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

Abstract

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 (≈10⁹ s⁻¹) > isotropic (≈10⁸ s⁻¹) > crystalline (≈10⁷ s⁻¹) phases, although these measurement temperatures were different.