S₁-State Decay Dynamics of Benzenediols (Catechol, Resorcinol, and Hydroquinone) and Their 1:1 Water Clusters

Abstract

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.

πσ*-Mediated Nonadiabatic Tunneling Dynamics of Thiophenols in S1: The Semiclassical Approaches

“…This also implies that the S–H tunneling dissociation of thiophenol (S 1 ) would be largely influenced by the upper-lying S 1 /S 2 conical intersection responsible for the shaping of the underlying adiabatic tunneling barrier, as it is located much closer to the S 1 minimum compared to the case of phenol. In this aspect, we focus (although the O–H tunneling dynamics of phenol and its analogs − have also been found to be extremely interesting) on the study of the S–H (or S–D) dissociation dynamics of the S 1 thiophenol or its chemical derivatives in terms of the mode specificity and the kinetic isotope effect as it may unravel the detailed dynamic influence of the conical intersection on the tunneling reactions.…”

Dynamic Role of the Intramolecular Hydrogen Bonding in the S1 State Relaxation Dynamics Revealed by the Direct Measurement of the Mode-Dependent Internal Conversion Rate of 2-Chlorophenol and 2-Chlorothiophenol

“…In the predissociation event (e.g., the S–CH 3 bond dissociation of the S 1 thioanisole), − the reactive flux placed in the proximity of the conical intersection either nonadiabatically funnels through the narrowly defined conical intersection or sticks to the adiabatic potential energy surfaces to explore the phase space for riding on the minimum energy reaction path. For the tunneling case, the light atom (H or D) escapes from the S 1 potential well via tunneling through the barrier which is dynamically shaped by the upper-lying conical intersection. − Notably, the tunneling dynamics of the S 1 state has recently been thoroughly investigated in a state-specific way for a number of heteroaromatic molecular systems such as phenol, ,− substituted phenols, − o -cresol, , thiophenol, , 2-fluorothiophenol, ,, 2-methoxythiophenol, − or 2-chlorothiophenol. ,, It has been found that the tunneling dynamics in terms of the tunneling rate (also the energy disposal dynamics or nonadiabatic transition probability in some cases) are quite mode-dependent, and their qualitative behaviors could be rationalized from the dynamic shaping of the tunneling barrier by the upper-lying S 1 (ππ*)/S 2 (πσ*) conical intersection in many cases. ,,, …”