The S1-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 S1 lifetime of catechol is found
to be short, giving τ ∼ 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 S1 lifetime gets lengthened with low-frequency vibrational
mode excitations, giving τ ∼ 9.0 ps for the 116 cm–1 band. The S1 lifetimes at the additional
vibronic modes of catechol are newly measured, showing the nonnegligible
mode-dependent fluctuations of the tunneling rate. When catechol is
complexed with water, the S1 lifetime is enormously increased
to τ ∼ 1.80 ns at the zero-point level while it shows
an unusual dip at the intermolecular stretching mode excitation (τ
∼ 1.03 ns at 146 cm–1). Otherwise, it is
shortened monotonically with increasing the internal energy, giving
τ ∼ 0.67 ns for the 856 cm–1 band.
Two different asymmetric or symmetric conformers of resorcinol give
the respective S1 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 S1 decaying rate is slightly accelerated
for both conformers. The S1 lifetimes of trans and cis forms of hydroquinone are measured to be
more or less same, giving τ ∼ 2.8 ns at the zero-point
level. When H2O is complexed with hydroquinone, the S1 decaying process is facilitated for both conformers, slightly
more efficiently for the cis conformer.