1242658-09-8Relevant articles and documents
Exploiting conformational dynamics to facilitate formation and trapping of electron-transfer photoproducts in metal complexes
Meylemans, Heather A.,Hewitt, Joshua T.,Abdelhaq, Mirvat,Vallett, Paul J.,Damrauer, Niels H.
supporting information; experimental part, p. 11464 - 11466 (2010/10/02)
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 θ between the bipyridine and the aryl substituent. Driving forces for photoinduced electron transfer (ΔGET) and back electron transfer (ΔGBET) 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 ΔGET and ΔGBET are invariant to within 0.02 eV across the series. Upon visible photoexcitation of each of the D-A systems with ～100 fs laser pulses at 500 ± 10 nm, an electron-transfer (ET) photoproduct is observed to form with a time constant of τET = 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 θ, 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 (τBET) are also reported with τBET = 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 θ changes during conformational dynamics provides a new strategy to facilitate the formation and storage of charge-separated excited states.
