Achieving a molecular-level understanding of how the structures and compositions of metal–organic frameworks (MOFs) influence their charge carrier concentration and charge transport mechanism—the two key parameters of electrical conductivity—is essential for the successful development of electrically conducting MOFs, which have recently emerged as one of the most coveted functional materials due to their diverse potential applications in advanced electronics and energy technologies. Herein, we have constructed four new alkali metal (Na, K, Rb, and Cs) frameworks based on an electron-rich tetrathiafulvalene tetracarboxylate (TTFTC) ligand, which formed continuous π-stacks, albeit with different π–π-stacking and S⋯S distances (d_π–π and d_S⋯S). These MOFs also contained different amounts of aerobically oxidized TTFTC˙⁺ radical cations that were quantified by electron spin resonance (ESR) spectroscopy. Density functional theory calculations and diffuse reflectance spectroscopy demonstrated that depending on the π–π-interaction and TTFTC˙⁺ population, these MOFs enjoyed varying degrees of TTFTC/TTFTC˙⁺ intervalence charge transfer (IVCT) interactions, which commensurately affected their electronic and optical band gaps and electrical conductivity. Having the shortest d_π–π (3.39 Å) and the largest initial TTFTC˙⁺ population (∼23%), the oxidized Na-MOF 1-ox displayed the narrowest band gap (1.33 eV) and the highest room temperature electrical conductivity (3.6 × 10⁻⁵ S cm⁻¹), whereas owing to its longest d_π–π (3.68 Å) and a negligible TTFTC˙⁺ population, neutral Cs-MOF 4 exhibited the widest band gap (2.15 eV) and the lowest electrical conductivity (1.8 × 10⁻⁷ S cm⁻¹). The freshly prepared but not optimally oxidized K-MOF 2 and Rb-MOF 3 initially displayed intermediate band gaps and conductivity, however, upon prolonged aerobic oxidation, which raised the TTFTC˙⁺ population to saturation levels (∼25 and 10%, respectively), the resulting 2-ox and 3-ox displayed much narrower band gaps (∼1.35 eV) and higher electrical conductivity (6.6 × 10⁻⁵ and 4.7 × 10⁻⁵ S cm⁻¹, respectively). The computational studies indicated that charge movement in these MOFs occurred predominantly through the π-stacked ligands, while the experimental results displayed the combined effects of π–π-interactions, TTFTC˙⁺ population, and TTFTC/TTFTC˙⁺ IVCT interaction on their electronic and optical properties, demonstrating that IVCT interactions between the mixed-valent ligands could be exploited as an effective design strategy to develop electrically conducting MOFs.

中文翻译：

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π-堆叠混合价四硫富瓦烯配体间间隔电荷转移相互作用对3D金属-有机骨架电导率的影响

实现对金属有机骨架 (MOF) 的结构和组成如何影响其电荷载流子浓度和电荷传输机制（导电性的两个关键参数）的分子级理解对于成功开发导电 MOF 至关重要。由于其在先进电子和能源技术中的多种潜在应用，最近成为最令人垂涎​​的功能材料之一。在此，我们基于富电子四硫富瓦烯四羧酸盐 (TTFTC) 配体构建了四种新的碱金属（Na、K、Rb 和 Cs）骨架，它们形成了连续的 π 堆积，尽管具有不同的 π-π 堆积和 S ⋯S 距离（d _π–π和d _S⋯S）。这些 MOF 还包含不同数量的有氧氧化 TTFTC˙ ⁺自由基阳离子，这些阳离子通过电子自旋共振 (ESR) 光谱进行量化。密度泛函理论计算和漫反射光谱表明，根据 π–π-相互作用和 TTFTC˙ ⁺布居，这些 MOF 享有不同程度的 TTFTC/TTFTC˙ ⁺间隔电荷转移 (IVCT) 相互作用，相应地影响了它们的电子和光学带隙和电导率。具有最短的d _π–π (3.39 Å) 和最大的初始 TTFTC˙ ⁺群体 (∼23%)，氧化的 Na-MOF 1-ox显示出最窄的带隙 (1.33 eV) 和最高的室温电导率 (3.6 × 10 ^-5 S cm ^-1 )，而由于其最长的d _π–π (3.68 Å) 和可忽略不计的 TTFTC˙ ⁺种群，中性Cs-MOF 4表现出最宽的带隙（2.15 eV）和最低的电导率（1.8 × 10 ^-7 S cm ^-1）。将新鲜制备的，但不能最佳地氧化K-MOF 2和RB-MOF 3最初显示中间带隙和导电性，但是，在长时间的需氧氧化，这提高了TTFTC˙ ⁺人口达到饱和水平（分别为~25% 和 10%），所得的2-ox和3-ox显示出更窄的带隙（~1.35 eV）和更高的电导率（6.6 × 10 ^-5和 4.7 × 10 ^-5 S cm ^-1，分别）。计算研究表明，这些 MOF 中的电荷运动主要通过 π-堆叠配体发生，而实验结果显示了 π–π-相互作用、TTFTC· ⁺群体和 TTFTC/TTFTC· ^{+ 的综合影响} IVCT 相互作用对其电子和光学性质的影响，表明混合价配体之间的 IVCT 相互作用可作为开发导电 MOF 的有效设计策略。