Photoresponse within dye-incorporated metal-organic architectures

Abstract

Recognition, involving weak non-covalent interactions, is an important event in biological systems, participating in the binding process of enzyme and substrate. Following this model, chemists have developed many recognition systems. Constructing optical signal recognizing systems based on supramolecular metal-organic architectures is one of their efforts to achieve fine biomimetic system beyond enzymes. Such supramolecular metal-organic hosts can be formed by the assembly of preorganized organic ligands and functionalized metal ions/clusters, of which the various combinations are conducive to the diversification and functionalization of these materials. They possess large cavities that can provide abundant interaction sites and synergistic effects of several non-covalent interactions and the hydrophilicity and hydrophobicity of cavities benefit to bind with analytes selectively to construct dye-containing host-guest recognition systems. The defined cavities and predetermined building blocks of metal-organic cages/frameworks also allow for encapsulating target guests and manipulating various interactions for selective binding and recognition of specific analytes. Most importantly, changes of optical signals in these supramolecular systems could indicate the binding of host and analyte molecules to realize the tangible detection. It depends on the excellent photo- chemical and/or physical properties of these host-guest systems. Analytes with matched size and shape bind to the inner pores by host-guest interactions and may promote or block the electron/energy transfer or redox processes existing in the hosts, which would produce special optical recognition signals for target analytes. Host-guest molecular recognition has been considered as an important contribution to the domain of supramolecular chemistry, and it facilitated the development of advanced supramolecular materials.In this review, we focus on the photoactive metal-organic cages/frameworks incorporating or encapsulating dyes to narrate the recognition process for specific molecules via optical signals controlled by electron/energy transfer processes, redox reactions or other processes within the confined pockets of such metal-organic hosts. The photoactive host-guest systems in this topic can be classified into three categories: metal-organic hosts incorporating organic dyes as ligand skeletons, metal-organic hosts incorporating organometallic dyes as a part of metal nodes, and metal-organic hosts encapsulating dye molecules within the inner cavities. These ways all further increase the diversity of photo- chemical and/or physical properties of supramolecular host-guest systems. And in the confined environment, the photoinduced electron/energy transfer (PET/PEnT) processes between donor-acceptor pair in supramolecular systems are promoted because of short through-space distance, thus leading to quicker and stronger photoresponsive signals. It is also necessary to design the hosts only matching to recognition target for convenient real-time detection through its sensitivity, quick and easy photo-response to analytes. We first briefly outline the potential methods for constructing photosensitive metal-organic hosts by incorporating dye molecules into the ligand backbone or as part of the metallic nodes or encapsulating dye molecules as guests. Then, the recent advances in molecule recognition by photoactive host-guest systems based on metal-organic cages/frameworks through optical ways involving in electron/energy transfer or redox mechanisms are summarized respectively.