Spin-orbit coupling, osmium complexes

The emission lifetimes of the bipy and phen complexes of ruthenlum(II) at 77°K are generally in the range t = 0.5-10 ps. (Table 7). Since these values are intermediate to those generally observed for the fluorescence and phosphorescence of organic compounds, the radiative transition in the ruthenium complexes was suggested to be a heavy-atom perturbed spin-forbidden process (168,169). From a determination of the absolute quantum yields as well as lifetimes of a series of ruthenium(II) and osmium(II) complexes, the associated radiative lifetimes were calculated (170). The variations in these inherent lifetimes within the series could be rationalized with a semi-emipirical spin-orbit coupling model thus affording further evidence that the radiative transitions are formally spin forbidden in these systems. 

The emissions from the osmium(II) complexes of bipy, and so on, have in general received assignments analogous to those of ruthenium(Il), that is, to formally spin-forbidden charge transfer (ir d) transitions (79,163,170). A notable difference in the behavior of the two sets of compounds is that the lifetimes of the osmiura(II) emitters are generally shorter than those of ruthenium(II) (Table 7). This difference is due to the greater affect of spin-orbital coupling on the radiative decay rates of the excited state osmium(II) species (170). 

Figure 2. Manifold of d7r emitting levels characteristic of complexes of ruthenium(II) ana osmium-ill). Splitting is calculated in terms of a parameter related to spin-orbit coupling (ki) and exchange integrals K(flj,aJ and K(e+,aJ.

Characteristic electronic spectral and redox couples of the osmium(II) and osmium(III) complexes are contained in Table I. All Os(III) complexes also exhibit transitions in the near-infrared (1500-2100 nm), due to the effects of spin-orbit coupling on the ground electronic state. 

Smec M et al (2008) Effect of spin-orbit coupling on reduction potentials of octahedral ruthenium(II/III) and osmium(ll/Ill) complexes. J Am Chem Soc 130(33) 10947-10954 Car R, Patrinello M (1985) Unified approach for molecular-dynamics and density-functional theory. Phys Rev Lett 55(22) 2471-2474... 

The use of osmium in place of ruthenium within the coordination architectures leads to changes in the properties of the LAs. Osmium possesses higher-energy n orbitals that lead to stabilization of the MLCT states and complexes with red-shifted absorptions and emissions relative to their ruthenium analogs. The higher degree of spin orbit coupling of osmium relative to ruthenium also modulates system properties. The Os(d7i)->bpy(7i ) MLCT transition of [Os(bpy)3] occurs red shifted relative to [Ru(bpy)3] at 640 nm. The Os(dTi)bpy(7i ) MLCT state emits at 746 nm in acetonitrile with x = 60 ns. The considerably shortened excited state lifetime relative to [Ru(bpy)3] is... 

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