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

Abstract

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°) than the molar rotation of the corresponding model compound (R)-BBEBPP ([ϕ]D 20 = +384°) 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 ∼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+·). Correspondingly, the CD spectrum of P4 (V+·) showed two new positive Cotton effects in the absorption bands of V+· at 404 and 520 nm. In comparison with the small molecular model compound that exhibited stronger negative Cotton effects in absorption bands of V+·, P4 (V+·) 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., [θ] from measuring nearly 0 to 1.2 × 104 deg cm2 dmol−1 at 404 nm or 5.9 × 103 deg cm2 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.

A novel multichromic copolymer via electrochemical copolymerization of (S)-1,1′-binaphthyl-2,2′-diyl bis(N-(6-hexanoic acid-1-yl) pyrrole) and 3,4-ethylenedioxythiophene

“…In the past few decades, attentions were focus on the preparation of EC small molecules containing binaphthyl units [24,25], only few literature was related to the synthesis of binaphthyl-based EC polymer aiming to design an redox-based chiroptically switching polymer [26]. It can be expected that if the binaphthyl unit is grafted onto an EC conducting polymer chain, multichromism may be obtained in the resulting Scheme 1.…”

The transformation of chiral signals into macroscopic properties of materials using chirality-responsive polymers

“…In 2008, Song et al 45,46 synthesized a helical poly(acetylene) with L-alanine and electrochromic viologens in the side chains. A new negative CD band at a long wavelength was observed upon reduction of the cationic viologens to radical cations.…”

Synthesis and Conformation of Substituted Chiral Binaphthyl-Azobenzene Cyclic Dyads with Chiroptical Switching Capabilities

“…Molecular scaffolds designed for disparate purposes may be linked to enhance the properties of one component or to confer altogether new properties. Stimulus-driven chiroptical switches such as light-, electrically-, pH-, ion-, and temperature-driven switches have received a great deal of attention in the past decade [1,2,3,4,5,6,7,8,9,10,11]. Their high sensitivity and fast response are of great interest for molecular devices.…”