The analysis of excited-state equilibria of weak electron donor–acceptor systems:: Tetracyanobenzene–toluene complex

Abstract

The electron donor–acceptor system of 1,2,4,5-tetracyanobenzene–toluene is a prime example of a weak charge-transfer complex which is stable in the ground state and which is also capable of forming an exciplex in diffusion-controlled reaction in liquid solution (in non- or low-polarity solvents). The potential energy surfaces calculated for the ground and excited state of this system reveal the presence of different orientational isomers of the isolated complex (of charge-transfer and of van der Waals character). An extended photokinetic scheme which includes excitation of these isomers as well as the diffusion-controlled formation of an exciplex has been proposed and analyzed. Computer simulations of the time profiles of emission of the investigated system performed within the framework of such a photokinetic scheme were in satisfactory agreement with experimental observations. This leads to the conclusion about the existence of different Franck–Condon excited states of the TCNB–toluene system in liquid solutions with relaxation pathways strongly controlled by the physical conditions of the solution.