Molecular Origins of Near-Infrared Luminescence in Molybdenum and Tungsten Oxyhalide Perovskites

Materials with near-infrared (near-IR) luminescence are
desirable
for applications in communications and sensing, as well as biomedical
diagnostics and imaging. The most used inorganic near-IR emitters
rely on precise doping of host crystal structures with select rare-earth
or transition metal ions. Recently, another class of materials with
intrinsic near-IR emission has been reported. The compositions of
these materials were initially described as vacancy-ordered halide
double perovskites Cs2MoCl6 and Cs2WCl6, but further investigation by some of us on the compound
reported as Cs2WCl6 revealed an oxyhalide instead,
with a composition Cs2WOxCl6–x, where 1 < x < 2. Here we demonstrate that the Mo compounds similarly possess
the composition Cs2MoOxCl6–x or Cs2MoOxBr6–x where 1
< x < 2. Preparing the pure halide appears
harder for Mo than for W, and we have not succeeded in doing so. The
distinctly different composition requires the coordination environment
and oxidation state for the Mo and W centers to be reconsidered from
what was assumed for the pure halides. In this work, we examine the
mechanism for near-IR emission in these materials given their true
structures and compositions. We demonstrate that the luminescence
is due to the specific d-orbital splitting caused by the presence
of oxygen in the distorted [MOX5]2– octahedra
(X is Cl or Br). The fine structure in the emission spectra at low
temperatures has been resolved and is attributed to vibronic coupling
to the Mo–O and W–O bond stretches. Understanding the
true structure and composition of these interesting materials, besides
explaining the near-IR luminescence, suggests how this desirable emission
can be realized and manipulated.