Bulk Assemblies of Lead Bromide Trimer Clusters with Geometry-Dependent Photophysical Properties

Abstract

Single crystalline bulk assemblies of metal halide clusters show great potential as highly efficient light emitters with tunable photophysical properties. However, synthetic control of the geometry of the clusters in a rational manner has not been well established, and the relationships between the photophysical properties and structures of this emerging class of zero-dimensional materials are still not well understood. Here, we report the synthesis and characterization of two bulk assemblies of lead bromide clusters, (bmpy)6[Pb3Br12] (T1) and (bmpy)9[ZnBr4]2[Pb3Br11] (T2) (bmpy: 1-butyl-1-methylpyrrolidinium), which contain metal halide trimer clusters with different geometries. T1 with chain-shaped [Pb3Br12]6– clusters is not emissive at room temperature, whereas T2 with triangle-shaped [Pb3Br11]5– clusters exhibits yellowish-green emission peaked at 564 nm with a photoluminescence quantum efficiency of 7% at room temperature. Detailed analysis of the structural and photophysical properties show that the photophysical properties and excited-state dynamics of these materials are highly dependent on the geometry of the metal halide clusters.

Pressure‐Engineered Photoluminescence Tuning in Zero‐Dimensional Lead Bromide Trimer Clusters

“…Under initial compression to ≈2 GPa, the [Pb 3 Br 11 ] 5− clusters are contracted and gradually distorted in both intra‐ and inter‐octahedral structures. In this case, the relatively rigid and distorted molecular environment of [Pb 3 Br 11 ] 5− clusters could prevent the transition of STEs from T1a to T1b (Figure 4 b), and thus the emission from T1a (peak I) is gradually enhanced [2, 4a] . This mechanism is similar to the emission variation of (bmpy) 9 [ZnBr 4 ] 2 [Pb 3 Br 11 ] at low temperature [2] .…”

“…The PL emission spectrum consists of two emission peaks, locating at ≈518 nm (peak I) and ≈570 nm (peak II), respectively. Previous studies have ascribed these two peaks to the STE emissions from spin‐triplet STE states [2, 4a] …”

“…The photoactive building blocks in 0D HMHs are totally isolated by the non‐photoactive components, leading to strong quantum confinement effect within the structure. Upon photoexcitation, the excitons are coupled with lattice and trapped in the distorted building blocks as STEs for emission [2] . Hence, structural geometries could greatly affect the emission properties of 0D HMHs [3] .…”

“…In (bmpy) 9 [ZnBr 4 ] 2 [Pb 3 Br 11 ], the individual building blocks of [ZnBr 4 ] 2− tetrahedra and bmpy + cations are nonluminous, acting as spacers and forming the periodically arranged “host” matrix (Figures 1 a and S1) [2, 4a] . The PL emission originates from the photoactive “guest” specie of triangle‐shaped [Pb 3 Br 11 ] 5− clusters (Figure 1 b).…”

“…Within the triplet states, the potential energy surface contains two energy minima of T1a and T1b. The highly symmetric geometry and soft molecular environment of [Pb 3 Br 11 ] 5− cluster contribute to the considerable transition of STEs from T1a (Peak I) to T1b (Peak II) [2, 4a] . Thus, the ambient PL emission spectrum of (bmpy) 9 [ZnBr 4 ] 2 [Pb 3 Br 11 ] is asymmetric and dominated by the STE emission from T1b.…”