Influence of bulky substituents on the photophysical properties of homoleptic iridium(III) complexes

A new series of homoleptic cyclometalated iridium(III) complexes based on a phenylpyridine (ppy) ligand containing bulky substituents have been synthesized and characterized. The phosphorescence behavior of the Ir complexes is investigated by steady-state and time-resolved emission spectroscopic techniques. Comparison of the results with those of the reference Ir(ppy)(3) reveals that the emission color and photophysical properties of other Ir complexes are influenced by the electron-donating groups (-CH3 and phenyl derivatives) attached to the ppy ligand. In particular, systematic red-shifts are observed by increasing the electron-donating ability. The emission spectrum of Ir(Me-ppy)(3), having a small electron-donating -CH3 group, is red-shifted; however, the emission quantum yield is low and the nonradiative decay constant is large. On the other hand, although bulky phenyl derivative-adducts (Ir(Ph-ppy)(3), Ir(MePh-ppy)(3), and Ir(diMePh-ppy)(3)) also exhibit red-shifted emission, their kinetic and photophysical behaviors are more optimal than those of Ir(Me-ppy)(3), whose behavior does not follow the energy gap law. This deviation may be attributed to the orthogonal structure associated with the steric hindrance of bulky substituents. The molecular structure, molecular orbitals in singlet/triplet manifolds, and energy band gap are verified by density functional theory calculations.