Transparent Photothermal Heaters from a Soluble NIR-Absorbing Diimmonium Salt

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.