Strongly Blue Luminescent Cationic Iridium(III) Complexes with an Electron-Rich Ancillary Ligand: Evaluation of Their Optoelectronic and Electrochemiluminescence Properties

Two strongly blue luminescent cationic heteroleptic iridium complexes 1b and 2b bearing a 4,4-bis(dimethylamino)-2,2-bipyridine (dmabpy) ancillary ligand and either 1-benzyl-4-(2,4-difluorophenyl)-1H-1,2,3-triazole (dFphtl) or 2-(2,4-difluorophenyl)-5-methylpyridine (dFMeppyH), respectively, have been synthesized and fully characterized. In comparison with other analogues, the interplay of the triazole unit with the dmabpy unit and methylation of the pyridine ring are discussed with respect to the photophysical, electrochemical, and electrochemiluminescent (ECL) properties of the complexes. The two complexes, 1b and 2b, are blue emitters with (max) = 495 and 494 nm, respectively. The nature of the excited states was established by various photophysical and photochemical experiments as well as DFT calculations. Both complexes emit from a ligand-centered state, however, the emission of 1b possesses significant charge-transfer character, which is absent in 2b. The presence of the methyl group on the cyclometalating ligand leads only to a modest increase in the radiative rate constant, k(r), but otherwise does not appreciably influence the optoelectronic properties of the complex compared with the non-methylated analogue. In contrast, the efficacy of the ECL emission when scanning to 2.50 V is strongly influenced by the presence of the methyl group. ECL emission is also enhanced in complexes bearing dmabpy ancillary ligands compared with those containing dtBubpy ligands. The two complexes exhibit similar electrochemical behavior. Incorporation of the dmabpy ligand shifts both the oxidation and reduction cathodically. The combination of the dmabpy and dFphtl groups increases the redox potential difference and thus the HOMO-LUMO gap but the emission is not further blueshifted. Thus, the structural modification of the cyclometalating ligand, although only modestly tuning the emission energy, modulates the nature of the excited state and the efficiency of the ECL process.