Stefan Lochbrunner, Christian Schriever, Eberhard Riedle

Observing Proton Transfer in Real Time The understanding of hydrogen dynamics is of crucial importance in chemistry and molecular biology. Excited state intramolecular proton transfer (ESIPT) is in particular well suited to study this subject. Due to its intramolecular character ESIPT can be investigated under well defined conditions and it can be directly initiated by absorption of photons from a laser pulse allowing time resolved experiments with an extremely high precision. In addition, ESIPT allows to study the basic features of a chemical reaction, i. e. bond fission and formation in a comparative simple situation. We investigated several molecules showing excited state intramolecular proton transfer (ESIPT) by pump probe absorption spectroscopy with a time resolution of 20 fs. They all contain an H-chelate ring as reaction center. After optical excitation the hydrogen atom of the hydroxy group in the chelate ring is transferred to a proton acceptor at the opposite site in the ring. Two of them, 2-(2'-hydroxyphenyl)benzothiazole (HBT) and 2-(2'-hydroxyphenyl)benzoxazole (HBO) have a nitrogen atom as proton acceptor. In the case of the other two, o-hydroxybenzaldehyde (OHBA) and 1-hydroxy-2-acetonaphtone (HAN) the proton switches between two oxygen atoms. For all molecules we encounter a similar situation. The emission rise after the optical excitation is delayed by about 50 fs and reflects the arrival of the launched wavepacket in the product configuration. Strong oscillatory signal contributions are the signature of coherently excited vibrational motions. The observed frequencies give direct insight in the reaction coordinate. The contributing modes are in-plane deformations of the H-chelate ring. The mode of lowest frequency is a bending motion of the entire molecule which reduces primarily the donor acceptor distance. It reflects the initial motion after the optical excitation. When the donor acceptor distance has shortened to a certain extent an electronic configuration change and a redistribution of bonds occur. The new bonds are associated with a new equilibrium geometry corresponding to the product well of the PES. At first the molecule has not yet adopted this geometry and is displaced from the product well. It accelerates towards the well and vibrational modes are coherently excited which reflect the changes in geometry leading to the equilibrium. ESIPT allows to study fundamental issues associated with the wavepacket dynamics in a reactive systems. By looking at double proton transfer we investigate the questions What happens in parallel reaction channels ? Are the vibrational modes coherently excited by the optical pump process or by the reaction itself ?

Former co-workers: Kai Stock, Vincent de Waele, Alexander Wurzer

"Ultrafast proton transfer of 1-hydroxy-2-acetonaphthone: Reaction path from resonance Raman and transient absorption studies" S. Lochbrunner, A. Szeghalmi, K. Stock, and M. Schmitt J. Chem. Phys. 122, 244315 (2005) Details "Direct observation of the nuclear motion during ultrafast intramolecular proton transfer" S. Lochbrunner, K. Stock, and E. Riedle J. Mol. Struct. 700, 13 - 18 (2004) Details "The microscopic mechanism of ultrafast excited state intramolecular proton transfer: a 30 fs study of 2-(2'-hydroxyphenyl)benzothiazole" S. Lochbrunner, A. J. Wurzer, and E. Riedle J. Phys. Chem. A 107, 10580 - 10590 (2003) Details "Ultrafast excited state proton transfer of 2-(2'-hydroxyphenyl)benzothiazole: Theoretical analysis of the skeletal deformations and the active vibrational modes" R. de Vivie-Riedle, V. De Waele, L. Kurtz, and E. Riedle J. Phys. Chem. A 107, 10591 - 10599 (2003) Details "Exploring the multidimensional character of ultrafast molecular processes" S. Lochbrunner and E. Riedle Recent Res. Devel. Chem. Physics 4, 31 - 61 (2003) Details "Proton transfer and internal conversion of o-hydroxybenzaldyde: Coherent versus statistical excited state dynamics" K. Stock, T. Bizjak, and S. Lochbrunner Chem. Phys. Lett. 354 (2002), 409-416 Details "Dynamics of excited-state proton transfer systems via time-resolvedphotoelectron spectroscopy" S. Lochbrunner, T. Schultz, M. Schmitt, J.P. Shaffer, M.Z. Zgierski and A. Stolow J. Chem. Phys. 114 (2001), 2519-2522 Details "Ultrafast excited state proton transfer and subsequent coherent skeletal motion of 2-(2'-hydroxyphenyl)benzothiazole" S. Lochbrunner, A. J. Wurzer, E. Riedle J. Chem. Phys. 112, 10699 - 10702 (2000) Details