CCB Colloquium presentation by Dr. Lin Chen, Northwestern University
Abstract: One of the fundamental processes in solar energy conversion is photoinduced electron transfer (PET) which is responsible for generating solar fuels and solar electricity. X-ray transient absorption spectroscopy has been used to study several systems in terms of excited state PET from the metal centers of transition metal complexes into electron acceptors or semiconductor nanoparticles (NPs). These studies have strong relevance to photocatalysis, and dye-sensitized solar cells (DSSCs). The MLCT states of Cu(I) diimine complexes undergo the Jahn-Teller distortion, have solvent-dependent lifetimes, and structure-dependent intersystem crossing (ISC). The excited state structural dynamics of several Cu(I) complexes alone have been studied by X-ray transient absorption spectroscopy (XTA) on both 100-ps and 100-fs time scales, 1-3 our study here intend to establish the control of structures for the dynamics and efficiency of interfacial electron injection from CuI complexes into TiO2 NPs in DSSC mimics. The electron transfer (ET) and charge recombination (CR) dynamics of a DSSC mimic, [CuI(dppS)2]+ (dppS=2,9-diphenyl-1,10-phenanthroline disulfonic acid disodium salt)–TiO2 nanoparticle (NP) hybrid have been investigated by combining optical transient absorption (TA), electron paramagnetic resonance (EPR), and XTA spectroscopies. We observed an efficient ultrafast ET (t = 0.4 ps) from the singlet 1MLCT state of [CuI(dppS)2]+ to TiO2 NPs and a slow CR (t ~ 2.5µs) dynamics in [CuI(dppS)2]+/TiO2. XTA studies revealed a flattened pseudo-tetrahedral geometry in [CuI(dppS)2]+, which weakened the spin-orbit coupling and resulted in a relatively slow intersystem crossing rate of (13.7 ps)-1. Consequently, it becomes achievable to have the fast electron injection from the singlet MLCT state of [CuI(dppS)2]+ to TiO2 NPs. These results demonstrate structural control of the excited state properties in designing efficient, stable and abundant light-harvesting materials.
(2) Chen, L. X et al., MLCT state structure and dynamics of a copper(I) diimine complex characterized by pump-probe X-ray and laser spectroscopies and DFT calculations. J. Am. Chem. Soc. 2003, 125, 7022-7034.
(3) Huang, J. et al., Highly Efficient Ultrafast Electron Injection from the Singlet MLCT Excited State of CuI-Diimine Complexes to TiO2 Nanoparticles, submitted, (2012)
(1) "Taking Snapshots of Photoexcited Molecular Structures in Disordered Media Using Pulsed X-rays" Lin X. Chen, Angew. Chemie. Intl. Ed., (Invited Review) 43, 2886-2905 (2004);
(2) "Probing Excited State Structures and Dynamics of Metal Complexes Using Time-resolved X-rays" Lin X. Chen, Ann. Rev. Phys. Chem.56, 221-254 (2005)