A study of the adsorption equilibrium of solution-phase CdS quantum dots (QDs) and acid-derivatized viologen ligands (N-[1-heptyl],N'-[3-carboxypropyl]-4,4'-bipyridinium dihexafluorophosphate, V(2+)) reveals that the structure of the surfaces of the QDs depends on their concentration. This adsorption equilibrium is monitored through quenching of the photoluminescence of the QDs by V(2+) upon photoinduced electron transfer. When modeled with a simple Langmuir isotherm, the equilibrium constant for QD-V(2+) adsorption, K(a), increases from 6.7 × 10(5) to 8.6 × 10(6) M(-1) upon decreasing the absolute concentration of the QDs from 1.4 × 10(-6) to 5.1 × 10(-8) M. The apparent increase in K(a) upon dilution results from an increase in the mean number of available adsorption sites per QD from 1.1 (for [QD] = 1.4 × 10(-6) M) to 37 (for [QD] = 5.1 × 10(-8) M) through desorption of native ligands from the surfaces of the QDs and through disaggregation of soluble QD clusters. A new model based on the Langmuir isotherm that treats both the number of adsorbed ligands per QD and the number of available binding sites per QD as binomially distributed quantities is described. This model yields a concentration-independent value for K(a) of 8.7 × 10(5) M(-1) for the QD-V(2+) system and provides a convenient means for quantitative analysis of QD-ligand adsorption in the presence of competing surface processes.

MADNESS (multiresolution adaptive numerical environment for scientific simulation) is a high-level software environment for solving integral and differential equations in many dimensions that uses adaptive and fast harmonic analysis methods with guaranteed precision based on multiresolution analysis and separated representations. Underpinning the numerical capabilities is a powerful petascale parallel programming environment that aims to increase both programmer productivity and code scalability. This paper describes the features and capabilities of MADNESS and briefly discusses some current applications in chemistry and several areas of physics.

We present a computational investigation into the line shapes of peaks in conductance histograms, finding that they possess high information content. In particular, the histogram peak associated with conduction through a single molecule elucidates the electron transport mechanism and is generally well-described by beta distributions. A statistical analysis of the peak corresponding to conduction through two molecules reveals the presence of cooperative effects between the molecules and also provides insight into the underlying conduction channels. This work describes tools for extracting additional interpretations from experimental statistical data, helping us better understand electron transport processes.

Cooperative effects between molecular wires affect conduction through the wires, and studies have yet to clarify the conditions under which these effects enhance (diminish) conduction. Using a simple but general model, we attribute this crosstalk to the duality of energetic splitting and phase interference between the wires’ conduction channels. In most cases, crosstalk increases (decreases) conductance when the Fermi level is far from (close to) an isolated wire’s resonance. Finally, we discuss strategies for controlling crosstalk between parallel molecular wires.

An ultrafast, nanoscale molecular switch is proposed, based on extension of the concept of nonadiabatic alignment to surface-adsorbed molecules. The switch consists of a conjugated organic molecule adsorbed onto a semiconducting surface and placed near a scanning tunneling microscope tip. A low-frequency, polarized laser field is used to switch the system by orienting the molecule with the field polarization axis, enabling conductance through the junction. Enhancement and spatial localization of the incident field by the metallic tip allow operation at low intensities. The principles of nonadiabatic alignment lead to switch on and off time scales far below rotational time scales.

We report the synthesis of several unique, boron-rich pincer complexes derived from m-carborane. The SeBSe and SBS pincer ligands can be synthesized via two independent synthetic routes, and are metallated with Pd(II). These structures represent unique coordinating motifs, each with a Pd-B(2) bond chelated by two thio-or selenoether ligands. This class of structures serves as the first example of boron-metal pincer complexes, and possesses several interesting electronic properties imposed by the m-carborane cage.Pincer complexes,1 formed from tridentate ligands (the "pincer") and metals or metalloids, are an exciting class of structures with utility in catalysis,2 molecular electronics,3 and medicine. 4 The pincer ligand provides tailorability with respect to 1) anchoring site on the base, the central moiety from which the arms extend,5a 2) metal-binding heteroatoms, which control the electronic nature of the complexed atom, 5b and 3) the auxiliary sites on the arms, 5c which can be modified to control the chirality of the resulting structure and the steric environment around the complexed atom. To date, the base has been made primarily from hydrocarbons, including aromatic groups such as benzene and heteroatom analogs, 6a aliphatic groups,6b-c and, more recently, carbene-type moieties. 6dIcosahedral carboranes represent one class of structures 7 that have not been explored as base units in pincer complexes. This is surprising considering these structures are highly tailorable, 8a exhibit extraordinary stability, 8b and provide a wide range of accessible chemical derivatization pathways. 8c Indeed, Hawthorne et al. have pioneered synthetic methods for chadnano@northwestern.edu. Supporting Information Available: Experimental and characterization for 4-7. X-ray crystallographic files for 5 and 7 in CIF format. Details on DFT studies of 5. These materials are available free of charge via the internet at http://pubs.acs.org. Herein we report the first carborane-based pincer ligand family and the complexes of these ligands metallated with Pd. The structure formed represents a new class of compounds with a previously unobserved Pd-B σ-coordination bond pincer motif. 10 Additionally, through density functional theory (DFT) calculations we show that the electronic structure of this novel complex, derived in part from this new ligand, is substantially different from its hydrocarbon analogues. The carborane-based pincer complex 5 was synthesized in four steps starting with the commercially available m-carborane 1 (Scheme 1a) to form the desired pincer ligand 4. Pincer complex 5 is made in 76% yield by reacting ligand 4 with Pd(CH 3 CN) 4 [BF 4 ] 2 in acetonitrile followed by 2 eq. of ( n Bu) 4 NCl. Importantly, complex 5 is stable indefinitely in air and can be chromatographed on silica. All spectroscopic data are consistent with the proposed structure for 5. ) and an isolated resonance at δ -0.7. Integration of the area under these peaks shows a 9:1 ratio, respectively. The 1 H coupled 11 B NMR spectrum of comple...

Complex band structure generalizes conventional band structure by also considering wavevectors with complex components. In this way, complex band structure describes both the bulk-propagating states from conventional band structure and the evanescent states that grow or decay from one unit cell to the next. Even though these latter states are excluded by translational symmetry, they become important when translational symmetry is broken via, for example, a surface or impurity. Many studies over the last 80 years have directly or indirectly developed complex band structure for an impressive range of applications, but very few discuss its fundamentals or compare its various results. In this work we build upon these previous efforts to expose the physical foundation of complex band structure, which mathematically implies its existence. We find that a material's static and dynamic electronic structure are both completely described by complex band structure. Furthermore, we show that complex band structure reflects the minimal, intrinsic information contained in the material's Hamiltonian. These realizations then provide a context for comparing and unifying the different formulations and applications of complex band structure that have been reported over the years. Ultimately, this discussion introduces the idea of examining the amount of information contained in a material's Hamiltonian so that we can find and exploit the minimal information necessary for understanding a material's properties.

We investigate possible causes of molecular rectification in electrode−molecule− electrode junctions. By using a simple model and simulated conductance histograms, we show that a molecular bias drop is responsible for rectification; conversely, asymmetric molecule−electrode couplings do not directly result in rectification. Instead, the degree of coupling (a)symmetry can be observed in the line shapes of the conductance histograms used to experimentally assess the current−voltage properties of such molecular junctions. More coupling asymmetry leads to less positively skewed histogram peaks.