Combination of Light and Ultrasound: Exploring Sono–Photochemical Activities of Phthalocyanine-Based Sensitizers

Recently, there has been increasing interest in photodynamic therapy (PDT) compared to traditional methods (immunotherapy, surgery, chemotherapy, or radiotherapy) in the treatment of cancer. Also, sonodynamic therapy (SDT) is a non-invasive therapeutic modality that enables high tissue penetration by triggered ultrasound (US). Phthalocyanines (Pcs) are known as one of the most important classes of sensitizers used in PDT and SDT. Motivated by these facts, in this study, to assess potentials for sono-PDT (SPDT), the preparation of peripherally substituted metal-free (2), zinc (3), and indium (4) Pcs was completed by 3-methoxybenzyloxy substituents. The resulting compounds were characterized by applying spectroscopic techniques including 1H NMR, FT-IR, UV–vis, and MS. In sono–photochemical studies, the calculated singlet oxygen quantum yield (ΦΔ) values were 0.68 for ZnPc (3) and 0.83 for InPc (4) after light irradiation; these values increased to 0.81 for 3 and 0.94 for 4 after combination US and light irradiation. The obtained data proved that the singlet oxygen production increased considerably by changing the method from light irradiation to US/light. This research will enrich the literature on increasing singlet oxygen production by the SPDT method. Theoretical calculations based on density functional theory (DFT) give further explanations of the optical and (photo)chemical processes behind the singlet oxygen generation. Our calculations are able to provide the energy of the excited states with excellent agreement with the experimental results, and they also allow us to understand and predict the activity of these Pcs based on their excited states and a clear enhancement of the spin–orbit coupling (SOC) when a metal center is included in the Pcs. This work thus exploits new insights into the advanced sono-photodynamic properties of potential sensitizers for efficient SPDT applications.