J. Phys. Chem. B 2010, 114, 14521–14527
14521
Influence of Ligand Substitution on Excited State Structural Dynamics in Cu(I) Bisphenanthroline Complexes† Jenny V. Lockard,*,‡ Sanaz Kabehie,| Jeffrey I. Zink,| Grigory Smolentsev,⊥,# Alexander Soldatov,⊥ and Lin X. Chen*,‡,§ Chemical Sciences and Engineering DiVision, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439, Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208, Department of Chemistry and Biochemistry, UniVersity of California at Los Angeles, Los Angeles, California 90095, Research Center for Nanoscale Structure of Matter, Southern Federal UniVersity, Sorge 5, RostoV-na-Donu, 344090 Russia, and Department of Chemical Physics, Lund UniVersity, P.O. Box 124, Lund, SE-22100, Sweden ReceiVed: March 12, 2010; ReVised Manuscript ReceiVed: May 26, 2010
This study explores the influences of steric hindrance and excited state solvent ligation on the excited state dynamics of CuI diimine complexes. Ultrafast excited state dynamics of Cu(I)bis(3,8-di(ethynyltrityl)-1,10phenanthroline) [CuI(detp)2]+ are measured using femtosecond transient absorption spectroscopy. The steady state electronic absorption spectra and measured lifetimes are compared to those of Cu(I)bis(1,10phenanthroline), [CuI(phen)2]+, and Cu(I)bis(2-9-dimethyl-1,10-phenanthroline), [CuI(dmp)2]+, model complexes to determine the influence of different substitution patterns of the phenanthroline ligand on the structural dynamics associated with the metal to ligand charge transfer excited states. Similarities between the [CuI(detp)2]+ and [CuI(phen)2]+ excited state lifetimes were observed in both coordinating and noncoordinating solvents and attributed to the lack of steric hindrance from substitution at the 2- and 9-positions. The solution-phase X-ray absorption spectra of [CuI(detp)2]+, [CuI(phen)2]+, and [CuI(dmp)2]+ are reported along with finite difference method calculations that are used to determine the degree of ground state dihedral angle distortion in solution and to account for the pre-edge features observed in the XANES region. Introduction Many transition metal complexes have intense metal-toligand-charge-transfer (MLCT) absorption bands in the visible spectral range that make them appealing for solar energy conversion and photocatalysis applications. While extensive and insightful studies of ruthenium complexes and their excited state dynamics have led to their important roles as dye sensitizers for photovoltaic devices and other solar energy applications,1-3 the limitation of systems based on such an expensive and nonabundant metal has long been recognized. Complexes that include abundant first row transition metals have attracted much attention as potential replacements.4-6 Copper(I) diimines, [CuI(NN)2]+ (where NN ) 1,10-phenanthroline or bipyridine derivatives), have been identified as promising candidates since their MLCT absorption in the visible region of the spectrum yields electron or energy donating capabilities comparable to those of ruthenium polypyridyl complexes,4,7-13 such as [CuI(dmp)2]+ compared to [RuII(bpy)3]2+, where dmp and bpy are 2,9-dimethyl-1,10-phenanthroline and bipyridyl, respectively. Despite their similar photoinduced MLCT processes, there is a major structural difference between the two metal complexes. The [CuI(NN)2]+ complexes have a pseudotetrahedral coordination geometry that permits intramolecular ligand motions that are largely restricted in those of the Ru(II) complexes with †
Part of the “Michael R. Wasielewski Festschrift”. * Corresponding author. E-mail:
[email protected]. Argonne National Laboratory. § Northwestern University. | University of California at Los Angeles. ⊥ Southern Federal University. # Lund University. ‡
octahedral coordination geometry. In addition, the coordination geometry of these copper complexes depends on the oxidation state of the copper center. The Cu(I) (3d10) complexes in the ground state have a tetrahedral (or near tetrahedral) D2d coordination geometry, whereas the Cu(II) (3d9) complexes in the MLCT excited state created upon photoexcitation prefer a more flattened tetrahedral (toward square planar) geometry with increased affinity for solvent ligation.7,9,13-17 The Cu(II) (3d9) center of the MLCT excited state is susceptible to a Jahn-Teller flattening distortion in which the dihedral angle between the phenanthroline ligand planes decreases from nearly orthogonal (i.e., D2d symmetry) in the Franck-Condon MLCT state to much smaller values (i.e., D2 symmetry). This distortion leads to pentacoordinated “exciplex” formation in different coordinating solvents as first reported by McMillin et al. using the evidence from their fundamental photophysical and photochemical studies.15-23 X-ray transient absorption (XTA) studies of photoexcited [CuI(dmp)2]+ presented by Chen et al. confirmed the exciplex model not only in a coordinating solvent, such as acetonitrile, but also in a so-called noncoordinating solvent, such as toluene, by providing direct evidence for additional coordination of a solvent molecule to the metal center while simultaneously showing the transient change in copper oxidation state from Cu(I) to Cu(II).24,25 More recently, several ultrafast transient spectroscopic studies have provided evidence for ultrafast structural rearrangements of [CuI(dmp)2]+ complexes.4,26-29 Despite some variation among the reported rate constants, there is consensus that a 10-15 ps rate constant is attributed to ISC (intersystem crossing), while the flattening distortion occurs on a subpicosecond time scale. The relatively slow ISC (compared to
