Effect of secondary ligands' size on energy transfer and electroluminescent efficiencies for a series of europium(iii) complexes, a density functional theory study

In this paper, a quantum chemistry method was used to investigate the effect of different sizes of substituted phenanthrolines on absorption, energy transfer, and the electroluminescent performance of a series of Eu(TTA) ₃L (L = [1,10] phenanthroline (Phen), Pyrazino[2,3-f][1,10] phenanthroline (PyPhen), 2-methylprrazino[2,3-f][1,10]phenanthroline(MPP), dipyrido[3,2-a:2′,3′-c]phenazine(DPPz), 11-methyldipyrido[3,2-a: 2′,3′c]phenazine(MDPz), 11.12-dimethyldipyrido[3,2-a:2′, 3′-c]phenazine(DDPz), and benzo[i]dipyrido[3,2-a:2′,3′-c] phenazine (BDPz)) complexes. Absorption spectra calculations show that different sizes of secondary ligands have different effects on transition characters, intensities, and absorption peak positions. The larger secondary ligands DPPz, MDPz, DDPz and BDPz lead to incomplete energy transfer from the triplet states of ligands to the ⁵D₀ of the Eu³⁺ ion compared with smaller ones (PyPhen and MPP) due to their lower S₁ or T ₁ state energy levels than that of TTA or ⁵D₀ of Eu³⁺. "Small polaron" stabilization energy (SPE) results reveal that electron trapping is the dominant electroluminescence (EL) mechanism in these materials due to their lower LUMO energies than 4,4′-N,N′-dicarbazolebiphenyl (CBP). Reorganization energy (λ) values show that these materials have better electron than hole transporting properties. In addition, the reasons for the origin of the 500 nm emission in Eu-PyPhen- and Eu-MPP-based OLED devices were investigated, and we suppose this emission may result from 2,9-dimethyl-4,7-diphenyl-1,10- phenanthroline (BCP), not from Alq₃.