Quantifiably Tuneable Luminescence by Ultra-Thin Metal-Organic Nanosheets via Dual-Guest Energy Transfer
Luminescent metal-organic frameworks (LMOFs) are promising materials for organic light-emitting diode (OLED) alternatives to silicate-based LEDs due to their tuneable structure and programmability. Yet, the 3D nature of LMOFs creates challenges for stability, optical transparency, and device integration. Metal-organic nanosheets (MONs) potentially overcome these limitations by combining the benefits of MOFs with an atomically thin morphology of large planar dimensions. Here, we report the bottom-up synthesis of atomically thin ZIF-7-III MONs via facile low-energy salt-templating. Employing guest@MOF design, the fluorophores Rhodamine B and Fluorescein were intercalated into ZIF-7 nanosheets (Z7-NS) to form light emissive systems exhibiting intense and highly photostable fluorescence. Aggregation and F\"orster resonance energy transfer, enabled by the MON framework, were revealed as the mechanisms behind fluorescence. By varying guest concentration, these mechanisms provided predictable quantified control over emission chromaticity of a dual-guest Z7-NS material and the definition of an 'emission chromaticity fingerprint' - a unique subset of the visible spectrum which a material can emit by fluorescence.