Dual Room-Temperature Fluorescent and Phosphorescent Emission in 8-Quinolinolate Osmium(II) Carbonyl Complexes: Rationalization and Generalization of Intersystem Crossing Dynamics

A new series of quinolinolate osmium carbonyl complexes were synthesized and characterized by spectroscopic
methods. Single-crystal X-ray diffraction studies indicate that these complexes consist of an octahedral ligand
arrangement with one chelating quinolinolate, one tfa or halide ligand, and three mutually orthogonal terminal CO
ligands. Variation of the substituents on quinolinolate ligands imposes obvious electronic or structural effects, while
changing the tfa ligand to an electron-donating iodide slightly increases the charge density on the central osmium
atom. These Os(II) complexes show salient dual emissions consisting of fluorescence and phosphorescence, the
spectral properties and relaxation dynamics of which have been studied comprehensively. The results, in combination
with the theoretical approaches, lead us to propose that the emission mainly originates from the quinolinolate ππ*
state. Both experimental and theoretical approaches generalize various types of intersystem crossing versus those
of the tris(quinolinolate) iridium Ir(Q)3, and their relative efficiencies were accessed on the basis of the associated
frontier orbital configurations. Our results suggest that 〈1dππ*|Hso|3ππ*〉 (or 〈3dππ*|Hso|1ππ*〉) in combination with
a smaller ΔES1-T1 gap (i.e., increasing the MLCT (dππ*) character) is the main driving force to induce the ultrafast
S1 → T1 intersystem crossing in the third-row transition metal complexes, giving the strong phosphorescent emission.