Disentangling the Electrochromic and Acidochromic Optical Switching in Multi-Stimuli-Responsive Spiropyran Copolymers

Stimuli-responsive polymers offer dynamic functionality via molecular reporters that provide optical, electronic, or electrochemical signaling following the application of optical, chemical, mechanical, or thermal stimuli. Spiropyran is a colorless compound capable of isomerizing to the vibrantly colored merocyanine, and it has been incorporated into non-conjugated polymer scaffolds for photochromic and mechanochromic applications. However, stimuli-responsive main-chain spiropyran copolymers with π-conjugated comonomers remain relatively undeveloped, and it is unclear how the conjugated comonomer identity and functionalization of spiropyran influence its isomerization to merocyanine. Here, the electrochemical and acid-induced isomerization of main-chain spiropyran copolymers are systematically probed where the identity of the comonomer (fluorene or carbazole) and the functionalization of spiropyran are altered to develop key structure-function relationships and mechanistic insight. The polymers PF-SP, PF-SPn, PCBz-SP, and PCBz-SPn were synthesized via Suzuki polymerization and fully characterized via NMR, GPC, TGA, DSC, and AFM. DFT and TD-DFT calculations were performed using model compounds to assess the electronic structure and changes in optical properties following oxidation and isomerization. The polymers were incorporated into electrochromic devices to evaluate the electrochemical isomerization to merocyanine. It is found that the identity of the comonomer plays a central role in achieving optimal ECD performance metrics (optical contrast, switching times, and coloration efficiency), and that the polymer remains in the merocyanine form following its initial isomerization. Acid-induced isomerization yields unique optical absorption spectra with distinct structure-property relationships in comparison to electrochemical isomerization.