Abstract
The application of donor-acceptor (D-A) conjugated polymer catalysts for hydrogen evolution reaction (HER) has shown great promise because of the tunability of such catalysts to have desired properties. Herein, we synthesized two polymer catalysts: poly[4,4′-(9-(4-aminophenyl)-9H-carbazole-3,6-diamine-alt-5-oxido-5-phenylbenzo[b]phosphindole-3,7-diyl)dibenzaldehyde] (PCzPO) and poly[N1,N1-bis(4-amino-2-fluorophenyl)-2-fluorobenzene-1,4-diamine-alt-5-oxido-5-phenylbenzo[b]phosphindole-3,7-diyl)dibenzaldehyde] (PNoFPO). The UV-vis absorption spectra showed that the less planar structure and the presence of electronegative fluorine atoms in the donor group of PNoFPO led to a higher optical gap compared to PCzPO, leading to almost five times faster HER rate using PCzPO compared to PNoFPO. However, density functional theory (DFT) calculations show that the frontier orbitals and the highest occupied molecular orbitals – lowest unoccupied molecular orbitals (HOMO-LUMO) gaps of PCzPO and PNoFPO D-A moiety models are very similar, such that, during light absorption, electrons move from donor to acceptor group where proton binding is preferred to happen thereafter. For both PCzPO and PNoFPO D-A moieties, H2 formation through an intramolecular reaction with a barrier of 0.6–0.7 eV, likely occurs at the acceptor group atoms where protons bind through electrostatic interaction. The intermolecular reaction has nearly zero activation energy but is expected to occur only when the repulsion is low between separate polymers chains. Finally, experimental and DFT results reveal the importance of extended configurations of D-A polymers on HER rate.
Highlights
The conversion of solar energy into stored chemical energy is forefront research for the development of renewable energy [1]
Sci. 2020, 10, 7017 produces water without any side products [2]. This motivates the search for photocatalyst materials for the photocatalysed hydrogen evolution reaction (HER) or water-splitting
For both PCzPO and PNoFPO, the electron density for high-lying occupied molecular orbitals is concentrated on the donor group while that of the low-lying unoccupied molecular orbitals is concentrated on the acceptor group
Summary
The conversion of solar energy into stored chemical energy is forefront research for the development of renewable energy [1]. Polymers made of alternating donor and acceptor units with highly conjugated structures have been shown to be more efficient photocatalysts for HER [4,5,6,7,8,9] With this design, photoexcitation of electrons is accompanied by an intramolecular charge transfer from donor to acceptor groups [10]. We synthesised two novel polymer catalysts (Figure 1) with different donor units, aiming to compare their HER performance and understand their H2 formation mechanisms Both structures have the same phosphindole analogue acceptor unit, 4,40 -(5-oxido-5-phenylbenzo [b]phosphindole-3,7-diyl)dibenzaldehyde (PO). DFT results show that the molecular orbitals of the polymer structures allow for the expected intramolecular charge transfer, leading to efficient proton binding and reduction on acceptor groups, followed by forming of H2 through an intramolecular reaction.
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