Strategic design and synthesis of osmium(II) complexes bearing a single pyridyl azolate π-chromophore: Achieving high-efficiency blue phosphorescence by localized excitation

We present the strategic design and synthesis of Os(II) complexes bearing a single pyridyl azolate π-chromophore with an aim to attain high efficiency blue phosphorescence by way of localized transition. It turns out that our proposal of localized excitation seems to work well upon anchoring a single π-chromophore on the Os(II) complexes such that the control of MLCT versus ππ* (or even LLCT) transitions is more straightforward. Among the titled complexes, [Os(CO)₃(tfa)(fppz)] (1) and [Os(CO) ₃(tfa)(fbtz)] (5) (tfa = trifluoroacetate, (fppz)H = 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole, and (fbtz)H = 3-(trifluoromethyl)-5- (4-tert-butyl-2-pyridyl)-1,2,4-triazole) give the anticipated blue phosphorescence with efficiencies of 0.26 (λ_max = 460 nm) and 0.27 (λ_max = 450 nm), respectively. For their halide analogues [Os(CO)₃(X)(fppz)] (2, X = Cl; 3, X = Br; 4, X = I) and phosphine-substituted isomeric derivatives [Os(tfa)(fppz)(PPh ₂Me)₂(CO)] (6-8), the localization of the excitation energy seems to populate at certain vibrational modes with weak bonding strength and hence an associated shallow potential energy surface to induce a facile radiationless transition. Furthermore, their ancillary ligands play an important role in fine-tuning not only the energy gap but also the emission intensity, i.e., in manifesting the radiationless transition pathways. Our results clearly show that there is always a tradeoff upon varying the parameters in an aim to optimize the hue and efficiency of phosphorescence toward blue.