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 3 showed 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.