Dependence of single-molecule conductance on molecule junction symmetry

The symmetry of a molecule junction has been shown to play a significant role in determining the conductance of the molecule, but the details of how conductance changes with symmetry have heretofore been unknown. Herein, we investigate a naphthalenedithiol single-molecule system in which sulfur atoms from the molecule are anchored to two facing gold electrodes. In the studied system, the highest single-molecule conductance, for a molecule junction of 1,4-symmetry, is 110 times larger than the lowest single-molecule conductance, for a molecule junction of 2,7-symmetry. We demonstrate clearly that the measured dependence of molecule junction symmetry for single-molecule junctions agrees with theoretical predictions.     

A mixed-valence Co7 single-molecule magnet with C3 symmetry

The synthesis, structure and magnetic properties of [Co(II)(4)Co(III)(3)(HL)(6)(NO(3))(3)(H(2)O)(3)](2+) [H(3)L = H(2)NC(CH(2)OH)(3)] are reported: the complex is an exchange-biased single molecule magnet.     

Metallacryptate single-molecule magnets: effect of lower molecular symmetry on blocking temperature

The structural characterization of complexes [Mn(II)4Mn(III)22(pdol)12(OCH3)12(O)16(N3)6] (1) and [Mn(II)4Mn(III)22(pdol)12(OCH3)12(O)16(OH)2(H3O)(OCH3)3].ClO4.5CH3OH (2), where pdol(2-) is di-2-pyridyl methanediol, reveals that each has a metallacryptand shell that encapsulates a manganese oxide core. Variable-temperature direct current magnetic susceptibility measurements on 2 indicate a paramagnetic ground state that results from an overall antiferromagnetic interaction in the cluster, with chiT values decreasing from 300 K (51.2 cm3 K mol(-1)) to 2 K (19.8 cm3 K mol(-1)). Variable-temperature alternating current magnetic susceptibility measurements imply that both 1 and 2 behave as single-molecule magnets. Fitting the frequency-dependent out-of-phase magnetic susceptibility to the Arrhenius equation yields an effective energy barrier, Ueff, to magnetization relaxation of 16.5 +/- 0.7 K (11.5 +/- 0.5 cm(-1)) for 1 and 36.2 +/- 2.0 K (25.1 +/- 1.4 cm(-1)) for 2. The larger value for 2 is in agreement with the lower molecular symmetry, larger magnetoanisotropy, and higher ground spin state of 2 compared to those of 1. This observation suggests a new strategy for increasing the blocking temperatures in high-nuclearity manganese clusters.     

The symmetry of azodicarbonitrile

The infrared and Raman spectra of azodicarbonitrile (I) made from cyanogen azide (II) were recorded under varied conditions of phase, temperature, concentration, and pressure and strongly indicate a C_2h symmetry (trans-(I)). Cis-(I) was not shown positively either by these techniques or by microwave spectra. The fundamental vibration frequencies tentatively suggested for trans-(I) are (cm⁻¹): v₁ = 2176, v₂ = 1422, v₃ = 1002, v₄= 741, v₅ = 282 (A_g), v₆ = 108 (B_g, v₇ = 2204, v₈ = 982, v₉ = 904, v₁₀ = 596 (B_u), v₁₁ = 574, v₁₂ = 133 (A_u). Of these, v₄ and v₆ were not observed directly, but inferred from an analysis of a complex band in the infrared spectrum.     

The use of site symmetry in constructing symmetry adapted functions

It is shown that the application of a projection operator from a given group to a function is equivalent to the successive application of projection operators from factor groups of the starting group to that function. When used with the factor groups representing the site symmetry of a position and the simplest group of interchanges of positions, this concept provides a very simple method for obtaining symmetry adapted linear combinations of basis functions.     

The combinatorics of symmetry adaptation

A method is developed for obtaining the generating functions for the equivalence classes of orbitals wherein only orbitals within an equivalence class participate in symmetry adaptation. It is shown that using Williamson's combinatorial theorem the generating functions for the symmetry species contained in each equivalence class can be obtained. The method is illustrated with Porphindianion.     

The chemical dynamics of nanosensors capable of single-molecule detection

Recent advances in nanotechnology have produced the first sensor transducers capable of resolving the adsorption and desorption of single molecules. Examples include near infrared fluorescent single-walled carbon nanotubes that report single-molecule binding via stochastic quenching. A central question for the theory of such sensors is how to analyze stochastic adsorption events and extract the local concentration or flux of the analyte near the sensor. In this work, we compare algorithms of varying complexity for accomplishing this by first constructing a kinetic Monte Carlo model of molecular binding and unbinding to the sensor substrate and simulating the dynamics over wide ranges of forward and reverse rate constants. Methods involving single-site probability calculations, first and second moment analysis, and birth-and-death population modeling are compared for their accuracy in reconstructing model parameters in the presence and absence of noise over a large dynamic range. Overall, birth-and-death population modeling was the most robust in recovering the forward rate constants, with the first and second order moment analysis very efficient when the forward rate is large (>10(-3) s(-1)). The precision decreases with increasing noise, which we show masks the existence of underlying states. Precision is also diminished with very large forward rate constants, since the sensor surface quickly and persistently saturates.     

Anion-sealed single-molecule capsules

A new approach to anion recognition utilizing electrostatic and hydrogen bonding interactions has been demonstrated by placement of the whole ion-pair in a molecular capsule.     

The behavior of single-molecule junctions predicted by atomistic simulations

Molecular dynamic simulations in combination with energy minimizations are used in order to understand the basis of the novel experiments reported recently by Haiss et al. (W. Haiss, C. Wang, I. Grace, A.S. Batsanov, D.J. Schiffrin, S.J. Higgins, M.R. Bryce, C.J. Lambert, R.J. Nichols, Nature Mater. 5 (2006) 995). Our model suggests that single-molecule junctions produced by the trapping method can be reached when the STM tip – substrate surface separation is smaller than 8 Å. Additionally, our model predicts that the effect of the electric field on the attachment/detachment process can be neglected.     

The symmetry of molecular polarization tensors

Polarization tensors are discussed in terms of their intrinsic symmetry group which is a direct product of the point group and the subgroup of the permutation group relevant to the experiment. The study of these latter groups is simplified by use of the isomorphism with certain point groups and permutations of suffixes can be visualized by rotations and reflections of the vertices of various objects in space. The approach unites the previous treatments and provides a means of constructing the bases for the irreducible tensor components. The difficulties introduced by Laplace's equation are explained and the information obtainable from induced birefringence experiments (Kerr and Cotton–Mouton effects) discussed for various systems.     

The symmetry group of nonrigid tetramethylsilane

Using nonrigid group theory, it is shown that the full nonrigid (f‐NRG) group of tetramethylsilane is isomorphic to the group C₃ S₄ of order 1944, where “” denotes the wreath product of groups, and C₃ and S₄ are the cyclic group of order three and the symmetric group of order four, respectively. This group has 51 conjugacy classes and irreducible representations. Then the character table of tetramethylsilane is derived for the first time. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Measurements of single-molecule electromechanical properties

The electromechanical properties of a single molecule covalently attached to two gold electrodes are studied by simultaneously measuring the conductance and the force during the stretching of the molecule. The conductance, the spring constant of the molecular junction, and the dependence of the conductance on the stretching force are determined. Like the conductance, the spring constant of a molecule depends also on the molecule-electrode contacts. The forces required to break the molecule-gold contacts are 1.5 nN for alkanedithiols and 0.8 nN for 4,4' bipyridine, indicating that the breakdowns take place at the Au-Au bond and at the N-Au bond, respectively.     

Heterodinuclear Cu-Tb single-molecule magnet

A strictly heterodinuclear Cu-Tb complex [LCu(O(2)COMe)Tb(thd)(2)] made with a Schiff base L, thd, and monomethylcarbonate ligands [L(2-) = N,N'-2,2-dimethylpropylenedi(3-methoxysalicylideneiminato); thd = tetramethylheptanedionato] behaves like a single-molecule magnet. The monomethylcarbonate ligand, which appears during the reaction pathway, bridges the Cu and Tb ions.     

Towards single-molecule optoelectronic devices

Benefiting from the development of molecular electronics and molecular plasmonics, the interplay of light and electronic transport in molecular junctions has attracted growing interest among researchers in both fields, leading to a new research direction of “single-molecule optoelectronics”. Here, we review the latest developments of photo-modulated charge transport, electroluminescence and Raman spectroscopy from single-molecule junctions, and suggest future directions for single-molecule optoelectronics.     

Heterometallic cubane single-molecule magnets

Two new heterometallic cubane molecules have been synthesized. High-frequency electron paramagnetic resonance and magnetization measurements indicate that [Mn(3)Ni(hmp)(3)O(N(3))(3)(C(7)H(5)O(2))(3)] (1) displays a well-isolated S = 5 ground state (DeltaE > 120 K), with g = 2.0, D = -0.23 cm(-1), and ferromagnetic Mn-Mn exchange interactions competing with antiferromagnetic Ni-Mn interactions. [Mn(3)Zn(hmp)(3)O(N(3))(3)(C(3)H(5)O(2))(3)] (2) possesses a S = 6 ground state (DeltaE > 105 K), with g = 2.0, D = -0.14 cm(-1), and ferromagnetic Mn-Mn exchange interactions. Magnetization vs magnetic field data for oriented single crystals of 1 and 2 indicate that both complexes are single-molecule magnets.