Mechanistic Studies of Hydrogen Evolution Reaction on Donor-Acceptor Conjugated Polymer Photocatalysts

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[N-1,N-1-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, H-2 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.