UV photodissociation dynamics of iodobenzene: Effects of fluorination

The UV photochemistry of various fluorinated iodobenzenes (4-fluoro-, 2,4-difluoro-, 3,5-difluoro-, and perfluoro-iodobenzene) has been investigated at many wavelengths by velocity map imaging, time-resolved near infrared absorption spectroscopy and (spin-orbit resolved) ab initio calculations of the ground and excited state potentials along the C-I stretch coordinate, RC-I. The textbook description of the near UV photochemistry of CH3I, i.e., sigma* <- n excitation to the (3)Q(0+) state, followed by direct dissociation (to yield spin-orbit excited iodine atom (I*) products) or by non-adiabatic coupling via a conical intersection (CI) with the (1)Q(1) potential (to yield ground state iodine (I) atoms) is shown to provide a good zero-order model for aryl iodide photochemistry also. However, the aryl halides also possess occupied pi and low-lying pi* orbitals, and have lower (C-2 upsilon or C-s) symmetry than CH3I. Both of these factors introduce additional subtleties. For example, excitations to and predissociation of pi pi* excited states provide additional routes to I products, most obviously at long UV wavelengths. n sigma*/pi sigma* configuration mixing stabilizes the (analogue of the) (3)Q(0+) potential energy surface (PES), to an extent that scales with the degree of fluorination; the corresponding 4A(1) PES in C6F5I is actually predicted to exhibit a minimum at extended RC-I. This has the effect of extending the long wavelength threshold for forming I* products. The lowered symmetry enables an additional (sloped) CI with the 5A(2) (9A '' in 2,4-difluorobenzene) PES, which provides an extra non-adiabatic route to (fast) ground state I atoms when populating the 4A(1) PES at shorter UV excitation wavelengths. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3696892]