Heteroleptic platinum(ii) complexes of 8-quinolinolates bearing electron withdrawing groups in 5-position

Abstract

A series of novel luminescent platinum(II) complexes bearing orthometalated 2-phenylpyridine ligands (C⁁N), namely 2-phenylpyridine (4) and 3-hexyloxy-2-phenylpyridine (5), and several 5-substituted quinolinolate ligands (5-X-Q), where X = NO₂ (a), X = CHO (b), X = Cl (bearing another Cl in 7-position of the Q-ligand) (c) and X = H (d) have been synthesized, characterized and their photophysical properties were studied. All complexes were obtained as a single isomer with N atoms of the C⁁N and Q ligands trans-coordinated to the platinum center as evidenced using single-crystal X-ray crystallography and NMR spectroscopy. Absorbance, luminescence as well as lifetime measurements in solution and in the solid state have been performed to establish a qualitative relationship between structure and luminescence properties. The compounds under investigation absorb intensively via an intraligand charge transfer (ILCT) in the visible range (460–480 nm) and emit from fluid solution and in the solid state at room temperature at 600–630 nm. The complexes show quantum yields up to 25% and lifetimes in the range of 20–30 μs in deoxygenated organic solvents at room temperature. The emitting state can be best described as a triplet intraligand charge-transfer state localized mainly on the quinolinolate ligand. In these complexes the phenylpyridine ligand can be essentially regarded as an ancillary ligand. Density functional theory (DFT) calculations were carried out on both the ground (singlet) and excited (triplet) states of these complexes and revealed the influence of the substitution of the quinolinolate ligand on the HOMO/LUMO energies and the oscillator strengths. Substitution on 3-position of the phenylpyridine ligand does not impact on the transition energies, and is thus suited to introduce other functional moieties, such as a solubilizing hexyloxy group.