Polyacene Molecules Research Articles

New promising class of anode materials for Ca-ion battery: polyaromatic hydrocarbons

Using first-principles calculations we propose new anode materials for calcium-ion batteries, namely anthracene (AN), tetracene (TN) and pentacene (PN) crystals. We show that adsorption of calcium atoms on isolated AN, TN and PN molecules is energetically favorable, as well as intercalation into bulk crystals in a wide range of calcium concentrations. For all crystals, volume expansion during intercalation is less than 20% and for pentacene, it is less than 8%. We assess the diffusion and electronic properties of AN, TN and PN. We show that calcium diffusion barriers along the polyacene molecules are less than 0.45 eV, but calcium diffusion is limited by “jumps” between the molecules. Sequential band filling upon increasing levels of intercalation leads to the reentrant semiconducting-metallic-semiconducting behavior. Our results point to the promise of anthracene, tetracene, and pentacene as anodes for calcium-ion batteries.

Perturbation theory with quantum signal processing

Perturbation theory is an important technique for reducing computational cost and providing physical insights in simulating quantum systems with classical computers. Here, we provide a quantum algorithm to obtain perturbative energies on quantum computers. The benefit of using quantum computers is that we can start the perturbation from a Hamiltonian that is classically hard to solve. The proposed algorithm uses quantum signal processing (QSP) to achieve this goal. Along with the perturbation theory, we construct a technique for ground state preparation with detailed computational cost analysis, which can be of independent interest. We also estimate a rough computational cost of the algorithm for simple chemical systems such as water clusters and polyacene molecules. To the best of our knowledge, this is the first of such estimates for practical applications of QSP. Unfortunately, we find that the proposed algorithm, at least in its current form, does not exhibit practical numbers despite of the efficiency of QSP compared to conventional quantum algorithms. However, perturbation theory itself is an attractive direction to explore because of its physical interpretability; it provides us insights about what interaction gives an important contribution to the properties of systems. This is in sharp contrast to the conventional approaches based on the quantum phase estimation algorithm, where we can only obtain values of energy. From this aspect, this work is a first step towards “explainable'' quantum simulation on fault-tolerant quantum computers.

Electronic spectroscopy of heptacene ions in the search for carriers of diffuse interstellar bands

Context. The absorption bands of the interstellar medium (ISM) in the optical and near-infrared regions called the diffuse interstellar bands (DIBs) have been known for almost a century, yet their origins remain largely unknown. Knowledge of molecular carriers of DIBs would allow for a much better understanding of the chemistry and physics of the ISM. Polycyclic aromatic hydrocarbons (PAHs) and, among them, polyacenes have been suggested as promising candidates for carriers of DIBs. Aims. In this paper, we report on the spectroscopy of heptacene (Hep), the polyacene molecule consisting of seven aromatic rings in a linear arrangement, in its cationic and anionic forms (Hep+/−). The performed spectroscopic studies made it possible to accurately determine the Hep+/− absorption band positions and to conduct a direct comparison of laboratory and observational spectra. Methods. We utilized helium-tagging action spectroscopy to measure the spectra of Hep+/− in a wide spectral range of 3000–13 000 Å. In most cases, the spectra obtained by this method can be directly compared with the observational spectra. By analyzing the spectral shift as a function of the number of attached helium atoms, we obtained precise estimates of the gas-phase band positions. Quantum-chemical computations were used to support and interpret the findings. Matrix isolation spectroscopy provided information on the spectral properties of neutral Hep and extended the spectral range for Hep+. Results. We found several absorption bands characterized by a rather large full width at half maximum in the spectra of Hep+/−. The two most intense bands were found at 4714 ± 5 Å and 12 250 ± 12 Å for Hep+ and at 4673 ± 14 Å and 11326 ± 4 Å for Hep−. We did not find any good match between laboratory and observational spectra. In particular, the intrinsic width of the absorption bands of Hep+/− is much higher than that of most observed DIBs. Conclusions. The non-detection of Hep+/− in the observational spectra excludes the bottom-up formation route for polyacenes in the ISM. Larger polyacene molecules could still be considered as potential carriers of DIBs in the case of an efficient top-down formation route. All currently measured polyacene ions exhibit relatively broad absorption bands. Therefore, additional spectroscopy studies of neutral polyacenes and larger polyacene ions as well as the study of possible top-down formation routes are suggested.

Mechanism of length-induced magnetism in polyacene molecules

Within the celebrated Su-Schrieffer-Heeger model including electron-lattice and electron-electron interactions, the electronic ground state of polyacene ($n$-acene) is studied as a function of molecular length $n$. The results demonstrate that the ground state exhibits a phase transition from a nonmagnetic state to an antiferromagnetic state at a critical length of $n=7$. The magnetism is explained by calculating the lattice distortion and the related average hopping rate of the \ensuremath{\pi} electrons. It is revealed that in polyacenes, there exist two competing mechanisms that minimize the molecular energy, namely the lattice dimerization and the formation of an antiferromagnetic spin density. Since the dimerization is restricted to the regions near the ends of the molecules the former mechanism is dominant for short molecules, which therefore assume a nonmagnetic state. When the length is increased beyond the critical value, the lattice dimerization in the middle of the molecule is reduced and a rapid drop of the average hopping rate occurs. This makes the second mechanism dominant and the associated antiferromagnetic state is stabilized. The effect of different interaction strengths and values of hopping integrals on the critical length is discussed. The results are confirmed by first-principles calculations. Suggestions for the design of organic antiferromagnets with a similar ladder structure are also given.

Length-induced large magnetoresistance in polyacene molecular spin valves

With the help of first-principles method, an unusual length effect in polyacene molecular spin valves is revealed. It is found that beyond a critical length, the magnitude of tunneling magnetoresistance is significantly enhanced from 10% to 2500%. Theoretical analysis demonstrates that such significant increase of magnetoresistance is related to the nonmagnetic-antiferromagnetic phase transition of polyacene molecules, which is determined by the relative magnetization orientation of the ferromagnetic electrodes. The molecular gap shows distinct length dependence with the change of molecular magnetic state, which keeps decreasing with length in the nonmagnetic state while a fine gap is obtained in the antiferromagnetic state. This work indicates a valid way to obtain considerable tunneling magnetoresistance in long molecular spin valves.

A first-principles study on transport properties of polyacene in zigzag graphene nanoribbon: Configuration, length and doping effects

Electronic transport properties of molecular junctions constructed by bridging a polyacene (PA) molecule between two zigzag graphene nanoribbons (ZGNR) are studied based on density functional theory and the nonequilibrium Green function method. It is found that the molecule-electrode coupling strength is related to the PA position with respect to the nanoribbon edge, which gives rise to the configuration dependency of transport properties. Negative differential resistance (NDR) is predicted in the junctions of which the PA molecule aligns with the inner part of the ZGNR. The on-set bias and current peak decrease as the PA molecule moves inward. The origin of NDR is presented by analyzing the transmission spectra, relative voltage-drop rate, and electron density difference of the junctions. The on-set bias is proportional to the energy of the resonance peak of the lowest unoccupied molecular orbital and can be tuned by the PA molecule length or by doping. This work provides a detailed discussion on PA-bridged ZGNR junctions, which may help to understand ZGNR-based molecular junctions and design negative differential resistance devices.

Cost-effective description of strong correlation: Efficient implementations of the perfect quadruples and perfect hextuples models.

Novel implementations based on dense tensor storage are presented for the singlet-reference perfect quadruples (PQ) [J. A. Parkhill et al., J. Chem. Phys. 130, 084101 (2009)] and perfect hextuples (PH) [J. A. Parkhill and M. Head-Gordon, J. Chem. Phys. 133, 024103 (2010)] models. The methods are obtained as block decompositions of conventional coupled-cluster theory that are exact for four electrons in four orbitals (PQ) and six electrons in six orbitals (PH), but that can also be applied to much larger systems. PQ and PH have storage requirements that scale as the square, and as the cube of the number of active electrons, respectively, and exhibit quartic scaling of the computational effort for large systems. Applications of the new implementations are presented for full-valence calculations on linear polyenes (CnHn+2), which highlight the excellent computational scaling of the present implementations that can routinely handle active spaces of hundreds of electrons. The accuracy of the models is studied in the π space of the polyenes, in hydrogen chains (H50), and in the π space of polyacene molecules. In all cases, the results compare favorably to density matrix renormalization group values. With the novel implementation of PQ, active spaces of 140 electrons in 140 orbitals can be solved in a matter of minutes on a single core workstation, and the relatively low polynomial scaling means that very large systems are also accessible using parallel computing.

Conductance Superposition Rule in Carbon Nanowire Junctions with Parallel Paths

Using first-principles calculations, we investigate conductance of molecular junctions consisted of single and double polyacene (PA) molecules bridging different carbon nanowire electrodes, including armchair carbon nanotubes (CNT) and zigzag graphene nanoribbons (GNR). Doubling the PA molecule enhances the junction conductance, except in the junction where a molecule contacts armchair CNT with nonvertical edges. Elongating the PA molecules change junction conductance. The different conductance scaling behaviors among various junctions are governed by the interface between the molecule and the electrode, the molecular length, and the edge states of zigzag GNR.

First-Principle Characterization for Singlet Fission Couplings.

The electronic coupling for singlet fission, an important parameter for determining the rate, has been found to be too small unless charge-transfer (CT) components were introduced in the diabatic states, mostly through perturbation or a model Hamiltonian. In the present work, the fragment spin difference (FSD) scheme was generalized to calculate the singlet fission coupling. The largest coupling strength obtained was 14.8 meV for two pentacenes in a crystal structure, or 33.7 meV for a transition-state structure, which yielded a singlet fission lifetime of 239 or 37 fs, generally consistent with experimental results (80 fs). Test results with other polyacene molecules are similar. We found that the charge on one fragment in the S1 diabatic state correlates well with FSD coupling, indicating the importance of the CT component. The FSD approach is a useful first-principle method for singlet fission coupling, without the need to include the CT component explicitly.

Light-Induced Field Enhancement in Nanoscale Systems from First-Principles: The Case of Polyacenes

Using first-principles calculations we studied the electric field enhancement in polyacene molecules upon illumination. These molecules can be seen as a specific class of C-based (i.e., graphene-derived) nanostructures, recently proposed as alternative materials for plasmonics. We demonstrate that optical transitions may generate oscillating dipolar response charge, giving rise to an induced electric field near the molecule, which thus acts as a plasmon-like nanoantenna. While the field amplification in the vicinity of single acenes is rather small and decreases when the size of the system is increased, it may be selectively enhanced in the case of acene’s assemblies. This paves the way for the design of more complex C-based architectures explicitly conceived to improve the amplification factor.

Microsolvation of anthracene inside superfluid helium nanodroplets

This article reports on the microsolvation of anthracene in superfluid helium nanodroplets as revealed by electronic spectroscopy. Among the polyacene molecules benzene, naphthalene, anthracene, tetracene and pentacene only anthracene and tetracene have been found to exhibit multiplet splitting of the electronic and vibronic transitions which can not be explained by a rotational fine structure. The experimental approach for the investigation of the multiplet is the combined investigation of the fluorescence excitation spectrum and dispersed emission spectra. New experimental data are presented on the microsolvation of anthracene in helium droplets. A detailed analysis of the anthracene data will be contrasted to corresponding data on tetracene. These data together with those reported for benzene, naphthalene and pentacene might serve to test and develop theoretical models which are needed to understand microsolvation of single molecules in superfluid helium droplets.

Fuzzy symmetries of two classes of linear polyacene molecules

The fuzzy symmetries of two kinds of linear polyacene molecules are probed into in the paper. In these molecules, any one of the benzene rings abreast connects at most other two rings in two ways: either its two opposite C–C bonds combine with two other rings, respectively, or its two meta-position C–C bonds connect two rings in cis- and trans-form, respectively. The former is called p-polyacenes (or straight polyacenes), and the latter is m-polyacenes (or kinked ones). It can be thought as the planar molecule with approximate one-dimensional space periodic transformation (parallel translation) symmetry, namely, group \({{\rm G}_{1}^{2}}\) symmetry, when the number of its benzene ring is very large; on the other hand, it can be considered as the fuzzy group \({{\rm G}_{1}^{2}}\) symmetry, if the benzene ring number is not large enough. The p-polyacene and m-polyacene with 20 benzene rings are analyzed as typical examples, and the energies of the π-molecular orbital (MO) and the fuzzy symmetry characters related to the space symmetry transformations are carefully examined. Moreover, the π-MOs of the p-polyacenes and m-polyacenes with different numbers of benzene ring are investigated to obtain the related rules.

Intramolecular ferromagnetic coupling in bis-oxoverdazyl and bis-thioxoverdazyl diradicals with polyacene spacers

We predict the intramolecular exchange coupling constant (J) for 10 different oxo- and thioxo-verdazyl-based hi-spin ground-state diradicals with linear polyacene couplers of varying length using the broken symmetry approach in an unrestricted DFT framework. The magnetic characteristics of these systems are explained using the spin-density distribution, and an analysis is made by “magnetic” orbitals. The nuclear independent chemical shift (NICS) values have been calculated for the diradicals. The average NICS(1) (1 A above the ring surface) value per benzenoid ring increases as the size of the coupler increases. So-called ΔNICS(1) values [the difference among average NICS(1) per benzenoid ring in the coupler and the NICS(1) of the linear polyacene molecule] are correlated with J values. Bond orders and hyperfine coupling constants have also been evaluated and analyzed for the diradicals.

The chemisorption of polyaromatic hydrocarbons on Si(1 0 0)H dangling bonds

The adsorption of polyacene molecules on a H-terminated Si(0 0 1)-2 × 1 surface where a few hydrogen atoms have been extracted is presented using the semi-empirical ASED+ method. To scale up the qualitative ASED+ method, the adsorption of benzene and pentacene on a clean silicon surface is first compared with DFT calculations together with their adsorption on a fully hydrogenated Si(1 0 0) surface. When a few hydrogen atoms have been selectively removed from the SiH(0 0 1) surface, ASED+ demonstrates a difficult chemisorption for the polyacenes series on one dangling bond and a large interaction on two dangling bonds compensating for the large molecule deformation required. The influence of the nearest H atoms when the molecule is adsorbed on two dangling bonds of a passivated Si(0 0 1) surface is very small. On a clean and on a partially hydrogenated surface, all the polyacenes need to overcome an energy barrier of 0.3–0.4 eV to reach a chemisorption state.

Thermodynamic properties of polyacenes

Carbon nanoribbons have been a subject of great interest lately because of their interesting physical properties, particularly their use as electrodes in lithium-based batteries, in a way to improve the performance of these batteries. By means of a diagrammatic perturbation expansion of the Hubbard model, the thermodynamic properties of a cyclical polyacene have been calculated. The entropy, magnetic susceptibility and specific heat curves present crossing points. The molecular hardness multiplied by the carbon number ( N C) in a polyacene molecule is a relevant parameter to determine its energy storage capability. Our results show that this parameter versus N C, for some cyclical polyacenes, presents a peak in N C=36 and it practically does not change for N C>60. The behavior for N C<30 is very similar to that of the available experimental curve for polyacenes.

Chemical bonding in metal sandwich molecules M nR 2 with R = pyrene C 16H 10 and tetracene C 18H 12

The structures and properties of neutral molecular complexes of palladium transition metal atoms sandwiched between pyrene C 16H 10 and tetracene C 18H 12 molecules have been studied using plane wave based density functional theory. Changes in symmetry and variations in the number of metal atom ( n = 4, 8 for pyrene and n = 4–9 for tetracene) were considered. The metal atoms prefer edge sites with low coordination to ring carbon atoms over “interior” sites with the high coordination as found in bis(benzene) palladium. This appears as a general motif for small polyacene molecules. All compounds examined had stable singlet ( S = 0) ground states relative to separate hydrocarbon and metal cluster, and a lowest triplet state ( S = 1) with spin on the metal atoms. When restricted to D 2 h symmetry the sandwich Pd 4(C 16H 10) 2 with pyrene had palladium atoms that were isolated from each other in η 2- and η 3-coordinated sites. The valence electron density was provided mainly by overlap of metal d xy - and d yz -functions with carbon p y ( y-axis ⊥ sandwich plane). The total charge density isometric surfaces showed that η 3-coordination involved higher charge density than η 2-coordination, and that both were weaker than C–C bond charge by an order of magnitude. In the pyrene n = 8 complex, weak metal–metal bonding was apparent around the molecular perimeter together with some metal bonding to the lone interior Pd atom. In sandwiches with tetracene the metal atoms had η 2- and η 3-coordination with ring edge carbon atoms. There were no metal atoms on interior sites. The n = 5 compound with a lopsided Pd distribution had a bonding pattern consistent with experimental work of Murahashi et al. [T. Murahashi, M. Fujimoto, M. Oka, Y. Hashimoto, T. Uemura, Y. Tatsumi, Y. Nakao, A. Ikeda, S. Sakaki, H. Kurosawa, Science 313 (2006) 1104]. Based on geometry (separation ⩽275 pm), total charge density and partial charge density components, we found evidence for metal–metal bonds. In the complex with highest loading ( n = 9) the metal–metal bonds encompassed the molecule. The Kohn–Sham partial charge densities of these bonds were identified.

Electron Dynamics at Polyacene/Au(111) Interfaces

Two-photon photoemission (2PPE) spectroscopy is used to examine the excited electronic structure and dynamics at polyacene/Au(111) interfaces. Image resonances are observed in all cases (benzene, naphthalene, anthrathene, tetracene, and pentacene), as evidenced by the free-electron like dispersions in the surface plane and the dependences of these resonances on the adsorption of nonane overlayers. The binding energies and lifetimes of these resonances are similar for the five interfaces. Adsorption of nonane on top of these films pushes the electron density in the image resonance away from the metal surface, resulting in a decrease in the binding energy (-0.3 eV) and an increase in the lifetime (from <20 to approximately 110 fs). The insensitivity of the image resonances to the size of polyacene molecules and the absence of photoinduced electron transfer from the metal substrate to molecular states both suggest that the unoccupied molecular orbitals are not strongly coupled to the delocalized metal states or image potential resonances.

Polyacene Spacers in Intramolecular Magnetic Coupling

We predict the intramolecular magnetic exchange coupling constant (J) for eleven nitronyl nitroxide diradicals (NN) with different linear and angular polyacene couplers from broken-symmetry density functional treatment. For the linear acene couplers, J initially decreases with increase in the number of fused rings. But from anthracene coupler onward, the J value increases with the number of benzenoid rings due to an increasing diradical character of the coupler moiety. The J value for the diradical with a fused bent coupler is always found to be smaller than that for a diradical with a linear coupler of the same size. The nuclear independent chemical shift (NICS) is calculated, and it is observed that the average of the NICS values per benzenoid ring in the diradical is less than that in the normal polyacene molecule. An empirical formula for the magnetic exchange coupling constant of a NN diradical with an aromatic spacer is obtained by combining the Wiberg bond order (BO), the angle of twist (phi) of the monoradical (NN) plane from the plane of the coupler, and the NICS values. A comparison of the formula with the computed values reveals that, from tetracene onward, the diradical nature of the linear acene couplers becomes prominent thereby leading to an increase in the ferromagnetic coupling constant. Isotropic hyperfine coupling constants are calculated by using a polarized continuum model for the diradicals in different solvents and in vacuum.

A semi-empirical study of polyacene molecules adsorbed on a Cu(1 1 0) surface

To describe the adsorption of large organic molecules on metal surfaces, to calculate the corresponding diffusion and rotation barriers, the semi-empirical mono-electronic Hamiltonian of the ASED molecular orbital method have been completed to take into account three body interaction terms. The full re-parametrization of this ASED+ version of ASED was determined on the specific case of benzene adsorbed on Cu(1 1 0) and a full transferability assumed for the member of the polyacene series also adsorbed on Cu(1 1 0). The adsorption energies, geometries, diffusion and rotation barriers are very well described by this new semi-empirical technique of calculation opening the way of optimizing larger conjugated molecule on surface for uni-molecular mechanics or electronics.

Third-order electric susceptibility of nonpolar azimuthal molecular rotors mounted on a covalent polymer grid

Polyacene molecules are constrained to rotate about the principal axis associated with the diagonal component of the molecular polarizability tensor that is intermediate in size. The rotation axes are oriented vertically (‘azimuthal rotors’) and are attached to a covalent monolayer grid such as that recently reported in the literature. We calculate the value of the relevant component of the third-order electric susceptibility of the system for a variety of grid spacings and determine that the optical nonlinearity of such a system is not particularly large, at best approaching that of carbon disulfide.