Importance of intermolecular interactions in assessing hopping mobilities in organic field effect transistors: pentacene versus dithiophene-tetrathiafulvalene

We report on a computational study to understand the high mobility found in dithiophene-tetrathiafulvalene (DT-TTF) transistors, with respect to that known for pentacene, using an extended measure of the reorganization energy (lambdareorg). We demonstrate the high importance of considering intermolecular interactions to assess hole-hopping mobilities. We find that due to the steric, polarizable environment and the structural changes induced by local intermolecular charge delocalization, the calculated lambdareorg for DT-TTF decreases from 0.574 eV in the isolated molecule to 0.042 eV in the crystal embedded molecule, which is of the same order of the previously reported value found for the isolated pentacene molecule, 0.098 eV.     

Orthogonal contacts between benzene rings: A special type of specific intermolecular interactions

The crystal structures of benzene and some of its derivatives are analyzed in detail. The structures are characterized by orthogonal contacts between the benzene cycles (OBzC). Although the orthogonal contacts conflict with the close packig law, they are an important structure-forming factor, being thus a specific type of intermolecular interactions. The OBzC system, which is inherent in the orthorhombic crystals of benzene, remains invariable in 2-aminophenol and 2-amino-4-chlorophenol crystals, notwithstanding other specific interactions (H-bonding and halogen...halogen contacts). OBzC are also observed in para- and ortho-cresol crystals. A classification of OBzC is given, and an especially effective projection method (rectangular projection) is suggested. M. V. Lomonosov Moscow State University, Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 5, pp. 775–789, September–October, 1995. Translated by L. Smolina     

Bistability in Fc-PTM crystals: the role of intermolecular electrostatic interactions

Fc-PTM is a valence tautomeric radical, where the ferrocene (Fc) group, a good electron donor, is linked by an ethylenic spacer to a perchlorotriphenylmethyl radical (PTM(*)), a good electron acceptor. In solution this compound exists mainly in the neutral Fc-PTM(*) form which can be photoexcited through an intramolecular electron transfer to the zwitterionic Fc(+*)-PTM(-) form. By contrast, in crystals of Fc-PTM at room temperature both the neutral and the zwitterionic forms coexist, pointing to a true bistability phenomenon. We rationalize these findings accounting for the role of intermolecular electrostatic interactions in Fc-PTM crystals. In fact the energy of the zwitterionic Fc(+*)-PTM(-) form is lowered in the crystal by attractive electrostatic intermolecular interactions and the cooperative nature of these interactions explains the observed coexistence of neutral Fc-PTM(*) and zwitterionic Fc(+*)-PTM(-) species. The temperature evolution of Mossbauer spectra of Fc-PTM is quantitatively reproduced adopting a bottom-up modeling strategy that combines a molecular model, derived from optical spectra of Fc-PTM in solution, with a model for intermolecular electrostatic interactions, supported by quantum-chemical calculations. Fc-PTM then offers the first experimental demonstration of bistability induced by electrostatic interactions in crystals of valence tautomeric donor-acceptor molecules.     

Screening for cocrystallization tendency: the role of intermolecular interactions

Pharmaceutical cocrystals have rapidly emerged as a new class of API solids with great promise and advantages. Much work has been focused on exploring the crystal engineering and design strategies that facilitate formation of cocrystals of APIs and ligands/cocrystal formers. However, fewer attempts have been made to understand the equilibrium phase behavior and phase transition kinetics of the cocrystallizing solutions. This limited knowledge on the solution physical chemistry often leads to difficulty in screening for potential molecular pairs of API and ligand that form cocrystals effectively. In this study, the long-time self-diffusivities measured using pulsed gradient spin-echo nuclear magnetic resonance (PGSE NMR) are used to characterize the particle interactions in solutions for pharmaceutical cocrystallizing systems. For the pairs of API and ligand that produce cocrystals, the heteromeric attractions between API and ligand are found to be stronger than the homomeric attractions between API molecules and between ligand molecules, suggesting that an energetically favorable condition is induced for the formation of cocrystals. To the best of our knowledge, this is the first report of using the pair contribution of the self-diffusivity as a screening tool for cocrystal formation.     

Calculation of intermolecular interactions in the benzene dimer using coupled-cluster and local electron correlation methods

Potential energy curves for the parallel-displaced, T-shaped and sandwich structures of the benzene dimer are computed with density fitted local second-order M?ller-Plesset perturbation theory (DF-LMP2) as well as with the spin-component scaled (SCS) variant of DF-LMP2. While DF-LMP2 strongly overestimates the dispersion interaction, in common with canonical MP2, the DF-SCS-LMP2 interaction energies are in excellent agreement with the best available literature values along the entire potential energy curves. The DF-SCS-LMP2 dissociation energies for the three structures are also compared with new complete basis set estimates of the interaction energies obtained from accurate coupled cluster (CCSD(T)) and DF-SCS-MP2 calculations. Since LMP2 is essentially free of basis set superposition errors, counterpoise corrections are not required. As a result, DF-SCS-LMP2 is computationally inexpensive and represents an attractive method for the study of larger pi-stacked systems such as truncated sections of DNA.     

Electron donor-acceptor complexes: Evaluation of MNDO as a computational tool to probe intermolecular interactions

Donor-acceptor pairs form EDA complexes that exist as conformational isomers exhibiting different ground-state and photochemical properties. We have sought a rapid, general, and accurate quantum mechanical computational method to generate potential energy surfaces that are representative of the donor-acceptor intermolecular interactions at the self-consistent field (SCF) level. The semiempirical molecular orbital (MO) method MNDO has been compared to ab initio methods to assess its behavior with respect to energy, dipole moment and ionization potential shifts. MNDO correctly distinguishes between repulsive and bound EDA complex states at the SCF level and produces potential curves that are smooth and free of spurious minima or cusps. MNDO curves are systematically more repulsive than those for ab initio STO-3G calculations; calculated interaction energies exhibit a mean absolute deviation of 2.90 kcal/mol. MNDO appears to provide a reliable qualitative estimate of the nondispersion portion of the interaction energy. Limitations and errors arising from minimal basis sets, single determinants, and neglect of dispersion are discussed.     

Modeling of anti-Langmuirian peaks in micellar electrokinetic chromatography: benzene and naphthalene

Peaks of benzene (bz) and naphthalene (np) having diffuse fronts and steep rears under overload conditions were studied quantitatively in MEKC with SDS surfactant. The retardation factors of these compounds, solubilized at microM to mM concentrations by either 10, 30, or 50 mM SDS, were determined by vacancy MEKC and frontal analysis MEKC. Isotherm coordinates were calculated from the retardation factors, and the equation for the concave upward anti-Langmuir isotherm was fit to them. Peak profiles were computed with the MacCormack algorithm from the isotherm fits and a simplified continuity equation appropriate to MEKC. These profiles were compared to ones generated in normal MEKC from samples of bz and np solubilized at muM to mM concentrations by either 10, 30, or 50 mM SDS. In all cases, the anti-Langmuir isotherm described the asymmetry of experimental peaks. For bz in 30 and 50 mM SDS and np in 10 and 50 mM SDS, good to excellent agreement was found between the experimental and predicted profiles. For bz in 10 mM SDS, the experimental profiles were more broadened than the predicted ones, although their asymmetries agreed. For np in 30 mM SDS, the experimental isotherm predicted greater peak asymmetry than was observed, and the correct anti-Langmuir isotherm for all sample concentrations and field strengths was calculated from the most asymmetrical peak by the inverse method. The relative decrease of zone velocity with increasing analyte concentration was calculated from the isotherm parameters, electrokinetic mobilities, retardation factors, surfactant concentrations, and CMC. The simplification of the continuity equation was justified.     

The role of intermolecular interactions in the viscoelastic properties of polymer melts

The exact microscopic expression for the stress tensor in polymer liquids contains a tensor product of the the segment position vector with the total, intra- plus inter-chain, force acting on the segment. On the other hand, the widely accepted theory of viscoelasticity of polymer melts¹⁾ is based on the assumption, that contributions from interchain interactions to the viscosity of polymer melts is negligible relative to the effectively intrachain entropic interactions. Starting from the exact Green-Kubo formula for the viscosity, the Rouse dynamic correlation functions, and Newton's second law, we show that the intrachain assumption is inadequate. Rather, the intrachain and interchain forces acting on polymer segments cancel each other largely. The intrachain contribution therefore cannot be dominant as anticipated in the usual treatment¹⁾, or, in other words, the interchain contribution cannot be ignored. The main contribution to viscoelastic properties of polymer melts can only arise from a part of the total stress tensor as already suggested by M. Fixman based on a different argument²⁾. It is concluded that the viscosity is of a purely interchain nature, and is determined by the tensor product of the vector connecting the centers-of-mass of neighboring macromolecules on the one hand, and the total force by which macromolecules interact, on the other, just in the case of simple liquids.     

Brillouin spectra and intermolecular potential of crystalline benzene

Benzene single crystals suitable for Brillouin scattering experiments have been prepared. The Brillouin spectra have been obtained at 0°C for all the crystal orientations needed for the determination of the nine elastic constants of the orthorhombic crystal. These elastic constants are compared to values determined from ultrasonic experiments, the Brillouin results are in general 10 to 15% higher than ultrasonic results. The dynamic properties of the crystal lattice have been evaluated, for small nonzero wave vector, using a Buckingham (exp—6) potential and a coulombic (exp—6—1) potential with parameters from different authors. The agreement is not satisfactory with the (exp—6) potential while the (exp—6—1) model is appropriate for calculation of the long-wave dynamics of the crystal.     

The intermolecular interactions in binaries

The recapitulation of systematical investigations of excess enthalpy of mixing in binary mixtures: pyridine base +n-alkane or some of arenes is presented. On the base of experimental results as well as model calculations (PFP, ERAS) the discussion of intermolecular interactions in pyridine bases is given.     

Polyelectrolyte solutions: Counterion condensation and intermolecular interactions

This article demonstrates that the neglect of nonlinear effects in the conventional counterion condensation theory for the double layer about a charged cylinder can be significant, especially for phenomena involving intramolecular or intermolecular interactions in dilute solutions. For concentrated solutions the Manning theory derives from a linearized superposition approximation for the potential, in contrast to the cylindrical-cell model, which explicitly treats interactions within an ordered array of parallel cylinders. A new theory which treats interactions explicitly while permitting disorder in two dimensions is presented, and predictions for the osmotic pressure are compared with those from the Manning and cylindrical-cell models.     

Ultrafast charge separation driven by differential particle and hole mobilities

The process of a local excitation evolving into an intramolecular charge-separated state is followed and compared for several systems by directly simulating the time propagation of the electronic wavefunction. The wavefunction and Hamiltonian are handled using the extended second-order algebraic diagrammatic construction (ADC(2)-x), which explicitly accounts for electron correlation in the dynamic many-particle state. The details of the charge separation can be manipulated according to the chemical composition of the system; atoms which dope the conjugated system with either particles or holes are shown to effect whether the particle or hole is more mobile. Initially, the charges oscillate between the ends of linear molecules (with different rates), separating periodically, but, at long times, both charges tend to spread over the whole molecule. Charge separation is also shown to occur for asymmetric systems, where it may eventually be experimentally feasible to excite a localized resonance (nonstationary state) on one end of the molecule preferentially and follow the ensuing dynamics.     

Structural effects on the hole mobilities of indenothiophene-embedded homologs

A systematic study of the relationship between the structures and the non-dispersive hole mobilities (up to 10⁻³ cm² V⁻¹ s⁻¹) of a homologous series of amorphous indenothiophene-containing materials is described. The hole mobilities were dependent mainly on the length and rigidity of the π-conjugated backbone and the peripheral substituents.     

Non-condon effects on radiationless triplet decay in benzene and naphthalene

Model calculations that avoid the Condon approximation are reported for T₁$\text{}\text{?}$ar S₀ intersystem crossing in normal and perdeuterated benzene and naphthalene. In both benzenes and in naphthalene-d₈, skeletal bending vibrations coupling T₁ to T₂ are found to be more efficient as accepting modes than CH-stretching vibrations.     

Singlet exciton interactions in crystalline naphthalene

The decay of prompt fluorescence in crystalline naphthalene at 300 K, excited by a picosecond 266 nm pulse, has been studied as a function of excitation intensity. Experimental decay curves can be fitted only when the exponential distribution in depth of excitation and the radial (gaussian) intensity profile of the excitation are both taken into account. From an analysis of the decay at early time (?5 ns) a best fit value of the singlet—singlet annihilation rate constant is found γ_SS = (4 ± 1) × 10^?10 cm³ s^?1. If the reaction is diffusion-limited, this rate implies an average singlet diffusivity D_S = (2 ± 1) × 10^?4 cm² s^?1.     

Spectra of the dications of benzene,naphthalene and azulene

Spectra of the dications of benzene, naphthalene and azulene produced by double photoionisation of the parent compounds are presented. All the electron distributions show evidence of inner-valence Auger processes, which enhance population of low energy states of the dications. It is confirmed that the ground state of the benzene dication is a triplet, contrary to a recent theoretical prediction. The double ionisation energies of the three title molecules are determined as 24.65 ± 0.03, 21.28 ± 0.03 and 20.2 ± 0.2 eV, respectively.