Effects of added salt on photochemical isomerization of azobenzene in alternate multilayer assemblies: Bipolar amphiphile-polyelectrolyte

The photochemical isomerization of azobenzene is investigated in systems in which symmetrically substituted chromophores are organized in a self-assem bled molecular film on fused silica. Alternate multilayer films of a cation ic bipolar amphiphile and an anionic polyelectrolyte are prepared by a self -assembly (SA) method based on layer-by-layer deposition using electrostati c attraction between oppositely charged ions. The bipolar amphiphile layer, which exists between polyelectrolyte layers, contains an azobenzene unit i n order to allow for a trans-cis photoisomerization to take place. We have studied the effect of added salt (NaCl) to the bipolar amphiphile, i.e., 4, 4'-bis((12-(trimethylammonio)dodecyl)oxy)-azobenzene dibromide, BA-12, on t he structure of the multilayer films (tilt angle of the chromophore, film t otal thickness) and on trans-cis or cis-trans photoisomerization. Optical d ichroism is induced in the SA film upon linearly polarized light irradiatio n. This dichroism could be reversibly erased and rewritten by irradiation w ith light of an appropriate wavelength. Our study suggests that the level o f birefringence and the photoisomerization in the SA multilayer films can b e controlled to a certain degree by a systematic variation of the supramole cular ordering in the photochromic layers. The trans-cis photoisomerization in the SA films that are obtained from higher ionic strength occurs almost 10 times slower.