Limitation of molecular twisting: Upgrading a donor-acceptor dye to drive H2 evolution

Abstract

Abstract The performance of dye-sensitized photoelectrochemical (DSPEC) cells is currently hampered by the low efficiency of the photocathode, predominantly due to ineffective charge separation. To promote efficiency, donor-acceptor (D-A) dyes for photosensitization of the p-type semiconductor have been designed, spatially separating electrons and holes. We further improve on the state of the art by manipulating photoinduced twisting of a D-A P1 dye adsorbed onto NiO by co-adsorption of myristic 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.