Molecular\nOrigins of Near-Infrared Luminescence in\nMolybdenum and Tungsten Oxyhalide Perovskites

Materials with near-infrared (near-IR) luminescence are\ndesirable\nfor applications in communications and sensing, as well as biomedical\ndiagnostics and imaging. The most used inorganic near-IR emitters\nrely on precise doping of host crystal structures with select rare-earth\nor transition metal ions. Recently, another class of materials with\nintrinsic near-IR emission has been reported. The compositions of\nthese materials were initially described as vacancy-ordered halide\ndouble perovskites Cs<sub>2</sub>MoCl<sub>6</sub> and Cs<sub>2</sub>WCl<sub>6</sub>, but further investigation by some of us on the compound\nreported as Cs<sub>2</sub>WCl<sub>6</sub> revealed an oxyhalide instead,\nwith a composition Cs<sub>2</sub>WO<sub><i>x</i></sub>Cl<sub>6–<i>x</i></sub>, where 1 < <i>x</i> < 2. Here we demonstrate that the Mo compounds similarly possess\nthe composition Cs<sub>2</sub>MoO<sub><i>x</i></sub>Cl<sub>6–<i>x</i></sub> or Cs<sub>2</sub>MoO<sub><i>x</i></sub>Br<sub>6–<i>x</i></sub> where 1\n< <i>x</i> < 2. Preparing the pure halide appears\nharder for Mo than for W, and we have not succeeded in doing so. The\ndistinctly different composition requires the coordination environment\nand oxidation state for the Mo and W centers to be reconsidered from\nwhat was assumed for the pure halides. In this work, we examine the\nmechanism for near-IR emission in these materials given their true\nstructures and compositions. We demonstrate that the luminescence\nis due to the specific d-orbital splitting caused by the presence\nof oxygen in the distorted [MOX<sub>5</sub>]<sup>2–</sup> octahedra\n(X is Cl or Br). The fine structure in the emission spectra at low\ntemperatures has been resolved and is attributed to vibronic coupling\nto the Mo–O and W–O bond stretches. Understanding the\ntrue structure and composition of these interesting materials, besides\nexplaining the near-IR luminescence, suggests how this desirable emission\ncan be realized and manipulated.