Preparation of Photochromic Wood Films Comprising Spiropyran-Based Wood Cellulose Scaffold Realized through Grafting and Densification

The development of smart color-changing wood could facilitate its functional application. Herein, photochromic wood films (PWFs) were synthesized by grafting 1-(2-hydroxyethyl)-3,3-dimethylindolino-6'-nitrobenzopyrylospiran onto the cellulosic skeleton structure of delignified wood using hexamethylene diisocyanate as a bridging linker, followed by the densification process. The microstructural morphology, chemical composition, photochromic properties, and mechanical properties of the prepared PWFs were characterized by scanning electron microscope, Fourier transform infrared spectroscopy, colorimeter, and universal mechanical testing machine. The photochromic capability of the wood film was positively correlated with the grafting amount of spiropyran; the color change is evident when the grafting amount is high. Furthermore, the photochromic and recovery capability of the wood film weakened after sequentially irradiating 40 times using a UV lamp and daylight lamp, which could be partially recovered after a period of rest. However, the wood film strength was negatively correlated with the grafting amount of spiropyran. The grafted spiropyran affected the hydrogen bonding between cellulose nanofibers of the wood film during densification, which caused a reduction in the film strength. However, the strength was still >120 MPa. Meanwhile, temperature and humidity affected the photochromic capability of the wood film. Thus, the developed high-strength PWF has potential applications in various fields, such as intelligent sensing, personalized design, indoor and outdoor architecture, and optoelectronics.