Excited-State Properties of the Ligand-Localized ³ππ* State of Cyclometalated Ruthenium(II) Complexes

Abstract

We report the results of an investigation on the absorption spectra, phosphorescence spectra, phosphorescence lifetimes, and magnetic properties of [Ru(bhq)(CO) 2 Cl(L)], where bhq - is the C-deprotonated forms of benzo[ h ]quinoline (bhqH) and L is either PEt 3, p -toluidine, or piperidine. The lowest singlet states of the Ru(II) complexes are metal-to-ligand charge-transfer 1 dπ* states. Vibrational structures of the phosphorescence spectra observed in the crystalline states at 4.2 K are similar to the structures of the phosphorescence spectra and the magnitude of the free bhqH ligand. Zero-field splittings indicate that the lowest triplet states of all the Ru(II) complexes are mainly characterized as ligand-localized 3 ππ* states. However, the phosphorescence lifetimes are significantly shorter for Ru(II) complexes as compared with free bhqH. This result suggests that the lowest triplet state of the Ru(II) complex includes 3 dπ* character due to configurational mixing with the bhq-localized 3 ππ* state. By intersystem crossing from the singlet excited state, the in-plane y sublevel is the most populated for bhqH, whereas the out-of-plane x sublevel is the most populated for the Ru(II) complexes. This dramatic change of the intersystem crossing route is satisfactorily interpreted within the framework of the theory of the intersystem crossing by considering the difference of the character of the lowest singlet state.