Excited state hydrogen transfer dynamics in substituted phenols and their complexes with ammonia: $ππ∗-πσ∗$ energy gap propensity and ortho-substitution effect

Lifetimes of the first electronic excited state $(S1)$ of fluorine and methyl (⁠$o-$⁠, $m-$⁠, and $p-$⁠) substituted phenols and their complexes with one ammonia molecule have been measured for the $00$ transition and for the intermolecular stretching $σ1$ levels in complexes using picosecond pump-probe spectroscopy. Excitation energies to the $S1$ (⁠$ππ∗$⁠) and $S2$ (⁠$πσ∗$⁠) states are obtained by quantum chemical calculations at the MP2 and CC2 level using the aug-cc-pVDZ basis set for the ground-state and the $S1$ optimized geometries. The observed lifetimes and the energy gaps between the $ππ∗$ and $πσ∗$ states show a good correlation, the lifetime being shorter for a smaller energy gap. This propensity suggests that the major dynamics in the excited state concerns an excited state hydrogen detachment or transfer (ESHD/T) promoted directly by a $S1/S2$ conical intersection, rather than via internal conversion to the ground-state. A specific shortening of lifetime is found in the o-fluorophenol-ammonia complex and explained in terms of the vibronic coupling between the $ππ∗$ and $πσ∗$ states occurring through the out-of-plane distortion of the C–F bond.