## 4 Conclusions  
In this work, we have successfully synthesised two novel donor-acceptor (D-A) type conjugated polymer photocatalysts, PCzPO and PNoFPO. Optoelectronic properties of the polymers were  
Figure 8: Energy diagram of intermolecular H\({}_{2}\) formation using two protonated PCzPO D-A moiety models, computed by DFT. Figure 7: Energy diagram of intramolecular H\({}_{2}\) formation using the PCzPO D-A moiety model protonated at C1 and C2 (red line) and C1 and O (black line), computed by DFT. characterised both experimentally and using density functional theory calculations. From the perspective of electronic structures, the measured optical gap of PCzPO was lower than that of PNoFPO by UV-vis absorption, which is inconsistent with the similar HOMO-LUMO gaps of PCzPO and PNoFPO D-A moieties, computed by DFT. This discrepancy might be associated with better-extended \(\pi\) conjugation and potential intermolecular stacking brought about by the more planar nature of the carbazole group of PCzPO compared to the triphenylamine group of PNoFPO; these structural effects were not considered in DFT D-A moiety models. Because of the intermolecular stacking of carbazole groups, leading to longer conjugation and higher electron transporting capability, PCzPO outperformed PNoFPO in HER performance. Further DFT analysis of the PCzPO model was then performed to investigate the details of H\({}^{+}\) binding, reduction and H\({}_{2}\) evolution mechanisms. First, it shows that the more natively charged C and O atoms of the phosphindole analogue acceptor group are the primary protonation sites. Secondly, we deduce that H\({}_{2}\) formation is likely taken place through an intramolecular reaction with similar activation energies of 0.6-0.7 eV for both PCzPO and PNoFPO. Though the intermolecular reactions to form H\({}_{2}\) is almost barrierless, this pathway is expected to occur when the repulsion between polymers is low. Presumably, the higher intermolecular stacking of PCzPO D-A polymers leads to a higher possibility of occurrence of intermolecular H\({}_{2}\) formation. Finally, from our experimental and DFT results, we highlight that the extended configurations of D-A conjugated polymers could be the crucial factor leading to higher HER rate while the donor structure determines the mechanisms involved in transforming H\({}^{+}\) to H\({}_{2}\). The detailed electron push-pull interactions of adding heteroatoms in either acceptor or donor groups on HER performance will be studied in the future. The following are available online at [[http://www.mdpi.com/2076-3417/10/20/7017/s1](http://www.mdpi.com/2076-3417/10/20/7017/s1)]([http://www.mdpi.com/2076-3417/10/20/7017/s1](http://www.mdpi.com/2076-3417/10/20/7017/s1)), Figure S1: UV-vis absorption spectra of PCzPO and PNoFPO D-A polymer, measured in Dicholoromethane Figure S2: Hydrogen evolution profile of PCzPO and PNoFPO using visible light for polymers (5 mg catalyst in 10 mL mixture (water: MeOH: triethylamine = 1:1:1), 1000 W/m\({}^{2}\), 380 nm-780 nm, 90 \(\mu\)L H\({}_{2}\)PtCl\({}_{6}\) (1M) was added). Figure S3: Energy diagram of intramolecular H\({}_{2}\) formation using PCzPO (left) and PNoFPO (right) D-A moiety models, computed by DFT. Figure S4: Formation of a phosphorous-phosphorous bond (P atoms are shown as orange spheres and the bond is shown as an orange line) preventing the interaction of bound H atoms (highlighted in light blue) for a complete hydrogen evolution, computed by DFT. Conceptualisation, Y.I.A.R., H.-H.C. and H.-Y.T.C. ; formal analysis, Y.I.A.R. and L.-Y.T. ; funding acquisition, H.-Y.T.C., H.-H.C. and C.C. ; methodology, Y.I.A.R., L.-Y.T., H.-Y.T.C., H.-H.C. and C.C. ; resources, H.-Y.T.C. and H.-H.C.; software, H.-Y.T.C. ; supervision, H.-Y.T.C., H.-H.C. and C.C. ; validation, Y.I.A.R., writing--original draft, Y.I.A.R. and L.-Y.T. ; writing--review and editing, Y.I.A.R., X.T., H.-Y.T.C., H.-H.C. and C.C. All authors have read and agreed to the published version of the manuscript. H.-Y.T.C.