Direct Evidence of Photoinduced Charge Transport Mechanism in 2D Conductive Metal Organic Frameworks

Abstract

Conductive metal organic frameworks (MOFs) represent a promising class of porous crystalline materials that have demonstrated potential in photo-electronics and photocatalytic applications. However, the lack of fundamental understanding on charge transport (CT) mechanism as well as the correlation of CT mechanism with their structure hampered their further development. Herein, we report the direct evidence of CT mechanism in 2D Cu-THQ MOFs and the correlation of temporal and spatial behaviors of charge carriers with their photoconductivity by combining three advanced spectroscopic methods, including time resolved optical and X-ray absorption spectroscopy and terahertz spectroscopy. In addition to Cu-THQ, the CT in Cu/Zn-THQ after incorporating Zn 2+ guest metal was also examined to uncover the contribution of through space pathway, as the presence of the redox inactive 3d 10 Zn 2+ is expected to perturb the long range in-plane CT. We show that the hot carriers in Cu-THQ generated after photoexcitation are highly mobile and undergo fast localization to a lower energy state (cool carriers) with electrons occupying Cu center and holes in ligands. The cool carriers, which have super long lifetime (>17 ns), are responsible for the long-term photoconductivity in Cu-THQ and transport through the O− Cu−O motif with negligible contribution from interlayer ligand π−π stacking, as incorporation of Zn 2+ in Cu-THQ significantly reduced photoconductivity. These unprecedented results not only demonstrate the capability to experimentally probe CT mechanism but also provide important insight in the rational design of 2D MOFs for photoelectronic and photocatalytic applications.

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

“…Moreover, the metallo-sp 2 c-COF-15 increases the mobility by 80 % compared to sp 2 c-COF-14 (Figure 6c), which might be associated with the formation of more efficient transport pathways by integrating the Cu II complex acceptor. [17] These results demonstrated that not only the band gap but also the carrier transport properties can be elaborated by the rational design of 2D sp 2 carbon frameworks. To further check the semiconducting properties of the as-synthesized sp 2 c-COFs, their conductivities were measured under vacuum and summarized in Figure S15.…”

Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks

“…19 Several strategies have been developed to improve the charge separation efficiency of COFs, for example incorporating donor-acceptor (D-A) moieties 20,21 or halogen atoms in COFs, 22,23 constructing novel πconjugated building blocks, 24 and generating junctions with other semiconductors. 25 In addition to the above strategies, reducing the particle size of COFs to nanometer scale is a more facile method to improve the charge separation efficiency due to the possibility of charge carrier recombination is reduced in the short diffusion distance 26 . Therefore, the single-layer COFs (SLCOFs) offering the minimum diffusion distance for charge carriers will be the perfect candidate for photocatalysis.…”

Broadband and High‐Sensitivity Time‐Resolved THz System Using Grating‐Assisted Tilted‐Pulse‐Front Phase Matching

“…Time‐resolved terahertz (THz) spectroscopy is an optical characterization technique now routinely used in the spectral range between 0.5 and 3 THz to monitor microscopic interactions and resolve key signature resonances in diverse materials such as superconductors, [ 1–4 ] semiconductors, [ 5,6 ] topological insulators, [ 7–10 ] heavy fermions, [ 11–13 ] ferroelectrics, [ 14,15 ] antiferromagnets, [ 16 ] and metal‐organic compounds. [ 17,18 ] One configuration commonly used for these experiments relies on a near infrared (NIR) ultrafast source and a pair of second‐order nonlinear crystals to generate and detect phase‐locked THz transients. [ 19 ]…”