Direct observation of the nuclear motion during ultrafast intramolecular proton transfer

The skeletal motions contributing to the reaction path of the ultrafast excited state intramolecular proton transfer (ESIPT) are determined directly from time resolved measurements. We investigate the ESIPT in the compounds 2-(20-hydroxyphenyl)benzothiazole, 2-(20-hydroxyphenyl)benzoxazole and ortho-hydroxybenzaldehyde by UV–visible pump-probe spectroscopy with 30 fs resolution. The proton transfer is observed in real time and a characteristic ‘ringing’ of the molecule in a small number of vibrational modes is found after the reaction. The results show that a bending motion of the molecular skeleton reduces the proton donor–acceptor distance and an electronic configuration change occurs at a sufficient contraction leading to the bonds of the product conformer. The process evolves as a ballistic wavepacket propagation on an adiabatic potential energy surface. The proton is shifted by the skeletal motions from the donor to the acceptor site and tunneling has not to be considered. q 2004 Elsevier B.V. All rights reserved