Conformational dynamics and spin-orbit interactions in a series of sterically hindered porphyrins in the lowest triplet state

We have studied the nature and unusual photophysical properties of triplet states of a series of sterically hindered porphyrins (meso-phenyl-substituted derivatives of octamethylporphyrin (PS-OMP)) using quantum-chemical calculations of the parameters that determine the probability of deactivation of the T ₁ state and modeling the T-T absorption spectra. We show that a decrease in the lifetime of the T ₁ state of PS-OMP is related with the enhancement of the channel of nonradiative deactivation (T ₁ → S ₀), which occurs (i) due to the conformational lability in the T ₁ state, as a result of which the energy gap T ₁-S ₀ considerably decreases, and (ii) because of an increase in the matrix element of the spin-orbit interaction due to a change in the hybridization of atoms of the macrocycle as a consequence of its nonplanar distortion. The value of the vibronically induced matrix element of the spin-orbit interaction between the S ₀ and T ₁ states of PS-OMP depends weakly on the type of the conformation and the value and the character of distortion of the porphyrin macrocycle. The conformational lability of PS-OMP clearly manifests itself in the spectra and kinetics of the T-T absorption of these compounds at room temperature and determines the nonmonoexponential character of the phosphorescence decay kinetics in frozen matrices. Using porphin as an example, we have shown that nonplanar distortions of the macrocycle facilitate a decrease in the phosphorescence rate constant at 77 K, which is caused, on the one hand, by an increase in the energy gaps T ₁-S _n and S ₀-T _n and, on the other hand, by an increase in the wavelength of the transition T ₁ → S ₀.