1,721,310 research outputs found
Irreversibility in redox molecular conduction: single versus double metal-molecule interfaces
No full textIn this work we analyze the onset and manifestation of irreversibility phenomena in the charge transport at single and double metal-redox molecule interfaces, with special emphasis on the role of the nuclear system reorganization energy in causing the distortion of cyclic voltammograms in the first case and the occurrence of hysteresis phenomena in the second case. Under physical conditions for which two states of the molecular system come into play, effects of irreversibility increase with the reorganization energy at a single interface, while an opposite trend is seen in the conduction through a molecular junction. The apparent contradiction between these two behaviors, which was raised in a previous work (Migliore, A.; Nitzan, A.; J. Am. Chem. Soc. 2013, 135, 9420-32) is here resolved through detailed investigation of the connections between molecule reorganization energy, bias-dependent population of the molecular redox site(s), and threshold voltage scan rate for the onset of irreversible behavior. Moreover, our investigation of the effects of the reorganization energy on the voltammogram peaks proposes a strategy for extracting the value of the reorganization energy of the molecular system from the experimental behavior
How to Extract Quantitative Information on Electronic Transitions from the Density Functional Theory "Black Box"
No full textElectronic couplings and vertical excitation energies are crucial determinants of charge and excitation energy transfer rates in a broad variety of processes ranging from biological charge transfer to charge transport through inorganic materials, from molecular sensing to intracellular signaling. Density Functional Theory (DFT) is generally used to calculate these critical parameters, but the quality of the results is unpredictable because of the semiempirical nature of the available DFT approaches. This study identifies a small set of fundamental rules that enables accurate DFT computation of electronic couplings and vertical excitation energies in molecular complexes and materials. These rules are applied to predict efficient DFT approaches to coupling calculations. The result is an easy-to-use guide for reliable DFT descriptions of electronic transitions
Nonorthogonality Problem and Effective Electronic Coupling Calculation: Application to Charge Transfer in π-Stacks Relevant to Biochemistry and Molecular Electronics
No full textA recently proposed method for the calculation of the effective electronic coupling (or charge-transfer integral) in a two-state system is discussed and related to other methods in the literature. The theoretical expression of the coupling is exact within the two-state model and applies to the general case where the charge transfer (CT) process involves nonorthogonal initial and final diabatic (localized) states. In this work, it is shown how this effective electronic coupling is also the one to be used in a suitable extension of Rabis formula to the nonorthogonal representation of two-state dynamical problems. The formula for the transfer integral is inspected in the regime of long-range CT and applied to CT reactions in redox molecular systems of interest to biochemistry and/or to molecular electronics: the guanine-thymine stack from regular B-DNA, the polyaromatic perylenediimide stack, and the quinol-semiquinone couple. The calculations are performed within the framework of the Density Functional Theory (DFT), using hybrid exchange-correlation (XC) density functionals, which also allowed investigation of the appropriateness of such hybrid-DFT methods for computing electronic couplings. The use of the recently developed M06-2X and M06-HF density functionals in appropriate ways is supported by the results of this work. © 2011 American Chemical Society
Full-electron calculation of effective electronic couplings and excitation energies of charge transfer states: Application to hole transfer in DNA π-stacks
No full textIn this work I develop and apply a theoretical method for calculating effective electronic couplings (or transfer integrals) between redox sites involved in hole or electron transfer reactions. The resulting methodology is a refinement and a generalization of a recently developed approach for transfer integral evaluation. In fact, it holds for any overlap between the charge-localized states used to represent charge transfer (CT) processes in the two-state model. The presented theoretical and computational analyses show that the prototype approach is recovered for sufficiently small overlaps. The method does not involve any empirical parameter. It allows a complete multielectron description, therefore including electronic relaxation effects. Furthermore, its theoretical formulation holds at any value of the given reaction coordinate and yields a formula for the evaluation of the vertical excitation energy (i.e., the energy difference between the adiabatic ground and first-excited electronic states) that rests on the same physical quantities used in transfer integral calculation. In this paper the theoretical approach is applied to CT in B-DNA base dimers within the framework of Density Functional Theory (DFT), although it can be implemented in other computational schemes. The results of this work, as compared with previous Hartree-Fock (HF) and post-HF evaluations, support the applicability of the current implementation of the method to larger π -stacked arrays, where post-HF approaches are computationally unfeasible. © 2009 American Institute of Physics
Irreversibility and Hysteresis in Redox Molecular Conduction Junctions
No full textIn this work we present and discuss theoretical models of redox molecular junctions that account for recent observations of nonlinear charge transport phenomena, such as hysteresis and hysteretic negative differential resistance (NDR). A defining feature in such models is the involvement of at least two conduction channels - a slow channel that determines transitions between charge states of the bridge and a fast channel that dominates its conduction. Using Marcus' theory of heterogeneous electron transfer (ET) at metal-molecule interfaces we identify and describe different regimes of nonlinear conduction through redox molecular bridges, where the transferring charge can be highly localized around the redox moiety. This localization and its stabilization by polarization of the surrounding medium and/or conformational changes can lead to decoupling of the current response dynamics from the time scale of the voltage sweep (that is, the current does not adiabatically follow the voltage), hence to the appearance of memory (thermodynamic irreversibility) in this response that is manifested by hysteresis in current-voltage cycles. In standard voltammetry such irreversibility leads to a relative shift of the current peaks along the forward and backward voltage sweeps. The common origin of these behaviors is pointed out, and expressions of the threshold voltage sweep rates are provided. In addition, the theory is extended (a) to analyze the different ways by which such phenomena are manifested in single sweep cycles and in ensemble averages of such cycles and (b) to examine quantum effects in the fast transport channel. © 2013 American Chemical Society
On the evaluation of the Marcus-Hush-Chidsey integral
No full textThe electrochemical rate constant obtained from the Marcus-Hush theory of heterogeneous electron transfer is given as a relatively complex integral. Recently, two apparently different expressions of this rate constant in the form of a series of analytical functions appeared in the literature. We demonstrate here the equivalence of these expressions and discuss their different approximations, resulting from the two distinct analytical derivations, which have implications in the practical calculation of electron transfer rate constants at electrode surfaces. © 2012 Elsevier B.V. All rights reserved
Nonlinear Charge Transport in Redox Molecular Junctions: A Marcus Perspective
No full textRedox molecular junctions are molecular conduction junctions that involve more than one oxidation state of the molecular bridge. This property is derived from the ability of the molecule to transiently localize transmitting electrons, implying relatively weak molecule-leads coupling and, in many cases, the validity of the Marcus theory of electron transfer. Here we study the implications of this property on the nonlinear transport properties of such junctions. We obtain an analytical solution of the integral equations that describe molecular conduction in the Marcus kinetic regime and use it in different physical limits to predict some important features of nonlinear transport in metal-molecule-metal junctions. In particular, conduction, rectification, and negative differential resistance can be obtained in different regimes of interplay between two different conduction channels associated with different localization properties of the excess molecular charge, without specific assumptions about the electronic structure of the molecular bridge. The predicted behaviors show temperature dependences typically observed in the experiment. The validity of the proposed model and ways to test its predictions and implement the implied control strategies are discussed. © 2011 American Chemical Society
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Hermitian and Unitary Almost-Companion Matrices of Polynomials on Demand
Full text linkWe introduce the concept of the almost-companion matrix (ACM) by relaxing the non-derogatory property of the standard companion matrix (CM). That is, we define an ACM as a matrix whose characteristic polynomial coincides with a given monic and generally complex polynomial. The greater flexibility inherent in the ACM concept, compared to CM, allows the construction of ACMs that have convenient matrix structures satisfying desired additional conditions, compatibly with specific properties of the polynomial coefficients. We demonstrate the construction of Hermitian and unitary ACMs starting from appropriate third-degree polynomials, with implications for their use in physical-mathematical problems, such as the parameterization of the Hamiltonian, density, or evolution matrix of a qutrit. We show that the ACM provides a means of identifying the properties of a given polynomial and finding its roots. For example, we describe the ACM-based solution of cubic complex algebraic equations without resorting to the use of the Cardano-Dal Ferro formulas. We also show the necessary and sufficient conditions on the coefficients of a polynomial for it to represent the characteristic polynomial of a unitary ACM. The presented approach can be generalized to complex polynomials of higher degrees
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