Remarkable Increase in the Rate of Trans–Cis Photoisomerization of Os(II)-Terpyridine Complexes via Oxidation and Reduction

Abstract

Luminescent homoleptic Os(II)-terpyridine complexes comprising stilbene-appended naphthalene, anthracene, and pyrene motifs are designed in this work, and their photophysical, electrochemical, and photoisomerization behaviors are extensively investigated. All complexes exhibit intense spin-allowed singlet metal-to-ligand charge transfer (¹MLCT) bands in the visible (496–500 nm) and weaker spin-forbidden singlet-to-triplet ³MLCT transitions in the 600–700 nm range. They display moderate emission at room temperature with lifetimes in the range of 84.5–112.5 ns. Electrochemical studies reveal a reversible Os²⁺/Os³⁺ oxidation couple within 0.93–0.96 V, alongside multiple reversible or quasi-reversible reduction peaks associated with terpyridine units in between −1.10 and −1.85 V. The stilbene motifs facilitate reversible trans–cis photoisomerization under alternative treatment with visible and UV light, enabling the complexes to function as photomolecular switches in the near-infrared domain. Interestingly, a remarkable increase in the rate of photoisomerization has been achieved via oxidation as well as reduction of the complexes, which, in turn, induces multistep switching involving reversible oxidation–reduction and trans–cis isomerization. Computational investigations are also conducted on all three conformations {trans–trans (t–t), trans–cis (t–c), and cis–cis (c–c)} of the complexes to gain insight into their electronic structures and for accurate assignment of their absorption and emission spectral bands.