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.