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 pi-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 pi-chromophore on the Os(II) complexes such that the control of MLCT versus pipi* (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 (lambdamax=460 nm) and 0.27 (lambdamax=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)(PPh2Me)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.