Ultra‐Thin 2D Ionic Salt Supported with Strong Hydrogen‐Bonding Assisted Ionic Interaction

Abstract 2D materials have attracted great interest since the report of graphene. However, because of the fragile stability of ultra‐thin nanosheets, most studies are restricted to sheets maintained by strong covalent or coordination bonds. The research on which kind of bonds can maintain the free‐standing existence of 2D nanosheets is still of great significance. Recently, 2D ionic salts are successfully synthesized on substrates, but whether 2D ionic salts can free‐stand is still a problem. Herein this problem is addressed by a free‐standing 2D ionic salt (thickness: ≈2 nm) exfoliated from a 4,4′‐bipyridinium hydrochloride salt crystal. The stability of this 2D salt is supported by a strong NH···Cl hydrogen (H)‐bonding assisted ionic interaction (17.99 kcal mol⁻¹), which is verified by density functional theory calculation and natural bond orbital analysis. The salt crystal has strong air‐stable radicals inside and the 2D ionic salt exhibits red fluorescence in solution and in solid‐state, especially in solution the stokes shifts are very large (≈ 386 nm). This breakthrough work is not only beneficial for the construction of novel 2D materials but also for the understanding of H‐bonding interactions.