(Invited) Limitation of Molecular Twisting: Upgrading a Donor-Acceptor Dye to Drive H2 Evolution

Abstract

Dye-sensitized photoelectrochemical cells (DSPECs) have attracted wide attention to convert solar energy into high-energy chemicals. A DSPEC consists of a photoanode performing the oxidation half reaction and aphotocathode performing the reduction half reaction. In this presentation I will demonstrate how the performance of a dye-sensitized photocathode can be manipulated to enhance performance and even produce hydrogen! We have used a state- of-the-art dye (D-A P1 dye) on the surface of optimized NiO, and co-adsorbed myristic acid (Tetradecanoic Acid), which has a carboxyl anchoring group and a long apolar alkyl chain. Time-resolved photoluminescence and Density Functional Theory studies show that twisting lowers the energy levels of the photoexcited D-A dye, while twisting is inhibited in case myristic acid is co-adsorbed on the NiO surface. The presence of myristic acid also favors light-induced charge separation, as apparent from femtosecond transient absorption, and increases the apparent photocurrent. Very interestingly, only in the presence of myristic acid, light-induced H2 evolution is observed in aqueous media, despite the absence of a H2 evolution catalyst. We assign the H2 generation to a synergetic effect of inhibited twisting of the D-A dye radical anion increasing its electrochemical potential, combined with charge transfer and conversion of H+ on the hydroxylated NiO surface. Our work illustrates the importance of understanding effects of photoinduced intramolecular twisting and demonstrates that control thereof offers a simple design approach for efficient solar fuel devices.