Turn-on luminescence from molecular rotor realignment in metal-organic framework thin films

Sub-unit motion within metal-organic frameworks (MOFs) offers unique opportunities for nanoscale sensing. However, achieving controlled partial rotation of bulky linkers remains a significant challenge. In this study, a 50-fold luminescence enhancement is observed from a MOF thin film when intra-pore solvent flow orients the linker chromophores. These MOF thin films can be prepared via a facile drop-casting method on various substrates. The MOFs structure consists of zinc-coordinated layers containing rotatable chromophores, separated by pillar molecules. Grazing-incidence wide-angle X-ray scattering analysis confirms the formation of highly oriented films. The deposition of a volatile organic compound, such as ethanol, triggers a significant enhancement in luminescence as the solvent nears complete evaporation. Photophysical characterization and quartz crystal microbalance measurements reveal that this phenomenon is driven by internal stress on the MOF's pore level generated during the final stages of evaporation. This stress can result in a realignment of the MOF chromophores at the molecular scale. Consequently, this dynamic turn-on luminescence behavior establishes a foundation for nanoscale platforms capable of indicating solvent volatilization in real time.