Electron Injection from Copper Diimine Sensitizers into TiO₂: Structural Effects and Their Implications for Solar Energy Conversion Devices

Copper­(I) diimine complexes have emerged as low cost replacements for ruthenium complexes as light sensitizers and electron donors, but their shorter metal-to-ligand-charge-transfer (MLCT) states lifetimes and lability of transient Cu­(II) species impede their intended functions. Two carboxylated Cu­(I) bis-2,9-diphenylphenanthroline (dpp) complexes [Cu­(I)­(dpp-O­(CH₂CH₂O)₅)­(dpp-(COOH)₂)]⁺ and [Cu­(I)­(dpp-O­(CH₂CH₂O)₅)­(dpp-(Φ-COOH)₂)]⁺ (Φ = tolyl) with different linker lengths were synthesized in which the MLCT-state solvent quenching pathways are effectively blocked, the lifetime of the singlet MLCT state is prolonged, and the transient Cu­(II) ligands are stabilized. Aiming at understanding the mechanisms of structural influence to the interfacial charge transfer in the dye-sensitized solar cell mimics, electronic and geometric structures as well as dynamics for the MLCT state of these complexes and their hybrid with TiO₂ nanoparticles were investigated using optical transient spectroscopy, X-ray transient absorption spectroscopy, time-dependent density functional theory, and quantum dynamics simulations. The combined results show that these complexes exhibit strong absorption throughout the visible spectrum due to the severely flattened ground state, and a long-lived charge-separated Cu­(II) has been achieved via ultrafast electron injection (<300 fs) from the ¹MLCT state into TiO₂ nanoparticles. The results also indicate that the TiO₂-phen distance in these systems does not have significant effect on the efficiency of the interfacial electron-transfer process. The mechanisms for electron transfer in these systems are discussed and used to develop new strategies in optimizing copper­(I) diimine complexes in solar energy conversion devices