Development of the dendrimer photochemistry and the reaction dynamics

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(1)Photoisomerization mechanismA cross-linked 1,3,5-tristyrylbenzene dendrimer was synthesized to study the photochemical isomerization mechanism. On irradiation, cross-linked dendrimer isomerized with the quantum yields of 0.063, which was not very much different from that for a model compound (0.080), supporting a volume-conserving mechanism during the photochemical E-Z isomerization.(2)Whole structural change of the dendrimers triggered by photoisomerizaition at the core partThe conformational dynamics and the energy-releasing process of stilbene dendrimers upon photoexcitation in water are investigated by the time-resolved transient grating method. Whereas the energy relaxation and the conformational change are completed within 30 ns for the first generation, slower dynamics are observed for the second and the third generations.(3)Increasing the fluorescence quantum yield in dendronized chromophoreThe photochemical properties of a series of newly synthesized dendrimers, having a 2- … More (2'-hydroxyphenyl)-benzoxazole (HBO) core, were studied in benzene. In general, the quantum yield for fluorescence of HBO and its analogues was small, However, the fluorescence quantum yields were determined to be 0.022,0.030, and 0.038 for G0,G2, and G3 dendrimers, respectively, increasing in higher generation dendrimers.(4)Increasing the quantum yield for generation of singlet oxygen upon photoexcitation of dendronized stilbenesSince the excited singlet state of the core moiety in stilbene-cored dendrimers G1-G4 has ca.10-ns lifetime, oxygen quenched their excited singlet state producing the singlet oxygen. The efficiency of singlet oxygen production based on the quenching of the stilbene core is the highest in G4 stilbene dendrimer.(5)Self-assembly of chromophore using amphiphilic dendronsThe self-assembly of amphiphilic pyrene-cored poly(aryl ether) dendrimers has been studied. Pyrene excimer emission from the higher generation is observed in KOH aqueous solution at low concentration. The results indicate that the self-aggregation in higher generation dendrimer takes place more efficiently than in lower generation in aqueous solution. Less