Synthesis and characterization of [(CH3)2NH2][Na0.5Cr0.5(HCOO)3]: a rare example of luminescent metal-organic frameworks based on Cr(III) ions.

A novel formate [(CH3)2NH2][Na0.5Cr0.5(HCOO)3] (DMNaCr) was prepared by a solvothermal method. This compound crystallizes in the perovskite-type metal formate framework (space group R3[combining macron]) with disordered dimethylammonium (DMA(+)) cations. X-ray diffraction, DSC, Raman and IR studies show that in contrast to the isostructural iron analogue [(CH3)2NH2][Na0.5Fe0.5(HCOO)3] (DMNaFe), DMNaCr does not exhibit any structural phase transition at low temperatures. This behavior has been attributed to the smaller flexibility of the perovskite-like framework in DMNaCr when compared with that of DMNaFe. Dielectric permittivity data reveal pronounced dielectric relaxation that is attributed to the dynamical rotation of DMA(+) ions. Electron absorption and photoluminescence studies show that this material exhibits efficient emission at low temperatures. A detailed analysis of the optical properties shows that chromium ions are located at the site of intermediate crystal field strength with Dq/B = 2.29.