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The nature and dynamics of the bound charge-transfer (CT) state in the exciton dissociation process in organic solar cells are of critical importance for the understanding of these devices. It was recently demonstrated that this state can be probed by a new experiment in which an infrared (IR) push-pulse is used to dissociate charges from the bound excited state. Here we proposed a simple quantum dynamics model to simulate the excitation of the IR pulse on the bound CT state with model parameters extracted from quantum chemical calculations. We show that the pulse dissociates the CT state following two different mechanisms: one, fairly expected, is the direct excitation of higher energy CT states leading to charge separation; the other, proposed here for the first time, is a rebound mechanism in which the negative charge is transferred in the opposite direction to form the neutral Frenkel exciton state from where it dissociates.

The dependence of charge-transfer states on interfacial geometry at the phthalocyanine/fullerene organic photovoltaic system is investigated. The effect of deviations from the equilibrium geometry of the donor-donor-acceptor trimer on the energies of and electronic coupling between different types of interfacial electronic excited states is calculated from first-principles. Deviations from the equilibrium geometry are found to destabilize the donor-to-donor charge transfer states and to weaken their coupling to the photoexcited donor-localized states, thereby reducing their ability to serve as charge traps. At the same time, we find that the energies of donor-to-acceptor charge transfer states and their coupling to the donor-localized photoexcited states are either less sensitive to the interfacial geometry or become more favorable due to modifications relative to the equilibrium geometry, thereby enhancing their ability to serve as gateway states for charge separation. Through these findings, we eludicate how interfacial geometry modifications can play a key role in achieving charge separation in this widely studied organic photovoltaic system.

This paper discusses the mechanism of generation of free charges in organic photovoltaic cells (OPV) from electrostatically bound electron–hole pairs. The efficiency of this process is explained when interfacial charge-transfer (CT) states are generated by direct optical excitation. We used semiclassical quantum dynamics at a short time scale (∼100 fs) and Redfield theory at a relatively long time scale (∼10–100 ps) to cover both the process of dissociation and the relaxation to the lowest energy state. Our calculations suggest that a CT state with an intermediate electron–hole separation can evolve into a charge-separated (CS) state on ultrafast time scales (∼100 fs) as a result of quantum diffusion. On long time scales, however, the CS states ultimately relax to the low-energy CT states due to the interaction with the thermal bath, indicating that the yield of free charge carrier generation is determined by the interplay between ultrafast charge separation, due to quantum diffusion, and the much slower quantum relaxation process.

Recently developed optimally tuned range-separated hybrid (OT-RHS) functionals within time-dependent density functional theory have been shown to address existing limitations in calculating charge transfer excited state energies. The RSH success in improving the calculation of CT states stems from enforcing the correspondence of the frontier molecular orbitals (FMOs) to physical properties, where the highest occupied MO energy relates to the ionization potential and the lowest unoccupied MO energy relates to the electron affinity. However, in this work, we show that a less accurate description of CT states that involves non-FMOs is afforded by the RSH approach. In order to achieve a high quality description of such higher energy CT states, the parameter tuning procedure, which lies at the foundation of the RSH approach, needs to be generalized to consider the CT process. We demonstrate the need for improved description of such CT states in donor-acceptor systems, where the optimal tuning parameter is accounting for the state itself.

In this paper, we adopt an approach suitable for monitoring the time evolution of the intramolecular contribution to the spectral density of a set of identical chromophores embedded in their respective environments. We apply the proposed method to the Fenna-Matthews-Olson (FMO) complex, with the objective to quantify the differences among site-dependent spectral densities and the impact of such differences on the exciton dynamics of the system. Our approach takes advantage of the vertical gradient approximation to reduce the computational demands of the normal modes analysis. We show that the region of the spectral density that is believed to strongly influence the exciton dynamics changes significantly in the timescale of tens of nanoseconds. We then studied the impact of the intramolecular vibrations on the exciton dynamics by considering a model of FMO in a vibronic basis and neglecting the interaction with the environment to isolate the role of the intramolecular exciton-vibration coupling. In agreement with the assumptions in the literature, we demonstrate that high frequency modes at energy much larger than the excitonic energy splitting have negligible influence on exciton dynamics despite the large exciton-vibration coupling. We also find that the impact of including the site-dependent spectral densities on exciton dynamics is not very significant, indicating that it may be acceptable to apply the same spectral density on all sites. However, care needs to be taken for the description of the exciton-vibrational coupling in the low frequency part of intramolecular modes because exciton dynamics is more susceptible to low frequency modes despite their small Huang-Rhys factors.

Charge transfer (CT) states formed at the donor/acceptor heterointerface are key for photocurrent generation in organic photovoltaics (OPV). Our calculations show that interfacial donor-to-donor CT states in the phthalocyanine-fullerene OPV system may be more stable than donor-to-acceptor CT states and that they may rapidly recombine, thereby constituting a potentially critical and thus far overlooked loss mechanism. Our results provide new insight into processes that may compete with charge separation, and suggest that the efficiency for charge separation may be improved by destabilizing donor-to-donor CT states or decoupling them from other states.

The feasibility of calculating photoinduced intramolecular electron transfer rate constants in realistic molecular donor–acceptor systems via Fermi’s golden rule, using inputs obtained from state-of-the-art electronic structure techniques, is demonstrated and tested. To this end, calculations of photoinduced electron transfer rate constants were performed on two benchmark systems: (1) phenylacetylene-bridged carbazole-naphthalimide (meta and para) and (2) C60-(N,N-dimethylaniline). Intramolecular input parameters such as normal-mode frequencies, Huang–Rhys factors, and electronic coupling coefficients were obtained via ground state, time-dependent, and constrained density functional theory. Good agreement between the intramolecular Fermi’s golden rule rate constants and the experimental rate constants is found for both systems without accounting for the solvent reorganization. The relative roles of intramolecular vs intermolecular modes at promoting electron transfer and the validity of several limits of ...

Graphene layers have been targeted in the last years as excellent host materials for sensing a remarkable variety of gases and molecules. Such sensing abilities can also benefit other important scientific fields such as medicine and biology. This has automatically led scientists to probe graphene as a potential platform for sequencing DNA strands. In this work, we use robust numerical tools to model the dynamic and electronic properties of molecular sensor devices composed of a graphene nanopore through which DNA molecules are driven by external electric fields. We performed molecular dynamic simulations to determine the relation between the intensity of the electric field and the translocation time spent by the DNA to pass through the pore. Our results reveal that one can have extra control on the DNA passage when four additional graphene layers are deposited on the top of the main graphene platform containing the pore in a 2 × 2 grid arrangement. In addition to the dynamic analysis, we carried electronic transport calculations on realistic pore structures with diameters reaching nanometer scales. The transmission obtained along the graphene sensor at the Fermi level is affected by the presence of the DNA. However, it is rather hard to distinguish the respective nucleobases. This scenario can be significantly altered when the transport is conducted away from the Fermi level of the graphene platform. Under an energy shift, we observed that the graphene pore manifests selectiveness toward DNA nucleobases.

The quaternary aluminium hydrides SrAlGeH and BaAlGeH were synthesized from either hydrogenating the intermetallic AlB2-type precursors SrAlGe and BaAlGe or reacting SrH2 with a mixture of Al and G ...

Vibrational properties of the gallium monohydrides SrGaGeH, BaGaSiH, BaGaGeH, and BaGaSnH (AeGaTtH) have been investigated by means of inelastic neutron scattering (INS) and first principles calculations. The compounds contain separated Ga–H units being part of a two dimensional polyanionic layer, [TtGaH]2− (Tt=Si, Ge, Sn). The INS spectra show internal Ga–H bending and stretching modes at frequencies around 900 and 1200 cm−1, respectively. While the stretching mode is virtually invariant with respect to the variable chemical environment of the Ga–H unit, the bending mode frequency varies and is highest for BaGaSiH and lowest for BaGaSnH. The stretching mode is a direct measure of the Ga–H bond strength, whereas the bending mode reflects indirectly the strength of alkaline earth metal–hydrogen interaction. Accordingly, the terminal Ga–H bond in solid state AeGaTtH is distinct, but—compared to molecular gallium hydrides—very weak.

Hydrogen bonding has a ubiquitous role in electron transport and in molecular recognition, with DNA base pairing being the best-known example. Scanning tunnelling microscope images and measurements of the decay of tunnel current as a molecular junction is pulled apart by the scanning tunnelling microscope tip are sensitive to hydrogen-bonded interactions. Here, we show that these tunnel-decay signals can be used to measure the strength of hydrogen bonding in DNA base pairs. Junctions that are held together by three hydrogen bonds per base pair (for example, guanine-cytosine interactions) are stiffer than junctions held together by two hydrogen bonds per base pair (for example, adenine-thymine interactions). Similar, but less pronounced effects are observed on the approach of the tunnelling probe, implying that attractive forces that depend on hydrogen bonds also have a role in determining the rise of current. These effects provide new mechanisms for making sensors that transduce a molecular recognition event into an electronic signal.

The vibrational properties of the recently discovered aluminum hydrides SrAl2H2 and SrAlSiH have been investigated by means of inelastic neutron scattering (INS) and first-principles calculations. Both compounds contain Al-H units being part of a two-dimensional polyanionic layer, [(AlH)(AlH)]2- and [Si(AlH)]2-, respectively. The INS spectrum of SrAlSiH is characterized by very weakly dispersed Al-H modes with well-resolved overtones, while SrAl2H2 yields a solid-state dispersed phonon spectrum. The frequency of the stretching mode of the Al-H unit in SrAlSiH is the hitherto lowest observed for a terminal Al-H bond. At the same time, SrAlSiH displays the highest decomposition temperature known for an aluminum hydride compound. It is proposed that the stability of solid-state aluminum hydrides correlates inversely with the strength of Al-H bonding.

Photoinduced charge-transfer (CT) processes play a key role in many systems, particularly those relevant to organic photovoltaics and photosynthesis. Advancing the understanding of CT processes calls for comparing their rates measured via state-of-the-art time-resolved interface-specific spectroscopic techniques with theoretical predictions based on first-principles molecular models. We measure charge-transfer rates across a boron subphthalocyanine chloride (SubPc)/C60 heterojunction, commonly used in organic photovoltaics, via heterodyne-detected time-resolved second-harmonic generation. We compare these results to theoretical predictions based on a Fermi's golden rule approach, with input parameters obtained using first-principles calculations for two different equilibrium geometries of a molecular donor-acceptor in a dielectric continuum model. The calculated rates (∼2 ps(-1)) overestimate the measured rates (∼0.1 ps(-1)), which is consistent with the expectation that the calculated rates represent an upper bound over the experimental ones. The comparison provides valuable understanding of how the structure of the electron donor-acceptor interface affects the CT kinetics in organic photovoltaic systems.

We report theoretical simulations of the effects of different binding sites and metal contact geometries on the conductance through single alkanedithiol molecules on gold. Examined are the current-voltage (I- V) characteristics and the projected density of states when a one-atom-thick filament is formed at the molecule-metal interface. Filaments similar to these are believed to be formed in gold STM break-junction experiments. I- V characteristics with these contacts are compared to those of planar gold contacts at hollow or on-top sites. The results reveal that a gold filament contact produces a large change in the Fermi level lineup which significantly affects its I-V curve. The conductance decreases by a factor of 4 ∼ 6 when gold atoms are placed between the molecule and the gold surface to form a filament, and non-linearities develop. The hollow and on-top planar contacts show similar conductivities to each other and produce linear I-V curve.

BaInGeH was synthesized by hydrogenating the intermetallic compound BaInGe. The crystal structure determination from the powder neutron diffraction data of BaInGeD [P3m1, Z = 1, a = 4.5354(3) A, c = 5.2795(6) A] reveals the presence of hydrogen in tetrahedral voids defined by three Ba atoms and one In atom.

It has been reported in recent years that vibronic resonance between vibrational energy of the intramolecular nuclear mode and excitation-energy difference is crucial to enhance excitation energy transport in light harvesting proteins. Here we investigate how vibronic enhancement induced by vibronic resonance is influenced by the details of local and non-local exciton-phonon interactions. We study a heterodimer model with parameters relevant to the light-harvesting proteins with the surrogate Hamiltonian quantum dynamics method in a vibronic basis. In addition, the impact of field-driven excitation on the efficiency of population transfer is compared with the instantaneous excitation, and the effect of multi-mode vibronic coupling is presented in comparison with the coupling to a single effective vibrational mode. We find that vibronic enhancement of site population transfer is strongly suppressed with the increase of non-local exciton-phonon interaction and increasing the number of strongly coupled high-frequency vibrational modes leads to a further decrease in vibronic enhancement. Our results indicate that vibronic enhancement is present but may be much smaller than previously thought and therefore care needs to be taken when interpreting its role in excitation energy transport. Our results also suggest that non-local exciton-phonon coupling, which is related to the fluctuation of the excitonic coupling, may be as important as local exciton-phonon coupling and should be included in any quantum dynamics model.

Vibronic coupling between the electronic and vibrational degrees of freedom has been reported to play an important role in charge and exciton transport in organic photovoltaic materials, molecular aggregates, and light-harvesting complexes. Explicitly accounting for effective vibrational modes rather than treating them as a thermal environment has been shown to be crucial to describe the effect of vibronic coupling. We present a methodology to study dissipative quantum dynamics of vibronically coupled systems based on a surrogate Hamiltonian approach, which is in principle not limited by Markov approximation or weak system-bath interaction, using a vibronic basis. We apply vibronic surrogate Hamiltonian method to a linear chain system and discuss how different types of relaxation process, intramolecular vibrational relaxation and intermolecular vibronic relaxation, influence population dynamics of dissipative vibronic systems.

We present a detailed study of the charge transport characteristics of double-stranded DNA oligomers including the oxidative damage 7,8-dihydro-8-oxoguanine (8-oxoG). The problem is treated by a hybrid methodology combining classical molecular dynamics simulations and semiempirical electronic structure calculations to formulate a coarse-grained charge transport model. The influence of solvent- and DNA-mediated structural fluctuations is encoded in the obtained time series of the electronic charge transfer parameters. Within the Landauer approach to charge transport, we perform a detailed analysis of the conductance and current time series obtained by sampling the electronic structure along the molecular dynamics trajectory, and find that the inclusion of 8-oxoG damages into the DNA sequence can induce a change in the electrical response of the system. However, solvent-induced fluctuations tend to mask the effect, so that a detection of such sequence modifications via electrical transport measurements in a liquid environment seems to be difficult to achieve.

Charge transport characteristics of short double-strand (ds) DNA including mismatches are stud- ied within a methodology combining molecular dynamics (MD) simulations and electronic structure calculations based on a fragment orbital approach. Electronic parameters and transmission prob- abilities are computed along the MD trajectory. We find that in the course of the MD simulation the energetic position of frontier orbitals may be interchanged. As a result, the highest occupied molecular orbital (HOMO) can temporarily have a large weight on the backbones as a function of time. This shows that care must be taken when projecting the electronic structure onto effective low-dimensional model Hamiltonians to calculate transport properties. Further, the transport cal- culations indicate a suppression of the charge migration efficiency when introducing a single GT or AC mismatch in the DNA sequence.

BaInGeH was synthesized by hydrogenating the intermetallic compound BaInGe. The crystal structure determination from the powder neutron diffraction data of BaInGeD [P3m1, Z = 1, a = 4.5354(3) A, c = 5.2795(6) A] reveals the presence of hydrogen in tetrahedral voids defined by three Ba atoms and one In atom.

Using a tunnel junction in which one electrode is guanidinium-functionalized (to trap DNA via hydrogen bonding to the backbone phosphates) and a second electrode which is functionalized with a base (to capture its complementary target on the DNA), current versus distance curves are obtained which yield an accurate measure of the base composition of DNA oligomers. With this long tunneling path, resolution is limited to sequence blocks of about twenty bases or larger, because of the need to form a large-area tunnel junction. A shorter hydrogen-bonded path across bases will be required for DNA sequencing. Nonetheless, these measurements point the way to a new type of nanoscale sensor.

We present the results of first-principles calculations for the electron tunnel current through hydrogen-bonded DNA base pairs and for (deoxy)nucleoside-nucleobase pairs. Electron current signals either through a base pair or through a deoxynucleoside-nucleobase pair are a potential mechanism for recognition or identification of the DNA base on a single-stranded DNA polymer. Four hydrogen-bonded complexes are considered: guanine-cytosine, diaminoadenine-thymine, adenine-thymine, and guanine-thymine. First, the electron tunneling properties are examined through their complex band structure (CBS) and the metal contact's Fermi-level alignment. For gold contacts, the metal Fermi level lies near the highest occupied molecular orbital for all DNA base pairs. The decay constant determined by the complex band structure at the gold Fermi level shows that tunnel current decays more slowly for base pairs with three hydrogen bonds (guanine-cytosine and diaminoadenine-thymine) than for base pairs with two hydrogen bonds (adenine-thymine and guanine-thymine). The decay length and its dependence on hydrogen-bond length are examined. Second, the conductance is computed using density functional theory Green's-function scattering methods and these results agree with estimates made from the tunneling decay constant obtained from the CBS. Changing from a base pair to a deoxynucleoside-nucleobase complex shows a significant decrease in conductance. It also becomes difficult to distinguish the current signal by only the number of hydrogen bonds.

Superconducting AlB2-type silicides CaAlSi, SrAlSi, and BaAlSi (MAlSi) absorb hydrogen and form semiconducting monohydrides where hydrogen is exclusively attached to Al. This induces a metal-nonmet ...

Vibrational properties of the gallium hydrides SrGa2H2 and BaGa2H2 have been investigated by means of inelastic neutron scattering (INS) and first-principles calculations. The compounds contain Ga-H units being part of a two-dimensional polyanionic layer, [(GaH)(GaH)]2-. The INS spectra are composed of dispersed internal Ga-H bending and stretching modes at frequencies above 600 cm(-1) and external lattice modes at frequencies below 220 cm(-1). Frequencies of the internal modes are not susceptible to the metal countercation, indicating a strong integrity of the polyanionic layer as a building unit in the structures of SrGa2H2 and BaGa2H2. The Ga-H stretching modes have frequencies between 1200 and 1400 cm(-1), which is very low compared to molecular gallium hydrides. The weak Ga-H bond in SrGa2H2 and BaGa2H2 is balanced by Sr(Ba)-H interactions.

Single-molecule conductance measurements made under potential control provide a critical link between chemical and molecular electronic data. These measurements are made possible by the STM break-junction method introduced recently, but questions remain about its reliability. Here we report the use of a logarithmic current-to-voltage converter to examine a wide range of currents in an STM break junction study of octanedithiol, clearly showing both the gold-quantum wire regime and the single molecule conductance regime. We find two sets of molecular currents that we tentatively ascribe to different bonding geometries of the molecules in the break junction.

The rates of interfacial charge transfer and recombination between the donor and acceptor layers play a key role in determining the performance of organic photovoltaic cells. The time scale and mechanism of these processes are expected to be impacted by the structure of the interface. In this paper we model the kinetics of those processes within the framework of a subphthalocyanine/fullerene donor/acceptor dimer model. Two likely configurations (on-top and hollow) in which the interfacial charge transfer and recombination may occur are studied. The corresponding rate constants are calculated within the fully quantum-mechanical framework of Fermi’s golden rule. All the input parameters (excitation energies, electronic coupling coefficients, normal-mode frequencies and coordinates, and Huang–Rhys factors) are obtained from density functional theory calculations with density functionals designed to yield accurate results in the case of noncovalently bound systems and charge transfer states. Multiple π–π* and...

We report a first-principles study of electron ballistic transport through a molecular junction containing deoxycytidine-monophosphate (dCMP) connected to metal electrodes. A guanidinium ion and guanine nucleobase are tethered to gold electrodes on opposite sides to form hydrogen bonds with the dCMP molecule providing an electric circuit. The circuit mimics a component of a potential device for sequencing unmodified single-stranded DNA. The molecular conductance is obtained from DFT Green's function scattering methods and is compared to estimates from the electron tunneling decay constant obtained from the complex band structure. The result is that a complete molecular dCMP circuit of 'linker((CH(2))(2))-guanidinium-phosphate-deoxyribose-cytosine-guanine' has a very low conductance (of the order of fS) while the hydrogen-bonded guanine-cytosine base-pair has a moderate conductance (of the order of tens to hundreds of nS). Thus, while the transverse electron transfer through base-pairing is moderately conductive, electron transfer through a complete molecular dCMP circuit is not. The gold Fermi level is found to be aligned very close to the HOMO for both the guanine-cytosine base-pair and the complete molecular dCMP circuit. Results for two different plausible geometries of the hydrogen-bonded dCMP molecule reveal that the conductance varies from fS for an extended structure to pS for a slightly compressed structure.

We present theoretical results for the electron transport properties of the organic molecule polyaniline, especially leucoemeraldine (LEB), the fully reduced form. The electron tunnelling characteristics of these chain-like molecules are described by their complex band-structure. We explore how the bandgap and tunnelling decay parameter β depend on the oxidation state of the molecule and on the torsion angle between rings. It is found that the metal Fermi level lies near the HOMO for gold contacts with a single leucoemeraldine molecule, which results in non-linear I–V characteristics. The conductance of a hepta-aniline (LEB) oligomer is obtained from a first-principles I–V curve and compared with the recent experimental results. We examine the effect of stretching of the molecule on its conductance to explain the discrepancy between the theoretical simulations and single-molecule conductance measurement experiment.