Are molecular 5,8-π-extended quinoxaline derivatives good chromophores for photoluminescence applications?

The synthesis of a new series of photoluminescent compounds, namely 5,8-diaryl quinoxaline derivatives (aryl = phenyl, 4-fluorophenyl, 4-methoxyphenyl, and 4-cyanophenyl), was achieved by a direct Suzuki cross-coupling reaction with the employment of a NCP-pincer palladacycle. The electrochemical and photophysical properties of these compounds were also investigated. Four new 4,8-diaryl-2,1,3-benzothiadiazoles were also synthesized in order to enable a comparison between the two types of nitrogen-containing pi-extended heterocycles. The substitution of a hydrogen atom at the 4-position of the aryl that is groups attached to the quinoxaline or benzothiadiazole base by either electron-donating or -withdrawing groups results in an increase in the bandgap energy (from 2.21 to 2.52 eV) of pi-extended 5,8-quinoxaline derivatives and a decrease in the bandgap energy (from 2.65 to 2.40 eV) of pi-extended 2,1,3-benzothiadiazoles. Moreover, pi-extension at the 5- and 8-positions of the quinoxaline core is not essential for the photoluminescence of these compounds and 4,7-pi-extended 2,1,3-benzothiadiazole derivatives are far better candidates for luminescence applications than are the quinoxaline derivatives. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).