Photophysical properties of a carbazolyl mesogen of 8PCzC as revealed by absorption, fluorescence, and transient absorption measurements.

Fluorescence spectra in three phases [crystalline, smectic A liquid crystal (SmA), and isotropic] of a mesogen including one carbazolyl (Cz) chromophore, 4'-n-octylphenyl carbazole-2-carboxylate (8PCzC), were measured at various temperatures and compared with fluorescence spectra in solvents with various dielectric constants. Further, for the first time, picosecond transient absorption spectroscopy was applied to elucidating photoinduced electron-transfer dynamics (charge separation, charge recombination, and hole transfer reactions) in the three phases of 8PCzC doped with a small amount of an electron acceptor. The fluorescence spectra showed clearly the presence of two crystalline phases, which was supported by DSC measurement. On the basis of a comparison of the fluorescence spectra and those in solution, the 8PCzC molecule in the first crystal structure was suggested to be in polar environment compared to that in the second one. The fluorescence yields of 8PCzC in both SmA and isotropic phases were remarkably small compared to that in crystalline one. This was ascribed to the formation of hydrogen bonds between 8PCzC molecules in both SmA and isotropic phases. As a first approximation, the picosecond transient absorption spectra in the three phases of 8PCzC doped with 2,4,7-trinitrofluorenone were analyzed by the simple model of one-dimensional hole migration. The rate constants obtained for the hole transfer from the cation state of Cz chromophore to neighboring Czs decreased in the order of SmA (approximately 10(9) s(-1)) > isotropic (approximately 10(8) s(-1)) > crystalline (approximately 10(7) s(-1)) phases, although these measurement temperatures were different.

Nonameric porphyrin assemblies--formation and intra-assembly energy transfer reactions.

The nonameric porphyrin assemblies constructed with the series of free base tetraphenylporphyrins Pn having four pyrazine moieties linked with alkyl chains of different lengths, (CH2)n (n = 1, 5, 9, 17, 30), and dimeric [meso-tetrakis(2-carboxy-4-nonylphenyl)porphyrinato]zinc(II), ZnP2, show the effective light-collection effect and the typical Forster-type energy transfer from ZnP2 to Pn.

Synthesis of directly linked zinc(II) porphyrin-imide dyads and energy gap dependence of intramolecular electron transfer reactions.

A series of zinc(II) porphyrin-imide dyads (ZP-Im), in which an electron donating ZP moiety is directly connected to an electron accepting imide moiety in the meso position, have been prepared for the examination of energy gap dependence of intramolecular electron transfer reactions with large electronic coupling. The nearly perpendicular conformation of the imide moiety towards the porphyrin plane has been revealed by Xray crystal structures. The energy gap for charge separation, 1ZP* - Im --> ZP+ - Im-, is varied by changing the electron accepting imide moiety to cover a range of about 0.8 eV in DMF. Definitive evidence for electron transfer has been obtained in three solvents (toluene, THF, and DMF) through picosecond-femtosecond transient absorption studies, which have allowed us to determine the rates of photoinduced charge separation, 1ZP* - Im --> ZP+ - Im-, and subsequent thermal charge recombination ZP+ - Im- --> ZP - Im. The free-energy gap dependence (energy gap law) has been probed from the normal to the nearly top region for the charge separation rate alone, and only the inverted region for the charge recombination rate. Although both of the energy gap dependencies can be approximately reproduced by means of the simplified semiclassical equation, when we take into consideration the effect of the high frequency vibrations replaced by one mode of averaged frequency, many features, including the effects of solvent polarity and the electron tunneling matrix element on the energy gap law, differ considerably from those of the previously studied porphyrin-quinone systems, which have weaker interchromophore electronic interactions.

Efficient photocyclization of dithienylethene dimer, trimer, and tetramer: quantum yield and reaction dynamics.

Multi-dithienylethene arrays, in which two, three, or four 1,2-bis(2,4-dimethylthiophen-3-yl)perfluorocyclopentenes are ethynylene-bridged, were synthesized. Upon irradiation with ultraviolet light the hexane solutions of the arrays turned violet-blue and the color disappeared by irradiation with visible light. The quantum yields of photocyclization reactions successively increased from 0.21 to 0.40 by increasing the number of the dithienylethene moieties in the arrays from one to four. Picosecond laser photolysis as well as the fluorescence depolarization experiment confirmed that efficient excited energy migration in the arrays from the photochemically inactive parallel conformer to the photoactive antiparallel conformer resulted in the high quantum yields.