Dual Photo- and Mechanochromisms of Graphitic Carbon Nitride/Polyvinyl Alcohol Film

An adaptable stimuli responsive system is proposed based on defective graphitic carbon nitride (d-GCN)/polyvinyl alcohol (PVA) composite attached to different substrates. Nitrogen vacancies, introduced during synthesis via potassium doping, act as internal electron donors to enhance the photoreduction of redox dyes for quick reversible photochromism. This system can be applied as visible light responsive rewritable paper that offers high resolution for printed text, rapid activation/recoloration, multicolor functionality, and high cyclability. The defects also inhibit the d-GCN particle growth resulting in smaller particle sizes over bulk samples, which enable the formation of transparent mechanochromic films. By altering the PVA/d-GCN ratio in the composites, two mechanical stimuli responses are introduced by taking advantage of wrinkle and crack formation on the film surface. With a low d-GCN concentration, mechanical scratching of the film causes the formation of surface wrinkles that alter the film transparency without damaging the film, allowing for text to be written on the surface that can be erased by moisture. With a high d-GCN concentration, cracks developed on the surface scatter light when stretched, enabling the development of a strain dependent smart window. Both these mechanochromisms can be combined with the photochromism to develop multifunctional smart materials with widespread application.