Room temperature blue phosphorescence: a combined experimental and theoretical study on the bis-tridentate Ir(iii) metal complexes

A series of new bis-tridentate Ir(iii) complexes (1/1b, 2/2b and 3/3b) incorporating both bis(imidazolylidene)benzene and dianionic functional pyrazolyl (or phenyl) pyridine chelates have been synthesized, among which complexes 2 and 2b exhibit intense and structural sky-blue emission in both solution and solid states. In stark contrast, 1/1b is non-emissive in solution, while 3/3b reveals highly red-shifted emission with a featureless spectral profile. This variation in photophysics is associated with the interchange of metal-chelate bonding in the selected tridentate chelate, which affects both the crystal field stabilization energy and the ππ* transition character of the resulting Ir(iii) metal complexes. In 1/1b, the stabilized metal-centered (MC) dd excited states induce a dominant radiationless channel that accounts for the lack of emission in solution. The appreciable ligand-to-ligand charge transfer (LLCT) in 3/3b rationalizes its broad and featureless emission, which is different from the dominant intraligand ππ* transition in 2/2b. The combination of experimental and theoretical approaches thus provides fundamental insight into the influence of chelates as well as the metal-chelate interaction, which is beneficial for the future design of efficient and robust Ir(iii) phosphors for OLED applications.