The structure-directing amine changes everything: structures and optical properties of two-dimensional thiostannates

Abstract

Two different two-dimensional thiostannates (SnS) were synthesized using tris(2-aminoethyl)amine (tren) or 1-(2-aminoethyl)piperidine (1AEP) as structure-directing agents. Both structures consist of negatively charged thiostannate layers with charge stabilizing cations sandwiched in-between. The fundamental building units are Sn3S4 broken-cube clusters connected by double sulfur bridges to form polymeric (Sn3S7 2−) n honeycomb hexagonal layers. The compounds are members of the R-SnS-1 family of structures, where R indicates the type of cation. Despite consisting of identical structural units, the band gaps of the two semiconducting compounds were found to differ substantially at 2.96 eV (violet–blue light) and 3.21 eV (UV light) for tren–SnS-1 and 1AEP–SnS-1, respectively. Aiming to explain the observed differences in optical properties, the structures of the two thiostannates were investigated in detail based on combined X-ray diffraction, solid-state 13C and 119Sn MAS NMR spectroscopy and scanning electron microscopy studies. The compound tren–SnS-1 has a hexagonal structure consisting of planar SnS layers with regular hexagonal pores and disordered cations, whereas 1AEP–SnS-1 has an orthorhombic unit cell with ordered cations, distorted hexagonal pores and non-planar SnS layers. In the formation of 1AEP–SnS-1, an intramolecular reaction of the structure-directing piperidine takes place to form an N-heterobicyclic cation through in situ C—H activation. Hirshfeld surface analysis was used to investigate the interaction between the SnS layers and cations in 1AEP–SnS-1 and revealed that the most nucleophilic part of the SnS sheets is one of the two crystallographically distinct double sulfur bridges.