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This review is an effort in putting together the latest results about room-temperature magnetoresistive (MR) effects in nanoscale/single-molecule electronic devices consisting of one (few) molecule(s) placed in electrical contact between two nanoscale electrodes. Molecules represent powerful building blocks for developing state-of-the-art MR devices, as they bring long spin relaxation timescales, low cost and high tunability of their electrical and magnetic properties via chemical modifications. The capability to control at room temperature and under bespoke electrodes’ magnetization the MR response of a single-molecule (SM) device has been a longstanding quest. Such SM platforms could serve as fundamental tools to understand what the main mechanistic ingredients of MR effects in a molecular device are, leading to their use as building-blocks for miniaturization in spintronic applications. The work carried out so far in this field has identified two key components directly involved in the MR response of a single(few)-molecule(s) device: (i) The molecule|electrode spinterface, defining the interplay between interfacial electrostatics and spin density, has been proven to play a fundamental role in the interpretation of the observed single-molecule junction's MR effects, which is governed by the electrode material and the electrode-molecule chemistry. (ii) Two aspects of the molecular structure have been demonstrated to be involved in the spin-dependent conduction mechanism: (1) the presence of paramagnetic metal centres in the molecular structure and how their orbitals bearing the unpaired electrons couple with the device electrodes, and (2), the degree of chirality within the molecular wire. This contribution will focus on the above points (i-ii) by making use of specific examples in the literature.

International audience; Three Pd(II) phthalocyanine-carotenoid dyads featuring chromophores linked by amide bonds were prepared in order to investigate the rate of triplet-triplet (T-T) energy transfer from the tetrapyrrole to the covalently attached carotenoid as a function of the number of conjugated double bonds in the carotenoid. Carotenoids having 9, 10 and 11 conjugated double bonds were studied. Transient absorption measurements show that intersystem crossing in the Pd(II) phthalocyanine takes place in 10 ps in each case and that T-T energy transfer occurs in 126, 81 and 132 ps in the dyads bearing 9, 10 and 11 double bond carotenoids, respectively. To identify the origin of this variation in T-T energy transfer rates, density functional theory (DFT) was used to calculate the T-T electronic coupling in the three dyads. According to the calculations, the primary reason for the observed T-T energy transfer trend is larger T-T electronic coupling between the tetrapyrrole and the 10-double bond carotenoid. A methyl group adjacent to the amide linker that connects the Pd(II) phthalocyanine and the carotenoid in the 9 and 11-double bond carotenoids is absent in the 10-double bond carotenoid, and this difference alters its electronic structure to increase the coupling.

We describe and evaluate a method for computationally predicting reduction potentials of a diverse group of organic molecules by linearly correlating calculated lowest unoccupied molecular orbital energies with ground state reduction potentials measured in acetonitrile. The approach is shown to provide a unique combination of extreme computational simplicity and excellent accuracy across a diverse range of organic structures and a wide window of reduction potentials. A disparate set of molecules (74 compounds belonging to six distinct structural families, comprised of molecules containing C, H, N, O, F, Cl, and Br, with functional groups including esters, ketones, halides, nitriles, quinones, alkenes, arenes, heteroarenes, and pyridinium and higher benzologs, all containing conjugated pi systems, spanning a 3.5-V range of reduction potentials) was used to build the correlations. This methodology was found to be computationally inexpensive compared with other approaches and to permit the useful prediction of reduction potentials of additional molecules of diverse structural types not included in the families used to determine the correlation parameters. The effects of varying the basis set used in the B3LYP electronic structure calculations and including solvent (compared with calculations in gas phase) were also examined. It was found that the inclusion of a continuum solvent model in the calculations was required for accurate results, particularly when including cationic species in the correlations (although when only neutral molecules were examined, reasonable results could even be obtained in vacuo). Several of the best correlations were used to predict the reduction potentials of seven much larger and structurally diverse chromophores that were not included in the correlation data set. Strong correlations (r 2 values>0.99) with very good predictive abilities (root mean square deviation

Comparative theoretical and experimental investigations are reported into chemically induced magnetism in atomically-precise, ligand-stabilized gold clusters Au25 , Au38 and Au55 . The results indicate that [Au25 (PPh3 )10 (SC12 H25 )5 Cl2 ](2+) and Au38 (SC12 H25 )24 are diamagnetic, Au25 (SC2 H4 Ph)18 is paramagnetic, and Au55 (PPh3 )12 Cl6 , is ferromagnetic at room temperature. Understanding the magnetic properties resulting from quantum size effects in such atomically precise gold clusters could lead to new fundamental discoveries and applications.

The role of quantum confinement, size, and solvent effects on the surface enhanced Raman spectra of biologically important molecules absorbed on semiconducting titanium dioxide (TiO2) nanoparticles is investigated using density functional calculations. The results obtained for both the gas phase and solvated systems indicate significant changes in the electronic structure and the Raman spectra of molecules like formic acid and dopamine, when they are adsorbed on small TiO2 nanoparticles. A number of distinctive features that are determined by the formation of a charge-transfer complex at the nanoparticle-molecule interface can be noted in the Raman spectra. Both the spectra and the electronic properties are strongly size dependent and are also sensitive to the presence of the solvent and the nature of adsorbate interaction. Although these calculations reinforce recent experimental findings on the role of quantum confinement, they also pose new questions about the extension of collective effects and the effect of pH and other environmental variables. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

We extend to second order in the field intensity a previously obtained first-order expression for the conductance of a tight-binding molecular wire, bridging two electrodes, illuminated by a weak laser. The higher-order term brings in corrections due to two-photon processes that, for low intensities and strong molecule–electrode contacts, are several orders of magnitude smaller than the first-order leading terms. For other ranges of parameters, the second-order correction may be significant mostly due to processes involving the absorption or emission of two photons. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

We review the theory of transport across molecular interfaces within the framework of the partitioning and inner projection techniques, developed by Per-Olov Lowdin. Using a one-particle approximation to the many-body equation for the current in a system consisting of a molecular bridge and two electrodes, it is shown that the idea of partitioning can be used advantageously to define a self-energy that contains the coupling of the molecule to the electrodes. Inner projections are then introduced as a systematic way of including electron correlation effects in the transport across molecular junctions. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

A convenient experimental system is described, with which electron transport through structurally well-defined, 2–5 nm-thick, organic films can be examined. Two types of junction J have been studied in which self-assembled monolayers (SAMs, for example, SAM(1) formed on Ag from aliphatic and aromatic thiols, and SAM(2), formed on Hg from hexadecanethiol) are in contact through either van der Waals interactions (see picture) or through covalent, hydrogen, or ionic bonds.

By molecular wires, one generally means molecular structures that transmit a signal between two termini. We discuss some theoretical models and analysis for electronically conductive molecular wires in which a single molecule conducts charge between two electrodes. This situation resembles both intramolecular non-adiabatic electron transfer, in which electronic tunneling between donor and acceptor is seen, and mesoscopic quantum transport. We discuss formal methods for predicting conductance in molecular wire circuits. The critical component that differs from the usual conductivity is the interface between electrode continuum and the discrete levels of the molecule. This can be described in several ways. We present an analysis based on the Bardeen tunneling formula. Specific problems (electron polarization, disorder, nuclear scattering, charge distribution) are discussed. Finally, the differing mechanisms expected for the conductance, ranging from ballistic tunneling to gated transfer, are outlined.

The electronic spin filtering capability of a single chiral helical peptide is measured. A ferromagnetic electrode source is employed to inject spin-polarized electrons in an asymmetric single-molecule junction bridging an α-helical peptide sequence of known chirality. The conductance comparison between both isomers allows the direct determination of the polarization power of an individual chiral molecule.

We have calculated the first-, second-, and third-order contribution to the refractive index of a dilute aqueous solution of three-level molecules irradiated by an intense one-frequency external field. Each solute molecule is described by two coupled harmonic curves of electronic energies with the same force constant and with minima displaced in energy and nuclear coordinate. We assume that the intramolecular coupling mixes only the states corresponding to two near degenerate of the three levels considered. Important linear and nonlinear contributions of the solute molecules to the refractive index have been found, mostly in the so-called weak and intermediate coupling regions. © 1993 John Wiley & Sons, Inc.

Local-scaling transformations are employed in order to formulate a desity functional theory where both electrons and nuclei are treated quantum mechanically. Because of the properties of these transformations, the ensuing version of density functional theory is N- and v- representable. In particular, we study the effect of these transformations on both densities and generating wave functions, and as a result, a functional for the energy expressed in terms of the electronic and nuclear densities is constructed. Plausible physical approximations for simplifying this electronic-nuclear functional are considered, and upon variation of the fuctional with respect to the electronic and nuclear densities, a system of coupled Euler–Lagrange equations is obtained.

Two methods for numerical solution of multidimensional diffusion problems are presented and applied to the two dimensional barrier crossing problem in the overdamped limit. One of these methods is based on evaluating the smallest non-vanishing eigenvalue of the Smoluchowski equation, and the other is based on an adaption of Chandler's steady state correlation function approach. Both methods make use of the fast Fourier transform algorithm for solving a transformed version of the Smoluchowski equation. The numerical solutions are compared to results based on the Kramers theory and some observations concerning effects of the dynamics of barrier crossing problems are made.

The partition of a molecule in different regions where virial relations are fulfilled has been proved in the literature to be a fruitful scheme for the description of molecular properties. However, the conceptual importance of this approach is somewhat diminished by the fact that nuclei are considered to occupy successive fixed positions. We show that this restriction can in fact be relaxed when such partitions of molecules are considered. The conditions that density functions must fulfill on the surfaces of such regions, when the nuclear motion is taken into account, are found. Using these conditions, we have examined general cases where the nuclear motion may introduce important modifications of the surfaces defined by the usual condition of zero flux on the one-particle density corresponding to the fixed nuclei model. Finally a further generalization is introduced by considering partitions of subsystems of particles of the system.

Starting from the Hohenberg-Kohn functional we show that when the energy density is given as a function of ρ and ∇ρ, i.e., ξ = ξ(ρ, ∇ρ), the condition ∇ρ · n = 0 (which was found by Bader et al. to define virial fragments), appears as a natural boundary condition for the variation of this functional. We also show that when the energy density includes second order derivatives (∇2ρ) this condition is necessary but not sufficient to guarantee the vanishing of the variation. The implications of these results are discussed in the context of a density functional theory for virial fragments.

The 1/N expansion is used to generate semiclassical effective potentials for two-electron ions. These potentials can be considered an improvement over the conventional ones computed by Lin and Fano for the full two-electron coulomb potential insofar as they incorporate some of the quantum mechanical effects. No falling into the nucleus is allowed. Furthermore, the minima of these effective potentials are in surprising agreement with those computed by Fano, Macek, and Lin. This would indicate that the collective variable R, associated with N = 6, leads to a much smaller expansion parameter N, making the adiabatic separation more justified. The role and significance of these broken symmetry solutions with geometric localization are discussed as well as generalizations. The procedure is semiclassical in the sense that the kinetic energy becomes negligible by having a large effective mass.