Labilizing the Photoinert: Extraordinarily Facile Photochemical Ligand Ejection in an [Os(N^N)₃]²⁺ Complex

Abstract

are unambiguously observed and characterised by NMR spectroscopy and mass spectrometry. Kinetically inert d6 complexes of second and third row transition metal elements (e.g. Re(I), Ru(II), Os(II), Ir(III)) have attracted enormous interest due to their attractive photophysical properties that make them potentially amenable to application in lightemitting technologies, [1] dye-sensitised photovoltaics, [2] phosphorescent biological imaging microscopy [3] and solar catalysis.[4] The utility in phosphorescent and electron/energy transfer applications stems from the relatively long-lived triplet metal-to-ligand charge-transfer ( 3 MLCT) states that complexes of these metals exhibit. These can however be deactivated by triplet metal-centred ( 3 MC) states if these are sufficiently close in energy to photoexcited 3 MLCT states [5] and can induce photochemical reactivity such as ligand-loss or isomerisation.[6] For example, photochemical reactivity can be designed into ruthenium(II) complexes; inclusion of steric bulk in the ligand set in order to weaken metal-ligand bonds lowers the 3 MC state making thermal population from 3 MLCT after photoexcitation achievable. This methodology has been exploited in the design of photoinitiated DNA binding complexes with anti-cancer activity. [7] We have recently reported results on the non-sterically promoted photochemical reactivity of the complexes [Ru(bpy)2(btz)] 2+ and [Ru(bpy)(btz)2] 2+ (bpy = 2,2'-bipyridyl).[8]Here, presence of the btz ligand appears to induce a destabilisation of the 3 MLCT state, bringing it into closer energetic proximity to the 3 MC state thereby enabling thermal population of the 3 MC state with consequential photochemical btz ligand ejection in acetonitrile solutions. In the case of the latter of these complexes, this is accompanied by the unprecedented observation of a metastable ligand-loss intermediate, trans-[Ru(bpy)(κ 2 -btz)(κ 1 -btz)(NCCH3)] 2+ , which can be formed quantitatively by photolysis in an NMR tube within minutes and has been crystallographically characterised.[9] The btz ligand has also been observed to induce photochemical decomposition in the iridium(III) complex [Ir(dfptz)2(btz)] + (dfptzH = 4-(2,4-difluorophenyl)triazole) [10] which one would expect to be far more inert due to the typically higher lying 3 MC states associated with the 5d metal centre.These intriguing results led us to explore osmium(II) triazole-based complexes. [Os(N^N)3] 2+ -type complexes are typically highly kinetically inert and require harsh conditions in their synthesis in order to effect ligand exchange. This is due to a large ligand-field splitting yielding high-energy 3 MC states regarded as thermally inaccessible. Consequently these complexes are photochemically stable, avoiding the ligand ejection photochemistry that sometimes afflicts, or can be designed into, Ru(II) analogues. Indeed, for such osmium complexes Meyer noted that "ligand loss photochemistry is either extremely inefficient or nonexistent", [11] a view that has prevailed for s...

Photophysical and Cellular Imaging Studies of Brightly Luminescent Osmium(II) Pyridyltriazole Complexes

“…32,33 More commonly reported are charge-neutral complexes with anionic 1,2,4-triazolate or pyrazolate ligand systems which are frequently combined with phosphine, arsine or carbonyl donors. [34][35][36][37][38] We have recently reported a series of tris-bidentate triazole-based osmium(II) complexes and their application in light-emitting electrochemical cell devices 39,40 and the bis-tridentate complex [Os(btzpy) 2 ] 2+ (btzpy = 2,6-bis(1-phenyl-1,2,3triazol-4-yl)pyridine) which exhibits phosphorescence at 595 nm. This latter complex was shown to be readily taken up by HeLa and U2OS cancer cell lines and displays a high degree of mitochondrial localisation.…”

Ruthenium(II) Complex‐based Highly Specific Luminescence Light‐up Probe for Detecting HOCl via C(sp2)‐H Chlorination

“…Ruthenium(II) polypyridine complexes possess several advantageous properties, such as visible‐light absorption, large Stokes shift, long‐lived excited states, high thermal, photo, and chemical stabilities, good solubility and stability in aqueous media, and low cytotoxicity [64–68] . Although the photophysical/chemical properties of d 6 ‐metal polypyridine complexes containing 1,2,3‐triazole ligand have been expansively studied by several research groups to date, [69–73] sensing and bio‐imaging using these complexes are rarely explored [74–75] . Recently, a few Ru(II) and Ir(III) based probes of 1,2,3‐triazole ligand have been reported by our group for sensing and bio‐imaging through C−H analyte interaction [76–80] .…”