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.