Crystal structures, polymerization, and thermochromic phase changes in urethane-substituted diacetylenes crystals with varying alkyl chain lengths

A series of urethane-substituted diacetylene monomers (DA-mUPh) with different alkyl chain length (m = 4, 5, and 6) in side-chain substituents of (CH₂)_m OCONHC₆H₅ was synthesized. X-ray structural analyses for the DA-mUPh single crystals show that the monomers are packed by hydrogen bonding between the urethane groups, but the arrangement of the diacetylene moieties depends on the alkyl chain length. The reactivity of the DA-mUPh crystals upon γ-irradiation is different depending whether the alkyl chain length (m) is odd or even. The difference can be explained by the distance between the neighboring carbon atoms of the diacetylene groups. The temperature dependence of reflectance spectra for the polydiacetylene crystals has also been investigated. The thermochromic behavior is argued in comparison with the analysis of the differential scanning calorimetry.