Regulating Photoluminescence of Aluminum Complexes by Substituents on Metal: From Non-luminescence to Room-Temperature Phosphorescence

Regulating Photoluminescence of Aluminum Complexes by Substituents on Metal: From Non-luminescence to Room-Temperature Phosphorescence | 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 Regulating Photoluminescence of Aluminum Complexes by Substituents on Metal: From Non-luminescence to Room-Temperature Phosphorescence Kazuo Tanaka, Shunichiro Ito, Takuya Hosokai, Yoshiki Chujo This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4633219/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 09 Sep, 2024 Read the published version in Communications Chemistry → Version 1 posted You are reading this latest preprint version Abstract In this manuscript, synthesis and optical properties, such as crystallization-induced emission (CIE) and room-temperature phosphorescence (RTP), and the substituent effect on the central element are reported based on β -diketiminate aluminum complexes. Although luminescent aluminum compounds have been utilized for emitting and electron transporting layers in organic light-emitting diodes, most of them often exhibit not phosphorescence but fluorescence with low photoluminescent quantum yields in the aggregated state than those in the amorphous state due to concentration quenching. In this study, the π-conjugated β -diketiminate ligand was employed for constructing four-coordinated complexes with dialkyl- or dihaloaluminum moieties, and the dihaloaluminum complexes were found to exhibit the CIE property. Moreover, we found that the diiodoaluminum complex provided RTP, while the dialkylaluminum complexes hardly showed emission at room temperature. From theoretical calculations, it was suggested that undesired structural relaxation in the singlet excited state of dialkyl complexes should be suppressed by introducing electronegative halogens instead of alkyl groups. Our findings might be useful for establishing a new molecular design not only for obtaining luminescent complexes but also for achieving triplet-harvesting materials. Physical sciences/Chemistry/Physical chemistry/Excited states Physical sciences/Chemistry/Inorganic chemistry/Organometallic chemistry/Ligands Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Organic and inorganic aluminum complexes have attracted much attention as a Lewis acid in organic reactions and polymerizations and as a platform for constructing optoelectronic materials. In particular, luminescent aluminum complexes have been widely investigated since tris(8-quinolinolato)aluminum (Alq 3 ) has been applied to an emitting layer in organic light-emitting diodes (OLEDs). 1 – 3 Their luminescence color can be tuned over whole visible region by modifying the electronic structure of their ligand moieties especially in the case of 8-quinolinolate 4 and salen 5 complexes. However, most of these complexes exhibit only fluorescence rather than phosphorescence and critically weaker emission intensity in the solid state than that in the dilute solution state although they are required to show luminescent properties in the condensed state for the device applications. In addition, there are limited series of stable aluminum complexes which are able to exhibit remarkable emission properties. 2 Thus, it is still of great significance to explore a new class of solid-state luminescent aluminum complexes, especially with room-temperature phosphorescence (RTP). β -Diketiminate ligands, also known as β -diiminate or “nacnac” ligands, have been utilized for isolating a wide range of unstable main-group metal complexes as well as transition metal ones. These complexes have been applied for diverse chemical reactions, e.g. , activation of inert chemical bonds and polymerization. 6 – 15 In particular, since the monomeric β -diketiminate aluminum(I) complex was isolated, 16 various kinds of four-coordinated aluminum(III) complexes have been applied for activation of a variety of inert chemical bonds and have attracted growing attention in this two decades. 17 – 20 Despite tremendous research efforts, the optical and/or electronic properties of aluminum(III) complexes have not been mainly focused on probably because they have very weak absorption bands in visible region due to the limited π-conjugation length through the β -diketiminate ligands. Recently, solid-state emissive β -diketiminate complexes composed of four-coordinate group 13 elements, such as boron and gallium, have been reported. 21 – 27 It has been clarified that extension of the π-conjugated system of β -diketiminate ligands lead the corresponding complexes to be luminescent in the visible region. More importantly, these complexes emit weakly in the dilute solution and amorphous solid, while they provide intense emission in the crystalline state. This phenomenon is called crystallization-induced emission (CIE) and has received much attention because of its potential applications in the field of OLEDs, fluorescent sensors, bioimaging and laser amplifiers. 28 – 30 The bulky peripheral aromatic groups probably consume the excited energy in the excited state and prohibit π-stacking which leads to critical concentration quenching in the solid state. 29 Thus, these complexes are expected to be a platform for developing stimuli-responsive optical materials with solid-state luminescent properties. 31 – 38 Herein, we envisioned that a class of the π-extended β -diketiminate ligand serves as a scaffold to obtain efficient solid-state emission properties from aluminum complexes. Results and Discussion Synthesis and characterization To assess the effects of substituents on aluminum on the photophysical properties of β -diketiminate complexes, we synthesized five compounds with different substituents (Fig. 1 a). The isopropyl groups on the aromatic groups of the ligand ( LH ) were employed for the kinetic stabilization of the complex. 16 , 39 – 41 The dialkylaluminum complexes, LAlMe and LAlEt , were successfully synthesized by reacting trialkylaluminum with LH in toluene at 100°C according to the literatures on the syntheses of the related complexes. 13 , 39 , 42 The dihaloaluminum complexes, LAlCl , LAlBr , and LAlI , were synthesized with the modified procedure of the related compounds through the reactions of LH and n -butyllithium followed by the treatment with the corresponding aluminum trihalides. 39 , 40 These molecular structures were characterized with 1 H, 13 C{1H} and 27 Al{ 1 H} NMR spectroscopies, HRMS spectrometry, elemental analysis, and single crystal X-ray diffraction analysis (Fig. 1 b–e). Photophysical properties at room temperature There are two significant effects from the substituents on the photoluminescent properties (Fig. 2 a): (i) The dihaloaluminum complexes emit light in solid states at room temperature, while the dialkyl ones hardly show emission under the same condition; (ii) the emission color of the dihaloaluminum complexes depends on the type of halogen ( LAlCl , blue; LAlBr , bluish white; LAlI , green). Meanwhile, all compounds exhibited almost no luminescence in the solution states at room temperature in the similar manner as shown in the previous reports. 22 – 24 , 43 The previous studies demonstrated that the conventional β -diketiminate complexes have the CIE property with blue emission in crystalline states at room temperature when neither electron-accepting nor donating substituent was introduced on the ligands. Therefore, the non-emissive nature of the alkylaluminum complexes and the halogen-dependent emission color are the peculiar features among this class of luminophores. We conducted spectroscopic studies under nitrogen at room temperature to elucidate the electronic structures of the complexes (Fig. 2 b–d and Table 1 ). Firstly, UV–vis absorption spectra were recorded in the 2-methylpentane (2MP)/toluene solutions (99/1, v/v, 1 × 10 –5 M) at room temperature (Fig. 2 b). All compounds showed similar absorption spectra with slight difference in the position of absorption maximum. The shapes and positions of the longest-wavelength absorption bands were similar to the typical β -diketiminate complexes and assignable mainly to the π–π* (S 0 –S 1 ) transition of the ligand moiety, suggesting that the electronic character of the ground-state structure of the complexes should not be affected by the substituents on the aluminum atom. Nevertheless, the substituents apparently change the S 0 –S 1 transition energy probably due to their different contribution to the frontier orbitals. 44 Secondly, photoluminescent (PL) spectra were measured for the same solutions and their crystalline powders (Fig. 2 c and d). As we presumed, quite weak and broad emission spectra were obtained from all solutions. Their absolute quantum yields ( Φ PL ) were lower than 0.01. In the crystalline state, LAlMe and LAlEt hardly exhibited emission enhancement ( Φ PL < 0.01), while LAlCl , LAlBr , and LAlI showed significant emission spectra in the visible region. Φ PL values were determined to be 0.48 for LAlCl , 0.44 for LAlBr , and 0.58 for LAlI . Importantly, the PL spectra of LAlBr and LAlI composed two distinct bands in the blue and green regions. PL lifetime ( τ PL ) measurements revealed that the shorter-wavelength bands possessed nanosecond-order τ PL , while the longer-wavelength ones had microsecond-order τ PL (Fig. 2 g and h, Table 2 ). Finally, their PL spectra recorded after several milliseconds after photoexcitation showed only longer-wavelength component (Figure S8). Consequently, the blue- and green-emission bands were assignable to fluorescence and phosphorescence, respectively. These data mean that the heavy atom effect of bromine and iodine should lead to the RTP properties of LAlBr and LAlI . To the best of our knowledge, the estimated phosphorescence quantum yield ( Φ P ) value of LAlI (0.54) is the highest one among the aluminum complexes. It is also of interest to note that the related boron complexes with iodine on the peripheral aromatic rings hardly show apparent RTP. 25 The heavy atoms directly attached on the central element might efficiently accelerate intersystem crossing and phosphorescence processes. Table 1 Results of photophysical measurements at room temperature a λ abs / nm ε / 10 4 M – 1 cm – 1 λ Fluo / nm λ Phos / nm Φ F Φ P LAlMe solution 403 2.5 460 n.d. < 0.01 – crystal – – 473 n.d. < 0.01 – LAlEt solution 410 2.0 458 n.d. < 0.01 – crystal – – 480 n.d. < 0.01 – LAlCl solution 386 2.8 436 n.d. < 0.01 < 0.01 crystal – – 442 n.d. 0.48 – LAlBr solution 385 2.9 441 n.d. < 0.01 < 0.01 crystal – – 447 511 0.25 0.19 LAlI solution 384 2.4 450 528 < 0.01 < 0.01 crystal – – 454 515 0.04 0.54 a Photoluminescence properties were recorded with photoexcitation at the absorption maximum wavelength in solution state at room temperature. Solution, 1 × 10 –5 M in 2-methylpentane/toluene (99/1, v/v); crystal, recrystallized from hexane; –, not determined. Phosphorescence spectra were recorded with pulsed excitation. Quantum yields of fluorescence and phosphorescence were estimated by absolute quantum yields and deconvoluted photoluminescence spectra with multi-component Gaussian functions. Table 2 PL lifetime and estimated rate constants at room temperature a / ns / ms k r S / 10 7 s – 1 k nr S / 10 7 s – 1 k ISC S / 10 7 s – 1 k r T / 10 2 s – 1 k nr T / 10 2 s – 1 LAlCl solution 0.03 – 3.3 3300 3.3 – – crystal 2.1 – 34 31 6.4 – – LAlBr solution 0.04 – 2.5 2500 25 – – crystal 2.0 4.0 13 16 23 1.1 1.4 LAlI solution 0.04 0.10 1.0 2200 250 0.4 100 crystal 0.14 0.29 29 10 680 20 15 a and , average fluorescence and phosphorescence lifetimes, respectively; k r S , radiative decay rate constant from singlet state (fluorescence); k nr S , nonradiative decay rate constant from singlet state (internal conversion); k ISC S , intersystem crossing rate constant from singlet state; k r T , radiative decay rate constant from triplet state (phosphorescence); k nr T , nonradiative decay rate constant from triplet state. From the fluorescence and phosphorescence quantum yields and PL lifetime measurements with the dihaloaluminum complexes at room temperature, rate constants of each photophysical processes were estimated (Table 2 , see the Supplementary Information). For all three complexes in the solution states, quite large nonradiative decay rate constants for singlet states ( k nr S ~ 10 10 s – 1 ) were obtained, suggesting that most excited molecules would be quenched nonradiatively through the internal conversion from S 1 to S 0 probably because this process could occur through conical intersections. 45 Only in the case of LAlI , the rapid intersystem crossing process derived from the strong heavy atom effect of iodine could occur to some extent ( k ISC / k nr S ~ 0.1). On the other hand, the crystals of these complexes exhibited at least 100 times smaller k nr S values than their solutions, leading to their CIE properties. In the cases of LAlBr and LAlI , the suppression of the internal conversion should open the intersystem crossing processes as well as fluorescence. It is worth noting that the radiative rate constant from singlet states ( k r S ) and k ISC S of some complexes were enhanced by the crystallization, which might originate from the intermolecular interactions and could contribute to their CIE properties. Photophysical properties at 77 K To gain further information about the photophysical processes, we recorded PL spectra of the solutions and crystalline powders at 77 K with a cryostat under nitrogen atmosphere (Fig. 2 e and Table 3 ). Importantly, all compounds clearly exhibited phosphorescence in the frozen solution state, probably because the nonradiative decay processes could be closed at the low temperature. Indeed, the hypsochromic shifts of the emission band were observed, indicating that structural changes in the excited state should be hampered under the frozen environment. In other words, it is suggested that there are significant structural relaxations in the excited state that cause nonradiative decay of the singlet excited states in room-temperature solutions. Interestingly, LAlMe and LAlCl exhibited phosphorescence at 77 K, despite the absence of heavy atoms, implying the intrinsic triplet-forming properties of these series of compounds. 43 In the crystalline states at 77 K, the slight hypsochromic shifts of emission bands were observed except for LAlMe . These shifts might originate from the tight packing of the crystals. On the other hand, the bathochromic shift for the crystal of LAlMe might be attributed to the weakening of the 0–0 band. Significantly, the apparent crystallization-induced phosphorescence enhancement was observed from LAlI . The estimated Φ Phos in crystal was 2.2 times higher than that in the frozen solution, possibly because of the acceleration of intersystem crossing and phosphorescence processes and because the restriction of nonradiative decay from excited triplet states. Table 3 Results of photoluminescence measurements at 77 K a λ Fluo / nm λ Phos / nm Φ Fluo Φ Phos LAlMe solution 445 541 0.96 0.04 crystal 474 569 0.25 n.d. LAlEt solution 429 530 n.d. n.d. crystal 429 n.d. n.d. n.d. LAlCl solution 428 510 0.93 0.03 crystal 422 535 0.96 0.04 LAlBr solution 435 514 0.56 0.35 crystal 422 506 0.56 0.26 LAlI solution 426 515 0.13 0.33 crystal 425 507 0.03 0.74 a Excited at the absorption maximum wavelength in solution state at room temperature. Solution, 1 × 10 –5 M in 2-methylpentane/toluene (99/1, v/v); crystal, recrystallized from hexane; n.d., not determined due to negligible phosphorescence. Phosphorescence spectra were recorded with pulsed excitation. Quantum yields of fluorescence and phosphorescence were estimated by absolute quantum yields and deconvoluted photoluminescence spectra with multi-component Gaussian functions. Theoretical calculations Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed with the Gaussian 16 package 46 to study the electronic structures of the dimethylaluminum and dihaloaluminum complexes (Fig. 3 ). Geometry optimization was performed for both S 0 and S 1 states with the CAM-B3LYP functional and the Lanl2DZ basis set for I and the 6-31G(d,p) one for the other atoms, followed by single-point transition energy calculations at the same level of theory except for the basis set for the light atoms (6-311 + + G(d,p)). The significant structural relaxation at the S 1 state was estimated only for LAlMe , leading to the narrow S 0 –S 1 energy gap (1.58 eV) and the small oscillator strength ( f = 0.0080). The most characteristic change in the relaxation was the elongation of one of the Al–C bonds from 1.98 Å (S 0 ) to 2.30 Å (S 1 ). At the S 0 geometry, its Kohn–Sham highest occupied molecular orbital (HOMO) was significantly delocalized over the Al–C bond as well as the ligand moiety, while the Kohn–Sham lowest unoccupied molecular orbital (LUMO) possessed little contribution from this bond because of a nodal plane passing through it. Hence, the electron density between aluminum and carbon atoms should decrease upon the photoexcitation from S 0 to S 1 , resulting in the weakening of the bond and the considerably large structural relaxation at the S 1 state. Similar photoinduced bond weakening or activation of Al–C bond have been reported in other systems of photochemical reactions. 48–51 For the resulted S 1 geometry, the HOMO mainly located at the Al–C moiety, where the HOMO–LUMO overlap much less effectively than that for the S 0 geometry, making the f value of the S 1 –S 0 transition smaller. Therefore, it is suggested that the structural relaxation should be responsible for the nonradiative decay process of the dialkylaluminum complexes. On the other hand, the dihaloaluminum complexes presented only smaller structural changes between the S 0 and S 1 states, probably because the contribution from the Al–halogen bond to their HOMO would be smaller. Natural bond orbital (NBO) analysis suggested that the orbital on the Al–C bond should be mainly composed of the NBO attributed to the 2p orbital of the carbon atom, which is located at the similar energy region with the HOMO of the β -diketiminate ligand. As the corresponding p orbitals of chlorine, bromine, and iodine should be located at the much lower energy region because of their large electronegativity than carbon, the Al–halogen bond would not strongly contribute to the HOMO of the dihaloaluminum complexes due to the weaker orbital interactions. As a result, the undesired structural relaxation causing the non-radiative quenching could hardly occur in the S 1 state and the S 1 –S 0 electronic transition would be no longer forbidden at its S 1 geometry ( e.g. , f = 0.5070 for LAlCl , Fig. 3 b). Consequently, it is suggested that the photophysical processes of β -diketiminate complexes could be drastically modulated by the substituents on the central element. In addition, it is worth noting that the electron-donating contribution from the Al–C bond destabilizes the HOMO level compared to the dihaloaluminum complexes, leading to the lower S 1 state consistent with the observed redshift of the absorption band. Indeed, LAlEt , with the more strongly electron-donating ethyl group, showed the absorption band in the lowest energy region among the complexes. We also calculated excited triplet state (T n ) energy and spin–orbit coupling (SOC) constants between S m and T n states, ξ (S m –T n ), for the dihaloaluminum complexes with the Q-Chem 5 package 47 to get deeper insights into their phosphorescent properties (Fig. 4 ). The S 1 and T 1 states of each complex were dominantly characterized by the locally excited (LE) state within the central N 2 C 3 moiety. As the S 1 –T 1 energy gap was calculated to be about 1.0 eV or larger, the ISC from S 1 to T 1 seemed to be less efficient. On the other hand, the T n (n = 2–4) states were located in the similar energy region of the S 1 state (± 100 meV). In addition, the SOC values were estimated to be large enough to accept the efficient ISC between S 1 and T n . For LAlCl and LAlBr , these large SOC values are attributable to the charge transfer (CT) character of these T n states with twisted conformations between the donor (aromatic rings) and acceptor (N 2 C 3 unit) as shown in Fig. 4 b. As the transitions between the S 1 (LE) to the T n (CT) occurs with the large change in orbital angular momentum derived from the twisted conformations, the electron-spin flipping is allowed with holding the angular momentum conservation. 48 On the other hand, the T 2 and T 3 states of LAlI significantly consist of the transition from the nonbonding orbitals (lone pairs) of the iodine atoms (HOMO–1 and HOMO–2) to its LUMO. Consequently, the heavy-atom effect of iodine could efficiently accelerate the ISC between S 1 to T 2 and T 3 . Importantly, it was suggested that the SOC constants not only between S 1 and T n but also between S 0 and T 1 significantly increased as the atomic number of the halogen atoms become larger because of the heavy atom effect. Therefore, both of ISC and phosphorescence processes should be enhanced in LAlBr and LAlI compared to LAlCl . Conclusion CIE-active four-coordinated β -diketiminate aluminum(III) complexes, LAlCl , LAlBr , and LAlI were discovered. From the spectroscopic measurements and the theoretical calculations, it was strongly suggested that the 2p orbital of the carbon atom in LAlMe significantly contributed to its HOMO and induced the undesirable structural relaxation in the S 1 state. As a result of the relaxation, internal conversion from S 1 to S 0 should occur even in the crystalline state at room temperature. The series of NBO analysis certainly proposed that more electronegative halogen substituents would not disturb the HOMO distribution of the β -diketiminate moiety, leading to suppression of the non-radiative quenching paths. Thus, the dihaloaluminum complexes exhibited efficient photoluminescence in the crystalline state at room temperature. In particular, LAlI exhibited room-temperature phosphorescence with 0.54 of the phosphorescence quantum yield as a result of the efficient heavy atom effect of iodine on the central element. Our strategy for constructing desired luminophores by modifying the substituents on the central element could be applicable not only for achieving further luminescent metal complexes but also for obtaining optoelectronic materials, reagents and catalysts. Declarations Data Availability The data that support the findings of this study are available in the supplementary material of this article. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers 1880035 for LAlCl , 1880036 for LAlMe , 2364980 for LAlBr , and 2364981 for LAlI . These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Acknowledgements Computation time was provided by the SuperComputer System, Institute for Chemical Research, Kyoto University. We thank Y. Okabayashi (AIST) for his support on time-resolved PL measurements. This work was partially supported by The Asahi Glass Foundation and a Grant-in-Aid for Early-Career Scientists (for S.I., JSPS KAKENHI Grant Numbers 21K14673 and 23K13793) and for Scientific Research (B) (for K.T., JSPS KAKENHI Grant Number, 24K01570). Competing Interests The authors declare no competing interests. References Tang, C. W. & VanSlyke, S. A. Organic electroluminescent diodes. Appl. Phys. Lett. 51, 913–915 (1987). Wang, S. Luminescence and electroluminescence of Al(III), B(III), Be(II) and Zn(II) complexes with nitrogen donors. Coord. Chem. Rev. 215, 79–98 (2001). Zhao, S.-B. & Wang, S. Luminescence and reactivity of 7-azaindole derivatives and complexes. Chem. Soc. Rev. 39, 3142–3156 (2010). Pohl, R. & Anzenbacher, P. 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The synthesis and structure of lithium derivatives of the sterically encumbered β-diketiminate ligand [{(2,6-Pr i 2 H 3 C 6 )N(CH 3 )C} 2 CH] – , and a modified synthesis of the aminoimine precursor. J. Chem. Soc., Dalton Trans. 3465–3469 (2001). Ito, S., Tanaka, K. & Chujo, Y. Characterization and photophysical properties of a luminescent aluminum hydride complex supported by a β-diketiminate ligand. Inorganics 7, 100 (2019). Aoyama, Y., Sakai, Y., Ito, S. & Tanaka, K. Effects of central elements on the properties of group 13 dialdiminate complexes. Chem. A Eur. J. 29, e202300654 (2023). Ito, S. et al. Highly efficient luminescence from boron β-dialdiminates and their π-conjugated polymers in both solutions and solids: significant impact of the substituent position on luminescence behavior. Mater. Chem. Front. 7, 4971–4983 (2023). Frisch, M. J. et al. Gaussian 16 Rev. C.01 . (Wallingford, CT, 2016). Shao, Y. et al. Advances in molecular quantum chemistry contained in the Q-Chem 4 program package. Mol. Phys. 113, 184–215 (2015). Shao, W. & Kim, J. Metal-free organic phosphors toward fast and efficient room-temperature phosphorescence. Acc. Chem. Res. 55, 1573–1585 (2022). Additional Declarations There is NO Competing Interest. Supplementary Files SupportingInformation.docx Cite Share Download PDF Status: Published Journal Publication published 09 Sep, 2024 Read the published version in Communications Chemistry → 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-4633219","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":321000185,"identity":"d26c063e-5ae3-49c1-b42f-37ef118e20c9","order_by":0,"name":"Kazuo Tanaka","email":"data:image/png;base64,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","orcid":"https://orcid.org/0000-0001-6571-7086","institution":"Kyoto University","correspondingAuthor":true,"prefix":"","firstName":"Kazuo","middleName":"","lastName":"Tanaka","suffix":""},{"id":321000186,"identity":"14df90fc-d1d1-4278-992e-c68bbc7540e7","order_by":1,"name":"Shunichiro Ito","email":"","orcid":"https://orcid.org/0000-0003-3822-0247","institution":"Kyoto University","correspondingAuthor":false,"prefix":"","firstName":"Shunichiro","middleName":"","lastName":"Ito","suffix":""},{"id":321000187,"identity":"d7ac76c2-c75e-4043-b0ec-52b190e8966f","order_by":2,"name":"Takuya Hosokai","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Takuya","middleName":"","lastName":"Hosokai","suffix":""},{"id":321000188,"identity":"e137a196-57a9-4aae-a6b5-65bae1497b0a","order_by":3,"name":"Yoshiki Chujo","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Yoshiki","middleName":"","lastName":"Chujo","suffix":""}],"badges":[],"createdAt":"2024-06-25 03:25:45","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4633219/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4633219/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s42004-024-01295-z","type":"published","date":"2024-09-09T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":60907426,"identity":"a5cfb413-7dcf-4275-81c9-fa4d3a5267ec","added_by":"auto","created_at":"2024-07-23 12:10:14","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":401217,"visible":true,"origin":"","legend":"\u003cp\u003eAluminum complexes investigated in this study. (a) Synthetic scheme of the dialkylaluminum and dihaloaluminum complexes. Single-crystal structures of (b) \u003cstrong\u003eLAlMe\u003c/strong\u003e, (c) \u003cstrong\u003eLAlCl\u003c/strong\u003e, (d) \u003cstrong\u003eLAlBr\u003c/strong\u003e, and (e) \u003cstrong\u003eLAlI\u003c/strong\u003e.\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-4633219/v1/a558dd83b85af4426eb59669.png"},{"id":60906728,"identity":"c54cd0a6-5fe1-4c31-bf91-ea4f05232b49","added_by":"auto","created_at":"2024-07-23 12:02:13","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1624655,"visible":true,"origin":"","legend":"\u003cp\u003ePhotophysical properties of aluminum \u003cem\u003eβ\u003c/em\u003e-diketiminate complexes. (a) Photographic images of solutions and crystals of the complexes. UV, 365 nm. (b,c) UV–vis absorption and PL spectra in solutions at room temperature, respectively. (d) PL spectra in crystalline states at room temperature. (e,f) PL spectra in solutions and crystalline states at 77 K, respectively. (g,h) Phosphorescence decay curve of \u003cstrong\u003eLAlBr\u003c/strong\u003e (detected at 511 nm) and \u003cstrong\u003eLAlI\u003c/strong\u003e (detected at 515 nm) in crystalline states at room temperature, respectively. Solid lines represent fitting curves.\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-4633219/v1/70a54bd9008d0ef62668503e.png"},{"id":60906729,"identity":"215adf11-eb06-48cf-89d1-764d08d1b95b","added_by":"auto","created_at":"2024-07-23 12:02:13","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":942502,"visible":true,"origin":"","legend":"\u003cp\u003eResults of (TD-)DFT calculations. (a) Calculated energies of Kohn–Sham HOMO and NBO of the corresponding Al–X (X = Me, Cl, Br, and I) bond. (b,c) Optimized geometries and energies of \u003cstrong\u003eLAlCl\u003c/strong\u003e and \u003cstrong\u003eLAlMe\u003c/strong\u003e, respectively, at S\u003csub\u003e0\u003c/sub\u003e and S\u003csub\u003e1\u003c/sub\u003e states. (d) Kohn–Sham HOMO and LUMO and NBO. Orange circles highlight the contribution from the substituent on aluminum to each HOMO.\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-4633219/v1/34358dadd3f8f7ff3dc89a36.png"},{"id":60906733,"identity":"5e5863a9-9599-4af3-85aa-626caed50e2c","added_by":"auto","created_at":"2024-07-23 12:02:14","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":749032,"visible":true,"origin":"","legend":"\u003cp\u003eExcited singlet and triplet states of the complexes. (a) Excitation energies of singlet and triplet states relative to each S\u003csub\u003e0\u003c/sub\u003e state. Orbital contributions to each state and SOC constants between S\u003csub\u003em\u003c/sub\u003e and T\u003csub\u003en\u003c/sub\u003e states are shown in gray texts. H and L denote HOMO and LUMO. (b) Kohn–Sham molecular orbital distributions (isovalue = 0.03).\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-4633219/v1/9d7e7f84f6ca6297d1b9377e.png"},{"id":64211273,"identity":"cac67d52-2c02-4c4a-93e5-ee42e2ef3855","added_by":"auto","created_at":"2024-09-10 07:31:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5337823,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4633219/v1/8f15ac10-b04a-456b-935e-53cab01b5d8c.pdf"},{"id":60906731,"identity":"19dcf0db-0c37-4f1f-82ec-32ac31dac0bb","added_by":"auto","created_at":"2024-07-23 12:02:14","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":11001665,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"SupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-4633219/v1/d406dc1fa8372015a53bc996.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Regulating Photoluminescence of Aluminum Complexes by Substituents on Metal: From Non-luminescence to Room-Temperature Phosphorescence","fulltext":[{"header":"Introduction","content":"\u003cp\u003eOrganic and inorganic aluminum complexes have attracted much attention as a Lewis acid in organic reactions and polymerizations and as a platform for constructing optoelectronic materials. In particular, luminescent aluminum complexes have been widely investigated since tris(8-quinolinolato)aluminum (Alq\u003csub\u003e3\u003c/sub\u003e) has been applied to an emitting layer in organic light-emitting diodes (OLEDs).\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 Their luminescence color can be tuned over whole visible region by modifying the electronic structure of their ligand moieties especially in the case of 8-quinolinolate\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e and salen\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e complexes. However, most of these complexes exhibit only fluorescence rather than phosphorescence and critically weaker emission intensity in the solid state than that in the dilute solution state although they are required to show luminescent properties in the condensed state for the device applications. In addition, there are limited series of stable aluminum complexes which are able to exhibit remarkable emission properties.\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e Thus, it is still of great significance to explore a new class of solid-state luminescent aluminum complexes, especially with room-temperature phosphorescence (RTP).\u003c/p\u003e \u003cp\u003e \u003cem\u003eβ\u003c/em\u003e-Diketiminate ligands, also known as \u003cem\u003eβ\u003c/em\u003e-diiminate or \u0026ldquo;nacnac\u0026rdquo; ligands, have been utilized for isolating a wide range of unstable main-group metal complexes as well as transition metal ones. These complexes have been applied for diverse chemical reactions, \u003cem\u003ee.g.\u003c/em\u003e, activation of inert chemical bonds and polymerization.\u003csup\u003e\u003cspan additionalcitationids=\"CR7 CR8 CR9 CR10 CR11 CR12 CR13 CR14\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e In particular, since the monomeric \u003cem\u003eβ\u003c/em\u003e-diketiminate aluminum(I) complex was isolated,\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e various kinds of four-coordinated aluminum(III) complexes have been applied for activation of a variety of inert chemical bonds and have attracted growing attention in this two decades.\u003csup\u003e\u003cspan additionalcitationids=\"CR18 CR19\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e Despite tremendous research efforts, the optical and/or electronic properties of aluminum(III) complexes have not been mainly focused on probably because they have very weak absorption bands in visible region due to the limited π-conjugation length through the \u003cem\u003eβ\u003c/em\u003e-diketiminate ligands. Recently, solid-state emissive \u003cem\u003eβ\u003c/em\u003e-diketiminate complexes composed of four-coordinate group 13 elements, such as boron and gallium, have been reported.\u003csup\u003e\u003cspan additionalcitationids=\"CR22 CR23 CR24 CR25 CR26\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e It has been clarified that extension of the π-conjugated system of \u003cem\u003eβ\u003c/em\u003e-diketiminate ligands lead the corresponding complexes to be luminescent in the visible region. More importantly, these complexes emit weakly in the dilute solution and amorphous solid, while they provide intense emission in the crystalline state. This phenomenon is called crystallization-induced emission (CIE) and has received much attention because of its potential applications in the field of OLEDs, fluorescent sensors, bioimaging and laser amplifiers.\u003csup\u003e\u003cspan additionalcitationids=\"CR29\" citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e The bulky peripheral aromatic groups probably consume the excited energy in the excited state and prohibit π-stacking which leads to critical concentration quenching in the solid state.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e Thus, these complexes are expected to be a platform for developing stimuli-responsive optical materials with solid-state luminescent properties.\u003csup\u003e\u003cspan additionalcitationids=\"CR32 CR33 CR34 CR35 CR36 CR37\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e Herein, we envisioned that a class of the π-extended \u003cem\u003eβ\u003c/em\u003e-diketiminate ligand serves as a scaffold to obtain efficient solid-state emission properties from aluminum complexes.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSynthesis and characterization\u003c/h2\u003e \u003cp\u003eTo assess the effects of substituents on aluminum on the photophysical properties of \u003cem\u003eβ\u003c/em\u003e-diketiminate complexes, we synthesized five compounds with different substituents (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). The isopropyl groups on the aromatic groups of the ligand (\u003cb\u003eLH\u003c/b\u003e) were employed for the kinetic stabilization of the complex.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan additionalcitationids=\"CR40\" citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e The dialkylaluminum complexes, \u003cb\u003eLAlMe\u003c/b\u003e and \u003cb\u003eLAlEt\u003c/b\u003e, were successfully synthesized by reacting trialkylaluminum with \u003cb\u003eLH\u003c/b\u003e in toluene at 100\u0026deg;C according to the literatures on the syntheses of the related complexes.\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e,\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e The dihaloaluminum complexes, \u003cb\u003eLAlCl\u003c/b\u003e, \u003cb\u003eLAlBr\u003c/b\u003e, and \u003cb\u003eLAlI\u003c/b\u003e, were synthesized with the modified procedure of the related compounds through the reactions of \u003cb\u003eLH\u003c/b\u003e and \u003cem\u003en\u003c/em\u003e-butyllithium followed by the treatment with the corresponding aluminum trihalides.\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e,\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e These molecular structures were characterized with \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eH, \u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC{1H} and \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003eAl{\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eH} NMR spectroscopies, HRMS spectrometry, elemental analysis, and single crystal X-ray diffraction analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb\u0026ndash;e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003ePhotophysical properties at room temperature\u003c/h3\u003e\n\u003cp\u003eThere are two significant effects from the substituents on the photoluminescent properties (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea): (i) The dihaloaluminum complexes emit light in solid states at room temperature, while the dialkyl ones hardly show emission under the same condition; (ii) the emission color of the dihaloaluminum complexes depends on the type of halogen (\u003cb\u003eLAlCl\u003c/b\u003e, blue; \u003cb\u003eLAlBr\u003c/b\u003e, bluish white; \u003cb\u003eLAlI\u003c/b\u003e, green). Meanwhile, all compounds exhibited almost no luminescence in the solution states at room temperature in the similar manner as shown in the previous reports.\u003csup\u003e\u003cspan additionalcitationids=\"CR23\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e The previous studies demonstrated that the conventional \u003cem\u003eβ\u003c/em\u003e-diketiminate complexes have the CIE property with blue emission in crystalline states at room temperature when neither electron-accepting nor donating substituent was introduced on the ligands. Therefore, the non-emissive nature of the alkylaluminum complexes and the halogen-dependent emission color are the peculiar features among this class of luminophores.\u003c/p\u003e \u003cp\u003eWe conducted spectroscopic studies under nitrogen at room temperature to elucidate the electronic structures of the complexes (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb\u0026ndash;d and Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Firstly, UV\u0026ndash;vis absorption spectra were recorded in the 2-methylpentane (2MP)/toluene solutions (99/1, v/v, 1 \u0026times; 10\u003csup\u003e\u0026ndash;5\u003c/sup\u003e M) at room temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). All compounds showed similar absorption spectra with slight difference in the position of absorption maximum. The shapes and positions of the longest-wavelength absorption bands were similar to the typical \u003cem\u003eβ\u003c/em\u003e-diketiminate complexes and assignable mainly to the π\u0026ndash;π* (S\u003csub\u003e0\u003c/sub\u003e\u0026ndash;S\u003csub\u003e1\u003c/sub\u003e) transition of the ligand moiety, suggesting that the electronic character of the ground-state structure of the complexes should not be affected by the substituents on the aluminum atom. Nevertheless, the substituents apparently change the S\u003csub\u003e0\u003c/sub\u003e\u0026ndash;S\u003csub\u003e1\u003c/sub\u003e transition energy probably due to their different contribution to the frontier orbitals.\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e Secondly, photoluminescent (PL) spectra were measured for the same solutions and their crystalline powders (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and d). As we presumed, quite weak and broad emission spectra were obtained from all solutions. Their absolute quantum yields (\u003cem\u003eΦ\u003c/em\u003e\u003csub\u003ePL\u003c/sub\u003e) were lower than 0.01. In the crystalline state, \u003cb\u003eLAlMe\u003c/b\u003e and \u003cb\u003eLAlEt\u003c/b\u003e hardly exhibited emission enhancement (\u003cem\u003eΦ\u003c/em\u003e\u003csub\u003ePL\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.01), while \u003cb\u003eLAlCl\u003c/b\u003e, \u003cb\u003eLAlBr\u003c/b\u003e, and \u003cb\u003eLAlI\u003c/b\u003e showed significant emission spectra in the visible region. \u003cem\u003eΦ\u003c/em\u003e\u003csub\u003ePL\u003c/sub\u003e values were determined to be 0.48 for \u003cb\u003eLAlCl\u003c/b\u003e, 0.44 for \u003cb\u003eLAlBr\u003c/b\u003e, and 0.58 for \u003cb\u003eLAlI\u003c/b\u003e. Importantly, the PL spectra of \u003cb\u003eLAlBr\u003c/b\u003e and \u003cb\u003eLAlI\u003c/b\u003e composed two distinct bands in the blue and green regions. PL lifetime (\u003cem\u003eτ\u003c/em\u003e\u003csub\u003ePL\u003c/sub\u003e) measurements revealed that the shorter-wavelength bands possessed nanosecond-order \u003cem\u003eτ\u003c/em\u003e\u003csub\u003ePL\u003c/sub\u003e, while the longer-wavelength ones had microsecond-order \u003cem\u003eτ\u003c/em\u003e\u003csub\u003ePL\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg and h, Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Finally, their PL spectra recorded after several milliseconds after photoexcitation showed only longer-wavelength component (Figure S8). Consequently, the blue- and green-emission bands were assignable to fluorescence and phosphorescence, respectively. These data mean that the heavy atom effect of bromine and iodine should lead to the RTP properties of \u003cb\u003eLAlBr\u003c/b\u003e and \u003cb\u003eLAlI\u003c/b\u003e. To the best of our knowledge, the estimated phosphorescence quantum yield (\u003cem\u003eΦ\u003c/em\u003e\u003csub\u003eP\u003c/sub\u003e) value of \u003cb\u003eLAlI\u003c/b\u003e (0.54) is the highest one among the aluminum complexes. It is also of interest to note that the related boron complexes with iodine on the peripheral aromatic rings hardly show apparent RTP.\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e The heavy atoms directly attached on the central element might efficiently accelerate intersystem crossing and phosphorescence processes.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eResults of photophysical measurements at room temperature\u003csup\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eλ\u003c/em\u003e\u003csub\u003eabs\u003c/sub\u003e / nm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eε\u003c/em\u003e\u003c/p\u003e \u003cp\u003e/ 10\u003csup\u003e4\u003c/sup\u003e M\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eλ\u003c/em\u003e\u003csub\u003eFluo\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e/ nm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003eλ\u003c/em\u003e\u003csub\u003ePhos\u003c/sub\u003e\u003c/p\u003e \u003cp\u003e/ nm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cem\u003eΦ\u003c/em\u003e\u003csub\u003eF\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u003cem\u003eΦ\u003c/em\u003e\u003csub\u003eP\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eLAlMe\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003esolution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e403\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e460\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003en.d.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ecrystal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e473\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003en.d.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eLAlEt\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003esolution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e410\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e458\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003en.d.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ecrystal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e480\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003en.d.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eLAlCl\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003esolution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e386\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e436\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003en.d.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ecrystal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e442\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003en.d.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eLAlBr\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003esolution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e385\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e441\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003en.d.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ecrystal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e447\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e511\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.19\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eLAlI\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003esolution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e384\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e450\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e528\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ecrystal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e454\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e515\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.54\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003csup\u003e \u003cem\u003ea\u003c/em\u003e \u003c/sup\u003ePhotoluminescence properties were recorded with photoexcitation at the absorption maximum wavelength in solution state at room temperature. Solution, 1 \u0026times; 10\u003csup\u003e\u0026ndash;5\u003c/sup\u003e M in 2-methylpentane/toluene (99/1, v/v); crystal, recrystallized from hexane; \u0026ndash;, not determined. Phosphorescence spectra were recorded with pulsed excitation. Quantum yields of fluorescence and phosphorescence were estimated by absolute quantum yields and deconvoluted photoluminescence spectra with multi-component Gaussian functions.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePL lifetime and estimated rate constants at room temperature\u003csup\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003e\u0026lt;τ\u003c/em\u003e\u003csub\u003eF\u003c/sub\u003e\u0026gt;\u003c/p\u003e \u003cp\u003e/ ns\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003e\u0026lt;τ\u003c/em\u003e\u003csub\u003eP\u003c/sub\u003e\u0026gt;\u003c/p\u003e \u003cp\u003e/ ms\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003ek\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e/ 10\u003csup\u003e7\u003c/sup\u003e s\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003ek\u003c/em\u003e\u003csub\u003enr\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e/ 10\u003csup\u003e7\u003c/sup\u003e s\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cem\u003ek\u003c/em\u003e\u003csub\u003eISC\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e/ 10\u003csup\u003e7\u003c/sup\u003e s\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u003cem\u003ek\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e\u003csup\u003eT\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e/ 10\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u003cem\u003ek\u003c/em\u003e\u003csub\u003enr\u003c/sub\u003e\u003csup\u003eT\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e/ 10\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eLAlCl\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003esolution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e3.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ecrystal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e6.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eLAlBr\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003esolution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2500\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ecrystal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e1.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eLAlI\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003esolution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e250\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ecrystal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e680\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003csup\u003e \u003cem\u003ea\u003c/em\u003e \u003c/sup\u003e\u0026lt;\u003cem\u003eτ\u003c/em\u003e\u003csub\u003eF\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;and\u0026thinsp;\u0026lt;\u0026thinsp;\u003cem\u003eτ\u003c/em\u003e\u003csub\u003eP\u003c/sub\u003e\u0026gt;, average fluorescence and phosphorescence lifetimes, respectively; \u003cem\u003ek\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e, radiative decay rate constant from singlet state (fluorescence); \u003cem\u003ek\u003c/em\u003e\u003csub\u003enr\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e, nonradiative decay rate constant from singlet state (internal conversion); \u003cem\u003ek\u003c/em\u003e\u003csub\u003eISC\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e, intersystem crossing rate constant from singlet state; \u003cem\u003ek\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e\u003csup\u003eT\u003c/sup\u003e, radiative decay rate constant from triplet state (phosphorescence); \u003cem\u003ek\u003c/em\u003e\u003csub\u003enr\u003c/sub\u003e\u003csup\u003eT\u003c/sup\u003e, nonradiative decay rate constant from triplet state.\u003c/p\u003e \u003cp\u003eFrom the fluorescence and phosphorescence quantum yields and PL lifetime measurements with the dihaloaluminum complexes at room temperature, rate constants of each photophysical processes were estimated (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, see the Supplementary Information). For all three complexes in the solution states, quite large nonradiative decay rate constants for singlet states (\u003cem\u003ek\u003c/em\u003e\u003csub\u003enr\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e ~ 10\u003csup\u003e10\u003c/sup\u003e s\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e) were obtained, suggesting that most excited molecules would be quenched nonradiatively through the internal conversion from S\u003csub\u003e1\u003c/sub\u003e to S\u003csub\u003e0\u003c/sub\u003e probably because this process could occur through conical intersections.\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e Only in the case of \u003cb\u003eLAlI\u003c/b\u003e, the rapid intersystem crossing process derived from the strong heavy atom effect of iodine could occur to some extent (\u003cem\u003ek\u003c/em\u003e\u003csub\u003eISC\u003c/sub\u003e/\u003cem\u003ek\u003c/em\u003e\u003csub\u003enr\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e ~ 0.1). On the other hand, the crystals of these complexes exhibited at least 100 times smaller \u003cem\u003ek\u003c/em\u003e\u003csub\u003enr\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e values than their solutions, leading to their CIE properties. In the cases of \u003cb\u003eLAlBr\u003c/b\u003e and \u003cb\u003eLAlI\u003c/b\u003e, the suppression of the internal conversion should open the intersystem crossing processes as well as fluorescence. It is worth noting that the radiative rate constant from singlet states (\u003cem\u003ek\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e) and \u003cem\u003ek\u003c/em\u003e\u003csub\u003eISC\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e of some complexes were enhanced by the crystallization, which might originate from the intermolecular interactions and could contribute to their CIE properties.\u003c/p\u003e\n\u003ch3\u003ePhotophysical properties at 77 K\u003c/h3\u003e\n\u003cp\u003eTo gain further information about the photophysical processes, we recorded PL spectra of the solutions and crystalline powders at 77 K with a cryostat under nitrogen atmosphere (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee and Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Importantly, all compounds clearly exhibited phosphorescence in the frozen solution state, probably because the nonradiative decay processes could be closed at the low temperature. Indeed, the hypsochromic shifts of the emission band were observed, indicating that structural changes in the excited state should be hampered under the frozen environment. In other words, it is suggested that there are significant structural relaxations in the excited state that cause nonradiative decay of the singlet excited states in room-temperature solutions. Interestingly, \u003cb\u003eLAlMe\u003c/b\u003e and \u003cb\u003eLAlCl\u003c/b\u003e exhibited phosphorescence at 77 K, despite the absence of heavy atoms, implying the intrinsic triplet-forming properties of these series of compounds.\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e In the crystalline states at 77 K, the slight hypsochromic shifts of emission bands were observed except for \u003cb\u003eLAlMe\u003c/b\u003e. These shifts might originate from the tight packing of the crystals. On the other hand, the bathochromic shift for the crystal of \u003cb\u003eLAlMe\u003c/b\u003e might be attributed to the weakening of the 0\u0026ndash;0 band. Significantly, the apparent crystallization-induced phosphorescence enhancement was observed from \u003cb\u003eLAlI\u003c/b\u003e. The estimated \u003cem\u003eΦ\u003c/em\u003e\u003csub\u003ePhos\u003c/sub\u003e in crystal was 2.2 times higher than that in the frozen solution, possibly because of the acceleration of intersystem crossing and phosphorescence processes and because the restriction of nonradiative decay from excited triplet states.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eResults of photoluminescence measurements at 77 K\u003csup\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eλ\u003c/em\u003e\u003csub\u003eFluo\u003c/sub\u003e / nm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eλ\u003c/em\u003e\u003csub\u003ePhos\u003c/sub\u003e / nm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eΦ\u003c/em\u003e\u003csub\u003eFluo\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003eΦ\u003c/em\u003e\u003csub\u003ePhos\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eLAlMe\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003esolution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e445\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e541\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ecrystal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e474\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e569\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003en.d.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eLAlEt\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003esolution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e429\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e530\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003en.d.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003en.d.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ecrystal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e429\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003en.d.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003en.d.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003en.d.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eLAlCl\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003esolution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e428\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e510\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ecrystal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e422\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e535\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eLAlBr\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003esolution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e435\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e514\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.35\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ecrystal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e422\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e506\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.26\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e\u003cb\u003eLAlI\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003esolution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e426\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e515\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.33\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ecrystal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e425\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e507\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.74\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003csup\u003e \u003cem\u003ea\u003c/em\u003e \u003c/sup\u003eExcited at the absorption maximum wavelength in solution state at room temperature. Solution, 1 \u0026times; 10\u003csup\u003e\u0026ndash;5\u003c/sup\u003e M in 2-methylpentane/toluene (99/1, v/v); crystal, recrystallized from hexane; n.d., not determined due to negligible phosphorescence. Phosphorescence spectra were recorded with pulsed excitation. Quantum yields of fluorescence and phosphorescence were estimated by absolute quantum yields and deconvoluted photoluminescence spectra with multi-component Gaussian functions.\u003c/p\u003e\n\u003ch3\u003eTheoretical calculations\u003c/h3\u003e\n\u003cp\u003eDensity functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed with the Gaussian 16 package\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e to study the electronic structures of the dimethylaluminum and dihaloaluminum complexes (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Geometry optimization was performed for both S\u003csub\u003e0\u003c/sub\u003e and S\u003csub\u003e1\u003c/sub\u003e states with the CAM-B3LYP functional and the Lanl2DZ basis set for I and the 6-31G(d,p) one for the other atoms, followed by single-point transition energy calculations at the same level of theory except for the basis set for the light atoms (6-311\u0026thinsp;+\u0026thinsp;+\u0026thinsp;G(d,p)). The significant structural relaxation at the S\u003csub\u003e1\u003c/sub\u003e state was estimated only for \u003cb\u003eLAlMe\u003c/b\u003e, leading to the narrow S\u003csub\u003e0\u003c/sub\u003e\u0026ndash;S\u003csub\u003e1\u003c/sub\u003e energy gap (1.58 eV) and the small oscillator strength (\u003cem\u003ef\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.0080). The most characteristic change in the relaxation was the elongation of one of the Al\u0026ndash;C bonds from 1.98 \u0026Aring; (S\u003csub\u003e0\u003c/sub\u003e) to 2.30 \u0026Aring; (S\u003csub\u003e1\u003c/sub\u003e). At the S\u003csub\u003e0\u003c/sub\u003e geometry, its Kohn\u0026ndash;Sham highest occupied molecular orbital (HOMO) was significantly delocalized over the Al\u0026ndash;C bond as well as the ligand moiety, while the Kohn\u0026ndash;Sham lowest unoccupied molecular orbital (LUMO) possessed little contribution from this bond because of a nodal plane passing through it. Hence, the electron density between aluminum and carbon atoms should decrease upon the photoexcitation from S\u003csub\u003e0\u003c/sub\u003e to S\u003csub\u003e1\u003c/sub\u003e, resulting in the weakening of the bond and the considerably large structural relaxation at the S\u003csub\u003e1\u003c/sub\u003e state. Similar photoinduced bond weakening or activation of Al\u0026ndash;C bond have been reported in other systems of photochemical reactions.\u003csup\u003e48\u0026ndash;51\u003c/sup\u003e For the resulted S\u003csub\u003e1\u003c/sub\u003e geometry, the HOMO mainly located at the Al\u0026ndash;C moiety, where the HOMO\u0026ndash;LUMO overlap much less effectively than that for the S\u003csub\u003e0\u003c/sub\u003e geometry, making the \u003cem\u003ef\u003c/em\u003e value of the S\u003csub\u003e1\u003c/sub\u003e\u0026ndash;S\u003csub\u003e0\u003c/sub\u003e transition smaller. Therefore, it is suggested that the structural relaxation should be responsible for the nonradiative decay process of the dialkylaluminum complexes.\u003c/p\u003e \u003cp\u003eOn the other hand, the dihaloaluminum complexes presented only smaller structural changes between the S\u003csub\u003e0\u003c/sub\u003e and S\u003csub\u003e1\u003c/sub\u003e states, probably because the contribution from the Al\u0026ndash;halogen bond to their HOMO would be smaller. Natural bond orbital (NBO) analysis suggested that the orbital on the Al\u0026ndash;C bond should be mainly composed of the NBO attributed to the 2p orbital of the carbon atom, which is located at the similar energy region with the HOMO of the \u003cem\u003eβ\u003c/em\u003e-diketiminate ligand. As the corresponding p orbitals of chlorine, bromine, and iodine should be located at the much lower energy region because of their large electronegativity than carbon, the Al\u0026ndash;halogen bond would not strongly contribute to the HOMO of the dihaloaluminum complexes due to the weaker orbital interactions. As a result, the undesired structural relaxation causing the non-radiative quenching could hardly occur in the S\u003csub\u003e1\u003c/sub\u003e state and the S\u003csub\u003e1\u003c/sub\u003e\u0026ndash;S\u003csub\u003e0\u003c/sub\u003e electronic transition would be no longer forbidden at its S\u003csub\u003e1\u003c/sub\u003e geometry (\u003cem\u003ee.g.\u003c/em\u003e, \u003cem\u003ef\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.5070 for \u003cb\u003eLAlCl\u003c/b\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). Consequently, it is suggested that the photophysical processes of \u003cem\u003eβ\u003c/em\u003e-diketiminate complexes could be drastically modulated by the substituents on the central element. In addition, it is worth noting that the electron-donating contribution from the Al\u0026ndash;C bond destabilizes the HOMO level compared to the dihaloaluminum complexes, leading to the lower S\u003csub\u003e1\u003c/sub\u003e state consistent with the observed redshift of the absorption band. Indeed, \u003cb\u003eLAlEt\u003c/b\u003e, with the more strongly electron-donating ethyl group, showed the absorption band in the lowest energy region among the complexes.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWe also calculated excited triplet state (T\u003csub\u003en\u003c/sub\u003e) energy and spin\u0026ndash;orbit coupling (SOC) constants between S\u003csub\u003em\u003c/sub\u003e and T\u003csub\u003en\u003c/sub\u003e states, \u003cem\u003eξ\u003c/em\u003e(S\u003csub\u003em\u003c/sub\u003e\u0026ndash;T\u003csub\u003en\u003c/sub\u003e), for the dihaloaluminum complexes with the Q-Chem 5 package\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e to get deeper insights into their phosphorescent properties (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The S\u003csub\u003e1\u003c/sub\u003e and T\u003csub\u003e1\u003c/sub\u003e states of each complex were dominantly characterized by the locally excited (LE) state within the central N\u003csub\u003e2\u003c/sub\u003eC\u003csub\u003e3\u003c/sub\u003e moiety. As the S\u003csub\u003e1\u003c/sub\u003e\u0026ndash;T\u003csub\u003e1\u003c/sub\u003e energy gap was calculated to be about 1.0 eV or larger, the ISC from S\u003csub\u003e1\u003c/sub\u003e to T\u003csub\u003e1\u003c/sub\u003e seemed to be less efficient. On the other hand, the T\u003csub\u003en\u003c/sub\u003e (n\u0026thinsp;=\u0026thinsp;2\u0026ndash;4) states were located in the similar energy region of the S\u003csub\u003e1\u003c/sub\u003e state (\u0026plusmn;\u0026thinsp;100 meV). In addition, the SOC values were estimated to be large enough to accept the efficient ISC between S\u003csub\u003e1\u003c/sub\u003e and T\u003csub\u003en\u003c/sub\u003e. For \u003cb\u003eLAlCl\u003c/b\u003e and \u003cb\u003eLAlBr\u003c/b\u003e, these large SOC values are attributable to the charge transfer (CT) character of these T\u003csub\u003en\u003c/sub\u003e states with twisted conformations between the donor (aromatic rings) and acceptor (N\u003csub\u003e2\u003c/sub\u003eC\u003csub\u003e3\u003c/sub\u003e unit) as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb. As the transitions between the S\u003csub\u003e1\u003c/sub\u003e(LE) to the T\u003csub\u003en\u003c/sub\u003e(CT) occurs with the large change in orbital angular momentum derived from the twisted conformations, the electron-spin flipping is allowed with holding the angular momentum conservation.\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e On the other hand, the T\u003csub\u003e2\u003c/sub\u003e and T\u003csub\u003e3\u003c/sub\u003e states of \u003cb\u003eLAlI\u003c/b\u003e significantly consist of the transition from the nonbonding orbitals (lone pairs) of the iodine atoms (HOMO\u0026ndash;1 and HOMO\u0026ndash;2) to its LUMO. Consequently, the heavy-atom effect of iodine could efficiently accelerate the ISC between S\u003csub\u003e1\u003c/sub\u003e to T\u003csub\u003e2\u003c/sub\u003e and T\u003csub\u003e3\u003c/sub\u003e. Importantly, it was suggested that the SOC constants not only between S\u003csub\u003e1\u003c/sub\u003e and T\u003csub\u003en\u003c/sub\u003e but also between S\u003csub\u003e0\u003c/sub\u003e and T\u003csub\u003e1\u003c/sub\u003e significantly increased as the atomic number of the halogen atoms become larger because of the heavy atom effect. Therefore, both of ISC and phosphorescence processes should be enhanced in \u003cb\u003eLAlBr\u003c/b\u003e and \u003cb\u003eLAlI\u003c/b\u003e compared to \u003cb\u003eLAlCl\u003c/b\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eCIE-active four-coordinated \u003cem\u003eβ\u003c/em\u003e-diketiminate aluminum(III) complexes, \u003cb\u003eLAlCl\u003c/b\u003e, \u003cb\u003eLAlBr\u003c/b\u003e, and \u003cb\u003eLAlI\u003c/b\u003e were discovered. From the spectroscopic measurements and the theoretical calculations, it was strongly suggested that the 2p orbital of the carbon atom in \u003cb\u003eLAlMe\u003c/b\u003e significantly contributed to its HOMO and induced the undesirable structural relaxation in the S\u003csub\u003e1\u003c/sub\u003e state. As a result of the relaxation, internal conversion from S\u003csub\u003e1\u003c/sub\u003e to S\u003csub\u003e0\u003c/sub\u003e should occur even in the crystalline state at room temperature. The series of NBO analysis certainly proposed that more electronegative halogen substituents would not disturb the HOMO distribution of the \u003cem\u003eβ\u003c/em\u003e-diketiminate moiety, leading to suppression of the non-radiative quenching paths. Thus, the dihaloaluminum complexes exhibited efficient photoluminescence in the crystalline state at room temperature. In particular, \u003cb\u003eLAlI\u003c/b\u003e exhibited room-temperature phosphorescence with 0.54 of the phosphorescence quantum yield as a result of the efficient heavy atom effect of iodine on the central element. Our strategy for constructing desired luminophores by modifying the substituents on the central element could be applicable not only for achieving further luminescent metal complexes but also for obtaining optoelectronic materials, reagents and catalysts.\u003c/p\u003e "},{"header":"Declarations","content":"\u003ch2\u003eData Availability\u003c/h2\u003e\n\u003cp\u003eThe data that support the findings of this study are available in the supplementary material of this article. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers 1880035 for \u003cstrong\u003eLAlCl\u003c/strong\u003e, 1880036 for \u003cstrong\u003eLAlMe\u003c/strong\u003e, 2364980 for \u003cstrong\u003eLAlBr\u003c/strong\u003e, and 2364981 for \u003cstrong\u003eLAlI\u003c/strong\u003e. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.\u0026nbsp;\u003c/p\u003e\n\u003ch2\u003eAcknowledgements\u003c/h2\u003e\n\u003cp\u003eComputation time was provided by the SuperComputer System, Institute for Chemical Research, Kyoto University. We thank Y. Okabayashi (AIST) for his support on time-resolved PL measurements. This work was partially supported by The Asahi Glass Foundation and a Grant-in-Aid for Early-Career Scientists (for S.I., JSPS KAKENHI Grant Numbers 21K14673 and 23K13793) and for Scientific Research (B) (for K.T., JSPS KAKENHI Grant Number, 24K01570).\u0026nbsp;\u003c/p\u003e\n\u003ch2\u003eCompeting Interests\u003c/h2\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eTang, C. W. \u0026amp; VanSlyke, S. A. Organic electroluminescent diodes. Appl. Phys. Lett. 51, 913\u0026ndash;915 (1987).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang, S. Luminescence and electroluminescence of Al(III), B(III), Be(II) and Zn(II) complexes with nitrogen donors. Coord. Chem. Rev. 215, 79\u0026ndash;98 (2001).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhao, S.-B. \u0026amp; Wang, S. Luminescence and reactivity of 7-azaindole derivatives and complexes. Chem. 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Res. 55, 1573\u0026ndash;1585 (2022).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"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":"","lastPublishedDoi":"10.21203/rs.3.rs-4633219/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4633219/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this manuscript, synthesis and optical properties, such as crystallization-induced emission (CIE) and room-temperature phosphorescence (RTP), and the substituent effect on the central element are reported based on \u003cem\u003eβ\u003c/em\u003e-diketiminate aluminum complexes. Although luminescent aluminum compounds have been utilized for emitting and electron transporting layers in organic light-emitting diodes, most of them often exhibit not phosphorescence but fluorescence with low photoluminescent quantum yields in the aggregated state than those in the amorphous state due to concentration quenching. In this study, the π-conjugated \u003cem\u003eβ\u003c/em\u003e-diketiminate ligand was employed for constructing four-coordinated complexes with dialkyl- or dihaloaluminum moieties, and the dihaloaluminum complexes were found to exhibit the CIE property. Moreover, we found that the diiodoaluminum complex provided RTP, while the dialkylaluminum complexes hardly showed emission at room temperature. From theoretical calculations, it was suggested that undesired structural relaxation in the singlet excited state of dialkyl complexes should be suppressed by introducing electronegative halogens instead of alkyl groups. Our findings might be useful for establishing a new molecular design not only for obtaining luminescent complexes but also for achieving triplet-harvesting materials.\u003c/p\u003e","manuscriptTitle":"Regulating Photoluminescence of Aluminum Complexes by Substituents on Metal: From Non-luminescence to Room-Temperature Phosphorescence","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-23 12:02:08","doi":"10.21203/rs.3.rs-4633219/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"communications-chemistry","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"commschem","sideBox":"Learn more about [Communications Chemistry](http://www.nature.com/commschem/)","snPcode":"","submissionUrl":"","title":"Communications Chemistry","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Communications Series","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"eaebcd9d-8fed-4bb7-a43f-bad0a9c955a3","owner":[],"postedDate":"July 23rd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":33933084,"name":"Physical sciences/Chemistry/Physical chemistry/Excited states"},{"id":33933085,"name":"Physical sciences/Chemistry/Inorganic chemistry/Organometallic chemistry/Ligands"}],"tags":[],"updatedAt":"2024-09-10T07:31:06+00:00","versionOfRecord":{"articleIdentity":"rs-4633219","link":"https://doi.org/10.1038/s42004-024-01295-z","journal":{"identity":"communications-chemistry","isVorOnly":false,"title":"Communications Chemistry"},"publishedOn":"2024-09-09 04:00:00","publishedOnDateReadable":"September 9th, 2024"},"versionCreatedAt":"2024-07-23 12:02:08","video":"","vorDoi":"10.1038/s42004-024-01295-z","vorDoiUrl":"https://doi.org/10.1038/s42004-024-01295-z","workflowStages":[]},"version":"v1","identity":"rs-4633219","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4633219","identity":"rs-4633219","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}