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

The quantum yields of I*(P-2(1/2)) production from iodobenzene and pentafluoroiodobenzene at five different dissociation wavelengths of 222, 236, 266, 280, and similar to305 nm are presented and compared with those obtained from nonaromatic cyclic iodides (i.e., cyclohexyl iodide and adamantyl iodide). The I(P-2(3/2)) and I*(P-2(1/2)) 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 sigma*<--n as well as pi*<--pi 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 pi*<--pi 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 5ppi nonbonding iodine electron to the pi* molecular orbital near the red edge of the A band. This charge-transfer state is coupled more strongly to the (3)Q(1) state of the sigma*<--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. (C) 2002 American Institute of Physics.