Due to ligand scrambling, the synthesis and investigation of the properties of heteroleptic Cu(I) complexes can be a challenging task. In this work, we have studied the optical and electrochemical properties of a series of homoleptic complexes, such as [Cu(dbda)₂]⁺, [Cu(dmp)₂]⁺, [Cu(Br-dmp)₂]⁺, [Cu(bcp)₂]⁺, [Cu(dsbtmp)₂]⁺, [Cu(biq)₂]⁺, and [Cu(dap)₂]⁺ in solution, and those of their heteroleptics [Cu(dbda)(dmp)]⁺, [Cu(dbda)(Br-dmp)]⁺, [Cu(dbda)(bcp)]⁺, [Cu(dbda)(dsbtmp)]⁺, [Cu(dbda)(biq)]⁺, [Cu(dbda)(dap)]⁺ adsorbed on the surface of anatase TiO₂ (dbda = 6,6′-dimethyl-2,2′-bipyridine-4,4′-dibenzoic acid; dmp = 2,9-dimethyl-1,10-phenanthroline; Br-dmp = 5-bromo-2,9-dimethyl-1,10-phenanthroline; bcp = bathocuproine or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline; dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline; biq = 2,2′-biquinoline; dap = 2,9-dianisyl-1,10-phenanthroline). We show that the maximum absorption wavelengths of the heteroleptic complexes on TiO₂ can be reasonably predicted from those of the homoleptic complexes in solution through a simple linear relation, whereas the prediction of their redox properties is less trivial. In the latter case, two different linear patterns emerge: one including the ligands bcp, biq, and dap and another one including the ligands dmp, Br-dmp, and dsbtmp. We offer an interpretation of the data based on the chemical structure of the ligands. On one hand, ligands bcp, biq, and dap possess a more extended π-conjugated system, which gives a more prominent contribution to the overall redox properties of the ligand dbda. On the other hand, the ligands dmp, Br-dmp, and dsbtmp are all phenanthroline-based containing alkyl substituents and contribute less than dbda to the overall redox properties.

A series of heteroleptic Cu(I) diimine complexes with different ancillary ligands and 6,6′-dimethyl-2,2′-bipyridine-4,4′-dibenzoic acid (dbda) as the anchoring ligand were self-assembled on TiO₂ surfaces and used as dyes for dye-sensitized solar cells (DSSCs). The binding to the TiO₂ surface was studied by hard X-ray photoelectron spectroscopy for a bromine-containing complex, confirming the complex formation. The performance of all complexes was assessed and rationalized on the basis of their respective ancillary ligand. The DSSC photocurrent–voltage characteristics, incident photon-to-current conversion efficiency (IPCE) spectra, and calculated lowest unoccupied molecular orbital (LUMO) distributions collectively show a push–pull structural dye design, in which the ancillary ligand exhibits an electron-donating effect that can lead to improved solar cell performance. By analyzing the optical properties of the dyes and their solar cell performance, we can conclude that the presence of ancillary ligands with bulky substituents protects the Cu(I) metal center from solvent coordination constituting a critical factor in the design of efficient Cu(I)-based dyes. Moreover, we have identified some components in the I^–/I₃^–-based electrolyte that causes dissociation of the ancillary ligand, i.e., TiO₂ photoelectrode bleaching. Finally, the detailed studies on one of the dyes revealed an electrolyte–dye interaction, leading to a dramatic change of the dye properties when adsorbed on the TiO₂ surface.