Ultrafast vibrational relaxation processes induced by intramolecular excited state hydrogen transfer

Vibrational spectra between 1000 and 1700 cm−1 are studied after ultrafast transfer of a hydrogen atom in the excited state of 2-(2′-hydroxyphenyl)benzothiazole. Femtosecond pump–probe experiments reveal new vibrational bands of the keto-S1 state, including the carbonyl stretching band formed by hydrogen transfer. Such bands display a negligible spectral reshaping but blue-shift by up to 7 cm−1 following biexponential kinetics with time constants of 700 fs and 15 ps. The blue-shift is attributed to the anharmonic coupling of the fingerprint vibrations to Raman-active low-frequency modes that are excited upon electronic excitation and depopulated by intramolecular redistribution and cooling to the solvent.