Self-adaptive photochromism

Abstract

Organisms with active camouflage ability exhibit changeable appearance with the switching of environments. However, manmade active camouflage systems heavily rely on integrating electronic devices, which encounters problems including a complex structure, poor usability, and high cost . In the current work, we report active camouflage as an intrinsic function of materials by proposing self-adaptive photochromism (SAP). The SAP materials were fabricated using donor-acceptor Stenhouse adducts (DASAs) as the negative photochromic phases and organic dyes as the fixed phases (nonphotochromic). Incident light with a specific wavelength induces linear-to-cyclic isomerization of DASAs, which generates an absorption gap at the wavelength and accordingly switches the color. The SAP materials are in the primary black state under dark and spontaneously switch to another color upon triggering by transmitted and reflected light in the background. SAP films and coatings were fabricated by incorporating polycaprolactone and are applicable to a wide variety of surfaces.

Fig. 1.. Design of SAP materials.

(A) Creatures with active camouflage (created with Photoshop 2023). (B) Schematic illustration of the mechanism of self-adaptive photochromism (SAP). (C) General chemical structure and isomerization between linear and cyclic donor-acceptor Stenhouse adducts (DASAs). (D) Normalized ultraviolet-visible (UV-vis) absorption spectra of the color-contributing units in fixed and negative photochromic phases [polycaprolactone (0.1 g/ml) in tetrahydrofuran/dichloromethane, 1:9 v/v]. (E) Relative color of DASAs under light irradiation with different wavelengths and distances.

Fig. 2.. Color accuracy of SAP materials in the dark.

(A) Simulated UV-vis absorption (left) and transmission (right) spectra of SAP solutions with various combinations. (B) Modified CIE 1931 a* b* values of SAP solutions; the closer to the origin indicates that the SAP solutions show higher color accuracy in black. (C) Simulated UV-vis transmission spectra of A4 solutions with various initial absorbances. (D) Modified CIE 1931 a* b* values of A4 solutions with the initial absorbance increasing from 0.5 to 50. (E) Calculated brightness (L*, colored solid) and deviation [SQRT(a*² + b*²), diagonal filled] values of the A4 solutions with initial absorbance between 0.5 and 50; the inset shows photographic images of A4 solutions with the initial absorbance of 0.5, 2, and 6. Lower L*+ SQRT(a*² + b*²) indicates higher color accuracy in black.

Fig. 3.. Color accuracy of SAP materials under light irradiation.

(A) Top half: Tunable range of color for A4 solutions with various initial absorbances. Down half: Modified CIE 1931 a* b* values of A4 solutions (A = 6) under 520-nm (green), 590-nm (yellow), 620-nm (orange), and 660-nm (red) light irradiation; the distance was kept at 5, 10, 30, 50, 75, 100, and 200 cm, and dash lines stand for the color of LED lights. (B) Modified CIE 1931 a* b* values of SAP solutions. (B) Accuracy of the color (Δθ) of A4 solutions under light irradiation with various conditions (wavelength and distance).

Fig. 4.. SAP and active camouflage in solutions.

(A) Schematic illustration of the philosophy for the active camouflage: The color of the environment depends on the transmitted and reflected light. (B) Normalized UV-vis absorption spectra of the SAP solutions in the dark and under 660-, 520-, and 590-nm light irradiation, inset shows the photographic images of each color-contributing unit and the resulted in SAP solutions (the spectra of 590- and 660-nm irradiation were obtained under the liquid nitrogen treatment). (C) Fatigue resistance of SAP solutions switching of black-red (red line, absorbance at 644 nm monitored), black-green (green line, absorbance at 556 nm monitored), and black-yellow (yellow line, absorbance at 602 nm monitored) for 20 cycles. (D) Color switching of SAP solutions in black, red, green, and yellow acrylic boxes; the left cuvette was loaded with SAP solutions, and the right cuvette with black ink as the control. Black dots represent the in situ distance between the average RGB values of regions C and B, and colored dots represent the distance between regions A and B. (E) Movie screenshots of the active camouflage of SAP solutions in red, green, and yellow bushes. (F) Schematic illustration and movie screenshots of realizing active camouflage by SAP solutions in a slender transparent tube (25-mm outer diameter, 0.8-mm wall thickness, 160-mm length) covered by sequentially arranged sticky notes.

Fig. 5.. SAP in films and coatings.

(A) UV-vis diffuse reflectance spectra of SAP films before (black, solid) and after (colored) 660-nm (95 mW/cm², 30 s), 520-nm (140 mW/cm², 30 s), and 590-nm (25 mW/cm², 120 s) light irradiation; the spectra were recorded under room temperature (solid) and liquid nitrogen (dashed), and the insets show photographic images of the SAP films before irradiation, immediately after irradiation, and after 85-s scanning. (B) Schematic illustration of spray coating an SAP solution on the surface of a white acrylonitrile butadiene styrene (ABS) model. (C) Movie screenshots of the SAP process of three black ABS models in response to 660-nm (top), 520-nm (middle), and 590-nm (bottom) light. (D) Schematic illustration of irradiating the “UESTC” by green and red light. (E) Photographic images of SAP coatings on smooth surfaces (glass and stainless steel) before (left) and after (right) light irradiation.