New π-Conjugated Polymers Derived from a Benzimidazole Unit. Preparation, Solvatochromism, and Oxidation of Poly(aryleneethynlene)s Composed of 2-(3,5-Di-tert-butyl-4-hydroxyphenyl)benzimidazole Bearing a Hindered Phenolic Substituent

New π-conjugated polymers constituted of a benzimidazole unit have been prepared. Palladium-catalyzed polycondensation between 4,7-dibromobenzimidazole bearing a hindered phenolic substituent (3,5-di- tert -butyl-4-hydroxyphenyl) and four kinds of diethynylaromatic compounds HC⋮C Ar C⋮CH (e.g., Ar = p -phenylene ( p -Ph), m -phenylene ( m -Ph), and pyridine-2,5-diyl (2,5-Py)) gives the corresponding poly(aryleneethynylene) (PAE) type polymers in high yields. The polymers with the p -Ph, m -Ph, and 2,5-Py groups have molecular weights of (0.4−3.3) × 10 4, as determined by GPC. The position of the π−π* absorption band as well as photoluminescence intensity of the polymers strongly depends on the kind of solvent. In general, the π−π* absorption peak of the PAE-type polymers with the p -Ph groups shifts to a shorter wavelength in acidic organic solvents, whereas that of the PAE-type polymers with the 2,5-Py unit shifts to a longer wavelength in the acidic solvent. Intensity of the photoluminescence increases in the acidic media. The polymers as well as a model compound are oxidized with PbO 2 . The oxidized products give rise to an IR peak at about 1680 cm -1, which is assigned to ν(C O) originated from resonance structures of the oxidized species, and their ESR signals appear at about g = 2.0 with a peak-to-peak line width of 8−11 G.