Photodissociation of C6H5I, C6F5I, and related iodides in the ultraviolet

Abstract

The quantum yields of I*(2P1/2) production from iodobenzene and pentafluoroiodobenzene at five different dissociation wavelengths of 222, 236, 266, 280, and ∼305 nm are presented and compared with those obtained from nonaromatic cyclic iodides (i.e., cyclohexyl iodide and adamantyl iodide). The I(2P3/2) and I*(2P1/2) atoms generated in the photolysis of the above iodides were monitored using a two-photon laser-induced fluorescence technique. From the measured I* quantum yields, two general observations are made for aryl iodides. They are that (i) the I* yield is influenced by the σ*←n as well as π*←π transitions at all photolysis wavelengths within the A band and (ii) there is a clear indication of a fluorine substitution effect on the dynamics of I* production. The contribution from the benzene type π*←π transition varies with excitation wavelength. Fluorine substitution in aryl iodides is found to increase the I* quantum yield similar to what is reported in alkyl iodides. The effect of fluorine substitution is more pronounced at the red edge of the A-band excitation than at any other wavelengths. This is explained by invoking the presence of a charge-transfer band arising due to the transition of a 5pπ nonbonding iodine electron to the π* molecular orbital near the red edge of the A band. This charge-transfer state is coupled more strongly to the Q13 state of the σ*←n transition in pentafluoroiodobenzene than in iodobenzene. The dynamics of I* formation is found to be unaltered by ring strain in cyclic iodides except at the blue wing excitation. At the blue wing, B-band transitions affect the dynamics of I* production in cyclic iodides, leading to the formation of more I* from adamantyl iodide.