title: Exploiting conformational dynamics to facilitate formation and trapping of electron-transfer photoproducts in metal complexes.
abstract: Three new photoinduced electron donor-acceptor (D-A) systems are reported which juxtapose a Ru(II) excited-state donor with a bipyridinium acceptor via a conformationally active asymmetric aryl-substituted bipyridine ligand participating in the bridge between D and A. Across the series of complexes 1-3, steric bulk is sequentially added to tune the inter-ring dihedral angle theta between the bipyridine and the aryl substituent. Driving forces for photoinduced electron transfer (DeltaG(ET)) and back electron transfer (DeltaG(BET)) are reported based on electrochemical measurements of 1-3 as well as Franck-Condon analysis of emission spectra collected for three new donor model complexes 1'-3'. These preserve the substitution patterns on the aryl substituent in their respective D-A complexes but remove the bipyridinium acceptor. Both DeltaG(ET) and DeltaG(BET) are invariant to within 0.02 eV across the series. Upon visible photoexcitation of each of the D-A systems with approximately 100 fs laser pulses at 500 +/- 10 nm, an electron-transfer (ET) photoproduct is observed to form with a time constant of tau(ET) = 29 ps (1), 37 ps (2), and 57 ps (3). That ET remains relatively rapid throughout this series, even as steric bulk significantly increases the inter-ring dihedral angle theta, is attributed to the effects of ligand-based torsional dynamics driven by intraligand electron delocalization in the D*-A excited state manifold prior to ET. The lifetimes of the charge-separated states (tau(BET)) are also reported with tau(BET) = 98 ps (1), 217 ps (2), and 789 ps (3), representing a more than 8-fold increase across the series. This is attributed to reverse conformational dynamics in D(+)-A(-) driven by steric repulsions, which serves to minimize electronic coupling to the ground state. Steric control of ligand geometry and the range over which theta changes during conformational dynamics provides a new strategy to facilitate the formation and storage of charge-separated excited states.
authors.affiliation: Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado 80309, USA.
authors.forename: Heather A
authors.initials: HA
authors.lastname: Meylemans
authors.forename: Joshua T
authors.initials: JT
authors.lastname: Hewitt
authors.forename: Mirvat
authors.initials: M
authors.lastname: Abdelhaq
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Vallett
authors.forename: Niels H
authors.initials: NH
authors.lastname: Damrauer
mesh_terms: D003198:Computer Simulation; D004563:Electrochemistry; D004583:Electrons; D008024:Ligands; D008968:Molecular Conformation; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D010777:Photochemistry; D011726:Pyridinium Compounds / Q000737:chemistry*; D013237:Stereoisomerism; D013816:Thermodynamics
chemical_list: D008024:Ligands; D009942:Organometallic Compounds; D011726:Pyridinium Compounds