u003c/span\u003e]\u003c/sup\u003e become visible and increase over time. Concurrently, the protonation level of N increases, as indicated by the rise in vibrational intensities of PyH\u003csup\u003e+\u003c/sup\u003e, C\u0026thinsp;=\u0026thinsp;NH\u003csup\u003e+\u003c/sup\u003e (1615 cm\u003csup\u003e-1\u003c/sup\u003e), and N-H\u003csup\u003e+\u003c/sup\u003e (2784 cm\u003csup\u003e-1\u003c/sup\u003e). The in-situ IR data suggests that BTBpy also follows a 2e\u003csup\u003e-\u003c/sup\u003e one-step redox process to produce H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, but without mediation by keto-to-enol tautomerism. Significantly, the initial levels of protonation in TpBpy and BTBpy are challenging to observe, and their degrees of protonation escalate as the reaction progresses (due to the presence of catalyst, O\u003csub\u003e2\u003c/sub\u003e, H\u003csub\u003e2\u003c/sub\u003eO, and UV-Vis light). This indicates that protonation occurs concurrently with the photocatalytic reactions and intensifies as the reaction proceeds. Compared to BTBpy, TpBpy exhibits higher intensities of intermediate species and protonation, which aligns with the observed performances of the photocatalytic reactions depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eThe in-situ IR spectra of TpBpy were measured under different conditions to investigate the factors affecting the occurrence and degree of keto-to-enol tautomerism. Condition 1 involved dispersing TpBpy in water vapor with O\u003csub\u003e2\u003c/sub\u003e flowing, as shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb. Condition 2 involved dispersing TpBpy in water vapor with Ar flowing, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee. Condition 3 involved dispersing TpBpy in Ar flowing, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef. It was observed that keto-to-enol tautomerism occurs, but to a lesser extent when there is no O\u003csub\u003e2\u003c/sub\u003e flowing (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee). Additionally, there is a slight enhancement in protonation. However, in the absence of O\u003csub\u003e2\u003c/sub\u003e and water vapor, even after 30 minutes of photoexcitation, there were minimal keto-to-enol tautomerism or protonation signals (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef). These findings suggest that UV-vis light is a crucial prerequisite for keto-to-enol tautomerism, and the presence of O\u003csub\u003e2\u003c/sub\u003e and water vapor as reactants significantly enhances the degree of keto-to-enol tautomerism\u003csup\u003e[\u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e72\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eConsidering the structure tautomerism, intermediate species, and the protonation of N, a keto-enol tautomerism-mediated 2e\u003csup\u003e-\u003c/sup\u003e one-step redox process leading to the generation of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e can be summarized as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg. To further investigate the impact of keto-enol tautomerism on charge separation, theoretical calculations of electronic configurations for the initial and intermediate structures presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg were performed. The thermodynamically stable β-ketoenamine (E\u003csub\u003e0\u003c/sub\u003eK\u003csub\u003e3\u003c/sub\u003e) structure serves as the initial structure, followed by the adsorption of water molecules on the Bpy active sites. Subsequently, the photoisomerization of \u003cem\u003eβ\u003c/em\u003e-ketoenamine (keto-to-enol tautomerism) results in the formation of E\u003csub\u003e1\u003c/sub\u003eK\u003csub\u003e2\u003c/sub\u003e and E\u003csub\u003e2\u003c/sub\u003eK\u003csub\u003e1\u003c/sub\u003e, utilizing photogenerated holes to produce hydrogen peroxide and protonation of pyridine. Notably, before the photocatalytic water oxidation reaction (WOR), the charge on the Tp unit was \u0026minus;\u0026thinsp;0.449 |e| and \u0026minus;\u0026thinsp;0.461 |e|, whereas after WOR, the formation of E\u003csub\u003e1\u003c/sub\u003eK\u003csub\u003e2\u003c/sub\u003e and E\u003csub\u003e2\u003c/sub\u003eK\u003csub\u003e1\u003c/sub\u003e through keto-to-enol tautomerism resulted in Tp unit charges of -0.539 |e| and \u0026minus;\u0026thinsp;1.072 |e|, respectively. Correspondingly, prior to WOR, the Bpy unit side charge in the E\u003csub\u003e0\u003c/sub\u003eK\u003csub\u003e3\u003c/sub\u003e structure was +\u0026thinsp;0.434 |e| and +\u0026thinsp;0.803 |e|, while after WOR, the positive charge on the Bpy unit side increased to +\u0026thinsp;0.908 |e| and +\u0026thinsp;1.011 |e|. In conjunction with the above transient absorption analysis, it is noticed that the enol-free Tp units act as electron traps, guiding the flow of electrons towards the Tp side and accumulating them, thereby promoting the flow of holes towards the Bpy active sites, where the adsorbed water was photocatalytically oxidized to generate H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. Similarly, comparing the charge density difference before and after the photocatalytic oxygen reduction reaction (ORR), the charge amounts of the Tp unit in E\u003csub\u003e1\u003c/sub\u003eK\u003csub\u003e2\u003c/sub\u003e and E\u003csub\u003e2\u003c/sub\u003eK\u003csub\u003e1\u003c/sub\u003e before ORR are \u0026minus;\u0026thinsp;0.517 |e| and \u0026minus;\u0026thinsp;1.285 |e|, respectively. After the ORR, the enol-to-keto tautomerism forms E\u003csub\u003e0\u003c/sub\u003eK\u003csub\u003e3\u003c/sub\u003e, with the charge amount on the Tp unit side being \u0026minus;\u0026thinsp;0.449 |e|. Correspondingly, in E\u003csub\u003e1\u003c/sub\u003eK\u003csub\u003e2\u003c/sub\u003e and E\u003csub\u003e2\u003c/sub\u003eK\u003csub\u003e1\u003c/sub\u003e before ORR, the positive charge amounts on the Bpy unit side are +\u0026thinsp;0.722 |e| and +\u0026thinsp;0.936 |e|, respectively, while in E\u003csub\u003e0\u003c/sub\u003eK\u003csub\u003e3\u003c/sub\u003e after ORR, the positive charge amount on the Bpy unit side decreases to +\u0026thinsp;0.434 |e|. It can be observed that the enol-containing Tp units formed by keto-to-enol tautomerism act as hole traps, guiding and trapping the holes on the Tp side, thereby promoting the flow of electrons towards the Bpy active sites for photocatalytic reduction reactions, where the adsorbed oxygen on Bpy-H\u003csup\u003e+\u003c/sup\u003e is reduced to H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. Notably, during oxidation and reduction processes, the charge flow direction in donor-acceptor (D-A) type COFs is consistent and unidirectional, with electrons moving towards the acceptor and holes transferring to the donor. In contrast, in Tp-derived imine COFs like TpBpy, when in the keto form (E\u003csub\u003e0\u003c/sub\u003eK\u003csub\u003e3\u003c/sub\u003e), electrons flow towards Tp causing oxidation on Bpy side; while in the enol-containing forms (E\u003csub\u003e1\u003c/sub\u003eK\u003csub\u003e2\u003c/sub\u003e and E\u003csub\u003e2\u003c/sub\u003eK\u003csub\u003e1\u003c/sub\u003e), holes move towards Tp resulting in reduction on Bpy side.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe ultimate goal of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis is industrial application. Therefore, the separation-free and continuous-flow reaction process should be the primary condition (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea), and natural sunlight excitation with real spectral range, zero energy consumption, and excellent environmental adaptability is another condition (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). Here, firstly, a flow reactor capable of continuously producing products without the need for subsequent separation processes was employed to assess the performance of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis (\u003cb\u003eFigure S83\u003c/b\u003e and \u003cb\u003eS84\u003c/b\u003e). In this setup, TpBpy served as the photocatalytic filling material (\u003cb\u003eFigure S83\u003c/b\u003e), while a peristaltic pump regulated the flow rate of ultrapure water throughout the system. Initially, a Xenon lamp was employed as the simulated sunlight source in a laboratory environment (\u003cb\u003eFigure S84\u003c/b\u003e). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec, the average H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e concentration recorded over a 20-hour period was 172 \u0026micro;M, and the concentration of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e produced by TpBpy remained relatively stable. The photocatalytic generation rate of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e in our custom-built flow reactor reached 1429 mM h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e for TpBpy, considerably higher than the recently reported sunlight-driven synthesis rate in a flow reaction system, such as TAPT\u0026ndash;FTPB COFs (376 mM h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e).\u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e Furthermore, TpBpy demonstrated a notably improved solar-to-chemical conversion (SCC) efficiency of 0.040%, exceeding the performance of TAPT\u0026ndash;FTPB COFs at 0.010%.\u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e Subsequently, to achieve the ultimate goal of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis, natural sunlight was used to as the light source in the open-air environment (\u003cb\u003eFigure S85\u003c/b\u003e). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed, the H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e concentration produced by photosynthesis and the corresponding solar-to-chemical conversion (SCC) efficiency were found to be positively and negatively correlated with natural sunlight intensity, respectively. The average photocatalytic generation rate of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e reached 1030 mM h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, and an SCC efficiency reached 0.038%. This consistent stability over the course of the 3-day outdoor experiment and 20-hour indoor experiment for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production demonstrates the remarkable cyclic stability of TpBpy.\u003c/p\u003e \u003cp\u003eTo date, there have been no reports on the use of COFs in flow reactions for the outdoor production of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e. Consequently, to compare the H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photocatalytic efficiency of the separation-free system under natural sunlight, we investigated all the reported immobilization systems (\u003cb\u003eTable S7\u003c/b\u003e). As shown in \u003cb\u003eFigure S86\u003c/b\u003e and \u003cb\u003eS87\u003c/b\u003e, by immobilizing the TpBpy catalyst onto a glass slide (0.3 m \u0026times; 0.4 m) as a coating film and subsequently submerging the catalyst-coated glass slide in stagnant ultrapure water (18.25 MΩ cm), the immobilization reactor utilizing TpBpy achieved a H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis SCC of 0.029%. This exceeds all the reported performance of immobilization systems, such as COF-2CN, (0.0075%)\u003csup\u003e[\u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e73\u003c/span\u003e]\u003c/sup\u003e, PI-BD-TPB (0.024%)\u003csup\u003e[\u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e74\u003c/span\u003e]\u003c/sup\u003e, and COF-N32 (0.019%)\u003csup\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e. Additionally, the SCC of dispersion systems under natural sunlight was also compared. TpBpy showed the highest SCC among all reported photocatalytic materials under similar conditions (\u003cb\u003eFigure S88\u003c/b\u003e and \u003cb\u003eTable S7\u003c/b\u003e). Therefore, whether in the flow phase, immobilization phase, or dispersion phase, TpBpy exhibited superior SCC performance compared to other photocatalysts under natural sunlight.\u003c/p\u003e \u003cp\u003eIt is evident that the SCC of all photocatalysts for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e production under natural sunlight remains far lower than the 0.10% associated with natural photosynthesis. Therefore, improving photocatalytic performance remains the foremost challenge for the industrial application of photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e synthesis. Although recent reports indicate that SCC values have exceeded 1.00% under laboratory conditions, these high SCC values often result from continuous optimization of various experimental parameters. Some studies even fail to provide complete experimental details and data, leading to a misleading assessment of the maturity of photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e synthesis technology. When utilizing fixed-bed reactors that mimic the structure of plant leaves and employing natural sunlight as the light source, the SCC for photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e synthesis holds greater scientific significance. Therefore, we advocate for the use of SCC values obtained under natural sunlight in immobilization or flowing phase reactors as a comparative metric for evaluating the performance of photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e synthesis.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this work, it was demonstrated that the photoinduced keto-enol tautomerism can act as variable electron/hole traps, promoting carrier separation for effective H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis. The keto-enol tautomerism facilitates exciton dissociation and charge transfer, resulting in shorter charge transfer duration and longer free carrier lifetime for photoredox reactions to take place. The Tp imine COFs exhibited increased photo-induced dynamic behavior, including elevated photocurrent densities, lower charge transfer resistances, and significantly improved photocatalytic activities compared to BT imine COFs with improvements ranging from several to dozens of times. Through in-situ FTIR spectra and fs-TA analysis, supported by theoretical calculations, it was observed that Tp moieties can transition between acting as electron traps and hole traps due to keto-enol tautomerism in TpBpy. In the keto form (E\u003csub\u003e0\u003c/sub\u003eK\u003csub\u003e3\u003c/sub\u003e), electrons migrate towards Tp trap and holes towards Bpy side, leading to adsorbed water been oxidized at the Bpy catalytic active sites. Conversely, in the enol-containing forms (E\u003csub\u003e1\u003c/sub\u003eK\u003csub\u003e2\u003c/sub\u003e and E\u003csub\u003e2\u003c/sub\u003eK\u003csub\u003e1\u003c/sub\u003e), holes move towards Tp trap and electrons towards Bpy side, resulting in adsorbed oxygen on Bpy-H\u003csup\u003e+\u003c/sup\u003e been reduced at the Bpy catalytic active sites. Based on this advanced carrier separation mechanism, TpBpy achieved an exceptionally rapid H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis rate of 3.79 mM h\u003csup\u003e-1\u003c/sup\u003e in pure water. And it also exhibits remarkable efficiency in the photocatalytic generation of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e in a flow-reactor system under natural sunlight, achieving a solar-to-chemical conversion efficiency of 0.038%, exceeding the performance of all previously documented photocatalysts to the best of our knowledge.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis work is supported by the National Natural Science Foundation of China (No. 22476109, 2476110), the Hubei Provincial Natural Science Foundation of China (No. 2022CFA065), and the 111 Project (D20015). X. Y. K. acknowledges the support from the Lee Kuan Yew Postdoctoral Fellowship with start-up grant (024042-00001). T. M. acknowledged the Australian Research Council (ARC) through Future Fellowship (FT210100298), Discovery Project (DP220100603), Linkage Project (LP210200504, LP220100088, LP230200897) and Industrial Transformation Research Hub (IH240100009) schemes, the Australian Government through the Cooperative Research Centres Projects (CRCPXIII000077), the Australian Renewable Energy Agency (ARENA) as part of ARENA's Transformative Research Accelerating Commercialisation Program (TM021), and European Commission's Australia-Spain Network for Innovation and Research Excellence (AuSpire).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eC\u0026ocirc;t\u0026eacute;, A. P., Benin, A. I., Ockwig, N. W., Keeffe, M. O., Matzger, A. J. Yaghi, O. M. Porous, crystalline, covalent organic frameworks. Science 310, 1166\u0026ndash;1170 (2005).\u003c/li\u003e\n \u003cli\u003eBanerjee, T., Podjaski, F., Kr\u0026ouml;ger, J., Biswal, B. P., Lotsch, B. V. 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Efficient photosynthesis of hydrogen peroxide by cyano-containing covalent organic frameworks from water, air and sunlight. Angew. Chem. Int. Ed. 63, e202318562 (2024).\u003c/li\u003e\n \u003cli\u003eChi, W., Dong, Y., Liu, B., Pan, C., Zhang, J., Zhao, H., Zhu, Y., Liu, Z. A photocatalytic redox cycle over a polyimide catalyst drives efficient solar-to-H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e conversion. Nat. Commun. 15, 5316 (2024).\u003c/li\u003e\n \u003cli\u003eLiu, R., Chen, Y., Yu, H., Položij, M., Guo, Y., Sum, T. C., Heine, T., Jiang, D. Linkage-engineered donor-acceptor covalent organic frameworks for optimal photosynthesis of hydrogen peroxide from water and air. Nat. Catal. 7, 195\u0026ndash;206 (2024).\u003cstrong\u003e\u003c/strong\u003e\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Covalent organic frameworks, keto-enol tautomerism, charge carrier separation, hydrogen peroxide photosynthesis","lastPublishedDoi":"10.21203/rs.3.rs-5211465/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5211465/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eCovalent organic frameworks (COFs) are excellent photocatalysts for hydrogen peroxide (H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) photosynthesis, but are often limited by retarded charge carrier separation. Presently, donor-acceptor (D-A) type COFs are commonly used to enhance the separation of photogenerated electrons and holes at the molecular level by promoting the migration of electrons towards the acceptor and the movement of holes towards the donor. However, the significantly slower kinetics of the synchronous water oxidation and oxygen reduction reactions (WOR and ORR) often result in the accumulation of photogenerated carriers, which induces strong Coulomb forces, in turn adversely affecting the carrier separation efficiency of D-A type COFs. Herein, it is observed that keto-enol tautomerism can function as dynamic traps for both electrons and holes, alternately capturing them, while the counterpart holes and electrons participate in and are consumed during asynchronous oxidation and reduction reactions. This represents the first example of T-C type COFs (T denotes traps units; C denotes catalytic active units), which can effectively weaken the Coulomb force by reducing charge carrier accumulation, resulting in rapid charge transfer and prolonged lifetimes of free charge carriers for efficient alternating photocatalytic WOR and ORR. Our in-depth research indicates that imine COFs based on 2,4,6-trihydroxybenzaldehyde-1,3,5-tricarbaldehyde (Tp series) exhibit enhanced photocatalytic activity compared to those based on 1,3,5-benzenetricarboxaldehyde (BT series), which can be attributed to the occurrence of keto-enol tautomerization. Notably, the optimal Tp imine COF (TpBpy) displays an ultrafast rate for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis, reaching 3.79 mM h\u003csup\u003e-1\u003c/sup\u003e, surpassing all previously reported photocatalysts. More importantly, when employed in a flow-reactor system, TpBpy also showcases exceptional effectiveness for continuous photocatalytic H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e synthesis, achieving a solar-to-chemical conversion (SCC) efficiency of 0.038%, representing the highest performance recorded to date under natural sunlight conditions. This work offers molecular-level guidance for designing efficient photocatalysts for H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e photosynthesis and proposes standardized criteria for obtaining reliable SCC values.\u003c/p\u003e","manuscriptTitle":"Keto-enol tautomerism as transformative electron/hole traps to promote charge carrier separation for record-high H2O2 photosynthesis in real world","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-12-30 06:03:17","doi":"10.21203/rs.3.rs-5211465/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"c70ce33a-0335-4e86-b8e0-c42f6648135a","owner":[],"postedDate":"December 30th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":42145954,"name":"Physical sciences/Chemistry/Catalysis/Photocatalysis"},{"id":42145955,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Photocatalysis"}],"tags":[],"updatedAt":"2024-12-30T06:03:17+00:00","versionOfRecord":[],"versionCreatedAt":"2024-12-30 06:03:17","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5211465","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5211465","identity":"rs-5211465","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}