Effects of Ring Fluorination on the Ultraviolet Photodissociation Dynamics of Phenol

The dynamics of photoinduced O–H bond fission in five fluorinated phenols (2-fluorophenol, 3-fluorophenol, 2,6-difluorophenol, 3,4,5-trifluorophenol, and pentafluorophenol) have been investigated by H Rydberg atom photofragment translational spectroscopy following excitation at many wavelengths in the range 220 ≤ λ ≤ 275 nm. The presence of multiple fluorine substituents reduces the efficiency of O–H bond fission (by tunneling) from the first excited (11ππ*) electronic state, whereas all bar the perfluorinated species undergo O–H bond fission when excited at shorter wavelengths (to the 21ππ* state). As in bare phenol, O–H bond fission is deduced to occur by non-adiabatic coupling at conical intersections between the photoprepared “bright” ππ* states and the 11πσ* potential energy surface. In all cases, the fluorophenoxyl photoproducts are found to be formed in a range of vibrational levels, all of which include an odd number of quanta (typically one) in an out-of-plane (a″) vibrational mode; this product vibration is viewed as a legacy of the parent out-of-plane motions that promote non-adiabatic coupling to the dissociative 11πσ* potential. The radical products also show activity in in-plane vibrations involving coupled (both in- and out-of-phase) C–O and C–F wagging motions, which can be traced to the impulse between the recoiling O and H atoms and, in detail, are sensitive to the presence (or not) of an intramolecular F···H–O hydrogen bond. Upper limit values for the O–H bond dissociation energies are reported for all molecules studied apart from pentafluorophenol.