Controlled Stilbene Photochemistry in Ammonium Bilayer Membranes

Long-chain ammonium amphiphiles containing the stilbene chromophore (C₁₂StbC_nN⁺) formed bilayer assemblies in water, as confirmed by electron microscopy and other means. Their absorption spectra showed blue shifts characteristic of parallel chromophore stacking in the bilayer. The extent of the blue shift was greater for the C₁₂StbC₁₀N⁺(C₁₀spacer methylene) bilayer than for the Ci₂StbC₄N⁺(C₄spacer methylene) bilayer. The emission spectra were red-shifted accordingly. The monomer—cluster equilibrium (phase separation) of the stilbene chromophore in the bilayer matrix of dialkylammonium bromides was readily detected by absorption spectral changes, and it was promoted in the crystalline matrix. The photoreactivity of the C₁₂StbC₁₀N⁺bilayer was much smaller than that of the C₁₂StbC₄N⁺bilayer, which underwent ready dimerization. This difference was apparently produced by typical parallel stacking of the stilbene unit in the former. The course of stilbene photochemistry was affected by phase separation. Bimolecular cycloaddition, cyclobutane formation, was dominant for the stilbene component in a phase-separated cluster, whereas unimolecular trans-cis isomerization and the subsequent phenanthrene generation were observed when the stilbene was molecularly dispersed either in a matrix membrane or in ethanol. The photochemical processes can be controlled by the membrane physical state.