Fe3+ Oxidation Strategy Enhances the Nonradiative Transition Ability of a Near-Infrared BODIPY Photosensitizer for Photothermal Therapy.

Near-infrared (NIR) BODIPY-based photosensitizer NIR-2BDP is constructed through an Fe3+ oxidation strategy. Theoretical calculations and experimental results show that intramolecular charge separation effectively reduces the HOMO-LUMO energy gap, broadens the light absorption range, and suppresses radiative transition (RT) processes. The decrease in the singlet-triplet energy gap enhances the intersystem crossing (ISC) ability of NIR-2BDP (the spin-orbit coupling constant (S1→T1) is 2.5 times that of NIR-BDP, and the singlet oxygen (1O2) quantum yield is 2.1 times that of NIR-BDP). The low RT (only 2.2% fluorescence quantum yield) and relatively low triplet generation (11.7% 1O2 quantum yield) suggest that NIR-2BDP should have a stronger contribution of nonradiative transition (NRT) processes (and hence a higher photothermal conversion capability). Nanoparticles constructed from NIR-2BDP and DSPE-mPEG2000 exhibit good photothermal conversion efficiency (41.4%) and killing efficiency to tumor cells in the NIR-II window (1000-1700 nm). Therefore, this study extends the sensitization window and improves the NRT ability, paving the way for the construction of multi-functional NIR BODIPY-like photosensitizers.