Two-stage ligand exchange in Mn(III)-based porphyrinic metal−organic frameworks for fluorescence water sensing

Metal−organic frameworks (MOFs) have emerged as promising materials for smart sensing and imaging applications. Here, a novel micron-biscuit Mn(III)-based porphyrinic MOF is synthesized with the modulation of 4,4′-biphenyldicarboxylic acid (BPDC). The anisotropic growth of MOF is induced by the increased steric hindrance, yielding micron-biscuit Mn-TCPP(BPDC) MOF crystals. Mn(III), as a sealer, quenched the intrinsic fluorescence of porphyrin because of the ligand-to-metal charge transfer (LMCT) effect, and this disappeared fluorescence can be regularly restored and enhanced due to LMCT effect inhibition and release of free porphyrin derived from the water-induced two-stage ligand exchange (TSLE) between BPDC, porphyrin and water. The as-prepared MOF acts as an excellent "signal-on" luminescent water-sensing probe with low amount (2.5 μg/mL) that can efficiently realize the trace detection of water in various organic media with high sensitivity, especially in dimethyl sulfoxide (DMSO), the detection limit is as low as 0.04% (v/v). Moreover, Mn-TCPP(BPDC) was successfully used for sensing water content in real commercial samples. This MOF with the guest-induced TSLE not only provides inspiration for the design of new luminescent materials, but also widens the path for smart sensing.