A New, More Versatile, Optical Switching Helical Polycarbodiimide Capable of Thermally Tuning Polarizations ±359°

Abstract

An in depth study of newly synthesized polycarbodiimides containing asymmetric aryl pendant groups has been performed. The focus of this study was to create new polymers capable of the low-energy, reversible, solvo-, and thermocontrollable conformational switching phenomenon previously discovered. Of the polymers synthesized, one polymer, N-(1-naphthyl)-N′-(n-octadecyl) polycarbodiimide, Poly-5, has been found to undergo reversible optical switching in various solvents, each of which alter the temperature at which the switching is observed. The switching is not due to helical inversions, but instead due to synchronous rearrangement of polyarene pendant groups around the helical backbone. This is further shown by anisotropic changes of the aromatic chemical shifts using variable temperature 1H NMR through the observed thermal switching range in THF-d8. The net change in specific optical rotation reaches a remarkable 1700° and changes in the direction of rotation. The polymer system is tunable and capable of holding the polarization output constant at constant temperature anywhere within the switching profile. Polarimetry and electronic circular dichroism (ECD) studies were performed to observe these conformational changes. Additionally reported are new polycarbodiimides that probe the effects of size and geometry of various arene pendant groups on the switching behavior of these polymer systems.