Theoretical studies on the photoisomerization mechanism of osmium(ii) sulfoxide complexes

Abstract

Photochromic compounds, employing photonic energy for excited-state bond rupture and bond construction processes, are excellent candidates for application as optical molecular information storage devices. In this work, a dispersion corrected density functional theory (DFT-D) study was carried out on the photoisomerization mechanism of the osmium sulfoxide complex, [Os(bpy)₂(DMSO)₂]²⁺ (bpy = 2,2′-bipyridine; DMSO = dimethyl sulfoxide), which features a pronounced photochromic character based on an ultrafast photo-triggered linkage isomerization located at the SO-ligand. Calculated results demonstrate that the Os–S → Os–O isomerization proceeds adiabatically on the potential energy surface (PES) of the lowest metal-to-ligand charge transfer excited states ³MLCT_S → ³MLCT_O, and the metal centered ligand field (³LF_S) excited state is not involved. However, attributed to the weakened Os–DMSO bond-strength upon Os–S to Os–O isomerization, the population of the ³LF_O excited state is thermally accessible. Therefore, photodissociation of DMSO from the Os center occurs via homolytic cleavage of the Os–O bond after an avoided curve crossing between the lowest ³MLCT_O state and the ³LF_O repulsive state along the stretching coordinate of the Os–O bond.