Luminescent lanthanide MOFs based on conjugated 1,1′-ethynebenzene-3,3′,5,5′-tetracarboxylate ligand: Syntheses, structures and photoluminescent properties

A family of novel lanthanide metal-organic frameworks (MOFs) with formula [Ln₂(EBTC)_1.5(CH₃OH)₄]·6H₂O [EBTC^4- = 1,1′-ethynebenzene-3,3′,5,5′-tetracarboxylate, Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8) and Dy (9)], have been synthesized via solvothermal reaction. All compounds are isostructural, crystallized in the monoclinic space group P2₁/n, and show a three-dimensional (4,6)-connected network with Schläfli symbol of {3¹⁰}. Photoluminescent measurements indicated that 1 and 7 emit the luminescence originating from the intraligand π←π∗ transition, 2 shows the broadband emission due to allowed 4f-5d transitions, and 4-9 show the emission of typical lanthanide f-f transition via the ligand "antenna effect" in the solid state at ambient temperature. Interestingly, compounds 6 and 8 showed microsecond time-scale fluorescence lifetimes with 0.84 ms for 6 and 0.39 ms for 8, respectively. Such unique spectroscopy features may have applications in biomacromolecule research.