Mechanism of Dependence of Fluorescent Properties of Tetraaryltetracyanoporphyrazine and Its Derivatives on Viscosity

The paper presents quantum chemical calculations of the conformational structure in the ground and excited states of tetraaryltetracyanoporphyrazine (H₂–Pz(Ph)₄(CN)₄) and its derivatives, which are considered promising photodynamic photosensitizers with the function of fluorescent control of the degree of destruction of cancer cells. 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 the excited S₁ states. Among the low-lying excited states, there are no those whose population can lead to fluorescence quenching due to a noticeable change in the position of the phenyl rings relative to the macrocycle in the torsion coordinate. This suggests that these compounds cannot be classified as fluorescent rotors, as was previously assumed. It has been found that H₂–Pz(Ph)₄(CN)₄ and its derivatives in the solution form solvate complexes with oxygen-containing solvent molecules (water, methanol, ethanol, glycerol, tetrahydrofuran) with the participation of the H and N atoms of the pyrrole and pyrrolenine rings, respectively. These complexes are characterized by out-of-plane distortion of the macrocycle, which increases significantly in the S₁ state and leads to large displacements of peripheral substituents perpendicularly to the macrocycle. The conformational dynamics in the S₁ state is accompanied by a reduction in the energy gap ΔЕ(S₀ – S₁), an increase in the spin–orbit interaction between the excited states, and anharmonicity of NH stretching vibrations. All these factors lead to a decrease in the fluorescence lifetime (τ_F) of H₂–Pz(Ph)₄(CN)₄ and their derivatives in the solution and contribute to the dependence of τ_F on the viscosity of the medium.