Excited vs. ground state equilibria in the formation and relaxation of a molecular exciplex

Abstract

Picosecond/nanosecond time-resolved emission spectra of weak electron donor-acceptor (EDA) systems in liquid solutions (of low polarity) were investigated. It was shown that, for a weak EDA system (e.g. molecular complexes of 1,2,4,5-tetra-cyanobenzene with methylated benzenes), the departure from a two-state equilibrium in the excited state and the observed lack of a relationship between the emission decay parameters in different spectral regions may be accounted for by the existence of different isomeric forms (orientational isomers) of the EDA system. This conclusion was successfully verified by the results of calculations of the excited and ground state potential energy surfaces of the EDA systems investigated.