REVERSIBLE ALIGNMENT CHANGE OF LIQUID-CRYSTALS INDUCED BY PHOTOCHROMIC MOLECULAR FILMS .13. FACTORS AFFECTING PHOTOINDUCED ALIGNMENT REGULATION OF CYCLOHEXANECARBOXYLATE-TYPE NEMATIC LIQUID-CRYSTALS BY AZOBENZENE MOLECULAR FILMS

Azobenzene units attached on quartz plates through Si-O bonding exhibit reversible photoisomerization and regulate the alignment of a nematic liquid crystal (LC) between homeotropic and planar modes when LC cells are constructed by putting the LC between two plates covered with the azo-modified surfaces which have been called "command surfaces". The efficiency of the command surfaces is affected crucially by the molecular structure and the occupied area of azobenzene unit. Concerning the molecular structural effect, the hydrophobic nature of a substituent at the azobenzene moiety holds an important role in the alignment regulation. The mode of silylation and the length of a spacer unit between the azobenzene moiety and the silylating group are key factors in commanding the alignment regulation as well. There is a critical area occupied by an azobenzene unit for the photoregulation of LC alignment. An occupied area larger than ca. 120 angstrom2 results in no photoregulation. The occupied area also affects the photoresponse pattern, and discussion is given on the existence of a threshold of the command surface property for the photoregulation. Effects of cell thickness and the exposure cycles on the photoresponse are also presented.