Unveiling the optical properties of the MOF UiO-66 through a time-resolved study

Metal–organic frameworks (MOFs) represent an emerging class of porous crystalline materials with highly customizable periodic structures, offering opportunities for innovation in areas such as sustainable energy and advanced healthcare. Luminescent MOFs are gaining attention as key nanomaterials in photonics due to their tunable optical properties, including controllable emission wavelengths and excellent photostability. Among them, the zirconium-based archetypal UiO-66 stands out for its structural robustness, though its optical properties remain largely unexplored.

In our research, emphasis is placed on understanding the mechanisms underlying its luminescence, especially its photophysical behaviour under thermal treatments and environmental conditions, such as hydration. The optical properties of UiO-66 in powdered form were systematically investigated using steady-state and nanosecond time-resolved photoluminescence (PL) spectroscopy.

For the first time, upon laser excitation at 4.43 eV, the PL spectra of UiO-66 revealed a double-peak emission band comprising two overlapping components, peaking at 2.8 eV and 3.2 eV, with lifetimes of 1.5 ns and 5 ns, respectively. In contrast, UiO-66 in aqueous solution exhibited a single emission peak at 3.1 eV with a lifetime of 5 ns, demonstrating the material’s sensitivity to environmental factors, as water molecules suppress the lower-energy emission transition.
Temperature-dependent effects included a decrease in emission intensity coupled with a notable increase in lifetime as the temperature rose. The radiative rate of the involved transitions was estimated and found to vary with temperature, suggesting possible alterations in the electronic configuration.

Based on these findings, we developed a model to illustrate the photophysical processes occurring within the ligand–metal complex of UiO-66, involving an excitation transfer (ET) from the light-absorbing linker to the zirconium metal node, with ET efficiency strongly affected by external environmental factors.
This study deepens our understanding of the photophysical behaviour of MOFs and paves the way for the tailored design of UiO-66 in advanced optical technologies.