Ultrafast temporary charge transfer in pyrrolidinyl–benzonitrile and pyrrolyl–benzonitrile in the gas phase

Abstract

4-(N-pyrrolidinyl)benzonitrile and 4-(N-pyrrolyl)benzonitrile were excited by an ultrashort pulse at 270 nm to their L_a (S₂) states and then probed by ionization at long wavelengths. Parent and fragment ion signals show components with time constants <100 fs which we attribute to ultrafast relaxation to the L_b (S₁) state. From this short time we infer a conical intersection between the L_b and L_a surfaces. The wave packet can branch there, one part going temporarily to a strongly displaced state. Its shift was concluded from an anisotropy observed only there. The only excited state known to have a large displacement is the charge-transfer (CT) state. The positive anisotropy indicates that the CT state belongs to the 2A species for both molecules. For pyrrolylbenzonitrile, this is in contrast to previous assignments. The anisotropy, and a coherent oscillation observed in pyrrolidinylbenzonitrile, support the idea that the amino-group twist is an important component of the CT reaction coordinate.