Photophysical and Cellular Imaging Studies of Brightly Luminescent Osmium(II) Pyridyltriazole Complexes
Abstract
The series of complexes [Os(bpy)3- n(pytz) n][PF6]2 (bpy = 2,2'-bipyridyl, pytz = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole, 1 n = 0, 2 n = 1, 3 n = 2, 4 n = 3) were prepared and characterized and are rare examples of luminescent 1,2,3-triazole-based osmium(II) complexes. For 3 we present an attractive and particularly mild preparative route via an osmium(II) η6-arene precursor circumventing the harsh conditions that are usually required. Because of the high spin-orbit coupling constant associated with the Os(II) center the absorption spectra of the complexes all display absorption bands of appreciable intensity in the range of 500-700 nm corresponding to spin-forbidden ground-state-to-3MLCT transitions (MLCT = metal-to-ligand charge transfer), which occur at significantly lower energies than the corresponding spin-allowed 1MLCT transitions. The homoleptic complex 4 is a bright emitter (λmaxem = 614 nm) with a relatively high quantum yield of emission of ∼40% in deoxygenated acetonitrile solutions at room temperature. Water-soluble chloride salts of 1-4 were also prepared, all of which remain emissive in aerated aqueous solutions at room temperature. The complexes were investigated for their potential as phosphorescent cellular imaging agents, whereby efficient excitation into the 3MLCT absorption bands at the red side of the visible range circumvents autofluorescence from biological specimens, which do not absorb in this region of the spectrum. Confocal microscopy reveals 4 to…
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