Vacuum-deposited films of mesogen of 4-n-pentyl-4″-cyano-p-terphenyl: their electronic spectra and molecular aggregate structures

Takafumi Sumiyoshi, Iwao Takahashi, Yasuyuki Tsuboi, Hiroshi Miyasaka, Akira Itaya, Tsuyoshi Asahi, Hiroshi Masuhara

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Absorption and fluorescence spectra of 4-n-pentyl-4″-cyano-p-terphenyl (5CT) films deposited on hydrophilic and hydrophobic quartz substrates were measured and compared with those of its various solutions and of its neat forms (crystalline, liquid crystal, and isotropic phases). Further, these spectra were compared with those of 4-n-alkoxy-4′-cyanobiphenyls (nOCB, n = 8, 9, 10, 12), which had been studied in detail already. The absorption and fluorescence spectra of the 5CT-deposited films were almost independent of substrate property and deposition temperature. This behavior was different from those observed for nOCB. The absorption spectra differed from those of its solution samples and suggested that cyanoterphenyl groups of 5CT molecules in the deposited films form H-aggregate-like structures with their long axes parallel to each other and relatively perpendicular to the stacking direction in the deposited films. The fluorescence spectra were also different from those of its neat and solution samples. Surface morphology of the deposited films was also investigated using atomic force microscopy (AFM). AFM images showed that the deposited films consisted of a number of small plate crystals, which was consistent with H-aggregate-like structures of cyanoterphenyl groups, and that the small plate crystal transformed to needle ones with time. The structural transformation correlated with the absorption and fluorescence spectral changes with time. The deposition processes of 5CT as revealed by in situ fluorescence spectra during deposition were simple compared with those of nOCB and are discussed on the basis of the in situ fluorescence spectra and the AFM images.

Original languageEnglish
Pages (from-to)285-293
Number of pages9
JournalThin Solid Films
Issue number1
StatePublished - 17 Jul 2000

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