Zinc Metal Complex of <i>tert</i>-butyl Substituted Phthalocyanine: Assessment of Photosensitizer Potential with Theoretical Calculations

Photodynamic therapy (PDT) is a medical treatment that uses a photosensitizer molecule and a light source to induce the formation of reactive oxygen species in tissue. Phthalocyanines (Pcs) are macrocyclic photosensitizers with useful photophysical and photochemical properties in PDT applications. Motivated by these facts, this study has experimentally and theoretically investigated the suitability of 2,6-di-(tert-butyl)-4-methylphenoxy substituted zinc Pc (dt-ZnPc) in the first steps of PDT applications. Photophysical and photochemical characteristics of dt-ZnPc were measured and validated with the literature. Also, the suitability of the dt-ZnPc complex substituted with 2,6-di-(tert-butyl)-4-methylphenoxy groups for PDT applications has been supported by theoretical calculations for the first time. Density Functional Theory (DFT) and time-dependent DFT (TDDFT) calculations, incorporating both vertical excitation and adiabatic processes, were employed to elucidate the optical and photochemical mechanisms underlying singlet oxygen generation by unsubstituted metal-free and zinc Pcs and dt-ZnPc. Calculated excited state energies agreed well with experimental results, enabling the prediction of photosensitizer activity based on excited state properties and enhanced spin-orbit coupling (SOC) imparted by the metal center.