Luminescent phosphine ruthenium(II) complexes with 8-hydroxyquinoline derivative ligands

Ruthenium(II) complexes with formulas [RuH(CO)(PPh₃)₂(L)] (where L=5-chloro-8-hydroxy-2-methylquinoline-7-carboxylic acid (L₁), 8-hydroxy-2,5-dimethylquinoline-7-carboxylic acid (L₂), 2-methyl-8-quinolinol (L₃) and 8-hydroxy-2-methyl-5,7-dinitroquinoline (L₄)) were synthesized from precursor complex [RuHCl(CO)(PPh₃)₃] and characterized by IR, NMR, UV–vis spectroscopy and X-ray crystallography. The emission properties of the complexes were studied in solutions and solid state at ambient temperature. The quantum yields of fluorescence, the lifetimes and the nature of excited states were described on the basis of molecular properties. To elucidate the structural, spectroscopic and bonding properties of the obtained complexes, calculations at the DFT level were undertaken. The complexes display green to yellow–orange emission depending mostly on the impact of substituents of the quinoline moiety. The emission spectra of the ligands were prepared in order to discuss fluorescence resonance energy transfer (FRET) between the metal center and the ligands. Excitation spectra of the complexes display a good agreement with the emission bands of the ligands. Thus, the ligand centered (LC) excited state, which interacts with metal to ligand charge transfer (MLCT) excited state plays a role in emission of complexes. Comparing to the ligands, decreasing of decay times and quantum yields in the received complexes are visible, which is connected with efficient non-radiative decay from the ³MLCT states in the complexes. - Highlights: • Quinolinato ruthenium(II) complexes display green to yellow-orange emission. • Wavelengths of emission depend mostly on substituents of quinoline moiety. • The intersystem crossing between ¹LC and ³LC excited states has place. • The emission coming from ³LC excited states interfering with ³MLCT excited states.