Highly Luminescent Crystalline Sponge: Sensing Properties and Direct X-ray Visualization of the Substrates

A phenomenon of crystalline sponge is represented by guest-dependent structural fluidity of the host polymeric lattice in highly crystalline sorbents, such as metal-organic frameworks, driven by multiple weak intermolecular interactions. Such induced fitting in MOFs is a valuable property in selective adsorption, guest determination by single-crystal XRD and in-situ structural analysis under external stimuli. In this work, a porous three-dimensional metal-organic framework [Eu₂(DMF)₄(ttdc)₃]·4.45DMF (1; DMF = N,N-dimethylformamide, ttdc^2- = trans-thienothiophenedicarboxylate anion) was applied as a crystalline sponge bearing luminescent functionality to couple its sensing properties with direct structural determination of the adsorbed molecules. As a result, the paper discusses crystal structures and luminescent properties for the successfully obtained new adducts with the crystallographic formulae [Eu₂(DMSO)₄(ttdc)₃]·2.5DMSO·2.2H₂O (1; DMSO = dimethylsulfoxide), [Eu₂(DMF)₄(ttdc)₃]·3phet (1; phet = phenylethanal) and [Eu₂(DMF)_3.5(cin)_0.5(ttdc)₃]·1.64cin (1; cin = trans-cinnamaldehyde). As a result of inclusion of DMSO into 1, a slight increase in the quantum yield and excited state phosphorescence lifetime was observed, while the adsorption of phet leads to a considerable (up to three times) decrease in the corresponding values. The incorporation of cinnamal results in a full quenching of QY, from 20% down to zero, and a more than order of magnitude diminishing of the excited state lifetime compared to the initial 1. The effective sensing of cinnamal was explained from the structural point of view by its direct coordination to the Eu³⁺ emitter, as well as by multiple weak intermolecular interactions with ttdc antenna ligand, both capable of enhancing the non-radiative energy dissipation.