Phosphorescent cationic iridium complexes with phenyl-imidazole type cyclometalating ligands: A combined experimental and theoretical study on photophysical, electrochemical and electroluminescent properties

• Cationic iridium complexes with phenyl-imidazole type CΛN ligands were developed.
• The CΛN ligands destabilize both the HOMO and LUMO levels of the complexes.
• Orange-red LEC using the complex gives a maximum current efficiency of 14.3 cd A−1.

Phosphorescent cationic iridium complexes with phenyl-imidazole type cyclometalating ligands have been synthesized for the first time and their photophysical, electrochemical properties have been comprehensively investigated. By changing the ancillary ligands, the complexes give orange-red or green-blue light. Compared to 2-phenylpyridine (ppy), the phenyl-imidazole ligands destabilize simultaneously the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals of the complexes. Their emitting triplet states show dominant charge-transfer (iridium/cyclometalating ligands→ancillary ligands) character. The complexes have been used to fabricate solid-state light-emitting electrochemical cells (LECs). The orange-red LEC gives a high peak current efficiency of 14.3 cd A−1, which is among the highest reported for orange-red LECs; the green-blue LEC gives a peak current efficiency of 6.3 cd A−1. It is shown that the phenyl-imidazole cyclometalating ligands hold promise for the invention of iridium-based cationic phosphorescent dyes with tunable energy levels and emission properties.

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