Ultrafine Spatial Modulation\nof Diazapyrene-Based\nTwo-Dimensional Conjugated Covalent Organic Frameworks

Tailormade\nbottom-up synthesis of covalent organic frameworks\n(COFs)\nfrom various functional building blocks offer not only tunable topology\nand pore size but also multidimensional properties. High crystallinity\nis one of the prerequisites for their structures and associated physicochemical\nproperties. Among different π-conjugated motifs for constructing\nCOFs, pyrene-based tetragonal structures are effective in achieving\nhighly ordered and crystalline states. In the present research, we\ndemonstrated that the substitution of pyrene with 2,7-diazapyrene\nproduces nearly “flat” structures of two-dimensional\n(2D) COF layers by controlling the torsional angle of linker molecules.\nFeaturing finite pore diameters and excellent thermodynamic stability\nof ∼500 °C, ordered face-to-face (slipped AA) stacking\narrangements were produced. Extended electrical conjugation spanning\n2D frames with modest optical bandgaps (<i>E</i><sub>g</sub>) of ∼2.1 eV shows the planar character of diazapyrene-based\nCOFs. The stacking of the conjugated 2D frames with small <i>E</i><sub>g</sub> values is also beneficial for the formation\nof highly stable conducting pathways in the crystalline state, which\nwas confirmed by the results of the microwave conductivity measurements.\nNitrogen centers in diazapyrene units also play a key role as the\nactive sites for proton transfer, and the maximum proton conductivity\nof σ = 10<sup>–2</sup> S cm<sup>–1</sup> was achieved\nalong the cocontinuous nanopore structures surrounded by the active\nsites. Results show that tetragonal COFs based on diazapyrene can\nbe used as a highly crystalline two-dimensional material with special\nelectrical and proton-conducting capabilities.