Ultrafast excited-state dynamics associated with the photoisomerization of trans-4-diethylaminoazobenzene in solution

•The photoisomerization of trans-4-DEAAB was investigated by femtosecond transient absorption spectroscopy and quantum chemical calculations.•Following excitation, the S2 state directly decays to the S1 state by internal conversion with time constant of ∼0.10 ps.•The isomerization occurs by inversion mechanism on the S1 state.•It is H-bond that results in the shorter timescale of τ3 in alcohols than in acetone.

The ultrafast dynamics of S2-excited trans-4-diethylaminoazobenzene (trans-4-DEAAB) is investigated in solution by femtosecond transient absorption spectroscopy combined with quantum chemical calculations. Following excitation, the internal conversion from the S2 state to the S1 state occurs directly with time constant of ∼0.10 ps. The cis isomer is observed by the partial recovery of ground-state bleach in 450–480 nm and the permanent positive absorption in 480–550 nm. The time constant of <1 ps is assigned to the lifetime of the S1 state which decays by isomerization to the cis-isomer hot S0 state and internal conversion to the trans-isomer hot S0 state. The photoisomerization is deduced by inversion mechanism because of the independence between the viscosity and the S1-state lifetime. The vibrational cooling of the hot S0 state of cis-4-DEAAB occurs with ∼15 ps in acetone but is shortened to ∼6 ps in alcohols solvents. This shortening is due to the H-bond which forms easily in protic solvent and accelerates the flow of vibrational energy between solute and solvent.

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