A theoretical study of phosphorescent Cu(I) complexes with 2-(2'quinolyl)imidazole and POP mixed ligands

Abstract

We herein report a theoretical study using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods to investigate Cu(I) complexes with 2-(2′-pyridyl/quinolyl)imidazole and bis[2-(diphenylphosphino)phenyl]ether mixed ligands. Based on the experimental data for complexes 1 and 2, we first benchmarked different functionals with different HF% and found B3PW91 to be the optimal functional for this system. The computational results indicate that complex 1, with a pyridyl unit, has a much larger radiative decay rate (kr) than complex 2, which has a quinolyl unit. This difference is presumably due to higher HOMO electronic distribution in the dx2-y2 orbital, which leads to a markedly shortened CuN2 bond, enhancing the metal-ligand interaction. However, a much smaller experimental value was found for the non-radiative decay rate (knr) in complex 2, rendering 1 a slightly weaker emitter than 2. We conclude that the difference is due to more effective suppression of deformation when the quinolyl unit is used instead of pyridyl. We sought to increase the photoluminescence quantum yield (PLQY) through modifying the ligand on complex 2, with the goal being to keep the small knr value while simultaneously increasing kr. The computational results indicate that our designed complexes 2a-2c, which possess modified ligands with electron-donating or withdrawing alkyl substituents on N3, increased the distributions of dx2-y2 and decreased that of the dyz compared to 2. Their coordinating abilities were therefore enhanced, with the kr values being 1.34, 22.70, and 0.16 times that of 2 for 2a, 2b and 2c, respectively. Higher PLQYs were achieved in 2a and 2b with the addition of electron-donating alkyl substituents on the ligands, which yielded complexes with significantly shortened CuN2 bonds and enhanced metal-ligand interaction. This investigation on the microscopic mechanism of the photoluminescent properties of these complexes can provide useful knowledge for experimentalists.