Zr₆(μ₃-O)₄(μ₃-OH)₄ node cores are indispensable building blocks for almost all zirconium-based metal–organic frameworks. Consistent with the insulating nature of zirconia, they are generally considered electronically inert. Contrasting this viewpoint, we present spectral measurements and calculations indicating that emission from photoexcited NU-601, a six-connected Zr-based MOF, comes from both linker-centric locally excited and linker-to-node charge-transfer (CT) states. The CT state originates from a hole transfer process enabled by favorable energy alignment of the HOMOs of the node and linker. This alignment can be manipulated by changing the pH of the medium, which alters the protonation state of multiple oxy groups on the Zr-node. Thus, the acid–base chemistry of the node has a direct effect on the photophysics of the MOF following linker-localized electronic excitation. These new findings open opportunities to understand and exploit, for energy conversion, unconventional mechanisms of exciton formation and transport in MOFs.

中文翻译：

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锆基金属-有机框架中意外的光驱动连接子到节点空穴转移


Zr₆（μ_3-O）₄（μ_3-OH）₄ 节点内核是几乎所有锆基金属有机框架不可或缺的构建模块。与氧化锆的绝缘性质一致，它们通常被认为是电子惰性的。与这一观点形成对比，我们提出了光谱测量和计算，表明光激发 NU-601（一种基于 Zr 的六连接 MOF）的发射来自以接头为中心的局部激发和接头到节点电荷转移 （CT） 状态。CT 状态起源于空穴转移过程，该过程由节点和连接子的 HOMO 的良好能量对齐实现。这种排列可以通过改变培养基的 pH 值来操纵，从而改变 Zr 节点上多个氧基团的质子化状态。因此，节点的酸碱化学对接头局域化电子激发后 MOF 的光物理学有直接影响。这些新发现为理解和利用 MOF 中激子形成和运输的非常规机制进行能量转换提供了机会。