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One of the most promising properties of lead halide perovskite nanocrystals (NCs) is their defect tolerance. It is often argued that, due to the electronic structure of the conduction and valence bands, undercoordinated ions can only form localized levels inside or close to the band edges (i.e., shallow traps). However, multiple studies have shown that dangling bonds on surface Br- can still create deep trap states. Here, we argue that the traditional picture of defect tolerance is incomplete and that deep Br- traps can be explained by considering the local environment of the trap states. Using density functional theory calculations, we show that surface Br- sites experience a destabilizing local electrostatic potential that pushes their dangling orbitals into the bandgap. These deep trap states can be electrostatically passivated through the addition of ions that stabilize the dangling orbitals via ionic interactions without covalently binding to the NC surface. These results shed light on the formation of deep traps in perovskite NCs and provide strategies to remove them from the bandgap.

We have studied the charge transport properties of self-assembled structures of semisynthetic zinc chlorins (ZnChls) in the solid state by pulsed radiolysis time-resolved microwave conductivity measurements. These materials can form either a two-dimensional (2D) brickwork-type slipped stack arrangement or a one-dimensional (1D) tubular assemblies, depending on the exact molecular structure of the ZnChls. We have observed efficient charge transport with mobilities as high as 0.07 cm(2) V(-1) s(-1) for tubular assemblies of 3(1)-hydroxy ZnChls and up to 0.28 cm(2) V(-1) s(-1) for 2D stacked assemblies of 3(1)-methoxy ZnChls at room temperature. The efficient charge transporting capabilities of these organized assemblies opens the way to supramolecular electronics based on biological systems.

We present a theoretical study of the positive charge transfer in stilbene-linked DNA hairpins containing only AT base pairs using a tight-binding model that includes a description of structural fluctuations. The parameters are the charge transfer integral between neighboring units and the site energies. Fluctuations in these parameters were studied by a combination of molecular dynamics simulations of the structural dynamics and density functional theory calculations of charge transfer integrals and orbital energies. The fluctuations in both parameters were found to be substantial and to occur on subpicosecond time scales. Tight-binding calculations of the dynamics of charge transfer show that for short DNA hairpins (

DNA-based molecular electronics will require charges to be transported from one site within a 2D or 3D architecture to another. While this has been shown previously in linear, π-stacked DNA sequences, the dynamics and efficiency of charge transport across DNA three-way junction (3WJ) have yet to be determined. Here, we present an investigation of hole transport and trapping across a DNA-based three-way junction systems by a combination of femtosecond transient absorption spectroscopy and molecular dynamics simulations. Hole transport across the junction is proposed to be gated by conformational fluctuations in the ground state which bring the transiently populated hole carrier nucleobases into better aligned geometries on the nanosecond time scale, thus modulating the π-π electronic coupling along the base pair sequence.

The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to investigate the nature of the relaxed S(1) state of 9,9'-bianthryl (AA), 10-cyano-9,9'-bianthryl (CAA), and 10,10'-dicyano-9,9'-bianthryl (CAAC). Changes in both the real, Deltaepsilon' (dielectric constant), and imaginary, Deltaepsilon' ' (dielectric loss), components of the complex permittivity have been measured. The dielectric loss transients conclusively demonstrate the dipolar nature of S(1) for all three compounds in the pseudopolar solvents benzene and 1,4-dioxane, and even in the nonpolar solvents n-hexane and cyclohexane. The required symmetry breaking is considered to result from density and structural fluctuations in the solvent environment. The dipole relaxation times for AA (CAAC) are ca. 2 ps for the alkanes and 7.9 (5.3) and 14 (14) ps for benzene and dioxane, respectively. The time scale of dipole relaxation for the symmetrical compounds is much shorter than that for rotational diffusion and is attributed to intramolecular, flip-flop dipole reversal via a neutral excitonic state. The dipole moment of the transient dipolar state is estimated to be ca. 8 D, that is much lower than the value of ca. 20 D determined from the solvatochromic shifts in the fluorescence in intermediate to highly polar solvents which corresponds to close to complete charge separation. For the asymmetric compound, CAA, a dipole moment close to 20 D is found in all solvents, including n-hexane. Dipole relaxation in this case occurs on a time scale of several hundred picoseconds and is controlled mainly by diffusional rotation of the molecules. The mechanism and kinetics of formation of the dipolar excited states are discussed in the light of these results.

In this work the mechanism of migration of positive charges through donor−DNA−acceptor systems is studied using a quantum mechanical model based on the tight-binding approximation. For DNA bridges containing only adenine-thymine (AT) base pairs the difference in ionization potential between the donor moiety and the AT base pairs (i.e., the injection barrier) is shown to determine the mechanism by which the charge migrates from the donor to the acceptor. For an injection barrier of 0.55 eV, corresponding to a guanine radical cation as the hole-donor, a β-value of 0.85 A-1 is found. This agrees reasonably with the value of β = 0.7 A-1 deduced from experimental studies on these sequences. For this injection barrier (0.55 eV) the charge density on the AT bridge was found to be very small, which is characteristic for charge transfer by single-step tunneling. For lower injection barriers the charge density on the AT bridge becomes substantial and the charge moves through the bridge according to a bandlike mecha...

DNA hairpins in which an electron donor and an electron acceptor are attached to the ends are excellent model systems for the study of charge transfer in weakly coupled pi-stacked systems. In this communication we report on a computational study of the effect of the base pair sequence in these DNA hairpins on the kinetics of charge transfer. We show that the rate of charge transfer strongly depends on the actual position of a GC base pair in a sequence that otherwise only contains AT base pairs. This can be explained by evaluating the energy landscape through which the charge travels. It is shown that including the electrostatic interaction between electron and hole can explain the experimentally observed dependence on the position of the GC in the DNA. We conclude that electrostatic interactions are important to consider when explaining the charge transfer kinetics in GC containing DNA sequences.

Discotic mesophases are known for their ability to self-assemble into columnar structures and can serve as semiconducting molecular wires. Charge carrier mobility along these wires strongly depends on molecular packing, which is controlled by intermolecular interactions. By combining wide-angle X-ray scattering experiments with molecular dynamics simulations, we elucidate packing motifs of a perylene tetracarboxdiimide derivative, a task which is hard to achieve by using a single experimental or theoretical technique. We then relate the charge mobility to the molecular arrangement, both by pulse-radiolysis time-resolved microwave conductivity experiments and simulations based on the non-adiabatic Marcus charge transfer theory. Our results indicate that the helical molecular arrangement with the 45 degrees twist angle between the neighboring molecules favors hole transport in a compound normally considered as an n-type semiconductor. Statistical analysis shows that the transport is strongly suppressed by structural defects. By linking molecular packing and mobility, we eventually provide a pathway to the rational design of perylenediimide derivatives with high charge mobilities.

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.

The transport of positive charges along porphyrin-based molecular wires has been probed in dilute solution using pulse radiolysis time-resolved microwave conductivity. The mobility of positive charges along isolated single-strand porphyrin wires was found to be 0.084 cm2/(V s). Addition of 4,4′-bipyridyl leads to the formation of a double-strand ladder complex, locking each polymer strand into a more planar structure and thus enhancing the porphyrin-porphyrin π-conjugation, leading to an increase in the charge carrier mobility by an order of magnitude to 0.91 cm2/(V s). Copyright © 2007 American Chemical Society.

The effective energy of a positive charge when it is localized at a specific guanine nucleobase in DNA was calculated using density functional theory. The results demonstrate that the efficiency of a guanine to act as a hole-trap in DNA strongly depends on the nature of the flanking nucleobases. The presence of a pyrimidine base at the 3′ position adjacent to a guanine significantly increases the localization energy of the positive charge. The calculated distributions of a positive charge in sequences of two or three adjacent guanines, flanked by other nucleobases, provide an explanation for experimental literature data on the site-selective oxidation of DNA. Copyright © 2003 American Chemical Society.