Synthesis and Redox-Driven Chiroptically Switching Properties of Viologen-Containing Optically Active Polymer with Main-Chain Axial Chirality

摘要

A novel type of redox-based chiroptically switching polymer was prepared by grafting electrochromic viologens onto side chains of a 1,1'-binaphthyl-6,6'-diyl-based optically active conjugated polymer. The resulting polymer exhibited much larger mean residue optical rotation ([m](D)(20) = -3308 degrees) than the molar rotation of the corresponding model compound (R)-BBEBPP ([phi](D)(20) = +384 degrees) and an intense negative CD signal in the absorption band of the polymer backbone, indicating a secondary chirality of the main chain. The viologen-grafting ratio was determined to be similar to 50% by proton NMR and thermogravimetric analysis, which corresponds to a predominant polymer structure with one viologen on each repeat unit. Due to the presence of viologen groups, the polymer exhibited a distinctive color change from light yellow to dark blue upon electrochemical or chemical reduction of cationic viologens (V2+) to radical cations (V+center dot). Correspondingly, the CD spectrum of P4 (V+center dot) showed two new positive Cotton effects in the absorption bands of V+center dot at 404 and 520 nm. In comparison with the small molecular model compound that exhibited stronger negative Cotton effects in absorption bands of V+center dot, P4 (V+center dot) has a quite different CD pattern, which was attributed to a different exciton coupling mechanism between the two. By taking advantage of the pronounced changes in the CD spectrum of P4 before and after reduction (e.g., [theta] from measuring nearly 0 to 1.2 x 10(4) deg cm(2) dmol(-1) at 404 nm or 5.9 x 10(3) deg cm(2) dmol(-1) at 520 nm), redox-driven chiroptically switching properties of P4 was probed in a DMF solution using tin as the reducing agent. After five redox cycles, no deterioration of the sample solution was observed and good reversibility in CD signals was demonstrated.