Interplay between Electronic, Magnetic, and Transport Properties in Metal Organic–Radical Frameworks

The development of modern electronic and spintronic technologies depends in large part on the ability to design materials exhibiting switchable magnetic and electrical properties. Here, motivated by the successful demonstration of reversible redox switching of magnetic order and electrical conductivity in two-dimensional metal–organic frameworks (MOFs) based on benzoquinoid linkers, we perform hybrid density functional theory calculations to investigate this phenomenon at the atomistic level. Electronic, magnetic, and charge transport properties have been systematically investigated for oxidized and reduced forms of Mn and Fe benzoquinoid frameworks (i.e., (Me4N)2[Mn2L3], (Me4N)2[Fe2L3] and Na3(Me4N)2[Mn2L3], Na­(Me4N)2[Fe2L3], respectively, with deprotonated chloranilic acid as L). We demonstrate that the experimentally observed large increase in electronic conductivity upon ligand-centered reduction in the Mn MOF (109 S·cm–1) is due to cooperative effects arising from band gap reduction and the presence of electrons with lower effective mass. Superior conductivity (by at least 3 orders of magnitude) of the redox pair of the Fe benzoquinoid framework as compared to the Mn analogue stems from similar factors and, notably, a large increase in electron delocalization for the reduced Fe compound.现代电子学与自旋电子学技术的发展，在很大程度上依赖于设计具备可切换磁学与电学特性材料的能力。受此前已成功证实的、基于苯醌类连接体的二维金属有机框架（metal-organic frameworks, MOFs）中磁序与电导率可逆氧化还原切换的研究启发，我们开展了杂化密度泛函理论（hybrid density functional theory）计算，在原子尺度下探究该现象的内在机制。我们系统研究了锰基与铁基苯醌类框架的氧化态与还原态的电子、磁学及电荷输运特性，对应化合物分别为(Me₄N)₂[Mn₂L₃]、(Me₄N)₂[Fe₂L₃]以及Na₃(Me₄N)₂[Mn₂L₃]、Na(Me₄N)₂[Fe₂L₃]，其中L为去质子化氯冉酸（chloranilic acid）。我们证实，实验中观测到的锰基MOF在配体中心发生还原反应后电子电导率大幅提升（达109 S·cm⁻¹），其原因源于带隙缩小与低有效质量电子共同带来的协同效应。与锰基同类框架相比，铁基苯醌类框架的氧化还原对具备更优异的电导率（至少高出3个数量级），这同样源于上述相似因素，尤为关键的是还原态铁基化合物的电子离域程度显著提升。