Substituent effects in the photocyclization of 2-styrylbiphenyls

Abstract

Quantum yields for the photocyclization of several substituted 2-styrylbiphenyls (4) to 9,10-dihydrophenanthrenes (6) have been measured. It appears that the values fit into a linear Hammett plot and also that the substituent effects cannot be explained by ascribing them to perturbations of the orbital energies which are most involved in the photoreaction; calculations with the extended Hückel MO method did not even give qualitatively correct results always. It is supposed that substituents affect the heights and positions of the extreme in the energy curves for ground and excited state along the reaction co-ordinate. The linear plot for quantum yields versusσ values for the ground state suggests that the main effect is on the maximum of the energy curve for S₀.