Are Molecular 5,8‐π‐Extended Quinoxaline Derivatives Good Chromophores for Photoluminescence Applications?

Abstract 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 π‐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 π‐extended 5,8‐quinoxaline derivatives and a decrease in the bandgap energy (from 2.65 to 2.40 eV) of π‐extended 2,1,3‐benzothiadiazoles. Moreover, π‐extension at the 5‐ and 8‐positions of the quinoxaline core is not essential for the photoluminescence of these compounds and 4,7‐π‐extended 2,1,3‐benzothiadiazole derivatives are far better candidates for luminescence applications than are the quinoxaline derivatives.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)