Fabrication of bimolecular-type photochromic transparent wood with spatially differentiated UV-responsive coloration

Transparent wood (TW) has attracted considerable attention due to its combination of high transmittance and environmental friendliness. However, conventional transparent wood exhibits significant limitations in dynamic light modulation and spatially resolved color switching, making it incapable of meeting the demand for multi-color synergistic responses in applications such as smart windows. This study proposes a strategy combining bulk impregnation with localized coating, successfully fabricating photochromic transparent wood (PRTW) featuring region-divergent reversible color-changing behaviors upon UV exposure. First, the delignified wood template was dyed with Reactive Red X-3B to obtain a red-colored substrate. Then, epoxy resin containing spirooxazine photochromic molecules was vacuum-impregnated to construct a bulk photochromic underlayer. Finally, a waterborne acrylic coating containing naphthopyran was applied onto the left half of the surface to achieve localized photochromic functionality. The effects of photochromic powder (PM) content on the microstructure, optical properties, color-switching behavior, surface wettability, and mechanical performance were systematically investigated. Results showed that at a PM loading of 4 wt%, PRTW-4 exhibited a color difference (ΔE) of 10.69 upon UV irradiation, faded to ΔE < 2.5 within 60 s, and retained acceptable reversible performance over 30 tested cycles. It achieved a transmittance of 83.8% and a haze of 82.3%, combining high transparency with uniform light diffusion. The water contact angle increased to 77.47°, indicating significantly improved water resistance. A moderate rise in tensile strength to approximately 19.7 MPa was observed for PRTW-4 relative to uncoated RTW. This study breaks through the functional bottleneck of single-color and bulk-only response in traditional transparent wood, providing a new design paradigm for multifunctional wood-based materials for smart windows and beyond.