German Edition: DOI: 10.1002/ange.201502179 Metal–Organic Frameworks International Edition: DOI: 10.1002/anie.201502179 Formation of a Syndiotactic Organic Polymer Inside a MOF by [2+2] Photo-Polymerization Reaction** In-Hyeok Park, Raghavender Medishetty, Hyeong-Hwan Lee, Caroline Evania Mulijanto, Hong Sheng Quah, Shim Sung Lee,* and Jagadese J. Vittal* Abstract: Getting suitable crystals for single-crystal X-ray crystallographic analysis still remains an art. Obtaining single crystals of metal–organic frameworks (MOFs) containing organic polymers poses even greater challenges. Here we demonstrate the formation of a syndiotactic organic polymer ligand inside a MOF by quantitative [2+2] photopolymeriza- tion reaction in a single-crystal-to-single-crystal manner. The spacer ligands with trans,trans,trans-conformation in the pillared-layer MOF with guest water molecules in the channels, undergo pedal motion to trans,cis,trans-conformation prior to [2+2] photo-cycloaddition reaction and yield single crystals of MOF containing two-dimensional coordination polymers fused with the organic polymer ligands. We also show that the organic polymer in the single crystals can be depolymerized reversibly by cleaving the cyclobutane rings upon heating. These MOFs also show interesting photoluminescent proper- ties and sensing of small organic molecules. Growing good quality crystals is important in determining the three-dimensional (3D) arrangements of atoms and molecules in the solid state by single-crystal X-ray diffraction (SC-XRD) techniques. Understanding of the crystal packing enables us to explore the structure–property relationship for creating new advanced solid-state materials. [1] Crystalline properties arising from the orderly arrangements of polymer chains are well-sought to control mechanical, chemical, optical, and electrical properties for various industrial appli- cations. [2] The tactic polymers, on the contrary, display a high degree of crystallinity and are usually synthesized in solution using metal catalysts. [3] Topochemical polymerization reac- tions have been used to make a number of highly crystalline organic polymers in the solid state, but making tactic polymers by a solid-state route is elusive. [4] Recently, an isotactic polymer ligand embedded inside single crystals of metal–organic frameworks (MOFs) was made through poly- merization of a well-aligned slip-stacked conjugated diene ligand 1,4-bis[2-(4’-pyridyl)ethenyl]benzene (bpeb) by [2+2] cycloaddition reaction under UV light. [5] Here we report the formation of a syndiotactic polymer ligand (shown in Figure 1) incorporated inside another MOF through single- crystal-to-single-crystal (SCSC) reaction, but the unexpected photoreaction could not be predicted from the crystal packing. In the pillared-layer MOF accommodating guest water molecules in the channels, the spacer ligands undergo pedal motion under UV light prior to photoreaction to meet Schmidt’s criteria for [2+2] cycloaddition reaction. As a result, single crystals of MOF containing a 2D coordination polymer (CP) integrated with the syndiotactic organic polymeric ligand were obtained in an SCSC fashion. The organic polymer in the single crystals can be depolymerized reversibly by cleaving the cyclobutane rings upon heating. A three-fold interpenetrated pillared-layer MOF with pcu topology, [Zn 2 (bpeb)(obc) 2 ]·2 DMF·H 2 O [1; H 2 obc = 4,4’- oxybis(benzoic acid)] has been shown to retain its single crystal nature when the guest solvents were exchanged in methanol to form 2. [6] Both 1 and 2 were found to be photo- inactive as the olefin bonds in bpeb ligands were separated by > 4.2 . The guest solvents could be removed without destroying its single crystallinity by heating 2 at 160 8C for 6 h, but on standing in air, it takes water from humid air to form [Zn 2 (bpeb)(obc) 2 ]·4 H 2 O(3) as shown by SC-XRD studies. The asymmetric unit of 3 contains a molecular formula unit. Each Zn II is bridged by two obc ligands and chelated to another obc ligand (Figure 2a). The highly distorted dimer unit extends through these four obc ligands and forms a layer structure (Figure 2b). The highly corrugated (4,4) layers of [Zn 2 (obc) 2 ] with honeycomb-like cavity are pillared by the bpeb ligands forming a pillared-layer structure as shown in Figure 2 c. The two pyridyl rings in the bpeb ligand are twisted with respect to the middle ring by 20.38 and 31.08 and the ligand has trans ,trans ,trans-conformation. The overall top- ology of the 3D structure is a pcu net. The large void Figure 1. Structural diagrams of the two tactic polymers from the solid-state [2+2] photopolymerization of bpeb ligand. [*] Dr. I.-H. Park, H.-H. Lee, Prof. S. S. Lee Department of Chemistry and Research Institute of Natural Science Gyeongsang National University Jinju 660-701 (Republic of Korea) E-mail: sslee@gnu.ac.kr Dr. R. Medishetty, C. E. Mulijanto, H. S. Quah, Prof. J. J. Vittal Department of Chemistry, National University of Singapore 3, Science Drive 3, Singapore 117543 (Singapore) E-mail: chmjjv@nus.edu.sg [**] This work was supported by the NRF (2012R1A4A1027750), S. Korea, and the Ministry of Education, Singapore through NUS FRC grant R-143-000-562-112. Dr. Srinivasulu Aitipamula of ICES is thanked for the pharmaceutical samples tested in the manuscript. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201502179. A ngewandte Chemi e 1 Angew. Chem. Int. Ed. 2015, 54,1–6  2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! Ü Ü