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 SCFCIINDO/S models

\This paper considers the relation between the mechanisms behind nonoptical energy deactivation of electron-vibrational excitation associated with inn er and spin-orbital conversion, governed by the nonadiabaticity operator (o r the operator of spin-orbital coupling) in series of N, O, S azocyclic mol ecules, which are capable of fluorescing within the range of wavelengths la mbda(fl)(max) approximate to 260-460 nm and lasing within the range of wave lengths lambda(osc)(max) approximate to 344-460 nm under different conditio ns. 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* \(H) over cap(S0)\T-f(alpha)] Of spin-orbital coupling; rate constants of r adiative decay, intercombination, and inner conversion; fluorescence quantu m yields; and cross sections of absorption and stimulated emission. It is d emonstrated that, as the number of subsystems in a molecular structure incr eases, i.e., we pass from mono- to bi- and tricyclic systems, the fluoresce nce wavelength displays a bathochromic shift from lambda(fl)(max) = 260 to 350 nm, which is accompanied by the increase in the energy of excited state s of the n pi* type, the decrease in the energy of pi pi*-type states, the lowering of rate constants of nonoptical excitation deactivation, and the g rowth of rate constants of radiative decay. It is shown that the inversion of n pi*- and pi pi*-type levels within the range of wavelengths lambda(fl) (max) = 320-330 nm and the growth in the oscillator strength of a fluoresce nt transition (as well as the oscillator strength of 0-0 transitions) from f(fl)(e) = 0.2 up to f(gl)(e) = 1.0 are accompanied by a separation of sele ctive bands corresponding to fluorescence (maximum gain) and reabsorption i nduced by optical pump (or a flux of particles) for S-1* --> S-n* and T-1 - -> T-n transitions in the optical spectrum. Frequency separation of the ban ds of stimulated emission and induced active losses in an excited organic s ubstance suggests the existence of molecules with a high gain (which implie s, within the framework of the proposed model, that the limiting duration o f the leading edge of the pumping pulse allowing the implementation of lasi ng can be increased). For laser-active molecules, all the excited states of the n pi* type lie above fluorescent states of the pi pi* type, and select ive spin-orbital interaction mainly couples high-lying singlet and triplet states. Therefore, such systems are characterized by a high fluorescence qu antum yield, gamma = 0.4-1.0, while low active losses in a medium allow one to minimize the threshold pumping energy density required for lasing, whic h improves the photostability of molecules.