THE PHOTOSENSITIZED DIMERIZATION OF 1,3-CYCLOHEXADIENE

The quantum yield of formation of [4 + 2] and [2 + 2]cyclohexadiene dimers through photosensitization by aromatic nitriles is determined as a function of diene concentration, solvent polarity, presence of salts. A special effect by halides on the radical ion pair is observed, and is different from the general salt effect (increase of ionic strength) caused by perchlorates, fluophosphates, and fluoborates. A scheme based on two reactive intermediates, the exciplex and the radical ion pair (not the free radical ions) is proposed. The two species are in equilibrium and this causes the observed complex dependence of reaction quantum yield and product ratio on diene concentration. Intersystem crossing from the radical ion pair is the origin of a significant part of triplet 2 + 2 dimerization.