PHOTOPHYSICS AND PHOTOCHEMISTRY OF METHYLATED PHENOLS IN BETA-CYCLODEXTRIN INCLUSION COMPLEXES

Absorption and induced circular dichroism (ICD) spectra as well as photophysical (fluorescence quantum yield, fluorescence lifetime, and triplet-triplet absorption) and photochemical (hydrated electron formation) properties have been measured in aqueous solutions of phenol, p-cresol, 2,6-dimethylphenol, 3,5-dimethylphenol, 2,4,6-trimethylphenol, and 3,4,5-trimethylphenol in the presence of beta-cyclodextrin and compared to their behaviors in pure aqueous and ethanolic solutions. Complex formation between the phenols and beta-cyclodextrin is evidenced by absorption and ICD spectroscopy. The photophysical and photochemical behaviors of phenol and p-cresol are weakly affected by complexation. In contrast, the fluorescence quantum yields and lifetimes of the dimethyl- and trimethylphenols are markedly increased in the presence of cyclodextrin, in agreement with the formation of 1:1 complexes with distinct photophysical properties. This allows determination of their ground-state association constants, which are somewhat lower than those of phenol and p-cresol. The study of triplet-triplet absorption and hydrated electron formation of the trimethylated phenols shows that intersystem crossing is slowed down by complexation, whereas the efficiency of one-photon hydrated electron formation is drastically reduced. A marked decrease of the rate constant of S1-S0 internal conversion upon complexation is shown to be mainly responsible for the increase of fluorescence lifetimes and quantum yields. These results are discussed in terms of complex structure, in particular of the location of the phenolic OH group near to the rim of the macrocycle, which confers to its environment properties which liken it to that of alcoholic solutions.