Tuning of gallery heights in a crystalline 2D carbon nitride network

Poly(triazine imide)-a 2D layered network-can be obtained as an intercalation compound with halides from the ionothermal condensation of dicyandiamide in a eutectic salt melt. The gallery height of the intercalated material can be tuned via the composition of the eutectic melt and by post-synthetic modification. Here, we report the synthesis of poly(triazine imide) with intercalated bromide ions (PTI/Br) from a lithium bromide and potassium bromide salt melt. PTI/Br has a hexagonal unit-cell (P6(3)cm (no. 185); a 8.500390(68) angstrom, c 7.04483(17) angstrom) that contains two layers of imide-bridged triazine (C3N3) units stacked in an AB-fashion as corroborated by solid-state NMR, FTIR spectroscopy and high-resolution TEM. By comparison with a recently reported material PTI/Li+Cl-, prepared from a LiCl/KCl eutectic, the layer-stacking distance in the analogous bromide material was expanded from 3.38 angstrom to 3.52 angstrom - an exceptionally large spacing for an aromatic, discotic system (cf. graphite 3.35 angstrom). Subsequent treatment of PTI/Br with concentrated ammonium fluoride yields poly(triazine imide) with intercalated fluoride ions (PTI/F) (P6(3)/m (no. 176); a 8.4212(4) angstrom, c 6.6381(5) angstrom) as a statistical phase mix with PTI/Br. Fluoride intercalation leads to a contraction of the gallery height to 3.32 A, demonstrating that the gallery height is synthetically tuneable in these materials.