Solvation Effects on the Electronic Structure of 4-N,N-Dimethylaminobenzonitrile:Mixing of the Local Pi(pi) and Charge Transfer States

摘要

The effect of polar solvents acetonitrile and water on the electronic excitedstates 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 photo-depletion studies demonstrate that the solvent molecule can bind to DMABN at two distinct sites for the 1:1 cluster. Both DMABN(H2O)1 clusters generate small blue shifts for the 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 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 (ca. 1000 per centimeter and significant broadening (>1000 per centimeter) 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.