Using the solvent-invariant LE fluorescence as an internal reference, the ratio of the intensities of the LE and CT bands as well as the ratios of the prompt and delayed emission lifetimes were used to calibrate against solvent polarity. Recently, Yin et al. reported a TADF turn-on chemosensor, DCF-MPYM-lev (Figure ), for sulfite ion SO3 2-detection. The fluorescence intensity of DCM-MPYM-lev solution in CH3CN/PBS buffer (1/1) significantly increased and dual emissions at 535 nm and 640 nm were observed after the addition of SO3 2-. DCF-MPYM-lev was also used to monitor exogenous SO3 2-in living cells. Qiu et al. reported the carbazole-triazine-based donoracceptor TADF emitter PhTRZ-OCHO (Figure ) as a fluorescence turn-off/fluorescence quenching sensor for the detection of Na + , Mg 2+ and Fe 3+ ions. The emission intensity at 470 nm of PhTRZ-OCHO decreased on the addition of many of the metal ions tested (Ba + , Ca + , Cd 2+ , Co 2+ , Cr 2+ , Cu 2+ , Fe 3+ , Hg 2+ , K + , Mg 2+ , Mn 2+ , Na + , Ni 2+ , Pb + ), the strongest emission quenching occurred in the presence of Na + , Mg 2+ and Fe 3+ . The remarkable fluorescence quenching behavior was attributed to the metal-binding aldehyde group present in PhTRZ-OCHO where, in the presence of these ions, the charge transfer (CT) state is destabilized and non-emissive.