CONTACT ION-PAIRS FORMED FROM PHOTOLYZED SOLUTIONS OF N,N,N',N'-TETRAMETHYLBENZIDINE IN CCL4

The time variation of microwave dielectric absorption has been examine d for photolyzed solutions of N,N,N',N'-tetramethylbenzidine (TMB) in CCl4. The dielectric absorption signals show a rapid growth followed b y a second-order decay approaching a flat level higher than the baseli ne. These are attributed to the formation of contact ion pairs (TMB+Cl -) by reaction of the excited state of TMB with CCl4 and a subsequent dimerization process. The dipole moment of the contact ion pair and th e rate constant for its dimerization are determined to be 12.4phi-1/2 D and 1.7 X 10(9)phi-1 M-1 s-1, respectively, where phi is the quantum yield for the ion-pair formation with a value larger than 0.26 (possi bly greater-than-or-equal-to 0.5). Most properties associated with the contact ion pairs well resemble those found in similar ion pairs invo lving a cation of N,N,N',N'-tetramethyl-p-phenylenediamine.