Polycarbodiimides are a relatively novel class of macromolecules with a repeating amidine structure containing two modifiable nitrogens that bear the functionality of the polymer. These polymers adopt a stable, helical structure in both solution and the solid state which can be biased using chiral monomers/initiators to form optically active materials with an excess single-handed screw sense. Specifically attaching bulky aromatic units (i.e., naphthyl, anthryl, pyrenyl, etc.) to the C=N imine nitrogen of the polymer backbone results in an interesting, unique thermo- and solvo-driven chiroptical switching phenomenon where the specific optical rotations (OR) were shown to be modulated reversibly up to 1300�. These chiroptical changes were determined to be caused by the rotation about the N-aryl bond but the specifics of the process are still widely unknown. Poly(N-1-naphthyl-N-octadecylcarbodiimide) displayed the most intense changes in specific OR so this polymer has been the focus of several new studies to determine the specific mechanism of action for this process. The 15N NMR of the isotopically enriched polymer revealed that the process is consequence of varying populations of two distinct states. Using VCD spectroscopy and DFT calculations, the specific molecular structure of each state was identified and confirmed. In addition to the small molecular alterations, these two states result in an altered supramolecular self-assembly which was studied and identified by several microscopy techniques such as optical polarized microscope, SEM, and AFM.

Helical molecular switches: Identification of the specific molecular and supramolecular rearrangement in a thermo- and solvo-driven chiroptical switching polycarbodiimide

247th National Meeting (2014)

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