Photophysical properties of iridium complexes containing diamine ancillary ligands in the presence of phenylpyridine as cyclometalated ligand

A set of iridium complexes of [Ir(ppy)2(diamino-containing ligand)]PF6, where diamino-containing ligand = 1,2-diaminobenzene (1), 4-nitro-1,2-diaminobenzene (2), 4-methoxy-1,2-diaminobenzene (3), N,N’-dimethyl-1,2-diaminobenzene (4), N-methyl-1,2-diaminobenzene (5), and N-phenyl-1,2-diaminobenzene (6) were synthesised and characterised by high-resolution mass spectrometry, 1H, 13C, 31P, and 19F NMRs spectroscopy. Their electrochemical and photophysical properties were determined using cyclic UV-vis, voltammetry (CV), fluorometer and density functional theory (DFT). In this study, three different approaches were used. First, the sigma donor pi-acceptor bipyridine ligand is replaced by the sigma donors only of diamine ligands, leading to a high lowest unoccupied molecular orbital (LUMO) energy. The second approach is using the hydrophobic substituent effect like methyl at the amine groups. Thirdly, studying the pi-stacking effect via appending phenyl at the amine group. All the complexes show hypsochromic shift of emission compared to the archetypal iridium complex [Ir(ppy)2bpy]PF6 with different emission nature, resulting in different photophysical properties. Interestingly, complex 2 shows a mixed transition of metal-to-ligand charge transfer (MLCT) and ligand to ligand charge transfer (LLCT) like the archetypal iridium complex, indicating of a strong metal ligand interaction. This can be observed with the broad featureless bands in the U.V spectra centred at 466, and 531 nm. In addition to the two emission bands centred at 517, and 422nm.