Abstract
A series of new mixed‐ligand copper(I) complexes [Cu(NN)POP]BF~4~, where NN = 1,10‐phenanthroline (phen; 1a), 2,9‐dimethyl‐phen (DMphen; 1b), 4,7‐diphenyl‐phen (DPpehn; 1c) and 2,2'‐bipyridine (bpy; 2a), have been synthesized. Density functional theory (DFT) was applied to study the ground‐ and excited‐state properties of these copper(I) complexes. The electronic structure variation is obtained by changing the substituted positions on the phenanthroline ligand. A time‐dependent‐DFT approach (TDDFT) was used to interpret the absorption and emission spectra in this system based on the optimized geometries at the B3LYP/LANL2DZ and CIS/LANL2DZ levels of theory, respectively. The results show that the lowest‐energy excitations of all complexes are dominated by dπ(Cu)→π*(phen) or bpy (MLCT) excitations. In particular, the lowest triplet state (T~1~) corresponds to an excitation from the HOMO to the LUMO in all considered complexes, and emissions occur from T~1~, assigned to ^3^MLCT. Steric effects exerted by the pendent groups at the 2‐ and 9‐positions of the phenanthroline ligands increase the HOMO–LUMO gaps and thus lead to a blue‐shift of both absorption and emission spectra and effectively prevent structural relaxation in the MLCT state, which may narrow the energy gap between the excited and ground states and therefore increase nonradiative decay. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)