Design of semiconducting materials for water-splitting photocathodes

We will design and synthesise new classes of calamitic shaped small molecules to be employed as electron acceptors in bulk heterojunction blends for high reduction potential photocathodes. The design strategy will focus on creating conjugated aromatic semiconductors with discrete separation of electron rich and poor molecular sections, drawing on a symmetric monomer design template used in donor polymer synthesis. This has been an established and successful strategy to aid efficient materials optimisation and is a core area of expertise in the McCulloch group. In the intended application, it is necessary for the photocathode to exhibit a high reduction potential to maximize the overpotential, which enhances hydrogen generation. A prominent feature therefore of all molecular design considerations in this task is to facilitate high reduction potential heterojunctions through ensure that the acceptor LUMO is of high enough energy. This requires subtle destabilization of the LUMO through low levels of conjugation, out of plane orbital twisting, and the use of soft electron withdrawing groups. Building on this approach, we will explore conjugated systems that may additionally provide improved electron transport. A high photoelectrode overpotential will be facilitated by destabilisation of the LUMO through both steric twisting, conjugation blocking and limiting the strength of the electron withdrawing units. Importantly, the molecular design must ensure stability in aqueous environments both in the neutral and charged state. Water tolerant moieties will be identified and incorporated in the molecular design, while approaches to minimise water penetration at the film surface, and diffusion within the bulk will be pursued.