Lead-MOFs derived from an aggregation-induced emission ligand: a heat-induced fluorescence “turn-on” thermometer and the role of Na+ cations

Abstract

Recently, metal–organic frameworks (MOFs) have made great achievements as fluorescence thermometers, but most of them exhibit responses to temperature through the turn-off effect due to the temperature-caused quenching effect. In this work, we reported the synthesis of two lead-based MOFs with the formula of [Pb₂(ETTC)(DMA)₂]·2DMA (1) and [(CH₃)₂NH₂]·[Pb₂Na(HETTC)₂]·5.5DMA (2) derived from an aggregation-induced emission (AIE) ligand 4′,4′′,4′′′,4′′′′-(ethene-1,1,2,2-tetrayl)tetrakis([1,1′-biphenyl]-4-carboxylic acid) (H₄ETTC), both of which could function as fluorescence “turn-on” thermometers. MOFs 1 and 2 were synthesized under similar conditions except for the introduction of Na⁺ for MOF 2. Structural analysis revealed that both MOFs 1 and 2 manifested three-dimensional (3D) coordination frameworks with 1D channels along the b-axis, but MOF 1 was constructed from the organic ligand ETTC⁴⁻ and Pb–O–Pb chains while MOF 2 was built from the organic ligand HETTC³⁻ and Pb–O–Na chains. Impressively, due to the presence of AIE fluorophores, both MOFs 1 and 2 exhibited superior fluorescence properties and their emission intensity could be significantly enhanced as the temperature increased from 10 to 300 K. Further experiments demonstrated that they could be employed as fluorescence turn-on thermometers and the presence of Na⁺ made a great difference to the relationship between emission intensity and temperature.