Capturing and Analyzing the Excited-State Structure of a Cu(I) Phenanthroline Complex by Time-Resolved Diffraction and Theoretical Calculations

Abstract

Time-resolved crystallography and density functional theory calculations are used to analyze the geometric and electronic changes that occur upon photoexcitation of [Cu(I)(dmp)(dppe)]⁺ in crystalline [Cu(I)(dmp)(dppe)][PF₆] [dmp = 2,9-dimethyl-1,10-phenanthroline; dppe = 1,2-bis(diphenylphosphino)ethane]. In the pump−probe experiment, laser and X-ray pulses are synchronized to capture an image of the instantaneous molecular distortions in the transient triplet state. Parallel theoretical calculations, with the phenyl groups replaced by methyl groups, yield information on the distortion of the isolated cation and the change in electron density upon excitation. The experimental distortions are significantly less than the calculated values and are different for the two independent molecules in the asymmetric unit; these findings are attributed to the constraining influence of the crystal matrix. The calculations indicate that the electron transfer upon excitation is mostly from the dmpe ligand to the dmp ligand, while the Cu atomic charge changes by only ∼+0.1e, although the charge distribution on Cu is significantly affected. As found for homoleptic [Cu(I)(dmp)₂]⁺, the change in the population of the Cu atom is close to the calculated difference between the corresponding Cu(II) and Cu(I) complexes. Charge density difference maps confirm these conclusions and show a large rearrangement of the electron density on the Cu atom upon excitation.