Unveiling the Power of Dark State Photocages: An Efficient Pathway to Triplet State Under Near-Infrared Light Irradiation.

The photocage platform provides a synergistic approach to photodynamic therapy (PDT) and chemotherapy. Before light activation, both pro-photosensitizers and pro-drugs remain inert, ensuring safety. Upon near-infrared (NIR) light irradiation, they undergo photoactivation and controlled release in deep tissues. However, the low photolysis efficiency of conventional singlet-state photocages limits their in vivo applications, necessitating an exploration of how excited-state dynamics influence photolysis. In this study, we introduce the concept of a "dark state photocage" and establish a photocage platform responsive to NIR light by incorporating phenoxazine or phenothiazine groups. Mechanistic investigations reveal that the dark state enhances triplet-state formation and facilitates intersystem crossing, thereby promoting photolysis. These photocages achieve functional molecule release under low-power NIR light (15 mW cm⁻²), with an efficiency of up to 90%. This advancement enables a sharp transition from an "OFF" to an "ON" tumor imaging signal and significantly enhances the therapeutic efficacy against triple-negative breast cancer under NIR light irradiation, effectively integrating PDT and chemotherapy.