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Time Resolved Electron Transfer in Porphyrin Coordinated Ruthenium Dimers – from Mixed Valence Dynamics to Hot Electron Transfer Jonas Petersson, Jane S Henderson, Allison Brown, Leif Hammarström, and Clifford P. Kubiak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp510782c • Publication Date (Web): 05 Feb 2015 Downloaded from http://pubs.acs.org on February 10, 2015
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The Journal of Physical Chemistry
Time-Resolved Electron Transfer in Porphyrin Coordinated Ruthenium Dimers – From Mixed-Valence Dynamics to Hot Electron Transfer Jonas Petersson†#, Jane Henderson‡#, Allison Brown†, Leif Hammarström†, Clifford P. Kubiak*‡
†Department of Chemistry, Ångström Laboratory, Uppsala University, Box 523, SE75120 Uppsala, Sweden ‡Department of Chemistry and Biochemistry, University of California—San Diego, 9500 Gilman Drive MC 0358, La Jolla, California 92093, United States
Abstract Presented here is the first effort to study the formation and dynamics of the triruthenium cluster (Ru3O) pyrazine-bridged dimer mixed-valence state. Femtosecond transient absorption spectroscopy was implemented to follow photoinduced electron transfer reactions in a series of asymmetric porphyrin-coordinated dyads, which form strongly coupled mixed-valence species upon single reduction. Excitation of the porphyrin sub-unit resulted in electron transfer to the Ru3O dimer with a time constant τ~0.6 ps. The intramolecular electron transfer was confirmed by excitation of the Ru3O MLCT, which resulted in the formation of a vibrationally unrelaxed porphyrin ground state. Under both excitation experiments, the back electron transfer was extremely fast (τCR < 0.1 ps), preventing complete time-resolved exploration of the mixed-valence state. These complexes enabled the observation of excited product states following electron transfer processes, resulting from porphyrin S1 and S2 excitation. Though the charge recombination itself could not be observed, the yield of unrelaxed ground states supports the conclusion that delocalization takes place at least partly on a sub-100 fs timescale.
Introduction Mixed-valence compounds containing metal centers in different oxidation states have been a major focus of electron transfer (ET) research for decades.1,2,3 The observation of intervalence charge-transfer (IVCT) transitions provides a useful experimental probe of the relationship 1 ACS Paragon Plus Environment
The Journal of Physical Chemistry
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between optical and thermal ET reactions occurring within mixed-valence molecules. The energy, shape and intensity of the IVCT absorption bands allows the electronic coupling ( ) to be determined using the Marcus-Hush description of mixed-valency. is related to the free energy of activation for the interconversion of states (∆ ∗ ) and the reorganization energy of the ET event (λ), given by ∆ ∗ =
(1)
Equation 1, which is appropriate for symmetric systems, provides insight into the ground state adiabatic potential energy surface; as ∆ ∗ decreases the ground state changes from a double minimum, charge localized state, to a single minimum, fully delocalized state.4 Mixed-valence states and their IVCT absorption bands have been studied extensively, but there have been very few time-resolved studies observing changes to IVCT transitions.5,6,7,8 Measuring the timescale of electronic coupling-derived relaxation inherent to a mixed-valence system is a thought provoking experiment. The ability to observe IVCT formation would provide a direct spectroscopic probe of delocalization. In order to resolve these delocalization dynamics, time resolution of
