Synthesis and Properties of Novel Highly Fluorescent Pyrrolopyridazine Derivatives

We report here the synthesis, X-ray structures, optical and electrochemical properties, and density functional calculations for new pyrrolopyridazine derivatives. Ten strongly luminescent heterocycles were synthesized by 1,3-dipolar cycloaddition reactions between alkylated pyridazine or cinnoline and naphthoquinone or dimethyl acetylenedicarboxylate ( DMAD ). They are benzo[ f ]pyridazino[6,1- a ]isoindole-5,10-diones ( BPID ) 1 − 4, benzo[ f ]pyridazino[6,1- a ]isoindole ( BPI ) 5, pyrrolo[1,2- b ]cinnolines ( PC ) 6 and 7, and pyrrolo[1,2- b ]pyridazines ( PP ) 8 − 10 . The relative luminescence quantum yield can be as high as 90% and the heterocycles are luminescent in the solid state, which indicates that there is little self-quenching in these systems. Furthermore, we have shown that the optical properties of pyrrolopyridazines can be tuned by substitutions in different ring positions. An unexpected blue shift and a large red shift with a different π conjugation system were explained by aromaticity arguments using DFT calculations and X-ray crystal structures. Electrochemical investigation has shown that the redo potentials of the new heterocycles could be controlled by design. The synthesis of a series of compounds allowed detailed analysis of substituent effects on structures, frontier orbitals, and luminescent and electrochemical properties.