Conformational changes upon S1←S excitation in 4-dimethylaminobenzonitrile and some of its chemical analogs

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Abstract
One‐color time‐of‐flight mass spectra (mass resolved excitation spectra) for jet‐cooled 4‐dimethylaminobenzonitrile (4‐DMABN) and some of its chemical analogs, dimethylaniline (DMA), 3‐dimethylaminobenzonitrile (3‐DMABN), N, N‐dimethyl‐4‐(trifluoromethyl)aniline (4‐CF3‐DMA), and 4‐(d6‐dimethylamino)benzonitrile (4‐d6‐DMABN), are presented and analyzed. Near the origin of the S1←S0 transition the low frequency modes can be assigned to motions of the dimethylamino group for this series of molecules. The inversion motion of the dimethylamino group and the dimethylamino group torsion about the Cipso–N bond (the twist coordinate) in S1 give rise to the most prominent peaks in this spectrum. The potential parameters for the twist coordinate of 4‐DMABN and DMA are quite similar in S1: B=0.546, V2=175, and V4=525 cm−1 for 4‐DMABN and B=0.546, V2=175, and V4=515 cm−1 for DMA. The V2 and V4 terms are slightly larger for 3‐DMABN and 4‐CF3‐DMA. The inversion motion is also similar for these molecules but is more anharmonic for the para‐substituted dimethylanilines, 4‐DMABN and 4‐CF3‐DMA, than for the meta and unsubstituted molecules. A Franck–Condon intensity analysis for the dimethylamino twist in these molecules suggests that this group in 4‐DMABN is displaced in the excited state by ∼30° with respect to its planar orientation in the ground state. In both solutions and monosolvate clusters of 4‐DMABN with polar aprotic solvents, a low lying charge transfer (CT) state is identified in addition to the usual ππ* excited state of the bare molecule. The relation between the bare molecule 4‐DMABN twisting displacement upon excitation and the low lying CT state is discussed.