Estudo da reatividade do estado excitado triplete de xantonas dimetoxiladas empregando a técnica de fotólise por pulso de laser

Abstract

Studies by UV/Vis absorption spectroscopy, fluorescence emission and 77K phosphorescence emission indicated that the dimethoxylated xanthones 1,2-dimethoxy-9H-xanthen-9-one (1,2MX), 2,3-dimethoxy- 9H-xanthen-9-one (2,3MX) and 3,4-dimethoxy-9H-xanthen-9-one (3,4MX) present triplet excited state with a ππ* configuration. Studies by laser flash photolysis (λexc=355 nm) were performed to investigate the reactivity of the triplet excited state of the dimethoxylated xanthones 1,2MX, 2,3MX and 3,4MX toward different types of quenchers. Quenching of 1,2MX, 2,3MX and 3,4MX by 1-methylnaphthalene and 1,3-cyclohexadiene, in acetonitrile, results in the formation of the corresponding triplet excited states through a triplet-triplet energy transfer process controlled by diffusion. The hydrogen transfer reaction of these dimethoxylated xanthones employing alcohols (methanol, ethanol and 2-propanol) and 1,4-cyclohexadiene occurs through the pure mechanism, with rate constants consistent with the involvement of a triplet excited state with ππ* character, confirming the experimental results of the phosphorescence studies. In all cases the formation of the corresponding ketyl radicals was observed. Using the electron donors DABCO and triethylamine as quenchers of the triplet excited state of 1,2MX, 2,3MX and 3,4MX the formation of the corresponding radical anion by electron transfer was observed. On the other hand, quenching with phenol and its derivatives led to the formation of the ketyl/phenoxyl radical pair, while using indole or the methyl ester of N-acetyl tryptophan (NATME) the ketyl/indolyl radical pair was formed. In all cases, the reaction takes place through a mechanism involving an electron transfer coupled to an ultra-fast proton transfer. Comparing the quenching rate constants for the dimethoxylated xanthones1,2MX, 2,3MX and 3,4MX, it can be concluded that there is no relevant difference between their reactivities. Photolysis of 2,3MX in H2O/acetonitrile solution (1:1 v/v) (λexc=266 nm) resulted in its photoionization with the formation of the corresponding radical cation (λmax~390 nm). The characterization of this radical cation through the use of the nucleophilic quenchers chloride, bromide and acetate lead to quenching rate constants with diffusion-controlled values. Finally, 1,2MX, 2,3MX and 3,4MX were used as photosensitizers for singlet oxygen formation, although presenting very low efficiency (Φ Δ1O2: 1,2MX = 0.19; 2,3MX = 0.12 and 3,4MX = 0.06).