SYNTHESIS OF HIGHLY FUNCTIONALIZED PYRIDINES FOR PLANAR POLYMERS - MAXIMIZED PI-CONJUGATION IN ELECTRON-DEFICIENT MACROMOLECULES

Synthetic routes to planar polypyridines are described. Two pyridine m onomers for the step growth polymerization are prepared starting from 2,5-lutidine via 2,5-dibromopyridine-3,6-dicarboxylic acid as the comm on 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 c ores are disclosed while preparing the monomers for the condensation p olymerizations. A bis(Curtius) rearrangement followed by tert-butyl al cohol capture of the bis(isocyanate) effects the high-yielding convers ion of carbonyl moieties to the tert-butoxycarbonyl-protected aryldiam ine. Lithium-halogen exchange on the protected diaminopyridine followe d by stannylation affords the required dimetalated diamine monomer. Tr eatment of the pyridine(dibromodiacid chloride) with mild cuprates or organocopper reagents affords the pyridine(dibromodiketones). Pd/Cu-ca talyzed couplings of dibromopyridines with distannylpyridines are util ized 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 s o that the newly formed polypyridines are soluble, unlike many other a romatic 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 b andwidths 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, dem onstrating the efficacy of planarization for band gap shortening.