Near-Infrared Fluorescent 2,3-Dicyanopyrazines

Novel non-ionic near-infrared (NIR) fluorescent 2,3-dicyanopyrazines were prepared. 5-[6-(9-julolidyl)-1,3,5-hexatrienyl]- and 5-[8-(9-julolidyl)-1,3,5,7-octatetraenyl]-2,3-dicyanopyrazines showed fluorescence maxima (Fmax) at 716 and 751 nm with fluorescence quantum yields (Φf) 0.12 and 0.03 in toluene, respectively. MO calculations showed that these compounds have an intramolecular charge-transfer chromophoric system from the julolidyl to dicyanopyrazine moieties. The calculations also showed that since the HOMO energy level was unstabilized, and at the same time, the LUMO energy level was stabilized by expanding the conjugated system at 5-position, NIR fluorescent derivatives were obtained. They showed clear positive solvatochromism in the fluorescence spectroscopy. The fluorescence intensity drastically decreased in polar solvents.