Stilbene Photoisomerization in Dendrimers

Photochemical and photophysical properties of a poly(propylene amine) dendri-mer (2) functionalized with -stilbene units have been studied [102]. Z-photoisome-rization and photocyclization of the Z-isomer of the stilbene units were investigated in air-equilibrated acetonitrile solutions. The quantum yields of the E Z photoisomerization reaction and the fluorescence quantum yield of the E were found to be equal to 0.30 and 0.014, respectively. Stilbene dendrimers prepared by coupling 4,4 -dihydroxystilbene with first-, second-, third-, or fourth-generation benzyl ether-type dendrons underwent photoisomerization with the same efficiency as that of 4,4 -dimethoxystilbene [103], The lifetime of the core structure was found to be shorter then 1 ns. According to [104], polyphenylene-based stilbene dendrimers, Gl, G2, and G3, underwent mutual cis-trans isomerization upon direct irradiation with 310 nm light at room temperature. In a solvent glass at 77 K, one-way cis-trans isomerization was observed for G2. 

Clear evidence for the large conformational change upon isomerization of stilbene (tetramethoxystilbene) dendrimers of generation GO, Gl, G2, and G4 was presented [105]. From the time-resolved studies on energy and molecular volumes changes, it was found that the conformational change upon the trans- to cis-isomerization completed with the decay of the Ti state for G0-G2. The diffusion coefficient (D) 

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