Effects of Electronic and Nuclear Interactions on the Excited-State Properties and Structural Dynamics of Copper(I) Diimine Complexes

The effects of structural constraints on the metal-to-ligand charge transfer (MLCT) excited state structural dynamics of cuprous bis-2,9-diphenyl-phenanthroline ([Cu­(I)­(dpp)₂]⁺) in both coordinating acetonitrile and noncoordinating toluene were studied using X-ray transient absorption (XTA) spectroscopy and density functional theory (DFT) calculations. The phenyl groups attached to the phenanthroline ligands not only effectively shield the Cu­(I) center from solvent molecules, but also force a flattened tetrahedral coordination geometry of the Cu­(I) center. Consequently, the MLCT state lifetime in [Cu­(I)­(dpp)₂]⁺ is solvent-independent, unlike the previously studied 2,9-methyl substituted bis-phenanthroline Cu­(I) complex. The MLCT state of [Cu­(I)­(dpp)₂]⁺ still undergoes a “pseudo Jahn-Teller distortion,” with the angle between the two phenanthroline ligand planes decreased further by 7°. The XTA results indicate that, in the MLCT excited state of [Cu­(I)­(dpp)₂]⁺, the phenyls at the 2, 9 positions of the phenanthroline rotate, breaking the π–π interaction with the phenanthroline ligands without ever rotating in-plane with the phenanthroline ligands. Hence, the transferred electron density from the Cu­(I) center is localized on the phenanthroline moiety with no charge density present on the phenyl rings. The insight about the effect of the structural constraints on the MLCT state properties will guide the design of Cu­(I) diimine complexes with suitable excited-state properties to function as earth-abundant dye sensitizers for solar electricity generation.