Reversible thermochromic polydiacetylene/Zn(II) ion assemblies prepared via co-assembling in aqueous phase: The essential role of pH

Abstract

Reversible thermochromism of polydiacetylene/zinc(II) ion (PDA/Zn2+ assemblies is achieved via co-assembling process in an aqueous medium. Our preparation method is quite easy compared to other previous reports. Diacetylene monomer, 1012-pentacosadiynoic acid PCDA, and ZnCl2 are simply mixed in water followed by an incubation in a refrigerator and then UV light irradiation. The resultant assemblies prepared using normal conditions (pH 6.5) exhibits partially reversible themochromism. Hydroxide ion (OH−) is a key component for promoting strong interaction between PCDA carboxylate headgroup and Zn2+ ions during the self-assembling process. We have found that increasing the pH above 8 yields fully reversible thermochromism. The reversible blue-to-purple color transition occurs at about 75 °C. X-ray diffraction reveals the Zn2+-intercalated bilayer structure while infrared spectroscopy probes strong interactions within the assemblies. The PCDA/Zn2+ mole ratio is also a very important parameter for achieving reversible thermochromism. An ability to prepare the PDA/Zn2+ assemblies in aqueous suspension allows the fabrication of solid-state colorimetric sensors by drop-casting on a glass slide, writing on a paper or incorporating into a polyvinylalcohol film. The reversible color-transition temperature can be tuned to about 40 °C by shortening the alkyl chain of constituent PDA. Our new approach for preparing the reversible thermochromic PDA-based materials is quite easy and can be utilized for large scale production of colorimetric sensor, smart ink or paint.