Tailoring Photophysical Properties of Diketopyrrolopyrrole Small Molecules with Electron-Withdrawing Moieties for Efficient Solar Steam Generation

The development of photothermal materials (PTMs) for solar steam generation (SSG) has gained tremendous attention in response to the global clean water scarcity issue. However, the investigation in employing organic small-molecule PTMs for SSG applications is rarely found due to their narrow optical absorption range to harvest solar energy and insufficient photostability for long-term use. Herein, we employ a diketopyrrolopyrrole (DPP) core unit together with electron-withdrawing (EW) endcaps and siloxane side chains to introduce stronger intramolecular charge transfer (ICT) characteristics as well as the hydrophobic character. The enhanced ICT characteristics of DPP derivatives render a broad optical absorption range, less emission, and a high nonradiative decay rate for efficient solar energy harvesting and photothermal effects. Meanwhile, the hydrophobic nature of these DPP derivatives allows the facile fabrication of novel Janus photothermal membranes for effective water vaporization and solar-to-vapor conversion efficiency. By embedding DPP derivatives to the SSG device, we showed that the solar-to-vapor efficiency can reach up to 71.8% under relatively low visible light power (∼700 W m-2), which is, on average, 2.66 times higher than that of bulk water of similar dimension. Moreover, this report demonstrates the great potential of conjugated small molecules for photothermal applications, owing to their versatility and flexibility in structural engineering and its diminishing radiative decay properties. This may inspire more innovation and advancement in SSG applications.