Solvation effects on the electronic structure of 4-N, N-dimethylaminobenzonitrile: Mixing of the local ππ* and charge-transfer states

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Abstract
The effect of polar solvents acetonitrile and water on the electronic excited states of 4‐N, N‐dimethylaminobenzonitrile (DMABN) is studied through the optical spectroscopy of small clusters of DMABN/solvent. The clusters are created in a supersonic jet expansion. The results of mass resolved excitation spectroscopy (MRES), fluorescence excitation (FE), dispersed emission (DE), and photodepletion studies demonstrate that the solvent molecule can bind to DMABN at two distinct sites for the one‐to‐one cluster. Both DMABN (H2O)1 clusters generate small blue shifts for the S1←S0 cluster transition and evidence low‐energy vibronic structure nearly identical to that found for the bare molecule. The DMABN (CH3CN)1 clusters behave quite differently. One cluster geometry induces a small blue shift of the S1←S0 electronic transition with little change in its vibronic structure and intensity pattern. We suggest this binding site involves the cyano end of the DMABN molecule. The second cluster geometry induces a large red shift (∼1000 cm−1) and significant broadening (>103 cm−1) of the lowest‐energy transition. This red shifted transition is associated with a charge‐transfer transition within the DMABN molecule lowered in energy due to the acetonitrile coordination with the DMABN aromatic ring. The lowering of the charge‐transfer state in DMABN (CH3CN)n, n=1,...,5 clusters is supported by the following data: long wavelength emission from clusters with broad red shifted absorption; distinct lifetimes for emission at 350 nm (4.6 ns) and 400 nm (6.0 ns); broad red shifted absorption for one geometry of the DMABN (CH3CN)1 cluster. These results support the idea that the charge‐transfer transition in DMABN is stabilized by short‐range dipole–dipole interactions between DMABN and polar nonhydrogen bonding solvents.