Photochemistry of halogenated anilines studied by time-variation of microwave dielectric absorption

The photochemistry of p-halogenated anilines (H2NC6H4X-p (XA)) such as p-chloroaniline (CA; X=Cl), p-bromoaniline (BA; X=Br), and p-iodoaniline (IA; X=I) in benzene has been studied by the time-variation of microwave dielectric absorption (MWDA) method with respect to the transient behavior of the excited states and intermediates with independent dipole moments. The formation and the transient behavior of CA in the triplet excited state (3CA∗) having the ion pair character of [p-aminophenyl cation/Cl−], an ion pair of [p-aminophenyl cation/Br−], and a p-aminophenyl radical were observed in benzene from the time profiles of the MWDA signals immediately after the laser flash during 308-nm laser irradiation of CA, BA, and IA. The yields of the cleavage of the CX bond were in the order of CA<BA<IA which reflects the order of the bond dissociation energy (BDE), CA>BA>IA. Decay of 3CA∗ having the character of [p-aminophenyl cation/Cl−] was observed with a lifetime of 3.8 μs. The homolytic cleavage of the CBr bond in 3BA∗ occurs to give a radical pair [p-aminophenyl radical/Br] which sequentially gives an ion pair [p-aminophenyl cation/Br−] through electron transfer between radicals. The dissociation of the [p-aminophenyl cation/Br−] to the p-aminophenyl cation and Br− was observed to occur on a 20 μs time scale. On the other hand, homolytic cleavage of the CI bond in 3IA∗ occurs efficiently to give the p-aminophenyl radical and I. Bimolecular reactions of 3CA∗ having the character of the [p-aminophenyl cation/Cl−] with oxygen and a p-aminophenyl radical with oxygen were found to occur at rate constants of 8.3×107 and 108–109 M−1 s−1, respectively.