Vacuum-deposited films of liquid crystal molecule of 4-dodecyloxy-4′-cyanobiphenyl: Their electronic spectra and molecular aggregate structures

Absorption and fluorescence spectra of 4-dodecyloxy-4′-cyanobiphenyl (12OCB) films deposited on hydrophilic and hydrophobic substrates were measured and compared with those of its neat form (crystalline, liquid crystal, and isotropic phases). Further, these spectra were compared with those of 4-octyloxy-4′-cyanobiphenyl (8OCB). In addition, surface morphology of the deposited films was investigated using atomic force microscopy (AFM). The absorption and fluorescence spectra of the deposited films were almost independent of substrate property and deposition temperature, which was different from the result of 8OCB. The absorption spectra of the 12OCB-deposited films were observed at short wavelengths compared with those of 12OCB in solution and the first band split into two bands, suggesting the formation of H-aggregate structures of cyanobiphenylyloxy groups in the deposited films. The fluorescence spectra of the deposited films were observed in the shorter wavelength region than those of its neat and solution samples and were never reported for other cyanobiphenyl-type liquid crystal compounds. This new fluorescence was responsible for the H-aggregate structures of the 12OCB-deposited films. AFM images showed that the deposited films consisted of a number of small plate crystals. The small plate crystals were considered to have smectic-like structure composed of bilayers, which was consistent with H-aggregate structures of cyanobiphenylyloxy groups suggested from the absorption and fluorescence spectra. The deposition processes of 12OCB were investigated by measuring in-situ fluorescence spectra during deposition and discussed in comparison with those of 8OCB.