Site Specific Electropolymerization To Form Transition-Metal-Containing, Electroactive Polythiophenes Jerry L. Reddinger and John R. Reynolds* Center for Macromolecular Science and Engineering, Department of Chemistry, University of Florida, Gainesville, Florida 32611 Received August 19, 1997. Revised Manuscript Received February 6, 1998 We report a series of novel bis(salicylidene) metal complexes that undergo oxidative electropolymerization to afford polymers comprised of three different backbone constitutions and a variety of film colors. The ligand architecture, being either N,N′-bis(salicylidene)- 3,4-diaminothiophene (SALOTH) or N,N′-bis(salicylidene)-3′,4′-diamino-2,2′:5′,2′′-terthiophene (BTh-SALOTH), is based upon a thiophene or 2,2′:5′,2′′-terthiophene core with the imine functionalities attached at the 3-, 4-, or 3′-,4′-positions, respectively. This monomer design possesses two switchable polymerization sites that yield phenylene-linked backbones with unblocked salicylidene rings or polythiophenes with the use of methyl blocking groups. As expected with metal centers in direct electronic communication with the polymer chains, the metal type has a dramatic influence over electrochromic properties, with colors unique to each metal being achieved. For example, the Ni-containing films exhibit an orange-to- green redox transition while the Cu-containing analogues display a light green/dark green pair. Details of monomer synthesis as well as full electrochemical details for the monomers and polymers are presented. Introduction The field of conducting and electroactive polymers continues to expand with the advent of new systems suitable for an ever-growing myriad of applications that include corrosion protection, electrochromics, chemical sensors, and batteries, along with ESD and EMI- shielding materials. 1 More specifically, in the past few years, there has been an increase in synthetic efforts aimed at obtaining metal-containing, conducting poly- mers, and these materials suggest an array of potential uses (e.g., electrocatalysis, electrochromic displays, and molecular recognition). There are many examples of π-conjugated polymers in the literature that possess pendant metal centers, attached to the backbone by insulating tethers. 2 For the most part metal/polymer interactions in these systems are inherently weak. Thus, to maximize the interaction of metal centers with the polymer’s extended π-system, the ideal structure would have the metal centers directly affixed to, and in direct electronic communication with, the polymer backbone. As yet, there are still only a few reported examples of conducting and electroactive polymers where metal centers are in conjugation with the polymer’s π-system. 3-7 All of these systems possess metal centers coordinated to bidentate, nitrogen-containing, heterocyclic units (2,2′-bithiazole or 2,2′-bipyridyl) incorporated into the polymer backbone. While the number of examples are few, these preliminary efforts have already shown the broad flexibility that metal coordination can impart to traditional organic systems. Peng and Yu have utilized the conjugated backbone of ruthenium-containing poly- mers to enhance the photosensitivity of the parent organic polymer. 5 Such an effect is important for the future development of photoconductive materials. Zhu and Swager have used metal/polymer complexation as a means to polyrotaxane formation via a Sauvage-type 8 template effect. 6b Wang and Wasielewski have shown the sensitivity of a pseudo-poly(phenylenevinylene) system to the complexation of various metal ions. 7 Their work utilized conformational changes of the polymer that are associated with the coordination of the metal ions, affording a system that can toggle between its conjugated and nonconjugated forms. Our group has reported on a number of low-oxidation- potential thiophene-based monomers that when polym- erized form electroactive materials possessing novel electronic properties. 9 We have recently reported on our initial efforts to tune the optical and electronic proper- ties of polymers via the incorporation of metal centers. 10 (1) For an up-to-date overview, see the Proceeding of ICSM 1996, Synth. Met. 1997, 84-87, and Handbook of Conducting Polymers, 2nd ed., T. A. Skotheim, R. L. Elsenbaumer, and J. R. 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