Excited state dynamics of a push–pull stilbene: A femtosecond transient absorption spectroscopic study

The excited state dynamics of a structurally flexible push–pull stilbene, namely, trans-4-(N,N-dimethylamino)-4′-nitrostilbene (DMANS), in solvents of varying polarities, H-bonding abilities and viscosities, has been investigated using femtosecond transient absorption spectroscopic technique. Results of DFT and TDDFT calculations have been used to explain the experimental data. Following photoexcitation of the planar DMANS molecule to the excited singlet (S1) state, it undergoes cis–trans isomerization via twisting of the double bond. However, in solvents of medium and large polarities, the twisted intramolecular charge transfer (TICT) process involving only the rotation of the nitrophenyl group, but not the rotation of the dimethyl aniline or nitro group, is the dominant relaxation process in the excited state of DMANS, in polar solvents.