title: Electron Injection from Copper Diimine Sensitizers into TiO2: Structural Effects and Their Implications for Solar Energy Conversion Devices.
abstract: 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 (1)MLCT 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.
authors.affiliation: †Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States.
authors.forename: Michael W
authors.initials: MW
authors.lastname: Mara
authors.affiliation: §Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United States.
authors.forename: David N
authors.initials: DN
authors.lastname: Bowman
authors.affiliation: ⊥Graduate School of Energy, Environment, Water and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea.
authors.forename: Onur
authors.initials: O
authors.lastname: Buyukcakir
authors.affiliation: †Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States.
authors.forename: Megan L
authors.initials: ML
authors.lastname: Shelby
authors.affiliation: ∥Centre for Molecular Movies, Department of Physics, NEXMAP Section, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
authors.forename: Kristoffer
authors.initials: K
authors.lastname: Haldrup
authors.forename: Jier
authors.initials: J
authors.lastname: Huang
authors.forename: Michael R
authors.initials: MR
authors.lastname: Harpham
authors.forename: Andrew B
authors.initials: AB
authors.lastname: Stickrath
authors.forename: Xiaoyi
authors.initials: X
authors.lastname: Zhang
authors.affiliation: †Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States.
authors.forename: J Fraser
authors.initials: JF
authors.lastname: Stoddart
authors.affiliation: ⊥Graduate School of Energy, Environment, Water and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea.
authors.forename: Ali
authors.initials: A
authors.lastname: Coskun
authors.affiliation: §Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United States.
authors.forename: Elena
authors.initials: E
authors.lastname: Jakubikova
authors.affiliation: †Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States.
authors.forename: Lin X
authors.initials: LX
authors.lastname: Chen