A planar–nonplanar conformational transition in conjugated polymer solutions

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.