The Orange Side of Disperse Red 1: Humidity‐Driven Color Switching in Supramolecular Azo‐Polymer Materials Based on Reversible Dye Aggregation

Humidity detection, and the quest for low‐cost facile humidity‐sensitive indicator materials is of great interest for many fields, including semi‐conductor processing, food transport and storage, and pharmaceuticals. Ideal humidity‐detection materials for a these applications might be based on simple clear optical readout with no power supply, i.e.: a clear color change observed by the naked eye of any untrained observer, since it doesn't require any extra instrumentation or interpretation. Here, the introduction of a synthesis‐free one‐step procedure, based on physical mixing of easily available commercial materials, for producing a humidity memory material which can be easily painted onto a wide variety of surfaces and undergoes a remarkable color change (approximately 100 nm blue‐shift of λ_MAX) upon exposure to various thresholds of levels of ambient humidity is reported. This strong color change, easily visible to as a red‐to‐orange color switch, is locked in until inspection, but can then be restored reversibly if desired, after moderate heating. By taking advantage of spontaneously‐forming reversible ‘soft’ supramolecular bonds between a red‐colored azo dye and a host polymer matrix, a reversible dye ‘migration’ aggregation appearing orange, and dis‐aggregation back to red can be achieved, to function as the sensor.