LCAO MO SCFCIINDO S quantum-chemical calculations of characteristics of electronic transitions involving excited states in UV absorption, stimulated emission, and pump-induced reabsorption spectra in N, O, S series of heteroaromatic molecules
This paper provides a detailed investigation of regularities of variations in characteristics of electronic transitions in UV absorption, fluorescence, phosphorescence, and pump-induced reabsorption spectra in quasi-homologous N, O, S series of mono-, bi-, tri-, penta-, and quinquicyclic compounds based on benzene, furan, thiophene, oxazole, and oxadiazole performed by means of the LCAO MO SCF CI INDO/S quantum-chemical method. Calculated spectra of reabsorption induced for S-1* --> S-n* and T-1 --> T-n electronic transitions are compared with characteristics of the electron-vibrational structure of emission spectra, as well as with fluorescence and lasing efficiencies of heteroaromatic molecules in the UV and visible spectral ranges. It is proved that the periodicity of variations in characteristics of organic molecules observed in fluorescence, phosphorescence, and lasing experiments is associated with characteristics of electronic transitions and the nature of the electron-vibrational interaction of structural subsystems (groups of atoms, bonds, or cycles) in singlet and triplet excited molecular states. The best photophysical characteristics are achieved for polyatomic systems, whose excited states are involved in a quasi-resonance related to electron-vibrational subsystems (atomic-bond groups), since the spectra of limiting gain and induced reabsorption through T-1 --> T-n transitions are well separated for such systems, while the absence of radiation generation is caused by a complete overlapping of these bands in an optical spectrum.