Exploring the Energy Disposal Immediately After Bond-Breaking in Solution: The Wavelength-Dependent Excited State Dissociation Pathways of para-Methylthiophenol

Abstract

A wavelength-resolved ( pump = 295, 285, 270 and 267 nm) photodissociation study of paramethylthiophenol (p-MePhSH) in ethanol solution has been performed using femtosecond transient absorption (TA) spectroscopy and the results compared with those from studies of the corresponding photodissociation in cyclohexane solution at 270 nm. Anisotropy spectra are used to identify the electronic character of the initially populated excited state(s). S-H bond fission is found to occur via the dissociative S 2 (1 1 πσ*) state, which can be populated directly, or by ultrafast non-radiative transitions from the S 3 (2 1 ππ*) state, or by very efficient tunneling from the S 1 (1 1 ππ*) state, depending on the excitation wavelength -in line with conclusions from

Molecular Photofragmentation Dynamics in the Gas and Condensed Phases

“…Extracting the initial product branching in a solution phase dissociation is challenging on account of competing dynamical processes like in-cage recombination, which will encourage any incipient 4-MePhS( Ã ) + H products to re-sample CI-2 and either follow the diabat to reform S 0 parent molecules or re-cross the CI to yield 4-MePhS( X ) + H radicals. In this specific case, kinetic analyses and decay associated spectra suggest a strong preference for initial bond extension to 4-MePhS( Ã ) + H products 98 -in clear contrast to gas-phase studies at the same wavelength. 76,99 The initial X / Ã product branching ratio estimated from equivalent studies of this same photolysis in ethanol is only ~0.8 -highlighting the different early-time dynamics prevailing in the different solution environments.…”

Photochemical reaction dynamics of 2,2′-dithiobis(benzothiazole): direct observation of the addition product of an aromatic thiyl radical to an alkene with time-resolved vibrational and electronic absorption spectroscopy

“…Photodissociation studies in the gas phase, transient absorption spectroscopy in the liquid phase, and ab initio calculations for the p-methylthiophenoxy radical generated from photolysis of pmethylthiophenol all suggest that absorption from the à state may contribute to transient spectra of photochemically generated thiyl radicals. [19][20][21][22] Careful inspection of the inset in Figure 3(a) identifies an absorption feature located at around 400 -480 nm at early time delays (~ 100 fs) that may be a candidate. However, this feature has its maximum absorbance at time zero and can be observed only while the pump and probe beams are overlapped in time ( Figure 3(b) inset).…”

The near ultraviolet photodissociation dynamics of 2- and 3-substituted thiophenols: Geometric vs. electronic structure effects

“…This comparison suggests a small bathochromic (red-)shift on going from the gas phase to n-hexane solution, similar to that found when comparing the near UV absorption spectra of 4-MePhSH in solution in cyclohexane (another weakly interacting solvent) and in the gas phase. 50 Guided by the previous studies of PhSH 26 and the various 4-YPhSH molecules, 38 and the vertical excitation energies and oscillator strengths calculated for the present molecules (Table I), we conclude the following: (i) absorption to the 1 1 ππ* state is the dominant contributor at the longest excitation wavelengths, (ii) this absorption is likely to be supplemented by 1 1 πσ* ← S 0 absorption as the excitation wavelength is reduced, and (iii) 2 1 ππ* ← S 0 absorption accounts for the more intense absorption at shorter wavelengths (peaking at ∼235 nm in all cases).…”

“…Such fragmentation dynamics have been demonstrated in phenol [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] and thiophenol, [24][25][26][27][28][29][30] in a range of substituted phenols [31][32][33][34][35][36] and thiophenols, [37][38][39] and in the methylated analogues anisole 40,41 and thioanisole, [42][43][44][45][46] in both the gas and condensed [47][48][49][50] phase. The present study focusses on thiophenols and, particularly, how the much-studied S-H bond fission process is affected by asymmetric substitution (i.e., in the 2-and 3-positions) of the aromatic ring.…”