Electronic structure, spectra, and nature of electronically excited states of the bifluorophores stilbene-CH₂-coumarine and phenyl-CH₂-coumarine

Abstract

Quantum-chemical calculations of trans-stilbene and benzene molecules as well as molecules of bifluorophores based on them — trans-stilbene-CH₂-coumarine 120 and phenyl-CH₂-coumarine — were performed by the INDO/S method. The form of the molecular orbitals, the distribution of the electron density in the ground and excited states, the spectra and nature of the electronically states of bifluorophore molecules were established in relation to the energy donor and acceptor molecules. Three conformations of the bifluorophore molecules were studied (planar, “edge,” “wing”). It was discovered that the energy and type of the molecular orbitals of the bifluorophores are independent of the relative orientation of the donor and acceptor subsystems. It is shown that the mixed character of the three lowest electronically excited states, participating in the formation of the spectrum, of the bifluorophores gives rise to quite appreciable overlapping of their electronic wave functions and, therefore, ensures that the internal-conversion processes are efficient.