  The zero-dimensional (0D) metal halides comprise periodically distributed and
isolated metal-halide polyhedra, which act as the smallest inorganic quantum
systems and can accommodate quasi-localized Frenkel excitons. These excitons
exhibit unique photophysics including broadband photon emission, huge Stokes
shift, and long decay lifetime. The polyhedra can have different symmetries due
to the coordination degree of the metal ions. Little is known about how the
polyhedron type affects the characteristics of the 0D metal halide crystals. We
synthesize and comparatively study three novel kinds of 0D organic-inorganic
hybrid tin halide compounds. They are efficient light emitters with a highest
quantum yield of 92.3%. Although they have the same compositional organic
group, the most stable phases are composed of octahedra for the bromide and
iodide but disphenoids (see-saw structures) for the chloride. They separately
exhibit biexponential and monoexponential luminescence decays due to different
symmetries (Ci group for octahedra and C2 group for disphenoids) and
corresponding different electronic structures. The chloride has the largest
absorption photon energy among the three halides, but it has the smallest
emission photon energy. A model regarding the unoccupied energy band degeneracy
is proposed based on the experiments and density functional theory
calculations, which explains well the experimental phenomena and reveals the
crucial role of polyhedron type in determining the optical properties of the 0D
tin halide compounds.
