Estudo teórico do efeito do solvente na fotofísica e na geração de oxigênio singleto de derivados porfirínicos

Abstract

Photodynamic Therapy, used as a form of treatment for various diseases, consists of exciting (with light) a photosensitizer; as consequence, the generation of cytotoxic species occurs, destroying the affected or disease-inducing cells. The cytotoxic species generated can be: i) molecular oxygen excited to a singlet electronic state, or, ii) radicals called reactive oxygen species. In this sense, recently, a student of the Group of Porphyrins, Porfirazines and Catalysis of UFMG evaluated the photophysical properties and antitumor activity (in vitro) of two porphyrinic derivatives. Between these two compounds, the porphyrin, H2T(3EPy)P4+, was capable of singlet oxygen generation in acetonitrile and water, the porphyrazine, H2(3EPy)8Pz8+, however, generated this species only in acetonitrile. To better understand these results, this work was carried out, focusing on DFT and TDDFT calculations to explain the photophysical mechanisms associated with these porphyrinic compounds and the reason for why the porphyrazine isn’t capable of singlet oxygen generation in water. Geometry optimizations of the ground and excited electronic states were carried out at the B3LYP/def2-SVP level and energy, transition dipole and spin-orbit coupling calculations were carried out at the ωB97X-D3/def2-TZVP level with implicit solvation through the CPCM model. Results of these calculations were analyzed with the Gouterman model, Marcus theory and Einstein model to study the electronic structure, the intersystem crossing and fluorescence rates of the studied compounds, respectively. Excited state dynamics calculations on a model porphyrazine without substituents, but with explicit hydrogen bonds were carried out to study intersystem crossing considering vibronic spin-orbit coupling. Results for H2T(3EPy)P4+ and H2(3EPy)8Pz8+ showed that the solvent effect on the photophysical processes was not adequately represented using the CPCM model and that Marcus theory underestimates the intersystem crossing speed of these compounds between the singlets and triplets excited states. The excited state dynamics calculations showed a large reduction in the intersystem crossing rate for the model porphyrazine caused by the presence of hydrogen bonds. Results for the model porphyrazine are in agreement with patterns already observed in the literature, therefore it is proposed that the low singlet oxygen generation by porphyrazines in water is caused by hydrogen bonds that inhibit vibronic spin-orbit coupling between states.