In this study charge transport along zinc porphyrin-based molecular wires is simulated, considering both bandlike and hopping mechanisms. It is shown that bandlike transport simulations yield significantly overestimated hole mobility values. On the basis of kinetic and thermodynamic considerations, it is inferred that charge transport along zinc porphyrin-based molecular wires occurs by small polaron hopping. Hole mobility values on the order of 0.1 cm(2) V(-1) s(-1) are found from small polaron hopping simulations, which agree well with previously reported experimental results. It is suggested that the experimentally observed increase of the charge carrier mobility on formation of supramolecular ladderlike structures is determined by two factors. One of these is an increase of charge transfer integrals between monomer units due to molecular wire planarization. A more important factor is the reduction of the amount of energetic disorder along the molecular wire and in its environment. General guidelines for determining the mechanism of charge transport along molecular wires are discussed.

Quantum interference effects occurring in molecules through which a charge can travel via multiple pathways can be the basis for new unconventional design principles in molecular scale electronics. However, these quantum interference effects can be reduced by interaction between the charge and molecular vibrations. In this work dephasing (decoherence) effects have been studied using a model that combines a (classical) molecular mechanics description of molecular vibrations with a quantum mechanical propagation of the charge. It is found that despite the clear effect of dephasing on the charge propagation, interference effects are largely retained at room temperature if vibrations are accounted for. Additionally, it is shown that taking electronic interactions between non-nearest neighbor atoms into account also diminishes interference effects but not sufficiently to destroy them completely. It is concluded that interference effects are strong enough to use them in a functional manner in molecular electronics. This opens up new ways to design molecular electronic components that exploit quantum interference.

A new generally applicable method to calculate the relative energetic stability of localized and delocalized charges in a system of two molecules is presented. The relative stability of localized and delocalized charges was calculated for π-stacked triphenylenes at varying twist angles and intermolecular distances. The reliability of the new method was validated by comparison with results from Hartree–Fock calculations on particular configurations. According to the calculations, charges are localized for larger twist angles that are typical for triphenylene derivatives in the liquid crystalline phase. In contrast, significant charge delocalization is expected for eclipsed stacking of triphenylene units in a covalent organic framework. This can give rise to band-like motion of delocalized charges with a mobility of the order of 10 cm2 V–1 s–1 or more.

This Letter proposes a realistic design of a single-molecule quantum-interference-based transistor. The transistor consists of a cross-conjugated donor–bridge–acceptor molecule and is chemically gated by a functional group that can be charged. Numerical simulations indicate that the device properties can be tuned to desired specifications by the choice of its constituting functional groups. The transistor does not require external contacts to control its operation. However, it can be chemically functionalized for easy integration into molecular electonic circuits, especially because its operation does not involve any conformational changes in the molecule. The upper operational frequency limit of the proposed device is found to be in the terahertz range.

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In organic materials, exciton dissociation into free charges requires overcoming an electron-hole Coulomb interaction that exceeds the thermal energy and may still be large after charge transfer at a donor/acceptor interface. We analyze the factors affecting efficiency of charge separation and subsequent removal of electrons and holes from such a donor-acceptor interface and suggest strategies for optimizing these processes. Energy transfer, charge separation and charge transfer in the vicinity of the donor-acceptor interface are studied within a common theoretical framework based on a quantum master equation, for a model system with realistic excitation energies and electronic couplings. We find that enhancing the efficiency of both charge transfer from the donor to the acceptor and of charge removal from the donor-acceptor interface requires an intricate balance between the extent of electronic delocalization throughout the material and rates of energy dissipation. For very large exciton binding energies, cascade charge separation in systems with more than one donor and one acceptor species, such as molecular polyads, is found to greatly facilitate the dissociation of geminate pairs. Our calculations predict charge separation on sub-picosecond timescales for several parameter combinations, leading to design principles for enhancing charge separation in multi-chromophore systems.

We review the principles of formation, physical properties, and current or envisaged applications for a wide range of carbon allotropic forms. We discuss experimental and theoretical advances relating to staple zero-, one-, and two-dimensional carbon structures, such as fullerenes, carbon nanotubes, and graphene. In addition we emphasize research on emerging carbon allotropes (carbon nanoscrolls, funnels, etc) that result from combining or deforming allotropic forms with well-defined dimensionality. Such materials fall in-between clearly delineated dimensional categories and consequently exhibit unique characteristics that are promising for electronic, optical, and mechanical applications. We also consider other approaches to tuning properties of carbon-based materials, such as chemical functionalization, intentional introduction of structural disorder, and placement of guest atoms or molecules inside hollow structures. Finally, we discuss the properties of and experimental methods for studying zero-dimensional systems (paramagnetic nitrogen impurity atoms) in diamond matrix. The review emphasizes the interplay between the various material properties of carbon-based nanostructures and the designs for nanoscale devices that rely on synergistic combinations of these properties. For example, an electromechanical vibrator, a strain sensor, a nanodynamometer, and a nanoelectromechanical memory cell that we describe exploit both electronic and nanomechanical properties of low-dimensional carbon structures, a reed switch and a magnetic field sensor use magnetic and nanomechanical properties, a maser based on nitrogen-doped diamond uses thermal and optoelectronic properties, etc. All presented device concepts have been validated by calculations, and some have been implemented experimentally.

Optimizing the optical properties of large chromophore aggregates and molecular solids for applications in photovoltaics and nonlinear optics is an outstanding challenge. It requires efficient and reliable computational models that must be validated against accurate theoretical methods. We show that linear absorption spectra calculated using the molecular exciton model agree well with spectra calculated using time-dependent density functional theory and configuration interaction singles for aggregates of strongly polar chromophores. Similar agreement is obtained for a hybrid functional (B3LYP), a long-range corrected hybrid functional (ωB97X), and configuration interaction singles. Accounting for the electrostatic environment of individual chromophores in the parametrization of the exciton model with the inclusion of atomic point charges significantly improves the agreement of the resulting spectra with those calculated using all-electron methods; different charge definitions (Mulliken and ChelpG) yield similar results. We find that there is a size-dependent error in the exciton model compared with all-electron methods, but for aggregates with more than six chromophores, the errors change slowly with the number of chromophores in the aggregate. Our results validate the use of the molecular exciton model for predicting the absorption spectra of bulk molecular solids; its formalism also allows straightforward extension to calculations of nonlinear optical response.