Excited states, spectroscopic and generation properties, and transition scheme of the complex molecule 1,4-phenylene-2,2'-bisoxazole

We investigate the spectroscopic and generation properties of a new active complex molecule, 1,4-phenylene-2,2'-bisoxazole (OPO), which is capable of fluorescing and generating light in solvents of various kinds within the wavelength region λ ≈ 340–420 nm with a high fluorescence quantum yield γ ≈ 0.45–0.93 and a low threshold pump density E_lp. Using the density matrix method, we show that the solvent affects the distribution of electron density among the individual atoms and fragments of the complex OPO molecule in the ground state, leading to systematic changes in geometry. As a result, there are changes in the distribution of bond lengths in the rings of the OPO molecule. Protonation of the nitrogen atoms in the OPO molecule changes the structure of the excited electronic singlet (S_i^*) and triplet (T_i) states and in the transitions S₀ → S_n^*, S₁^* → S₀, S₁^* → S_n^*, T₁ → T_n , T₁ → S₀, which determine the spectroscopic and generation characteristics of the complex compounds. The spectral, fluorescence, and generation characteristics of this complex heteroatomic molecule in solvents of various types and also in the vapor phase were measured and calculated. The results show that the decrease or increase of E_lp in the switch from solution to vapor, or as the properties of the solvent change, stems from dynamic separation or overlap of the lasing and induced absorption bands of the S₁^* → S_n^* and T₁ → T_n transitions. Lasing does not occur in concentrated acids because of the nearly complete overlap of the limiting-gain and induced absorption (T₁ → T_n) bands. We show that the method of a priori structural modeling for quantum mechanical control of the properties of molecular excited states and transitions can be used in practical applications.