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

[Image: see text] 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(NH(2))(3)(+)] and mixed cesium–guanidinium cations: G(2)SnBr(4), CsGSnBr(4), and Cs(2)GSn(2)Br(7). G(2)SnBr(4) has a one-dimensional structure that consists of chains of corner-sharing [SnBr(5)](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: CsGSnBr(4) and Cs(2)GSn(2)Br(7). Furthermore, the flat shapes of the guanidinium cations induce anisotropic out-of-plane tilts of the [SnBr(6)](4–) octahedra in the CsGSnBr(4) and Cs(2)GSn(2)Br(7) compounds. In G(2)SnBr(4), the Sn lone pair is highly stereoactive and favors non-octahedral, that is, square pyramidal coordination of Sn(II). G(2)SnBr(4) exhibits bright broad-band emission from self-trapped excitonic states, owing to its soft lattice and electronic localization. This emission in G(2)SnBr(4) 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.