Photoinduced Electron Transfer in the System Eosin–Europium–Cucurbit[6–8]urils in Water

Resumo

Quenching of the eosin triplet state by europium(III) in water has been studied in the absence and in the presence of cucurbit[6–8]uril by means of phosphorescence and triplet–triplet absorption in a deoxygenated aqueous solution at room temperature. The quenching is due to photoinduced electron transfer from eosin to europium and to its complexes with cucurbit[6–8]urils. The value of the quenching rate constant of eosin phosphorescence by europium in the absence of cavitands is 1.9 × 10⁶ L mol^–1 s^–1, whereas in the presence of CB6, CB7, and CB8, the values of the rate constant increase to 2.1, 2.2, and 2.6 × 10⁶ L mol^–1 s^–1, respectively. Simultaneously with the decrease in the lifetime of the eosin molecule in the triplet state, an increase in the yield of the eosin semioxidized form was observed as a product of photoinduced electron transfer. It has been also shown that with an increase in the concentration of CB7 and CB8, the quenching rate constant increases two times. The difference in the values of the quenching rate constants may be due to the fact that europium, being as a quencher, is present in the form of complexes of different compositions with cucurbit[6–8]urils.