Stefan Lochbrunner, Christian Schriever, Eberhard Riedle

Coherent Wavepacket Dynamics on Parallel Reaction Paths Molecules exhibiting excited state intramolecular proton transfer (ESIPT) are excellent model systems to study the evolution of an initially prepared wavepacket and the role of skeletal motions in ultrafast reactions. For systems with parallel reaction channels, the question arises if in this situation coherent wavepacket dynamics can be used to characterize the different channels and get an improved understanding of the mechanisms. We investigate the double proton transfer system [2,2'-Bipyridyl]-3,3'-diol (BP(OH)2) dissolved in cyclohexane. The molecule contains two H-chelate rings where at each ESIPT can occur. After optical excitation the molecule can undergo double proton transfer in a concerted or sequential fashion leading to the diketo form respectively to a monoketo intermediate. The monoketo yield increases with excitation energy. The monoketo intermediate transforms in a second step to the final diketo within 10 ps. In the spectral region of the product fluorescence at 630 nm we find an emission rise after a significant delay with respect to the pump pulse. The delay is interpreted as the time the optically prepared wavepacket needs to travel from the Franck-Condon region to the product minimum. Strong oscillations are observed in all traces. The initial motion is driven by the electronic reconfiguration due to the optical excitation. A further configuration change occurs shortly before the product minimum, namely the change from the enol to the keto form. The oscillating signal components originate from coherently excited vibrational modes in the product form of the proton transfer. The Fourier transform of the oscillatory components shows two dominating contributions, one at 196 cm-1 and the other at 295 cm-1.We performed DFT calculations of BP(OH)2 in the electronic ground state and found two vibrational normal modes fitting nicely to the observed frequencies. Bending motion with 196 cm-1. Stretching motion with 295 cm-1. Changing the excitation from 375 nm to 350 nm increases the yield for the monoketo intermediate by a factor of 3 from 10 % to 30 %. Correspondingly, the amplitude of the signal oscillations at 196 cm-1 is much smaller for pumping at 375 nm than at 350 nm. Obviously, the 196 cm-1 mode is more strongly excited if more monoketo is generated, demonstrating that this mode contributes in fact to the single proton transfer. The oscillatory contribution at 295 cm-1 is of comparable strength for both excitation wavelengths. Since the yield for the concerted transfer varies only by a factor of 1.3 the strength of the 295 cm-1 mode is much less affected by the excitation wavelength.

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

"Ultrafast double proton transfer: symmetry breaking wavepacket motion and absence of deuterium isotope effect" S. Lochbrunner, K. Stock, C. Schriever, and E. Riedle Ultrafast Phenomena XIV, T. Kobayashi, T. Okada, T. Kobayashi, K. Nelson, S. De Silvestri, eds. (Springer-Verlag, Berlin, 2005), 491 - 495 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 "Symmetry Breaking Wavepacket Motion and Absence of Deuterium Isotope Effect in Ultrafast Excited State Proton Transfer" S. Lochbrunner, K. Stock, C. Schriever, and E. Riedle Femtochemistry and Femtobiology: Ultrafast Events in Molecular Science, M. M. Martin and J. T. Hynes, eds. (Elsevier, Amsterdam, 2004), 193 - 196 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 "Highly localized vibronic wavepackets in large reactive molecules" A. J. Wurzer, S. Lochbrunner, E. Riedle Applied Physics B 71, 405 - 409 (2000) Details