title: Ultrafast structural dynamics of Cu(I)-bicinchoninic acid and their implications for solar energy applications.
abstract: In this study, ultrafast optical transient absorption and X-ray transient absorption (XTA) spectroscopy are used to probe the excited-state dynamics and structural evolution of copper(I) bicinchoninic acid ([Cu(I)(BCA)2](+)), which has similar but less frequently studied biquinoline-based ligands compared to phenanthroline-based complexes. The optical transient absorption measurements performed on the complex in a series of polar protic solvents demonstrate a strong solvent dependency for the excited lifetime, which ranges from approximately 40 ps in water to over 300 ps in 2-methoxyethanol. The XTA experiments showed a reduction of the prominent 1s → 4pz edge peak in the excited-state X-ray absorption near-edge structure (XANES) spectrum, which is indicative of an interaction with a fifth ligand, most likely the solvent. Analysis of the extended X-ray absorption fine structure (EXAFS) spectrum shows a shortening of the metal-ligand bond in the excited state and an increase in the coordination number for the Cu(II) metal center. A flattened structure is supported by DFT calculations that show that the system relaxes into a flattened geometry with a lowest-energy triplet state that has a dipole-forbidden transition to the ground state. While the short excited-state lifetime relative to previously studied Cu(I) diimine complexes could be attributed to this dark triplet state, the strong solvent dependency and the reduction of the 1s → 4pz peak in the XTA data suggest that solvent interaction could also play a role. This detailed study of the dynamics in different solvents provides guidance for modulating excited-state pathways and lifetimes through structural factors such as solvent accessibility to fulfill the excited-state property requirements for efficient light harvesting and electron injection.
authors.affiliation: Chemical Sciences and Engineering Division, Argonne National Laboratory , 9700 South Cass Avenue, Argonne, Illinois 60439-3113, United States.
authors.forename: Kelly A
authors.initials: KA
authors.lastname: Fransted
authors.forename: Nicholas E
authors.initials: NE
authors.lastname: Jackson
authors.forename: Ruifa
authors.initials: R
authors.lastname: Zong
authors.forename: Michael W
authors.initials: MW
authors.lastname: Mara
authors.forename: Jier
authors.initials: J
authors.lastname: Huang
authors.forename: Michael R
authors.initials: MR
authors.lastname: Harpham
authors.forename: Megan L
authors.initials: ML
authors.lastname: Shelby
authors.forename: Randolph P
authors.initials: RP
authors.lastname: Thummel
authors.forename: Lin X
authors.initials: LX
authors.lastname: Chen