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Charge Dynamics In A Donor-acceptor Covalent Organic Framework With Periodically Ordered Bicontinuous Heterojunctions.
Published 2013 · Materials Science, Medicine
The donor–acceptor heterojunction is a key structure in current technologies, including transistors, light-emitting diodes, and photovoltaics, because it controls the charge dynamics in the devices. Covalent organic frameworks (COFs) are crystalline molecular skeletons that allow atomically precise integration of building blocks into periodic array structures. In this regard, we have demonstrated arene, porphyrin, and phthalocyanine COFs that provide periodically ordered columnar arrays of p-components and show outstanding semiconducting and photoconductive properties. We recently synthesized a donor–acceptor COF that gives rise to a periodically ordered bicontinuous heterojunction structure and self-sorted donor and acceptor columnar arrays separated at nanometer-scale intervals. This nanoscopic segregation morphology forms a broad interface for charge separation, provides ambipolar pathways for charge collection, and would be ideal for the current semiconducting devices that involve photoenergy transformations; however, the charge dynamics, which is a key mechanism that controls the energy transformation, remains unclear. Here, we report the charge dynamics of a donor–acceptor COF, which were determined using time-resolved spectroscopy to elucidate the photochemical processes of the free charges from their generation to delocalization and retention. In the COF, the heterojunctions allow an ultrafast electron transfer from the donor to the acceptor columns. Consequently, the light absorption is directly coupled with charge dissociation to generate free charges in the donor and acceptor p-columns within 2 ps. On the other hand, the stacked p-columns delocalize the charges, suppress charge recombination, and retain the charges for a prolonged period of time. We show that both solvated and solid-state COFs enable rapid charge separation and exceptional long-term charge retention, thereby providing a key mechanistic basis to envisage the high potential of donor–acceptor COFs for photoelectric applications. The donor–acceptor COF (Scheme 1a, DZnPc-ANDI-COF) is a tetragonal, mesoporous 2D framework that is composed of zinc phthalocyanine as an electron donor and naphthalene diimide as an acceptor. In the COF, the two p-units are alternately linked within an electron-transfer distance and at a dihedral angle of approximately 428. The COF provides selfsorted, bicontinuous columnar arrays and constitutes periodically structured heterojunctions in which each donor column is interfaced with four acceptor columns that are equally active in capturing photo-generated electrons (Scheme 1b). The DZnPc-ANDI-COF absorbs light over a broad visible and near-infrared region up to 1100 nm (Figure S1 in the Supporting Information). Elemental analysis, infrared spectroscopy, nuclear magnetic resonance spectroscopy, and electron microscopy confirmed the formation of the COF (Figure S2–S4 and Table S1). The same COF has been reported as a thin film. The DZnPc-ANDI-COF exhibited a type IV nitrogen sorption curve that is characteristic of mesoporous frameworks (Figure 1a). The Brunauer–Emmett–Teller surface area and pore volume were calculated as 1410 mg 1 and 1.25 cmg , respectively. The pore-size distribution profile with a range up