Phenylene–2,5-dimethylpyrazine co-oligomers: synthesis by Suzuki couplings, X-ray structures of neutral and diprotonated teraryl species and efficient blue emission

Phenylene–2,5-dimethylpyrazinyl co-oligomers and a dipyridylpyrazine derivative have been synthesised by Suzuki cross-coupling reactions starting from 2,5-dibromo-3,6-dimethylpyrazine. X-Ray crystal structures are reported for two teraryl derivatives, viz. 2,5-bis(2-methoxyphenyl)-3,6-dimethylpyrazine 2 and 2,5-bis(6-methoxypyridin-3-yl)-3,6-dimethylpyrazine 6, and a diprotonated pyrazinyl dication salt, viz. 2,5-bis(2-methoxyphenyl)-3,6-dimethylpyrazinium bis(tetrafluoroborate) salt 3. Compounds 2 and 6 and the dication in 3 have crystallographic Ci symmetry and adopt twisted conformations: dihedral angles between the aryl and pyrazine rings are 74.0° (2), 56.4° (3) and 44.6° (6). Violet-blue photoluminescence is seen for 2λmax 372 nm, 5λmax 418 nm and 6λmax 387 nm in ethanol solution. [Compound 5 is 1,4-dimethoxy-2,5-bis{2-(5-tert-butylphenyl-3,6-dimethylpyrazinyl)benzene}]. Blue electroluminescence, λmax 444 nm, is observed for the device structure ITO/PEDOT/5/Ca with no long-wavelength emission from π-aggregates or exciton states.