Preparation and characterization of mixed thin films containing spiropyrans and long chain alkyl silanes: towards a command surface for liquid crystal realignment

Spiropyran derivatives containing variable-length linking chains and either tri- or mono-alkoxysilane end groups were synthesized. Thin films on glass slides were formed with mixture of the spiropyran compounds and either octadecyltrimethoxysilane or octadecyldimethylmethoxysilane. The photochemistry of the spiropyran in the films was investigated by UV-Vis spectroscopy. Irradiation forms the merocyanine, which is detected by absorption spectroscopy. The rate of the thermal reversion of the merocyanine to the spiropyran was studied and found to show no dependence on chain length and resemble that found in solid polymers, not solution. Sessile drop contact angle measurements show a decrease from 70 +/- 3 to 63 +/- 2 degrees for films prepared from 4, and a drop from 50 to 46 +/- 2 degrees for films prepared from 6 upon irradiation with UV light. Evaluation of this system as a surface to control the realignment of the nematic liquid crystal in a nematic liquid crystal proved unsuccessful, though the surface energy of the system can be changed solely by irradiation with light. (C) 1998 Elsevier Science Limited.