Photochromic covalent organic polymers (COPs) with dual response based on fast fluorescence and color transformation

Abstract

Photochromic COP materials have attracted more and more attention due to their intelligent photophysical properties and specific responsive capabilities. However, developing photochromic COP materials with visible light triggering and dual responses (fluorescence quenching and color transformation) remains quite challenging. Here, we combined photochromic 4,4-(cyclopentadienyl-1-en-1,2-alkyl)bis(5-methyl thiophene-2-formaldehyde) (DAE) with 1,3,5-tris(4-cyanomethylbenzene)benzene (CPTD) for the first time to synthesize a new photochromic COP through a Knoevenagel condensation reaction. The COP exhibited excellent fluorescence performance in both solid and liquid states. The COP retained the unique photochromic behavior of DAE, exhibiting a color change (from orange to green) under the irradiation of visible light. The function of the COP could be further regulated by 460 nm visible light irradiation (with a power of 30 W), resulting in fluorescence tuning and color redshift. The COP also exhibited good reversible thermal recovery performance, and its fluorescence spectrum and color returned to the pre-irradiation state after heating at 135 °C for a specific time. The response time of the COP was shorter than that of most reported visible light triggered photochromic diarylethene derivatives. The fluorescence was significantly quenched after exposure to 460 nm visible light (with a power of 30 W) for 3 s. The color changed to green followed by fluorescence quenching. The unique light triggered response properties of the COP–PLA film make it promising for optical anti-counterfeiting applications.