Synthesis and Properties of a Novel Cross-Linked Electroactive Polymer Formed from a Bipolar Starburst Monomer

This Article describes the synthesis and physical properties of two bipolar starburst monomers: 1,3,5-tris{5-(7-(carbazol-9-yl)-(9,9'-spirobifluoren-2-yl)-1,3,4-oxadiazol-2-yl}benzene (OXD-CBZ) and 1,3,5-tris[7-diphenyl-(9,9'-spirobifluorene)-1,3,4-oxadiazoyl]benzene (OXD-DPA) featuring an electron-deficient tris(1,3,4-oxadiazole)phenylene ring as an interior core bridged by rigid spirobifluorene units to terminal electroactive carbazole (CBZ) and diphenylamino (DPA) groups. The electronic absorption spectra of OXD-CBZ and OXD-DPA depend only slightly on the solvent polarity, revealing that weak electronic coupling existed between the donor (CBZ and diphenylamino groups) and acceptor (1,3,4-oxadiazole) moieties in the ground state. In contrast, the emission spectra of these starburst bipolar molecules were strongly dependent on the solvent polarity, a phenomenon that we attribute to the polarized excited states generated through intramolecular charge transfer. The starburst monomer OXD-CBZ exhibited a redox gradient, where the peripheral CBZ moieties exhibited lower oxidation potentials than those of the fluorene units in the interior, allowing us to obtain a new electropolymerized macromolecule. The polymer films derived from OXD-CBZ exhibited good conductivity, reversible electrochemical processes, and stable color changes (from transparent to green to light blue) with high coloration efficiency upon electro-oxidation. The radical cations of OXD-CBZ were localized within the CBZ rings; the lack of substituents at the C3 and C6 positions of the CBZ unit led to the effective electrochemical cross-linking process. In contrast, electro-oxidation of OXD-DPA, which was end-capped with DPA groups, produced stable radical cations that did not undergo associated electrochemical reactions. The higher stability of the OXD-DPA radical cations through resonance stabilization with the fluorene ring impeded the progress of typical triphenylamine dimerization reactions.