CIDNP STUDIES OF THE PHOTO-REDUCTION OF AROMATIC KETONES BY AROMATIC-AMINES - SINGLET VS TRIPLET RADICAL ION-PAIR FORMATION BY SELECTIVE EXCITATION

Photoreduction of acetonitrile solutions of 4-substituted acetophenones by N, N-dimethylanilines hasbeen investigated by observing the CIDNP signals of regenerated ketone and amine. Irradiation in the wavelength range absorbed by theketone gives CIDNP signals of opposite polarity to those produced by excitation of the amine. Addition of biphenyl as tripletquencher enhances the CIDNP signals produced by amine excitation, but effectively annihilates those produced by ketone absorption. These results are interpreted in terms of two pathways for radical ion pair formation. Ketone excitation results principally in electron transfer from ground-state amine to triplet-state ketone, with formation of triplet radical ion pairs. Singletion pairs are formed by electron transfer from singlet excited amine to ground-state ketone. This is the dominant process subsequent to amine excitation, but it is accompanied by energy transfer to ketone and triplet ion pair formation. This model is consistent with the known energies of the species involved and the rate constants for their interconversion. Similar results are obtained for benzophenone-aromatic amine systems. An example of reverse electron transfer which yields triplet-state productsis also reported. © 1979, American Chemical Society. All rights reserved.