Vibrational population relaxation of hydrogen-bonded phenol complexes in solution: Investigation by ultrafast infrared pump-probe spectroscopy

We investigated the vibrational population relaxation of the OH stretching mode of hydrogen-bonded phenol complexes in solution by ultrafast infrared spectroscopy. For a phenol-benzonitrile complex, vibrational population relaxation takes place at around 5 ps. The decay time constants of the pump-probe signals do not depend on the probe frequency. We also measured the pump-probe signals of the OH stretching vibrations for phenol-acetone, phenol-diethylether and phenol-tetrahydrofuran complexes. We found that the time scales of vibrational population relaxation for these complexes are correlated with hydrogen bond strength; i.e., stronger hydrogen bonding leads to faster vibrational population relaxation. We did not observe a clear probe frequency dependence in relaxation rates. For these complexes, rapid evolution between different conformations causes fast fluctuation in the vibrational frequency of the OH stretching modes. This fluctuation averages out the vibrational population relaxation times of different conformations. In contrast, the decay time constants of pump-probe signals for hydrogen-bonded phenol oligomers were found to depend significantly on probe frequency. The decay time constant at the blue side (3521 cm−1) was 2.1 ps, while that at the red side (3320 cm−1) was 0.7 ps. We discuss the probe frequency dependence of vibrational population relaxation times for hydrogen-bonded phenol oligomers in solution.