Alkaline-earth metal based MOFs with second scale long-lasting phosphor behavior

Abstract

Three novel 3D metal–organic frameworks (MOFs) based on alkaline-earth (AE) metal ions and aromatic carboxylic linkers, i.e. {[Ca₃(btb)₂(DMF)₃]·7DMF}_n (1), {[Ca(bdc)(DMF)(H₂O)]·2H₂O}_n (2) and {[Ba(NH₂-bdc)(DMF)]}_n (3) [where btb = 1,3,5-tris(4-carboxyphenyl)benzene, bdc = 1,4-benzenedicarboxylate and NH₂-bdc = 2-amino-1,4-benzenedicarboxylate] have been synthesized under mild solvothermal conditions. All the compounds contain three-dimensional frameworks built up from rod-like AE-carboxylate based secondary building units joined by the aromatic carboxylate linkers. Moreover, their crystal structures present intriguing physical features such as chirality (1 and 3) or large voids occupied by solvent molecules (1 and 2), in addition to less frequent long-lasting phosphorescence (LLP) originating from the ligands' triplet states, as confirmed by DFT calculations. This persistent photoluminescence is characterized by steady-state blue fluorescence that leads to a pale blue (compound 2) and green (1 and 3) afterglow, which lasts for seconds. Exhaustive characterization of long-lasting phosphorescence at low temperature by means of time-resolved spectroscopy reveals remarkably long lifetimes (τ) of ca. 835 and 375 ms (for 1 and 3), and as high as the record-breaking τ ≈ 2 seconds for 2, which are remarkably higher than those recorded for free ligands. Interestingly, compound 2 preserves excellent room temperature phosphorescence (RTP) characterized by an emission lifetime of nearly 1 second.