Photochemically induced isotopic exchange between iodobenzene and molecular iodine

The photochemical isotopic exchange between iodobenzene and radioactive molecular iodine (I¹³¹I) was studied in methylcyclohexane solutions. The limiting exchange yields, φ_EX⁰, measured at several excitation wave-lengths in the uv range were found to be identical with the corresponding deiodinatipn limiting yields, φ_DE⁰. This observation and a study of the effects of dissolved oxygen and iodine concentrations suggest that the exchange is initiated by photocleavage of iodobenzene to phenyl (Ph-) and iodine radicals, with O₂ and I₂ competing in the scavenging of Ph· (k_I2+Ph./k_O2+Ph.= 2.1 ± 0.5). The photocleavage of iodobenzene was found to be wavelength dependent in the near-uv range (e.g., 334 and 365 nm), where part of the absorption leads directly to the iodobenzene triplet. It is suggested that dissociation takes place from the lowest excited singlet, but practically not from the triplet state. It is shown that iodine atoms, produced by selective excitation in the visible range, do not exchange with iodobenzene. Neither does excitation of the charge-transfer (CT) complex between iodobenzene and iodine lead to exchange or deiodination.