Donor-acceptor Coupling Research Articles

Photoinduced Electron Transfer in α‐Cyclodextrin‐Based Supramolecular Dyads: A Free‐Energy‐Dependence Study

Photoinduced electron transfer (PET) between alpha-cyclodextrin-appended pyrene (PYCD) and a few acceptor molecules was studied in aqueous solutions. The pyrene moiety in PYCD is located above the narrower rim of the alpha-CD and is fully exposed to water. The acceptors are monocyclic organic molecules and, upon dissolution in water in the presence of PYCD, a fraction of the donor-acceptor systems is present as supramolecular dyads and the remaining fraction as free molecules. Free-energy-dependence studies showed that electron transfer in the supramolecular dyads follows the Marcus equation. The donor-acceptor coupling and the reorganization energy were determined from fits of the data to the Marcus equation. The electronic coupling was found to be similar to those reported for hydrogen-bonded systems. It appears that the actual lambdaout values are somewhat lower than values calculated with the continuum model. The experimental design has also allowed, for the first time, a visual demonstration of the inverted region on the basis of the raw fluorescence lifetime data.

Excitation Energy Transfer in Ion Pairs of Polymethine Cyanine Dyes: Efficiency and Dynamics

The present work deals with singlet excitation energy transfer (EET) occurring in contact ion pairs (CIPs) of several anionic oxonol analogues (acting as EE donors) and cationic cyanines (acting as acceptors) characterized by off resonance individual transitions. Combining conductometric and spectroscopic measurements with decreasing solvent polarity, we were able to observe a progressive ion pairing leading first to solvent-separated ion pairs (SSIPs) and then to CIPs. Analysis of the absorption spectra of three selected salts (A2,C1, A2,C2, and A1,C4) in chloroform-toluene mixtures showed that the transformation of SSIP into CIP involves the appearance of a certain exciton coupling, the extent of which decreases regularly with increasing gap between the local excitation energies. Fluorescence excitation spectra showed that EET occurs in CIP, and EET efficiencies were evaluated with a procedure expressly devised for weakly emitting donors. These were between 0.2 and 0.65 for the examined ion pairs involving anions A1 and A2. The spectroscopic study was complemented by a theoretical investigation aimed at establishing the dynamic regime of the observed EET. From classical MD simulations and local full geometry optimizations, A2,C1 and A2,C2 were found to form rather stable sandwich-type CIP structures with interchromophore distances (R) of about 0.45-0.50 nm. The donor-acceptor electronic coupling was calculated in terms of Coulombic interactions between atomic transition charges. For CIP, the electronic coupling was decidedly beyond the limit of the weak coupling required for an incoherent Förster-type mechanism. Thus, we tried to arrange the EET dynamics within the theory developed by Kimura, Kakitani, and Yamato (J. Phys. Chem. B 2000, 104, 9276) for the intermediate coupling case, which provides analytical expressions of time-dependent occupation probability, EET rate, and coherency in terms of two basic quantities: the electronic coupling and a correlation time related to the Franck-Condon factor. The latter was shown to be primarily modulated by Förster's spectral overlap integral (related in turn to the excitation energy gap). Calculations were carried out for the three sample systems using three values of the electronic coupling roughly corresponding to CIP, 1.0, and 2.0 nm interchromophore distances. At the CIP distance, EET in both A2,C1 and A2,C2 was predicted to occur with a partial exciton mechanism, very short transfer times (about 10 fs), and high degree of coherence. In A1,C4 (having the largest energy gap), EET was found to occur with a hot-transfer mechanism. More or less hot-transfer dynamics appeared to be retained by all three systems at R = 1.0 nm. Fully incoherent EET appeared to become operative only at distances larger than 2.0 nm.

Donor‐Substituted 1,1,4,4‐Tetracyanobutadienes (TCBDs): New Chromophores with Efficient Intramolecular Charge‐Transfer Interactions by Atom‐Economic Synthesis.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Estimation of electronic coupling in π-stacked donor-bridge-acceptor systems: Correction of the two-state model

Comparison of donor-acceptor electronic couplings calculated within two-state and three-state models suggests that the two-state treatment can provide unreliable estimates of V(da) because of neglecting the multistate effects. We show that in most cases accurate values of the electronic coupling in a pi stack, where donor and acceptor are separated by a bridging unit, can be obtained as V(da) = (E(2)-E(1))mu(12)R(da) + (2E(3)-E(1)-E(2))2mu(13)mu(23)R(da) (2), where E(1), E(2), and E(3) are adiabatic energies of the ground, charge-transfer, and bridge states, respectively, mu(ij) is the transition dipole moments between the states i and j, and R(da) is the distance between the planes of donor and acceptor. In this expression based on the generalized Mulliken-Hush approach, the first term corresponds to the coupling derived within a two-state model, whereas the second term is the superexchange correction accounting for the bridge effect. The formula is extended to bridges consisting of several subunits. The influence of the donor-acceptor energy mismatch on the excess charge distribution, adiabatic dipole and transition moments, and electronic couplings is examined. A diagnostic is developed to determine whether the two-state approach can be applied. Based on numerical results, we showed that the superexchange correction considerably improves estimates of the donor-acceptor coupling derived within a two-state approach. In most cases when the two-state scheme fails, the formula gives reliable results which are in good agreement (within 5%) with the data of the three-state generalized Mulliken-Hush model.

Donor‐Substituted 1,1,4,4‐Tetracyanobutadienes (TCBDs): New Chromophores with Efficient Intramolecular Charge‐Transfer Interactions by Atom‐Economic Synthesis

A wide variety of monomeric and oligomeric, donor-substituted 1,1,4,4-tetracyanobutadienes (TCBDs) have been synthesized by [2+2] cycloaddition between tetracyanoethylene (TNCE) and donor-substituted alkynes, followed by electrocyclic ring opening of the initially formed cyclobutenes. Reaction yields are often nearly quantitative but can be affected by the electron-donating power and steric demands of the alkyne substituents. The intramolecular charge-transfer (CT) interactions between the donor and TCBD acceptor moieties were comprehensively investigated by X-ray crystallography, electrochemistry, UV-visible spectroscopy, and theoretical calculations. Despite the nonplanarity of the new chromophores, which have a substantial twist between the two dicyanovinyl planes, efficient intramolecular CT interactions are observed, and the crystal structures demonstrate a high quinoid character in strong donor substituents, such as N,N-dimethylanilino (DMA) rings. The maxima of the CT bands shift bathochromically upon reduction of the amount of conjugative coupling between strong donor and acceptor moieties. Each TCBD moiety undergoes two reversible, one-electron reduction steps. Thus, a tri-TCBD derivative with a 1,3,5-trisubstituted benzene core shows six reversible reduction steps within an exceptionally narrow potential range of 1.0 V. The first reduction potential E(red,1) is strongly influenced by the donor substitution: introduction of more donor moieties causes an increasingly twisted TCBD structure, a fact that results in the elevation of the LUMO level and, consequently, a more difficult first reduction. The potentials are also strongly influenced by the nature of the donor residues and the extent of donor-acceptor coupling. A careful comparison of electrochemical data and the correlation with UV-visible spectra made it possible to estimate unknown physical parameters such as the E(red,1) of unsubstituted TCBD (-0.31 V vs Fc+/Fc) as well as the maxima of highly broadened CT bands. Donor-substituted TCBDs are stable molecules and can be sublimed without decomposition. With their high third-order optical nonlinearities, as revealed in preliminary measurements, they should become interesting chromophores for ultra-thin film formation by vapor deposition techniques and have applications in opto-electronic devices.

Syntheses and Quadratic Optical Nonlinearities of Ruthenium(II) Complexes with Ethynyl-ConnectedN-Methylpyridinium Electron Acceptors

We have prepared a number of new dipolar complexes containing ethynyl or buta-1,3-diynyl units linking electron-rich {Ru(II)(NH3)5}2+, trans-{Ru(II)(NH3)4L}+ (L = pyridine or N-methylimidazole), or trans-{Ru(II)Cl(pdma)2}+ [pdma = 1,2-phenylenebis(dimethylarsine)] centers to pyridinium electron acceptors. In acetonitrile solutions at 295 K, the new complexes display unusual blue-shifting of their metal-to-ligand charge-transfer (MLCT) bands as the conjugation is extended, in a fashion similar to that of the corresponding ethenyl systems. Hyper-Rayleigh scattering (HRS) and Stark spectroscopic measurements provide direct and indirect estimates of static first hyperpolarizabilities beta0, and both the linear and nonlinear optical (NLO) properties are temperature- and medium-dependent. Thus, at 77 K in butyronitrile glasses, the MLCT bands display more normal red shifts upon conjugation extension. While the Stark-derived beta0 values generally increase as n (the number of ethynyl units) increases from 0 to 2, the HRS data show maximization at n = 1 for two of the ammine series but an increase upon moving from n = 1 to 2 for the pdma complexes. Comparisons with the analogous ethenyl chromophores show that the latter generally display larger beta0 values, whether determined via HRS or Stark data, and the inferiority of the ethynyl systems in terms of NLO response is more pronounced when n = 2. This differing behavior is attributable primarily to larger increases in the transition dipole moment mu12 (and, hence, donor-acceptor pi-electronic coupling) on elongation in the ethenyl chromophores.

Electron Spin Dynamics as a Probe of Molecular Dynamics: Temperature-Dependent Magnetic Field Effects on Charge Recombination within a Covalent Radical Ion Pair

The electron spin-spin exchange interaction, 2J, in radical pairs (RPs) is exquisitely sensitive to the details of molecular structure and can thus serve as an important probe of structural dynamics in RPs of potential interest to photonic and electronic devices. Photoinitiated ultrafast two-step charge separation produces (1)(MeOAn(+)(*)-6ANI-NI(-)(*)), where MeOAn = p-methoxyaniline, 6ANI = 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, and NI = naphthalene-1,8:4,5-bis(dicarboximide). Radical pair intersystem crossing subsequently produces (3)(MeOAn(+)(*)-6ANI-NI(-)(*)), and the total RP population decays with approximately 10 ns lifetime at 140 K, which increases to nearly 30 ns at 300 K in toluene. The activation energy observed for this process is negative and can be explained by a mechanism involving a conformational preequilibrium of the RP followed by charge recombination. Over the same temperature range, the magnetic field effect (MFE) on yield of the triplet recombination product, MeOAn-6ANI-(3)()NI, yields the magnitude of 2J, which directly monitors the superexchange electronic coupling for charge recombination. A single resonance in the MFE plot is observed at 300 K, which splits into two resonances at temperatures below 230 K, suggesting that there are two distinct groups of RP conformations at low temperature. The magnitude of 2J for the lower field resonance (10 mT) at 140 K is 5 times smaller than that of the high field resonance. At 300 K the equilibrium is shifted almost entirely to the set of conformers with the stronger electronic coupling. The motion that couples these two groups of conformations is the motion that most effectively gates the donor-acceptor electronic coupling.

Static and dynamic aspects of DNA charge transfer: a theoretical perspective

In this work, we approach the impact of dynamic and static disorder on DNA charge transfer from a theoretical and numerical perspective. Disordered or defect geometries are either realized via molecular dynamics simulations using a classical force field or by experimentally determined DNA bulge structures. We apply a chemically specific, atomically resolved extended Su-Schrieffer-Heeger model to compute the energy parameters relevant to DNA charge transfer. For both models studied here, the effective donor-acceptor couplings--and hence the charge transfer rates--significantly depend upon the geometry. Dynamic disorder leads to a correlation time in this quantity of the order of 30 fs, and the transfer rates universally exhibit a broad, yet well-defined, exponential distribution. For DNA bulges, the angle characterizing the defect controls the charge transfer efficiency. The results are discussed and extensively compared to experimental findings and other calculations.

Non-Condon theory of nonadiabatic electron transfer reactions in V-shaped donor–bridge–acceptor complexes

The rate of nonadiabatic long-distance electron transfer (ET) is derived for the direct and superexchange electronic coupling between the donor and acceptor. The model takes into account a non-Condon thermal modulation of the electronic coupling through the interaction of the system transition dipoles with the polarization fluctuations of the solvent. Going from a linear donor–bridge–acceptor complexes to a bent, V-shaped geometry lowers the system symmetry resulting in several novel properties of the ET matrix element based on the fact that permanent and transition dipoles in the system are not polarized along the direction of ET. The effective ET matrix element HET gains two zeros as a function of the donor–acceptor vertical energy gap. The positions of zeros of HET depend on the sign relations between the donor–bridge and bridge–acceptor electronic couplings and corresponding transition dipoles. The ET matrix element becomes dependent on solvent through the solvent refractive index and the inhomogeneous width of the donor–acceptor optical transition. The former factor is mainly responsible for the temperature slope of |HET|. The derivative d|HET|/dT switches its sign at the zero points of HET. The distance decay parameter of the donor–acceptor coupling is shown to vary linearly with the inhomogeneous width of the optical transition between the donor and acceptor.

Photophysics of novel donor–{saturated rigid hydrocarbon bridge}–acceptor systems exhibiting efficient excitation energy transfer

Intramolecular singlet–singlet energy transfer is reported in a series of novel donor–{saturated rigid hydrocarbon bridge}–acceptor (D–B–A) molecules containing dimethoxynaphthalene (DMN) or dimethoxybenzene (DMB) as the energy donor chromophore, and a dione group as the acceptor. In the case of the DMN donor, the efficiency of energy transfer has been studied as a function of the bridge length with the number of sigma bonds linking the donor and acceptor groups ranging from 6 to 12. Highly efficient energy transfer is observed in all the D–B–A molecules studied, and the results are discussed with reference to the role of dipole–dipole and through-bond donor–acceptor coupling mechanisms. The energy transfer process facilitates efficient photosensitized decomposition of the dione moiety. These results are compared to data published on related donor–acceptor compounds. The chemical structure of the backbone linking the donor and acceptor is shown to have a marked effect on the photophysics of the molecules.

Femtosecond Fluorescence Anisotropy Kinetics as a Signature of Ultrafast Electronic Energy Transfer in Bichromophoric Molecules

Ultrafast time resolved fluorescence anisotropy decay measurements were performed to gain insight into the energy gap dependence of donor-acceptor inter-chromophoric coupling within one supra-molecule. Three new compounds, each consisting of two semi-rigidly linked and strongly coupled chromophores, were designed and synthesized for this study. Their general structure is donor-spacer-acceptor, where "donor" is phenylamino, pyrenylamino, or benzanthronylamino moiety, and acceptor is aminobenzanthrone. While being similar structurally, the compounds differ significantly in the excitation energy difference of the two chromophores in a supra-molecule. Experimental data show an ultrafast initial fluorescence emission anisotropy decrease (within less then 1 ps) when the excited state energies of the interacting chromophores are close to each other or equal. No such fast fluorescence anisotropy dynamics is observed for a compound with a large energy gap.

Isolation of individual components in the electronic absorption spectra of polyaniline from the spectroelectrochemical data

The absorption spectra of polyaniline (PAN) films obtained by electrochemical polymerization on transparent SnO2-glass electrodes are analyzed. The spectra are recorded at steady-state potentials of 0.0 to 0.8 V (Ag/AgCl) in an aqueous HC1 solution. Individual absorption bands, isolated by the Alentsev-Fok method, include both earlier known bands and some new absorption bands. The former correspond to π-π* transitions in the benzenoid rings (300-320 nm), radical cations (∼435 nm), localized polarons (∼755 nm), and quinoid structures (∼655 nm). By comparing these results with the spectroelectrochemical data obtained earlier for electrochemically synthesized PAN in solutions of various acids and for vacuum-deposited PAN, the newly found bands are assigned to different intermolecular interactions, specifically, a donor-acceptor coupling of the quinoid moieties with counter anions (at 570 nm) and an electron exchange between highly conducting domains of the film (∼900 nm)

Green's function calculation of through-bond electronic coupling in donor–bridge–acceptor model systems

The Green's function formalism is applied for the calculation of the effective through-bond donor–acceptor coupling in model molecular systems. The calculation is performed at a Hartree–Fock (self-consistent) level, by using semiempirical AM1 and CNDO/S, and ab initio STO-3G methods. The results are compared with that obtained from the splitting of the appropriate levels, by using the Koopmans' theorem, within each one of the selected quantum chemical methods.

Perturbed ground state method for electron transfer

A novel ab initio method is presented for characterization of electron transfer (ET). The method utilizes perturbed ground state (PGS) properties of the ET systems in order to evaluate the electron donor–acceptor coupling and the donor–acceptor energy splitting. Since no excited states are involved in calculations, density functional implementation of the method provides an efficient way to include electron correlation effects for ET in large chemical systems. The PGS method is applied to two model systems and is compared with high-level ab initio results. The PGS method performs very well for the test systems. The method is more general than traditional techniques, providing both the ET coupling and the donor–acceptor energy splitting.

Ground-State, Mode-Dependent Vibronic Coupling in Some Simple, Cyanide-Bridged Transition-Metal Donor-Acceptor Complexes.

Patterns of the shifts in bridging cyanide-stretching frequencies have been examined in several fully saturated, &mgr;-cyano, bi- or trimetallic transition-metal donor-acceptor (D/A) complexes. An earlier (Watzky, M. A.; et al.Inorg. Chem. 1996, 35, 3463) inference that the bridging ligand nuclear and the D/A electronic coordinates are entangled is unequivocally demonstrated by the 32 cm(-)(l) lower frequency of nu(CN) for (NH(3))(5)Cr(CNRu(NH(3))(5))(4+) than for the cyanopentaamminechromium(III) parent. This contrasts to the 41 cm(-)(1) increase in nu(CN) upon ruthenation of (NH(3))(5)RhCN(2+). More complex behavior has been found for cis and trans trimetallic, donor-acceptor complexes. The symmetric combination of CN(-) stretching frequencies in trans-Cr(III)(MCL)(CNRu(II)(NH(3))(5))(2)(5+) complexes (MCL = a tetraazamacrocyclic ligand) shifts 100-140 cm(-)(1) to lower frequency, and the antisymmetric combination shifts less than about 30 cm(-)(1). This contrast in the shifts of the symmetric and the antisymmetric combinations of the CN stretches persists even in a trans complex with no center of symmetry. Two CN stretches have also been resolved in an analogous cis complex, and both shift to lower frequency by about 60 cm(-)(1). The net shift, summed over all the CN-stretching frequencies, is about the same for the bis-ruthenates of related dicyano complexes. A simple, symmetry-adapted perturbation theory treatment of the coupled vibrations is employed to deal with the opposing effects of the "kinematic" shifts (delta) of nu(CN) to higher frequency, expected in the absence of D/A coupling, and shifts ( f ) of nu(CN) to lower frequency that occur when D/A coupling is large. The Rh(III)- and Cr(III)-centered complexes correspond to different limits of this model: delta > f and delta < f, respectively. When referenced by means of this model to complexes with Rh(III) acceptors, the shifts in trimetallic complexes, summed over the symmetric and antisymmetric combinations of CN stretches, are about twice those of bimetallic complexes. Similarly referenced and summed over all bridging CN frequencies, the shifts of nu(CN) to lower energies are proportional to the oscillator strength of the electronic, donor-acceptor charge-transfer transition. The simplest interpretation of this correlation is that the donor-acceptor coupling in these systems is a function of the nuclear coordinates of the bridging ligand. This behavior of these complexes is semiquantitatively consistent with expectation for CN(-)-mediated vibronic (pseudo-Jahn-Teller) coupling of neighboring donors and acceptors, and the observed Ru(II)/CN(-) CT absorption parameters can be used in a simple, semiclassical vibronic model to predict shifts in nu(CN) that are in reasonable agreement with those observed.

Electron tunneling in biological molecules

Abstract

Lanthanide ions as redox probes of long-range electron transfer in proteins

Occupancy of the two calcium-binding sites of codfish parvalbumin by the redox-active probe ions Yb 3+ and Eu 3+ causes the average tryptophan (Trp) fluorescence lifetime in this protein to decrease and become non-exponential from the single exponential values found for Ca 2+ and La 3+ of 4.45 ns. These observations are interpreted in terms of an electron transfer (ET) deexcitation mechanism wherein excited singlet state Trp transfers an electron to the Ln 3+ ion, reducing it to the +2 oxidation state and producing a Trp cation radical. Back ET reestablishes the initial system. The driving forces, Δ G°, for the Eu 3+ and Yb 3+ ET systems are different, whereas the nuclear rearrangement factor, λ, and electron donor-acceptor coupling, H AB, of semiclassical ET theory should be nearly the same for both ions. This allows the λ-value to be determined from the measured rates (∼2.05 eV). Temperature dependence studies show that the rate constant for the Eu 3+ system is near the activationless maximum value. Yb 3+ and Eu 3+ are established as redox probes of long-range ET in proteins making Ln 3+-substituted calcium-binding proteins convenient model systems for studying the distance-dependence of ET.

Synergetic electron transfer in molecular electronic and photosynthetic mechanisms

The flow of energy may be enhanced in non-linear systems, when the order is increased through export of entropy, facilitated by autocatalytic (feedback) loops. This principle has been applied to derive a far-from-equilibrium ‘self-organized’ mechanism of electron transfer as well as a synergetic multi-electron transfer. A key requirement is the functioning of feedback loops within the macromolecules or their environment, which can transfer small amounts of energy required for self-organization. Possible macromolecular mechanisms are discussed, by which an active friction force can be exerted on electrons during transfer. A possible active role of intervening protein structure in contrast to the passive role discussed for conventional distant donor acceptor coupling is investigated. Evidence for self-organized electron transfer in photosynthetic structures is discussed. The last step in oxygen evolution is considered to be a synergetic multi-electron transfer reaction, with oscillating behavior being a consequence of it. Quasi-symmetric molecular geometries (reaction centers, manganese complex) are considered as mechanistically essential elements facilitating symmetry breaking mechanisms.

Intramolecular Electron-Transfer Studies as a Function of Bridge Topology: The Importance of Solvent-Mediated Donor-Acceptor Electronic Coupling

The donor-acceptor electronic coupling matrix elements, |V|, for photoinduced, intramolecular electron-transfer (ET) reactions in one linear and three C-shaped molecules have been determined from the temperature dependence of ET rate constants. The coupling matrix element in the linear molecule was found to be solvent-independent. By contrast, the coupling matrix elements in two of the three C-shaped molecules exhibit significant solvent dependence. Donor-acceptor coupling matrix elements were calculated for the linear and C-shaped molecules in the absence and presence of solvent molecules using the generalized Mulliken-Hush theory. Together, the experimental and theoretical results indicate that solvent molecules, and not the covalent bridge, mediate the electronic coupling in the C-shaped molecules. Preliminary studies of ET rate constants as a function of solvent bulk are also described.

A method to analyze multi-pathway effects on protein mediated donor-acceptor coupling interactions

Current strategies for analyzing donor-acceptor interactions involve mapping the dominant coupling pathways or families of pathways, followed by a numerical estimate of the interaction mediated by the paths. This strategy succeeds because it properly balances the through-bond versus through-space aspects of the protein electron transfer problem. Yet, existing analytical tools do not provide general predictions of how specific protein motifs might favor or disfavor electron transfer. This issue is best addressed with methods that incorporate multiple interfering pathways and that allow the coupling to be dissected systematically into contributing factors. We present such a method that probes collective effects arising from secondary and tertiary motifs. This analysis indicates when (and why) single pathway strategies succeed or fail. Applications of this new method to model secondary structures are described.