Theories of Structure–Function Relationships for Bridge‐Mediated Electron Transfer Reactions

Skourtis, Spiros S.; Beratan, David N.

doi:10.1002/9780470141656.ch8

Cited by 107 publications

(129 citation statements)

References 170 publications

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“…For the inhomogeneous system in Fig. S2, the decay of the D-to-A rate for the superexchange mechanism would be proportional to the square of the B-mediated D-A tunneling matrix element V 2 DA ∝ e −βR dA , where β ≈ (2/3.4 Å) ln [(ΔE B /V rms )] in the perturbative limit (1,2,26,27). Substituting V rms = 0.1 eV and ΔE B = 1 eV gives β ≈ 1.4 Å −1 .…”

Section: The Multisite Matching Probabilitymentioning

confidence: 99%

Biological charge transfer via flickering resonance

Zhang¹,

Liu²,

Balaeff³

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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Biological electron-transfer (ET) reactions are typically described in the framework of coherent two-state electron tunneling or multistep hopping. However, these ET reactions may involve multiple redox cofactors in van der Waals contact with each other and with vibronic broadenings on the same scale as the energy gaps among the species. In this regime, fluctuations of the molecular structures and of the medium can produce transient energy level matching among multiple electronic states. This transient degeneracy, or flickering electronic resonance among states, is found to support coherent (ballistic) charge transfer. Importantly, ET rates arising from a flickering resonance (FR) mechanism will decay exponentially with distance because the probability of energy matching multiple states is multiplicative. The distance dependence of FR transport thus mimics the exponential decay that is usually associated with electron tunneling, although FR transport involves real carrier population on the bridge and is not a tunneling phenomenon. Likely candidates for FR transport are macromolecules with ET groups in van der Waals contact: DNA, bacterial nanowires, multiheme proteins, strongly coupled porphyrin arrays, and proteins with closely packed redox-active residues. The theory developed here is used to analyze DNA charge-transfer kinetics, and we find that charge-transfer distances up to three to four bases may be accounted for with this mechanism. Thus, the observed rapid (exponential) distance dependence of DNA ET rates over distances of K K K K15 Å does not necessarily prove a tunneling mechanism.vibronic coupling | resonant tunneling pathways | superexchange | coherence | gated transport C hemical structure and, importantly, structural fluctuations determine the mechanism and kinetics of charge transfer. Redox energy fluctuations are of particular significance when transport barrier heights and the energy fluctuations are of similar magnitude. Indeed, the sensitivity of biological electrontransfer (ET) rates to conformational fluctuations and consequent (transient) delocalization is the topic of intense interest (1-3). Resonant enhancement of biological ET rates is consistent with a growing body of physical and structural data found in DNA ET through stacked nucleobases (4), extended delocalized structures of bacterial photosynthesis (including the special pair, bridging chlorophyll and pheophytin) (5), the polaronic states of oxidized porphyrin arrays up to seven porphyrin diameters in spatial extent (6), micrometer-scale bacterial nanowires (7, 8), multiheme oxidoreductases (9, 10), amino acid side chains in ribonucleotide reductase (11), engineered protein-based hopping-chains (12), and centimeter-scale charge-transport chains in filamentous bacteria (13). Here, we describe a transient or flickering resonance (FR) mechanism for ET. The FR mechanism arises when thermal fluctuations produce geometries that enable charge delocalization across the entire structure by bringing the donor (D), bridge (B), and acceptor (...

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Section: The Multisite Matching Probabilitymentioning

confidence: 99%

Biological charge transfer via flickering resonance

Zhang¹,

Liu²,

Balaeff³

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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164

263

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“…Such an IR excitation-induced elongation is expected to affect the H-bond strength and to reduce the DA coupling matrix element that determines the ET rate. 7 While the lifetime of the excited bending modes is expected to be ca. 1 ps, 12 other modes with lower frequencies will be populated via vibrational relaxation.…”

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confidence: 99%

“…13 The planarity of the bridge hydrogenbonded pair is expected to influence the DA coupling and the CS rate significantly. 7,14,15 Modulating the planarity of the G and C rings may also enhance the nonradiative relaxation of the excited state to the ground state, reducing the quantum yield of the CS reaction. 15 While the Watson-Crick structure is the dominant one, the presence of other tautomers in quantities up to 2% has been predicted, most notably the imino-enol double hydrogen atom transfer structure.…”

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confidence: 99%

“…[1][2][3][4]7 In systems with two interfering ET pathways, the excitation of a pathway-specific bridge vibration, which may induce electron-vibration energy exchange, labels the ET pathway and therefore modifies pathway interferences. [1][2][3][4]7 Affecting ET rates using mid-IR radiation 3 is generally attractive because ultrashort laser pulses offer subpicosecond perturbation, and radiation in the mid-IR is chemically innocent. In addition to inelastic tunneling, excitation of bridge vibrations can perturb elastic-tunneling kinetics.…”

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confidence: 99%

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Modulating Unimolecular Charge Transfer by Exciting Bridge Vibrations

et al. 2009

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Nonadiabatic electron transfer (ET) measures the rate of electron tunneling, often facilitated by intervening covalent and nonbonded interactions. [1][2][3][4] Bridge structure therefore influences the ET kinetics, and bridge thermal fluctuations are predicted to modulate the tunneling propensity. 4,5 Structure defines the coupling pathways, and thermal fluctuations enable the system to find configurations that enhance the interaction strength.6 An open and crucial question is whether or not bridge motion can be manipulated (driven) by an external field to control pathway interactions and ET kinetics. Here, we show that mid-IR driving of bridge vibrations produces ET kinetic slowing of a photoinduced charge separation reaction.Recent theoretical analysis indicates that ET kinetics can be changed by controlling the coherence of inelastic tunneling pathway interferences in a molecular analogue of the double-slit experiment. [1][2][3][4]7 In systems with two interfering ET pathways, the excitation of a pathway-specific bridge vibration, which may induce electron-vibration energy exchange, labels the ET pathway and therefore modifies pathway interferences. [1][2][3][4]7 Affecting ET rates using mid-IR radiation 3 is generally attractive because ultrashort laser pulses offer subpicosecond perturbation, and radiation in the mid-IR is chemically innocent. In addition to inelastic tunneling, excitation of bridge vibrations can perturb elastic-tunneling kinetics. The donor-acceptor (DA) coupling may be modulated by exciting a bridge vibrational mode without electron-phonon energy exchange. Related ideas for the control of currents in molecular wires are being addressed in the context of molecular electronics and inelastic-tunneling spectroscopy. 8 Here, we report the first real-time observation of ET rate modulation by mid-IR excitation in a donor-bridge-acceptor (DBA) ensemble. 9This ensemble consists of an anthracene-derived acceptor linked to a dimethylaniline-containing donor by guanosine-cytidine (GC) hydrogen bonding ( Figure 1A). The ET is probed in a 3-pulse experiment, performed with a sequence of UV, mid-IR, and visible pulses ( Figure 1B), each of ca. 100 fs duration. The first pulse at 400 nm creates the acceptor-localized electronic excited state (ES) that then "captures" an electron from the donor with an ET rate constant, k CS , of ca. (30 ps) -1 . 9 After a small time delay, τ, the second pulse centered at 1670 cm -1 (and ∼120 cm -1 in width) is applied, targeting vibrational modes in DBA labeled in Figure 1A. The third pulse in the visible spectral region probes the sample absorbance as a function of the probe's delay time, T ( Figure 1B).The absorbance changes in the 3-pulse measurements were calculated using the equation ∆abs ) D IR -D ) log(I/I IR ), where D IR and D are the optical densities and I IR and I are the probe signals with the IR pump on and off, respectively. Since the mid-IR pulses were chopped at half of the laser repetition rate, and the two consecutively recorded spectra were processed...

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“…To calculate the ET coupling matrix element T DA , we employ two methods; generalized Mulliken-Hush (GMH) [27] and bridge Green function (BGF) [28][29][30].…”

Section: B Et Coupling and Pathway Analysismentioning

confidence: 99%

FMO3-LCMO study of electron transfer coupling matrix element and pathway: Application to hole transfer between two tryptophans through cis- and trans-polyproline-linker systems

Kitoh-Nishioka

Ando

2016

The Journal of Chemical Physics

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The linear-combination of fragment molecular orbitals with three-body correction (FMO3-LCMO) is examined for electron transfer (ET) coupling matrix elements and ET pathway analysis, with application to hole transfer between two triptophanes bridged by cis-and trans-polyproline linker conformations. A projection to the minimal-valence-plus-core FMO space was found to give sufficient accuracy with significant reduction of computational cost while avoiding the problem of linear dependence of FMOs stemming from involvement of bond detached atoms.

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Theories of Structure–Function Relationships for Bridge‐Mediated Electron Transfer Reactions

Cited by 107 publications

References 170 publications

Biological charge transfer via flickering resonance

Biological charge transfer via flickering resonance

Modulating Unimolecular Charge Transfer by Exciting Bridge Vibrations

FMO3-LCMO study of electron transfer coupling matrix element and pathway: Application to hole transfer between two tryptophans through cis- and trans-polyproline-linker systems

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