Existence of the second excited triplet state of 1,8-dibromoanthraquinone observed by picosecond laser photolysis in solutions at room temperature

Picosecond laser photolysis of 1,8-dibromoanthraquinone in solutions at room temperature has revealed the existence of a new absorption band which is interpreted to be the absorption of the second excited nπ* triplet [T2(nπ*)] state with a localized charge-transfer character between the bromine and oxygen atoms. The inter-system crossing (ISC) time from the lowest excited nπ* singlet [S1(nπ*)] state to the T2(nπ*) state is ca. 15–20 ps and the internal conversion (IC) time from the T2(nπ*) state to the lowest excited ππ* triplet [T1(ππ*)] state is ca. 70–110 ps. In comparison with the previous result for 1,8-dichloroanthraquinone, it is proposed that the S1(nπ*)→ T2(nπ*) ISC time is affected not only by the internal heavy-atom effect of halogen atom but also by the conformational distortion of molecules, and that both the T2–T1 energy gap and the conformational distortion of molecules affect the T2(nπ*)→ T1(ππ*) IC time.