Limiting Molecular Twisting: Upgrading a Donor–Acceptor Dye to Drive H₂ Evolution

The donor–acceptor (D–A) dye 4-(bis-4-(5-(2,2-dicyano-vinyl)-thiophene-2-yl)-phenyl-amino)-benzoic acid (P1) has been frequently used to functionalize NiO photocathodes and induce photoelectrochemical reduction of protons when coupled to a suitable catalyst. Photoinduced twisting of the P1 dye is steered on NiO by co-adsorption of tetradecanoic acid (C₁₄, myristic acid (MA)). Density Functional Theory and time-resolved photoluminescence studies confirm that twisting lowers the energy levels of the photoexcited D–A dye. The apolar environment provided by the MA suppresses photoinduced D–A twisting, retards charge recombination following photoinduced charge separation between P1 and NiO, and provides a larger electrochemical potential increasing the photocurrent. Very interestingly, co-adsorption of MA induces H₂ evolution upon photoexcitation without the presence of an H₂ evolution catalyst. Based on prior art, the formation of H₂ is assigned to the dissolution of Ni²⁺, followed by reduction and re-deposition of Ni nanoparticles acting as the catalytically active site. It propose that only excited P1 with suppressed twisting provides the sufficient electrochemical potential to induce deposition of Ni nanoparticles. The work illustrates the importance of understanding the effects of photoinduced intramolecular twisting and highlights the promise of designing twisting-limited D–A dyes to create efficient solar fuel devices.