Zirconium-based Metal-Organic Frameworks with N-Confused Porphyrins: Synthesis, Structures, and Optical Properties Yufeng Yang, 1 Ryuichi Sakashita, 1 Kazuhisa Yamasumi, 1 Masatoshi Ishida,* 1 Teppei Yamada, 1,2 and Hiroyuki Furuta* 1 1 Department of Chemistry and Biochemistry, Graduate School of Engineering, and Center for Molecular Systems, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395 2 PRESTO, JST, Honcho 4-1-8, Kawaguchi, Saitama 332-0012 (E-mail: ishida.masatoshi.686@m.kyushu-u.ac.jp, hfuruta@cstf.kyushu-u.ac.jp) Metalorganic frameworks (MOFs) composed of zirconium (Zr 6 ) and N-confused porphyrins (NCPs) were synthesized. The cubic structures with ftw topology were determined by powder X-ray diffraction and microscopic analyses. These complexes represent high chemical stability toward various pH ranges and NIR absorption/emission derived from the NCP moieties. Keywords: Metal–organic frameworks | Porphyrin analogues | Near-IR properties Zirconium-based metalorganic frameworks (Zr-MOFs), a class of porous coordination polymers consisting of multinuclear Zr-cluster ions and functional organic molecules, have attracted increasing attention due to their intrinsic chemical and thermal stability. 1 These Zr-MOF materials are applicable toward catalysis (driven by light, acids, oxidants, etc.), gas storage/ separation, molecular sensing, drug delivery, etc. Among various organic building blocks for MOFs, porphy- rin compounds have been extensively utilized due to their large π-conjugated backbone that can provide unique π-space and functionality for various applications. With their inherent large absorption coefficients in the visible to near-infrared (NIR) energy regions, 2 use of porphyrin ligands is advantageous for developing potential NIR-responsive materials. In addition, coordination of a large variety of metals in the porphyrinic core leads to the various metal-dependent heterogeneous catalysts. 3 Stable MOF materials with efficient NIR light harvesting property have been demanded for photocatalytic systems as well. 4 To achieve the stable and robust porphyrinic MOF materials with specific functions (e.g., catalysts, sensors, etc.), synthesisof porphyrin-containing Zr-MOFs in a topologically pure phase is necessary. However, due to the flexible coordination modes of the high-valent zirconium(IV) clusters and the slow exchange reactions of the Zr IV -cation and carboxylate ligands, the rational synthesisof topology-controlled Zr-MOF remains challenging. Recently, Zhou et al. have reported the synthesisof various porphyrinic Zr-MOFs (e.g., a series of PCN-221225). 5 The change in the synthetic conditions like substrate ratio, zirconium salt, temperature, reaction time, or solvent, brought a huge diversity in the resulting MOF topology. 6 The structures of porphyrinic linkers also affected the crystalline topology by changing the dihedral angles of the meso-aryl groups. 7 Yaghi et al. have independently reported the synthesisof Zr-MOFs (e.g., MOF-525) with porphyrins under different conditions, resulting in the formation of an ftw-type cubic structure with a 4,12-coordinated Zr 6 cluster. 8 It is quite similar to PCN-221 5a containing 4,12-coordinated Zr 8 blocks. Thus, the topology of porphyrin-based MOFs largely relies on the reaction conditions. In this study, we have synthesized novel “N-confused” porphyrinic MOFs containing the most-substituted ftw topolog- ical Zr 6 clusters under the appropriate conditions (Scheme 1). The Zr 6 node (i.e., Zr 6 O 4 (OH) 4 (COO) 12 ) being targeted in this study has the highest connectivity in Zr-based MOFs, which shows excellent chemical stability. 8 The resulting MOFs could exhibit unique NIR optical properties due to the significant electronic perturbation of the porphyrinic π-block via N- confusion (NCP, in Scheme 1a). 9 In fact, NCP derivatives have an inverted pyrrolering with its nitrogen atom pointing outward. Thisdirect structural modification of the distinct 18π conjuga- tion circuitallows breaking the degeneracy of the typical HOMO and LUMO pairs, resulting in the narrower HOMO LUMO energy gaps (i.e., lower energy absorption) than those of regular porphyrins. 10 In addition, the conspicuous NH tautomer- ism in NCP where a hydrogen atom shifts between the inner and outer nitrogen sites, reflects the difference in the optical properties of the bistable tautomers (Scheme 1a). The dynamic nature of the electronic structure of NCPs may lead to (a) (b) Scheme 1. (a) NH tautomerism of NCP, (b) synthesisof NCP- MOF-525, and MeNCP-MOF-525. The structures of MOFs shown here are the theoretical structures based on Rietveld refinement. The structure of Zr 6 cluster present in (Me)NCP- MOF-525 is drawn from the ref 8. CL-170461 Received: May 4, 2017 | Accepted: May 26, 2017 | Web Released: June 3, 2017 1230 | Chem. Lett. 2017, 46, 1230–1232 | doi:10.1246/cl.170461 © 2017 The Chemical Society of Japan