The people involved: O. F. Mohammed, J. Dreyer, E. T. J. Nibbering, T. Elsaesser
Former team members: C. Chudoba, F. Tschirschwitz, M. Rini, A. Kummrow, M. Pfeiffer, A. Usman

Excited state intramolecular hydrogen transfer

1. Vibrational perspective of hydrogen/proton transfer: the idea is to probe the hydrogen/proton transfer reaction by inspection of specific vibrational marker modes of either the reactant or product states. In the case of Excited State Hydrogen Transfer (ESIHT) in 2-(2'-hydroxyphenyl) benzothiazole (HBT) we monitor the infrared active vibrations in the fingerprint region, in particular the C=O stretching band of the keto*-state located at 1530 cm-1.

2.  2-(2'-hydroxyphenyl)benzothiazole (HBT) exists in the enol form in the electronic ground state. After electronic excitation to the S1 state a rapid hydrogen transfer occurs towards the keto*-state. The latter product state can be probed by inspection of the infrared active C=O stretching band.

3. Typical features observed in spectrally resolved femtosecond UV pump / IR probe spectroscopy. At negative delay times contributions occur due to the perturbed free induction decay of vibrational bands in the electronic ground state. Around zero delay solvent signals appear that are frequency independent with comparable magnitudes as the solute bands. At postive delay times the bleach signals of the reactant modes and absorbance signals due to product bands contribute to the pump-probe results.

4. The C=O stretching band marking the appearance of the keto*-state shows a delayed rise, indicating a delayed hydrogen transfer process. Subsequent blue shifting of the C=O band is clearly seen in the transient spectra.

5. After the hydrogen transfer process the C=O band position is a) modulated by anharmonically coupled low frequency modes that have been coherently excited by the UV pump pulse or by the hydrogen transfer process. The blue-shifting behaviour with clear sub-picosecond and picosecond components indicate the ongoing intramolecular vibrational redistribution and vibrational cooling processes through which the excess energy of the hydrogen transfer is channeled, thereby making the hydrogen transfer process ultrafast and irreversible.

infrared active out-of-plane twisting mode

infrared-active twisting mode

Raman active in-plane deformation mode

6. The coherent modulations with frequencies of 60 and 120 cm-1 are ascribed to the infrared active out-of-plane twisting mode (top) and the Raman active in-plane deformation mode (bottom) respectively. The latter is coherently excited by the UV pump pulse, the other is impulsively excited before/during the hydrogen transfer process, but not by the optical pump pulse. The twisting mode (middle) is assumed not to contribute to the coherent oscillations in the pump-probe signals as it does not significantly modulate the hydrogen bond distance.