Control of Crystallinity and Porosity of Covalent Organic Frameworks by Managing Interlayer Interactions Based on Self-Complementary π‑Electronic Force Xiong Chen, † Matthew Addicoat, ‡ Stephan Irle, ‡ Atsushi Nagai, † and Donglin Jiang* ,†,§ † Department of Materials Molecular Science, Institute for Molecular Science, National Institutes of Natural Sciences, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan ‡ Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan § PRESTO, Japan Science and Technology Agency (JST), Chiyoda-ku, Tokyo 102-0075, Japan * S Supporting Information ABSTRACT: Crystallinity and porosity are crucial for crystalline porous covalent organic frameworks (COFs). Here we report synthetic control over the crystallinity and porosity of COFs by managing interlayer interactions based on self-complementary π-electronic forces. Fluoro- substituted and nonsubstituted aromatic units at different molar ratios were integrated into the edge units that stack to trigger self-complementary π-electronic interactions in the COFs. The interactions improve the crystallinity and enhance the porosity by maximizing the total crystal stacking energy and minimizing the unit cell size. Consequently, the COF consisting of equimolar amounts of fluoro-substituted and nonsubstituted units showed the largest effect. These results suggest a new approach to the design of COFs by managing the interlayer interactions. P orous materials have attracted great attention in many fields of science and technology. Among them, covalent organic frameworks (COFs) are a unique class because they are composed of lightweight elements linked by strong covalent bonds. 1-7 The most intriguing trait is their atomically precise integration of building blocks into periodic two-dimensional (2D) and three-dimensional structures, which endow COFs with high flexibility in the design of skeleton and polygon morphologies. 3i COFs have emerged as predesignable porous materials for gas adsorption 1-7 and provide useful skeletons for the design of a new class of organic semiconductors that feature columnar π-arrays periodically aligned with a nanometer-scale precision. 3,6 In this sense, 2D COFs serve as new platforms for the design of organic 2D materials with structural periodicity that is difficult to achieve with other molecular architectures. However, control over the crystallinity and porosity, which are key parameters in their applications, has been elusive. Here we report the synthetic control of COFs by managing interlayer interactions based on self-complementary π-electronic forces. We demonstrated the strategy using imine-linked porphyrin COFs in which fluoro-substituted and nonsubstituted arenes at different molar ratios were integrated into the edge units. The porphyrins occupy the vertices and the arene units are located on the edges of mesoporous two-dimensional COFs. Mixtures containing 2,3,5,6-tetrafluoroterephthalaldehyde (TFTA) and terephthalaldehyde (TA) at different molar ratios were utilized in polycondensation with copper 5,10,15,20-tetrakis(p- tetraphenylamino)porphyrin (CuP) under solvothermal con- ditions, generating five new imine-linked COFs [Chart 1; also see the Supporting Information (SI)]. These reactions exhibited similar isolated yields, indicating that the reactivities of TFTA and TA are similar under the solvothermal conditions. The edge units of the COFs were composed of tetrafluor- ophenyl (TFPh) and phenyl (Ph) groups at molar ratios of 100/0, 75/25, 50/50, 25/75, and 0/100, respectively. IR spectroscopy, elemental analysis, field-emission scanning electron microscopy, and transmission electron microscopy confirmed the formation of the COFs (Figures S1-S3 and Table S1 in the SI). Figure 1A shows the X-ray diffraction (XRD) patterns of the five COFs. Each COF exhibited diffraction peaks at 3.4, 6.9, and 20-22°, which were assigned to the (100), (200), and (001) facets, respectively. A significant feature is that the XRD peak intensities were highly dependent on the edge components. For example, the COF bearing only Ph units in the edges (CuP-Ph) exhibited the lowest XRD intensity of ∼15000 cps for the (100) facet (red curve). When the content of TFPh units was increased to 25 mol % (CuP-TFPh 25 ), the intensity increased to 21000 cps (purple curve). The most explicit increment was observed for CuP-TFPh 50 , which showed an intensity of 30300 cps (blue curve). In this case, the two edge units are present in an equimolar ratio and produce the largest number of self-complementary π-stacked pairs. Consequently, CuP-TFPh 50 shows the strongest self- complementary electronic interactions. Further increments in the TFPh content eventually caused decrements in the XRD intensity (green and black curves). The distinct edge-depend- ent intensity changes reflect the effective control over the crystallinity of the COF through self-complementary electronic interactions. Such interactions have been employed for the crystal engineering of arene arrays through strengthened π-π interactions between fluoro-substituted and nonsubstituted arenes. 8 In the present π-array systems, the enhanced π- interactions improved the crystallinity of the COFs. Received: October 11, 2012 Published: December 27, 2012 Communication pubs.acs.org/JACS © 2012 American Chemical Society 546 dx.doi.org/10.1021/ja3100319 | J. Am. Chem. Soc. 2013, 135, 546-549