Electric Rectification by a Redox-Conductive Metal-Organic Framework Bilayer Electrode

Electric Rectification by a Redox-Conductive Metal-Organic Framework Bilayer Electrode | 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 Article Electric Rectification by a Redox-Conductive Metal-Organic Framework Bilayer Electrode Sascha Ott, Amol Kumar, Jingguo Li This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9056510/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Unidirectional electron flow is essential for applications in electron storage and reconfigurable electronics, and traditionally realized in semiconductor junctions and even single-molecule concepts. Combining aspects from both technologies, redox-conductive metal-organic frameworks (RC-MOFs) exhibit molecule-like behavior in a crystalline, porous matrix. Herein, we show that bilayer RC-MOF electrodes composed of sequentially deposited Zn(PMDI) and Zn(NDI) on fluorine-doped tin oxide (FTO) function as chemical free-energy-based rectifying junctions. Unidirectional electron flow arises from thermodynamically allowed, and spatially organized redox reactions at the Zn(PMDI)|Zn(NDI) interface. Showcasing the rectifying function, electrons that reach the outer Zn(NDI) layer in the FTO|Zn(PMDI)|Zn(NDI) configuration are trapped as NDI•− and cannot be recovered by applying an oxidative bias. Introduction of [Co(bpy)3]3+ to the electrolyte creates a source–drain situation that reveals the potential-dependent directional electron flow across the bilayer. These results position RC-MOF bilayers as programmable electrochemical diodes, with rectification governed by layer sequence and redox accessibility. Physical sciences/Materials science/Materials for devices/Electronic devices Physical sciences/Chemistry/Electrochemistry Physical sciences/Materials science/Materials for energy and catalysis/Metal–organic frameworks Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Main Redox-conductive metal-organic frameworks (RC-MOFs) are an emerging class of electrically conducting MOFs in which electrons are transported through hopping events between electronically isolated redox active units. 1, 2 Different from MOFs that operate through the band-type conduction, 3, 4 a unique characteristic of RC-MOFs is that the molecular properties of the redox active building blocks are retained. 1 This means that some of the most intriguing properties of a discrete molecule, such as, absorption and emission, redox chemistry, magnetism, etc., can be introduced into tailor-made materials. 3, 5-9 Contemporary research on RC-MOFs has mainly concentrated on monolayer MOF thin films that are grown on conducting substrates. This design is beneficial for applications such as electrocatalysis, optoelectronics and energy storage devices, since the redox state of the MOF films can be easily controlled and modified through an applied potential. 10-15 Also, such architectures facilitate fundamental studies of the electron hopping transport which is inherently coupled to the diffusion/migration of charge-balancing counterions. Significant progress has been made along these lines of research, and many electrochromic and electrocatalytic MOFs have been reported, 16-19 along with mechanistic studies of the underlying cation-coupled electron hopping process. For example, a wide range of MOF-borne parameters like anisotropicity, 12 pore size, 20 hopping distances, 21, 22 and experimental parameters like counterions, 11, 23, 24 solvents, 25 etc., have systematically been investigated for their influence on the rate of electron transport. An aspect of RC-MOFs that has to the best of our knowledge not been addressed yet lies in the combination of two different RC-MOFs with distinct redox properties. If the two RC-MOFs are spatially separated, a bilayer electrode can be envisaged in which the different redox levels of the participating MOFs give rise to a chemical free-energy-based rectifying junction. 26 Such systems have been demonstrated in context of redox active polymers, 27-30 but never in the context of RC-MOFs. In a broader picture, such a RC-MOF-based rectifying junction lies in between a single-molecule junction, 31-36 and a conventional PN junction (Fig. 1). 37, 38 While diode-like behavior in the former arises from electron-rich and electron-deficient portions of the same molecule (Fig. 1a), 39 electron flow between two RC-MOF layers includes a redox reaction at the interface between the two RC-MOFs (Fig. 1b). The cation-coupled electron transfer reaction is also fundamentally different to the situation in a conventional PN junction, where unidirectional electron flow at the interface of two semiconductors is caused by an electric field (carrier depletion) in the space charge region (Fig. 1c). 28, 40-42 In this regard, bilayer RC-MOF electrodes are unique alternatives, as the energy levels of the participating redox couples can, in principle, be chosen at will, allowing for high rectification ratios in desired potential windows. If successful, this new bilayer RC-MOF design may find applications in energy harvesting and storage, (photo)electrocatalysis, and electronic devices. 1, 3, 43, 44 Fig. 1 Schematic representation of unidirectional electron flow at a) a molecular rectifier; b) the interface of a bilayer redox-conducting (RC) MOF, and c) in the space charge region of a conventional PN semiconductor junction. With different redox properties of the constituting linkers of the RC-MOF bilayer (b), electron flow from redox-conductive MOF-1 to MOF-2 at the interface is thermodynamically feasible only in one direction. This is in contrast to the situation in conventional PN junctions (c) in which the directional electron flow is electronic in nature and driven by the presence of a depletion region. In this work, we demonstrate unidirectional electron flow in a bilayer RC-MOF system on the surface of an FTO substrate for the first time. The bilayer RC-MOF electrodes in this study consist of Zn(NDI) (NDI = naphthalene diimide bis-pyrazole) and Zn(PMDI) (PMDI = pyromellitic diimide bis-pyrazoles) that are grown on top of each other in a FTO|Zn(NDI)|Zn(PMDI), or the reverse FTO|Zn(PMDI)|Zn(NDI) configuration. It is shown that the bilayer RC-MOF electrodes support directional electron transport as long as the redox levels of the constituting linkers in the two layers are aligned in a way that enables thermodynamically downhill electron transfers at the interface. In the opposite case, electron transport through the entire electrode is impeded, giving rise to the postulated free-energy-based rectifying junction. The directional electron flow in both configurations is followed by operando UV-vis spectroscopy, and the unidirectionality of the electron transport is demonstrated by adding an electron acceptor, [Co(bpy) 3 ] 3+ , to the supporting electrolyte, thereby mimicking an Ohmic contact in a source/drain-type measurement. The configuration that allows electron transport throughout the entire electrode can be used to reduce [Co(bpy) 3 ] 3+ , while the reverse configuration requires higher potential pathways, showcasing the diode-like behavior of the RC-MOF bilayer electrode. Results and Discussion Bilayer RC-MOF electrode preparation and basic characterization Bilayer RC-MOF electrodes were prepared through a two-step solvothermal synthesis process. Briefly, the inner Zn(NDI) MOF was first grown as a thin film on the surface of the FTO substrate, following a previously published protocol. 17 Subsequently, Zn(PMDI) MOF was grown on top of the Zn(NDI) film by a secondary solvothermal synthesis (see ESI for details), resulting in the target FTO|Zn(NDI)|Zn(PMDI) bilayer electrode (Fig. 2a). The inverse geometry, FTO|Zn(PMDI)|Zn(NDI), was prepared is a similar fashion with Zn(PMDI) as inner and Zn(NDI) as outer layer. The bilayer MOFs were compared to a previously published reference system consisting of a mixed-linker MOF (FTO|Zn(PMDI) 0.5 (NDI) 0.5 ), where the two different linkers are spatially unresolved. 22 The crystalline structure of the bilayer electrode was confirmed by thin-film XRD (Fig. 2b) which shows a preferred orientation and separate features corresponding to both FTO|Zn(NDI) and FTO|Zn(PMDI) films. In comparison, the diffraction pattern of the mixed-linker FTO|Zn(PMDI) 0.5 (NDI) 0.5 shows as an average of both individual Zn(NDI) and Zn(PMDI) components. The two MOF layers grow as compact thin films with individual thicknesses of approximately 600-700 nm, as determined by cross-section scanning electron microscopy (SEM) (Fig. 2c, Fig S1-2). Fig. 2. (a) Schematic illustration of the two consecutive solvothermal reactions to prepare the bilayer RC-MOF architecture with different linkers in each layer. (b) Thin-film XRD pattern of the bilayer FTO|Zn(PMDI)|Zn(NDI) (in red) showing diffraction peaks corresponding to the individual simulated Zn(NDI) and Zn(PMDI) XRD patterns (in blue and green, respectively). The diffraction peaks are more pronounced in the 2 nd and 3 rd order peaks in the (110) crystalline direction. The XRD of FTO|Zn(PMDI) 0.5 (NDI) 0.5 where the linkers are statistically distributed across the film shows one diffraction peak as average of both individual FTO|Zn(NDI) and FTO|Zn(PMDI) films. (c) SEM cross-section of the FTO|Zn(NDI)|Zn(PMDI) bilayer electrode, showing the two RC-MOF layers. Electrochemistry The electrochemical properties of the FTO|Zn(PMDI)|Zn(NDI) and FTO|Zn(NDI)|Zn(PMDI) electrodes were evaluated by cyclic voltammetry (CV) in a conventional three electrode set-up (see ESI for details). The scan rate in all experiments was chosen at v = 25 mV/s which is sufficiently slow to explore the finite diffusion regime of the electrodes. 1, 45 The observed waves were assigned in analogy to those of the mixed linker FTO|Zn(PMDI) 0.5 (NDI) 0.5 system. Fig. 3a-c illustrate the available redox levels in the different electrodes, their spatial position relative to the substrate, together with the voltametric responses during cathodic scans. The CV of FTO|Zn(PMDI) 0.5 (NDI) 0.5 (Fig. 3a) features four reversible one-electron redox events, with the 1 st and 3 rd wave at -0.97 V and -1.34 V corresponding to the NDI 0/•− and NDI •−/2− redox couples, respectively, while the 2 nd and 4 th wave at -1.17 V and -1.60 V originate from the PMDI 0/•− and PMDI •−/2− couples, respectively. 22 The presence of four waves in the CV of the mixed linker system arises from the fact that both PMDI and NDI linkers are in direct contact with the underlying FTO substrate. Fig. 3. Schematic representation of the mixed linker and bilayer electrodes with the available redox levels (including their spatial position), and the corresponding cyclic voltammograms (0.5 M KPF 6 in Ar-saturated DMF, v = 25 mV s -1 ). a) FTO|Zn(PMDI) 0.5 (NDI) 0.5 mixed linker MOF: four waves of equal current density are observed as all four redox levels are in direct contact with the FTO substrate. b) FTO|Zn(NDI)|Zn(PMDI): only waves at the potentials of the two NDI couples are observed; Zn(PMDI) reduction (grey levels) is mediated through NDI 2 - , while re-oxidation of the produced PMDI •− occurs through NDI 0 . c) FTO|Zn(PMDI)|Zn(NDI): first scan leads to the irreversible one-electron reduction of the outer layer NDI (grey levels). Subsequent scans show about half the current density for the first cathodic event. In contrast, the CVs of the bilayer electrodes are fundamentally different (Fig. 3b and c). In the FTO|Zn(NDI)|Zn(PMDI) configuration (Fig, 3b), the Zn(PMDI) outer layer is physically separated from the conducting substrate, and can only be electrochemically accessed through mediated electron transport via the underlying Zn(NDI) redox levels. Consequently, only two waves at potentials of the NDI 0/•− and NDI •−/2− redox couples are observed in the cathodic CV scan (Fig. 3b, Fig. S3). The first reduction produces the NDI •− which is re-oxidized in an electrochemically reversible process in case the scan is directly reversed. No electrons are injected into the outer PMDI layer as the reduction potential of the latter is more negative than that of the NDI •− . When the cathodic scan is extended beyond the NDI 0/•− couple, a second reduction is observed that is characterized by approximately twice the current density of the first. This behavior is consistent with the formation of NDI 2− which is strong enough of a reductant to drive PMDI reduction, leading to the concomitant formation of PMDI •− . Further reduction of PMDI •− to PMDI 2− is not possible in the FTO|Zn(NDI)|Zn(PMDI) configuration due to the thermodynamic mismatch of the PMDI •−/2− couple being around 250 mV uphill to the NDI •−/2− couple (Fig. 3b). In the reverse sweep of the extended scan (orange CV in Fig. 3b), two re-oxidation peaks are resolved at potentials of the NDI 2−/•− and NDI •−/0 redox levels. The more cathodic feature corresponds to the re-oxidation of NDI 2− to NDI •− while the second re-oxidation wave is characterized by about twice the current, consistent with both NDI •− to NDI 0 , and PMDI •− to PMDI 0 oxidations. The latter proceeds through a mediated electron transport pathway via the NDI •−/0 redox level. Subsequent CV scans are identical to the first scan, pointing towards the absence of significant diffusion barriers or kinetic limitations (Fig. S3). In the reverse configuration, the CV of the FTO|Zn(PMDI)|Zn(NDI) bilayer electrode features solely reductions at potentials that correspond to the PMDI 0/•− and PMDI •−/2− couples (Fig. 3c). Mirroring the discussion above, the NDI linkers in the outer layer are not in direct contact with the substrate, thus do not give any direct voltametric response, and can only be addressed electrochemically through mediated electron transport processes of the underlying PMDI linkers (Fig. 3c). In the first reductive process, Zn(PMDI) is reduced to Zn(PMDI) •− which is sufficiently reducing to reduce NDI to the NDI •− radical anion in the outer layer. The wave thus corresponds to the one-electron reduction of both Zn(PMDI) and Zn(NDI). Similarly, the second reduction wave at more negative potential corresponds to the formation of Zn(PMDI) 2− and Zn(NDI) 2− . In the first cathodic scan, both waves are thus of equal current density. In the reverse re-oxidation sweep, the more cathodic feature around -1.5 V is only half the current density of that of the reduction wave. This behavior reflects the alignment of the available redox levels, as the Zn(PMDI) 2− is directly re-oxidized to Zn(PMDI) •− which is however not strong enough of an oxidant to oxidize the outer layer Zn(NDI) 2− . The latter is oxidized through a mediated process of the Zn(PMDI) 0/•− couple which is observed at more positive potentials of -1.0 V. Comparing the charge that is passed during the reductive scan with that during the re-oxidation scan reveals that the film has irreversibly stored one molar equivalent of electrons. These electrons are trapped in the outer layer as Zn(NDI) •− , and do not have a thermodynamically feasible pathway for re-oxidation through the Zn(PMDI) layer. With the NDI linkers being reduced after the first scan, the second scan features a first reduction that is characterized by significantly less current compared to that of the first scan (Fig S4). The persisting NDI •− in the FTO|Zn(PMDI)|Zn(NDI) bilayer electrode gives rise to a characteristic yellow color of Zn(NDI) •− after the first CV cycle, 17 indicating that a substantial amount of electrons remain trapped in the Zn(NDI) outer layer (vide infra). Fundamentally, the charge trapping in the NDI •− state is the result of the molecular nature and hence fixed redox levels of the bilayer RC-MOF electrode, which forms a rectifying interface. This junction only permits thermodynamically allowed unidirectional electron transport, with no indication of a breakdown in the experimental potential window. Noteworthy is that the NDI •− state persists for days, showcasing the high quality of the bilayer electrode. Spectro-electrochemistry (SEC) Spectroscopic evidence for the assignments of the waves in the CV experiments was obtained from UV-Vis spectroelectrochemical measurements (see ESI and Fig. S8-9). As reported earlier, the spatially unresolved, mixed linker FTO|Zn(PMDI) 0.5 (NDI) 0.5 electrode can be prepared in five distinct redox states, 22 all of which can be individually accessed at their formal potentials, as both linkers can be electrochemically addressed directly by the underlying FTO substrate. Importantly, each redox state of each linker (NDI •− , PMDI •− , NDI 2− and PMDI 2− ) exhibits characteristic electronic absorptions in the visible part of the spectrum (Fig. S8), allowing the unambiguous assignment of the redox composition of the electrode at any given applied potential (see ESI and reference 22 for details). Fig. 4. Selected operando UV-vis measurements of the FTO|Zn(NDI)|Zn(PMDI) and FTO|Zn(PMDI)|Zn(NDI) bilayer electrodes during key stages of the CV scans. a) Spectroscopic changes during the second reduction in the cathodic scan, which shows the disappearance of the NDI •− signature, and the coupled appearance of NDI 2− and PMDI •− . b) Spectroscopic changes during the anodic reverse sweep, in which the NDI •− and PMDI •− are oxidized simultaneously to revert to the redox ground state. c) Presence of the different NDI and PMDI redox states as a function of applied potential. Highlighted in the green regions are the simultaneous appearance of the NDI 2− (418 nm, red curve) and PMDI •− states (714 nm, green curve) around -1.34 V in the cathodic sweep, and the simultaneous disappearance of NDI •− (472 nm, blue curve) and PMDI •− (714 nm, green curve) during the reverse oxidative sweep at -1V. NDI 0 (359 nm, black curve) is for the reference. The redox states were assigned based on their characteristic absorptions (see ESI for complete spectra). d) Pictures taken of the FTO|Zn(PMDI)|Zn(NDI) bilayer electrode before, during and after the first CV. Shown are the different colors at the different redox states of the bilayer; most importantly, the differences at 0 V applied potential, where the electrode is colorless before the first scan, but remains yellow after the first scan. This characteristic is due to the electron trapping in the outer Zn(NDI) layer. e) Spectroscopic evidence of the charge trapping: after the first scan, the signals of the NDI •− remains, irrespective of the applied potential. The spectroelectrochemical analyses of the bilayer electrodes are fully consistent with the assignments from the CV section (vide supra, Fig. S9). For example, in the FTO|Zn(NDI)|Zn(PMDI) electrode, Zn(PMDI) reduction can be observed along with the emergence of the spectroscopic signatures of the NDI 2− (Fig. 4a). In the re-oxidation sweep, NDI 2− is oxidized selectively, and PMDI •− re-oxidation occurs as a more anodic process together with NDI •− re-oxidation (Fig. 4b). The appearance and decay of the different redox states as a function of applied potential is summarized in Fig. 4c (see Fig. S9e-h for the complete spectra). For the reverse FTO|Zn(PMDI)|Zn(NDI) electrode configuration, the unidirectional electron transport through the bilayer electrode can be observed by the bare eye as different colors emerging during the cathodic and anodic scans (Fig. 4d, see Fig. S9a-d for complete spectra). Most importantly, electron trapping in the outer Zn(NDI) layer can be observed after the first scan, and its spectroscopic signature remains throughout any subsequent experiments (Fig. 4e, Fig. S10-11). The operando characterization thus provides direct spectroscopic evidence for the charge trapping, and the importance of the bilayer film design. The storage of electrons in the form of NDI •− in the outer layer is the result of unidirectional electron flow from PMDI •− species to NDI species across the Zn(PMDI)|Zn(NDI) interface. This phenomenon offers an interesting opportunity for such bilayer MOF systems in (photo)electrocatalysis applications, where photoelectrons that are extracted from illuminated semiconductor (SC) substrates can be stored remote from the SC|MOF interface, and available for chemistry in the electrolyte. 46, 47 Electrochemical diode The unidirectional electron flow in the bilayer MOF electrodes make them potential candidates for electric rectification. To demonstrate this potential, an electron acceptor, [Co(bpy) 3 ] 3+ , with an E 1/2 = -0.15 V was added to the supporting electrolyte (Fig. S12). This species is too large to enter the MOF pores, but will scavenge electrons at the MOF-electrolyte interface, 48 thereby mimicking an Ohmic contact in a source/drain type measurement. In the presence of excess [Co(bpy) 3 ] 3+ in the electrolyte (ca. 36 mM), the CV of the FTO|Zn(NDI)|Zn(PMDI) bilayer displays a pseudo-catalytic wave at the NDI •−/2− redox level (Fig. 5a, middle), consistent with a direct conduction pathway between electrode and acceptor in solution (Fig. 5a, left). No current enhancement is observed in the first reduction wave, even though the NDI •− is sufficiently reducing to promote electron transfer to the Co acceptor thermodynamically. The absence of this process is due to the insolating nature of the Zn(PMDI) outer layer that prevents contact between the NDI •− and the acceptor. The onset potential at which the pseudo-catalytic reaction occurs is best visualized in a series of steady state chronoamperometry experiments at different potentials in the presence and absence of [Co(bpy) 3 ] 3+ (Fig. 5a, right, Fig. S14-16). The steady state current density in the absence of acceptor approaches zero (Fig. S13) due to the exhaustive reduction of all spatially and thermodynamically available redox levels. In presence of the acceptor, a significant increase in steady state current density is observed at applied potentials that allow for conduction throughout the entire bilayer architecture, i.e. electrons from the inner layer can be transported to the outer layer, where they are extracted by the terminal [Co(bpy) 3 ] 3+ acceptor. The steady state current densities of the FTO|Zn(NDI)|Zn(PMDI) electrode exhibit a sharp increase at the potential of the NDI •−/2− redox level (Fig. 5a, right), consistent with the requirements mentioned above. Almost no current is observed at the NDI 0/•− level due to the thermodynamic inaccessibility of the outer layer (Fig. 5a, right, Fig S15). Fig. 5. Schematic energy level diagram, CVs and steady state current densities obtained from chronoamperometry experiments at varying step potentials in the presence and absence of [Co(bpy) 3 ] 3+ electron acceptor, mimicking an Ohmic contact for a) FTO|Zn(NDI)|Zn(PMDI) and b) FTO|Zn(PMDI)|Zn(NDI). CVs in the absence (blue) and presence (red) of an excess of [Co(bpy) 3 ] 3+ at 10 mV s -1 . In the reverse configuration, CVs of the FTO|Zn(PMDI)|Zn(NDI) electrode show a current enhancement in the presence of the [Co(bpy) 3 ] 3+ acceptor already at the potential of the first reduction, i.e. the PMDI0 0/•− couple (Fig. 5b, middle, Fig. S17). This behavior is expected, as the redox levels of the contributing PMDI 0/•− and NDI 0/•− couples are spatially and thermodynamically aligned to promote electron transport through the entire bilayer system already at the potential of the first PMDI reduction. Optically, charge extraction by the [Co(bpy) 3 ] 3+ acceptor can be noted by the absence of the yellow color of the NDI •− state. When the steady state current densities are plotted against the step-potentials for the two bilayer architectures (Fig. 5, right), it becomes apparent that the applied potentials to achieve half-maximum conductivity differ by about 200 mV. At an applied potential of -1.12 V, the FTO|Zn(PMDI)|Zn(NDI) bilayer sustains a current density of ~0.20 mA, while the inverse bilayer promotes only 0.02 mA at the same potential, i.e. a factor of ca. 10 times less. It should be noted that this difference is directly coupled to the difference in redox potentials of the contributing Zn(PMDI) and Zn(NDI) layers, and can be altered basically at will by changing the contributing linkers to adjust for a given application. Another noteworthy point is that the steady state current densities under pseudo-catalytic conditions are very similar for the two architectures, which again points to very similar redox conductivity kinetics of the two contributing MOF layers. In fact, the steady-state current closely matches that of FTO|Zn(NDI) monolayer films in the presence of the same acceptor, 48 indicating that the dominant kinetic limitation arises from charge transport within the individual MOF layers rather than from any interfacial electron transport. Conclusion In this work, we have successfully prepared RC-MOF bilayer electrodes that feature intrinsic redox gradients due to the unique redox characteristics of the employed linkers. Unidirectional electron transport is observed when the cross-film electron flow is a thermodynamic downhill redox reaction in nature (Fig. 1b). Consequently, characteristic electrochemistry profiles with distinct CV features are observed in comparison with mixed linker FTO|Zn(PMDI) 0.5 (NDI) 0.5 . Specifically, the unidirectional electron transport in the FTO|Zn(PMDI)-Zn(NDI) electrode gives rise to trapped electrons in the form of NDI •− species in the outer MOF film. Operando UV-vis spectroscopy unambiguously confirmed the presence of trapped NDI •− species, positioning RC-MOF bilayer electrodes as chemical free energy-based rectifying junctions. The rectification effect can be exploited to design switchable charge/color memory elements, chemical sensors and neurochemical devices, and is a significant step toward realizing scalable, solid-state molecular electronic devices. Methods All electrochemical experiments were conducted on MOF thin films that were grown on fluorine-doped tin oxide (FTO) substrates. The successful fabrication of the electrodes was verified by thin film X-ray diffraction (XRD) to assure crystallinity, and scanning electron microscopy (SEM) to examine the morphology of the MOF films, including the determination of film thicknesses. Preparation of bilayer RC-MOF electrodes Prior to MOF growth, a horizontal line was scratched into the FTO substrate using a diamond pencil. The line isolates the bottom 15-20% of the FTO slide from the upper portion which is where all subsequent measurements are performed. In other words, the lower 15-20% of the slide are not in electric contact any longer, and do not contribute to any of the electrochemical phenomena. This procedure is necessary to produce high quality data, as the bilayer electrode in the upper portion of the material exhibits more uniform coverage, minimal intermixing of the two layers or short-circuiting. The cut-off of the lower part of the electrode also prevents direct electrical contact between exposed FTO and the [Co(bpy) 3 ] 3+ acceptor, which would short-circuit the source-drain experiments. For the preparation of the FTO|Zn(NDI)|Zn(PMDI) bilayer electrode, a Zn(NDI) thin film was first grown directly on the FTO substrate, followed by the growth of a Zn(PMDI) layer on top of FTO|Zn(NDI). An analogous procedure was employed to obtain the reverse bilayer architecture, FTO|Zn(PMDI)|Zn(NDI), by simply interchanging the sequence of MOF layer growth (details in supplementary material). Electrochemistry Cyclic voltammetry Cyclic voltammetry (CV) was carried out using a PGSTAT 204 potentiostat equipped with NOVA 2.1.4 software (Autolab-Metrohm). The mixed-linker and bilayer RC-MOF electrodes were used as working electrodes in a custom-built single-compartment electrochemical cell, containing also a glassy carbon counter and a standard non-aqueous Ag/Ag(NO) 3 reference electrode. All measurement were performed in deaerated, dry DMF containing 0.5 M KPF 6 as the supporting electrolyte under an atmosphere of argon that was continuously purged through the cell headspace. All potentials are calibrated against the Fc +/0 redox couple (details supplementary material). Spectroelectrochemistry Operando time-resolved UV–Vis spectroelectrochemistry was performed using a diode-array spectrophotometer (Agilent 8453) coupled to a potentiostat (Autolab PGSTAT100) controlled by NOVA 2.1.4 software. Redox processes were carried out in a single-compartment electrochemical cell consisting of a quartz cuvette (1 cm pathlength) equipped with a home-made stopper designed to hold three electrodes (Fig. S5). The bilayer electrodes were used as working electrodes, with a Pt rod counter electrode and a non-aqueous Ag/AgNO 3 as the reference electrode. Only the working electrode was positioned in the optical path. Absorption spectra were collected in kinetic mode during electrochemical operation to monitor electrogenerated species in real time. Operando spectroelectrochemistry enables direct correlation of spectral signatures with applied potentials, allowing concerted and sequential reduction processes to be distinguished through monitoring evolution and decay of their distinct absorption profile. All measurements were performed under argon using deaerated electrolytes as described in the CV section, and background spectra were obtained using bare FTO under identical conditions. Source-Drain Experiment The diode-like behavior was evaluated by CV and chronoamperometry at varying potentials in the presence and absence of electron acceptor Co(bpy) 3 ](PF 6 ) 3 in the supporting electrolyte. The conditions for the measurements were identical to the ones described above in the CV section. The [Co(bpy) 3 ] 3+ complex is too large to enter the MOF pores and was present in excess to act as an interfacial electron sink, effectively mimicking an Ohmic contact analogous to a source-drain electrode configuration. CV measurements were first recorded in the presence of an excess electron acceptor at a scan rate of 10 mV s -1 . Step-potential chronoamperometry (200-300s per step) was then performed over the potential range of -0.2 to -1.8 V in 0.1 V increments. This approach enables the precise probing of each redox state and ensures steady state current under thermodynamically feasible and specially allowed charge transport conditions. Declarations Data availability All data supporting the findings of this study are available in the Article or its Electronic Supplementary Information (ESI). The ESI includes full details of the linker preparation, synthetic methods, electrochemical, spectroelectrochemical studies, and other supporting characterization data. Acknowledgements Financial support for the work was provided by the Knut & Alice Wallenberg Foundation (KAW 2019.0071), the Olle Engkvist Foundation (212-0147), and the Swedish Research Council (VR 2023-03395). Author contributions J.L. and A.K. contributed equally to this work. J.L. A.K. and S.O. planned and designed the experiments. A.K. synthesized the linkers, J.L. and A.K. prepared the bilayer electrodes on FTO. A.K. performed the characterization. A.K. and J.L. carried out the experiments and, together with S.O. analysed and interpretated the data. S.O supervised and directed the project, including funding acquisition. J.L., A.K and S.O. co-wrote the manuscript. Competing interests The authors declare no competing interests. References Li, J.; Ott, S. The Molecular Nature of Redox-Conductive Metal–Organic Frameworks. Acc. Chem. Res. 2024 , 57 (19), 2836-2846. Kung, C.-W.; Goswami, S.; Hod, I.; Wang, T. C.; Duan, J.; Farha, O. K.; Hupp, J. T. 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Beyond diffusion: ion and electron migration contribute to charge transport in redox-conducting metal–organic frameworks. Chem. Sci. 2025 , 16 (12), 5214-5222. Additional Declarations There is NO Competing Interest. Supplementary Files Lietal.SupplementaryInformation.pdf Electric Rectification by a Redox-Conductive Metal-Organic Framework Bilayer Electrode Cite Share Download PDF Status: Posted 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. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9056510","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":609055742,"identity":"1303f7de-e4b0-43f5-a684-55e9b3a4eb50","order_by":0,"name":"Sascha Ott","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA6ElEQVRIiWNgGAWjYBAC+QYGBmYgLQekDgNpCcJaDA5AtBjzsLElE6mFAaIlsYeNx5g4hxmwH3/4uaCGIXG/fM9ng485FgwGNxKYP3zAo0W+J8dYesYxkC28mxNnbpMAaWGTnIHPmgM5bMw8bBAth3mhWph58Gk5//wZM88/sF8eH/4L0cL8+Q8+LTcSzJh528BamJMZIVoYpPHpMLjxxliat0/CmOdYmrFh7zYJHskzD9ske/Boke9Pf/iZ55uNHHvz4ccSP7fVyfEdTz784Qc+ayAAEYNAjzM2ENYwCkbBKBgFowAvAABxmEZlsQTNlgAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-1691-729X","institution":"Uppsala University","correspondingAuthor":true,"prefix":"","firstName":"Sascha","middleName":"","lastName":"Ott","suffix":""},{"id":609055743,"identity":"ffeb659b-1575-4c27-8bbc-35d46c32debb","order_by":1,"name":"Amol Kumar","email":"","orcid":"","institution":"Uppsala University","correspondingAuthor":false,"prefix":"","firstName":"Amol","middleName":"","lastName":"Kumar","suffix":""},{"id":609055744,"identity":"6b416514-1e76-4b8b-bb93-14b609a09c2f","order_by":2,"name":"Jingguo Li","email":"","orcid":"https://orcid.org/0000-0002-3058-5164","institution":"Uppsala universitet","correspondingAuthor":false,"prefix":"","firstName":"Jingguo","middleName":"","lastName":"Li","suffix":""}],"badges":[],"createdAt":"2026-03-07 07:50:32","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9056510/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9056510/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":105563747,"identity":"9879e990-cd12-416c-beb2-0581886003d8","added_by":"auto","created_at":"2026-03-27 12:47:42","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":75324,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic representation of unidirectional electron flow at a) a molecular rectifier; b) the interface of a bilayer redox-conducting (RC) MOF, and c) in the space charge region of a conventional PN semiconductor junction. With different redox properties of the constituting linkers of the RC-MOF bilayer (b), electron flow from redox-conductive MOF-1 to MOF-2 at the interface is thermodynamically feasible only in one direction. This is in contrast to the situation in conventional PN junctions (c) in which the directional electron flow is electronic in nature and driven by the presence of a depletion region.\u003c/p\u003e","description":"","filename":"Picture1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9056510/v1/22741df259c44684b6e42d29.jpg"},{"id":105198631,"identity":"537314e1-8569-4e94-a1be-1b8d51ef622f","added_by":"auto","created_at":"2026-03-23 10:48:23","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":224235,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Schematic illustration of the two consecutive solvothermal reactions to prepare the bilayer RC-MOF architecture with different linkers in each layer. (b) Thin-film XRD pattern of the bilayer FTO|Zn(PMDI)|Zn(NDI) (in red) showing diffraction peaks corresponding to the individual simulated Zn(NDI) and Zn(PMDI) XRD patterns (in blue and green, respectively). The diffraction peaks are more pronounced in the 2\u003csup\u003end\u003c/sup\u003e and 3\u003csup\u003erd\u003c/sup\u003e order peaks in the (110) crystalline direction. The XRD of FTO|Zn(PMDI)\u003csub\u003e0.5\u003c/sub\u003e(NDI)\u003csub\u003e0.5\u003c/sub\u003e where the linkers are statistically distributed across the film shows one diffraction peak as average of both individual FTO|Zn(NDI) and FTO|Zn(PMDI) films. (c) SEM cross-section of the FTO|Zn(NDI)|Zn(PMDI) bilayer electrode, showing the two RC-MOF layers.\u003c/p\u003e","description":"","filename":"Picture2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9056510/v1/d0c1c3e32a084ec85b04857c.jpg"},{"id":105198630,"identity":"4df804b5-08f4-427e-9da3-a37227a268cc","added_by":"auto","created_at":"2026-03-23 10:48:23","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":192286,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic representation of the mixed linker and bilayer electrodes with the available redox levels (including their spatial position), and the corresponding cyclic voltammograms (0.5 M KPF\u003csub\u003e6\u003c/sub\u003e in Ar-saturated DMF, \u003cem\u003ev\u003c/em\u003e = 25 mV s\u003csup\u003e-1\u003c/sup\u003e). a) FTO|Zn(PMDI)\u003csub\u003e0.5\u003c/sub\u003e(NDI)\u003csub\u003e0.5\u003c/sub\u003e mixed linker MOF: four waves of equal current density are observed as all four redox levels are in direct contact with the FTO substrate. b) FTO|Zn(NDI)|Zn(PMDI): only waves at the potentials of the two NDI couples are observed; Zn(PMDI) reduction (grey levels) is mediated through NDI\u003csup\u003e2-\u003c/sup\u003e, while re-oxidation of the produced PMDI\u003csup\u003e•−\u003c/sup\u003e occurs through NDI\u003csup\u003e0\u003c/sup\u003e. c) FTO|Zn(PMDI)|Zn(NDI): first scan leads to the irreversible one-electron reduction of the outer layer NDI (grey levels). Subsequent scans show about half the current density for the first cathodic event.\u003c/p\u003e","description":"","filename":"Picture3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9056510/v1/714011d5f0c4b6c0cd65901b.jpg"},{"id":105198633,"identity":"98009cac-d79b-42fd-aa43-456e68c00ed8","added_by":"auto","created_at":"2026-03-23 10:48:23","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":191723,"visible":true,"origin":"","legend":"\u003cp\u003eSelected operando UV-vis measurements of the FTO|Zn(NDI)|Zn(PMDI) and FTO|Zn(PMDI)|Zn(NDI) bilayer electrodes during key stages of the CV scans. a) Spectroscopic changes during the second reduction in the cathodic scan, which shows the disappearance of the NDI\u003csup\u003e•−\u003c/sup\u003e signature, and the coupled appearance of NDI\u003csup\u003e2−\u003c/sup\u003e and PMDI\u003csup\u003e•−\u003c/sup\u003e. b) Spectroscopic changes during the anodic reverse sweep, in which the NDI\u003csup\u003e•−\u003c/sup\u003e and PMDI\u003csup\u003e•−\u003c/sup\u003e are oxidized simultaneously to revert to the redox ground state. c) Presence of the different NDI and PMDI redox states as a function of applied potential. Highlighted in the green regions are the simultaneous appearance of the NDI\u003csup\u003e2−\u003c/sup\u003e (418 nm, red curve) and PMDI\u003csup\u003e•−\u003c/sup\u003e states (714 nm, green curve) around -1.34 V in the cathodic sweep, and the simultaneous disappearance of NDI\u003csup\u003e•−\u003c/sup\u003e (472 nm, blue curve) and PMDI\u003csup\u003e•−\u003c/sup\u003e (714 nm, green curve) during the reverse oxidative sweep at -1V. NDI\u003csup\u003e0\u003c/sup\u003e (359 nm, black curve) is for the reference. The redox states were assigned based on their characteristic absorptions (see ESI for complete spectra). d) Pictures taken of the FTO|Zn(PMDI)|Zn(NDI) bilayer electrode before, during and after the first CV. Shown are the different colors at the different redox states of the bilayer; most importantly, the differences at 0 V applied potential, where the electrode is colorless before the first scan, but remains yellow after the first scan. This characteristic is due to the electron trapping in the outer Zn(NDI) layer. e) Spectroscopic evidence of the charge trapping: after the first scan, the signals of the NDI\u003csup\u003e•−\u003c/sup\u003e remains, irrespective of the applied potential.\u003c/p\u003e","description":"","filename":"Picture4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9056510/v1/1ef00d25de7e86aec95f70a2.jpg"},{"id":105198634,"identity":"a6cdf8e2-218f-44b9-99d3-d0919a5f6ded","added_by":"auto","created_at":"2026-03-23 10:48:24","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":184282,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic energy level diagram, CVs and steady state current densities obtained from chronoamperometry experiments at varying step potentials in the presence and absence of [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e electron acceptor, mimicking an Ohmic contact for a) FTO|Zn(NDI)|Zn(PMDI) and b) FTO|Zn(PMDI)|Zn(NDI). CVs in the absence (blue) and presence (red) of an excess of [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e at 10 mV s\u003csup\u003e-1\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"Picture5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9056510/v1/ca305fda7209e199ba5e7530.jpg"},{"id":106591379,"identity":"e7cf8b18-2784-4b43-b459-4ab579803d40","added_by":"auto","created_at":"2026-04-10 08:42:48","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1517288,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9056510/v1/88ef5d51-f0ec-485e-9959-2904a9abb4a2.pdf"},{"id":105198635,"identity":"6aa02fb8-fc79-4cbb-9c84-c5804a379cda","added_by":"auto","created_at":"2026-03-23 10:48:24","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1495681,"visible":true,"origin":"","legend":"Electric Rectification by a Redox-Conductive Metal-Organic Framework Bilayer Electrode","description":"","filename":"Lietal.SupplementaryInformation.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9056510/v1/3daeb78bd8c9411953fe40d9.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Electric Rectification by a Redox-Conductive Metal-Organic Framework Bilayer Electrode","fulltext":[{"header":"Main","content":"\u003cp\u003eRedox-conductive metal-organic frameworks (RC-MOFs) are an emerging class of electrically conducting MOFs in which electrons are transported through hopping events between electronically isolated redox active units.\u003csup\u003e1, 2\u003c/sup\u003e Different from MOFs that operate through the band-type conduction,\u003csup\u003e3, 4\u003c/sup\u003e a unique characteristic of RC-MOFs is that the molecular properties of the redox active building blocks are retained.\u003csup\u003e1\u003c/sup\u003e This means that some of the most intriguing properties of a discrete molecule, such as, absorption and emission, redox chemistry, magnetism, etc., can be introduced into tailor-made materials.\u003csup\u003e3, 5-9\u003c/sup\u003e Contemporary research on RC-MOFs has mainly concentrated on monolayer MOF thin films that are grown on conducting substrates. This design is beneficial for applications such as electrocatalysis, optoelectronics and energy storage devices, since the redox state of the MOF films can be easily controlled and modified through an applied potential.\u003csup\u003e10-15\u003c/sup\u003e Also, such architectures facilitate fundamental studies of the electron hopping transport which is inherently coupled to the diffusion/migration of charge-balancing counterions. Significant progress has been made along these lines of research, and many electrochromic and electrocatalytic MOFs have been reported,\u003csup\u003e16-19\u003c/sup\u003e along with mechanistic studies of the underlying cation-coupled electron hopping process. For example, a wide range of MOF-borne parameters like anisotropicity,\u003csup\u003e12\u003c/sup\u003e pore size,\u003csup\u003e20\u003c/sup\u003e hopping distances,\u003csup\u003e21, 22\u003c/sup\u003e and experimental parameters like counterions,\u003csup\u003e11, 23, 24\u003c/sup\u003e solvents,\u003csup\u003e25\u003c/sup\u003e etc., have systematically been investigated for their influence on the rate of electron transport.\u003c/p\u003e\n\u003cp\u003eAn aspect of RC-MOFs that has to the best of our knowledge not been addressed yet lies in the combination of two different RC-MOFs with distinct redox properties. If the two RC-MOFs are spatially separated, a bilayer electrode can be envisaged in which the different redox levels of the participating MOFs give rise to a chemical free-energy-based rectifying junction.\u003csup\u003e26\u003c/sup\u003e Such systems have been demonstrated in context of redox active polymers,\u003csup\u003e27-30\u003c/sup\u003e but never in the context of RC-MOFs. In a broader picture, such a RC-MOF-based rectifying junction lies in between a single-molecule junction,\u003csup\u003e31-36\u003c/sup\u003e and a conventional PN junction (Fig. 1).\u003csup\u003e37, 38\u003c/sup\u003e While diode-like behavior in the former arises from electron-rich and electron-deficient portions of the same molecule (Fig. 1a),\u003csup\u003e39\u003c/sup\u003e electron flow between two RC-MOF layers includes a redox reaction at the interface between the two RC-MOFs (Fig. 1b). The cation-coupled electron transfer reaction is also fundamentally different to the situation in a conventional PN junction, where unidirectional electron flow at the interface of two semiconductors is caused by an electric field (carrier depletion) in the space charge region (Fig. 1c).\u003csup\u003e28, 40-42\u003c/sup\u003e In this regard, bilayer RC-MOF electrodes are unique alternatives, as the energy levels of the participating redox couples can, in principle, be chosen at will, allowing for high rectification ratios in desired potential windows. If successful, this new bilayer RC-MOF design may find applications in energy harvesting and storage, (photo)electrocatalysis, and electronic devices.\u003csup\u003e1, 3, 43, 44\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 1\u003c/strong\u003e Schematic representation of unidirectional electron flow at a) a molecular rectifier; b) the interface of a bilayer redox-conducting (RC) MOF, and c) in the space charge region of a conventional PN semiconductor junction. With different redox properties of the constituting linkers of the RC-MOF bilayer (b), electron flow from redox-conductive MOF-1 to MOF-2 at the interface is thermodynamically feasible only in one direction. This is in contrast to the situation in conventional PN junctions (c) in which the directional electron flow is electronic in nature and driven by the presence of a depletion region.\u003c/p\u003e\n\u003cp\u003eIn this work, we demonstrate unidirectional electron flow in a bilayer RC-MOF system on the surface of an FTO substrate for the first time. The bilayer RC-MOF electrodes in this study consist of Zn(NDI) (NDI = naphthalene diimide bis-pyrazole) and Zn(PMDI) (PMDI = pyromellitic diimide bis-pyrazoles) that are grown on top of each other in a FTO|Zn(NDI)|Zn(PMDI), or the reverse FTO|Zn(PMDI)|Zn(NDI) configuration. It is shown that the bilayer RC-MOF electrodes support directional electron transport as long as the redox levels of the constituting linkers in the two layers are aligned in a way that enables thermodynamically downhill electron transfers at the interface. In the opposite case, electron transport through the entire electrode is impeded, giving rise to the postulated free-energy-based rectifying junction. The directional electron flow in both configurations is followed by operando UV-vis spectroscopy, and the unidirectionality of the electron transport is demonstrated by adding an electron acceptor, [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e, to the supporting electrolyte, thereby mimicking an Ohmic contact in a source/drain-type measurement. The configuration that allows electron transport throughout the entire electrode can be used to reduce [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e, while the reverse configuration requires higher potential pathways, showcasing the diode-like behavior of the RC-MOF bilayer electrode.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003eBilayer RC-MOF electrode preparation and basic characterization\u003c/p\u003e\n\u003cp\u003eBilayer RC-MOF electrodes were prepared through a two-step solvothermal synthesis process. Briefly, the inner Zn(NDI) MOF was first grown as a thin film on the surface of the FTO substrate, following a previously published protocol.\u003csup\u003e17\u003c/sup\u003e Subsequently, Zn(PMDI) MOF was grown on top of the Zn(NDI) film by a secondary solvothermal synthesis (see ESI for details), resulting in the target FTO|Zn(NDI)|Zn(PMDI) bilayer electrode (Fig. 2a). The inverse geometry, FTO|Zn(PMDI)|Zn(NDI), was prepared is a similar fashion with Zn(PMDI) as inner and Zn(NDI) as outer layer. The bilayer MOFs were compared to a previously published reference system consisting of a mixed-linker MOF (FTO|Zn(PMDI)\u003csub\u003e0.5\u003c/sub\u003e(NDI)\u003csub\u003e0.5\u003c/sub\u003e), where the two different linkers are spatially unresolved.\u003csup\u003e22\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eThe crystalline structure of the bilayer electrode was confirmed by thin-film XRD (Fig. 2b) which shows a preferred orientation and separate features corresponding to both FTO|Zn(NDI) and FTO|Zn(PMDI) films. In comparison, the diffraction pattern of the mixed-linker FTO|Zn(PMDI)\u003csub\u003e0.5\u003c/sub\u003e(NDI)\u003csub\u003e0.5\u003c/sub\u003e shows as an average of both individual Zn(NDI) and Zn(PMDI) components. The two MOF layers grow as compact thin films with individual thicknesses of approximately 600-700 nm, as determined by cross-section scanning electron microscopy (SEM) (Fig. 2c, Fig S1-2).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 2.\u003c/strong\u003e (a) Schematic illustration of the two consecutive solvothermal reactions to prepare the bilayer RC-MOF architecture with different linkers in each layer. (b) Thin-film XRD pattern of the bilayer FTO|Zn(PMDI)|Zn(NDI) (in red) showing diffraction peaks corresponding to the individual simulated Zn(NDI) and Zn(PMDI) XRD patterns (in blue and green, respectively). The diffraction peaks are more pronounced in the 2\u003csup\u003end\u003c/sup\u003e and 3\u003csup\u003erd\u003c/sup\u003e order peaks in the (110) crystalline direction. The XRD of FTO|Zn(PMDI)\u003csub\u003e0.5\u003c/sub\u003e(NDI)\u003csub\u003e0.5\u003c/sub\u003e where the linkers are statistically distributed across the film shows one diffraction peak as average of both individual FTO|Zn(NDI) and FTO|Zn(PMDI) films. (c) SEM cross-section of the FTO|Zn(NDI)|Zn(PMDI) bilayer electrode, showing the two RC-MOF layers.\u003c/p\u003e\n\u003cp\u003eElectrochemistry\u003c/p\u003e\n\u003cp\u003eThe electrochemical properties of the FTO|Zn(PMDI)|Zn(NDI) and FTO|Zn(NDI)|Zn(PMDI) electrodes were evaluated by cyclic voltammetry (CV) in a conventional three electrode set-up (see ESI for details). The scan rate in all experiments was chosen at \u003cem\u003ev\u003c/em\u003e = 25 mV/s which is sufficiently slow to explore the finite diffusion regime of the electrodes.\u003csup\u003e1, 45\u003c/sup\u003e The observed waves were assigned in analogy to those of the mixed linker FTO|Zn(PMDI)\u003csub\u003e0.5\u003c/sub\u003e(NDI)\u003csub\u003e0.5\u003c/sub\u003e system. Fig. 3a-c illustrate the available redox levels in the different electrodes, their spatial position relative to the substrate, together with the voltametric responses during cathodic scans. The CV of FTO|Zn(PMDI)\u003csub\u003e0.5\u003c/sub\u003e(NDI)\u003csub\u003e0.5\u003c/sub\u003e (Fig. 3a) features four reversible one-electron redox events, with the 1\u003csup\u003est\u003c/sup\u003e and 3\u003csup\u003erd\u003c/sup\u003e wave at -0.97 V and -1.34 V corresponding to the NDI\u003csup\u003e0/\u0026bull;\u0026minus;\u003c/sup\u003e and NDI\u003csup\u003e\u0026bull;\u0026minus;/2\u0026minus;\u003c/sup\u003e redox couples, respectively, while the 2\u003csup\u003end\u003c/sup\u003e and 4\u003csup\u003eth\u003c/sup\u003e wave at -1.17 V and -1.60 V originate from the PMDI\u003csup\u003e0/\u0026bull;\u0026minus;\u003c/sup\u003e and PMDI\u003csup\u003e\u0026bull;\u0026minus;/2\u0026minus;\u003c/sup\u003e couples, respectively.\u003csup\u003e22\u003c/sup\u003e The presence of four waves in the CV of the mixed linker system arises from the fact that both PMDI and NDI linkers are in direct contact with the underlying FTO substrate.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 3.\u003c/strong\u003e Schematic representation of the mixed linker and bilayer electrodes with the available redox levels (including their spatial position), and the corresponding cyclic voltammograms (0.5 M KPF\u003csub\u003e6\u003c/sub\u003e in Ar-saturated DMF, \u003cem\u003ev\u003c/em\u003e = 25 mV s\u003csup\u003e-1\u003c/sup\u003e). a) FTO|Zn(PMDI)\u003csub\u003e0.5\u003c/sub\u003e(NDI)\u003csub\u003e0.5\u003c/sub\u003e mixed linker MOF: four waves of equal current density are observed as all four redox levels are in direct contact with the FTO substrate. b) FTO|Zn(NDI)|Zn(PMDI): only waves at the potentials of the two NDI couples are observed; Zn(PMDI) reduction (grey levels) is mediated through NDI\u003csup\u003e2\u003c/sup\u003e\u003csup\u003e-\u003c/sup\u003e, while re-oxidation of the produced PMDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e occurs through NDI\u003csup\u003e0\u003c/sup\u003e. c) FTO|Zn(PMDI)|Zn(NDI): first scan leads to the irreversible one-electron reduction of the outer layer NDI (grey levels). Subsequent scans show about half the current density for the first cathodic event.\u003c/p\u003e\n\u003cp\u003eIn contrast, the CVs of the bilayer electrodes are fundamentally different (Fig. 3b and c). In the FTO|Zn(NDI)|Zn(PMDI) configuration (Fig, 3b), the Zn(PMDI) outer layer is physically separated from the conducting substrate, and can only be electrochemically accessed through mediated electron transport via the underlying Zn(NDI) redox levels. Consequently, only two waves at potentials of the NDI\u003csup\u003e0/\u0026bull;\u0026minus;\u003c/sup\u003e and NDI\u003csup\u003e\u0026bull;\u0026minus;/2\u0026minus;\u003c/sup\u003e redox couples are observed in the cathodic CV scan (Fig. 3b, Fig. S3). The first reduction produces the NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e which is re-oxidized in an electrochemically reversible process in case the scan is directly reversed. No electrons are injected into the outer PMDI layer as the reduction potential of the latter is more negative than that of the NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e. When the cathodic scan is extended beyond the NDI\u003csup\u003e0/\u0026bull;\u0026minus;\u003c/sup\u003e couple, a second reduction is observed that is characterized by approximately twice the current density of the first. This behavior is consistent with the formation of NDI\u003csup\u003e2\u0026minus;\u003c/sup\u003e which is strong enough of a reductant to drive PMDI reduction, leading to the concomitant formation of PMDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e. Further reduction of PMDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e to PMDI\u003csup\u003e2\u0026minus;\u003c/sup\u003e is not possible in the FTO|Zn(NDI)|Zn(PMDI) configuration due to the thermodynamic mismatch of the PMDI\u003csup\u003e\u0026bull;\u0026minus;/2\u0026minus;\u003c/sup\u003e couple being around 250 mV uphill to the NDI\u003csup\u003e\u0026bull;\u0026minus;/2\u0026minus;\u003c/sup\u003e couple (Fig. 3b).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn the reverse sweep of the extended scan (orange CV in Fig. 3b), two re-oxidation peaks are resolved at potentials of the NDI\u003csup\u003e2\u0026minus;/\u0026bull;\u0026minus;\u003c/sup\u003e and NDI\u003csup\u003e\u0026bull;\u0026minus;/0\u003c/sup\u003e redox levels. The more cathodic feature corresponds to the re-oxidation of NDI\u003csup\u003e2\u0026minus;\u003c/sup\u003e to NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e while the second re-oxidation wave is characterized by about twice the current, consistent with both NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e to NDI\u003csup\u003e0\u003c/sup\u003e, and\u0026nbsp;PMDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e to PMDI\u003csup\u003e0\u003c/sup\u003e oxidations. The latter proceeds through a mediated electron transport pathway via the NDI\u003csup\u003e\u0026bull;\u0026minus;/0\u003c/sup\u003e redox level. Subsequent CV scans are identical to the first scan, pointing towards the absence of significant diffusion barriers or kinetic limitations (Fig. S3).\u003c/p\u003e\n\u003cp\u003eIn the reverse configuration, the CV of the FTO|Zn(PMDI)|Zn(NDI) bilayer electrode features solely reductions at potentials that correspond to the PMDI\u003csup\u003e0/\u0026bull;\u0026minus;\u003c/sup\u003e and PMDI\u003csup\u003e\u0026bull;\u0026minus;/2\u0026minus;\u003c/sup\u003e couples (Fig. 3c). Mirroring the discussion above, the NDI linkers in the outer layer are not in direct contact with the substrate, thus do not give any direct voltametric response, and can only be addressed electrochemically through mediated electron transport processes of the underlying PMDI linkers (Fig. 3c). In the first reductive process, Zn(PMDI) is reduced to Zn(PMDI)\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e which is sufficiently reducing to reduce NDI to the NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e radical anion in the outer layer. The wave thus corresponds to the one-electron reduction of both Zn(PMDI) and Zn(NDI). Similarly, the second reduction wave at more negative potential corresponds to the formation of Zn(PMDI)\u003csup\u003e2\u0026minus;\u003c/sup\u003e and Zn(NDI)\u003csup\u003e2\u0026minus;\u003c/sup\u003e. In the first cathodic scan, both waves are thus of equal current density. In the reverse re-oxidation sweep, the more cathodic feature around -1.5 V is only half the current density of that of the reduction wave. This behavior reflects the alignment of the available redox levels, as the Zn(PMDI)\u003csup\u003e2\u0026minus;\u003c/sup\u003e is directly re-oxidized to Zn(PMDI)\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e which is however not strong enough of an oxidant to oxidize the outer layer Zn(NDI)\u003csup\u003e2\u0026minus;\u003c/sup\u003e. The latter is oxidized through a mediated process of the Zn(PMDI)\u003csup\u003e0/\u0026bull;\u0026minus;\u003c/sup\u003e couple which is observed at more positive potentials of -1.0 V. Comparing the charge that is passed during the reductive scan with that during the re-oxidation scan reveals that the film has irreversibly stored one molar equivalent of electrons. These electrons are trapped in the outer layer as Zn(NDI)\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e, and do not have a thermodynamically feasible pathway for re-oxidation through the Zn(PMDI) layer. With the NDI linkers being reduced after the first scan, the second scan features a first reduction that is characterized by significantly less current compared to that of the first scan (Fig S4). The persisting NDI\u003csup\u003e\u0026bull;\u0026minus;\u0026nbsp;\u003c/sup\u003ein the FTO|Zn(PMDI)|Zn(NDI) bilayer electrode gives rise to a characteristic yellow color of Zn(NDI)\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e after the first CV cycle,\u003csup\u003e17\u003c/sup\u003e indicating that a substantial amount of electrons remain trapped in the Zn(NDI) outer layer (vide infra).\u003c/p\u003e\n\u003cp\u003eFundamentally, the charge trapping in the NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e state is the result of the molecular nature and hence fixed redox levels of the bilayer RC-MOF electrode, which forms a rectifying interface. This junction only permits thermodynamically allowed unidirectional electron transport, with no indication of a breakdown in the experimental potential window. Noteworthy is that the NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e state persists for days, showcasing the high quality of the bilayer electrode.\u003c/p\u003e\n\u003cp\u003eSpectro-electrochemistry (SEC)\u003c/p\u003e\n\u003cp\u003eSpectroscopic evidence for the assignments of the waves in the CV experiments was obtained from UV-Vis spectroelectrochemical measurements (see ESI and Fig. S8-9). As reported earlier, the spatially unresolved, mixed linker FTO|Zn(PMDI)\u003csub\u003e0.5\u003c/sub\u003e(NDI)\u003csub\u003e0.5\u003c/sub\u003e electrode can be prepared in five distinct redox states,\u003csup\u003e22\u003c/sup\u003e all of which can be individually accessed at their formal potentials, as both linkers can be electrochemically addressed directly by the underlying FTO substrate. Importantly, each redox state of each linker (NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e, PMDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e, NDI\u003csup\u003e2\u0026minus;\u003c/sup\u003e and PMDI\u003csup\u003e2\u0026minus;\u003c/sup\u003e) exhibits characteristic electronic absorptions in the visible part of the spectrum (Fig. S8), allowing the unambiguous assignment of the redox composition of the electrode at any given applied potential (see ESI and reference\u003csup\u003e22\u003c/sup\u003e for details).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 4.\u003c/strong\u003e Selected operando UV-vis measurements of the FTO|Zn(NDI)|Zn(PMDI) and FTO|Zn(PMDI)|Zn(NDI) bilayer electrodes during key stages of the CV scans. a) Spectroscopic changes during the second reduction in the cathodic scan, which shows the disappearance of the NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e signature, and the coupled appearance of NDI\u003csup\u003e2\u0026minus;\u003c/sup\u003e and PMDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e. b) Spectroscopic changes during the anodic reverse sweep, in which the NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e and PMDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e are oxidized simultaneously to revert to the redox ground state. c) Presence of the different NDI and PMDI redox states as a function of applied potential. Highlighted in the green regions are the simultaneous appearance of the NDI\u003csup\u003e2\u0026minus;\u003c/sup\u003e (418 nm, red curve) and PMDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e states (714 nm, green curve) around -1.34 V in the cathodic sweep, and the simultaneous disappearance of NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e (472 nm, blue curve) and PMDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e (714 nm, green curve) during the reverse oxidative sweep at -1V. NDI\u003csup\u003e0\u003c/sup\u003e (359 nm, black curve) is for the reference. The redox states were assigned based on their characteristic absorptions (see ESI for complete spectra). d) Pictures taken of the FTO|Zn(PMDI)|Zn(NDI) bilayer electrode before, during and after the first CV. Shown are the different colors at the different redox states of the bilayer; most importantly, the differences at 0 V applied potential, where the electrode is colorless before the first scan, but remains yellow after the first scan. This characteristic is due to the electron trapping in the outer Zn(NDI) layer. e) Spectroscopic evidence of the charge trapping: after the first scan, the signals of the NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e remains, irrespective of the applied potential.\u003c/p\u003e\n\u003cp\u003eThe spectroelectrochemical analyses of the bilayer electrodes are fully consistent with the assignments from the CV section (vide supra, Fig. S9). For example, in the FTO|Zn(NDI)|Zn(PMDI) electrode, Zn(PMDI) reduction can be observed along with the emergence of the spectroscopic signatures of the NDI\u003csup\u003e2\u0026minus;\u003c/sup\u003e (Fig. 4a). In the re-oxidation sweep, NDI\u003csup\u003e2\u0026minus;\u003c/sup\u003e is oxidized selectively, and PMDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e re-oxidation occurs as a more anodic process together with NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e re-oxidation (Fig. 4b). The appearance and decay of the different redox states as a function of applied potential is summarized in Fig. 4c (see Fig. S9e-h for the complete spectra). For the reverse FTO|Zn(PMDI)|Zn(NDI) electrode configuration, the unidirectional electron transport through the bilayer electrode can be observed by the bare eye as different colors emerging during the cathodic and anodic scans (Fig. 4d, see Fig. S9a-d for complete spectra). Most importantly, electron trapping in the outer Zn(NDI) layer can be observed after the first scan, and its spectroscopic signature remains throughout any subsequent experiments (Fig. 4e, Fig. S10-11). The operando characterization thus provides direct spectroscopic evidence for the charge trapping, and the importance of the bilayer film design. The storage of electrons in the form of NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e in the outer layer is the result of unidirectional electron flow from PMDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e species to NDI species across the Zn(PMDI)|Zn(NDI) interface. This phenomenon offers an interesting opportunity for such bilayer MOF systems in (photo)electrocatalysis applications, where photoelectrons that are extracted from illuminated semiconductor (SC) substrates can be stored remote from the SC|MOF interface, and available for chemistry in the electrolyte.\u003csup\u003e46, 47\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eElectrochemical diode\u003c/p\u003e\n\u003cp\u003eThe unidirectional electron flow in the bilayer MOF electrodes make them potential candidates for electric rectification. To demonstrate this potential, an electron acceptor, [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e, with an \u003cem\u003eE\u003c/em\u003e\u003csub\u003e1/2\u003c/sub\u003e = -0.15 V was added to the supporting electrolyte (Fig. S12). This species is too large to enter the MOF pores, but will scavenge electrons at the MOF-electrolyte interface,\u003csup\u003e48\u003c/sup\u003e thereby mimicking an Ohmic contact in a source/drain type measurement. In the presence of excess [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e in the electrolyte (ca. 36 mM), the CV of the FTO|Zn(NDI)|Zn(PMDI) bilayer displays a pseudo-catalytic wave at the NDI\u003csup\u003e\u0026bull;\u0026minus;/2\u0026minus;\u0026nbsp;\u003c/sup\u003eredox level (Fig. 5a, middle), consistent with a direct conduction pathway between electrode and acceptor in solution (Fig. 5a, left). No current enhancement is observed in the first reduction wave, even though the NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e is sufficiently reducing to promote electron transfer to the Co acceptor thermodynamically. The absence of this process is due to the insolating nature of the Zn(PMDI) outer layer that prevents contact between the NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e and the acceptor.\u003c/p\u003e\n\u003cp\u003eThe onset potential at which the pseudo-catalytic reaction occurs is best visualized in a series of steady state chronoamperometry experiments at different potentials in the presence and absence of [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e (Fig. 5a, right, Fig. S14-16). The steady state current density in the absence of acceptor approaches zero (Fig. S13) due to the exhaustive reduction of all spatially and thermodynamically available redox levels. In presence of the acceptor, a significant increase in steady state current density is observed at applied potentials that allow for conduction throughout the entire bilayer architecture, i.e. electrons from the inner layer can be transported to the outer layer, where they are extracted by the terminal [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e acceptor. The steady state current densities of the FTO|Zn(NDI)|Zn(PMDI) electrode exhibit a sharp increase at the potential of the NDI\u003csup\u003e\u0026bull;\u0026minus;/2\u0026minus;\u003c/sup\u003e redox level (Fig. 5a, right), consistent with the requirements mentioned above. Almost no current is observed at the NDI\u003csup\u003e0/\u0026bull;\u0026minus;\u003c/sup\u003e level due to the thermodynamic inaccessibility of the outer layer (Fig. 5a, right, Fig S15).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 5.\u003c/strong\u003e Schematic energy level diagram, CVs and steady state current densities obtained from chronoamperometry experiments at varying step potentials in the presence and absence of [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e electron acceptor, mimicking an Ohmic contact for a) FTO|Zn(NDI)|Zn(PMDI) and b) FTO|Zn(PMDI)|Zn(NDI). CVs in the absence (blue) and presence (red) of an excess of [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e at 10 mV s\u003csup\u003e-1\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eIn the reverse configuration, CVs of the FTO|Zn(PMDI)|Zn(NDI) electrode show a current enhancement in the presence of the [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e acceptor already at the potential of the first reduction, i.e. the PMDI0\u003csup\u003e0/\u0026bull;\u0026minus;\u003c/sup\u003e couple (Fig. 5b, middle, Fig. S17). This behavior is expected, as the redox levels of the contributing PMDI\u003csup\u003e0/\u0026bull;\u0026minus;\u003c/sup\u003e and NDI\u003csup\u003e0/\u0026bull;\u0026minus;\u003c/sup\u003e couples are spatially and thermodynamically aligned to promote electron transport through the entire bilayer system already at the potential of the first PMDI reduction. Optically, charge extraction by the [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e acceptor can be noted by the absence of the yellow color of the NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e state.\u003c/p\u003e\n\u003cp\u003eWhen the steady state current densities are plotted against the step-potentials for the two bilayer architectures (Fig. 5, right), it becomes apparent that the applied potentials to achieve half-maximum conductivity differ by about 200 mV. At an applied potential of -1.12 V, the FTO|Zn(PMDI)|Zn(NDI) bilayer sustains a current density of ~0.20 mA, while the inverse bilayer promotes only 0.02 mA at the same potential, i.e. a factor of ca. 10 times less. It should be noted that this difference is directly coupled to the difference in redox potentials of the contributing Zn(PMDI) and Zn(NDI) layers, and can be altered basically at will by changing the contributing linkers to adjust for a given application.\u003c/p\u003e\n\u003cp\u003eAnother noteworthy point is that the steady state current densities under pseudo-catalytic conditions are very similar for the two architectures, which again points to very similar redox conductivity kinetics of the two contributing MOF layers. In fact, the steady-state current closely matches that of FTO|Zn(NDI) monolayer films in the presence of the same acceptor,\u003csup\u003e48\u003c/sup\u003e indicating that the dominant kinetic limitation arises from charge transport within the individual MOF layers rather than from any interfacial electron transport.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this work, we have successfully prepared RC-MOF bilayer electrodes that feature intrinsic redox gradients due to the unique redox characteristics of the employed linkers. Unidirectional electron transport is observed when the cross-film electron flow is a thermodynamic downhill redox reaction in nature (Fig. 1b). Consequently, characteristic electrochemistry profiles with distinct CV features are observed in comparison with mixed linker FTO|Zn(PMDI)\u003csub\u003e0.5\u003c/sub\u003e(NDI)\u003csub\u003e0.5\u003c/sub\u003e. Specifically, the unidirectional electron transport in the FTO|Zn(PMDI)-Zn(NDI) electrode gives rise to trapped electrons in the form of NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e species in the outer MOF film. Operando UV-vis spectroscopy unambiguously confirmed the presence of trapped NDI\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e species, positioning RC-MOF bilayer electrodes as chemical free energy-based rectifying junctions. The rectification effect can be exploited to design switchable charge/color memory elements, chemical sensors and neurochemical devices, and is a significant step toward realizing scalable, solid-state molecular electronic devices.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eAll electrochemical experiments were conducted on MOF thin films that were grown on fluorine-doped tin oxide (FTO) substrates. The successful fabrication of the electrodes was verified by thin film X-ray diffraction (XRD) to assure crystallinity, and scanning electron microscopy (SEM) to examine the morphology of the MOF films, including the determination of film thicknesses.\u003c/p\u003e\n\u003cp\u003ePreparation of bilayer RC-MOF electrodes\u003c/p\u003e\n\u003cp\u003ePrior to MOF growth, a horizontal line was scratched into the FTO substrate using a diamond pencil. The line isolates the bottom 15-20% of the FTO slide from the upper portion which is where all subsequent measurements are performed. In other words, the lower 15-20% of the slide are not in electric contact any longer, and do not contribute to any of the electrochemical phenomena. This procedure is necessary to produce high quality data, as the bilayer electrode in the upper portion of the material exhibits more uniform coverage, minimal intermixing of the two layers or short-circuiting. The cut-off of the lower part of the electrode also prevents direct electrical contact between exposed FTO and the [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e acceptor, which would short-circuit the source-drain experiments.\u003c/p\u003e\n\u003cp\u003eFor the preparation of the FTO|Zn(NDI)|Zn(PMDI) bilayer electrode, a Zn(NDI) thin film was first grown directly on the FTO substrate, followed by the growth of a Zn(PMDI) layer on top of FTO|Zn(NDI). An analogous procedure was employed to obtain the reverse bilayer architecture, FTO|Zn(PMDI)|Zn(NDI), by simply interchanging the sequence of MOF layer growth (details in supplementary material).\u003c/p\u003e\n\u003cp\u003eElectrochemistry\u003c/p\u003e\n\u003cp\u003eCyclic voltammetry\u003c/p\u003e\n\u003cp\u003eCyclic voltammetry (CV) was carried out using a PGSTAT 204 potentiostat equipped with NOVA 2.1.4 software (Autolab-Metrohm). The mixed-linker and bilayer RC-MOF electrodes were used as working electrodes in a custom-built single-compartment electrochemical cell, containing also a glassy carbon counter and a standard non-aqueous Ag/Ag(NO)\u003csub\u003e3\u003c/sub\u003e reference electrode. All measurement were performed in deaerated, dry DMF containing 0.5 M KPF\u003csub\u003e6\u003c/sub\u003e as the supporting electrolyte under an atmosphere of argon that was continuously purged through the cell headspace. All potentials are calibrated against the Fc\u003csup\u003e+/0\u0026nbsp;\u003c/sup\u003eredox couple (details supplementary material).\u003c/p\u003e\n\u003cp\u003eSpectroelectrochemistry\u003c/p\u003e\n\u003cp\u003eOperando time-resolved UV–Vis spectroelectrochemistry was performed using a diode-array spectrophotometer (Agilent 8453) coupled to a potentiostat (Autolab PGSTAT100) controlled by NOVA 2.1.4 software. Redox processes were carried out in a single-compartment electrochemical cell consisting of a quartz cuvette (1 cm pathlength) equipped with a home-made stopper designed to hold three electrodes (Fig. S5). The bilayer electrodes were used as working electrodes, with a Pt rod counter electrode and a non-aqueous Ag/AgNO\u003csub\u003e3\u003c/sub\u003e as the reference electrode. Only the working electrode was positioned in the optical path. Absorption spectra were collected in kinetic mode during electrochemical operation to monitor electrogenerated species in real time. Operando spectroelectrochemistry enables direct correlation of spectral signatures with applied potentials, allowing concerted and sequential reduction processes to be distinguished through monitoring evolution and decay of their distinct absorption profile. All measurements were performed under argon using deaerated electrolytes as described in the CV section, and background spectra were obtained using bare FTO under identical conditions.\u003c/p\u003e\n\u003cp\u003eSource-Drain Experiment\u003c/p\u003e\n\u003cp\u003eThe diode-like behavior was evaluated by CV and chronoamperometry at varying potentials in the presence and absence of electron acceptor Co(bpy)\u003csub\u003e3\u003c/sub\u003e](PF\u003csub\u003e6\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e in the supporting electrolyte. The conditions for the measurements were identical to the ones described above in the CV section. The [Co(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e3+\u003c/sup\u003e complex is too large to enter the MOF pores and was present in excess to act as an interfacial electron sink, effectively mimicking an Ohmic contact analogous to a source-drain electrode configuration. CV measurements were first recorded in the presence of an excess electron acceptor at a scan rate of 10 mV s\u003csup\u003e-1\u003c/sup\u003e. Step-potential chronoamperometry (200-300s per step) was then performed over the potential range of -0.2 to -1.8 V in 0.1 V increments. This approach enables the precise probing of each redox state and ensures steady state current under thermodynamically feasible and specially allowed charge transport conditions.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data supporting the findings of this study are available in the Article or its Electronic Supplementary Information (ESI). The ESI includes full details of the linker preparation, synthetic methods, electrochemical, spectroelectrochemical studies, and other supporting characterization data.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFinancial support for the work was provided by the Knut \u0026amp; Alice Wallenberg Foundation (KAW 2019.0071), the Olle Engkvist Foundation (212-0147), and the Swedish Research Council (VR 2023-03395).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eJ.L. and A.K. contributed equally to this work. J.L. A.K. and S.O. planned and designed the experiments. A.K. synthesized the linkers, J.L. and A.K. prepared the bilayer electrodes on FTO. A.K. performed the characterization. A.K. and J.L. carried out the experiments and, together with S.O. analysed and interpretated the data. S.O supervised and directed the project, including funding acquisition. J.L., A.K and S.O. co-wrote the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eLi, J.; Ott, S. The Molecular Nature of Redox-Conductive Metal\u0026ndash;Organic Frameworks. \u003cem\u003eAcc. Chem. Res. \u003c/em\u003e\u003cstrong\u003e2024\u003c/strong\u003e, \u003cem\u003e57\u003c/em\u003e (19), 2836-2846.\u003c/li\u003e\n\u003cli\u003eKung, C.-W.; Goswami, S.; Hod, I.; Wang, T. C.; Duan, J.; Farha, O. K.; Hupp, J. T. Charge Transport in Zirconium-Based Metal\u0026ndash;Organic Frameworks. \u003cem\u003eAcc. Chem. Res. \u003c/em\u003e\u003cstrong\u003e2020\u003c/strong\u003e, \u003cem\u003e53\u003c/em\u003e (6), 1187-1195.\u003c/li\u003e\n\u003cli\u003eXie, L. S.; Skorupskii, G.; Dincă, M. 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Switching between Limiting Charge Extraction Regimes in an Illuminated Semiconductor\u0026ndash;Metal\u0026ndash;Organic Framework Junction. \u003cem\u003eJ. Am. Chem. Soc. \u003c/em\u003e\u003cstrong\u003e2025\u003c/strong\u003e.\u003c/li\u003e\n\u003cli\u003eJohnson, B. A.; Castner, A. T.; Agarwala, H.; Ott, S. Beyond diffusion: ion and electron migration contribute to charge transport in redox-conducting metal\u0026ndash;organic frameworks. \u003cem\u003eChem. Sci. \u003c/em\u003e\u003cstrong\u003e2025\u003c/strong\u003e, \u003cem\u003e16\u003c/em\u003e (12), 5214-5222.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-9056510/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9056510/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Unidirectional electron flow is essential for applications in electron storage and reconfigurable electronics, and traditionally realized in semiconductor junctions and even single-molecule concepts. Combining aspects from both technologies, redox-conductive metal-organic frameworks (RC-MOFs) exhibit molecule-like behavior in a crystalline, porous matrix. Herein, we show that bilayer RC-MOF electrodes composed of sequentially deposited Zn(PMDI) and Zn(NDI) on fluorine-doped tin oxide (FTO) function as chemical free-energy-based rectifying junctions. Unidirectional electron flow arises from thermodynamically allowed, and spatially organized redox reactions at the Zn(PMDI)|Zn(NDI) interface. Showcasing the rectifying function, electrons that reach the outer Zn(NDI) layer in the FTO|Zn(PMDI)|Zn(NDI) configuration are trapped as NDI•− and cannot be recovered by applying an oxidative bias. Introduction of [Co(bpy)3]3+ to the electrolyte creates a source–drain situation that reveals the potential-dependent directional electron flow across the bilayer. These results position RC-MOF bilayers as programmable electrochemical diodes, with rectification governed by layer sequence and redox accessibility.","manuscriptTitle":"Electric Rectification by a Redox-Conductive Metal-Organic Framework Bilayer Electrode","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-23 10:48:14","doi":"10.21203/rs.3.rs-9056510/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"901c64bf-51b0-4215-9665-9c631f80578b","owner":[],"postedDate":"March 23rd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":64809443,"name":"Physical sciences/Materials science/Materials for devices/Electronic devices"},{"id":64809444,"name":"Physical sciences/Chemistry/Electrochemistry"},{"id":64809445,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Metal\u0026#x2013;organic frameworks"}],"tags":[],"updatedAt":"2026-04-10T08:42:29+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-23 10:48:14","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9056510","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9056510","identity":"rs-9056510","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}