MECHANISM OF CONTACT ION-PAIR FORMATION IN PHOTOLYZED MIXTURES OF N,N,N',N'-TETRAMETHYLBENZIDINE WITH HALOGEN-CONTAINING COMPOUNDS IN NONPOLAR SOLVENT

Time variation of microwave dielectric absorption has been examined for photolyzed solutions of N,N,N'N'-tetramethylbenzidine (TMB) admixed with a halogen-containing compound AX (AX = CCl4, C2H5I, C2H5Br, C6H5I, and C6H5Br) in benzene. In each solution the dielectric absorption signals show rapid growth after photoirradiation, indicating formation of a contact ion pair (CIP) TMB(+)X(-) (X denotes a halogen atom). The amplitudes of the signals are dependent on the AX concentration and are large for CCl4 but relatively small in other molecules when compared at the same concentration. The growth rate corresponds to the response time of the apparatus and are not dependent on the concentration of the solute molecule, which excludes the possibility of electron transfer from an isolated excited triplet state of TMB to the AX molecule, as opposed to the case of N,N,N'N'-tetramethyl-p-phenylenediamine (TMPD). A mechanism has been suggested that the CIPs are formed from direct excitation of complexes (TMB.AX) present in the solution. The difference in the efficiencies of the CIP formation among the electron-acceptor molecules can be explained by assuming nonadiabatic transitions for the electron-transfer processes.