Warren F. Beck

Vibrational coherence in the excited state dynamics of Cr(acac)3: probing the reaction coordinate for ultrafast intersystem crossing

Vibrational coherence was observed following excitation into the lowest-energy spin-allowed 4A2 → 4T2 ligand-field absorption of Cr(acac)3. The transient kinetics were fit to a rapidly damped 164 cm−1 oscillatory component, the frequency of which is not associated with the ground state of the molecule. The signal is assigned as an excited-state vibrational coherence; the timescale of the event suggests that this vibrational coherence is retained during the 4T2 → 2E intersystem crossing that immediately follows 4A2 → 4T2 excitation. DFT calculations indicate that the 164 cm−1 oscillation likely corresponds to a combination of Cr–O bond stretching in the ligand-field excited state as well as large amplitude motion of the ligand backbone. This hypothesis is supported by ultrafast time-resolved absorption measurements on Cr(t-Bu-acac)3 (where t-Bu-acac is the monoanionic form of 2,2,6,6-tetramethyl-3,5-heptanedione) – an electronically similar but more sterically encumbered molecule – which exhibits a 4T2 → 2E conversion that is more than an order of magnitude slower than that observed for Cr(acac)3. These results provide important insights into the nature of the reaction coordinate that underlies ultrafast excited-state evolution in this prototypical coordination complex.

RADIATIONLESS DECAY FROM THE LIGAND-TO-METAL CHARGE-TRANSFER STATE IN THE BLUE COPPER PROTEIN PLASTOCYANIN

Abstract The cysteine S(p π ) → Cu 2+ d x 2 − y 2 ligand-to-metal charge-transfer (LMCT) state in plastocyanin exhibits a time-resolved pump-probe spectrum that has excited-state absorption and stimulated-emission components to the blue and red, respectively, of the absorption maximum. The LMCT state returns to the ground state by populating the d xz + yz → d x 2 − y 2 ligand-field (LF) state. The lack of hole-burned features in the time-resolved spectra implies the presence of intramolecular vibrational redistribution and/or protein-matrix solvation dynamics on a time scale that is shorter than the 125 fs lifetime of the LMCT state.

Excited state conformational dynamics in carotenoids: Dark intermediates and excitation energy transfer

A consideration of the excited state potential energy surfaces of carotenoids develops a new hypothesis for the nature of the conformational motions that follow optical preparation of the S2 (1(1)Bu(+)) state. After an initial displacement from the Franck-Condon geometry along bond length alternation coordinates, it is suggested that carotenoids pass over a transition-state barrier leading to twisted conformations. This hypothesis leads to assignments for several dark intermediate states encountered in femtosecond spectroscopic studies. The Sx state is assigned to the structure reached upon the onset of torsional motions near the transition state barrier that divides planar and twisted structures on the S2 state potential energy surface. The X state, detected recently in two-dimensional electronic spectra, corresponds to a twisted structure well past the barrier and approaching the S2 state torsional minimum. Lastly, the S(∗) state is assigned to a low lying S1 state structure with intramolecular charge transfer character (ICT) and a pyramidal conformation. It follows that the bent and twisted structures of carotenoids that are found in photosynthetic light-harvesting proteins yield excited-state structures that favor the development of an ICT character and optimized energy transfer yields to (bacterio)chlorophyll acceptors.

Oxidation of exogenous substrates by the O2-evolving center of photosystem II and related catalytic air oxidation of secondary alcohols via a tetranuclear manganese(IV) complex

Abstract The O2-evolving center of Photosystem II (PSII) contains atetranuclear Mn complex that acts as the catalyst for photosynthetic H2O oxidation. The catalytic cycle for H2O oxidation is thought to involve the light-driven generation of a very strongly oxidizing state of the Mn complex prior to the formation of the OO bond. However, the Mn complex also exhibits oxidation chemistry in its lower oxidation states. Exogenous ligands, such as primary amines and hydroxylamines, have been shown previously to interact with the O2-evolving center in the dark S1 oxidation state by coordinating to a Cl−-binding site near the Mn complex. In this paper, we present evidence that primary and secondary amines irreversibly inactivate the Mn complex in the S1 state by a reductive mechanism, leading to the liberation of Mn(II) ions and a concomitant loss of O2-evolution activity. Hence, the Mn complex in the O2-evolving center acts as a strong oxidant even in the dark-stable S1 state. The synthetic tetranuclear Mn(IV) complex [(TACN)4Mn4O6]Br4, which has been suggested as a model for the Mn complex in the O2-evolving center, is a strong oxidant as well. We find that it catalyzes the air oxidation of secondary alcohols to ketones and triphenylphosphine to triphenylphosphine oxide.

Nanosecond-regime correlation time scales for equilibrium protein structural fluctuations of metal-free cytochrome c from picosecond time-resolved fluorescence spectroscopy and the dynamic Stokes shift.

We used picosecond time-resolved fluorescence spectroscopy to characterize the fluorescence Stokes shift (FSS) response function of metal-free (or free-base, fbCytc) cytochrome c under the solution conditions that favor the native states of ferricytochrome c (FeCytc) and Zn(II)-substituted cytochrome c (ZnCytc). The intrinsic porphyrin chromophore serves in these experiments as a fluorescent probe of the structural fluctuations of the surrounding protein and solvent. Demetalation of the porphyrin destabilizes the folded structure of cytochrome c owing to the loss of the axial metal-histidine and metal-methionine bonds. Thus, these experiments examine how the time scales detected in a dynamic solvation experiment in a chromoprotein report changes in the character of motion. The FSS response function in fbCytc in water and pH 7 is well described by a biexponential response over the 100 ps to 50 ns regime with time constants of 1.4 and 9.1 ns; under similar conditions, ZnCytc exhibits a biexponential FSS response with time constants of 250 ps and 1.5 ns [Lampa-Pastirk and Beck, J. Phys. Chem. B 2004, 108, 16288]. These time constants correspond, respectively, to the correlation time scales for motions of the hydrophobic core and the solvent-contact layer of the protein. Both of the time constants observed in fbCytc are further lengthened upon addition of glycerol to the external solvent so that a significant fraction of the protein dynamics is rendered effectively static on the fluorescence time scale. The solvation reorganization energy, the time-integrated Stokes shift of the fluorescence spectrum, is reduced by about a third to 33 cm(-1) in 50% glycerol from 43 cm(-1) in water. These results are interpreted structurally using a model for Brownian diffusive motion with thermally activated barrier crossings on the protein-folding energy landscape. The results suggest that the mean-squared deviations of the structural fluctuations exhibited by fbCytc are nearly a factor of 10 larger than those of ZnCytc. This conclusion is consistent with the suggestion that fbCytc assumes a dynamic, partially unfolded structure with some of the characteristics of a molten globule.

Rapid-scanning interferometer for ultrafast pump–probe spectroscopy with phase-sensitive detection

We describe a rapid-scanning modified Mach–Zehnder interferometer suitable for use in femtosecond pump–probe or dichroism spectroscopy with self-mode-locked titanium–sapphire oscillators. A galvanometer-type rapid-scanning translation stage is employed in the pump arm of the interferometer. The intensity or polarization of the pump beam is modulated by a fused-silica photoelastic modulator employed as a half-wave retarder. The detection system exploits the rapid-scanning stage and photoelastic modulator by combining phase-sensitive detection with transient digitization. The system we describe permits pump–probe or dichroism spectroscopy to be conducted without temporal distortion of the laser pulses used in the experiment, with high scan-to-scan reproducibility, and with all of the previously noted advantages of rapid-scanning methods in terms of sensitivity. The approach is especially well suited for use with pulse-picked or cavity-dumped sources, which are often required to be operated at moderate pulse...

Femtosecond Heterodyne Transient-Grating Studies of Nonradiative Decay of the S2 (11Bu+) State of β-Carotene: Contributions from Dark Intermediates and Double-Quantum Coherences

Femtosecond transient-grating spectroscopy with heterodyne detection was employed to characterize the nonradiative decay pathway in β-carotene from the S2 (1(1)Bu(+)) state to the S1 (2(1)Ag(-)) state in benzonitrile solution. The results indicate definitively that the S2 state populates an intermediate state, Sx, on an ultrafast time scale prior to nonradiative decay to the S1 state. Numerical simulations using the response function formalism and the multimode Brownian oscillator model were used to fit the absorption and dispersion components of the transient-grating signal with a common set of parameters for all of the relevant Feynman pathways, including double-quantum coherences. The requirement for inclusion of the Sx state in the nonradiative decay pathway is the observed fast rise time of the dispersion component, which is predominantly controlled by the decay of the stimulated emission signal from the optically prepared S2 state. The finding that the excited-state absorption spectrum from the Sx state is significantly red-shifted from that of S2 and S1 leads to a new assignment for the spectroscopic origin of the Sx state. Rather than assigning Sx to a discrete electronic state, such as the (1)Bu(-) state suggested in previous work, it is proposed that the Sx state corresponds to a transition-state-like structure on the S2 potential surface. In this hypothesis, the 12 fs time constant for the decay of the S2 state corresponds to a vibrational displacement of the C-C and C═C bond-length alternation coordinates of the conjugated polyene backbone from the optically prepared Franck-Condon structure to a potential energy barrier on the S2 surface that divides planar and torsionally displaced structures. The lifetime of the Sx state would be associated with a subsequent relaxation along torsional coordinates over a steep potential energy gradient toward a conical intersection with the S1 state. This hypothesis leads to the idea that twisted structures with intramolecular charge-transfer character along the S2 torsional gradient are active in excitation energy-transfer mechanisms to (bacterio)chlorophyll acceptors.

Vibrationally Coherent Preparation of the Transition State for Photoisomerization of the Cyanine Dye Cy5 in Water.

Femtosecond pump-continuum probe spectroscopy with impulsive excitation was employed to observe coherent wavepacket motions of the cyanine dye Cy5 in water that promote photoisomerization after optical preparation of the first excited singlet state, S1. The chief component in the excited-state vibrational coherence is a resonance Raman-inactive, 273 cm(-1) mode of mixed carbon-carbon bond length alternation and out-of-plane or twisting character. The ultrafast (30 fs) damping of these motions arises from trajectories that irreversibly cross the transition state barrier; after several recurrences to the transition state region, vibrational cooling traps a significant fraction of the excited-state molecules in the planar, Franck-Condon region of the potential energy surface. Motion in the 273 cm(-1) promoting mode is apparently launched by a change in conformation of the conjugated polyene backbone during the first few vibrations of the high-frequency C-C and C═C bond length alternation coordinates that principally contribute to the initial displacement from the Franck-Condon structure. To our knowledge, this work provides the first direct observations of the intramolecular redistribution of excited-state potential energy into reactive motions that are rapidly damped by transition state barrier-crossing events leading to photoisomerization in a conjugated polyene. These results provide insight into the vibrational dynamics that contribute to the photoisomerization of retinal protonated Schiff bases in the rhodopsins and to the formation of intramolecular charge transfer character in carotenoids in photosynthetic light-harvesting proteins.

Intermolecular vibrational coherence in the bacteriochlorophyll proteins B777 and B820 from Rhodospirillum rubrum.

The low-frequency vibrational coherence in the bacteriochlorophyll (BChl)-containing subunit proteins B777 and B820 from the LH1 light-harvesting complex isolated from Rhodospirillum rubrum G9 exhibits rapidly damped modulation components arising from intermolecular, formally nonbonding interactions between the BChl macrocycle and polar groups in the surrounding detergent or protein. The vibrational coherence observed in the monomeric B777 system resembles that observed previously with BChl in acetone because it contains a pair of broad overlapping line shapes with a mean frequency of 191 cm(-1), but the 10:1 intensity ratio of the librational and translational components is distinctive of the motions of the polar head groups in the nonionic detergent micelle that solvates the BChl macrocycle. In contrast, the vibrational coherence observed with the dimeric B820 complex is almost 20 times weaker in intensity and exhibits narrower line shapes and lower average frequencies than observed in B777. The structure of the B820 complex sterically protects the pair of BChl macrocycles from the surrounding solvent, so modulation components assigned to intrinsic interactions between the BChl and the protein and between the pair of BChls are revealed. A relatively well-ordered interaction between the BChl macrocycle and a tryptophan residue in each alpha-helical polypeptide accounts for a 28 cm(-1) component with a narrow line shape, but most of the intensity arises from a broader 46 cm(-1) component that is assigned to the interaction between the paired BChl macrocycles. The breadth of the line shape for this component is a measure of the disorder in the ensemble of B820 subunits. The results support the hypothesis that the excited-state vibrational dynamics and the optical and/or Marcus charge-transfer reorganization energies of BChl in photosynthetic light-harvesting proteins and reaction centers are strongly controlled by van der Waals modes with neighboring molecules, with dominant contributions to the intermolecular potential arising from the London dispersion and dipole-dipole interactions.

Electron Spin-Lattice Relaxation of the Stable Tyrosine Radical D+ in Photosystem II

Photosystem II (PSII) contains a remarkably stable tyrosine radical D+, located at tyr-160 in the D2 polypeptide, which exhibits a well-known electron paramagnetic resonance (EPR) spectrum, EPR Signal IIS (1). The function of D+ in the mechanism of photosynthetic H2O-oxidation remains unclear despite evidence that it can oxidize the Mn complex, the H2O-oxidation catalyst in PSII, from the S0 state to the normally dark-stable S1 state (2). Several studies indicate that the oxidation state of the Mn complex influences the electron spin-lattice relaxation rate of D+ (3–5), perhaps via a weak dipolar coupling, as suggested by Evelo et al. (5). Hence, the relaxation properties of D+ may provide a probe for the topology of redox-active sites in the O2-evolving center (OEC) and of the magnetic properties of the Mn complex.