Identification of the specific, shutter-like conformational reorientation in a chiroptical switching polycarbodiimide by VCD spectroscopy

Abstract

The specific conformational states responsible for the unique, reversible temperature- and solvent-driven chiroptical switching process experienced by poly(N-1-naphthyl-N'-octadecyl-carbodiimide) (PNOC) have been identified using VCD spectroscopy and DFT calculations. The distinct VCD spectra of PNOC corresponding to the two specific conformations were obtained for the polymer dissolved in DCM-d2 (state A) and CDCl3 (predominantly state B). To specifically assign the structures of both conformations, two simplified 7mer models were constructed and optimized using DFT calculations. The theoretical spectra associated with these model conformations show a high level of agreement when compared to the experimental VCD spectra. The two states consist of the naphthyl pendant groups aligned directionally opposing the helical rotation (model A) and aligned with the helicity of the backbone (model B). This pendant reorientation causes very large OR and ECD Cotton effect inversions upon modification to the temperature or solvent composition of dilute (+)-PNOC solutions in specific solvents. In addition, the pendant group equilibrium from state A to B causes a contraction of the helical pitch from the more expanded 5/1 pitch to the more contracted 7/2 pitch resulting in increased disorder of the solvation sphere surrounding the polymer chain.

Developments in synthesis, characterization, and self‐assembly of polycarbodiimide systems

“…The final backbone displayed an angle change on the N 4 dihedral angles of −70° for model A and −103° for model B, indicating a helical pitch change from 5/1 to a contracted 7/2. Both the theoretical and experimental VCD and IR spectra match very closely correlating the orientation of the naphthyl pendant groups in model A and model B to the conformations of State A (DCM) and State B (CHCl 3 ), respectively …”

“…The theoretical VCD spectra for models A′ and B′ reflected models A and B, which were not optimized to the helical pitch change. This concludes that the changes in SOR and ECD values are dependent on the orientation of the sterically bulky pendant groups on the imine nitrogen, while changes in the VCD were due to the change in the helical pitch of the backbone …”

“…To propose specific macromolecular confirmations associated with States A and B, experimental vibrational circular dichroism (VCD) analysis and theoretical DFT calculations were conducted on Poly‐13 (Fig. ) . A Poly‐13 solution in DCM, containing a backbone in the State A conformation, was found to have an imine stretch at 1614 cm −1 with a strong ± bisignate VCD peak and a positive VCD peak at 1572 cm −1 associated with the naphthyl C=C aryl breathing mode.…”

DFT investigation on dihydrogen-bonded amine-borane complexes

“…Vibrational circular dichroism (VCD) spectra As a powerful method to probe the binding topology of DHB systems, VCD spectra have been utilized both for the determination of absolute configurations and for structural studies of a wide range of systems ranging from small molecules to polymers [40][41][42][43]. The VCD spectra of the monomer and dimer complexes are shown in Fig.…”

Coil‐helix and sheet‐helix block copolymers via macroinitiation from telechelic ROMP polymers

“…Polymeric scaffolds comprising architecturally well‐defined building blocks such as helices and sheets are interesting building blocks for bcp fabrication not only owing to their structural relevance to biomolecules, but also because of their intrinsic properties. Bcps containing helical features, for instance, offer potential in applications such as chiroptical switching and liquid crystals . Bcps can be engineered via both noncovalent and covalent strategies, both of which are modular and can afford polymeric scaffolds comprising multiple different polymer blocks within a single macromolecular species.…”