GROUND AND EXCITED-STATE ISOMERIC FORMS OF THE ANTHRACENE-DIETHYLANILINE MOLECULAR EXCIPLEX

The potential energy surfaces for the molecular complex formed between anthracene (the electron acceptor) and N,N-diethylaniline (DEA) (the electron donor) were computed as the quasi-adiabatic states resulting from the configuration interaction between the ground (AD), locally ex cited (AD) and charge-transfer (A(-)D(+)) excited electronic configur ations, The results clearly indicate the existence of three geometrica lly and energetically different isomeric forms of the complex in the g round state. In the excited state, the potential energy surfaces revea l the existence of five well-defined equilibrium configurations separa ted by energy barriers and characterized by different admixtures of th e (AD) and (A(-)D(+)) electronic configurations. Such a variety of eq uilibrium configurations in the ground and excited states is, in part, accounted for by the existence of two different conformational forms of DEA that can form complexes with anthracene, and are characterized by different balances between steric effects and interactions of elect ronic charge distributions in the complex components. The energies of transitions between the relevant ground and excited state equilibrium configurations were calculated and compared with spectroscopic data of a jet-cooled complex obtained in supersonic beam experiments. These t ransitions were successfully assigned to the observed resonance-like a nd exciplex-like spectra, and this enabled interpretation of observed changes in the fluorescence excitation and fluorescence spectra of the complex upon excess excitation energy.