Weak Distance Dependence Research Articles

Energy Dissipation from Confined States in Nanoporous Molecular Networks.

Crystalline nanoporous molecular networks are assembled on the Ag(111) surface, where the pores confine electrons originating from the surface state of the metal. Depending on the pore sizes and their coupling, an antibonding level is shifted upward by 0.1-0.3 eV as measured by scanning tunneling microscopy. On molecular sites, a downshifted bonding state is observed, which is occupied under equilibrium conditions. Low-temperature force spectroscopy reveals energy dissipation peaks and jumps of frequency shifts at bias voltages, which are related to the confined states. The dissipation maps show delocalization on the supramolecular assembly and a weak distance dependence of the dissipation peaks. These observations indicate that two-dimensional arrays of coupled quantum dots are formed, which are quantitatively characterized by their quantum capacitances and resonant tunneling rates. Our work provides a method for studying the capacitive and dissipative response of quantum materials with nanomechanical oscillators.

A Simple, Semiclassical Mechanism for Activationless, Long RangeCharge Transport in Molecular Junctions

Past reports on photocurrents in molecular junctions consisting of aromatic oligomers between electrical contacts reveal very low activation energies (<1 meV) and weak distance dependence for molecular layer thicknesses of 20–60 nm. Photocurrent transport mediated by sequential tunneling between adjacent subunit orbitals represents a “super highway” for charge transport with low activation barrier, field dependence and long range of at least 60 nm. In addition to photocurrents, such transport may be involved in dark currents for distances >10 nm, previously reported biological transport across μm in bacterial nanowires, and >1 cm in cable bacteria.

Weak Distance Dependence of Hot-Electron-Transfer Rates at the Interface between Monolayer MoS2 and Gold.

Electron transport across the transition-metal dichalcogenide (TMD)/metal interface plays an important role in determining the performance of TMD-based optoelectronic devices. However, the robustness of this process against structural heterogeneities remains unexplored, to the best of our knowledge. Here, we employ a combination of time-resolved photoemission electron microscopy (TR-PEEM) and atomic force microscopy to investigate the spatially resolved hot-electron-transfer dynamics at the monolayer (1L) MoS2/Au interface. A spatially heterogeneous distribution of 1L-MoS2/Au gap distances, along with the sub-80 nm spatial- and sub-60 fs temporal resolution of TR-PEEM, permits the simultaneous measurement of electron-transfer rates across a range of 1L-MoS2/Au distances. These decay exponentially as a function of distance, with an attenuation coefficient β ∼ 0.06 ± 0.01 Å-1, comparable to molecular wires. Ab initio simulations suggest that surface plasmon-like states mediate hot-electron-transfer, hence accounting for its weak distance dependence. The weak distance dependence of the interfacial hot-electron-transfer rate indicates that this process is insensitive to distance fluctuations at the TMD/metal interface, thus motivating further exploration of optoelectronic devices based on hot carriers.

A quantum mechanical model for hole transport through DNA: predicting conditions for oscillatory/non-oscillatory behavior

A quantum mechanical model that considers tunneling and inelastic scattering has been applied to explain the hole transfer reaction from a G (Guanine) base to a GGG base cluster through a barrier of Adenine bases, (A)_n (<em>n</em> = 1-16). For <em>n</em> = 1, the ratio of tunneling to inelastic scattering is about 6, which is sharply decreased to around 0.23 and 5.23 × 10^-8 for <em>n</em> = 4 and 16 respectively, suggesting dominance of inelastic scattering for <em>n</em> ≥ 4. As in experiment, the calculated product yield ratios (P_GGG) exhibit a strong distance dependence for <em>n</em> < 4, and a weak distance dependence for <em>n</em> ≥ 4. We also predict conditions under which oscillatory or non-oscillatory charge transfer (CT) yield are expected. .

Tracking Photoinduced Charge Separation in DNA: from Start to Finish.

The initial studies of the dynamics of photoinduced charge separation conducted in our laboratories 20 years ago found strongly distance-dependent rate constants over short distances but failed to detect intermediates in the transport of positive charge (holes). These observations were consistent with the single-step superexchange or tunneling mechanism that had been observed for numerous donor-bridge-acceptor systems at that time. Subsequent studies found weak distance dependence for hole transport over longer distances in DNA, characteristic of incoherent hopping of either localized or delocalized holes. The introduction of synthetic DNA capped hairpin constructs possessing hole donor and acceptor chromophores (or purine bases) at opposite ends of a base-pair domain made it possible to determine the time required for transit of charge from one chromophore to the other and, in some cases, to distinguish between the transit time and the much faster initial charge injection time. These studies eliminated conventional tunneling as a viable mechanism for charge transport in DNA except at very short donor-acceptor separations; however, they did not establish the presence or nature of intermediates in the charge separation process. Recent studies in our laboratories have succeeded in identifying key intermediates as well as untangling the dynamics and efficiency of the charge separation process from start to finish. The dynamics of the initial charge injection process is dependent upon both its free energy and the stacking of the hole donor chromophore and adjacent purine base. The transport of positive charge (holes) over multiple base pairs in duplex DNA occurs most efficiently via repeating adenine bases, known as A-tracts. The transit time across an A-tract is strongly dependent upon the free energy for hole injection, whereas the efficiency of charge separation depends on the competition between charge delocalization and charge recombination in the contact radical ion pair. The guanine cation radical has been detected both by femtosecond transient absorption and by stimulated Raman spectroscopies when the guanine is located near the chromophore employed for hole injection into an A-tract. Replacement of guanine by its derivative 8-phenylethynylguanine (EG), permits tracking of hole transport across longer poly(purine) sequences as a consequence of the stronger transient absorption and stimulated Raman scattering for EG+• vs G+•. We have recently obtained evidence based on femtosecond transient absorption spectroscopy for the formation of delocalized A-polarons in A-tracts possessing four or more A-T base pairs. Similar methods have been used to track hole transport across less-common DNA structures including diblock and triblock poly(purines), locked nucleic acids, three-way junctions, and G-quadruplexes. Similar methods are have been applied to the study of photoinduced electron transport in DNA.

Fluctuation in Interface and Electronic Structure of Single-Molecule Junctions Investigated by Current versus Bias Voltage Characteristics.

Structural and electronic detail at the metal-molecule interface has a significant impact on the charge transport across the molecular junctions, but its precise understanding and control still remain elusive. On the single-molecule scale, the metal-molecule interface structures and relevant charge transport properties are subject to fluctuation, which contain the fundamental science of single-molecule transport and implication for manipulability of the transport properties in electronic devices. Here, we present a comprehensive approach to investigate the fluctuation in the metal-molecule interface in single-molecule junctions, based on current-voltage ( I- V) measurements in combination with first-principles simulation. Contrary to conventional molecular conductance studies, this I- V approach provides a correlated statistical description of both the degree of electronic coupling across the metal-molecule interface and the molecular orbital energy level. This statistical approach was employed to study fluctuation in single-molecule junctions of 1,4-butanediamine (DAB), pyrazine (PY), 4,4'-bipyridine (BPY), and fullerene (C60). We demonstrate that molecular-dependent fluctuation of σ-, π-, and π-plane-type interfaces can be captured by analyzing the molecular orbital (MO) energy level under mechanical perturbation. While the MO level of DAB with the σ-type interface shows weak distance dependence and fluctuation, the MO level of PY, BPY, and C60 features unique distance dependence and molecular-dependent fluctuation against the mechanical perturbation. The MO level of PY and BPY with the σ+π-type interface increases with the increase in the stretch distance. In contrast, the MO level of C60 with the π-plane-type interface decreases with the increase in the stretching perturbation. This study provides an approach to resolve the structural and electronic fluctuation in the single-molecule junctions and insight into the molecular-dependent fluctuation in the junctions.

Electronic characterization of Geobacter sulfurreducens pilins in self-assembled monolayers unmasks tunnelling and hopping conduction pathways.

The metal-reducing bacterium Geobacter sulfurreducens produces protein nanowires (pili) for fast discharge of respiratory electrons to extracellular electron acceptors such as iron oxides and uranium. Charge transport along the pili requires aromatic residues, which cluster once the peptide subunits (pilins) assemble keeping inter-aromic distances and geometries optimal for multistep hopping. The presence of intramolecular aromatic contacts and the predominantly α-helical conformation of the pilins has been proposed to contribute to charge transport and rectification. To test this, we self-assembled recombinant, thiolated pilins as a monolayer on gold electrodes and demonstrated their conductivity by conductive probe atomic force microscopy. The studies unmasked a crossover from exponential to weak distance dependence of conductivity and shifts in the mechanical properties of the film that are consistent with a transition from interchain tunneling in the upper, aromatic-free regions of the helices to intramolecular hopping via aromatic residues at the amino terminus. Furthermore, the mechanistic stratification effectively "doped" the pilins at the amino terminus, favoring electron flow in the direction opposite to the helix dipole. However, the effect of aromatic dopants on rectification is voltage-dependent and observed only at the low (100 mV) voltages that operate in biological systems. The results thus provide evidence for a peptide environment optimized for electron transfer at biological voltages and in the direction needed for the respiration of external electron acceptors. The implications of these results for the development of hybrid devices that harness the natural abilities of the pilins to bind and reduce metals are discussed.

The local electronic properties of individual Pt atoms adsorbed on TiO2(110) studied by Kelvin probe force microscopy and first-principles simulations.

Noble metal nanostructures dispersed on metal oxide surfaces have applications in diverse areas such as catalysis, chemical sensing, and energy harvesting. Their reactivity, chemical selectivity, stability, and light absorption properties are controlled by the interactions at the metal/oxide interface. Single-atom metal adsorbates on the rutile TiO2(110)-(1 × 1) surface have become a paradigmatic model to characterize those interactions and to understand the unique electronic properties of these supported nanostructures. We combine Kelvin probe force microscopy (KPFM) experiments and density functional theory (DFT) calculations to investigate the atomic-scale variations in the contact potential difference of individual Pt atoms adsorbed on a hydroxylated (h) TiO2(110)-(1 × 1) surface. Our experiments show a significant drop in the local contact potential difference (LCPD) over Pt atoms with respect to the TiO2 surface, supporting the presence of an electron transfer from the Pt adsorbates to the substrate. We have identified two characteristic regimes by LCPD spectroscopy. At far tip-sample distances, LCPD values show a weak distance dependence and can be attributed to the intrinsic charge transfer from Pt to the oxide support. Beyond the onset of short-range chemical interactions, LCPD values exhibit a strong distance dependence that we ascribe to the local structural and charge rearrangements induced by the tip-sample interaction. These findings also apply to other electropositive adsorbates such as potassium and the hydrogen atoms forming the OH groups that are present on the h-TiO2(110) surface, promoting KPFM as a suitable tool for the understanding of electron transfer in catalytically active materials.

Deep-hole transfer leads to ultrafast charge migration in DNA hairpins.

Charge transport through the DNA double helix is of fundamental interest in chemistry and biochemistry, but also has potential technological applications such as for DNA-based nanoelectronics. For the latter, it is of considerable interest to explore ways to influence or enhance charge transfer. In this Article we demonstrate a new mechanism for DNA charge transport, namely 'deep-hole transfer', which involves long-range migration of a hole through low-lying electronic states of the nucleobases. Here, we demonstrate, in a combined experimental and theoretical study, that it is possible to achieve such transfer behaviour by changing the energetics of charge injection. This mechanism leads to an enhancement in transfer rates by up to two orders of magnitude and much weaker distance dependence. This transfer is faster than relaxation to the lowest-energy state, setting this mechanism apart from those previously described. This opens up a new direction to optimize charge transfer in DNA with unprecedented charge-transfer rates.

Medium Recoil and Jet Modification in Heavy Ion Collisions

A complete set of elastic processes and induced gluon radiation within the higher-twist approach have been implemented in the Linear Boltzmann Transport model for jet propagation and interaction with quark-gluon plasma in high-energy heavy-ion collisions. We impose global energy momentum conservation in the 2→n processes of induced radiation which will influence the final gluon spectra. We will compare the elastic and the radiative energy loss of partons and their effects on reconstructed jets. The energy loss of a leading parton is found to have a quadratic distance dependence only for a short distance, but will have much weak distance dependence because of the accumulated energy loss and the strong energy dependence of the local energy loss rate. Since reconstructed jets recover some of the energy lost by the leading parton, the quadratic path length dependence persists for a longer distance. The spatial distribution and time evolution of the jet-induced medium excitation are also discussed.

Electron Transfer Rate Maxima at Large Donor-Acceptor Distances.

Because of their low mass, electrons can transfer rapidly over long (>15 Å) distances, but usually reaction rates decrease with increasing donor-acceptor distance. We report here on electron transfer rate maxima at donor-acceptor separations of 30.6 Å, observed for thermal electron transfer between an anthraquinone radical anion and a triarylamine radical cation in three homologous series of rigid-rod-like donor-photosensitizer-acceptor triads with p-xylene bridges. Our experimental observations can be explained by a weak distance dependence of electronic donor-acceptor coupling combined with a strong increase of the (outer-sphere) reorganization energy with increasing distance, as predicted by electron transfer theory more than 30 years ago. The observed effect has important consequences for light-to-chemical energy conversion.

Emission Quenching of Magnetic Dipole Transitions near a Metal Nanoparticle

The emission quenching of magnetic dipole transitions due to electromagnetic coupling to a metal nanoparticle is studied theoretically. We show that, at nanometer distances to the nanoparticle surface, the quenching is much weaker than that of electric dipole transitions, resulting in far higher radiative quantum efficiencies. This difference is explained by the fact that the electric field induced by an oscillating magnetic dipole and responsible for the energy transfer to the metal has a weaker distance dependence than the electric field of an electric dipole. Our results imply that magnetic dipole transitions may be superior to electric ones if coupling to a metallic nanoantenna over sub-10 nm distances is used to enhance optical emission from a quantum emitter.

Unusual Weak Interparticle Distance Dependence in Raman Enhancement from Nanoparticle Dimers

Here, we report surprisingly weak distance dependence in Raman enhancement from a raspberry-like gold nanoparticle termed raspberry-MM. A raspberry-MM is composed of closely packed gold nanobeads assembled on a polymer core. Due to the abundant “built-in” hot spots between adjacent gold nanobeads, bright and uniform Raman signals were observed from isolated single raspberry-MMs. Interestingly, dimers of raspberry-MMs also showed highly reproducible Raman signals, indicating that the dimer SERS signal is not strongly dependent on the nanoparticle separation. Finite-difference time-domain (FDTD) modeling shows that a strong hot spot is created at the dimer-gap, as expected. However, since there are many more built-in hot spots in each raspberry-MM, the contribution of the dimer-gap hot spot to the total Raman enhancement remains low even for 2 nm separation, which explains the observed weak distance dependence. This result is in stark contrast with many previous SERS studies on nanoparticle dimers and clusters, and provides an important guideline on how to design bright and highly reproducible Raman substrates.

Hopping versus Tunneling Mechanism for Long-Range Electron Transfer in Porphyrin Oligomer Bridged Donor-Acceptor Systems.

Achieving long-range charge transport in molecular systems is interesting to foresee applications of molecules in practical devices. However, designing molecular systems with pre-defined wire-like properties remains difficult due to the lack of understanding of the mechanism for charge transfer. Here we investigate a series of porphyrin oligomer-bridged donor-acceptor systems Fc-Pn-C60 (n = 1-4, 6). In these triads, excitation of the porphyrin-based bridge generates the fully charge-separated state, Fc(•+)-Pn-C60(•-), through a sequence of electron transfer steps. Temperature dependence of both charge separation (Fc-Pn*-C60 → Fc-Pn(•+)-C60(•-)) and recombination (Fc(•+)-Pn-C60(•-) → Fc-Pn-C60) processes was probed by time-resolved fluorescence and femtosecond transient absorption. In the long triads, two mechanisms contribute to recombination of Fc(•+)-Pn-C60(•-) to the ground state. At high temperatures (≥280 K), recombination via tunneling dominates for the entire series. At low temperatures (<280 K), unusual crossover from tunneling to hopping occurs in long triads. This crossover is rationalized by the increased lifetimes of Fc(•+)-Pn-C60(•-), hence the higher probability of reforming Fc-Pn(•+)-C60(•-) during recombination. We demonstrate that at 300 K, the weak distance dependence for charge transfer (β = 0.028 Å(-1)) relies on tunneling rather than hopping.

Blending Through-Space and Through-Bond π–π-Coupling in [2,2′]-Paracyclophane-oligophenylenevinylene Molecular Wires

A series of ZnP-pCp-oPPV-C60 conjugates covalently connected through [2,2']-paracyclophane-oligophenylenevinylene (pCp-oPPV) bridges containing one, two, and three [2,2']-paracyclophanes (pCps) has been prepared in multistep synthetic procedures involving Horner-Wadsworth-Emmons olefination reactions and/or Heck type Pd-catalyzed reactions. Molecular modeling suggests that charge transfer is effectively mediated by the pCp-oPPVs through a predominant hole-transfer mechanism. Photophysical investigation supports molecular modeling and reveals two major trends. On one hand, C60 excitation of 1, 2, and 3 leads exclusively to charge transfer between pCp and C60 to afford a ZnP-(pCp-oPPV)(•+)-C60(•-) radical ion pair state without giving rise to a subsequent charge shift to yield the ZnP(•+)-pCp-oPPV-C60(•-) radical ion pair state. On the other hand, ZnP excitation of 1, 2, and 3 results in a rather slow charge transfer between ZnP and C60, after which the ZnP(•+)-pCp-oPPV-C60(•-) radical ion pair state evolves. In temperature-dependent ZnP fluorescence experiments, which were performed in the temperature range from 273 to 338 K, two domains are discernible: low and high temperature behaviors. In the low temperature range (i.e., below 30 °C) the rate constants do not change, suggesting that a superexchange mechanism is the modus operandi. In the high temperature range (i.e., >30 °C) the rate constants increase. Moreover, we find rather strong distance dependence for 1 and 2 and weak distance dependence for 2 and 3. A damping factor of 0.145 Å(-1) is derived for the former pair and 0.012 Å(-1) for the latter.

Fluorescence Quenching over Short Range in a Donor‐DNA‐Acceptor System

A new donor-DNA-acceptor system has been synthesized containing Nile red-modified 2'-deoxyuridine as charge donor and 6-N,N-dimethylaminopyrene-modified 2'-deoxyuridine as acceptor to investigate the charge transfer in DNA duplexes using fluorescence spectroscopy and time-resolved femtosecond pump-probe techniques. Fluorescence quenching experiments revealed that the quenching efficiency of Nile red depends on two components: 1) the presence of a charge acceptor and 2) the number of intervening CG and AT base pairs between donor and acceptor. Surprisingly, the quenching efficiency of two base pairs (73% for CG and the same for AT) is higher than that for one base pair (68% for CG and 37% for AT), while at a separation of three base pairs less than 10% quenching is observed. A comparison with the results of time-resolved measurements revealed a correlation between quenching efficiency and the first ultrafast time constant suggesting that quenching proceeds via a charge transfer from the donor to the acceptor. All transients are satisfactorily described with two decays: a rapid charge transfer with 600 fs (∼10(12) s(-1)) that depends strongly and in a non-linear fashion on the distance between donor and acceptor, and a slower time constant of a few picoseconds (∼10(11) s(-1)) with weak distance dependence. A third time constant on a nanosecond time scale represents the fluorescence lifetime of the donor molecule. According to these results and time-dependent density functional theory (TDDFT) calculations a combination of single-step superexchange and multistep hopping mechanisms can be proposed for this short-range charge transfer. Furthermore, significantly less quenching efficiency and slower charge transfer rates at very short distances indicate that the direct interaction between donor and acceptor leads to a local structural distortion of DNA duplexes which may provide some uncertainty in identifying the charge transfer rates in short-range systems.

Distance dependence of hole transfer rates from G radical cations to GGG traps in DNA

Relative reaction rates for hole transfer between G radical cations and GGG triplets in DNA, through different bridges of varying lengths, are numerically calculated and the obtained results are compared with corresponding experimental observations [Giese et al., 2001, Nature, 412, 318; Angew. Chem., Int. Ed., 1999, 38, 996]. Hole donors and acceptors are separated either by (T-A)n bridges or by N repeated barriers consisting of (T-A,T-A) double base-pairs which are connected through single G-C base-pairs. In the former case, hole transfer rates show a strong exponential decrease with the length of the bridge for short bridges, while a switching to weak distance dependence has been observed for longer bridges. In the latter case, a power law seems to better describe the distance dependence of charge transfer rates. All these experimental observations are qualitatively reproduced by our simulations without any adjustable parameter, considering only tunneling as the charge transfer mechanism. Physical insights into the mechanism providing the switching behavior in the case of (T-A)n bridges are presented through an analysis of the eigenfunctions of the system.

Interenzyme Substrate Diffusion for an Enzyme Cascade Organized on Spatially Addressable DNA Nanostructures

Spatially addressable DNA nanostructures facilitate the self-assembly of heterogeneous elements with precisely controlled patterns. Here we organized discrete glucose oxidase (GOx)/horseradish peroxidase (HRP) enzyme pairs on specific DNA origami tiles with controlled interenzyme spacing and position. The distance between enzymes was systematically varied from 10 to 65 nm, and the corresponding activities were evaluated. The study revealed two different distance-dependent kinetic processes associated with the assembled enzyme pairs. Strongly enhanced activity was observed for those assemblies in which the enzymes were closely spaced, while the activity dropped dramatically for enzymes as little as 20 nm apart. Increasing the spacing further resulted in a much weaker distance dependence. Combined with diffusion modeling, the results suggest that Brownian diffusion of intermediates in solution governed the variations in activity for more distant enzyme pairs, while dimensionally limited diffusion of intermediates across connected protein surfaces contributed to the enhancement in activity for closely spaced GOx/HRP assemblies. To further test the role of limited dimensional diffusion along protein surfaces, a noncatalytic protein bridge was inserted between GOx and HRP to connect their hydration shells. This resulted in substantially enhanced activity of the enzyme pair.

Role of Bridge Energetics on the Preference for Hole or Electron Transfer Leading to Charge Recombination in Donor-Bridge-Acceptor Molecules

The impact of donor-acceptor electronic coupling and bridge energetics on the preference for hole or electron transfer leading to charge recombination in a series of donor-bridge-acceptor (D-B-A) molecules was examined. In these systems, the donor is 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) and acceptor is naphthalene-1,8:4,5-bis(dicarboximide) (NI), while the bridges are either oligo(p-phenyleneethynylene) (PE(n)P, where n = 1-3) 1-3 or oligo(2,7-fluorenone) (FN(n), where n = 1-3) 4-6. Photoexcitation of 1-3 and 4-6 produces DMJ(+•)-An-PE(n)P-NI(-•) and DMJ(+•)-An-FN(n)-NI(-•), respectively, which undergo radical pair intersystem crossing followed by charge recombination to yield both (3*)An and (3*)NI, which are observed by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. (3*)NI is produced by hole transfer from DMJ(+•) to NI(-•), while (3*)An is produced by electron transfer from NI(-•) to DMJ(+•), using the agency of the bridge HOMOs and LUMOs, respectively. By monitoring the initial population of (3*)NI and (3*)An in 1-6, the data show that charge recombination occurs preferentially by selective hole transfer when the bridge is PE(n)P, while it occurs by preferential electron transfer when the bridge is FN(n). Over time, the initial population of (3*)NI decreases, while that of (3*)An increases, indicating that triplet-triplet energy transfer (TEnT) occurs. The observed distance dependence of TEnT from (3*)NI to An is weakly exponential with a decay parameter β = 0.08 Å(-1) for the PE(n)P series and β = 0.03 Å(-1) for the FN(n) series. In the PE(n)P series, this weak distance dependence is attributed to a transition from the superexchange regime to hopping transport as the energy gap for triplet energy injection onto the bridge becomes significantly smaller as n increases, while in the FN(n) series the corresponding energy gap is small for all n resulting in triplet energy transport by the hopping mechanism.

Theoretical Study of Long-Range Electron Transport in Molecular Junctions

A study of long-range electron transport through a series of molecular junctions comprised of π-conjugated molecules is presented. A theoretical model is built by using a quantum chemistry based Landauer theory combined with Green’s function formalism and the D’Amato−Pastawski model to describe the electronic conduction in the presence of incoherent scattering effects. Effects of incoherent transport on the total conductance under the nonbiased, stationary, and nondissipative conditions are investigated. Calculation results with the Newns−Anderson approximation show that the distance dependence of electron conduction through molecular wires smoothly interpolates the transition of the dominant transporting mechanism from coherent tunneling with the exponential decay in short distance to incoherent tunneling associated with sequential hopping with weak distance dependence.