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 (S-1) 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 sigma(1) levels in complexes using picosecond pump-probe spectroscopy. Excitation energies to the S-1 (pi pi*) and S-2 (pi sigma*) 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 S-1 optimized geometries. The observed lifetimes and the energy gaps between the (pi pi*) and (pi sigma*) 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 S-1/S-2 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 pi pi* and pi sigma* states occurring through the out-of-plane distortion of the C-F bond. (C) 2010 American Institute of Physics. [doi:10.1063/1.3480396]