Reactions of radiation-excited states of tetramethyl-p-phenylenediamine with electron acceptors

Abstract

Tetramethyl-p-phenylenediamine (TMPD) engagesin several different kinds of interaction with electron acceptors depending upon the' particular acceptor involved and the electronic state of the TMPD molecule. The charge-transfer (C-T) and electron-transfer reactions between ground-state TMPD and ?r-acceptors in nonpolar and polar media are well documented.' In nonpolar and polar rigid media, TMPD undergoes two-quantum ionization via the triplet state but this does not depend on the proximity of an acceptor molecule.2J Donor-acceptor collision is important, however, in fluid solutions; for example, the fluorescence spectrum of TMPD in a hydrocarbon solvent at 20' is progressively weakened by systematic addition of a-methylnaphthalene and a new emission band appears at longer wavelength which is assigned to a transient C-T complex on the basis of its lack of structure and its variation with temperature, viscosity, and incident light intensity.a Flash photolysis of these solutions produces triplet state a-methylnaphthalene but no triplet TMPD or TMPD*+ and the sequence of steps remains o b s~u r e .~ Analogous experiments with dimethylnaphthylamine (donor) and m-dimethylphthalate (acceptor) indicate that C-T emission is appreciable only in nonpolar solvents but that in polar solvents it is absent and the donor-acceptor interaction is one of electron transfer to produce radi~al-ions. ~ We report here that the radio-excited fluorescence of TMPD in benzene solution is strongly quenched by electron acceptors including NZO, SF6, COz, chloracetic acid, and benzyl acetate (Table I and Figures 1 and 2). k,/kt for chloracetic acid and benzyl acetate are, respectively, 244 M-1 (4') and 75 M-1 (9'). Quenching of fluorescence of 2,5-diphenyloxazole in benzene by these electron acceptors is very much less marked under (1) R. Foster and T.