Thermochromism and Structural Change in Polydiacetylenes Including Carboxy and 4‑Carboxyphenyl Groups as the Intermolecular Hydrogen Bond Linkages in the Side Chain

We investigated the thermochromic behavior of polydiacetylenes
including the carboxy and 4-carboxyphenyl groups as the side-chain
substituents adjacent to the conjugated main chain, and then, the
thermal stability and the thermochromism reversibility of the polymers
were related to changes in the polymer conformations monitored by
IR and Raman spectroscopies and powder X-ray diffractions. The polydiacetylenes
with no or a phenylene spacer between the main chain and the carboxylic
acid moiety were revealed to exhibit a thermal resistance for maintaining
reversible thermochromism in a high temperature range, rather than
polydiacetylenes with a conventional structure with a flexible alkylene
spacer. The molecular stacking structures of the diacetylenes and
the corresponding polymers in the crystals were discussed based on
the results of an X-ray single-crystal structure analysis as well
as the powder X-ray diffraction measurements.