Coordination Compounds with Photochromic Ligands: Ready

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Coordination Compounds with Photochromic Ligands: Ready Tunability and Visible Light-Sensitized Photochromism Chi-Chiu Ko†,‡ and Vivian Wing-Wah Yam*,† †

Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee (Hong Kong)) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Pokfulam, Hong Kong, P. R. China ‡ Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, P. R. China CONSPECTUS: Photochromic compounds are well-known for their promising applications in many areas. In this context, many different photochromic families have been developed. As the early study of these photochromic compounds was mainly focused on the organic system, their photochromic reactivity was mainly derived from the singlet excited state. We hypothesized that the incorporation of the photochromic ligand to the transition metal complex and coordination complex systems would not only render the triplet state of the organic photochromic system more readily accessible due to the large spin− orbit coupling of the heavy metal center but also would lead to ready extension of the excitation wavelength to less destructive longer wavelength low-energy excitation. On the other hand, the long-lived triplet excited states of the metal complexes are also suitable for energy or electron transfer processes, which should lead to new photochromic behavior and photoswitchable functional properties. Through the incorporation of the stilbene-, azo-, spirooxazine-, and dithienylethene-containing ligands to transition metal complex systems with heavy metal centers and suitable excited states, triplet state photosensitized photochromism has been achieved. With the triplet state photosensitization, the photochromism of these compounds could be extended from the high energy UV region to the visible region. In the development of dithienylethene-containing ligands, we have adopted an alternative strategy, which involves the incorporation of nitrogen and sulfur heterocycles that directly form part of the dithienylethene framework as ligands to exert a much stronger perturbation and influence on the excited state properties of the photochromic unit by the metal center. On the basis of the new design, wide ranges of dithienylethene-containing ligands, including phenanthrolines, 2-pyridylimidazoles, N-pyridylimidazol-2-ylidenes, cyclometalating thienylpyridines, β-diketonates, and βketoiminates have been designed and incorporated into various coordination systems. Apart from the photosensitization, tuning of the closed form absorption and photochromic behavior based on the perturbation of the metal center, coordination-assisted planarization, modification of the ancillary ligands and introduction of various electronic excited states derived from the coordination system have been successfully demonstrated. This strategy can be used for developing NIR photochromic dithienylethenes. With the above effects observed upon the coordination to different transition metal centers and central atoms, this strategy offers a simple and effective way for the modification of the photochromic characteristics. Moreover, the emission and other functional properties of the coordination systems could also be photoswitched by the photochromic reactions.

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INTRODUCTION

1970, but they only became popular in the 1980s when their excellent reversibility (fatigue-resistance) was reported.2−4,7 The colorless closed form of spirooxazines is thermally stable and would undergo reversible photoinduced ring-opening reaction from the cleavage of the spiro-C−O bond to give a planar and extensively conjugated colored open form (photomerocyanine) (Scheme 1b).2−4,7 Diarylethenes represent one of the most popular and extensively studied photochromic families.2−4,8 Their photochromism has been derived from the reversible photoinduced cyclization and cycloreversion reactions (Scheme 1c). These reactivities were first discovered as photochemical side-reactions in the study of photoinduced cis−

Photochromic compounds can undergo reversible color changes controlled by photochemical reactions. These compounds have found many important applications such as in optical materials, memories, and photoswitches.1−3 Therefore, a number of organic photochromic families such as stilbenes, azobenzenes, spiropyrans, spirooxazines, and diarylethenes have been developed.2−4 The long history of the wellknown reversible photoinduced trans−cis isomerizations of C C and NN double bonds has led to the families of stilbenes and azobenzenes.2−6 The different absorption properties between the cis- and trans-isomers can be ascribed to their differences in molecular planarity and thus the extent of πconjugation (Scheme 1a).2−6 Spirooxazines, which are structurally related to spiropyrans, were first reported in © XXXX American Chemical Society

Received: August 31, 2017

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DOI: 10.1021/acs.accounts.7b00426 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

moieties, near-infrared, gated, and multicolor photochromism as well as materials with photoswitchable functional properties have been demonstrated.8,12−19 Despite the excellent performance and many successful applications of these photochromic families, studies were mainly confined to that of the organic system in their early development.3 Systematic investigations on the design of photochromic ligands and their transition metal complexes have not been reported until recently.20−24 In this Account, we have summarized our recent works on the design of photochromic ligands, the study of the effect of the coordination systems on the photochromic reactivity, and their applications in the development of photoswitchable materials.

Scheme 1. Photochromism of (a) Stilbenes and Azobenzenes, (b) Spiropyrans and Spirooxazines, and (c) Dithienylethenes

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PHOTOCHROMISM BY TRIPLET EXCITED STATE PHOTOSENSITIZATION Owing to the spin-forbidden nature, the intersystem crossing between the singlet state and the triplet state is very inefficient in organic systems. Therefore, the photochromic reactions in organic systems mainly occur from the lowest-lying singlet excited state. Using organic triplet photosensitizers, intermolecular sensitization of the photoinduced trans−cis isomerizations of stilbenes and ring-opening reactions of spirooxazines have been reported to occur from their triplet excited states.7,25−27 With the phosphorescent transition metal complex systems as the triplet donor, the triplet pathway can be further enhanced by spin−orbit coupling.28,29 However, the quantum efficiency is rather limited as the bimolecular triplet−triplet energy transfer between the photosensitizer and the photochromic compounds is limited by diffusion. By incorporating the photochromic moieties into the photosensitizer, the efficiency of the energy transfer process can be improved because it becomes an intramolecular process. To achieve the intramolecular triplet state photosensitized photochromism, we have designed a series of stilbene- and azocontaining pyridine ligands (Figure 1a,b) and coordinated these ligands to the tricarbonylrhenium(I) bipyridine complex system,30,31 which is well-known to possess a triplet metal-toligand charge transfer (3MLCT) excited state.32 Upon coordination to the rhenium(I) complexes, photoinduced trans-to-cis isomerization can be probed by excitation into the MLCT transition. A triplet photoisomerization mechanism, which involves an intramolecular energy transfer from 3MLCT to triplet intraligand (3IL) excited state of the trans-isomer prior to the isomerization to the cis-form, was suggested. The proposed energy transfer process from 3MLCT to 3IL has also been supported by the very weak 3MLCT phosphorescence observed in the trans-isomer. After conversion to the cis-isomer,

trans isomerizations of stilbenes.5,6 With a cyclic ethene backbone to restrict the cis−trans isomerization, the pericyclic ring-closing and ring-opening reactions become the major reversible photoreactions occurring in diarylethenes. Due to the extended π-conjugation in the ring-closed forms, they usually show a broad absorption in the visible region and hence are highly colored. As the ring-opening and ring-closing reactions do not induce significant geometrical structural variations, diarylethenes are also well-known for their photochromism in the crystalline state.8,9 Pioneering work by Irie and co-workers has developed design strategies for the photochromic diarylethenes with excellent reversibility (fatigue-resistance) and thermal stability.8,10,11 Through experimental and theoretical investigations of the photochromic reactivity of a series of diarylethenes with different types of aryl groups, the correlation between the aromatic stabilization energy of the two aryl groups and the thermal stability of the photocyclized form has been established.10 Moreover, when perfluorocyclopentene is used as the cyclic ethene backbone, the diarylethenes generally show outstanding photochromic performance in terms of quantum yield, fatigue resistance and stability.8 Subsequently, a large number of thermally irreversible bistable photochromic diarylperfluorocyclopentenes with heteroaryl groups of low aromaticity, particularly thienyl groups, have been designed and extensively studied.8 Owing to the popularity of this subclass, the term “dithienylethenes” is also widely used to describe the bistable photochromic diarylethenes with two thienyl groups. Through the functionalization of the thiophene groups with a wide variety of substituents and functional

Figure 1. (a, b) Schematic drawing of stilbene- and azo-containing pyridine ligands. (c) Emission spectral traces of [Re(CO)3(phen)(NSP)]+ upon irradiation at λ = 330 nm in degassed CH2Cl2. Reproduced with permission from ref 31. Copyright 1998 The Royal Society of Chemistry. B


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Accounts of Chemical Research which has a higher-lying 3IL state due to less extensive πconjugation, the energy transfer from the 3MLCT state to the 3 IL state was blocked and the typically observed phosphorescence of the rhenium(I) system was restored (Figure 1c). As a result, MLCT phosphorescence of these complexes can be photomodulated with the trans−cis isomerization.31 Apart from the design of azo- and stilbene-containing ligands, a spirooxazine-containing pyridine ligand (SOPY) (Chart 1) Chart 1. Structure of SOPY

was also designed and incorporated into the rhenium(I) tricarbonyl diimine system, [Re(CO)3(N∧N)(SOPY)]+ (N∧N = tBu2bpy, Me2bpy, phen).33 The growth of the characteristic absorption of the open form peaking at 604 nm upon excitation into the MLCT transition of the complexes suggested that the photosensitization of the photochromic ring-opening reaction could also be achieved with the MLCT excited state of the rhenium(I) complexes (Scheme 2). Similarly, an intramolecular energy transfer from the 3MLCT state to the 3IL state, which undergoes the ring-opening reaction, was suggested in the MLCT photosensitized ring-opening reaction. The intramolecular energy transfer from the 3MLCT state to the 3IL state was evidenced by the fact that all the complexes display diimine-ligand insensitive SOPY LC phosphorescence instead of the typical MLCT phosphorescence (Figure 2). With the MLCT excitation, the photochromic ring-opening reaction of the spirooxazine in the complexes can be achieved with visible light excitation at ca. 420 nm instead of UV light, typically required for spirooxazines. To achieve photosensitization, it is important to consider the energy of the triplet state of the photosensitizer.20 For the rhenium(I) complexes with another series of spirooxazinecontaining bipyridine ligands (Chart 2),34 which have the 3 MLCT excited state (166−188 kJ mol−1 estimated based on the MLCT phosphorescence) lower-lying than those of the spirooxazine (210−255 kJ mol−1), quenching of the reactive excited state for ring-opening reaction by the 3MLCT excited state instead of photosensitization was observed.34 In these complexes, they displayed the typical MLCT phosphorescence, and the quantum yields of the photochromic ring-opening reaction decreased with the energies of the MLCT excited state. On the other hand, as the 3IL states of the photomerocyanines are much lower-lying in energy, the emission of the complexes

Figure 2. Emission spectra of (a) free ligand, SOPY, and its rhenium(I) complexes, [Re(CO)3(N∧N)(SOPY)]+ [N∧N = (b) phen, (c) Me2bpy, or (d) tBu2bpy] in degassed CH2Cl2 solution at room temperature. The asterisks denote artifacts, which are due to laser scattering and its harmonics. Reproduced with permission from ref 33. Copyright 2000 American Chemical Society.

Chart 2. Structures of Spirooxazine-Containing Bipyridine Ligands

could be reversibly switched from MLCT phosphorescence in the closed form to IL phosphorescence in the photomerocyanine form by the photochromic reactions (Figure 3). On the contrary, by coordinating one of these spirooxazinecontaining bipyridine ligands to the bis(alkynyl)platinum(II)

Scheme 2. Photosensitized Photochromism of [Re(CO)3(N∧N)(SOPY)]+ (N∧N = tBu2bpy, Me2bpy, or phen)

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lethenes has hindered the rotation of the thiophene units, which leads to very slow interconversion between the two welldocumented conformations in the open form, namely, the parallel and the antiparallel conformations. According to the Woodward−Hoffmann rule,38 only the antiparallel conformation is active in the photocyclization. By monitoring the photocyclization reactions of the photochromic dithienylphenanthroline with 1H NMR spectroscopy, it was confirmed that only the antiparallel conformation in the open form was converted to the closed form during the photoirradiation.37 On coordination to the tricarbonylrhenium(I) system, the complexes in their open form show both intense intraligand (IL) π−π* transitions in the high energy UV region and moderately intense MLCT [dπ(Re)→ π*(phen)] transition in the visible region with the absorption tail extended to ca. 480− 500 nm. The colorless solution of the free ligand gradually turned red, as characterized by the evolution and growth of new absorption peaking at 366, 510, and 540 nm, corresponding to the absorption of the closed form. For the rhenium(I) complex, the photocyclization, which can also be monitored spectrophotometrically by the closed-form absorption peaking at 390, 546, and 580 nm (Figure 5a), can occur upon excitation into

Figure 3. Normalized emission spectra of the closed forms () and the open forms (−−−) of (a) [Re(CO)3(SOBPY)Cl] and (b) [Re(CO)3(DSOB)Cl] in 77 K EtOH−MeOH−CH2Cl2 (4:1:1 v/v/v) glass. Reproduced with permission from ref 34. Copyright 2004 WileyVCH.

complex system (Figure 4a) with higher MLCT/LL′CT excited state, photosensitized ring-opening reaction of the spirooxazine

Figure 4. (a) Schematic diagram for [Pt(SOOB)(CCR2)2]. (b) Photochromic gel formed by the cholesteryl platinum(II) bipyridine complex. Reproduced with permission from ref 35. Copyright 2011 Wiley-VCH.

Figure 5. (a) Photochromism of the rhenium(I) complex with dithienylphenanthroline ligand. (b) UV−vis absorption spectral changes of the complex in degassed benzene (7.16 × 10−5 M) upon MLCT excitation at 440 nm. (c) Overlaid normalized corrected emission spectra of the open form () and the closed form (−−−) of the complex in EtOH−MeOH glass (4:1 v/v) at 77 K. Reproduced with permission from ref 36. Copyright 2004 American Chemical Society.

can be achieved.35 By introducing a cholesteryl functional group to the ancillary alkynyl ligands, the spirooxazinecontaining platinum(II) complex is capable of forming stable gels in various aliphatic organic solvents such as heptane, octane, decane, and dodecane at room temperature.35 In the metallogel state, photosensitized photochromic ring-opening reaction has been observed (Figure 4b). As an extension of our work on the development of photoswitching materials and the photosensitization of photochromic stilbenes, azobenzenes, and spirooxazines with the triplet excited state of transition metal complexes,30,31,33−35 efforts have been made to photosensitize dithienylethenecontaining ligands with transition metal complexes.36,37 Unlike most other earlier design of ligands with photochromic moieties, which were attached as pendants,20−22 our design involves the modification of the cyclic ethene backbone of the dithienylethene framework with a heteroaryl ligand, the phenanthroline ligand (phen).36,37 The incorporation of the 1,10-phenanthroline in the ethene backbone of the dithieny-

both the high-energy IL (π−π*) in the UV region and less destructive lower-energy MLCT transitions in the visible region (λ ≤ 480 nm). The slight red-shift of the closed form absorption in the complex is attributed to perturbation of the metal center. With the reversible photochromic reactions, the emission of the complex could also be photoswitched (Figure 5b). Photosensitization from the 3MLCT excited state was suggested for the photocyclization with the MLCT excitation. Ultrafast transient absorption and time-resolved emission spectroscopic study on the rhenium(I) complex was performed to characterize all the excited states involved from the MLCT excitation to the formation of the closed form (Figure 6). Recent study has revealed that the use of lower-energy visible light excitation through the triplet-state-photosensitized pathway in organic diarylethenes would minimize the formation of byproducts, leading to higher fatigue resistance.39 Apart from D


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Figure 6. (a) Proposed qualitative energy level diagram for the photosensitized photochromism by MLCT excitation. (b−d) Transient absorption spectra of rhenium(I) complex with dithienylphenanthroline ligand in MeCN obtained with 400 nm excitation at a series of pump−probe delays: (b) 1−50 ps; (c) 50 ps−1.5 ns; (d) 1.5−6 ns. The insets show absorption−time profiles at (b) 460 nm, (c) 420 nm, and (d) 386 nm. The lines are bestfit exponential curves to the data. The sharp features around 400 nm are due to artifacts from the pump pulses. Reproduced with permission from ref 37. Copyright 2006 Wiley-VCH.

complexes.43,44 In addition to the photoswitching of photophysical properties, the photochromic dithienylphenanthroline 36,37,40,42 has recently been incorporated into bis[dihydrobis(1-pyrazolyl)borato]iron(II) complex system to develop photoswitchable spin-crossover and magnetic materials.45,46 Extension of this work has been made to investigate the modification of the phenanthroline ligand with different types of bidentate N-heterocyclic ligands such as pyridylimidazoles.47−50 Depending on the substituent position of the 2pyridyl group on the imidazole ring, N,N-coordinating diimine [2-pyridylimidazoles (py-Im)]47,48 and N,C-coordinating pyridine-N-heterocyclic carbene [N-pyridylimidazol-2-ylidenes (py-NHC)]49,50 ligands have been developed. These ligands have been coordinated to transition metal complex systems with the lowest-lying excited states of different nature and energy (Chart 4). For the tricarbonylrhenium(I) and bis(alkynyl)platinum(II) complexes with the dithienylethene-

the extension of the wavelength of excitation for the photochromism and red-shifted absorption in the closed form, the quantum yield for the sensitized photocyclization of the complex was also improved compared to that of the free ligand. To tune the photochromism, it has been coordinated to various coordination complex systems, such as bis(alkynyl)platinum(II), 40 tris(bipyridyl)ruthenium(II), 41 and bis(thiolate)zinc(II) complexes42 (Chart 3). However, triplet Chart 3. Structures of Platinum(II), Ruthenium(II), and Zinc(II) Complexes with Dithienylphenanthroline Ligands

Chart 4. Structures of (a) Rhenium(I) and Platinum(II) Complexes with Dithienylethene-Containing py-Im Ligands and (b) Ruthenium(II) Complexes with DithienyletheneContaining py-NHC Ligands

state photosensitized photochromism was only observed in the platinum(II) and ruthenium(II) complexes but not in the zinc(II) complexes. Similar to the rhenium(I) complexes, the luminescence behavior of these complexes could be photoswitched by the photochromic reaction. The photoswitching of photoluminescence properties of the metal complexes based on the photochromic reactions of the diarylethene-containing ligands, which can serve as a nondestructive readout of the photochromic state when they are used as memory materials, have also been reported in the f−f emission of lanthanide E


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NHC catalysts with photoswitchable functions, activities, and regio- and enantioselectivities.51 The photoswitchable catalytic properties of dithienyl-substituted NHC ligands and their Ru(II) complexes for ammonia activation and olefin metathesis have been subsequently demonstrated.52,53 Triplet state photosensitized photochromism of other ligands containing dithienylperfluorocyclopentene, dithienylcyclopentene, or dithienylmaleimide pendants with various transition metal complexes have also been reported.54−59 The photosensitization processes have been demonstrated to show strong dependence on the linkage between the ligand and the photochromic dithienylethene pendants.55,56 By the control of energy transfer processes through the rational design of dithenylperfluorocyclopentene-containing ligands and metal complex systems, multistage photochromism has been achieved.57−59

containing py-Im ligands and tetracyanoruthenate(II) with dithienylethene-containing py-NHC ligands, photocyclization can be photosensitized by the corresponding MLCT excited states of the transition metal complex system. Moreover, their photocyclization reactions occur with improved quantum yields as compared to the corresponding free ligand upon excitation into their IL and MLCT transitions.47−49 In the case of η6mesitylene ruthenium(II) complexes with the metal-centered (MC) d−d excited state as the lowest-lying excited state, photocyclization of dithienylethene-containing py-NHC ligands via the triplet-state-sensitization pathway cannot be achieved. Instead, the reactive excited state for the cyclization is likely to be quenched by the lower-lying MC excited state, leading to a low quantum efficiency ( F) on the B(III) center. Such a strong perturbation was also observed in the boron(III) complexes with β-ketoiminate ligands (Chart 7).65 The absorptions of the open forms and closed forms of the βketoimine ligands are slightly blue-shifted with respect to the βdiketone analogues. A larger blue shift is observed for the boron(III) β-ketoiminate relative to the β-diketonate analogues. This has been ascribed to the more electron-rich nature of the nitrogen donor, which lowers the extent of the donor−acceptor ICT, leading to a higher-energy ICT band. The replacement of one of the oxygen atoms in β-diketonates with N−R′ in βketoiminates allows for structural modifications. Interestingly, the photocyclization and luminescence behavior of the phenyl substituted β-ketoiminate and its boron(III) compounds can be suppressed by the intramolecular rotation process of the Nphenyl moiety (Scheme 3). This explanation is supported by the significant enhancement of the emission upon aggregation formation, which restricts the rotation of the phenyl group, induced by the presence of the poor solvent (H2O) for the boron(III) compound. In addition, the replacement of the

phenyl substituent with the sterically bulky mesityl group could restore the photochromic activity and lead to enhancement of photoluminescence quantum yields. These findings represent an example of a unique strategy for modulating photochromic behavior through variation of the peripheral steric environment, that is the steric environment of the N-substituents of the ketoiminate and the ligand on the boron(III) center, without the need for applying the structural modification directly on the dithienylethene unit. Extension of the work on the utilization of photochromic βdiketonatoboranes to form multifunctional photochromic organogelators through the linking of the photochromic unit to organogelating moieties has also been made and demonstrated.66 Interestingly, the compounds are found to exhibit drastic color change even in gel state under photoirradiation.

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CONCLUSION REMARKS Ligands with different photochromic families including stilbene, azobenzene, spirooxazine, and dithienylethene have been designed and investigated. The incorporation of these ligands into different transition metal complex systems can serve as a simple and effective approaches for the modification and tuning of the photochromic characteristics. Through judicious choice of the transition metal complexes with heavy metal centers and suitable excited states, photosensitized photochromism via the triplet excited states can be achieved so that the photochromism can be extended from the UV region in the pure organic system to the lower-energy visible light region in their coordination compounds. For the dithienylethene-containing ligands, metal coordination could also extend the absorption of H


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Scheme 3. Schematic Representation Showing the Influence of the N-Substituents on the Photochromism of Boron(III) Ketoiminate Derivatives

Reproduced with permission from ref 65. Copyright 2016 Wiley-VCH

the closed form through metal perturbation, extended πconjugation due to conformational alteration, and introduction of new characteristic electronic transitions in the coordination systems. This has been demonstrated to be effective in developing NIR photochromic dithienylethenes. In addition, it also enables the tuning of the closed form absorption and photochromic behavior through the coordination of the dithienylethene-containing ligands to different metal centers as well as the modification of the ancillary ligands. As a result, the photochromic behavior of the dithienylethenes can be easily modified without the need for tedious preparative procedures. Apart from tuning of the photochromic behavior, the emission characteristics of the coordination systems could also be photoswitched by the photochromic reactions. By connecting the cyclic ethene backbone of the dithienylethenes to the boron(III) center, donor−acceptor charge transfer systems can be established. By perturbation of the boron(III) coordination system through the electronic properties as well as the steric environment of the ancillary ligands, tunable photochromic properties and switchable photochromism can be achieved. All in all, it has been demonstrated that visible light-driven photoresponsive systems can be achieved through the incorporation of metal−ligand chromophores. Readily tunable photochromism has been realized by straightforward metal coordination without tedious organic modifications and can be extended to the NIR region with the judicious choice of different coordination centers. Incorporation of metal center may also render a more robust photochromic system through the rigidification of the organic framework as well as the advantageous less destructive lower-energy excitation upon metal coordination. Hence, improved photoresponsiveness and fatigue resistance could be facilely attained. With the above design strategies together with the rich photophysical and photochemical behaviors of a wide variety of coordination systems, it is anticipated that new types of photochromic coordination compounds with novel and improved photochromic properties triggered by visible light excitation can be developed. Potential applications in preparing functional materials with efficient photoswitchable luminescent and catalytic behavior through the prudent combination of metal−ligand framework as well as fabrication of optical memory devices for 3D data storage using NIR-responsive photochromic systems with the potential for two-photon excitation are also foreseeable in the near future.

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ORCID

Vivian Wing-Wah Yam: 0000-0001-8349-4429 Notes

The authors declare no competing financial interest. Biographies Chi-Chiu Ko is Associate Professor in the Department of Chemistry at City University of Hong Kong (CityU). He received his B.Sc. (Hons) (1999) and Ph.D. (2003) from the University of Hong Kong. He joined CityU in September 2006. His current research interests include the design and synthesis of photofunctional materials, such as luminescent materials, photochromic materials, and photocatalysts. Vivian Wing-Wah Yam obtained both her B.Sc. (Hons) and Ph.D. degrees from The University of Hong Kong and is currently the Philip Wong Wilson Wong Professor in Chemistry and Energy and Chair Professor of Chemistry there. Her research interests include photophysics and photochemistry of transition metal complexes, supramolecular chemistry, and metal-based molecular functional materials for luminescence sensing, optoelectronics, optical and organic resistive memories, and solar energy conversion.

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ACKNOWLEDGMENTS V.W.-W.Y. acknowledges support from The University of Hong Kong und URC Strategic Research Theme on New Materials. This work has been supported by the University Grants Committee Areas of Excellence Scheme (AoE/P-03/08) and the General Research Fund (HKU 17305614) from the Research Grants Council of Hong Kong Special Administrative Region, People’s Republic of China. All students and postdoctoral researchers involved in the work are gratefully acknowledged. Technical assistance on the preparation of the manuscript by Dr. Eugene Y. H. Hong and Dr. Cheok-Lam Wong is also gratefully acknowledged.

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ABBREVIATIONS bpy, 2,2′-bipyridine; Me2bpy, 4,4′-dimethyl-2,2′-bipyridine; phen, 1,10-phenanthroline; Mes, mesityl; Th, thienyl; Ph, phenyl; MLCT, metal-to-ligand charge transfer; IL, intraligand; MC, metal-centered; ICT, intramolecular charge-transfer; ILCT, intraligand charge transfer; LLCT, ligand-to-ligand charge transfer transitions

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AUTHOR INFORMATION

REFERENCES

(1) Crano, J. C.; Guglielmetti, R. Organic Photochromic and Thermochromic Compounds, Vol. 2: Physicochemical Studies, Biological Applications, and Thermochromism; Plenum Press, New York, 1999.

Corresponding Author

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