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

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 Sn3 S4 broken-cube clusters connected by double sulfur bridges to form polymeric (Sn3 S7 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 13 C and 119 Sn 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.