Synthesis and properties of highly fluorescent indolizino[3,4,5-ab] isoindoles

We report here the synthesis, X-ray structures, optical and electrochemical properties, fabrication of light-emitting devices, and density functional calculations for indolizino[3,4,5-ab]isoindole (INI) derivatives. Strongly luminescent heterocycles based on the INI unit were synthesized by 1,3-dipolar cycloaddition reactions between pyrido[2,1-a]isoindole (PIS) and acetylene or ethylene derivatives. They are indolizino-[3,4,5-ab]isoindoles 2-9 and 14-15, benzo[1′,2′-1,2]indolizino[3,4,5-ab]isoindoles 10, pyridazino[4′,5′:1,2]-indolizino[3,4,5-ab]isoindoles 12-13, and 2,3-hydropyridazino[4′,5′:1,2]indolizino[3,4,5-ab]isoindole-1, 4-dione 11. The relative luminescence quantum yield can be as high as 90%. Their reduction and oxidation potentials and high luminescence can make these heterocycles possible alternatives to tris(8-hydroxyquinolinato)aluminum (Alq₃). The brightness of the light-emitting device reached as high as 10⁴ cd/m² and indolizino[3,4,5-ab]isoindole 3 emits beautifully blue light. The X-ray crystal structures of INI derivatives were obtained for the first time. The geometries obtained from X-ray data and density functional theory calculations shed more light on an interesting formally antiaromatic 16π system, which is divided into 10π and 6π aromatic systems. We also report a relatively easy protonation of INI, which occurs at a carbon, rather than nitrogen atom.