The inherent porous nature and facile tunability of metal-organic frameworks (MOFs) make them ideal candidates for use in multiple fields. MOF hybrid materials are derived from existing MOFs hybridized with other materials or small molecules using a variety of techniques. This led to superior performance of the new materials by combining the advantages of MOF components and others. In this review, we discuss several hybridization methods for the preparation of various MOF hybrids with representative examples from the literature. These methods include covalent modifications, noncovalent modifications, and using MOFs as templates or precursors. We also review the applications of the MOF hybrids in the fields of catalysis, drug delivery, gas storage and separation, energy storage, sensing, and others. Crystals 2018, 8, 325 2 of 23to coordinate to other materials. In addition to covalent modifications, MOF hybrids can also be made via noncovalent interactions, such as encapsulation [19,20], layer-by-layer deposition [21], and in situ growth [22]. These methods take advantage of noncovalent interactions between MOFs and the hybridizing species by trapping the species within the MOF pores, layering them on top of the parent MOF, or growing MOFs crystals in situ with the species. Noncovalent modification allows the individual characteristics of the MOF and hybridizing materials to work synergistically in the resultant MOF hybrids while requiring less synthetic efforts than covalent modifications. These methods can be used to achieve materials with MOF coating/protection, multi-layered membranes, and the controlled growth of MOF structures with superior performance than individual parent materials. Finally, hybridizing MOFs through use as either sacrificial templates [23] or precursors [24] utilizes the ordered structure of MOFs to afford porous materials with high surface areas and uniform pore sizes. This method eliminates the metal node and/or the organic linker, leaving behind only the newly synthesized materials with the inherited uniform nanoframe of the template/precursor MOF.In this review, we discuss each hybridization method with representative MOF hybrids from literature, as well as the hybrid materials' superior performances and applications. At the end of this review, we also summarize all reported MOF hybrid materials. Crystals 2018, 8, x FOR PEER REVIEW 2 of 22

Metal–organic frameworks (MOFs) have emerged as heterogeneous photosensitizers in applications ranging from photodynamic therapy to photocatalysis. However, singlet oxygen (1O2) quantum yields of MOF photosensitizers are largely unknown. Herein, we report a reliable method to quantify the 1O2 quantum yield of a porphyrin MOF PCN-222/MOF-545 (free base). To accurately measure the optical density of the MOF suspensions in chloroform, a UV–vis spectrometer equipped with an integrating sphere detector was employed. However, no 1O2 near-infrared luminescence signal from the excited MOF could be detected and the amount of 1O2 produced by the MOF photosensitizer was quantified using a 1O2 trap 9,10-dimethylanthracene. Using C70 as a reference, the 1O2 quantum yield of PCN-222/MOF-545 (free base) was determined to be 0.35 ± 0.02. This method should be applicable to the determination of 1O2 quantum yields of other solid-state materials.