An Enantiomeric Pair of Alkaline-Earth Metal Based Coordination Polymers Showing Room Temperature Phosphorescence and Circularly Polarized Luminescence

This work reports on the synthesis and characterization of an enantiomeric pair of 3D coordination polymers (CPs) based on heavy alkaline-earth metal ions and cyclic aliphatic chiral linkers, i.e. [Ba2(μ-L-pyro)2(μ4-NO3)(μ3-form)]n (1-L) and [Ba2(μ-D-pyro)2(μ4-NO3)(μ3-form)]n (1-D) [where pyro = pyroglutamate and form = formate]. These materials present a dense framework built up from the junction of barium(II) ions by means of three bridging ligands showing different connectivity, which affords robustness and, in turn, large stability to the crystal building. L- or D-pyro ligands are formed in situ during the solvothermal synthesis through the cyclization of the corresponding chiral glutamine species, which does not affect the chirality of the resulting ligand such that the structures preserve an enantiopure nature. A deep study of the photophysical properties reveals that these CPs display intriguing photoluminescence (PL) properties involving intense fluorescence and long-lasting phosphorescence, even at room temperature, which have been deeply characterized by diffuse reflectance, variable-temperature emission analysis and time-resolved emission spectra (TRES). The experimental results are sustained by DFT calculations that decipher the PL mechanism. Moreover, these materials exhibit intense pyro-based circularly polarized luminescence in agreement with their chiral nature, which constitutes the first example of an AE-based CP able to display CPL signals.