The photoreduction of p-fluoranil in dioxane was studied by combined CIDNP and CIDEP techniques. It was established that the photoreactive species is the excited triplet of the quinone and the phototriplet mechanism is mainly responsible for the CIDEP observation. The CIDNP observations revealed the cage recombination of the primary radical-pair resulting in the formation of the cross-combination product from the semiquinone and the dioxane radicals. The cage recombination cannot be monitored by the conventional kinetic esr technique. The field dependent CIDNP intensity of the p-fluoranil in the photoreduction by fluorohydroquinone was also studied and a calculation based on Lawler's scheme showed that the results can be adequately explained by the radical-pair theory including both the scavenged and cage combination processes.
