Guanidinium and Mixed Cesium–Guanidinium Tin(II) Bromides: Effects of Quantum Confinement and Out-of-Plane Octahedral Tilting

Hybrid organic–inorganic main-group metal halide compounds are the subject of intense research owing to their unique optoelectronic characteristics. In this work, we report the synthesis, structure, and electronic and optical properties of a family of hybrid tin (II) bromide compounds comprising guanidinium [G, C­(NH₂)₃⁺] and mixed cesium–guanidinium cations: G₂SnBr₄, CsGSnBr₄, and Cs₂GSn₂Br₇. G₂SnBr₄ has a one-dimensional structure that consists of chains of corner-sharing [SnBr₅]^2– square pyramids and G cations situated in between the chains. Cs⁺ exhibits a pronounced structure-directing effect where a mixture of Cs⁺ and G cations forms mono- and bilayered two-dimensional perovskites: CsGSnBr₄ and Cs₂GSn₂Br₇. Furthermore, the flat shapes of the guanidinium cations induce anisotropic out-of-plane tilts of the [SnBr₆]^4– octahedra in the CsGSnBr₄ and Cs₂GSn₂Br₇ compounds. In G₂SnBr₄, the Sn lone pair is highly stereoactive and favors non-octahedral, that is, square pyramidal coordination of Sn­(II). G₂SnBr₄ exhibits bright broad-band emission from self-trapped excitonic states, owing to its soft lattice and electronic localization. This emission in G₂SnBr₄ is characterized by a photoluminescence (PL) quantum yield of 2% at room temperature (RT; 75 ± 5% at 77 K) and a fast PL lifetime of 18 ns at room temperature.