Highly localized vibronic wavepackets in large reactive molecules

Abstract

The ultrafast dynamics of the internal conversion of S1 azulene and the excited-state intramolecular proton transfer in 2-(2′-hydroxyphenyl)benzothiazole (HBT) were investigated by pump–probe spectroscopy with tunable pulses as short as 20 fs. In both cases we find very pronounced oscillatory contributions to the transients, which are due to vibrational wavepacket motion in the excited state. The damping times are on the order of 1 ps even for these large reactive molecules in solution at room temperature. For azulene only 2 out of 48 vibrational modes participate and in HBT only 4 out of 69. This high degree of localization of the wavepacket seems to be a general feature, and supports hopes that even for systems of chemical interest coherent control might be possible.