## 1 Introduction  
The conversion of solar energy into stored chemical energy is forefront research for the development of renewable energy [1]. On this note, hydrogen fuel (H\({}_{2}\)) is known as the best fuel form because of its energy conversion efficiency and the fact that it can be generated from water and its oxidation only produces water without any side products [2]. This motivates the search for photocatalyst materials for the photocatalysed hydrogen evolution reaction (HER) or water-splitting. Although inorganic materials are the current choice for photocatalysts, polymer photocatalysts are of great interest owing to their cheaper cost, sustainability and tunability [3]. Specifically, 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]. This results in the accumulation of charge in the acceptor group, where efficient proton binding and reduction is expected to occur. By varying the donor and acceptor structure, the catalytic properties for HER can be controlled and improved.  
Donor-acceptor type conjugated polymers have shown effective photocatalysis performance [11]. Recent designs used donor structures based on fluorene and triazine analogues [12; 13; 14]. The important aspect of successful catalysts seems to be the presence of high \(\pi\) conjugation and the introduction of heteroatoms. For instance, nitrogen and fluorine substitution on fluorene-based analogues have shown improved performance for HER associated with charge transfer efficiency [14]. However, the mechanism for binding of a proton, its reduction and formation of H\({}_{2}\) is still unclear. To understand how to design better D-A polymer photocatalysts for HER, it is important to understand how varying the structure affects the optoelectronic properties and the mechanism of HER to, in turn, get better HER performance. Density functional theory (DFT) calculations have been valuable towards this end. DFT has been used to predict the optoelectronic properties and photocatalytic efficiency of polymer photocatalysts [15; 16]. Recently, the H\({}^{+}\) binding and H\({}_{2}\) formation mechanisms involved in HER using polymer photocatalysts have been studied by DFT as well [17; 10].  
In this work, we synthesised two novel polymer catalysts (Figure 1) with different donor units, aiming to compare their HER performance and understand their H\({}_{2}\) formation mechanisms. Both structures have the same phosphindole analogue acceptor unit, 4,4\({}^{\prime}\)-(5-oxide-5-phenylbenzo [b]phosphindole-3,7-diyl)dibenzaldehyde (PO). This acceptor structure features a phenylphosphine oxide group with a polar P = O moiety that has been associated with improved HER performance in previous studies [9; 18]. A carbazole analogue, 9-(4-aminophenyl)-9H-carbazole-3,6-diamine (PCz), was used as a donor group for the first polymer while a fluorine substituted triphenylamine group, N\({}^{1}\),N\({}^{1}\)-bis(4-amino-2-fluorophenyl)-2-fluorobenzene-1,4-diamine (PNoF) was used as a donor for the second. These polymers are herein referred to as PCzPO and PNoFPO, respectively. Their respective optoelectronic properties and HER rate were measured. We further performed DFT calculations to determine the frontier orbital density distributions and mechanism of the reaction to gain insight into this type of catalyst design. 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 H\({}_{2}\) through an intramolecular reaction. The intermolecular reaction of forming H2 could take place only when the repulsion is low between polymers. Both experimental and DFT results indicate the importance of extended configurations of D-A polymers on HER performance.