Electroabsorption Spectroscopy
Electroabsorption (Stark) spectroscopy has proven to be a powerful
tool for understanding mixed valence systems, as it can be used to determine
the distance an electron is transferred. (Click here
to see how Stark works.) In our group, it has been utilized to interrogate
localized intervalence charge transfer systems (see figure) for the effects
of metal substitution (ref.
139) as well as the effects of ligand substitution (ref.
152).
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Shown is the structure of Na[(NC)5Fe-CN-RU(NH3)4py]*6H2O, obtained from x-ray crystallography (ref. 152). Two of the H2O molecules have been omitted for clarity (they were disorded in the structure).
The charge transfer distance, or change in dipole moment (Dm), can further be used to predict the molecular hyperpolarizability (b) of a molecule. This has been recently applied (ref. 127) to porphyrin based push-pull chromophores 1 and 3 (in collaboration with Michael Therien, U. Penn) to help explain their large b values as measured by hyper-Rayleigh scattering.
Resonance Raman Spectroscopy
We make use of theoretical and experimental resonance Raman techniques to explore the excited state properties of dimacrocyclic mixed valence systems (provided by Spreer and co-workers). The metal centers in these molecules are strongly coupled, leading to delocalized (class III) behavior for the valence electrons (ref. 129).
The coupling constant, Hab, between the adiabatic ground and excited state potential surfaces of these molecules is believed to be quite large. Thus, the surfaces are thought to perturb each other to the point where an excited state potential surface is nested within the ground state potential.
We search for those modes in the Raman spectrum which are resonantly enhanced. Then, based on the work of Heller and co-workers on time dependent scattering theory, one can obtain the mode displacements of the vibrations which produce the electronic spectrum of the molecule. Thereafter, Franck-Condon factors can be calculated, and a rate constant for the excited state lifetime is obtainable via Fermi's Golden Rule. We are performing both experiments and modeling to better understand the properties of the excited state of these molecules as well as their bonding character.
Ultrafast Transient Absorption Spectroscopy(joint
project with Prof. Ken Spears)
An ultrafast laser system with continual tunability from the near UV
to the mid IR is near completion which will allow for kinetic measurements
of both electronic and vibrational dynamics on subpicosecond time scales
and excitation capabilities from the near UV through the near IR.
We use this system to perform pump/probe transient absorption measurements
which essentially allow us to watch electrons jump between energy levels
and watch vibrations exchange their energy with the environment.
From a kinetic measurement, we can extrapolate a rate constant and compare it to that obtained from the previously discussed resonance Raman calculations.
References
dimacrocyclic mixed valence systems--
Zhou, J.; Li, A.; Lange, C.; Allan, C.B.; Spreer, L.O.; Otvos, J.W.;
Calvin, M. Inorg. Chem. Acta. 1996, 246, 241.
Mountford, H.S.; MacQuenn, D.B.; Li, A.; Otvos, J.W.; Calvin, M.; Frankel,
R.B.; Spreer, L.O. Inorg. Chem. 1994, 33, 1748.
Spreer, L.O.; Li, A.; MacQueen, D.B.; Allan, C.B.;Otvos, J.W.; Calvin,
M.; Frankel, R.B.; Papaefthymiou, G.C. Inorg. Chem. 1994,
33,
1753.
time dependent scattering methods--
Heller, E.J.; Sundberg, R.L.; Tannor, D. J. Phys. Chem. 1982,
86,
1822.
Tannor, D.J.; Heller, E.J. J. Chem. Phys. 1982, 77,
202.