Transparent Photothermal Heaters from a Soluble NIR‐Absorbing Diimmonium Salt

Abstract While numerous near‐infrared (NIR) materials have emerged, most of them are strongly colored or black due to the absorption band or tails in the visible region. Here, a highly transparent and soluble NIR‐absorbing ionic salt, isobutyl‐substituted diimmonium borate (IDI), is synthesized and fabricated, through a solution process, as a thin film that shows a transmittance of over 93% in the whole visible region. A transparent photothermal (PT) film heater is fabricated with the IDI‐doped polymer solution, which shows a photothermal conversion efficiency (η_PT) of 75.2%. Additionally, the prepared PT heater shows a high water evaporation conversion efficiency (η_w) of 68.8% upon exposure to a 1064 nm laser. Furthermore, the transparent IDI film affords the development of a wireless transparent actuator for the first time, generating a bending angle over 75°, with over 2700 bending cycles. The transparent IDI film creates a hot transparent Venus flytrap and a colorful or fluorescent actuator upon the addition of colorants without losing the actuation properties.