Theoretical research on the effect of regulated pi-conjugation on the photophysical properties of Ir(III) complexes

In this work, the effect of regulated host and auxiliary ligand pi-conjugation on the photophysical properties of a series of Ir(III) carbene complexes is examined by using the start-of-the-art theoretical methods. According to our results, all of the lowest-lying and strongest absorption peaks can be assigned as having a mixed ligand-to-ligand/metal-to-ligand charge transfer (LLCT/MLCT) character, but the different ways of introducing phenyl have a great effect on the absorption wavelength variation. In addition, the charge transfer characteristics of lowest-lying emission have some minute differences. In addition, when the extended pi-conjugation is broken, the emission wavelength can be effectively retained due to the similar emission charge transfer related electronic density distribution of occupied molecular orbitals and unoccupied molecular orbitals. However, the larger pi-conjugation can give rise to remarkably blue-shifted emission. This blue-shifted emission can be attributed to the alteration in the transition character due to intense interactions between nearly degenerate unoccupied molecular orbitals. Through the evaluation of the spin-orbit coupling (SOC) effect, we can gain a deeper understanding of the radiative decay rate processes. These results reveal that the larger pi-conjugation can also lead to higher quantum efficiency due to the larger radiative decay and the smaller nonradiative decay rate. Our theoretical studies highlight the role of pi-conjugation of the host and auxiliary ligand, and thus, can pave the way for the design of novel and efficient blue phosphorescent materials.