To obtain a qualitative understanding of the donor-acceptor interactions in our polymers, we performed density functional theory (DFT) calculations of the frontier orbitals in a model system. This model comprises the triarylborane core connected to three thiophene moieties and represents a large segment of the polymeric repeats common to all three SBPs. The obtained Kohn-Sham highest occupied molecular orbital (HOMO) shows a node on the boron atom, whereas the lowest unoccupied molecular orbital (LUMO) has a large coefficient on boron, and notably smaller coefficients on the other atoms (Figure ). From these results we conclude that, after vertical excitation, the excited electron has the highest probability density at the boron center. On the basis of these calculations, we propose the following mechanism for photoexcitation and hydrogen evolution: (i) photoexcited charges separate and form a boryl radical anion and thiophene radical cation, and (ii) the generated cation is reduced by single-electron transfer from ascorbate. (iii) A second photoexcitation event transfers another electron to the boron center and the emerging electron pair is protonated, forming a borohydride species. Finally, (iv) a second proton induces elimination of hydrogen (H2) and the catalyst regenerates via a second, single-electron transfer from ascorbate (Figure ). We are aware that this proposed mechanism does not account for the role of Pd co-catalyst and that further studies on the mechanism are required to make a definitive statement. To a first approximation, Pd co-catalyst separates photoexcited electron-hole pairs spatially and for long enough to become photocatalytically active.