Improvement of quantum yields for photoinduced energy/electron transfer by isolation of self-aggregative zinc tetraphenyl porphyrin-pendant polymer using cyclodextrin inclusion in aqueous solution

Cyclodextrin inclusion for self-aggregative sensitizers was found to improve quantum yields for photoinduced energy/electron transfer by the isolation of sensitizers in an aqueous solution. In water, cyclodextrin derivatives can prevent self-aggregation of sensitizers, which decreases the total quantum yields for photoinduced energy/electron transfer by fast self-quenching of singlet excited states, and anionic water-soluble zinc tetraphenyl porphyrin-pendant polymers (poly-ZnP) show self-aggregation and self-quenching of the singlet excited state of the zinc tetraphenyl porphyrin moiety. With the addition of 2,3,6-tri-O-methyl-βcyclodextrin (TMBCD) to an aqueous solution of poly-ZnP, cyclodextrin inclusion induces isolation of the zinc tetraphenyl porphyrin moiety, which extends extensively the lifetime of the singlet excited state. In the presence of TMBCD, the quantum yield for photoinduced triplet-triplet (T-T) energy transfer (Φ_en) from photoexcited poly-ZnP to O₂ was enhanced remarkably from 0.11 (without TMBCD) to 0.57 (with TMBCD) in air-saturated water. Also, the quantum yield for electron transfer (Φ_et) from photoexcited poly-ZnP to water-soluble γ-cyclodextrin-bicapped [60]fullerene (C₆₀/γ-CD) increased from 0.095 (without TMBCD) to 0.20 (with TMBCD) in Ar-saturated water. Through quantitative photolysis, it has been revealed that the quantum yield for the formation of triplet excited poly-ZnP (Φ_T) is increased greatly to produce large Φ_en and Φ_et values with the addition of TMBCD to the solution. Demonstrated is a new strategy to improve quantum yields for photoinduced reactions using cyclodextrin derivatives as an isolator/stabilizer of photoexcited self-aggregative dyes.