The influence of electronically excited triplet states on the photophysical properties of polyatomic heterocyclic azoles as laser active elements

This paper considers series of azoles (organic compounds whose structure in cludes benzene, furan, thiophene, oxazole, and oxadiazole cycles chemically coupled with each other) to investigate the dependence of the main lasing characteristics of these compounds (the wavelength and the threshold pump d ensity) on the magnitude of pump-induced triplet losses within the fluoresc ence band. It is shown that only organic compounds with a definite type of spatial structure satisfy the condition of maximum separation of the band o f limiting gain (which coincides with the maximum of the fluorescence band) and the spectra of pump-induced triplet-triplet reabsorption in excited st ates. The latter condition simultaneously ensures (a) the minimum threshold pump densities required for lasing, (b) hypsochromic shift of the fluoresc ence band, (c) the maximum fluorescence quantum yield, and (d) the invarian ce of the ratio gamma (fl)/tau (fl) const in different solvents. In LCAO MO CSF CI quantum-chemical models, this phenomenon is attributed to the fact that only certain compounds of bisoxazoles, oxadizoles, oxazolyls, and oxad iazolyls in excited fluorescent (and phosphorescent) states allow the exces sive electron-vibrational excitation to be delocalized exclusively on atoms and bonds with similar positions and nature. We will consider practical ex amples showing that, when excitation corresponding to S-1* --> S-0 and T-1 --> S-0 optical transitions and the spectra of pump-induced losses through S-1* --> S-n* and T-1 --> T-n transitions is localized within the bonds of the same type, emission spectra overlap with the spectra of induced losses. This effect gives rise to a strong increase in the threshold pump density. Optical radiation in the ultraviolet range of wavelengths shorter than lam bda (max)(GEN) approximate to = 330-320 nm cannot be generated under these conditions directly (without nonlinear frequency conversion). We will perfo rm a comparative analysis of the methods for the investigation of lasing bu ild-up with the use of two-, three-, and five-level models of heteroaromati c azoles as laser active elements.