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

Abstract

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- A nd 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.