Ultrafast charge transfer via a conical intersection in dimethylaminobenzonitrile

Abstract

The L_a-like S₂ state (2A) of 4-(dimethylamino)benzonitrile was pumped at 267 nm in the gas phase at 130 °C. Nonresonant multiphoton ionization at 800 nm with mass-selective detection then probed the subsequent processes. Whereas ionization at the Franck–Condon geometry only gave rise to the parent ion, fragmentation increased on motion towards the charge-transfer (CT) state. This useful difference is ascribed to a geometry-dependent resonance in the ion. The time constants found are interpreted by ultrafast (≈68 fs) relaxation through a conical intersection to both the CT and the L_b-type S₁ state (1B). Then the population equilibrates between these two states within 1 ps. From there the molecule relaxes within 90 ps to a lower excited state which can only be a triplet state (T_n) and then decomposes within 300 ps. Previous experiments either investigated only 1B→CT relaxation—which does not take place in the gas phase or nonpolar solvents for energetic reasons—or, starting from S₂ excitation, typically had insufficient time resolution (>1 ps) to detect the temporary charge transfer. Only recently temporary population of the CT state was found in a nonpolar solvent (Kwok et al., J. Phys. Chem. A, 2000, 104, 4188), a result fully consistent with our mechanism. We also show that S₂→S₁ relaxation does not occur vertically but involves an intermediate strong geometrical distortion, passing through a conical intersection.