Fluorine Substitution Effects on the Photodissociation Dynamics of Iodobenzene at 304 nm

Abstract

The photodissociation dynamics of pentafluoroiodobenzene are investigated by state-selective one-dimensional translation spectroscopy at 304 nm. We have determined the one-dimensional recoil distribution and the spatial distribution in the form of the anisotropy parameter, β, as well as the photodissociation relative yields of both ground-state I(3P3/2) and excited-state I*(2P1/2) iodine photofragments. The results are compared to those observed for iodobenzene at 304 nm. As in iodobenzene, two velocity distributions were observed for the dissociation channel which gives ground-state iodine: a sharp, high recoil velocity peak assigned previously to n,σ* excitation and a slow recoil velocity distribution peak assigned previously to π,π* excitation. Unlike in C6H5I, the I* distribution is relatively strong and its spatial anisotropy can be measured. The fluorine perturbation has led to a number of different observations that can be summarized as follows: (1) The high velocity distribution has a lower average value and much broader width, suggesting more rapid energy redistribution to the fluorinated phenyl ring prior to and during the dissociation process, resulting from stronger coupling between the n,σ* and π,π* states and/or a longer excited-state lifetime; (2) the slow distribution is weaker and has an almost isotropic spatial distribution (the anisotropy parameter β ≈ 1.0), while in the iodobenzene spectrum β is correlated with the recoil velocity; (3) the I* quantum yield for C6F5I is 14 times larger than that for iodobenzene; and (4) β is correlated with the velocity in the I* spectrum found for C6F5I which is not observed for iodobenzene. These observed fluorine perturbations are attributed to an increased mixing between the charge-transfer state (resulting from electron transfer from the iodine nonbonding electrons to the π* orbitals of the fluorinated benzene ring) and both the n,σ* and the ring π,π* states. This leads to two effects: (1) a decrease in the nonbonding electron density on the iodine, which decreases the spin-orbit interaction between the n,σ* states themselves, resulting in a decrease in the curve-crossing probability (thus increasing the I* yield) and (2) an increase in the coupling between the repulsive n,σ* states and the fluorinated phenyl π,π* states, leading to an increase in the rate of energy redistribution.