Synthesis and applications of poly N-heterocyclic carbenes and investigation of aldimine coupling
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The design, synthesis, characterization and application of carbene-based metallopolymers are described herein. Metallopolymers have found wide applications in the fields of photovoltaics, energy storage and electrochromic windows. The incorporation of N-heterocyclic carbene (NHC) functionalities into a polymerizable scaffold would allow for many different metals to be attached in a facile and high-yielding manner. Such complexes could be functionalized onto surfaces and utilized as either spectroscopic or antimicrobial devices. Early attempts in our lab focused on utilizing bis(thiophene) diimines (instead of NHCs) as scaffolds for metal chelation and polymerization. This approach was unsuccessful due to the lability of the diimine moiety under electrochemical cycling and the thiophene moieties were not able to undergo polymerization. In order to more fully understand the key transformation in synthesizing the thiophene-substituted diimines, a comprehensive investigation of the aldimine coupling transformation was undertaken. A high concentration of substrate and catalyst was determined to be the most important factor in obtaining high yields of the dimerized products. Green solvents such as acetonitrile and hexanes could be used for the dimerization reaction when the cyanide counteranion was changed from sodium to tetrabutylammonium. The steric limitations were systematically identified and a series of possible substrates have been ruled out as viable candidates for dimerization. Applying the experience gleaned from earlier reports, the first example of an NHC polymer was prepared in which the monomer features an NHC functional group orthogonally connected to its main chain. A polymerizable imidazolylidene-AuCl complex containing pendant bithiophene moieties was prepared by a high yielding, multistep procedure. Oxidative electropolymerization of this monomer afforded the desired polymer (Au[NHC]Cl)n, which was characterized on the basis of electrochemical studies as well as by X-ray crystallography, photoelectron and UV-vis spectroscopy. The methodology described above was expanded to develop a series of analogous poly(N-heterocyclic carbene) complexes with appended entities (M = Ir, Au, Ag, or S)and found to be electrochromic. Most of the polymers exhibit an intense absorbance wave at 700 nm under oxidative conditions which is attributable to the formation of polaron excitations along the polymer main chain. The presence of a transition metal significantly increased the electrochromic character of the polycarbene system. The iridium-containing polymer was found to possess significant near-infrared (NIR) absorbance at 1100 nm in which the metal moiety effectively functions as an electron sink. Electrochemical analysis of the polymer thin films revealed that they exhibit highly reversible electrochromic activities.