The mechanism of the dependence of the fluorescent properties of tetraaryltetracyanoporphyrazine and its derivatives on the viscosity

Quantum-chemical calculations of the conformational structure in the ground and in excited states of tetraaryltetracyanoporphyrazine (H$_{2}$–Pz(Ph)$_{4}$(CN)$_{4}$) and its derivatives, which are considered as promising photodynamic photosensitizers with the function of fluorescent control of the degree of destruction of cancer cells, have been carried out. It was shown that, in the absence of specific interactions with the solvent, these compounds are characterized by a planar macrocycle structure both in the ground and in the excited $S_{1}$ state. Among the low-lying excited states, there are no ones, the population of which can lead to quenching of fluorescence due to a noticeable change in the position of the phenyl rings relative to the macrocycle along the torsion coordinate. This makes it possible to exclude the assignment of these compounds to fluorescent rotors, as previously assumed. It was found that H$_{2}$–Pz(Ph)$_{4}$(CN)$_{4}$ and its derivatives in solution form solvate complexes with oxygen-containing solvent molecules (water, methanol, ethanol, glycerol, tetrahydrofuran) with the participation of H and N atoms of pyrrole and pyrrolene rings, respectively. These complexes are characterized by out-of-plane distortion of the macrocycle, which significantly increases in the $S_{1}$ state and leads to large displacements of peripheral substituents perpendicular to the macrocycle. The conformational dynamics in the $S_{1}$ state is accompanied by a decrease in the energy gap $\Delta E(S_0-S_1)$, an increase in the spin-orbit interaction between excited states, as well as anharmonicity of the NH stretching vibrations. All these factors lead to a decrease in the fluorescence lifetime $(\tau_{\mathrm{F}})$ of H$_{2}$–Pz(Ph)$_{4}$(CN)$_{4}$ and their derivatives in solution, and contribute to the dependence of $\tau_{\mathrm{F}}$ on the viscosity of the medium.