Chemical reactions in isolated clusters: Excited state electron transfer in 3- and 4-dimethylaminobenzonitrile

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Abstract
Two‐color time‐of‐flight mass spectra of 3‐ and 4‐dimethylaminobenzonitrile (3‐ and 4‐DMABN) bare molecules and clusters with methane, water, acetone, dichloromethane, and acetonitrile are reported and discussed. The clusters and molecules are isolated and cooled in a supersonic expansion. Both 3‐ and 4‐DMABN bare molecules display significant changes in geometry, associated with rotation–inversion coordinates of the dimethylamino group, upon excitation from the ground electronic stateS 0 to the first excited singlet state S 1. Cluster spectra for the monosolvates [CH4, H2O, (CH3)2CO, CH2Cl2, CH3CN] of 3‐ and 4‐DMABN evidence two general types of behavior. (1) Cluster spectra have both red and blue shifts from their respective bare molecule origins which are relatively small (in general less than 200 cm− 1). These cluster spectra are nearly identical with the bare molecule spectra; the solvation process seems to have little effect on the DMABN molecule, especially the –N(CH3)2 moiety, for these clusters or cluster states. (2) Cluster spectra have shifts that are large (∼500–1000 cm− 1) and to low energy of the bare molecule spectra. These cluster spectra are composed of both sharp and broad features which bear little resemblance to those observed in the respective bare DMABN molecule spectra. Results of calculations of cluster geometry suggest that thedifferent cluster spectra may be associated with clusters of different geometry. These data are interpreted in terms of solution behavior of 3‐ and 4‐DMABN. The relationship between the twisted intramolecular charge transfer (TICT) state model proposed for 4‐DMABN in dipolar aprotic solvents and these cluster results is explored.