Chiroptically Active Photonics Polymers: Synthesis and Chiroptically Switching Properties of Helical Polyacetylene Bearing Electrochromic Viologens in the Side Chains

Toward near-infrared chiroptically active photonics polymers, a new type of optically active polyacetylene bearing l -alanine and electrochromic viologen in the side chains was prepared and carefully studied for the chiroptical properties. The resulting polymer exhibited much larger optical rotation ([m] 25 D = −1310°, DMF) than the model compound ([φ] 25 D = 201°, DMF) as well as a strong negative circular dichroism (CD) signal attributable to backbone absorption, indicating the formation of a single-handed helical structure of the polymer main chain. The content of viologen in the polymer was determined to be >70 mol % relative to the repeat unit by NMR and X-ray photoelectron spectrscopy. Because of the chiral perturbation of the helical main chain, the viologens in the side chains are asymmetrically positioned and interacted to yield a new strong negative CD band at long wavelengths (i.e., 450−800 nm) upon reduction to radical cations. The intensity of this CD signal could be easily switched between nearly 0 and −1.0 × 10 4 deg cm 2 dmol −1 by redox cycles, which could be repeated for five times without incurring any deterioration of the sample solution. The rotatory strength of the CD signal was calculated to be 1.3 × 10 −40 esu 2 cm 2 according to Moffitt and Moscowitz’s equation, which should contribute nontrivially to the chiroptical properties of the polymer at the near-infrared spectral region.