O-H bond fission in 4-substituted phenols: S₁ state predissociation viewed in a Hammett-like framework

The photofragmentation dynamics of various 4-substituted phenols (4-YPhOH, Y = H, MeO, CH₃, F, Cl and CN) following π* ← π excitation to their respective S₁ states have been investigated experimentally (by H Rydberg atom photofragment translational spectroscopy) and/or theoretically (by ab initio electronic structure theory and 1- and 2-D tunnelling calculations). Derived energetic and photophysical properties such as the O–H bond strengths, the S₁–S₀excitation energies and the S₁ predissociation probabilities (by tunnelling through the barrier under the conical intersection between the S₁(1¹ππ*) and S₂(1¹πσ*) potential energy surfaces in the R_O–H stretch coordinate) are considered within a Hammett-like framework. The Y-dependent O–H bond strengths and S₁–S₀ term values are found to correlate well with a simple descriptor of the electronic perturbation caused by the aromatic substituent Y (the Hammett constant, σ⁺_p). We also identify clear correlations between σ⁺_p and the probability of a photochemical process (predissociation). Such a finding is unsurprising, given that Y substitution will perturb the entire potential energy landscape, but appears not to have been demonstrated hitherto. The predictive capabilities of this approach are explored by reference to existing energetic data for larger 4-substituted phenols like 4-ethoxyphenol, tyramine, L-tyrosine and tyrosine containing di- and tri-peptides.