Ultrafast relaxation dynamics of the excited states of Michler's thione

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 absorption and emission as well as femtosecond transient absorption and fluorescence up-conversion spectroscopic techniques. Steady-state fluorescence measurements, following photoexcitation to the S₂ state of MT, reveal weak fluorescence from the S₂ state (φ_F~10_-3 in nonpolar and 10^-4 in polar solvents) but much weaker fluorescence from the S₁ state. Yield of fluorescence from the S₂ state is reduced in polar solvents because of reduced energy gap between the S₂ and S₁ states, ΔE(S₂-S₁), as well as interaction with the solvent molecules. Occurrence of S₂-fluorescence in polar solvents, despite small energy gap, suggests that symmetry allowed S₂(¹A₁)→S₀ (¹A₁) radiative and symmetry forbidden S₂(¹A₁)→S₁ (¹A₁) nonradiative transitions are the factors responsible for the S₂ fluorescence in MT. Lifetime of the S₂ state is shorter (varying in the range 0.28-3.5 ps in different solvents) than that predicted from the ΔE(S₂-S₁) value and this can be attributed to its flexible molecular structure, which promotes an efficient intramolecular radiationless deactivation pathways. The lifetime of the S₁state (~1.9-6.5 ps) is also very short because of small energy difference between the S₁ and T₁ states (ΔE(S₁-T₁) ~300 cm^-1) in cyclohexane and hydrogen-bonding interaction as well as the presence of the isoenergetic T1(pp) state to enhance the rate of the intersystem crossing process from the S₁(nπ^∗) state in protic solvents.