D–π–A and (D–π–)₂A fluorescent dyes (OUY-1 and OUY-2) with a 3-pyridyl group as an electron-withdrawing anchoring group have been newly developed as photosensitizers for dye-sensitized solar cells (DSSCs). We found that both dyes OUY-1 and OUY-2 were adsorbed onto a TiO₂ electrode through the formation of pyridinium ions and/or hydrogen bonding of the pyridyl group with the Brønsted-acidic sites on the TiO₂ surface. However, D–π–A dye NI-6, with a 4-pyridyl group, was predominantly adsorbed onto the TiO₂ surface through coordinate bonding between the pyridyl group and the Lewis-acidic sites on the TiO₂ surface. The photovoltaic performance of DSSC based on dye OUY-1 is comparable to that of the dye NI-6. On the other hand, the photovoltaic performance of DSSC based on dye OUY-2 is much lower than that of the dye OUY-1. This work revealed that the binding mode of D–π–A dyes with pyridyl groups on the TiO₂ surface can be changed by control of the basicity and electron density of pyridyl groups.

In order to provide a direction in molecular design of catechol (Cat) dyes for type II dye-sensitized solar cells (DSSCs), the dye-to-TiO₂ charge-transfer (DTCT) characteristics of Cat dyes with various substituents and their photovoltaic performance in DSSCs are investigated. The Cat dyes with electron-donating or moderately electron-withdrawing substituents exhibit a broad absorption band corresponding to DTCT upon binding to TiO₂ films, whereas those with strongly electron-withdrawing substituents exhibit weak DTCT. This study indicates that the introduction of a moderately electron-withdrawing substituent on the Cat moiety leads to not only an increase in the DTCT efficiency, but also the retardation of back electron transfer. This results in favorable conditions for the type II electron-injection pathway from the ground state of the Cat dye to the conduction band of the TiO₂ electrode by the photoexcitation of DTCT bands.

Specific solvatochromic D-π-A-type pyridinium dyes were designed and developed as photosensitizers for dye-sensitized solar cells (DSSCs). The dyes have N-sulfobutylpyridinium or N-(carboxybutyl)pyridinium bromide as an electron-withdrawing anchoring group. The two dyes show specific solvatochromism, leading to a large bathochromic shift of the absorption band in halogenated solvents. Moreover, a dye-adsorbed TiO₂ film immersed into halogenated solvents exhibits specific solvatochromism, as does a dye solution of halogenated solvents. DSSCs based on specific solvatochromic D-π-A-type pyridinium dyes and halogenated solvents as electrolyte solvent were prepared and their photovoltaic performance investigated. It was found that the appropriate combination of solvatochromic dyes with electrolyte solution and the effective interaction between solvatochromic dyes and TiO₂ surface can lead to not only an enhancement of light-harvesting efficiency (LHE), but also efficient electron injection from the dye to the conduction band (CB) of TiO₂. This work demonstrates that the solvatochromism of organic dyes is a key consideration for high-performance DSSCs based on organic dye sensitizers.

Carbazole-type donor-π-acceptor (D-π-A) fluorescent dyes (SO1 and SO2), each containing a diphenylamino system as an electron-donating group and a nitro moiety as an electron-accepting group, have been designed and synthesized, and their photophysical properties in solution and in the solid state have been investigated. The absorption and fluorescence properties of SO1 and SO2 in solution are similar, and both dyes exhibited moderate fluorescence quantum yields. In the solid state, however, the dye SO2, with a butyl substituent on the carbazole ring, exhibited red fluorescence at around 620 nm, whereas the dye SO1, with no substituent on the carbazole ring, did not exhibit these solid-state fluorescence properties. Furthermore, the dye SO2 exhibited weak mechanofluorochromic (MFC) properties: grinding of as-recrystallized dyes induces slight bathochromic shifts of the fluorescence excitation and emission maxima. To elucidate the effects of molecular and crystal structures on the solid-state fluorescence properties, we performed semiempirical molecular orbital calculations (PM3 and INDO/S) and single-crystal X-ray structural analysis. The X-ray crystal structures of SO1 and SO2 demonstrated that a continuous intermolecular π-stacking between the fluorophores was observable in the crystal structure of SO1, but not in that of SO2. The MO calculations revealed that SO1 and SO2 have similarly large dipole moments in the ground state (μ_g ≈ 8 D). The relationship between the observed solid-state photophysical properties and the molecular and crystal structures of the carbazole-type D-π-A fluorescent dyes are discussed on the basis of experimental results and MO calculations. It is found that the formation of a continuous intermolecular π-stacking between the fluorophores causes a drastic fluorescence quenching in the solid state and that D-π-A fluorescent dyes with very large dipole moments can reduce the MFC properties, due to strong dipole–dipole interactions between the fluorophores in the solid state.

Dye-sensitized solar cells (DSSCs) based on organic dyes adsorbed on oxide semiconductor electrodes, such as TiO₂, ZnO, or NiO, which have emerged as a new generation of sustainable photovoltaic devices, have attracted much attention from chemists, physicists, and engineers because of enormous scientific interest in not only their construction and operational principles, but also in their high incident-solar-light-to-electricity conversion efficiency and low cost of production. To develop high-performance DSSCs, it is important to create efficient organic dye sensitizers, which should be optimized for the photophysical and electrochemical properties of the dyes themselves, with molecular structures that provide good light-harvesting features, good electron communication between the dye and semiconductor electrode and between the dye and electrolyte, and to control the molecular orientation and arrangement of the dyes on a semiconductor surface. The aim of this Review is not to make a list of a number of organic dye sensitizers developed so far, but to provide a new direction in the epoch-making molecular design of organic dyes for high photovoltaic performance and long-term stability of DSSCs, based on the accumulated knowledge of their photophysical and electrochemical properties, and molecular structures of the organic dye sensitizers developed so far.

The cover picture shows dye-sensitized solar cells (DSSCs) based on organic dyes adsorbed on a nanocrystalline TiO₂ electrode. DSSCs have received considerable attention because of their high incident solar-light-to-electricity conversion efficiency and low cost of production. To create high-performance DSSCs, it is necessary to design and synthesize new and efficient organic dye photosensitizers with effective chromophores and substituents for the performance of DSSCs, which will be made possible by the exquisite molecular design and synthetic strategy of organic chemists. The background shows the architecture of the Department of Engineering, Hiroshima University, which is associated with the arrangement of organic dyes adsorbed on TiO₂ electrodes. The designs and synthesis of organic dyes for DSSCs are presented in the Microreview by Y. Ooyama and Y. Harima on p. 2903 ff.

Dye-sensitized solar cells (DSSCs) based on organic dyes adsorbed on nanocrystalline TiO₂ electrodes have received considerable attention because of their high incident solar light-to-electricity conversion efficiency and low cost of production. A number of organic dyes have so far been developed, and the relationship between their chemical structures and the photovoltaic performances of DSSCs based on the dyes has been examined. To create high-performance DSSCs, it is important to develop effective organic dye sensitizers, which should be optimized for the chemical structures to possess good light-harvesting features, to provide good electron communication between the dyes and a TiO₂ electrode, and to control the molecular arrangements on the TiO₂ electrode. The aim of this microreview is to highlight the design and synthesis of organic dyes for DSSCs based on recent work of organic chemists. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The crystals of phenanthro[9,10-d]imidazole-type fluorescent host 1 exhibit drastic fluorescence enhancement behaviour with a redshift in the emission maximum upon enclathration of various carboxylic acids such as formic acid, acetic acid and propionic acid. The optical changes are greatly dependent on the identity of the enclathrated carboxylic acids. The fluorescent clathrate compounds are formed not only by cocrystallization from carboxylic acid solutions but also by solid (fluorescent host)–gas (carboxylic acid vapour) contact. Furthermore, when the acetic acid inclusion crystals are exposed to propionic acid vapour, acetic acid is gradually replaced by propionic acid. The guest exchange of the inclusion crystals was accompanied with colour and fluorescent intensity changes. The X-ray structural analyses of the guest-free and carboxylic acid inclusion compounds demonstrated that the destructions of the π–π interactions, and the intermolecular hydrogen bonds binding fluorophores were induced by the enclathrated carboxylic acid molecules. Moreover, the imidazole ring of the host is protonated by the enclathrated carboxylic acid proton. On the basis of the spectroscopic data and the crystal structures, the effects of the enclathrated carboxylic acid on the solid-state photophysical properties of the clathrate compounds are discussed.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The crystals of benzo[b]naphtho[1,2-d]furan-6-one-type fluorophores (1) exhibit sensitive colour and fluorescence change upon enclathration of organic solvent molecules. The crystal of fluorophore 1b (R = Bu) exhibits fluorescence decrease upon inclusion of cyclohexane; however, the crystal of fluorophore 1c (R = Ph) exhibits a drastic fluorescence enhancement upon inclusion of chloroform. To elucidate the enclathrated guest effects on the photophysical properties of the crystals, the X-ray crystal structures of the guest-free and guest-inclusion compounds were determined. On the basis of the spectroscopic data and the crystal structures, the effects of the enclathrated guest on the solid-state photophysical properties of the clathrate compounds are discussed.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Heterocyclic fluorophores 5,5-dibutyl-9-dibutylamino-5H-benzo[b]naphtho[1,2-d]furan-6-one (4) and 3,3-dibutyl-9-dibutylamino-3H-benzo[kl]xanthen-2-one (7) with dialkyl substituents linked in a nonconjugated fashion to the chromophore skeleton have been derived from quinol-type compounds 1 and 2, and their photophysical properties were investigated in solution and in the solid state. Fluorophores 4 and 7 exhibit strong fluorescence intensity in the solid-state relative to that of quinols 1 and 2; however, 1 and 4 or 2 and 7 exhibit similar fluorescence intensities in solution. To elucidate the dramatic effect of the dialkyl substituent on the solid-state fluorescence excitation and emission spectra, we performed semiempirical molecular orbital calculations (AM1 and INDO/S) and X-ray crystallographic analysis. On the basis of the results of the calculations and the X-ray crystal structures, the relation between the solid-state photophysical properties and the chemical and crystal structures of 4 and 7 were discussed. It was confirmed that introduction of bulky dialkyl substituents to the fluorophore skeleton can efficiently prevent the short π–π contact between the fluorophores in molecular aggregation states and cause a dramatic solid-state fluorescence enhancement. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

A new class of fluorescent dye for sensing water in organic solvents by photo-induced electron transfer (PET), based on a (phenylamino)naphtho[1,2-d]oxazol-2-yl-type fluorophore with both proton binding and proton donating sites, has been designed developed. The fluorophore exhibits a weak emission in organic solvents but a drastic enhancement in the fluorescence intensity is observed with increasing water content in organic solvents. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Novel imidazo[4,5-a]naphthalene-type fluorescent clathrate host 2-(4-cyanophenyl)-5-[4-(diethylamino)phenyl]-3H-imidazo[4,5-a]naphthalene (2), with two possible tautomeric forms (A and B) of the imidazole ring, was developed. The crystal of fluorophore 2 exhibits sensitive colour change and fluorescence enhancement behaviour with a blueshift in the emission maximum upon enclathration of various kinds of organic solvent molecules. The crystal structures of the guest-free and clathrate compounds of 2 were determined by X-ray analysis. On the basis of spectroscopic data and crystal structures, the effects of the enclathrated guest on the solid-state photophysical properties of the clathrate compounds are discussed.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)