Solutions of certain urethane substituted polydiacetylenes undergo dramatic, reversible color changes on addition of a nonsolvent. In some cases, the lowest energy optical transition of the polymer shifts by more than 5000 cm−1 (21 300 cm−1 to 15 900 cm−1 when the nonsolvent is added). The color changes are shown to be due to a planar–nonplanar conformational transition of the polymer backbone. The planar, fully-conjugated conformation is stabilized by intramolecular hydrogen bonding between urethane functionalities on adjacent substituent groups.

The optical properties of single crystals and solution-cast films of two polydiacetylenes, poly3BCMU and poly4BCMU where the substituent group is-(CH2h .. OCONHCH2COOC~, are characterized. Visible absorption and reflection spectra for the polymer crystals are typical of those observed for other urethane substituted polydiacetylenes. The optical properties of the polymer films are controlled primarily by intramolecular hydrogen bonding between the N-H and C=O of the urethane functionalities on adjacent substituent groups. Hydrogen bonding stabilizes the planar, fully conjugated conformation of the individual polymer chains in the films. Increased temperature causes a disruption of the hydrogen bond network and a destabilization of the planar polymer conformation. Dramatic color changes result because of the sensitivity of the optical properties to backbone conformation.

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A novel planar ↔ nonplanar visual thermochromic conformational transition of polydiacetylene molecules in poor solvents is reported. The conformational transition is associated with both a color change (blue or red ↔ yellow) and a change in the state; the yellow solution (liquid) transforms to a blue or red gel (solid). The color transition occurs within a narrow range of temperature and has a large associated hysteresis. The enthalpy of the conformational transition is 29 kJ/mole of repeat unit. Fourier‐transform infrared studies show that molecules acquire a planar conformation in red or blue gels by formation of intramolecular H bonds between the adjacent substituent groups. Virtually all H bonds break (a nonplanar conformation) when the gels turn into yellow solutions.

The extent of π electron delocalization in solid-state polymerized diacetylenes is derived from observed Raman vibrational frequencies. Raman intense C=C and C≡C vibrations indicate that the polymer backbone is best represented by (=C–C≡C–C=)n. However, vC=C is linearly correlated with vC≡C over a wide frequency range and both frequencies increase with decreasing phase perfection. This suggests a significant fractional contribution δ from the mesomeric structure (–C=C=C=C–)n, which decreases the π bond order of C=C to 1-δ and C≡C to 2-δ. Both vC=C and vC≡C are calculated as a function of δ and compared with observed frequencies for variously substituted polydiacetylenes in different physical states. Resulting ΔvC=C/ΔvC≡C values obtained using two different bond-length to bond-order approximations are in good agreement with the observed value (1.19±0.12).