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

The ultrafast dynamics of S 2 -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 S 2 state to the S 1 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 S 1 state which decays by isomerization to the cis -isomer hot S 0 state and internal conversion to the trans -isomer hot S 0 state. The photoisomerization is deduced by inversion mechanism because of the independence between the viscosity and the S 1 -state lifetime. The vibrational cooling of the hot S 0 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.