Stilbene Dendrimer Fluorescence

The stilbene carbon unit has also been peripherally bound to POPAM cores. Although it does not strictly belong to the hydrocarbon dendrimers, the formula of a G2 dendrimer of this type (Fig. 4.21) is depicted here as an example. It was obtained by alkylation of the corresponding eightfold mono-sulphonamide with 4-(bromomethyl)stilbene. Its fluorescence, E/Z isomerisation, photoisomerisation (see Section 5.2.2), and excimer formation were compared with those of non-dendritic stilbenes. The quantum yields of photoisomerisation (0.30) and fluorescence of the E isomer (0.014) of the dendrimer proved to be substantially lower [38]. 

The electronic spectroscopy of trans-isomers of 3-(N-phenylamino)stilbene (mlc), 3-(N-methyl-N-phenylamino)stilbene (mid), 3-(N,N-diphenylamino)stilbene (mle), and 3-(N-(2,6-dimethylphenyl)amino)stilbene (mlf) and their double-bond constrained analogues, m2a-m2c and m2e, were studied [71]. When compared with trans-3-aminostilbene (mla), mlc-mle displayed a redshift of the So —> Si absorption and fluorescence spectra, lower oscillator strength and fluorescence rate constants, and more effldent Si —> Ti intersystem crossing. The N-Ph derivatives mlc-mle had lower fluorescence quantum yields and higher photoisomerization quantum yields. The role of Si —> Ti transition in the amino-substituted stilbenes as the predominant nonradiative decay pathway was discussed. The excited triplet (Ti) state formation of stilbene dendrimers (tetramethoxystilbene (generation G) GO, Gl,... 

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. 

---