Synthesis, Structure, Photophysical Properties, and Redox Behavior of Cyclometalated Complexes of Iridium(III) with Functionalized 2,2‘-Bipyridines

Abstract

The new functionalized polypyridine ligands 4‘-(4-chlorophenyl)-6‘-phenyl-2,2‘-bipyridine (clpbpy), 4‘-(4-tolyl)-6‘-phenyl-2,2‘-bipyridine (tpbpy), and 4‘-(4-carboxyphenyl)-6‘-phenyl-2,2‘-bipyridine (cpbpy), together with the known 4‘-(4-hydroxyphenyl)-6‘-phenyl-2,2‘-bipyridine (hpbpy) and 4‘-(4-tolyl)-2,2‘:6‘,2‘ ‘-terpyridine (ttpy) have been used to prepare a new series of Ir(III) cyclometalated compounds [Ir(ppy) 2 (HL-X)][PF 6 ] (ppy is the monoanion of 2-phenylpyridine; HL-X = hpbpy ( 1 ), clpbpy ( 2 ), tpbpy ( 3 ), cpbpy ( 4 ), and ttpy ( 5 )). All the new species have been characterized by IR and 1 H NMR, and the crystal structure of 4 is also presented and discussed. All the metal complexes exhibit oxidation mainly centered on an orbital derived from an Ir−(C - ) σ-bond and ligand-centered reduction processes; all of them are luminescent from 3 MLCT levels both at 77 K in a rigid matrix and at 298 K in fluid solution. The redox and absorption properties are more or less insensitive to the remote substituents on the rotationally free 4‘-phenyl ring, whereas fine-tuning of the luminescence properties is observed on changing substituents. The results show that the “energy gap law” for radiationless decay in the weak coupling limit is obeyed by this series of complexes, when complex 5 is excluded. Interestingly, the slope of the linear relationship between ln k nr and the emission energy at room temperature is significantly smoother than that reported for other luminescent polypyridine complexes containing different metals. Because of the high luminescence quantum yield and the presence of functionalities in the polypyridine ligand framework, the complexes reported may be considered as useful building blocks for light- and redox-active, multicomponent supramolecular systems.