Disphenoidal Zero-Dimensional Lead, Tin, and Germanium Halides: Highly Emissive Singlet and Triplet Self-Trapped Excitons and X‑ray Scintillation

Low-dimensional
metal halides have been researched as optoelectronic
materials for the past two decades. Zero-dimensional halides of ns2 elements (Sn, Pb, Sb) have recently gained attention as highly
efficient broadband light emitters. These compounds comprise discrete
metal halide centers, isolated by bulky organic cations. Herein, we
report isostructural halide complexes of Ge­(II), Sn­(II), and Pb­(II)
with a 1-butyl-1-methyl-piperidinium cation (Bmpip), featuring unusual
disphenoidal coordination with a highly stereoactive lone pair. Spectrally
broad, bright emission from highly localized excitons, with quantum
efficiencies of up to 75%, is observed in blue to red spectral regions
for bromides (for Pb, Sn, and Ge, respectively) and extends into the
near-infrared for Bmpip2SnI4 (peak at 730 nm).
In the case of Sn­(II) and Ge­(II), both singlet and triplet excitonic
emission bands have been observed. Furthermore, Bmpip2SnBr4 and Bmpip2PbBr4 exhibit X-ray-excited
luminescence (radioluminescence) with brightness being commensurate
with that of a commercial inorganic X-ray scintillator (NaI:Tl).