Vacuum-deposited films of liquid crystal molecules of 4-n-alkoxy-4′-cyanobiphenyls:: Their electronic spectra and molecular aggregate structure

Deposited films of 4-nonyloxy- and 4-decyloxy-4'-cyanobiphenyls (9OCB and 10OCB, respectively) were prepared under various conditions with measuring in situ fluorescence spectra during deposition. Their absorption and fluorescence spectra were investigated in comparison with those of 4-octyloxy and 4-dodecyloxy-4'-cyanobiphenyls (80CB and 12OCB, respectively), whose alkoxy chains were short or long compared with those of the present samples. In addition, surface morphology of the deposited films was investigated using atomic force microscopy (AFM). 8-, 9- and 10OCB-films deposited on hydrophilic substrates at room temperature were needle-like crystalline and showed monomer-like fluorescence, and when:as films on hydrophobic substrates were amorphous, they showed excimer fluorescence, and had similar absorption spectra to those in solution. These phenomena were different from those of 12OCB-deposited films (their absorption and fluorescence spectra were almost independent of substrate property and deposition temperature and were ascribable to H-aggregate structures of cyanobiphenylyloxy groups). The crystal form of the 8-, 9- and 10OCB-deposited films was also different from that of 12OCB (plate-like). The comparison of the absorption and fluorescence spectra of 8-, 9- and 10OCB-deposited films on hydrophilic substrates with those of 12OCB suggested the formation of H-aggregate structures of cyanobiphenylyloxy groups in 8-, 9-, and 10OCB films deposited at low temperature. A discontinuous change found in the series of fluorescence spectra observed during deposition of 8-, 9- and 10OCB on hydrophilic substrates was ascribable to structural transformation from amorphous to crystalline phases. On the contrary, such a change was not found not only for deposition of these nOCB on hydrophobic substrates but also for deposition of 12OCB on both the substrates. These interesting behaviors were discussed in terms of differences in van der Waals attraction between these nOCB molecules and its relative strength to interactions between the substrate and the compound. (C) 1999 Elsevier Science S.A. All rights reserved.