Modification of photochemical reactivity by nafion. Photocyclization and photochemical cis-trans isomerization of azobenzene

The photophysical and photochemical behavior of azobenzene (AB) incorporated into solvent-swollen acid form Nafion (Nafion-H+) membranes were examined. In water-swollen Nafion-H+ membrane AB exhibits strong fluorescence at room temperature, which has never been observed in isotropic solvents. Photolysis of AB adsorbed in water-swollen Nafion-H+ results in its cyclization to give benzo[c]cinnoline (BC) and benzidine (BZ) in quantitative yield. The product distribution is very dependent upon the number of AB molecules in each water cluster of the Nafion membrane (occupancy number). In the case of the occupancy number greater than 2, BC and BZ are formed equimolarly, while when only one AB molecule exists in each water cluster, BC is exclusively produced. The AB molecules incorporated into methanol-swollen Nafion-H+ membrane do not emit fluorescence and only undergo cis-trans isomerization when they are photoirradiated. These observations suggest that in water-swollen Nafion-H+ AB molecules are solublized in the fluorocarbon/water interface, and the protons of the Nafion-H+ participate in the photochemical and photophysical processes of AB. On the other hand, in methanol-swollen Nafion-H+ membrane AB molecules are located in the methanol pools, and their photochemical and photophysical behaviors are not intervened by the Nafion protons.