Highly resolved emission of osmium-deuterated bipyridine compound [Os(bpy-h8)n(bpy-d8)3-n]2+ (n = 0-3): evidence for electronic delocalization

Highly resolved emission spectra of the title compounds doped into single-crystal [Zn(bpy-h₈)₃](ClO₄)₂ have been recorded at 1.3 K. The partially deuterated dopants (n = 1, 2) occupy different sites in the host matrix. It is possible to excite site selectively the dominant site (designated B) for both compounds. Properties of the lowest excited electronic states of ³MLCT character of these sites of the partially deuterated dopants (n = 1, 2) are compared to the properties of the fully protonated (n = 3) and the fully deuterated (n = 0) species. The electronic origins lie at 14223 cm⁻¹ (n = 3), 14238 cm⁻¹ (site B, n = 2), 14233 cm⁻¹ (site B, n = l), and 14256 cm⁻¹ (n = 0). Within the experimental error of ±1 cm⁻¹ they are found at exactly the same energies in emission and excitation. The emission spectra of the four compounds exhibit rich vibrational satellite structures, being connected with these origins. A comparison of the satellite structures reveals that in the vibrational energy range of the metal-ligand modes the metal couples the different ligands, while in the energy range of the ligand modes (above ca. 500 cm⁻¹) the modes can be attributed to the individual ligands. This leads to the interesting result that the partially deuterated complexes exhibit (to the same electronic origin) vibrational lines, which occur also in the fully protonated and the fully deuterated compounds. For a given compound, the emission lifetimes τ are exactly the same, when measured at the energy of the origin and at any vibrational line, respectively. The decay is strictly monoexponential for every compound. The lifetimes increase upon deuteration [τ values: n = 3, 22 μs; n = 2 (site B), 26 μs; n = 1 (site B), 31 μs; n = 0, 46 μs]. These results lead to the conclusion - contrary to currently accepted models - that the excited electronic state is delocalized over all three ligands.