Photophysical properties of meso-substituted nitro derivatives of octaethylporphyrin and their Zn complexes

Using the density functional theory methods, quantum chemical calculations of the excited states of monosubstituted and disubstituted meso-nitro derivatives of octaethylporphyrin (OEP) and their Zn complexes are performed. The obtained data indicate that the charge transfer states are located substantially higher than the locally excited states and do not play a considerable role in the fluorescence quenching of these compounds, which was assumed earlier. It is shown that conformational dynamics take place for all studied compounds in the triplet state, which gives rise to the formation of structures with a common conjugated system of bonds between the porphyrin macrocycle and the nitro group. It is characterized by a small activation barrier ($<$ 200 cm$^{-1}$) of structural rearrangement and a noticeable decrease in the $\Delta(T_1-S_0)$ energy gap between the ground and lower triplet states. The obtained data indicate that H$_{2}$-$\alpha$-NO$_{2}$-OEP, H$_{2}$-$\alpha$,$\gamma$-(NO$_{2}$)$_{2}$-OEP, and Zn-$\alpha$,$\gamma$-(NO$_{2}$)$_{2}$-OEP in the triplet state in toluene solutions have conformations with the $\Delta(T_1-S_0)$ energy gap noticeably less than 7800 cm$^{-1}$. This makes it possible to explain the decrease in the quantum yield of interconversion of these compounds when estimating from the results of measuring the luminescence intensity of singlet oxygen. The fact that the sum of the interconversion and fluorescence quantum yields for the compounds under study is not equal to unity can be explained by conformational transformations in the excited states, which can have an effect on the probability of internal conversion in the singlet state and the accuracy of determining the interconversion quantum yield by different methods.