Photoinduced Electron Transfer in Multicomponent Truxene-Quinoxaline Metal–Organic Frameworks

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（Metal–organic frameworks, MOFs）可通过多种有趣的方式对光产生响应。光致变色（Photochromism）指框架因吸光诱导结构变化，进而引发颜色改变的现象。本研究表明，将喹喔啉配体引入MUF-7与MUF-77（MUF = 梅西大学框架，Massey University Framework）中，可得到光致变色金属有机框架，该材料在吸收405 nm波长的光后，会从黄色转变为红色。仅当喹喔啉单元被整合至框架结构中时，才可观察到此光致变色现象，而固态下的游离配体则无此现象。电子顺磁共振（Electron paramagnetic resonance, EPR）光谱分析显示，MOFs受辐照后会生成有机自由基；EPR信号的强度与存续时长取决于配体与框架的精确结构细节。光生自由基在黑暗环境中可长期稳定存在，但经可见光照射后可恢复至抗磁状态。单晶X射线衍射分析表明，辐照后框架发生键长变化，这与电子转移过程相符。此类框架的多组分特性，使其可通过空间电子转移、精准定位框架构筑单元，以及兼容配体的官能团修饰，从而实现光致变色现象。