Cornelio Joel, Lee Seok June, Zhou Tian-You, Alkaş Adil, Thangavel Kavipriya, Pöppl Andreas, Telfer Shane G
School of Natural Sciences, MacDiarmid Institute of Advanced Materials and Nanotechnology, Massey University, Palmerston North 4410, New Zealand.
Felix Bloch Institute for Solid State Physics, Leipzig University, Linnestrasse 5, Leipzig D-04103, Germany.
Chem Mater. 2022 Sep 15;34(18):8437-8445. doi: 10.1021/acs.chemmater.2c02220. eCollection 2022 Sep 27.
Metal-organic frameworks (MOFs) can respond to light in a number of interesting ways. Photochromism is observed when a structural change to the framework is induced by the absorption of light, which results in a color change. In this work, we show that introducing quinoxaline ligands to MUF-7 and MUF-77 (MUF = Massey University Framework) produces photochromic MOFs that change color from yellow to red upon the absorption of 405 nm light. This photochromism is observed only when the quinoxaline units are incorporated into the framework and not for the standalone ligands in the solid state. Electron paramagnetic resonance (EPR) spectroscopy shows that organic radicals form upon irradiation of the MOFs. The EPR signal intensities and longevity depend on the precise structural details of the ligand and framework. The photogenerated radicals are stable for long periods in the dark but can be switched back to the diamagnetic state by exposure to visible light. Single-crystal X-ray diffraction analysis reveals bond length changes upon irradiation that are consistent with electron transfer. The multicomponent nature of these frameworks allows the photochromism to emerge by allowing through-space electron transfer, precisely positioning the framework building blocks, and tolerating functional group modifications to the ligands.
金属有机框架（MOFs）能够以多种有趣的方式对光做出响应。当框架结构因光吸收而发生变化并导致颜色改变时，就会观察到光致变色现象。在这项工作中，我们表明，将喹喔啉配体引入MUF - 7和MUF - 77（MUF = 梅西大学框架）会产生光致变色的MOFs，它们在吸收405 nm光后会从黄色变为红色。仅当喹喔啉单元被纳入框架时才会观察到这种光致变色现象，而对于固态的独立配体则不会出现。电子顺磁共振（EPR）光谱表明，MOFs在辐照时会形成有机自由基。EPR信号强度和寿命取决于配体和框架的精确结构细节。光生自由基在黑暗中能长时间保持稳定，但通过暴露在可见光下可以切换回抗磁状态。单晶X射线衍射分析揭示了辐照时键长的变化，这与电子转移一致。这些框架的多组分性质通过允许空间电子转移、精确排列框架结构单元以及耐受配体上的官能团修饰，使得光致变色现象得以出现。
