Multistimuli-Responsive Molecular Photoswitches Based on Bimetallic Ru(II) and Os(II) Terpyridine Complexes

Two symmetrical luminescent bimetallic complexes of Ru(II) and Os(II) having the composition, [(tpy-PhCH₂PPh₃Br)₂M₂(tpvpvpt)]⁴⁺, where tpvpvpt = 2-(4-(4-(4-(4-(2,6-di (pyridin-2-yl)pyridin-4-yl)styryl)styryl)phenyl)-6-(pyridin-2-yl)pyridin-2-yl)pyridine and tpy-PhCH₂PPh₃Br = [4'-(p-triphenylphosphoniummethylphenyl)-2,2':6',2″-terpyridine] bromide are designed and thoroughly characterized in the present study. These complexes incorporate a bridging ligand (tpvpvpt) consisting of two consecutive stilbene units along with a capping ligand featuring active methylene and phosphonium groups (tpy-PhCH₂PPh₃Br). The complexes exhibit intense absorption across the entire UV-vis spectral region and display emission in the visible to NIR range. The -CH₂ protons within the molecular frameworks exhibit enhanced acidity due to the combined electronic effects of the adjacent PPh₃⁺ moiety and metal coordination, rendering the complexes responsive to anionic species such as F^-, OAc^-, as well as OH^-. Upon irradiation with visible light, the complexes undergo trans-trans to trans-cis photoisomerization, while the reverse trans-cis to trans-trans conversion is facilitated by UV light. Furthermore, in the presence of chemical oxidant (ceric ammonium nitrate), reductant (metallic sodium), or anionic species (F^-), as well as light of a specific wavelength, a significant enhancement in both the rate and quantum yield of photoisomerization is observed. This multistimuli-responsive behavior enables efficient and tunable photoswitching, combining broad spectral adaptability with enhanced photoisomerization efficiency, making these complexes promising candidates for advanced photochemical applications.