Coherent and Homogeneous Intramolecular Charge-Transfer Dynamics of 1-tert-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6), a Rigid Analogue of DMABN

Abstract

We report the intramolecular charge-transfer (ICT) dynamics of 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6), a planar analogue of 4-(dimethylamino)benzonitrile (DMABN), by using time-resolved fluorescence (TRF) and TRF spectra (TRFS). TRFS allow accurate determination of the ICT dynamics free from the spectral relaxation caused by the solvation and vibronic relaxation. For NTC6 in tetrahydrofuran (THF), the locally excited (LE) state is populated exclusively presumably via a conical intersection from the initial photoexcited S2 (La) state, and the LE state undergoes ICT single exponentially with a time constant of 1.8 ± 0.2 ps. In acetonitrile, however, both LE (22%) and ICT (78%) states are populated from the S2 state, and the population in the LE state undergoes ICT in 800 ± 100 fs. The ICT state undergoes further relaxation in 1.2 ps along the solvation and the intramolecular nuclear coordinates involving the rotation of the amino group to form a twisted ICT state. Coherent nuclear wave packet motions of 130 cm(-1), which can be assigned to the -C≡N group bending mode, were observed in the TRF of the reactant (LE) and product (ICT) states, indicating that the ICT reaction is partially coherent. Compared with DMABN, the ICT dynamics of NTC6 are quite homogeneous, and we speculated on the narrow conformational distribution of NTC6 in the ground state along the rotation of the amino group due to its rigid structure.