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

Absorption and induced circular dichroism (ICD) spectra as well as pho tophysical (fluorescence quantum yield, fluorescence lifetime, and tri plet-triplet absorption) and photochemical (hydrated electron formatio n) properties have been measured in aqueous solutions of phenol, p-cre sol, 2,6-dimethylphenol, 3,5-dimethylphenol, 2,4,6-trimethylphenol, an d 3,4,5-trimethylphenol in the presence of beta-cyclodextrin and compa red to their behaviors in pure aqueous and ethanolic solutions. Comple x formation between the phenols and beta-cyclodextrin is evidenced by absorption and ICD spectroscopy. The photophysical and photochemical b ehaviors of phenol and p-cresol are weakly affected by complexation. I n contrast, the fluorescence quantum yields and lifetimes of the dimet hyl- and trimethylphenols are markedly increased in the presence of cy clodextrin, in agreement with the formation of 1:1 complexes with dist inct photophysical properties. This allows determination of their grou nd-state association constants, which are somewhat lower than those of phenol and p-cresol. The study of triplet-triplet absorption and hydr ated electron formation of the trimethylated phenols shows that inters ystem crossing is slowed down by complexation, whereas the efficiency of one-photon hydrated electron formation is drastically reduced. A ma rked decrease of the rate constant of S1-S0 internal conversion upon c omplexation is shown to be mainly responsible for the increase of fluo rescence lifetimes and quantum yields. These results are discussed in terms of complex structure, in particular of the location of the pheno lic OH group near to the rim of the macrocycle, which confers to its e nvironment properties which liken it to that of alcoholic solutions.