Dependence between luminescence properties of Cu(i) complexes and electronic/structural parameters derived from steric effects

Abstract

A series of heteroleptic Cu(I) complexes of type [Cu(N^N)(POP)]⁺, with POP: bis-[2-(diphenylphosphino)-phenyl]ether and different phenanthroline-based N^N ligands were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). The complexes comprise N^N ligands with different bulky substituents at the 2,9 positions, and the addition of phenyl substituents at the 4,7 positions is also investigated. Furthermore, in the complex with the bulkier N^N ligand, the effect of unsaturated hydrocarbon chains added and the presence of dimethylamine electron donor groups were analyzed. The composition/energies of the frontier orbitals and the emission energies were determined for all complexes. Besides, we evaluated the steric effects induced by different N^N ligands in electronic (Pauli repulsion energy) and structural (root-mean-square deviation) terms to determine their influence on the photoluminescence efficiency comparing the properties of the proposed Cu(I) complexes with four reported complexes. The analyzed data allow us to find simple relationships to predict the radiative (k_r) and non-radiative (k_nr) constants based on the steric effects that each complex undergoes according to the type of N^N ligand used, obtaining the best balance (higher k_r and lower k_nr) in complexes with bulkier substituents. Therefore, these complexes could be proposed as suitable luminescent materials for optical applications.