A Remarkable Ligand Orientational Effect in Osmium-Atom-Induced Blue Phosphorescence

A new series of Os^II-based carbonyl complexes cis(CO),trans-(N_py,N_py),cis(N_tz,N_tz)-[Os(CO)₂(bptz)₂] (1), cis(CO),cis(N _py,N_py),trans(N_tz,N_tz)-[Os(bptz)₂(CO)₂] (2), and cis(CO),-trans(N_py,N_py),cis(N_tz,N_tz)-[Os(CO)₂-(fptz)₂] (3), where bptz and fptz denote 3-tert-butyl-5-(2-pyridyl)- and 3-trifluoromethyl-5-(2-pyridyl)-1,2,4-triazolate, respectively, have been designed and synthesized in an effort to achieve high efficiency, room-temperature blue phosphorescence. Although 1 and 2 are geometric isomers, remarkably different excited-state relaxation pathways were observed. Complex 1 exhibits strong phosphorescence in CH₃CN (Φ_p∼0.47) and as a single crystal at room temperature, whereas complex 2 is nearly nonemissive under similar conditions. The associated relaxation dynamics have been comprehensively investigated by spectroscopic and relaxation dynamics as well as by theoretical approaches. Our results lead us to the conclusion that for complex 2, the "loose bolt" effect of metal-ligand bonding interactions plays a crucial role in the fast radiationless deactivation of this type of geometrical isomer. Fine adjustment can also be achieved by functionalizing the ligands so that the electron-withdrawing nature of the CF₃ group in 3 stabilizes the HOMO of the triazolate moiety, thus moving the emission further into the pure "blue" region; this results in highly efficient phosphorescence and renders 3 particularly attractive for application in blue OLED devices.