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The electron-transfer rate constant is measured in two U-shaped donor-bridge-acceptor molecules over a wide range of temperature in acetonitrile and N-methylacetamide (NMA). The electron-transfer rate at high temperature can be well described by a nonadiabatic model of the reaction, but at low temperatures the rate in NMA becomes controlled by the solvent. The results are discussed in terms of theoretical models for the change in reaction mechanism and its dependence on the solute-solvent frictional coupling. [ABSTRACT FROM AUTHOR]

The electronic coupling matrix elements attending the charge separation reactions of a C-shaped molecule containing an excited pyrene as the electron acceptor and a dimethylaniline as the donor are determined in aromatic, ether, and ester solvents. Band shape analyses of the charge-transfer emission spectra (CT→S[SUB0]) provide values of the reaction free energy the solvent reorganization energy, and the vibrational reorganization energy in each solvent. The free energy for charge separation in benzene and toluene solvents is independenttv determined from the excited state equilibrium established between the locally excited pyrene s[SUB1] state and the charge-transfer state. Analyses of the charge separation kinetics using the spectroscopically determined reorganization energies and reaction free energies indicate that the electronic coupling is solvent independent, despite the presence of a cleft between the donor and acceptor. Hence, solvent molecules are not involved in the coupling pathway. The orientations of the donor and acceptor units, relative to the spacer, are not rigidly constrained, and their torsional motions decrease solvent access to the cleft. Generalized Mulliken-Hush calculations show that rotation of the pyrene group about the bond connecting it to the spacer greatly modulates the magnitude of through-space coupling between the S[SUB1] and CT states. The relationship between the torsional dynamics and the electron-transfer dynamics is discussed. [ABSTRACT FROM AUTHOR]

This work demonstrates that chiral imprinted CdSe quantum dots (QDs) can act as spin selective filters for charge transport. The spin filtering properties of chiral nanoparticles were investigated by magnetic conductive-probe atomic force microscopy (mCP-AFM) measurements and magnetoresistance measurements. The mCP-AFM measurements show that the chirality of the quantum dots and the magnetic orientation of the tip affect the current-voltage curves. Similarly, magnetoresistance measurements demonstrate that the electrical transport through films of chiral quantum dots correlates with the chiroptical properties of the QD. The spin filtering properties of chiral quantum dots may prove useful in future applications, for example, photovoltaics, spintronics, and other spin-driven devices. [ABSTRACT FROM AUTHOR]

Single-stepnonadiabatic electron tunneling models are widely usedto analyze electrochemical rates through self-assembled monolayerfilms (SAMs). For some systems, such as nucleic acids, long-rangecharge transfer can occur in a “hopping” regime thatinvolves multiple charge transfer events and intermediate states.This report describes a three-step kinetic scheme to model chargetransfer in this regime. Some of the features of the three-step modelare probed experimentally by changing the chemical composition ofthe SAM. This work uses the three-step model and a temperature dependenceof the charge transfer rate to extract the charge injection barrierfor a SAM composed of a 10-mer peptide nucleic acid that operatesin the hopping regime. [ABSTRACT FROM AUTHOR]

Charge transfer (CT) properties are compared between peptide nucleic acid structures with an aminoethylglycine backbone (aeg-PNA) and those with a y-methylated backbone (γ-PNA). The common aeg-PNA is an achiral molecule with a flexible structure, whereas γ-PNA is a chiral molecule with a significantly more rigid structure than aeg-PNA. Electrochemical measurements show that the CT rate constant through an aeg-PNA bridging unit is twice the CT rate constant through a γ-PNA bridging unit. Theoretical calculations of PNA electronic properties, which are based on a molecular dynamics structural ensemble, reveal that the difference in the CT rate constant results from the difference in the extent of backbone fluctuations of aeg- and γ-PNA. In particular, fluctuations of the backbone affect the local electric field that broadens the energy levels of the PNA nucleobases. The greater flexibility of the aeg-PNA gives rise to more broadening, and a more frequent appearance of high- CT rate conformations than in γ-PNA. [ABSTRACT FROM AUTHOR]

This work explores the sensitization of luminescent lanthanide Tb3+ and Eu3+ cations by the electronic structure of zinc sulfide (ZnS) semiconductor nanopartides. Excitation spectra collected while monitoring the lanthanide emission bands reveal that the ZnS nanopartides act as an antenna for the sensitization of Tb3+ and Eu3+. The mechanism of lanthanide ion luminescence sensitization is rationalized in terms of an energy and charge transfer between trap sites and is based on a semiemnirical model, proposed by Dorenbos and co-workers (Dorenbos, P. J. Phys.: Condens. Matter 2003, 15, 8417-8434; J. Lumin. 2004, ' 108, 301-305; J. Lumin. 2005, 111, 89-104. Dorenbos, P., van der Kolk, E. Appl Phys. Lett. 2006, 89, 061122-1-061122-3; Opt. Mater. 2008, 30, 1052-10S7. Dorenbos, P. J. Alloys Compd. 2009, 488, 568-573; references 1-6.) to describe the energy level scheme. This model implies that the mechanisms of luminescence sensitization of Tb3+ and Eu3+ in ZnS nanopartides are different; namely, Tb3+acts as a hole trap, whereas Eu3+ acts as an electron trap. Further testing of this model is made by extending the studies from ZnS nanopartides to other II-VI semiconductor materials; namely, CdSe, CdS, and ZnSe. [ABSTRACT FROM AUTHOR]

We present evidence for a near-resonant mechanism of charge transfer in short peptide nucleic acid (PNA) duplexes obtained through electrochemical, STM break junction (STM-BJ), and computational studies. A seven base pair (7-bp) PNA duplex with the sequence (TA)3-(XY)-(TA)3 was studied, in which XV is a complementary nucleobase pair. The experiments showed that the heterogeneous charge transfer rate constant (k0) and the single-molecule conductance (σ) correlate with the oxidation potential of the purine base in the XV base pair. The electrochemical measurements showed that the enhancement of k0 is independent, within experimental error, of which of the two PNA strands contains the purine base of the XV base pair. 7-bp PNA duplexes with one or two GC base pairs had similar measured k0 and conductance values. While a simple superexchange model, previously used to rationalize charge transfer in single stranded PNA (Paul et al. J. Am. Chem. Soc. 2009, 131, 6498-6507), describes some of the experimental observations, the model does not explain the absence of an enhancement in the experimental k0 and a upon increasing the G content in the duplexes from one to two. Moreover, the superexchange model is not consistent with other studies (Paul et al. J. Phys. Chem. B 2010, 114, 14140), that showed a hopping charge transport mechanism is likely important for PNA duplexes longer than seven base pairs. A quantitative computational analysis shows that a near-resonant charge transfer regime, wherein a mix of superexchange and hopping mechanisms are expected to coexist, can rationalize all of the experimental results. [ABSTRACT FROM AUTHOR]

Self-assembled monolayers of single-stranded (ss) peptide nudeic acids (PNAs) containing seven nucleotides (TTTXTTT), a C-terminus cysteine, and an N-terminus ferrocene redox group were formed on gold electrodes. The PNA monomer group (X) was selected to be either cytosine (C), thymine (T), adenine (A), guanine (G), or a methyl group (Bk). The charge transfer rate through the oligonucleotides was found to correlate with the oxidation potential of X. Kinetic measurements and computational studies of the ss- PNA fragments show that a nucleobase mediated charge transport mechanism in the deep tunneling superexchange regime can explain the observed dependence of the kinetics of charge transfer on the PNA sequence. Theoretical analysis suggests that the charge transport is dominantly hole-mediated and takes place through the filled bndge orbitals. The strongest contribution to conductance comes from the highest filled orbitals (HOMO, HOMO-1, and HOMO-2) of indMdual bases, with a rapid drop off in contributions from lower lying filled orbitals. Our studies further suggest that the linear correlation observed between the experimental charge transfer rates and the oxidation potential of base X arises from weak average interbase couplings and similar stacking geometries for the four TTTXTTT systems. [ABSTRACT FROM AUTHOR]

U-shaped donor–bridge–acceptor molecules with different electronic couplings have been investigated as a function of temperature in solvents with slow polarization relaxation, in particular, N-methylacetamide (NMA) and N-methylpropionamide (NMP). At high temperature, the electron-transfer rate is well described by a nonadiabatic model; however, the rate at low temperature is controlled by the solvent friction. The change of the electron-transfer mechanism is discussed and compared with theoretical models. [ABSTRACT FROM AUTHOR]

A correction is presented to the article "Effect of Backbone Flexibility on Charge Transfer Rates in Peptide Nucleic Acid Duplexes," by Emil Wierzbinski and team, which was published in volume 134, 2012.