Contrasting the excited state reaction pathways of phenol and para-methylthiophenol in the gas and liquid phases

Abstract

To explore how the solvent influences primary aspects of bond breaking, the gas and solution phase photochemistries of phenol and of para-methylthiophenol are directly compared using, respectively, H (Rydberg) atom photofragment translation spectroscopy and femtosecond transient absorption spectroscopy. Approaches are demonstrated that allow explicit comparisons of the nascent product energy disposals and dissociation mechanisms in the two phases. It is found, at least for the case of the weakly perturbing cyclohexane environment, that most aspects of the primary reaction dynamics of the isolated molecule are reproduced in solution. Specifically, in the gas phase, both molecules can undergo fast X–H (XO, S) bond dissociation upon excitation with short wavelengths (193 < λ_pump < 216 nm), following population of the dissociative S₂ (1¹πσ*) state. Product electronic branching, vibrational and translational energy disposals are determined. Photolysis of phenol and para-methylthiophenol in solution at 200 nm results in formation of vibrationally excited radicals on a timescale shorter than 200 fs. Excitation of para-methylthiophenol at 267 nm reaches close to the S₁ (1¹ππ*)/S₂ (1¹πσ*) conical intersection (CI): ultrafast dissociation is observed in both the isolated and solution systems—again indicating direct dissociation on the S₂ potential energy surface. Comparing results for this precursor at different excitation energies, the extent of geminate recombination and the derived H-atom ejection lengths in the condensed phase photolyses are in qualitative agreement with the translational energy release measured in the gas phase studies. Conversely, excitation of phenol at 267 nm prepares the system in its S₁ state at an energy well below its S₁/S₂ CI; the slow O–H bond fission inferred in the gas phase experiments is observed directly in the time-resolved studies in cyclohexane solution via the appearance of phenoxyl radical absorption after ∼1 ns, with only S₁ excited state absorption discernible at earlier delay times. The slow O–H bond fission in solution provides additional evidence for a tunnelling dissociation mechanism, where the H atom tunnels beneath the lower diabats of the S₂/S₁ CI. Finally, the photodissociation of phenol clusters in solution is considered, where evidence is presented that the O–H dissociation coordinate is impeded in H-bonded dimers.