Odd–Even and Hydrophobicity Effects of Diacetylene Alkyl Chains on Thermochromic Reversibility of Symmetrical and Unsymmetrical Diyndiamide Polydiacetylenes

Two series of symmetrical (<b>S<i>x</i></b>) and unsymmetrical (<b>U<i>y</i></b>) diacetylene monomers containing diamide groups with different methylene units are successfully prepared. Photopolymerization of their nanovesicles dispersed in water is carried out by irradiation at 254 nm affording blue sols of the corresponding PDAs. The degree of thermochromic reversibility (%DR) of the PDA sols are determined using UV–vis spectroscopy in order to probe effects of the number of the methylene units, <b><i>x</i></b> and <b><i>y</i></b>, within the linker and hydrophobic tail, respectively. The complete color reversibility (%DR > 89%) is observed only when <b><i>x</i></b> is an even number while partially reversible or irreversible thermochromism (%DR < 65%) is displayed in the case of odd <b><i>x</i></b> number. For the <b>U<i>y</i></b> series, the color recovering ability within the heating and cooling process increases along with the <b><i>y</i></b> number; %DR = 3, 62, and 90% for <b><i>y</i></b> = 0, 4, and 16, respectively. This work is the first direct demonstration of the roles of number of methylene units within the diacetylene monomers on the thermochromic reversibility of their PDAs that provide additional dimensions for rational molecular design in the development of PDA thermal sensors