Nonemissive Iridium(III) Solvent Complex as a Self-Reporting Photosensitizer for Monitoring Phototherapeutic Efficacy in a “Signal on” Mode

Photodynamic therapy (PDT) has long been receiving increasing attention for the minimally invasive treatment of cancer. The performance of PDT depends on the photophysical and biological properties of photosensitizers (PSs). The always-on fluorescence signal of conventional PSs makes it difficult to real-time monitor phototherapeutic efficacy in the PDT process. Therefore, functional PSs with good photodynamic therapy effect and self-reporting properties are highly desired. Here, two nonemissive iridium(III) solvent complexes, [(dfppy)2Ir(DMSO)]Cl (Ir-DMSO, dfppy = 2,4-difluorophenyl)pyridine, DMSO = dimethyl sulfoxide) and [(dfppy)2Ir(ACN)]Cl (Ir-ACN, ACN = acetonitrile) as PSs, were synthesized. Both of them exhibit intense high-energy absorption bands, low photoluminescence (PL) emission, and low dark toxicity. Thanks to the lower dark toxicity of Ir-DMSO, we chose it as a PS for further PDT. In this work, Ir-DMSO functions as a specific PL "signal on" PS for self-reporting therapeutic efficacy during its own PDT process. Colocalization experiments indicated that Ir-DMSO accumulated in the endoplasmic reticulum and mitochondria. Under light irradiation, Ir-DMSO not only exhibited the ability to kill cancer cells but also presented a "signal on" PL response toward cell death. During Ir-DMSO-induced PDT, cell death modality was further investigated and immunogenic cell death was revealed, in which main hallmarks, including ROS generation, upregulation of surface-exposed calreticulin, high-mobility group box 1, and adenosine triphosphate secretion, were observed. Thanks to the specific coordination reaction between Ir-DMSO and histidine (His)/His-containing proteins, the phototherapeutic efficacy can be monitored in real time without other signal probes. This work provides a new and promising strategy for the development of PSs with self-reporting ability, which is of great importance for imaging-guided PDT.