Module‐Patterned Polymerization towards Crystalline 2D sp²‐Carbon Covalent Organic Framework Semiconductors

Abstract

Despite rapid progress over the past decade, most polycondensation systems even upon a small structural variation of the building units eventually result in amorphous polymers other than the desired crystalline covalent organic frameworks. This synthetic dilemma is a central and challenging issue of the field. Here we report a novel approach based on module‐patterned polymerization to enable efficient and designed synthesis of crystalline porous polymeric frameworks. This strategy features a wide applicability to allow the use of various knots of different structures, enables polycondensation with diverse linkers, and develops a diversity of novel crystalline 2D polymers and frameworks, as demonstrated by using the C=C bond‐formation polycondensation reaction. The new sp2‐carbon frameworks are highly emissive and enable up‐conversion luminescence, offer low band gap semiconductors with tunable band structures, and achieve ultrahigh charge mobilities close to theoretically predicted maxima.

Outstanding Charge Mobility by Band Transport in Two-Dimensional Semiconducting Covalent Organic Frameworks

“…Along with the band-like transport nature, the observed high mobility also originates from the largely reduced scattering events as demonstrated by the long charge scattering time of up to 70 fs. This value is superior to that of the previously reported 2D COFs, whose scattering times range from 30 to 50 fs (with non-Drude transport characteristics). ,,− Temperature-dependent THz photoconductivity measurements confirm the band-like charge transport in TPB–TFB COF thin films, by showing a negative temperature dependence of the mobility. Furthermore, there remains considerable room for further enhancing the mobility, as impurity scattering plays a nontrivial role in limiting the charge mobility.…”

“…To the best of our knowledge, our results represent the first observation of the Drude transport of charge carriers in 2D COFs. Furthermore, the estimated mobility constitutes a record high in COFs and is almost one order of magnitude higher than the other reported conductive 2D COFs ,,− , (see Table S5). Note that the obtained mobility combines the contributions of in- and out-of-plane conduction.…”

“…The fast and slow relaxation components can be tentatively assigned to trapping and electron–hole recombination processes, respectively. The weighted fast and slow relaxation components yield an averaged charge carrier lifetime of t ∼ 40 ps, significantly longer than that of the other COFs characterized by the same technique, ,,− where the photoconductivity becomes negligible within sub-10 ps. To explore the role of crystallinity in determining the charge transport properties, we compare thin-film photoconductivity with that of the TPB–TFB COF powder (the sample morphology that has been extensively used in previous photoconductivity measurements).…”

Mesoporous Polyimide‐Linked Covalent Organic Framework with Multiple Redox‐Active Sites for High‐Performance Cathodic Li Storage

“…Covalent organic frameworks (COFs) are an emerging class of ordered crystalline and porous organic polymers built from modular organic building blocks through strong covalent bonds [10] . In addition to their applications revealed for gas separation and storage, [11] semiconductors, [12] catalysis, [13] and proton conduction, [14] efforts have also been paid to exploring the potentials of COFs as efficient organic electrode materials due to their enhanced chemical/thermal stability associated with the strong covalent bonding interactions [15–20] . In 2015, a two‐dimensional (2D) COF containing redox‐active naphthalene diimide was integrated with carbon nanotube and firstly used as cathodes of LIBs, which acquired a specific capacity of 68 mAh g −1 and more importantly robust cycle stability [16] .…”

Phonon-Limited Electron Transport in a Highly Conductive Two-Dimensional Covalent Organic Framework: A Computational Study

“…As a result, some highly conductive 2D COFs have been experimentally available, such as TPB-TFB COF (with electron/hole mobility of 165 ± 10 cm 2 V –1 s –1 ), COF-366 (with hole mobility of 8.1 cm 2 V –1 s –1 ), and CS-COF (with hole mobility of 4.2 cm 2 V –1 s –1 ) . Research efforts were also devoted to the rational design of new 2D COFs with a high electron/hole mobility, as well as the development of methods for improving the charge transport properties of existing 2D COFs. − For instance, iodine oxidation was demonstrated to be able to enhance the conductivity of 2D sp 2 -carbon-hybridized COFs by as much as 12 orders of magnitude because of hole and ion transfer. , Molecular iodine doping was proved to be effective in enhancing the charge mobility from 5 to 22 cm 2 V –1 s –1 in metal-phthalocyanine-based pyrazine-linked 2D COFs as a result of increased carrier relaxation time . A multiscale-simulation approach was developed to reveal band-like charge transport in highly ordered 2D TP-COFs, and this mechanism was also demonstrated to be operational in some other crystalline 2D COFs .…”