Photochemistry and mobility of stilbenoid dendrimers in their neat phases

Selectively deuterated, dodecyloxy substituted stilbenoid dendrimers of the first and second generation were synthesized by a convergent synthesis, using the Wittig-Horner reaction. The photochemistry and the fluorescence in the different crystalline and liquid crystalline phases were investigated. Molecules deuterated at the alpha-position of the alkoxy chains were used to study the photoreactions in the neat phases by H-1 NMR. Reactions of the double bonds are exclusively observed in the liquid crystal phases. No photoreactions occur in the crystalline state. The mobility of the dendrimers was studied by means of H-2 solid-state NMR spectroscopy. The onset of the photochemistry for dendrimer 1 [all-(E)-1,3,5-tris[2-(3,4,5-tridodecyloxyphenyl)ethenyl]benzene] corresponds to the increasing mobility at the Cr/LC transition. The first generation dendrimers still show large angle motion, whereas dendrimers of the second generation 2 [all-(E)-1, 3,5-tris(2-{3,5-bis[2-(3,4,5-tridodecyloxyphenyl) ethenyl] phenyl}ethenyl) benzene] are restricted to librational motions. Photochemical conversion and fluorescence quenching for first and second generation dendrimers 1 and 2 increase with increasing molecular motion and reach a maximum in the isotropic phase.