The previously discussed fluorescence is based on the radiative decay from S1 to S0. However, the population of excited triplet states Tn via intersystem crossing (ISC) from S1 competes with the radiative deactivation, thus decreasing the fluorescence intensity (Figure ). The low absolute quantum yield for BDP-TFP-COF may imply the population of T1, which motivated us to test this COF as a triplet photosensitizer for the generation of singlet oxygen. As a quantitative probe, we used 1,3-diphenyl-iso-benzofuran (DPBF), which forms an endoperoxide via [4+2] cycloaddition of singlet oxygen followed by an irreversible decomposition into 1,2-dibenzoylbenzene (DBB). Triplet sensitization with BDP-TFP-COF was analyzed by monitoring the time-dependent decay via UV/Vis absorption for a DPBF solution (1×10 -4 M in DMF, λmax = 415 nm). Upon irradiation with a green LED light (~7 W, 528 nm) in the presence of BDP-TFP-COF (0.2 mg mL -1 ), a continuous decrease of the DPBF absorption was observed over time, thus implying the efficient generation of singlet oxygen (Figures and).