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 (λmax = 460 nm) and 0.27 (λmax = 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.