Naphthobipyrrole: Versatile Synthesis and Electropolymerization

A facile synthetic route to a new polycyclic pyrrole derivative, 3,8-diethyl-1,10-dihydro-benzo[e]pyrrolo-[3,2-g]indole (1), is reported. This annulated bipyrrole acts as a monomer for electropolymerization and forms an electrochromic conducting polymer (poly1) when electrooxidized at low potentials (0.4 V vs Ag/Ag+) in acetonitrile. The presence of alkyl substituents at the 3 and 8 carbons (beta-pyrrolic positions) induces regioselective 2,5'-coupling of the pyrrole repeat units and gives rise to the more uniform polymeric product, poly1. Poly1 exhibits globular morphology, as judged from SEM pictures. Its spectroelectrochemical features can be attributed to the formation of four possible states: neutral, polaron, bipolaron, and transverse bipolaron. The relatively low switching potentials (-0.6 to +0.9 V vs Ag/Ag+ in MeCN) displayed by poly1 leads us to suggest that 1 has a role to play as a polymerizable moiety for the development of multicolor electrochromic materials.