Metal-Organic Frameworks with Multiple Luminescence Emissions: Designs and Applications.

Emissive species are powerful for luminescent detection with high sensitivity and simple procedure and for light-emitting diode (LED) lighting because of their high efficiency, long lifetime, and low energy consumption. Here we propose the concept of multiple luminescence emissions from a single matrix or species under single-wavelength excitation. Multiemission not only realizes the high sensitivity of luminescence sensing but also possesses the capacity of self-reference for environment-free interferences. The color change is also convenient for visible detection. In multiemission species, every emissive center responds to a specific analyte to improve the efficiency for multiple-target detection. Multiemission also extends the applications to anticounterfeiting, colorful LEDs, and information storage. To date, it is still challenging to combine more than one type of emissive center in a single matrix or species. Obtaining multiemission under single-wavelength excitation also needs exquisite design. Metal-organic frameworks (MOFs) are porous hybrid assemblies prepared with metal ions and organic ligands. Metal nodes and ligands with large π-conjugated systems have the potential for the construction of luminescent MOFs. Abundant and diverse precursors provide the possibility to prepare MOFs with multiple luminescence emissions. The pores or channels of MOFs also act as hosts to encapsulate luminescent guest species as additional emissive sites. In this Account, we propose the concept of multiple-luminescence MOFs (ML-MOFs) and summarize the recent research progress on their designs, constructions, and applications reported by our group and others. ML-MOFs are MOFs that possess more than one emissive center under single-wavelength excitation. Six different kinds of construction strategies of ML-MOFs are introduced: (1) multiemission from both metal nodes and ligands in single MOFs; (2) use of mixed-metal nodes as multiemission centers in single MOFs; (3) combination of different emissive MOFs as a whole to achieve multiemission application; (4) host-guest emissions from emissive MOFs after encapsulation of luminescent guest species; (5) organization of different emissive ligands in a single MOF for multiemission; and (6) use of single ligands exhibiting dual emission to prepare ML-MOFs. We also discuss the mechanisms that realize multiple emissions from MOFs under single-wavelength excitation, such as the antenna effect and excited-state intramolecular proton transfer. The applications of ratiometric sensing, LED lighting, anticounterfeiting, and information storage are summarized. With this Account, we hope to spark new ideas and to inspire new endeavors in the design and construction of ML-MOFs, especially with postsynthetic techniques such as postsynthetic modification, postsynthetic exchange, and postsynthetic deprotection, to promote the applications of MOFs in sensing, lighting, information storage, and others.