Photo-isomerization of polymer monolayers and multi-layers on water

The kinetics of trans-cis photo-isomerization of azo-dye-functionalized polymer films deposited at the air-water interface is studied by optical second harmonic generation. The azo dyes are found to isomerize with two distinct rate constants depending on their local environment. For the first polymer monolayer absorbed on water, isomerization occurs with roughly the same rate as in dilute solutions. For polymers deposited above this monolayer, the reaction rate is nearly two orders of magnitude slower, resembling that in a bulk polymer film. Environmental differences leading to the difference in the amount of free volume is presumably responsible for the two different reaction rates.