A Nanographene‐Based Two‐Dimensional Covalent Organic Framework as a Stable and Efficient Photocatalyst

Abstract

Synthesis of covalent organic frameworks (COFs) with desirable organic units furnishes advanced materials with unique functionalities. As an emerging class of twodimensional (2D) COFs, sp 2 -carbon-conjugated COFs provide a facile platform to build highly stable and crystalline porous polymers. Herein, a 2D olefin-linked COF was prepared by employing nanographene, namely, dibenzo-[hi,st]ovalene (DBOV), as a building block. The DBOV-COF exhibits unique ABC-stacked lattices, enhanced stability, and charge-carrier mobility of � 0.6 cm 2 V À 1 s À 1 inferred from ultrafast terahertz photoconductivity measurements. The ABC-stacking structure was revealed by the high-resolution transmission electron microscopy and powder X-ray diffraction. DBOV-COF demonstrated remarkable photocatalytic activity in hydroxylation, which was attributed to the exposure of narrow-energy-gap DBOV cores in the COF pores, in conjunction with efficient charge transport following light absorption.

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

“…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). 31 , 34 , 37 − 39 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.…”

“…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).…”

Effective Photocatalytic Initiation of Reactive Oxygen Species by a Photoactive Covalent Organic Framework for Oxidation Reactions

“…In the UV-vis spectrum and the solid-state UV–vis diffuse reflectance spectrum (DRS) of the constructed CPTPA-COF (Figure S11), a broad absorption band ranging from ∼300 nm to 420 nm was observed. Based on UV-vis DRS, the Kubelka–Munk-transformed reflectance spectrum was obtained (Figure b), − ,− and the corresponding optical bandgap of constructed CPTPA-COF was 2.41 eV, suggesting that the TPA-incorporated COF possessed a semiconductor feature. , In order to explain the energy band of CPTPA-COF, a Mott–Schottky experiment was then performed, based on which its flat-band potential was determined to be −0.95 eV (Figure c), and hence the conduction band potential was estimated to be −0.95 eV. Based on the values of optical bandgap and conduction band potential, the valence band potential was estimated to be 1.46 eV (see Figures c and d).…”

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

“…Based on the design strategy of modular condensation polymerization, at least 9 kinds of topologies of 2D monolayers, i.e., hcb, 5,52 sql, 53,54 kgm, [55][56][57][58] cem, 59 hxl, 60 cpi, 61 bex, 62,63 kgd, [64][65][66] and tth, 63,66 have been reported by combining more than one organic monomer or making the pore structure dissymmetric via changing the edge lengths and vertex angles. 67 Moreover, the 2D monolayers may adopt different stacking modes in space, e.g., AA, 5 AB, 5,68,69 and ABC, [70][71][72][73][74] further enriching the structural diversity of 2D COFs. Conversely, less than 100 different 3D COFs have been reported since the first report in 2007.…”

“…67 Moreover, the 2D monolayers may adopt different stacking modes in space, e.g. , AA, 5 AB, 5,68,69 and ABC, 70–74 further enriching the structural diversity of 2D COFs. Conversely, less than 100 different 3D COFs have been reported since the first report in 2007.…”