Tunable Redox and Optical Properties Using Transition Metal-Complexed Polythiophenes Jerry L. Reddinger and John R. Reynolds* Center for Macromolecular Science and Engineering, Department of Chemistry, University of Florida, Gainesville, Florida 32611 Received November 15, 1996 We report the synthesis and electropolymerization of the first salicyclidene-based transition metal-complexed thiophene, in which the metal is in direct electronic communication with the conjugated π-backbone, yield- ing a highly electroactive and electrochromic material. In addition, this system possesses multiple polymeri- zation sites that can be selectively activated to give polymers composed of different backbones. While there are several examples of pyrrole and thiophene-based heterocycles containing transition metal complexes at- tached via pendant side chains, these systems display little electronic interaction between the metal and conjugated chain. 1 Metal-containing polymers, in which the metal is coordinated to the polymer chain, are interesting from the standpoint that the conjugated backbone can help stabilize redox activity, affording a continuum of accessible states. Directly complexed pyridyl and bis(thiazolyl) polymers have been studied; however, metal complexation was possible only after polymerization of the metal-free monomer. 2 With this methodology difficulty can arise in assessing the degree to which coordination occurs, and it would be of great convenience if such polymers could be obtained from prepurified, complexed monomers. Peng and Yu have reported a poly(p-phenylenevinylene)-based polymer containing ionic ruthenium centers bound to bipyridyl (BPY) units incorporated into the polymer backbone. 3 Chemical polymerization was effected by utilizing a metal-complexed monomer, thus affording a material for which the metal content was known. In this instance polymerization was not possible with uncom- plexed monomer. Furthermore, while this is a very attractive system, the choice of metal for this system is limited to those which will form stable complexes with the BPY monomer and does not bind the palladium catalyst. During the preparation of this manuscript, Zhu and Swager reported on Ru-BPY 3 complexes having terminal thiophenes attached as electropolymer- izable sites. 4 These considerations propelled us toward synthesizing directly-complexed, metal-containing, elec- tropolymerizable molecules where the metal type will have a dramatic influence on the polymer’s resultant properties. We have specifically designed these poly- mers for their metal-dependent electrochromic proper- ties. Our group has long been interested in the use of low- oxidation potential thiophene-based monomers which, when polymerized, form electroactive materials pos- sessing novel electronic properties. 5 Such polymers have proved useful as components in dual-polymer electrochromic devices constructed in our laboratories, 6 and introduction of metal centers into these systems could afford a highly compatible material possessing the ability to be tuned over a wide color range. (Lifetimes of polymer-based electrochromic devices are controlled by this chemical compatibility.) Using this background for the incorporation of transition metals into such polymers, we elected to append the tetradentate coor- dinating bis(salicylidene)-type ligand onto a thiophene core. The selection of Schiff base ligand systems was founded upon their ease of synthesis, their ability to complex many transition metals indiscriminantly, and their robust nature, which inhibits decomplexation under a broad set of conditions. 7 Utilizing thiophene as the keystone of this macrocyclic ligand, the 3- and 4-positions of the ring were used as assembly points for imine functionalities, leaving the more reactive 2- and 5-sites open for electrochemical polymerization or fur- ther functionalization. These materials present syn- thetic flexibility not possible with other systems. The ligand SALOTH (1) was synthesized, as outlined in Scheme 1, via the condensation of salicylaldehyde with 3,4-thiophenediamine, 8 which was obtained by reduction of 2,5-dibromo-3,4-dinitrothiophene. 9 Metal complexation was achieved by addition of the ligand dissolved in a hot ethanolic solution to nickel or copper acetate in hot ethanol, and subsequent cooling afforded microcrystals of the target molecules 2 and 3 in good yield. 10 As the uncomplexed ligand is somewhat un- stable in solution, we opted to prepare subsequent complexes by the more direct route 7c of first combining the salicylaldehyde species and metal acetate and then condensing this complex with the corresponding di- amine, as illustrated in Scheme 2. The electrochemical properties of Cu-SALOTH (3) were investigated to determine its propensity toward electropolymerization. Repeated potential cycling between -0.5 V and +0.9 V versus Ag/Ag + (all further potentials will be referenced versus this reference electrode) of monomer 3 in 0.1 M TBAClO 4 /CH 2 Cl 2 showed both monomer oxidation and electropolymerization to form an electroactive film, as shown in eq 1. As illustrated by the current response from the repeated scans in Figure 1, a clean monomer oxidation is observed with a sharp onset at 0.72 V and a peak potential for monomer oxidation (E p,m ) at 0.89 V. The first return scan showed a substantial reduction with a peak at 0.55 V. Repeated scanning to +0.9 V showed the rapid growth of the polymer redox at 0.6-0.7 V and 0.4-0.55 V for the oxidation and reduction, respectively. Scheme 1 Scheme 2 673 Macromolecules 1997, 30, 673-675 S0024-9297(96)01689-0 CCC: $14.00 © 1997 American Chemical Society