S-1-State Decay Dynamics of Benzenediols (Catechol, Resorcinol, and Hydroquinone) and Their 1:1 Water Clusters

Abstract

The S-1-state decaying rates of the three different benzenediols, catechol, resorcinol, and hydroquinone, and their 1:1 water clusters have been state-specifically measured using the picosecond time-resolved parent ion transients obtained by the pump (excitation) and probe (ionization) scheme. The S-1 lifetime of catechol is found to be short, giving tau similar to 5.9 ps at the zero-point level. This is ascribed to the H-atom detachment from the free OH moiety of the molecule. Consistent with a previous report (J. Phys. Chem. Lett. 2013, 4, 3819-3823), the S-1 lifetime gets lengthened with low-frequency vibrational mode excitations, giving tau similar to 9.0 ps for the 116 cm(-1) band. The S-1 lifetimes at the additional vibronic modes of catechol are newly measured, showing the nonnegligible modedependent fluctuations of the tunneling rate. When catechol is complexed with water, the S-1 lifetime is enormously increased to tau similar to 1.80 ns at the zero-point level while it shows an unusual dip at the intermolecular stretching mode excitation (t similar to 1.03 ns at 146 cm(-1)). Otherwise, it is shortened monotonically with increasing the internal energy, giving t similar to 0.67 ns for the 856 cm(-1) band. Two different asymmetric or symmetric conformers of resorcinol give the respective S-1 lifetimes of 4.5 or 6.3 ns at their zero-point levels according to the estimation from our transients taken within the temporal window of 0-2.7 ns. When resorcinol is 1:1 complexed with H2O, the S-1 decaying rate is slightly accelerated for both conformers. The S-1 lifetimes of trans and cis forms of hydroquinone are measured to be more or less same, giving tau similar to 2.8 ns at the zero-point level. When H2O is complexed with hydroquinone, the S-1 decaying process is facilitated for both conformers, slightly more efficiently for the cis conformer.