Structural Design of Hybrid Manganese(II) Halides for High Quantum Efficiency and Specific Response to Methanol

Manganese(II) halides have been a new generation of optoelectronic materials due to their fascinating luminescent properties, however, lacking specific solvent-responsive analogues with high quantum efficiency. Herein, we prepared three single crystals, [Pr(MIm)2][MnBr4] ([Pr(MIm)2]2+= 1,3-di(methylimidazolium)-propane, Compound 1), [Pr(EIm)2][MnBr4] ([Pr(EIm)2]2+= 1,3-di(ethylimidazolium)-propane, Compound 2), and [Bu(MIm)2][MnBr4] ([Bu(MIm)2]2+= 1,4-di(methylimidazolium)-butane, Compound 3), where different Bola-type cations were chosen as organic components to separate [MnBr4]2–tetrahedrons. All three compounds emitted bright green light with excellent quantum yields of 95.3, 80.0, and 96.2%, benefiting from the large Mn···Mn distance. More interestingly, Compound 3showed a highly selective response to methanol in a series of tested organic solvents, with a rapid and reversible change in emission color from green to red. The single crystal of [Bu(MIm)2][MnBr4]·CH3OH with red emission proved that the luminescence switching was attributed to the adsorption of CH3OH molecules into the lattice space in the form of the O–H···Br hydrogen bonds. To our knowledge, for tetrahedrally coordinated Mn(II) species, the reversible emission color switching between green and red triggered by a solvent without the change of coordination number is achieved for the first time, providing promising applications for the specific detection of methanol.