Ratiometric Fluorescence Detection of Mg2+ Based on Regulating Crown-Ether Modified Annihilators for Triplet–Triplet Annihilation Upconversion

Detection of magnesium ion has been of great significance considering its critical physiological activities. Herein, we report ratiometric fluorescence detection of Mg²⁺ with high sensitivity and selectivity based on triplet-triplet annihilation (TTA) upconversion for the first time. Crown-ether functionalized anthracene derivatives were synthesized, which bifunctionally acted as not only annihilators to construct TTA upconversion systems but also the recognition probes for Mg²⁺ based on the photoinduced electron transfer (PET) mechanism. Their photophysical properties with the absence and presence of Mg²⁺ were comprehensively studied. It was found that solvents strongly influenced the photophysical properties and Mg²⁺-responsiveness. TTA upconversion systems with PtOEP as the sensitizer were further established and investigated. It turned out PtOEP/9-AEC in DCM exhibited an excellent linear relationship (R² = 0.9979) between the intensity ratio (the integrated upconverted luminescence intensity (I_UC) over the integrated downshifted phosphorescence intensity (I_PL), I_UC/I_PL) and the concentration of Mg²⁺ under the excitation of 532 nm with a limit of detection value of 2.52 μM and a high selectivity to Mg²⁺. This work opened a new perspective of designs and applications of TTA-upconversion-based ratiometric fluorescence for ion detection.