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

Three novel 3D metal-organic frameworks (MOFs) based on alkaline-earth (AE) metal ions and aromatic carboxylic linkers, i.e. {[Ca 3 (btb) 2 (DMF) 3 ]·7DMF} n ( 1 ), {[Ca(bdc)(DMF)(H 2 O)]·2H 2 O} n ( 2 ) and {[Ba(NH 2 -bdc)(DMF)]} n ( 3 ) [where btb = 1,3,5-tris(4-carboxyphenyl)benzene, bdc = 1,4-benzenedicarboxylate and NH 2 -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. Three MOFs based on alkaline-earth metals and aromatic carboxylate linkers show blue fluorescence that turns into green long-lasting phosphorescence.