Chiroptical Absorption and Luminescence Spectra of a Dissymmetric Osmium(II)-Polypyridyl Complex Containing an Optically Active Bis(bipyridine)-Type Ligand of Well-Defined Structural Chirality

Chiroptical absorption and luminescence spectra are reported for a novel Os(II)-polypyridyl complex in which Os(II) is coordinated to one achiral bidentate ligand and a chiral quadridentate ligand. The achiral ligand is 4,4&-dimethyl-2,2′-bipyridine (DMbpy), and the chiral ligand is a bisbipyridine-type molecule formed by linking two optically active (-)-[4,5]-pineno-2,2′-bipyridine molecules together via a (CH2)6 chain bridge. This chiral ligand is one of a family of ligands often referred to as chiragens, and here it is denoted by CG[6], where [6] identifies the number of carbon atoms in the chain bridge. The coordination core of the complex has a threebladed propeller-like structure formed by three Os(II)-bipyridyl chelate rings. This tris(bipyridyl)Os(II) core structure has approximate trigonal-dihedral (D3) symmetry, and the propeller-like assembly of its three chelate rings has a right-handed screw sense (or helicity). The overall complex, denoted by Δ-[Os(CG[6])(DMbpy)]2+, has C2 point-group symmetry, with a 2-fold symmetry axis that bisects both the CG[6] and DMbpy ligands and coincides with one of the digonal (2) axes of the tris(bipyridyl)Os(II) core structure. The overall structural chirality of the complex is dictated by the inherent chirality of the CG[6] ligand, which in turn is dictated by the absolute configuration about each of the six stereogenic carbon atoms in the CG[6] ligand structure. The chiroptical absorption and luminescence measurements performed in this study were carried out on solution samples of Δ-[Os(CG[6])(DMbpy)](PF6)2 dissolved in acetonitrile. These measurements yielded circular dichroism and absorption dissymmetry factor data over the 12500-30000 cm-1 spectral range, and circularly polarized luminescence and emission dissymmetry factor data over the 11 100-16 000 cm-1 spectral region. Comparisons between the absorption and emission dissymmetry factor data indicate chirality-related structural differences between the ground and emitting states of Δ-[Os(CG[6])(DMbpy)]2+. Both the unpolarized and circularly polarized absorption spectra of Δ-[Os(CG[6])(DMbpy)]2+ are qualitatively similar to the corresponding spectra reported for Δ-[Os(bpy)3]2+ (in acetonitrile) throughout the 12500-30300 cm-1 spectral region. However, there are significant quantitative differences between the spectra observed for these two complexes, and these differences are most apparent in the absorption dissymmetry factor data. This indicates that ligand structure outside the tris(bipyridyl)Os(II) coordination core of Δ-[Os(CG[6])(DMbpy)]2+ exerts a significant influence on the chiroptical absorption properties of this complex over the 12500-30300 cm spectral region, although all the optical absorption processes that occur within this spectral region may be assigned to one-electron d-π*-type metalto-ligand charge-transfer (MLCT) transitions localized within the tris(bipyridyl)Os(II) chelate structure. It appears that the noncoordinated chiral moieties of the CG[6] ligand exert a direct perturbative influence on both the dipole strengths and rotatory strengths of the MLCT transitions in Δ-[Os(CG[6])(DMbpy)]2+.