Fluorescence and Photoisomerization of Azobenzene-Containing Bilayer Membranes

Spectroscopic and photoisomerization behavior of aqueous bilayer aggregates of azobenzene-containing amphiphiles was examined. the azobenzene bilayers assume different chromophore orientations, depending on the component structure. Some of the azobenzene bilayers were found to be fluorescent, and the fluorescence intensity decreased as the chromophore orientation changed from the tilted head-to-tail type to the parallel type. Emission quenching was observed in the presence of extremely small amounts of a bound cyanine dye. In the trans-to-cis photoisomerization of the bilayers, the rate in the gel state decreased with changing chromophore orientations from the head-to-tail type to the parallel type. the rate was much larger and unaffected by the molecular structure, in the case of the liquid-crystalline bilayers and of the azobenzene amphiphiles isolated in inert bilayer matrices. In the phase-separated system, photoisomerization occurred between the unclustered isomers. the emission was quickly lost by the formation of the cis isomer. the photoisomerization was suppressed in the presence of the cyanine, probably due to energy transfer to the cyanine and sensitization of the reverse photoisomerization by the cyanine. An energy level diagram was constructed which includes excited states characteristic of the bilayer and explains the photophysical and photochemical processes. Finally, implications of the present finding in relation to light energy harvesting systems were discussed.