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We advance a theory for the effects of bridge configurational fluctuations on the electronic coupling for electron transfer reactions in donor-bridge-acceptor systems. The theory of radiationless transitions was applied for activationless electron transfer, where the nuclear Franck–Condon constraints are minimized, with the initial vibronic state interacting directly with the final vibronic manifold, without the need for thermal activation. Invoking the assumption of energy-independent coupling, the time-dependent initial state population probability was analyzed in terms of a cumulant expansion. Two limiting situations were distinguished, i.e. the fast configurational fluctuation limit, where the electron transfer rate is given in terms of the configurational average of me squared electronic coupling, and the slow configurational fluctuation limit, where the dynamics is determined by a configurational averaging over a static distribution of electron transfer probability densities. The correlation times for configurational fluctuations of the electronic coupling will be obtained from the analysis of molecular dynamics, in conjunction with quantum mechanical calculations of the electronic coupling, to establish the appropriate limit for electron transfer dynamics.

We explore the relationship between the electronic-nuclear level structure, the electronic couplings, and the dynamics of hole hopping transport in DNA. We utilized the electronic coupling matrix elements for hole transfer between nearest-neighbor nucleobases in DNA [Voityuk, A. A.; Jortner, J.; Bixon, M.; Rösch, N. J. Chem. Phys. 2001, 114, 5614] to evaluate intrastrand and interstrand superexchange electronic couplings, which determine hole hopping rates within the framework of a semiempirical quantum mechanical-kinetic model. Calculations of the exponential distance (R) dependence of the superexchange mediated intrastrand electronic couplings |Vsuper|² ∝ exp(−βR) between guanines (G) in “short” G⁺(T−A)nG (n ≲ 3) duplexes result in β = 0.8−0.9 Å^-1. We interpret the experimental data on time-resolved hole transport in the presence of a site-specifically bound methyl transferase mutant in DNA [Wagenknecht, H.-A.; Rajski, S. R.; Pascally, M.; Stemp, E. D. A.; Barton, J. K. J. Am. Chem. Soc. 2001, 123, 4400] in terms of composite sequential, interstrand and intrastrand superexchange mediated, and direct interstrand hole hopping. This mechanism accounts for the rate determining step, for the weak duplex size dependence of the rate, and for the long-range charge transport induced by interstrand superexchange via short (T−A) bridges, containing a single mediating nucleobase. For hole transfer via longer (T−A)n (n ≳ 3) bridges, the superexchange mechanism is replaced by the parallel mechanism of thermally induced hole hopping (TIH) via long (A)n chains. A kinetic analysis of the experimental data for hole transport through seven GG pairs separated by (T−A)n (n = 2−5) bridges across the 3‘−5‘ strand of the DNA duplex [Sartor, V.; Boone, E.; Schuster, G. B. J. Phys. Chem. B, 2001, 105, 11057] reveals that the superexchange−TIH crossover occurs at n = nx = 3. The explorations of the range of applicability and the breakdown of the superexchange mechanism in DNA lay the foundations for the scrutiny of the universality and system specificity of this mechanism in large-scale chemical and biophysical systems.

In this paper we present a self-consistent kinetic-quantum mechanical analysis of chemical yield data for hole trapping/detrapping in G⁺(T−A)mGGG duplexes (with free energy gaps Δt) and for hole hopping/trapping/detrapping in G⁺[(T)mG]n(T)mGGG duplexes of DNA. Bridge specificity of hole trapping/detrapping by GGG traps was specified by superexchange electronic contributions, inferred from electronic coupling matrix elements between nearest-neighbor nucleobases and semiempirical energy gaps, and energetic contributions, which determine the nuclear Franck−Condon factors. Unistep hole-trapping yields are accounted for by a weak bridge length dependence for short (N = 1, 2) bridges, due to detrapping. Marked bridge specificity is manifested for short (N = 1, 2) bridges, being distinct for (T)N and for [(A)m+1(T)m‘]n (m, m‘ ≥ 0 and N = n(m + m‘ + 1)) bridges. For long (N > 2) bridges an exponential bridge size dependence of the trapping yields prevails. Multistep hole transport results in different reaction rates of G⁺ (rate kd) and of (GGG)⁺ (rate kdt) with water, i.e., kd/kdt = 1.6, which, in conjunction with the unistep trapping/detrapping data, results in the free energy gaps for hole trapping of Δt = 0.096 eV in the G⁺(T)NGGG duplexes and of Δt = 0.062 eV in the G⁺[(A)m+1(T)m‘]nGGG duplexes.

We calculated electronic matrix elements for hole transfer between adjacent nucleobases in DNA. Calculations of the matrix elements for intrastrand and interstrand transfer were performed at the Hartree−Fock level employing the 6-31G* and 6-311G** basis sets. The matrix elements for intrastrand hole transfer, for which a wealth of experimental solution data is available, are almost independent of the basis set and exhibit an exponential interbase distance dependence, sensitivity to the donor−acceptor geometry, and dependence on 5‘ → 3‘ direction base sequence. The calculated intrastrand hole transfer matrix elements between adjacent thymines, v+(T,T) = 0.16 eV, is in good agreement with the experimental estimate, v+(T,T) = 0.18 eV, inferred from hole hopping in G⁺(T)mGGG (m = 1−3). The features of the nucleobase bridge specificity for superexchange-induced hole hopping between guanines in G⁺XY...G (X,Y = T or A) were elucidated, with the prediction of enhanced efficiency of thymine relative to adenine as mediator. Information on superexchange-mediated intrastrand and direct interstrand hole hopping between guanine bases was also inferred. Our results for interstrand, adjacent G⁺G coupling predict the existence of zigzagging pathways for hole hopping, in line with experiment.

Two elements of energetic control of charge migration in DNA involve the donor−bridge and the intrabridge energetics. These were applied for hole (positive ion) hopping transport via the guanines (G) (i.e., the nucleobase with the lowest oxidation potential) along the strand G⁺(T)mG(T)mG...G(T)pGGG (m = 1−3, p = 1−4) of the duplex (containing N G bases), where hole trapping occurs via the GGG triple unit. The individual hopping rates and the trapping rate are mediated by off-resonance superexchange coupling with the thymine (T) bases. The size dependence of the chemical yield ratios reveals a crossover from an algebraic to an exponential asymptotic N dependence. From the asymptotic relation for the yield we infer that maximal distances for hole hopping are 70, 175, and 380 Å for the TTT, TT, and T bridges, respectively, which specify the initiation of chemistry over a large distance of several hundreds of angstroms in DNA. Time-resolved data serve as fingerprints for the diffusive−reactive processes of hole hopping. Finally, we examine the parallel superexchange&sbd;thermally induced hopping in a system characterized by a positive donor−bridge energy gap.

We explore the interrelationship between (i) the kinetic data for the primary electron transfer (ET) from the excited singlet state (1P*) of the bacteriochlorophyll dimer (P) to bacteriopheophytin (H) in the reaction centers (RCs) of purple photosynthetic bacteria and (ii) the energetics and dynamics of the primary radical pair, P+H, i.e., its singlet‐triplet splitting rate, J, and triplet recombination rate, kT. We present a critical scrutiny of the ET mechanisms with regard to the role of the accessory bacteriochlorophyll (B). On the basis of the very weak temperature dependence of J, which sets a lower limit to the energy of (hypothetic) transient short‐lived ionic intermediate states, we argue against the applicability of sequential ET mechanisms involving P+B−H or P+BH−. The nonadiabatic/adiabatic ET mechanism is fraught with difficulties pertaining to a very low medium reorganization energy for the nonadiabatic crossing and an exceedingly high rate of the adiabatic crossing, together with a predicted order‐of‐magnitude enhancement of the low‐temperature (77 K) fluorescence quantum yield in high electric fields (5–10 mV/Å) which seem not to be borne out by experiment We propose that primary ET occurs via the unistep superexchange 1P*BH r̊ P+H−, which is mediated by electronic coupling with P+B−H. The analysis of the competition between superexchange and thermally activated ET resulted in large values of the intermolecular electronic couplings at the equilibrium nuclear configuration of 1P*BH. We suggest that the structural relaxation of one of the prosthetic groups (H) accompanying the primary ET results in the decrease of the intermolecular electronic coupling at the equilibrium configuration of P+BH−. It was established that the superexchange‐predicted kinetic data are consistent with the magnetic data for the radical pair. The small value of Jfor P+BH−originates from the cumulative effect of: (i) an essential, model‐independent cancellation between the triplet contribution, which is related to kTby a self‐consistency relationship, and the singlet contribution; (ii) the effect of configurational relaxation on the singlet contribution. Our analysis supports the superexchange mechanism for primary ET in bacterial photosynthetic RCs.

Energy storage and disposal by electron transfer in molecular and supermolecular systems may provide a conceptual and technical basis for the construction of molecular electronic devices, e.g., molecular wires and switches. We consider structural, intramolecular, medium energetic and medium dynamic control of electron transfer in synthetic and neutral supermolecules. Optimization principles are formulated for ultrafast, highly efficient and stable charge separation.

We analyse the superexchange model for the primary charge separation from the electronically excited singlet state (1P*) of the bacteriochlorophyll dimer (P) to the bacteriopheophytin (H) across the A branch of the bacterial photosynthetic reaction centers, which is mediated by the accessory bacteriochlorophyll (B). The dominant contribution to the superexchange electronic interaction between the initial 1P*BH and the final P+BH−states originates from the mixing with the mediating electronic state P+P−H, the energy of which is above 1P*. The superexchange electronic interaction is V=VPBVBH/δE, where VPBand VBHare the electronic couplings of 1P*BH with P+B−H and P+B−H with P+BH−, respectively, while δEis the vertical energy difference. The nonadiabatic electron-transfer rate is proportional to V2F, where Fis the nuclear Franck-Condon factor, which is determined by the (free) energy gap ΔG=−2000 cm−1, the medium reorganization energy λ (λ^lt;2500 cm−1) and the medium characteristic frequency ω≈100 cm−1. Indirect information on the constituents of the effective electronic coupling V≈25 cm−1was inferred from the ration |VBH/VPB| calculated from the intermolecular overlap approximation in conjunction with an activated sequential channel and the utilization of kinetic constraints on the dynamics of the primary electron transfer, which result in VPB≥60 cm−1, VBH≥360 cm−1and δE≥1100 cm−1. We discuss several physical phenomena and observables, i.e., electric field effects on the prompt fluorescence, the unidirectionality of charge separation across the A branch and magnetic interactions in the primary radial pair in the framework of the superexchange mechanism. The electric field (∈) dependence of the fluorescence quantum yield (Yf(∈)) for isotropic samples at 75 K predicts Yf(∈)=5 mV/Å)/Yf(0)=1.39 and Yf(∈=9 mV/Å)/Yf(0)=3.5. The fluorescence polarization data at constant field (Lock-hart, D.J., Goldstein R.F. and Boxer, S.G. (1988) J. Chem. Phys. 89, 1408–1415) can be well accounted for in terms of the energetic parameters λ=1600 cm−1and ΔG=−2000 cm−1together with the value ψ=61ofor the angle between the dipole P+H−and the transition moment of P. The unidirectionality of the charge separation across the A branch originates predominantly from structural symmetry breaking, which affects the electronic coupling, while the contribution of the nuclear contribution has been shown to be small. The predicted ratio of the electronic transfer rates k(A)/k(B)=82(+190; −70) at T=80 K is consistent with the recent experimental result k(A)/k(B)≥25 at this temperature. Finally we examined magnetic interactions of the primary P+H−radical pair, establishing the interrelationship between the singlet energy shifts and the triplet energy shift with the primary electron transfer rate, k, and the triplet recombination rate kTwhereupon the singlet-triplet splitting of P+H−is J=αk−βkTwhere the coefficients α and β depend on energetic parametes and Franck-Condon factors. The estimate of Jwithin the superexchange mechanism rests on the incorporation of an assumed configurational relaxation and essential cancellation effects.

The role of tryptophan (W) in the quinone reduction in the reaction centers of R. viridisand Rb. sphaeroidesis studied by a quantum chemical calculation of the electronic coupling matrix elements within the bacteriopheophytin-W-quinone subsystem, which is based on the X-ray structural data. The results favor the superexchange mechanism via W over the direct coupling. Uncertainties in the available X-ray coordinates and structural relaxation accompanying the formation of the P +H −state result in large changes in the relative contributions of the electronic couplings due to electron and hole superexchange. Aromatic amino acid residues may serve as essential functional components in electron transfer.

The role of tryptophan (W) in the quinone reduction in the reaction centers of R. viridisand Rb. sphaeroidesis studied by a quantum chemical calculation of the electronic coupling matrix elements within the bacteriopheophytin‐W‐quinone subsystem, which is based on the X‐ray structural data. The results favor the superexchange mechanism via W over the direct coupling. Uncertainties in the available X‐ray coordinates and structural relaxation accompanying the formation of the P+H−state result in large changes in the relative contributions of the electronic couplings due to electron and hole superexchange. Aromatic amino acid residues may serve as essential functional components in electron transfer.

This paper surveys the current ‘state-of-art’ of the theoretical understanding of electron transfer dynamics in donor-acceptor systems, which provide the conceptual and technical basis for solar energy conversion via optical and optoelectronic molecular devices and for the primary charge separation in photo-synthesis.

Radiationless transitions in isolated molecules are examined from the point of view of the breakdown of the Born‐Oppenheimer approximation. The intramolecular non‐radiative decay is considered in various limits and the radiative decay of coherently excited molecular levels is analyzed.