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) andN ,N-diethylaniline (DEA) (the electron donor) were computed as the quasi-adiabatic states resulting from the configuration interaction between the ground (AD), locally excited (A^*D) and charge-transfer (A⁻D⁺) excited electronic configurations. The results clearly indicate the existence of three geometrically and energetically different isomeric forms of the complex in the ground state. In the excited state, the potential energy surfaces reveal the existence of five well-defined equilibrium configurations separated by energy barriers and characterized by different admixtures of the (A^*D) and (A⁻D⁺) electronic configurations. Such a variety of equilibrium 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 electronic 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 transitions were successfully assigned to the observed resonance-like and exciplex-like spectra, and this enabled interpretation of observed changes in the fluorescence excitation and fluorescence spectra of the complex upon excess excitation energy.