Thiophene‐Bridged Donor–Acceptor sp²‐Carbon‐Linked 2D Conjugated Polymers as Photocathodes for Water Reduction

Abstract

Photoelectrochemical (PEC) water reduction, which can convert solar energy into clean, storable hydrogen fuel, has attracted considerable attention to address energy and environmental issues. [1-3] A PEC water splitting cell is an innovative H 2-production device consisting of solar energy collection (semiconductors) and water electrolysis (catalysts) units. [4-6] Principally, the semiconductors need to comply with several requirements for efficient PEC applications, such as a wide-range light harvesting ability, efficient charge transfer, corrosion stability, and a higher-lying lowest unoccupied molecular orbital (LUMO) energy level than the reduction potential of the proton (H + /H 2). [1,7] 2D covalent organic frameworks (2D COFs), as crystalline, layer-stacked, 2D porous polymers, have emerged as a promi sing class of materials for photocatalysis and PEC applications recently. [3,8-12] Their energy bandgaps, positions of the frontier orbitals, and active centers can be facilely tailored by bottom-up organic synthesis with abundant building blocks, linkages, and topologies. [6,13,14] In particular, 2D π-conjugated COFs, which belong to the class of 2D conjugated polymers, show notable advantages for PEC applications due to the π-stacked columns Photoelectrochemical (PEC) water reduction, converting solar energy into environmentally friendly hydrogen fuel, requires delicate design and synthesis of semiconductors with appropriate bandgaps, suitable energy levels of the frontier orbitals, and high intrinsic charge mobility. In this work, the synthesis of a novel bithiophene-bridged donor-acceptorbased 2D sp 2-carbon-linked conjugated polymer (2D CCP) is demonstrated. The Knoevenagel polymerization between the electron-accepting building block 2,3,8,9,14,15-hexa(4-formylphenyl) diquinoxalino[2,3a:2′,3′-c]phenazine (HATN-6CHO) and the first electron-donating linker 2,2′-([2,2′-bithiophene]-5,5′-diyl)diacetonitrile (ThDAN) provides the 2D CCP-HATNThDAN (2D CCP-Th). Compared with the corresponding biphenyl-bridged 2D CCP-HATN-BDAN (2D CCP-BD), the bithiophenebased 2D CCP-Th exhibits a wide light-harvesting range (up to 674 nm), a optical energy gap (2.04 eV), and highest energy occupied molecular orbital-lowest unoccupied molecular orbital distributions for facilitated charge transfer, which make 2D CCP-Th a promising candidate for PEC water reduction. As a result, 2D CCP-Th presents a superb H 2-evolution photocurrent density up to ≈7.9 µA cm −2 at 0 V versus reversible hydrogen electrode, which is superior to the reported 2D covalent organic frameworks and most carbon nitride materials (0.09-6.0 µA cm −2). Density functional theory calculations identify the thiophene units and cyano substituents at the vinylene linkage as active sites for the evolution of H 2. The ORCID identification number(s) for the author(s) of this article can be found under

All sp2 carbon covalent organic frameworks

“…Kagome COFs usually comprise two different types of pores, hexagonal mesopores and trigonal mesopores or micropores [17]. The [C 3 + C 2 ] diagram with a C 3 -symmetric knot but six reactive sites and a C 2 -symmetric linker allows the design of trigonal skeletons with two different pore sizes (Figure 1E) [19,24,37,38]. These topology diagrams constitute the chemical base for the design of 2D sp 2 carbon COFs.…”

“…The 2D CCP-Th (Figure 5F) with a band gap of 2.04 eV serves as a photoelectrode for photoelectrochemical water reduction ( [37] and the supplemental information online).…”

Light-Driven Hydrogen Evolution Assisted by Covalent Organic Frameworks

“…This includes both the organic part (COF) and the metallic (co-catalyst) part. Regarding the former, during the last 6 years it has been made clear that imine-derived [18], triazine-containing [9,[19][20][21][22], diacetylene-containing [23], β-ketoenamine-derived [23][24][25], and fully π-conjugated sp 2 C [26][27][28] COFs are very resistant under Despite the thermodynamic and kinetic aspects related to the photocatalytic process itself described above, the stability and fate of the whole system under turnover conditions cannot be disregarded. This includes both the organic part (COF) and the metallic (co-catalyst) part.…”

Covalent organic frameworks in Lieb lattice for tunable photocatalytic water splitting under visible light

“…30 A photocatalytic reaction usually involves a series of consecutive photoelectrochemical steps: (1) absorption of photons to generate electron-hole pairs, namely excitons, (2) dissociation of excitons into free charge carriers and charge transport to the catalyst-electrolyte interface, and (3) redox reactions driven by electrons and holes at the interfaces. 6,31 Although strategies that combine donor (D) and acceptor (A) building blocks have been used to tune band gaps of COFs, [32][33] it is not clear how band edge energies of COFs and their alignment with the redox potentials of water will be modified, and they should meet the thermodynamical requirement for water splitting reactions. Moreover, photo-generated electron-hole pairs are strongly bounded in organic materials due to the weak Coulombic screening effect, so it is essential to reduce the exciton binding energy and promote charge separation in COFs in order to attain high quantum efficiency.…”