Reversible Photoluminescence Switching in Manganese Hybrids Induced by Water-Chloride Exchange

Mn-based hybrids are emerging stimuli-responsive luminescent materials, whose response mechanism is generally based on chromic behavior induced by a transition from octahedral (Oh) to tetrahedral (Td) coordination. Upon a reversible phase transition driven by H2O/Cl– ligand exchange in an octahedral configuration (Oh to Oh), an alternative chromic mechanism is reported herein. Two zero-dimensional (0D) hybrid manganese chlorides, (C4H8N4)2[MnCl4(H2O)2]·2Cl (Mn-c) and (C4H8N4)2MnCl6 (Mn-r), were synthesized, featuring 4,6-diaminopyrimidinium cations and isolated Mn-centered octahedral motifs. Structural, spectroscopic, and theoretical analyses reveal that the cyan emission of Mn-c originates from the organic cations, whereas the red emission of Mn-r stems from Mn-centered d-d transitions. Interconversion between Mn-c and Mn-r can be triggered via the H2O/Cl– ligand exchange induced by heating (>108 °C) or by soaking in hydrochloric acid, resulting in a reversible phase transition and switchable photoluminescence (PL) behavior. Leveraging this reversible chromism, anticounterfeiting patterns were fabricated. This novel Oh-Oh phase transition and the related PL switching of 0D hybrid Mn-based halides provide a new platform for developing intelligent, multicolor-responsive luminescent materials.