Ultrafast charge transfer and coherent oscillations in 4-piperidino- benzonitrile

The L_a-like S₂ state of 4-(1-piperidino) -benzonitrile and the corresponding 3,5-dimethyl-piperidino derivative was excited at 270 nm in the gas phase at 130°C. Nonresonant multiphoton ionization at infrared wavelengths (810 and 1300 nm) with mass-selective detection of the ion yields then probed the subsequent processes. Two groups of time constants were found: in the range <2 ps and in the 100 ps range. In addition, a coherent oscillation with period 1.06 and 1.18 ps for the unsubstituted and substituted species were monitored until about 10 ps. The sub-picosecond time constants can be assigned to relaxation from S₂ through a conical intersection to both S₁ (L_b-like) and a charge transfer (CT) state. These two states then equilibrate within about 1.9 ps. The oscillation must be a vibration in L_b, because this is the only state still populated at 10 ps. It is suggested to assign it to the amino-group wagging (planarization/pyramidalization). It indicates a component of the direction into which the conical intersection is displaced from the L_b minimum. The ultrafast relaxation times suggest, however, that additional coordinates are involved in the process, probably amino-group twist and bond stretching in the benzene ring.