Non-linear characteristics of the photoinduced deformation in azobenzene-containing monolayers at the air-water interface

Abstract

Poly(vinyl alcohol)s having all azobenzene (Az) side chain (6Azn-PVA, n being the carbon number of the methylene spacer connecting PVA and Az unit) form a mechanically stable monomolecular film on a water surface. Alternate irradiation of UV (365 nm) and visible (436 nm) light induces large expansion and contraction of this monolayer in a reversible way. The photoinduced deformation of an isolated monolayer domain could be visualized using a Brewster angle microscope combined with a specially designed mobile mini trough. 6Azn-PVA with the longer spacers (n = 5 and 10) clearly exhibited a non-linear response in UV induced expansion with respect to the trans-to-cis photoisomerization proceeding with an induction time before starting expansion, whereas that of the shortest spacer (n = 1) did not show such a behavior. For 6Az10-PVA monolayer, intermittent UV light irradiation showed an forward (UV) and backward (dark) motion. The above non-linear deformation behavior can be the consequence of a cooperativity stemming from the self-assembling nature of the trans-Az side chains with the longer spacer.