Guest Encapsulation in Metal-Organic Frameworks for Photonics

Photonic functional materials have received mounting attention because of their employment in luminescent sensing, display, white-light-emission (WLE), data storage, bioimaging, etc. Metal-organic frameworks (MOFs), usually fabricated by metal nodes and organic linkers, offer many intriguing properties such as permanent porosity, structural designability, diversity of morphology, excellent crystallinity, multiple luminescent centers, and modifiable chemical functionality. These properties make MOFs versatile hosts in various environments, especially in developing advanced photonic functional materials. Furthermore, the pores within MOFs endow them as hosts to confine diverse guest molecules (e.g., organic dyes, perovskite quantum dots (QDs), photochromic molecules, lanthanide ions, metal complexes and so forth), so plenty of photonic functional guest encapsulation in MOF materials can be successfully fabricated.It is worth noting that the advantages of guest-encapsulated MOF materials for photonics are abundant. First, combining guest units and MOFs can result in host-guest MOFs with multiple photonic centers, which can be successfully used to realize diverse photonics properties. Second, apart from the intrinsic photonic properties of MOFs and guests, the isolation and protection of MOFs could optimize and enhance the photonic functionality of guests, such as enhancing the photoluminescence quantum yield (PLQY), stability, and so on. Third, the confinement and orientation of guest molecules within the ordered pores of MOFs undoubtedly enables a promising strategy for generating novel photonic functionalities. It is precisely due to the enormous combination possibilities, synergistic effects, and controllable and directional assembly of guest photonic units in host-guest MOF materials that provide a new avenue for designing and constructing tailored photonic functional materials and applications.In this Account, we describe some of our efforts to encapsulate the guest molecules within MOFs to construct photonic functional materials. Given the unique intermolecular interactions between guest molecules and MOFs, we have put an emphasis on rational encapsulation design to optimize and generate photonic functionality, such as introducing new photonic units in MOFs and then using them as ratiometric luminescent thermometers, luminescent film sensors, and luminescent sensor array; enhancing photonic performance through the isolation and protection of MOF pores and achieving WLE, multiphoton excited luminescence (MPEL); and generating novel photonic functionality including high-order nonlinear optical (NLO) response, multiphoton-pumped polarized lasing, controllable laser switch performance, photostimuli-responsive dynamic fluorescence, and so on. We also discuss the most important measurement techniques [confocal laser scanning microscope (CLSM) and single crystal X-ray diffraction (SCXRD)] that can be applied for proper determination of orientation and position of guests within MOFs. Various synthetic routes using the confinement strategies to yield the MOFs superset of guest composites were also discussed. Finally, we point out future design principles and research directions of guest-encapsulated photonic MOF materials.