Ichiro Imae

Dye‐Sensitized Solar Cells Based on Donor‐π‐Acceptor Fluorescent Dyes with a Pyridine Ring as an Electron‐Withdrawing‐Injecting Anchoring Group

A new-type of donor-acceptor π-conjugated (D-π-A) fluorescent dyes NI3-NI8 with a pyridine ring as electron-withdrawing-injecting anchoring group have been developed and their photovoltaic performances in dye-sensitized solar cells (DSSCs) are investigated. The short-circuit photocurrent densities and solar energy-to-electricity conversion yields of DSSCs based on NI3-NI8 are greater than those for the conventional D-π-A dye sensitizers NI1 and NI2 with a carboxyl group as the electron-withdrawing anchoring group. The IR spectra of NI3-NI8 adsorbed on TiO(2) indicate the formation of coordinate bonds between the pyridine ring of dyes NI3-NI8 and the Lewis acid sites (exposed Ti(n+) cations) of the TiO(2) surface. This work demonstrates that the pyridine rings of D-π-A dye sensitizers that form a coordinate bond with the Lewis acid site of a TiO(2) surface are promising candidates as not only electron-withdrawing anchoring group but also electron-injecting group, rather than the carboxyl groups of the conventional D-π-A dye sensitizers that form an ester linkage with the Brønsted acid sites of the TiO(2) surface.

Dye-sensitized solar cells based on D-π-A fluorescent dyes with two pyridyl groups as an electron-withdrawing-injecting anchoring group.

D-π-A fluorescent dye with two pyridyl groups as an electron-withdrawing-injecting anchoring group possessing a high coordinate bonding ability to Lewis acid sites on the TiO2 surface, which can lead to high dye loading on the TiO2 film and efficient electron injection, has been developed as a new type of D-π-A dye sensitizer for dye-sensitized solar cells.

New molecular design of donor-π-acceptor dyes for dye-sensitized solar cells: control of molecular orientation and arrangement on TiO2 surface

A series of benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes OH1, OH2, OH4, OH7, and OH17 with carboxyl groups on different positions of a chromophore skeleton are synthesized and applied to dye-sensitized solar cells (DSSCs) as a new class of donor-π-acceptor (D-π-A) photosensitizers. In the dye OH1, a carboxyl group acts as not only the anchoring group for attachment on TiO2 surface but also the electron acceptor. For OH2, OH4, and OH7, on the other hand, a carboxyl group is an anchoring group, but the electron acceptor is not a carboxyl group but a cyano group. The dye OH17 has two carboxyl groups, which are located at the same positions as those of OH1 and OH7. The absorption and fluorescence spectra and cyclic voltammograms of these fluorescent dyes resemble very well, showing a negligible influence of the position of carboxyl group on photophysical and electrochemical properties of these dyes. When these dyes are used in DSSCs, however, their photovoltaic performances differ considerably. In order to elucidate the difference in the DSSC performance among the five dyes, kinetics of the electron injection from the conduction band of TiO2 to dye cation and to I3− ions in solution were studied by employing the transient absorption spectroscopy and the transient photovoltage technique, respectively. It is found that the charge recombination rate for OH2 is similar to that of OH1 and is much slower than those of OH4 and OH17, consistent with the high short-circuit photocurrent densities for OH1 and OH2. On the basis of the MO calculations (AM1 and INDO/S) and the charge recombination kinetics, the differences of the photovoltaic performances among the five dyes are discussed from the viewpoint of configuration of the dyes adsorbed on TiO2 surface, and a new molecular design for D-π-A dye sensitizers based on a control of molecular orientation and arrangement of dye adsorbed on TiO2 surface is proposed.

Unique photoluminescence property of a novel perfectly carbazole-substituted POSS

The photoluminescence spectrum of newly synthesized octakis[2-(carbazol-9-yl)ethyldimethylsiloxy]silsesquioxane in the solid state shows a strong monomeric emission peak, which suggests the suppression of the formation of excimers even in the solid state.

Photovoltaic performance of dye-sensitized solar cells based on donor–acceptor π-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes with a carboxyl group at different positions of the chromophore skeleton

Donor-acceptor pi-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes 3a, 3b, 8a, and 8b with a carboxyl group at different positions of the chromophore skeleton have been designed and synthesized. The absorption and fluorescence spectra and cyclic voltammograms of the fluorescent dyes agree very well, showing that the position of the carboxyl group has a negligible influence on the photophysical and electrochemical properties of these dyes. When these dyes are used in dye-sensitized solar cells, however, their photovolatic performances are considerably different. The short-circuit photocurrents and energy conversion efficiencies under a simulated solar light increase in the order: 3a (2.12 mA cm(-2), 1.00%) approximately 3b (2.10 mA cm(-2), 1.06%) > 8b (1.50 mA cm(-2), 0.67%) > 8a (0.84 mA cm(-2), 0.34%). Based on semi-empirical molecular orbital calculations (AM1 and INDO/S) together with spectral analyses and their photovolatic performance, the relationships between the observed photovolatic properties and the chemical structures of the benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes are discussed. It is found that strong interaction between a TiO(2) surface and the electron accepting moiety of the dye leads to a high photovoltaic performance.

Dye-sensitized solar cells based on novel donor–acceptor π-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes exhibiting solid-state fluorescence

Novel donor–acceptor π-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes (2a–2d) with substituents (R = H, butyl, benzyl and 5-nonyl) on the nitrogen of carbazole ring have been designed and synthesized as sensitizers in dye-sensitized solar cells (DSSCs). Absorption and fluorescence properties of 2a–2d are similar in solution. In the solid state, however, the dyes 2c (R = benzyl) and 2d (R = 5-nonyl) with bulky substituents exhibit strong solid-state fluorescence properties, which are attributed to the reduction of the π–π interaction between the dyes. Photovoltaic parameters of DSSCs based on 2a–2d are measured with the amount of dyes adsorbed on TiO2 film as a parameter. It is found that the 2d exhibits a 100% efficiency for absorbed photon-to-current conversion, demonstrating that bulky substituents in 2d can efficiently prevent intermolecular energy transfer between the dyes in molecular aggregation states.

Stereospecific formation of optically active trialkylsilyllithiums and their configurational stability

Optically active trialkylsilyllithiums, (R)-(n-butyl)methylphenylsilyllithium (63% ee) and (S)-methyl(1-naphthyl)phenylsilyllithium (96% ee), were prepared by cleavage of the siliconsilicon bond of (R)-1-(n-butyl)-1-methyl-1-phenyl-2,2-diphenyl-2-methyldisilane with lithium metal, or the silicontin bond of (S)-methyl(1-naphthyl)phenylsilyltrimethylstannane with methyllithium, respectively. Optical purity of the silyllithiums was evaluated as corresponding silanes by HPLC on optically active stationary phase after hydrolysis. The formation of silyllithium was found to be highly stereospecific (>94, >99% retention, respectively). (S)-Methyl(1-naphthyl)phenylsilyllithium is configurationally stable for at least 1 h at −78°C in tetrahydrofuran.

Polymers containing pendant oligothiophenes as a novel class of electrochromic materials

Vinyl polymers containing oligothiophenes, e.g. 2,2′ : 5′,2″-terthiophene and 2,2′ : 5′,2″ : 5″,2‴-quaterthiophene, as pendant groups have been found to constitute a novel class of potential electrochromic materials. The electrochemically doped polymers undergo reversible, clear colour changes from bluish purple to pale yellowish orange, and from green to pale yellow, respectively, and vice versa on electrochemical dedoping and doping.

Synthesis, stereochemistry and chiroptical properties of naphthylphenyl-substituted optically active oligosilanes with α, ω-chiral silicon centers

Abstract Novel naphthylphenyl-substituted optically active oligosilanes with α,ω-chiral silicon centers were synthesized. The absolute configuration of (1 R ,2 R )-1,2-dimethyl-1,2-di(1-naphthyl)-1,2-diphenyldisilane ( R , R )- 3 , one of the oligosilanes, was determined by X-ray diffraction, which gave a direct evidence for the retention and inversion stereochemistry of attacking silylanion and attacked chlorosilane. The intense π–π interaction of aryl substituents on chiral silicon centers enhanced by σ–π conjugation with oligosilane unit made it possible to clearly assign the absolute configuration and stable conformation of the optically active oligosilanes. The disilane ( R , R )- 3 showed positive exciton chirality originated from π–π interactions of 1 B b,Np transition bands of two naphthyl chromophores on the adjacent silicon atoms. Contrary to this, (1 R ,3 R )-1,3-di(1-naphthyl)-1,3-diphenyl-2-trimethylsilyl-1,2,3-trimethyltrisilane ( R , R )- 5 , having silylene spacer with bulky trimethylsilyl group as branched substituent between two chiral centers, showed positive exciton chirality by interaction between red-shifted 1 L a,Ph and 1 B b,Np on the same silicon atom. (1 R ,3 R )-1,3-Di(1-naphthyl)-1,3-diphenyl-1,2,2,3-tetramethyltrisilane ( R , R )- 4 , which has dimethylsilylene group between two chiral silicon centers, showed two exciton chiralities, namely, positive chirality by 1 L a,Ph and 1 B b,Np on the same silicon atom, and negative chirality by two 1 B b,Np on the adjacent silicon atoms. The intensified negative Cotton effect at 234 nm was caused by overlapping of Cotton effects of two exciton chiralities.

Solvatochromism of novel donor–π–acceptor type pyridinium dyes in halogenated and non-halogenated solvents

Donor–π–acceptor type pyridinium dyes that have been newly synthesized showed bathochromic shift of the absorption band with decreasing dielectric constant (er) of solvent (negative solvatochromism), and the bathochromic shifts in halogenated solvents were larger than those in non-halogenated solvents of similar er values. The influences of halogenated solvents on the large bathochromic shifts are studied on the basis of 1H NMR measurements, semi-empirical molecular calculations (AM1 and INDO/S using the SCRF Onsager model), and absorption spectral measurements of the dyes adsorbed on nanocrystalline TiO2 film immersed in solvents.