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 pi* <- pi excitation to their respective S-1 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 S-1-S-0 excitation energies and the S-1 predissociation probabilities (by tunnelling through the barrier under the conical intersection between the S-1(1(1)pi pi*) and S-2(1(1)pi sigma*) potential energy surfaces in the RO-H stretch coordinate) are considered within a Hammett-like framework. The Y-dependent O-H bond strengths and S-1-S-0 term values are found to correlate well with a simple descriptor of the electronic perturbation caused by the aromatic substituent Y (the Hammett constant, sigma(+)(p)). We also identify clear correlations between sigma(+)(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.