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-assembled molecular film on fused silica. Alternate multilayer films of a cationic bipolar amphiphile and an anionic polyelectrolyte are prepared by a self-assembly (SA) method based on layer-by-layer deposition using electrostatic attraction between oppositely charged ions. The bipolar amphiphile layer, which exists between polyelectrolyte layers, contains an azobenzene unit in 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 the structure of the multilayer films (tilt angle of the chromophore, film total thickness) and on trans-cis or cis-trans photoisomerization. Optical dichroism is induced in the SA film upon linearly polarized light irradiation. This dichroism could be reversibly erased and rewritten by irradiation with light of an appropriate wavelength. Our study suggests that the level of birefringence and the photoisomerization in the SA multilayer films can be controlled to a certain degree by a systematic variation of the supramolecular 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.