Bruce S. Brunschwig

Energy surfaces, reorganization energies, and coupling elements in electron transfer

Abstract The effects of changes in the shapes and intersections of the reactant and product free-energy surfaces on the vertical reorganization parameter and the free energy of activation for an electron self-exchange reaction are considered. Parabolic free energy surfaces provide a very good description of the inner-shell reorganization process even when the stretching force constants for the oxidized and reduced forms of the redox couple differ by a factor of two. The activation energy depends on the reorganization criterion and the contributions of the individual reactants to the inner-shell barrier are quite sensitive to the model used. Optical charge transfers and the consequences of reactant, product, and transition state stabilization in weakly interacting and very strongly interacting systems are also considered. Relationships between the equilibrium constant for the comproportionation reaction forming the mixed-valence complex and the optical charge transfer parameters are presented.

Electroabsorption spectroscopy of charge transfer states of transition metal complexes

Abstract The use of electroabsorption spectroscopy to determine the dipole-moment changes that occur in the metal-to-ligand, ligand-to-metal, and metal-to-metal charge-transfer transitions in mononuclear and binuclear transition metal complexes is reviewed. The ground-excited state dipole-moment differences are much smaller than expected for the transfer of unit electronic charge between the donor and acceptor centers. The results are discussed in terms of a model in which two factors, electron delocalization and polarization of the acceptor, donor or bridging ligand electrons in response to the changed charge on the metal centers, are considered to be primarily responsible for the relatively small dipole-moment changes. The implications of the results for electronic coupling elements and reorganization energies are also discussed.

Rate-Constant Expressions for Nonadiabatic Electron-Transfer Reactions

Abstract Expressions for the rate constants for nonadiabatic electron-transfer reactions are presented. These expressions are valid when the electronic coupling between the two redox sites is small and the energy surfaces for the initial and final states are harmonic with identical force constants. High- and low-temperature limiting forms of the rate equations are presented and closed-form expressions which are good approximations to the Franck-Condon sums appearing in these equations are described. Systems in which the electron transfer causes a displacement in one or more vibrational modes are considered. When several vibrational modes are “active,” the higher frequency modes have different effects on the rate constants in the normal and inverted free-energy regions. In the inverted region nuclear tunneling effects are large when hwf > 2kT. Under these conditions the rate has only a weak dependence on temperature and the rate constants can exhibit “quantum beats.” Nuclear tunneling effects are smaller i...

Characterization of transient species and products in photochemical reactions of Re(dmb) (CO)3 Et with and without CO2

Transient FT-IR spectra of fac-Re(dmb)(CO)3(Et) after laser excitation (355 nm) were investigated in THF under Ar and CO2 atmospheres. The CO stretching bands of Re(dmb⊙)(CO)3(THF) grow (2008 and 1897 cm−1) and those of Re(dmb)(CO)3(Et) bleach (1987 and 1875 cm−1) at times <1 μs, consistent with clean cleavage of the Re-Et bond. Under a CO2 atmosphere, the long-lived radical (τ>100 ms) converts slowly to the formato complex Re(dmb)(CO)3(OC(O)H) (2020, 1916, 1873 and 1630 cm−1). When the solvent is slightly wet, the bicarbonato complex, Re(dmb)(CO)3(OC(O)OH), is also observed after photolysis under CO2.

Temperature dependence of the lifetimes of the ligand-field states of tris(1,10-phenanthroline)iron(II)

Abstract The lifetimes of the excited states produced by 527 nm excitation of tris(1,10-phenanthroline)iron(II) have been measured in a 4:1 (v/v) ethanol/methanol mixture in the 14–295 K temperature range. The results are analyzed in terms of the ⩽10 ps population of ligand-field (LF) states from the initially generated metal-to-ligand charge-transfer (MLCT) states.

Mechanistic and kinetic studies of cobalt macrocycles in a photochemical CO2 reduction system

Photoreduction of CO 2 using p-terphenyl as a photosensitizer and a tertiary amine as a sacrificial electron donor is efficiently catalyzed by cobalt macrocycles in methanolic acetonitrile solutions. Studies of the mechanism of CO 2 reduction in the catalytic system using Co II HMD 2+ (HMD = 5,7,7, 12, 14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) have been carried out using continuous and flash photolysis techniques. The transient spectra reveal the formation of p-terphenyl radical anion, the Co I HMD + complex, the 5-coordinate [Co I HMD-CO 2 ] + complex, and the six coordinate [S-Co III HMD-(CO 2 2− )] + complex (S = solvent) in the catalytic system. The importance of changes in the Co II/I reduction potential and the geometry around the Co center was studied by the use of several different complexes

Visible light photolysis of tris(bipyridine)ruthenium(II)-titanium(III) solutions

Abstract Hydrochloric acid solutions containing tris(bipyridine)ruthenium(II) and titanium(III) were irradiated with visible light. Contrary to a published report, we find that virtually no molecular hydrogen is produced during the photolysis (quantum yield ⩽ 1O −5 mol einstem −1 ). The mechanism of the quenching of the charge-transfer excited state of the ruthenium(II) complex by titanium(III) is discussed.

Quadratic nonlinear optical properties of novel pyridinium salts

A number of novel salts of dipolar cations containing dimethylamino electron donor and pyridinium electron acceptor groups have been prepared and studied by using various physical techniques including hyper-Rayleigh scattering and Stark spectroscopy. In addition to showing very large molecular static first hyperpolarizabilities, several of these salts exhibit pronounced bulk quadratic nonlinear optical effects. Related compounds in which the electron donor is a ruthenium(II) ammine centre have also been investigated, allowing interesting comparisons to be made between transition metal-containing and purely organic nonlinear optical chromophores.

Determination of the molecular quadratic non-linear optical responses of V-shaped metallochromophores by using Stark spectroscopy

The static molecular first hyperpolarizabilities β0 of a series of V-shaped RuII chromophores with two overlapping visible metal-to-ligand charge-transfer absorptions have been determined by using data obtained from Stark spectroscopy.

Photochemical carbon dioxide reduction with metal complexes: Differences between cobalt and nickel macrocycles

Problems related to increases of green house gases in the atmosphere and the depletion of fossil fuels have made the conversion of CO{sub 2} into useful chemicals and fuels an important area of research. However, CO{sub 2} reduction poses many scientific challenges. Despite intense interest in photochemical and electrochemical CO{sub 2} reduction, the kinetics and mechanism of the reduction remain unclear in many systems. This research focuses on mechanistic and kinetic studies of photochemical and electrochemical CO{sub 2} reduction that involves metal complexes as catalysts. This work makes use of UV-vis, NMR, and FTIR spectroscopy, flash photolysis, pulse radiolysis, X-ray diffraction, XANES (X-ray absorption near-edge spectroscopy) and EXAFS (extended X-ray absorption fine structure). Here the authors summarize their research on photochemical carbon dioxide reduction with metal macrocycles.