Exciton Self-Trapping in Hybrid Lead Halides: Role of Halogen

Low-dimensional
hybrid lead halides have recently been reported
as efficient white light emitters. However, unlike lead halide 3D
perovskites, most of the reported low-dimensional materials with broad-band
emission crystallize in different structure types according to their
halogen composition (i.e., Cl, Br, and I) for a selected organic molecule.
Because of the absence of isostructural halide series, the role
of chemistry in the self-trapping of the excitons at the origin of
the broad-band emission remains unclear. Among the most efficient
white phosphors, hybrid lead bromide (TDMP)­PbBr4 (TDMP
= trans-2,5-di­methyl­piper­azinium)
built of post-perovskite type chains exhibits a record photoluminescence
quantum yield for hybrid lead halides. In this article, the two new
isostructural (TDMP)­PbX4 chloride and iodide analogues
could be synthesized and structurally characterized. A comparison
of the optical properties of the lead halide series reveals a strong
dependence of the nature of the halogen (Cl, Br, or I) on the trapping/detrapping
of the excitons and the resulting emission intensities, wavelengths,
and band broadness.