Abstract
We investigate the emission property and relaxation mechanism of excited states of Iridium biscarbene complexes with varied ancillary N^N ligands; they are blue complexes bis(1-(4-fluorophenyl)-3-methylimidazolin-2-ylidene-C,C2’)(2-(pyrazol -5-yl)pyridine)iridium(III), Ir(fpmi)2(pypz) B1, bis(1-(4-fluorophenyl) -3-methylimidazolin-2-ylidene-C,C2’)(4-methyl-2-(pyrazol-5-yl)pyridine)iridium(III), Ir(fpmi)2(mpypz) B2, bis(1-(4-fluorophenyl)-3-methylimidazolin-2-ylidene-C,C2’) (3,5-dimethyl-2-(pyrazol-5-yl) pyridine)iridium(III), Ir(fpmi)2(dmpypz) B3, and green complex bis(1-(1-(4-tolyl)-3-methyl-imidazolin2-ylidene-C,C2’)(2-(pyridine-2-yl) benzo[d]imidazole), Ir(mpmi)2(pybi) G. At 266-nm excitation the ancillary + 1MLCT (fpmi) states are accessed. According to the results of nano-microsecond measurements and quantum yields at long wavelengths, we obtained the radiative rate constants kr of these complexes to be 0.5-1.5105 s-1. According to the values of kr, long lifetimes, and the experimental data we then assigned the emitting states to be triplet interligand 3(ancillary)* mixed extensively with triplet metal-to-ligand charge transfer (3MLCT). From picosecond lifetime measurements, we observe that B3 and G have a rise time constant ~34 ps, ~20 ps, faster than the B1 and B2 ~90 ps, ~76 ps, respectively in the emission curves. These fast rises are also confirmed with the results of transient absorption measurements. The fast rise corresponds to rapid conversion from singlet manifold centered at biscarbene ligand to the triplet states centered at the ancillary ligand. More rapid conversion rates in B3 and G may partially explain excellence external quantum efficiency that can avoid quenching and annihilation processes. We employed the quantum chemical calculations-density functional theory (DFT) to obtain their electronic structures and to aid in interpreting the experimental data.