Localization of spin-orbital coupling and transitions in spectra of pump-induced reabsorption in excited singlet and triplet states of laser-active molecules in LCAO MO SCF CI INDO/S models

This paper considers the relation between the mechanisms behind nonoptical energy deactivation of electron-vibrational excitation associated with inner and spin-orbital conversion, governed by the nonadiabaticity operator (or the operator of spin-orbital coupling) in series of N, O, S azocyclic molecules, which are capable of fluorescing within the range of wavelengths λmaxfl ≈ 260-460 nm and lasing within the range of wavelengths λmaxosc ≈ 344-460 nm under different conditions. The semiempirical LCAO MO SCF CI INDO/S method was applied to calculate the energies of singlet and triplet quantum states; matrix elements <S*i|̂s0|Tαf> of spin-orbital coupling; rate constants of radiative decay, intercombination, and inner conversion; fluorescence quantum yields; and cross sections of absorption and stimulated emission. It is demonstrated that, as the number of subsystems in a molecular structure increases, i.e., we pass from mono- to bi- and tricyclic systems, the fluorescence wavelength displays a bathochromic shift from λmaxfl = 260 to 350 nm, which is accompanied by the increase in the energy of excited states of the nπ* type, the decrease in the energy of ππ*-type states, the lowering of rate constants of nonoptical excitation deactivation, and the growth of rate constants of radiative decay. It is shown that the inversion of nπ*- and ππ*-type levels within the range of wavelengths λmaxfl = 320-330 nm and the growth in the oscillator strength of a fluorescent transition (as well as the oscillator strength of 0-0 transitions) from fefl ≈ 0.2 up to fefl ≈ 1.0 are accompanied by a separation of selective bands corresponding to fluorescence (maximum gain) and reabsorption induced by optical pump (or a flux of particles) for S*l → S*n and Tl → Tn transitions in the optical spectrum. Frequency separation of the bands of stimulated emission and induced active losses in an excited organic substance suggests the existence of molecules with a high gain (which implies, within the framework of the proposed model, that the limiting duration of the leading edge of the pumping pulse allowing the implementation of lasing can be increased). For laser-active molecules, all the excited states of the nπ* type lie above fluorescent states of the ππ* type, and selective spin-orbital interaction mainly couples high-lying singlet and triplet states. Therefore, such systems are characterized by a high fluorescence quantum yield, γ=0.4-1.0, while low active losses in a medium allow one to minimize the threshold pumping energy density required for lasing, which improves the photostability of molecules.