Cluster-Based Metal-Organic Frameworks: Modulated Singlet-Triplet Excited States and Temperature-Responsive Phosphorescent Switch

Luminescent metal-organic frameworks (MOFs) have received much attention due to their applications in color displays, sensors, and smart materials. However, how to balance the energy distribution between singlet and triplet excited states for a new generation of persistent luminescent MOFs is still a challenging goal. In this work, we report that the construction of cluster-based MOFs can supply an effective way to modulate the fluorescence and room-temperature phosphorescence (RTP) emission based on adjustable pi-pi stacking, halogen-bonding interaction, and metal-cluster units. Compared to the pristine ligand (5-bromoisophthalic acid) with obvious spin-orbit coupling, Zn-s and Zn-3 cluster-based MOFs exhibit tunable photoluminescence (such as fluorescence and RTP wavelength, lifetime, and quantum yield). The ultralong-lived RTP visualization and temperature-dependent luminescence also provide the Zn-s cluster-based MOF as a new type of anticounterfeiting and temperature-responsive phosphorescent switch material. Therefore, this work highlights the first example of cluster-based MOFs as ultralong-lived persistent luminescent materials for tuning singlet and triplet excited states, which may be extended to other similar systems for developing ultralong RTP and delayed fluorescence materials.