Synthesis of highly functionalized pyridines for planar polymers.: Maximized π-conjugation in electron deficient macromolecules

Synthetic routes to planar polypyridines are described. Two pyridine monomers for the step growth polymerization are prepared starting from 2,5-lutidine via 2,5-dibromopyridine-3,6-dicarboxylic acid as the common intermediate. The dicarboxylic acid serves as the key intermediate for the preparation of both the A and B components for the step-growth polymerization. Several novel transformations on sensitive pyridine cores are disclosed while preparing the monomers for the condensation polymerizations. A bis(Curtius) rearrangement followed by tert-butyl alcohol capture of the bis(isocyanate) effects the high-yielding conversion of carbonyl moieties to the tert-butoxycarbonyl-protected aryldiamine. Lithium-halogen exchange on the protected diaminopyridine followed by stannylation affords the required dimetalated diamine monomer. Treatment of the pyridine(dibromodiacid chloride) with mild cuprates or organocopper reagents affords the pyridine(dibromodiketones). Pd/Cu-catalyzed couplings of dibromopyridines with distannylpyridines are utilized for the polymerization schemes. This approach permits the ladder linkages of the planar polymers to (i) form in high yields upon proton activation once the polymer backbone is intact, (ii) be substituted so that the newly formed polypyridines are soluble, unlike many other aromatic ladder polymers, and (iii) contain double-bonded ladder units to keep the consecutive aryl moieties planar which maximizes extended pi-conjugation through the polymer backbones, thereby increasing the bandwidths and lowering the optical band gaps. The planar polypyridines here have optical band gaps of 1.5-2.0 eV which represent 1.3-1.8 eV smaller gaps than nonplanarized polypyridines in similar solvents, demonstrating the efficacy of planarization for band gap shortening.