| Abstract
| - Ultrafast relaxation dynamics of the S2 and S1 states of 4,4‘-bis(N,N-dimethylamino)thiobenzophenone(Michler's thione, MT) have been investigated in different kinds of solvents, using steady-state absorptionand emission as well as femtosecond transient absorption and fluorescence up-conversion spectroscopictechniques. Steady-state fluorescence measurements, following photoexcitation to the S2 state of MT, revealweak fluorescence from the S2 state (φF ∼ 10-3 in nonpolar and 10-4 in polar solvents) but much weakerfluorescence from the S1 state. Yield of fluorescence from the S2 state is reduced in polar solvents becauseof reduced energy gap between the S2 and S1 states, ΔE(S2−S1), as well as interaction with the solventmolecules. Occurrence of S2-fluorescence in polar solvents, despite small energy gap, suggests that symmetryallowed S2(1A1) → S0 (1A1) radiative and symmetry forbidden S2(1A1) → S1 (1A2) nonradiative transitions arethe factors responsible for the S2 fluorescence in MT. Lifetime of the S2 state is shorter (varying in the range0.28−3.5 ps in different solvents) than that predicted from the ΔE(S2−S1) value and this can be attributed toits flexible molecular structure, which promotes an efficient intramolecular radiationless deactivation pathways.The lifetime of the S1 state (∼1.9−6.5 ps) is also very short because of small energy difference between theS1 and T1 states (ΔE(S1−T1) ∼ 300 cm-1) in cyclohexane and hydrogen-bonding interaction as well as thepresence of the isoenergetic T1(ππ*) state to enhance the rate of the intersystem crossing process from theS1(nπ*) state in protic solvents.
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