Fluorescent In based MOFs showing “turn on” luminescence towards thiols and acting as a ratiometric fluorescence thermometer

Abstract

Herein, two indium-based metal organic frameworks (In-MOFs), namely [(CH₃)₂NH₂][In(bpdc)₂]·(DMF)₂ (1) and [(CH₃)₂NH₂]₃[(In₃Cl₂)(bpdc)₅]·(H₂O)₅(DMF)_2.5 (2, bpdcH₂ = 4,4′-biphenyldicarboxylic acid), have been synthesized and characterized. By introducing a Cl⁻ ion as a terminal ligand to break the symmetrical coordination environment of a typical InO₈ dodecahedron as that in a three dimensional (3D) compound 1 with a 4-connected and four-fold interpenetrating dia net topology, a chiral 2D compound 2 with unique double-helical chains along the c axis is generated. Fluorescence (FL) measurements indicate that both compounds demonstrate excellent FL sensing behaviour towards CS₂, a trace amount of Fe³⁺ ions and nitro-containing explosives. Notably, the layered 2 exhibits a unique “turn on” FL sensing behaviour towards thiol molecules, e.g., cysteine and 1-butylmercaptan, by exfoliating 2 into a few-layer FL material. The detection mechanism was investigated in detail by multiple characterizations including but not limited to EDS, SEM and AFM. The [(CH₃)₂NH₂]⁺ cations in 2 can be replaced by an Eu³⁺ ion to generate 2@Eu³⁺ which exhibits a new characteristic sharp red emission of Eu³⁺. More importantly, 2@Eu³⁺ can serve as a convenient ratiometric FL sensor for temperature, demonstrating obvious color changes from weak purple to red under UV light as the temperature increases. Furthermore, cationic green- and red-light emitting dyes incorporated with 2@dyes emit white light with 17.28% quantum yield.