A competition between 2D and 3D magnetic orderings in novel mixed valent copper frameworks

Low-dimensional hybrid inorganic-organic frameworks exhibit high structural flexibility and allow for the inclusion of various magnetic and optically-active species into their host structures. The emergence of copper-based hybrid structures for various optical applications provides a promising foundation for exploring the integration of magnetic sublattices, paving the way for advancements in magneto-optical coupling and multifunctional materials. Herein, we introduce a novel class of hybrid copper frameworks with covalently-connected alternating magnetic 2D copper(ii) formate and non-magnetic copper(i) bromide layers. The anionic framework is stabilized by A⁺ cations to form ACu₅Br₄(COOH)₄ (A⁺ = Na⁺, K⁺, Rb⁺, NH₄ ⁺) semiconductors (bandgaps 2.1-2.2 eV) with optical transitions suitable for optoelectronic applications. Comprehensive magnetometry studies show that ACu₅Br₄(COOH)₄ compounds exhibit low-dimensional 2D short-range antiferromagnetic order within the formate layers, characterized by strong exchange coupling (J/k _B ∼ -100 K). Upon further temperature reduction, interactions between Cu(ii) layers give rise to 3D long-range magnetic order at ∼40 K, despite the large (8.6-8.8 Å) spatial separation of the magnetic Cu(ii) formate layers by nonmagnetic Cu(i)-Br bridging layers. This transition is further supported by electron paramagnetic resonance (EPR) spectroscopy. This study expands our understanding of low-dimensional hybrid frameworks and opens new avenues for the design of 2D multifunctional materials.