Ultrafast relaxation dynamics of osmium-polypyridine complexes in solution

We present steady-state absorption and emission spectroscopy and femtosecond broadband photoluminescence up-conversion spectroscopy studies of the electronic relaxation of Os(dmbp)₃ (Os1) and Os(bpy) ₂(dpp) (Os2) in ethanol, where dmbp is 4,4′-dimethyl-2, 2′-biypridine, bpy is 2,2′-biypridine, and dpp is 2,3-dipyridyl pyrazine. In both cases, the steady-state phosphorescence is due to the lowest ³MLCT state, whose quantum yield we estimate to be ≤5.0 × 10^-3. For Os1, the steady-state phosphorescence lifetime is 25 ns. In both complexes, the photoluminescence excitation spectra map the absorption spectrum, pointing to an excitation wavelength-independent quantum yield. The ultrafast studies revealed a short-lived (≤100 fs) fluorescence, which stems from the lowest singlet metal-to-ligand-charge-transfer (¹MLCT) state and decays by intersystem crossing to the manifold of ³MLCT states. In addition, Os1 exhibits a 50 ps lived emission from an intermediate triplet state at an energy ∼2000 cm^-1 above that of the long-lived (25 ns) phosphorescence. In Os2, the ¹MLCT-³MLCT intersystem crossing is faster than that in Os1, and no emission from triplet states is observed other than the lowest one. These observations are attributed to a higher density of states or a smaller energy spacing between them compared with Os1. They highlight the importance of the energetics on the rate of intersystem crossing.