The Photochemical Specific Isomerization of Dendritic Stilbenes

The photochemical properties of stilbene dendrimers 1–3 in THF, as well as the corresponding potassium salts (water-soluble stilbene dendrimers = WSDs) 4–6 in aqueous solution were studied. Unlike the stilbene dendrimers 1–3, the environment of the core moiety in WSDs 4–6 depended on the generation. The first generation WSD 4 suffers from the water atmosphere around the stilbene core, as can be understood by the molecular structure. The largest Stokes shift observed in 4 among WSDs indicated that the dipole moment changed considerably upon photoexcitation. The Stokes shift tended to become smaller in higher generations. This means that the surrounding dendron should block the water molecule from reaching the stilbene core. The core stilbene in G3 WSD 6 may have an environment almost free of water, based on the observation of a similar Stokes shift of that of stilbene dendrimer 3, only soluble in organic solvents in which the core stilbene has a hydrophobic environment. These findings can be extended to the preparation of water soluble organic molecules with hydrophobic interiors with a photoresponsive chromophore at the core as a single molecule. Most interestingly, 6 underwent a photochemical one-way trans-to-cis isomerization. In addition to the photochemical studies of 1–6, the fluorescence properties of G3 dendrimer 3 were compared to those of the other series of stilbene dendrimer 7, which have no carboxylate groups at the periphery.