Color-tunable, multifunctional photochromic composites for wearable UV monitoring and biomechanical energy harvesting

Organic-inorganic hybrid photochromic materials have been considered one of the most promising photochromic candidates for light detection and information anti-counterfeiting. However, challenges still exist, such as single-color variation, long decoloration time, and lack of in-depth understanding of the photochromic mechanism. Here, a three-component photochromic composite system was designed, composed of two polymer matrices (electron donors) and phosphotungstic acid. By adjusting the ratio of one electron donor, the composite can initiate color change from transparent to deep blue, near-colorless, light mauve, mauve, and grey mauve, respectively, upon UV light irradiation, which has never been reported. These composites also feature flexibility, mechanical robustness, high transparency, faster photochromic reversibility, as well as dispensing printing. Importantly, the underlying multi-color photochromic mechanism is elucidated by leveraging the theoretical simulation and characterization techniques. A UV detection bracelet for real-time UV monitoring and a proton conductor for a wearable biomechanical energy harvester were fabricated using the photochromic composites, indicating application potentials for smart wearable electronics. This work introduces a new type of organic-inorganic photochromic composite and explores the photochromic mechanism, which may guide the design of new photochromic systems for future wearable technologies.