Synthesis of azafluoranthenes by iridium-catalyzed [2 + 2 + 2] cycloaddition and evaluation of their fluorescence properties
Takahiro Sawano, Kaho Takamura, Tomoka Yoshikawa, Kayo Murata, Marina Koga, Risa Yamada, Takahide Saito, Kazumasa Tabata, Yugo Ishii, Wataru Kashihara, Tatsuya Nishihara, Kazuhito Tanabe, Tadashi Suzuki, Ryo Takeuchi
Abstract
We report a method for the synthesis of azafluoranthenes under neutral reaction conditions in a highly atom-economical manner by the iridium-catalyzed [2 + 2 + 2] cycloaddition of 1,8-dialkynylnaphthalenes with nitriles. A variety of nitriles react with methyl- or phenyl-substituted 1,8-dialkynylnaphthalenes to give a wide range of azafluoranthenes. Azafluoranthenes bearing an amino group show intense fluorescence at around 500 nm. Comparison of the fluorescence properties of amine-substituted azafluoranthenes with related compounds revealed the importance of the amine moiety for obtaining a high fluorescence quantum yield. The choice of the solvent affected the emission maxima and the fluorescence quantum yield. Azafluoranthenes bearing pyrrolidine exhibited blue-shifted emission bands in a non-polar solvent and gave a fluorescence quantum yield of 0.76 in toluene. A Lippert-Mataga plot and computational studies provide insight into the origin of the fluorescence of azafluoranthenes. Furthermore, cellular experiments using human breast adenocarcinoma cells SK-BR-3 demonstrated the feasibility of using azafluoranthenes as fluorescent probes.