Phosphorescent iridium complexes based on 2-phenylimidazo[1,2-a]pyridine ligands: tuning of emission color toward the blue region and application to polymer light-emitting devices

A series of new blue-phosphorescent iridium(III) complexes 1-14 with ligands of 2-phenylimidazo1,2-a]pyridine (pip) derivatives were successfully prepared, and their electrochemical, photophysical, and electroluminescent (EL) properties were systematically investigated. It was found that the emission maxima are significantly dependent on the substituents on the phenyl ring in the range of 489-550 nm. For instance, electron-withdrawing groups such as F and CF3 shift the emission maxima to shorter wavelengths by lowering the HOMO levels (complexes 4-8), whereas the extended pi-conjugation leads to bathochromic shifts (2, 3). To obtain further information about the frontier orbital, substitution effects on the imidazole part were also investigated here, and it was found that electron-withdrawing or -donating substituents on the imidazole ring affected the emission maxima (9, 557 nm; 10, 525 nm). These results including their oxidation potentials suggest that the HOMO of the pip-based complex is a mixture of Ir-d, phenyl-pi, and imidazole-pi orbitals. From this viewpoint, combination of electron-withdrawing substituents on the phenyl ring with the use of another ancillary ligand enabled further blue shifts (13, 468, 499 nm; 14, 464, 494 nm). This new system based on pip is one of the rare examples of iridium complexes whose emissions can be tuned to the blue region. Preliminary polymer light-emitting devices (PLEDs) employing the Ir complexes were fabricated, and the devices showed moderate EL efficiencies.