2-STEP LASER-INDUCED-FLUORESCENCE STUDY OF 2',3',4',5',6'-PENTAMETHYL-3-HYDROXYFLAVONE IN SOLUTION AT ROOM-TEMPERATURE - CONTRIBUTION OF TRIPLET-STATES AND CONFIRMATION OF GROUND-STATE TAUTOMER

The S1'--> S0' fluorescence spectrum (lambda(max) = 505 nm) was induce d by both blue and red probe pulse excitations of transient species ge nerated by the 308-nm pump pulse excitation of 2',3',4',5',6'-pentamet hyl-3-hydroxyflavone (PM3HF) undergoing the S1 --> S1' excited-state i ntramolecular proton transfer in degassed 3-methylpentane at room temp erature. The tautomeric lowest triplet state (T1') is predominantly re sponsible for the two-step laser-induced fluorescence (TSLIF) induced by red probe pulse excitation, whereas both the primary lowest triplet state (T1) and the tautomeric ground state (S0') are mainly responsib le for the TSLIF induced by blue probe pulse excitation. The T2' --> S 1' and the T(n) --> S(m) reverse intersystem crossings are considered as the key processes for the appearance of the S1' --> S0' fluorescenc e upon the excitation of T1' and T1, respectively. In addition to the S1 --> S1' excited-state proton transfer followed by the S0' --> S0 gr ound-state reverse proton transfer, the longer sequence, S1 --> S1' -- > T1' --> S0' --> S0, was confirmed for the overall S1 --> S0 relaxati on. The respective lifetimes of S1', T1', and S0' in the sequence were determined to be 1.5 ns, 20 mus, and 1.4 mus, and the lifetime of T1 was determined to be 7.5 mus. From the triplet quenching study, the T1 -S0 energy gap is higher than 18 690 cm-1, and the T1'-S0' energy gap is between 11 650 and 10 900 cm-1.