Designing Molecular Bistability in Ruthenium Dimethyl Sulfoxide Complexes

Compounds of the type [Ru(tpy)(L2)(dmso)]z+ (tpy is 2,2‘:6‘,2‘ ‘-terpyridine; L2 can be 2,2‘-bipyridine (bpy), N,N,N‘,N‘-tetramethylethylenediamine (tmen), 2-pyridine carboxylate (pic), acetylacetonate (acac), malonate (mal), or oxalate
(ox)) have been studied by X-ray crystallography, electrochemistry, NMR, IR, and UV−vis spectroscopy. When L2
is bpy, tmen, or pic, the dmso ligand can be intramolecularly isomerized either electrochemically or photochemically.
Isomerization is not observed when L2 is acac, mal, or ox. Isomerization results in a drastic change in the absorption
spectrum, as well as in the voltammetry. Absorption maxima shift by 3470 (419−490 nm), 4775 (421−527 nm),
and 4440 cm-1 (429−530 nm) for the bpy, pic, and tmen complexes, respectively. Reduction potentials for S-bonded
and O-bonded complexes differ by 0.57, 0.75, and 0.62 V for the bpy, pic, and tmen complexes, respectively.
Quantum yields of isomerization (φS→O) were determined for the bpy (0.024 ± 1), pic (0.25 ± 1), and tmen (0.007
± 1) complexes. In comparison of these data to photosubstitution quantum yields, it appears that the isomerization
mechanism does not involve the ligand field states. This result is surprising given the importance of these states
in the photochemistry of ruthenium and osmium polypyridine complexes. These results and details of the mechanism
are discussed.