Ionic iridium complexes with conjugated phenyl substituent: Synthesis and DFT calculation on the electrochemical and electrochemiluminescent properties

• Two novel ionic iridium complexes with high electrochemiluminescent efficiency were synthesized and characterized.
• The photophysical, electrochemical and electrochemiluminescent properties of the ionic iridium complexes were investigated.
• The DFT calculations were carried out to elucidate the difference between ionic and neutral iridium complexes.

Two ionic iridium complexes, (bpq)2Ir(bpy)+PF6− (1) and (bpq-OCH3)2Ir(bpy)+PF6− (2), where bpq is 6-methyl-2,4-diphenyl quinolone anion, bpy is 2,2′-bipyridine, were synthesized and structurally characterized. Their photophysical, electrochemical and electrochemiluminescence properties were investigated. With the conjugated phenyl substitution, the π–π ligand center transitions in the emission process were enhanced, resulting in a broad and strong photoluminescence emission. The photoluminescence quantum yields were measured to be 0.214, 0.209, respectively for complex 1 and 2. The complex 1 also exhibited greater electrochemiluminescence efficiency than the complex 2. To further elucidate the influence of different structures on the electrochemical and electrochemiluminescence properties, the density functional theory calculations were performed on the complexes 1, 2 along with other four well documented bis-cyclometalated iridium complexes. The calculation revealed that about 90% electronic density in the lowest unoccupied molecular orbital were distributed in N∧N auxiliary ligand for ionic complexes, whereas most electronic density were distributed in C∧N main ligand for neutral complexes.

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