Theoretical study on the neutral and ionic Cu(I) phosphorescent complexes with 2-(2′-quinolyl)benzimidazole and phosphine mixed ligand

• MLCT hardly detection in neutral form due to strong ILCT π(NˆN)→π*(NˆN).
• The charge transfer and equilibrium property are improved in neutral form.
• Ether linkage together with stronger conjugation NˆN leads to higher PLQY.

The electronic structures and photophysical properties of a series of the neutral and ionic Cu(I) complexes with 2-(2′-quinolyl)benzimidazole and phosphine mixed ligand were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. To investigate the differences between the neutral and ionic complexes, the highest occupied molecular orbital (HOMO) energy, the lowest virtual molecular orbital (LUMO) energy, ΔEH-L (the energy gap between HOMO and LUMO), the ionization potentials (IPs), electron affinities (EAs) and reorganization energies (λ) were computed. To explain the reason of the metal-to-ligand charge transfer (MLCT) hardly detection in the neutral complexes, the absorption and emission spectra were studied in detail. To evaluate the photoluminescence quantum yield (PLQY), the non-radiative and radiative decay rate constants (knr and kr) were also presented. As a result, these calculations reveal that the structural distortion between the ground and excited states plays an important role in governing the PLQY. The charge transfer and the transport equilibrium property were markedly improved due to the lack of mobile counterions in the neutral form. Introducing the ether linkage together with the enhanced NˆN π-conjugation in 2b had obviously faster kr but slower knr, which led to its higher PLQY relative to the introducing the ether linkage only or lacking of a proton and the counterions in complexes.

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