Sequential Induction of Chirality in Helical Polymers: From the Stereocenter to the Achiral Solvent

Abstract

Several steps of chiral induction have been detected in poly(phenylacetylene)s among their different hierarchical levels of chirality by vibrational circular dichroism, namely, (i) from the stereogenic centers to the innermost polyacetylene helical covalent backbone (helix), (ii) from this to the external helix (helix) formed by the side phenyl pendants that form a complementary helix or counter-helix, and (iii) from this pendant helix to the helical solvation sphere (helix), the last one being observed along this work. The pendant to polyene backbone chiral induction determines the helical structure adopted by the polymer and therefore the solvation helix. This helical structure is promoted by two mechanisms: steric effects and hydrogen bonding. An important finding concerns the demonstration by VCD of how an achiral solvent becomes chirally organized owing to the template effect of the covalent polymer helices, an effect that is silent to other structural techniques such as ECD or AFM and that hence significantly broadens the scope of these previous analyses.

Photochemical Electrocyclization of Poly(phenylacetylene)s: Unwinding Helices to Elucidate their 3D Structure in Solution

“…Hence, to build up the 3D‐structure of a PPA, it is necessary to combine the information extracted by different techniques in solution and in the solid‐state: nuclear magnetic resonance (NMR), [26, 52–56] differential scanning calorimetry (DSC), [56] Raman, [57] Raman optical activity (ROA), [58] vibrational circular dichroism (VCD), [59] electronic circular dichroism (ECD), [60–62] atomic force microscopy (AFM), [50, 63–72] X‐ray diffraction, [73–82] as well as time‐dependent density functional theory (TD‐DFT) calculations (Figure 1). [83–85] Nevertheless, a correct assignment of the cis‐cisoidal ( c‐c ) or cis‐transoidal ( c‐t ) configuration of the polyene backbone is far from solved because the DSC thermogram is affected by the pendant groups or even by the dynamic behavior of the polymer, [56] rendering the technique useless in many cases.…”

Photochemical Electrocyclization of Poly(phenylacetylene)s: Unwinding Helices to Elucidate their 3D Structure in Solution

“…Thus,i ncis-cisoidal helices (w 1 < 908 8), the two helices rotate in the same sense (P int /P ext or M int /M ext ), while in cis-transoidal (w 1 > 908 8) polymers,both helices rotate in opposite directions (P int /M ext or M int /P ext ). [51] Hence,t ob uild up the 3D-structure of aP PA,i ti s necessary to combine the information extracted by different techniques in solution and in the solid-state:nuclear magnetic resonance (NMR), [26,[52][53][54][55][56] differential scanning calorimetry (DSC), [56] Raman, [57] Raman optical activity (ROA), [58] vibrational circular dichroism (VCD), [59] electronic circular dichroism (ECD), [60][61][62] atomic force microscopy (AFM), [50,[63][64][65][66][67][68][69][70][71][72] Xray diffraction, [73][74][75][76][77][78][79][80][81][82] as well as time-dependent density functional theory (TD-DFT) calculations ( Figure 1). [83][84][85] Nevertheless,acorrect assignment of the cis-cisoidal (c-c)o rcistransoidal (c-t)c onfiguration of the polyene backbone is far from solved because the DSC thermogram is affected by the pendant groups or even by the dynamic behavior of the polymer, [56] rendering the technique useless in many cases.…”

Raman Optical Activity (ROA) as a New Tool to Elucidate the Helical Structure of Poly(phenylacetylene)s

“…Another restriction of AFM is that it can only provide structural information when 2D crystals or self‐assembled monolayers are available due to the need for high‐resolution images . Vibrational circular dichroism (VCD) has also been used to obtain information about the orientation of the external helix in PPAs . Here, the solvent in which the experiments are performed interacts with the pendant groups of the PPA, forming another supramolecular helix with the same orientation as the external part of the PPA helix.…”