Divergent Donor-Excited Photoinduced Electron Transfer Effect: Unsuppressed Second Near-Infrared Fluorescence but Unexpected Role in Activatable Imaging-Guided Photothermal Therapy

Activatable near-infrared (NIR)-II multimodal probes enable precise theranostics via microenvironment-responsive mechanisms, yet long-wavelength detection remains challenging. Herein, we report <b>CL-DNBS</b>, a donor-excited photoinduced electron transfer (d-PeT)-based NIR-II probe that can be activated by biothiols. Contrary to conventional d-PeT-based probes, the 2,4-dinitrobenzenesulfonyl of <b>CL-DNBS</b> enhances the fluorescence intensity instead of quenching. Theoretical studies reveal that the lowest unoccupied molecular orbital energy of the NIR-II chromophore impedes electron transfer to nitro-based acceptors. Frontier orbital analysis identified an energy-level crossover point for xanthene-derived d-PeT function, validated through designed model molecules. Notably, biothiol-triggered cleavage activates new behaviors: the product <b>CL-OH</b> boosts photothermal conversion efficiency (60.5%) by enhancing nonradiative decay and induces a red-shifted "off-on" photoacoustic response at 920 nm. <b>CL-DNBS</b> achieves real-time <i>in vivo</i> biothiol-activatable photoacoustic imaging and photothermal therapy, demonstrating the innovative exploitation of d-PeT mechanisms in long-wavelength probe design. Our findings provide theoretical insights and practical paradigms for developing activatable NIR-II theranostic platforms.