Effect of electronically excited singlet and triplet states on the photophysical properties in the series of N, O, S heterocyclic compounds
The dependence of the main spectral fluorescence and lasing characteristics of heterocyclic compounds on the pumping-induced singlet-singlet and triplet-triplet losses within fluorescence band is investigated. The condition of maximum spectral separation of limit amplification and pumping-induced singlet-singlet and triplet-triplet reabsorption in excited states is satisfied for organic compounds with spatial structures of definite types. In LCAO MO SCF CI INDO/S and PPP/S quantum-chemical models, this phenomenon is attributed to the fact that only certain heteroaromatic azoles exhibit delocalization of the excessive vibronic excitation on similar atoms and bonds. When organic molecules are pumped by various UV excitation sources with the energy Eex exceeding 4-7 eV, and when the excitation pulse duration is comparable to the excited state lifetime, the spectra of pumping-induced reabsorption at S1* → Sn* and T1 → Tn transitions are formed in systems of excited states prior to light emission, and an effective channel of active losses is generated by the pumping pulse. The increase in the excitation energy that occurs within the time interval no less than the characteristic time of initiation of multiphoton processes in excited molecules with kST≈knr = 1012-1013 s-1 is shown to give rise to S1* ⇔ Sn* transitions populating higher Sn* states involved in spin-orbital interaction with triplet states Tn. Therefore, in this case, the drop in the fluorescence quantum yield γ ≤ 0.01 is several orders of magnitude stronger and the limit time is t1p < 0.01 ns. The Robinson-Frosch formula modified for fundamental problems of laser physics is employed to calculate the rates of optical and nonoptical deactivation of vibronic excitation energy, stimulated emission and secondary reabsorption cross sections, radiative decay and interconversion rates, and fluorescence quantum yields.