Reevaluation of electronic polarization energies in organic molecular crystals

Effective electronic polarization energies for positive P⁺_eff and negative P⁻_eff charge carriers in polyacene crystals have been reevaluated. By comparing the P⁺_eff and P⁻_eff values obtained from the analyses of reported energy parameters with calculated data, it is shown that the widely accepted assumption that P⁺_eff = P⁻_eff in polyacene crystals is not valid. By applying recently reported data on optical E^opt_G and adiabatic E^ad_G energy gap values, the contribution of molecular (vibronic) polarization in the total effective polarization energies W⁺_eff and W⁻_eff has been estimated, and modified energy diagrams for polyacene crystals have been presented. Further, due to practically constant values of observed and calculated P⁺_eff and P⁻_eff in polyacenes, the gap energies between positive and negative charge carrier conduction levels have been estimated for several related aromatic hydrocarbons.     

Non-empirical molecular orbital theory of the electronic structure of molecular crystals

A non-empirical molecular orbital treatment of molecular crystals, based on SCF perturbation theory and matrix partitioning methods is presented.     

Experimental and theoretical electronic charge densities in molecular crystals

Electronic charge density distribution in molecular systems has been described in terms of the topological properties. After briefly reviewing methods of obtaining charge densities from X-ray diffraction and theory, typical case studies are discussed. These studies include rings and cage systems, hydrogen bonded solids, polymorphic solids and molecular NLO materials. It is shown how combined experimental and theoretical investigations of charge densities in molecular crystals can provide useful insights into electronic structure and reactivity.     

Effect of electronic polarization on charge-transport parameters in molecular organic semiconductors

Theoretical investigations of charge transport in organic materials are generally based on the "energy splitting in dimer" method and routinely assume that the transport parameters (site energies and transfer integrals) determined from monomer and dimer calculations can be reliably used to describe extended systems. Here, we demonstrate that this transferability can fail even in molecular crystals with weak van der Waals intermolecular interactions, due to the substantial (but often ignored) impact of polarization effects, particularly on the site energies. We show that the neglect of electronic polarization leads to qualitatively incorrect values and trends for the transfer integrals computed with the energy splitting method, even in simple prototypes such as ethylene or pentacene dimers. The polarization effect in these systems is largely electrostatic in nature and can change dramatically upon transition from a dimer to an extended system. For example, the difference in site energy for a prototypical "face-to-edge" one-dimensional stack of pentacene molecules is calculated to be 30% greater than that in the "face-to-edge" dimer, whereas the site energy difference in the pentacene crystal is vanishingly small. Importantly, when computed directly in the framework of localized monomer orbitals, the transfer integral values for dimer and extended systems are very similar.     

Influence of polarization fluctuations on the electronic structure of molecular solids

Fluctuations in the intermolecular polarization energies of charge states in molecular solids can lead to broad (ΔE ≈ 0.5 eV) distributions of localized states, especially in polymers. Such fluctuations are caused by defects (e.g. surfaces) and thermal vibrations in molecular crystals and also by variations in the local structure in polymers. The resulting energy distributions yield natural interpretations of such diverse observations on the broadening of the photoemission spectra of molecular solids and the contact charge exchange spectra of polymers.     

Molecular orbital theory of the electronic structures of one-dimensional molecular crystals

A non-empirical tight-binding LCAO SCF MO treatment of one-dimensional molecular crystals based on the SCF perturbation theory is presented. The simpler version of this method at the level of the CNDO/2 approximation is also given.     

On polarization in ionic crystals

Permittivities (ɛ) of LiF, NaCl, and KBr ionic crystals of various dispersities were measured. A progressive increase of ɛ up to ∼10⁵ with the dispersity of crystals was detected. A conclusion was drawn that this effect needs to be considered when determining effective charges on atoms by Szigeti’s method in the case of measuring ɛ of the powders.     

Quantum control of molecular motion including electronic polarization effects with a two-stage toolkit

A method for incorporating strong electric field polarization effects into optimal control calculations is presented. A Born-Oppenheimer-type separation, referred to as the electric-nuclear Born-Oppenheimer (ENBO) approximation, is introduced in which variations of both the nuclear geometry and the external electric field are assumed to be slow compared with the speed at which the electronic degrees of freedom respond to these changes. This assumption permits the generation of a potential energy surface that depends not only on the relative geometry of the nuclei but also on the electric field strength and on the orientation of the molecule with respect to the electric field. The range of validity of the ENBO approximation is discussed in the paper. A two-stage toolkit implementation is presented to incorporate the polarization effects and reduce the cost of the optimal control dynamics calculations. As an illustration of the method, it is applied to optimal control of vibrational excitation in a hydrogen molecule aligned along the field direction. Ab initio configuration interaction calculations with a large orbital basis set are used to compute the H-H interaction potential in the presence of the electric field. The significant computational cost reduction afforded by the toolkit implementation is demonstrated.     

Non-linear polarization effects in guest-host liquid crystals

We report on an experimental investigation of the non-linear polarization effect in guest-host nematic liquid crystals (GH-NLCs) with uniaxial alignment. The degree of polarization of a laser beam transmitting the GH-NLCs and an aperture behind them is strongly dependent on the incident beam intensity. The mechanism is discussed on the basis of the laser-induced photothermal self-phase modulation. The origin of the effect is a strong anisotropy of the intensity-induced change or refractive indices.     

Variational calculation of quantum mechanical/molecular mechanical free energy with electronic polarization of solvent

Quantum mechanical/molecular mechanical (QM/MM) free energy calculation presents a significant challenge due to an excessive number of QM calculations. A useful approach for reducing the computational cost is that based on the mean field approximation to the QM subsystem. Here, we describe such a mean-field QM/MM theory for electronically polarizable systems by starting from the Hartree product ansatz for the total system and invoking a variational principle of free energy. The MM part is then recast to a classical polarizable model by introducing the charge response kernel. Numerical test shows that the potential of mean force (PMF) thus obtained agrees quantitatively with that obtained from a direct QM/MM calculation, indicating the utility of self-consistent mean-field approximation. Next, we apply the obtained method to prototypical reactions in several qualitatively different solvents and make a systematic comparison of polarization effects. The results show that in aqueous solution the PMF does not depend very much on the water models employed, while in nonaqueous solutions the PMF is significantly affected by explicit polarization. For example, the free energy barrier for a phosphoryl dissociation reaction in acetone and cyclohexane is found to increase by more than 10 kcal/mol when switching the solvent model from an empirical to explicitly polarizable one. The reason for this is discussed based on the parametrization of empirical nonpolarizable models.     

Large polarization and record-high performance of energy storage induced by a phase change in organic molecular crystals

Dielectrics that undergo electric-field-induced phase changes are promising for use as high-power electrical energy storage materials and transducers. We demonstrate the stepwise on/off switching of large polarization in a series of dielectrics by flipping their antipolar or canted electric dipoles via proton transfer and inducing simultaneous geometric changes in their π-conjugation system. Among antiferroelectric organic molecular crystals, the largest-magnitude polarization jump was obtained as 18 μC cm⁻² through revisited measurements of squaric acid (SQA) crystals with improved dielectric strength. The second-best polarization jump of 15.1 μC cm⁻² was achieved with a newly discovered antiferroelectric, furan-3,4-dicarboxylic acid. The field-induced dielectric phase changes show rich variations in their mechanisms. The quadruple polarization hysteresis loop observed for a 3-(4-chlorophenyl)propiolic acid crystal was caused by a two-step phase transition with moderate polarization jumps. The ferroelectric 2-phenylmalondialdehyde single crystal having canted dipoles behaved as an amphoteric dielectric, exhibiting a single or double polarization hysteresis loop depending on the direction of the external field. The magnitude of a series of observed polarizations was consistently reproduced within the simplest sublattice model by the density functional theory calculations of dipole moments flipping over a hydrogen-bonded chain or sheet (sublattice) irrespective of compounds. This finding guarantees a tool that will deepen our understanding of the microscopic phase-change mechanisms and accelerate the materials design and exploration for improving energy-storage performance. The excellent energy-storage performance of SQA was demonstrated by both a high recoverable energy-storage density W_r of 3.3 J cm⁻³ and a nearly ideal efficiency (90%). Because of the low crystal density, the corresponding energy density per mass (1.75 J g⁻¹) exceeded those derived from the highest W_r values (∼8–11 J cm⁻³) reported for several bulk antiferroelectric ceramics , without modification to relaxor forms.

Electric-field induced phase changes, which are promising for use in high-power electrical energy storage, can be realized in a series of organic dielectrics by flipping the antipolar or canted electric dipoles via proton transfer.     

Excavations in molecular crystals

Single crystals built from porous molecular networks can react with agents that penetrate the crystals, cleave fragments from the network, and thereby increase the volume available for guests, all without loss of crystallinity.     

Continuum treatment of electronic polarization effect

A continuum treatment of electronic polarization has been explored for in molecular mechanics simulations in implicit solvents. The dielectric constant for molecule interior is the only parameter in the continuum polarizable model. A value of 4 is found to yield optimal agreement with high-level ab initio quantum mechanical calculations for the tested molecular systems. Interestingly, its performance is not sensitive to the definition of molecular volume, in which the continuum electronic polarization is defined. In this model, quantum mechanical electrostatic field in different dielectric environments from vacuum, low-dielectric organic solvent, and water can be used simultaneously in atomic charge fitting to achieve consistent treatment of electrostatic interactions. The tests show that a single set of atomic charges can be used consistently in different dielectric environments and different molecular conformations, and the atomic charges transfer well from training monomers to tested dimers. The preliminary study gives us the hope of developing a continuum polarizable force field for more consistent simulations of proteins and nucleic acids in implicit solvents.     

Low frequency polarization of ferroelectric liquid crystals

The possibility of the separation of free charge carriers in ferroelectric liquid crystals has been investigated in terms of the kinetic equation for the charge density. The non-uniformity of the charge distribution leads to an additional term in the free energy of the system, which can be essential for low-frequency external fields. The corresponding correction to the dielectric response of the system is proportional to the spontaneous polarization of the sample and thus has an anomalous temperature dependence near the phase transition.     

Fifth-rank molecular polarization tensors

Fifth-rank molecular polarization tensors are discussed in terms of their tensor group which includes both the spatial symmetry and the permutation symmetry of the suffixes. This latter may be described by certain four-dimensional point groups, which in the nontotally symmetric cases may be projected into three-dimensional space. The study leads to a determination of the number of independent components of these tensors and a discussion of the types of phenomena which they may be expected to produce.     

Measurement of spontaneous polarization in ferroelectric smectic liquid crystals

This paper presents measurements of spontaneous polarization made on two ester-bridged materials (SCE1 and CE8). The two materials are chosen to be representative of long pitch (SCE1) and short pitch (CE8) materials. The measurements were made by two different methods, Diamant bridge and field reversal; these methods are compared and the results shown to be in good agreement. The variation in results between different samples of these materials is discussed and the results compared (where possible) with the work of other authors. Special attention is paid to the variation which occurs between the published spontaneous polarization measurements of different authors. The reasons for this variation are examined and variation in sample alignment and the determination of the transition temperature by different authors are shown to be major causes.     

Dynamics of anharmonic surfaces in harmonic crystals

Explicit expressions are given for the effective equations of motion for surface atoms placed in a general anharmonic potential and attached to semi-infinite harmonic bulk substrates. Three types of one-dimensional substrates are considered: i) continuous dispersionless, ii) discrete with nearest neighbours harmonic interactions, and iii) a substrate showing a strong spatial dispersion due to next-nearest neighbours interactions. The elimination of the harmonic degrees of freedom of the dispersionless substrate i) reduces the problem to the damped Duffing oscillator in the case of an external force applied to the surface atom, and to an extended Duffing oscillator with time-dependent coefficients in the case of scattering of a phonon incoming from the bulk. In both remaining cases [ii) and iii)]the resulting equations of motion are Volterra integro-differential equations. The equations of motion obtained allow for the study of transitive regimes, of higher harmonics generation and of chaotic behaviour, due to the surface anharmonicity.     

Sonofragmentation of molecular crystals

Possible mechanisms for the breakage of molecular crystals under high-intensity ultrasound were investigated using acetylsalicylic acid (aspirin) crystals as a model compound for active pharmaceutical ingredients. Surprisingly, kinetics experiments ruled out particle-particle collisions as a viable mechanism for sonofragmentation. Two other possible mechanisms (particle-horn and particle-wall collisions) were dismissed on the basis of decoupling experiments. Direct particle-shock wave interactions are therefore indicated as the primary mechanism of sonofragmentation of molecular crystals.