Redox-triggered switching of helical chirality of poly(phenylacetylene)s bearing riboflavin pendants

Abstract

Two novel optically active poly(phenylacetylene)s bearing riboflavin (vitamin B₂) residues as the pendant through different covalent linkages (poly-1 and poly-2) were prepared by the polymerization of the corresponding monomers (1 and 2) with a rhodium catalyst, and their chiroptical properties were investigated by UV-visible and circular dichroism (CD) spectroscopies. Riboflavin-linked poly-2 through the acetal linkage with the ribityl group to the phenylacetylene exhibited a relatively intense induced CD (ICD) in the polymer backbone region. The ICD was temperature-dependent, indicating that the main chain of poly-2 adopts a dynamic preferred-handed helical conformation induced by the optically active ribityl group. In contrast, poly-1 having the riboflavin pendant via the N³-methylene linker showed almost no apparent ICD due to the main-chain helical conformation, probably because the optically active ribityl group of poly-1 is located relatively far from the polymer backbone compared with poly-2. The cyclic voltammetric measurements of poly-1 and poly-2 revealed that these polymers possess reversible redox properties originating from the electron transfer process of the riboflavin pendants. The switching of the chiroptical properties in response to the redox stimuli was also investigated using a chemical reductant (Na₂S₂O₄) and oxidant (O₂).