Dual changes in conformation and optical properties of fluorophores within a metal-organic framework during framework construction and associated sensing event

Microsized chemosensor particle (CPP-16, CPP means coordination polymer particle), which is made from a metal-organic framework (MOF), is synthesized using pyrene-functionalized organic building block. This building block contains three important parts, a framework construction part, a Cu²⁺ detection part, and a fluorophore part. PXRD studies have revealed that CPP-16 has a 3D cubic structure of MOF-5. During both MOF formation and sensing event, fluorophores within CPP-16 undergo dual changes in conformation and optical properties. After MOF construction, pyrene moieties experience an unusual complete conversion from monomer to excimer form. This conversion takes place due to a confinement effect induced by space limitations within the MOF structure. The selective sensing ability of CPP-16 on Cu²⁺ over many other metal ions is verified by emission spectra and is also visually identified by fluorescence microscopy images. Specific interaction of Cu²⁺ with binding sites within CPP-16 causes a second conformational change of the fluorophores, where they change from stacked excimer (CPP-16) to quenched excimer states (CPP-16·Cu²⁺).