Theoretical studies on the redox-stimulated isomerization in electrochromic osmium sulfoxide complexes

Redox-stimulated intramolecular isomerization of the DMSO ligand in [Os(bpy)2(DMSO)2](2+) (bpy = 2,2'-bipyridine; DMSO = dimethyl sulfoxide) was explored theoretically for better understanding the electrochromic properties of osmium sulfoxide complexes. It is found that the HOMO-LUMO gap is decreased because the electron transfer amount from DMSO1 ligand to Os center using Os-S1 linkage is larger than that using Os-O1 linkage, which makes the absorption of such electrochromic Os(II) sulfoxide complexes red-shifted. Moreover, it is observed that Os-O linkage is preferred by the "hard" Os(III) metal and the "soft" Os(I) metal prefers Os-S linkage, compared with Os(II). Intrinsic reaction pathway calculation results demonstrate that Os-S1 → Os-O1 isomerization is favored by Os(II) oxidation, while Os-O1 → Os-S1 isomerization is much easier to be triggered by reduction of Os(III) or Os(II). In addition, DMSO2 linkage isomerization becomes much harder to proceed attributed to the increased bond-strength between DMSO2 and Os center upon Os(II)-O1 → Os(II)-S1 rearrangement, which makes only one DMSO ligand isomerized observed experimentally.