Tailoring Photochromism Beyond Human Eye Visibility: On/Off Switchable NIR Absorption in an Aluminosilicate.

Photochromic materials are able to change reversibly between two colors. The color change is triggered by ultraviolet radiation and reversed by white light and/or heating. Inorganic photochromes have been thought to operate with fixed wavelengths only, and because of this, their usage has been considered less convenient than that of their organic counterparts, even if the inorganic ones would show a much higher durability in applications. High-tunability within the whole visible wavelength range was recently demonstrated by some of us for hackmanites, a family of inorganic aluminosilicates. Here, we demonstrate that a davyne-type structure constitutes the first photochromic material exhibiting absorption exclusively in the near-infrared region, without accompanying visible color changes, thereby enabling fully invisible photochromism beyond human perception. Furthermore, we show that the invisible photochromic changes can be detected with a simple camera setup. This is the first material that exhibits invisible near-infrared reversible photochromism, and we discuss its potential for tagging applications.