Photomechanical Response of Azobenzene Containing Monolayers on Water Surface

Abstract

Abstract Monolayers of amphiphilic poly(vinyl alcohol)s and related low-molecular-mass compounds containing photochromic azobenzene (Az) were spread on a water surface. Their photomechanical responses induced by photoirradiation of UV (365 nm) and visible (436 nm) light were investigated. Illumination by UV light induced area expansion of Az monolayers up to ca. three times in magnitude, which strongly depended on the material, monitoring surface pressure, and subphase conditions. The expanded film of these molecular films reverted to the original area upon illumination with visible light. Use of a polymeric material was found to be advantageous to attain sufficient mechanical strength for full reproducibility and faster response. At a fixed area, reversible surface pressure changes were observed upon alternate UV and visible light illumination, and at 10—15 mN m−1 with the 6Az10-PVA monolayer the changes could be visualized plainly by watching motions of camphor grains. In the case of an Az amphiphile having the dissociative carboxylic acid head group, the photoresponsiveness was altered by pH changes and addition of Cd2+ ion in the subphase. Variations in the magnitude and response time of the photoinduced deformation were related to the packing state of Az monolayers evaluated by UV-visible absorption spectroscopy at the air–water interface.