Highly branched stilbene dendrimers were synthesized and their photochemical behavior was studied. Even the stilbene dendrimer with molecular weight over 6500 underwent trans-cis isomerization in the excited singlet state within the lifetime of 10 ns. The photoisomerization of C=C double bond of stilbene dendrimers in the excited state may proceed by a volume-conserving novel mechanism such as hula-twist rather than conventional 180 degrees rotation around the C=C double bond based on fluorescence and isomerization experiments.

Highly branched stilbene dendrimers underwent trans–cis isomerization in the excited singlet state within the lifetime of 10 ns.

The energy-releasing process and the conformational dynamics of stilbene dendrimers (tetramethoxystilbene (generation (G) 0, G1, G2, and G4) upon photoexcitation are studied by the time-resolved transient grating method. For all dendrimers, the excited triplet (T1) state formation was observed. From the time-resolved studies on energy and the molecular volumes changes, it is found that the conformational change completes with the decay of the T1 state for G0−G2. However, the dynamics is slightly slower for G4, which is attributed to the conformational change of the dendron part. The molecular volume change due to the conformational rearrangement of the dendron upon the trans to cis isomerization is slightly positive (molecular expansion). The expansion is explained in terms of the creation of the void volume surrounded by the dendron side chains. Although the diffusion coefficient (D) of the trans and cis forms of G0 is almost the same, the difference becomes larger with increasing generation. This is clear evidence for the large conformational change upon isomerization. The larger D of the trans form compared with that of the cis form is explained in terms of the surface roughness of the dendrimers.

Photochemical properties of trans-3,3',5,5'-tetramethoxystilbene (TMST) have been studied in various polar solvents. The Stokes shift of trans-TMST was found to be increased with increasing solvent polarity. The fluorescence lifetime of trans-TMST experienced a large solvent effect changing from 2.3 ns in cyclohexane to 16.6 ns in acetonitrile. These results indicate that the excited singlet state of trans-TMST has a charge-transfer (CT) character. On the basis of the obtained results, the interior polar environment of a water-soluble TMST dendrimer is discussed.