Ram S. Bhatta

Improved Force Field for Molecular Modeling of Poly(3-hexylthiophene)

An ab initio-based improved force field is reported for poly(3-hexylthiophene) (P3HT) in the solid state, deriving torsional parameters and partial atomic charges from ab initio molecular structure calculations with explicit treatment of the hexyl side chains. The force field is validated by molecular dynamics (MD) simulations of solid P3HT with different molecular weights including calculation of structural parameters, mass density, melting temperature, glass transition temperature, and surface tension. At 300 K, the P3HT crystalline structure features planar backbones with non-interdigitated all-trans hexyl side chains twisted ~90° from the plane of the backbone. For crystalline P3HT with infinitely long chains, the calculated 300 K mass density (1.05 g cm(-3)), the melting temperature (490 K), and the 300 K surface tension (32 mN/m) are all in agreement with reported experimental values, as is the glass transition temperature (300 K) for amorphous 20-mers.

Nanostructures and electronic properties of a high-efficiency electron-donating polymer.

The development of organic photovoltaic (OPV) solar cells has seeded a bright hope of achieving low-cost solar energy harvesting. Practical realization and successful commercialization require enhancing the efficiency of solar energy harvesting, which, in turn, relies on the core understanding of structure-property relationships in OPV materials. Here, we report the first large-scale density functional calculations of the nanoconformational and electronic properties of the thieno[3,4-b]thiophene-alt-benzodithiophene copolymer (PTB7), a high-efficiency OPV material. These first-principles results include the chain length dependence of the torsional potential, the nearest-neighbor torsional coupling, the band gap, and the electronic conjugation length. Importantly, PTB7 was found to have a torsional potential almost independent of chain length, very weak nearest-neighbor torsional coupling, a low band gap (∼1.8 eV), and a very long conjugation length (∼147 Å) compared to the other conjugated polymers like polythiophene and poly(3-alkylthiophene). These results suggest that PTB7 can be an efficient electron donor for OPV devices.

Molecular Structure of Poly(methyl methacrylate) Surface. I. Combination of Interface-Sensitive Infrared–Visible Sum Frequency Generation, Molecular Dynamics Simulations, and ab Initio Calculations

The chemical composition and molecular structure of polymeric surfaces are important in understanding wetting, adhesion, and friction. Here, we combine interface-sensitive sum frequency generation spectroscopy (SFG), all-atom molecular dynamics (MD) simulations, and ab initio calculations to understand the composition and the orientation of chemical groups on poly(methyl methacrylate) (PMMA) surface as a function of tacticity and temperature. The SFG spectral features for isotactic and syndiotactic PMMA surfaces are similar, and the dominant peak in the spectra corresponds to the ester-methyl groups. The SFG spectra for solid and melt states are very similar for both syndiotactic and isotactic PMMA. In comparison, the MD simulation results show that both the ester-methyl and the α-methyl groups of syndiotactic-PMMA are ordered and tilted toward the surface normal. For the isotactic-PMMA, the α-methyl groups are less ordered compared to their ester-methyl groups. The backbone methylene groups have a broad angular distribution and are disordered, independent of tacticity and temperature. We have compared the SFG results with theoretical spectra calculated using MD simulations and ab initio calculations. Our analysis shows that the weaker intensity of α-methyl groups in SFG spectra is due to a combination of smaller molecular hyperpolarizability, lower ordering, and lower surface number density. This work highlights the importance of combining SFG spectroscopy with MD simulations and ab initio calculations in understanding polymer surfaces.

A brief review of Badger–Bauer rule and its validation from a first-principles approach

Understanding the acid-base interactions is important in chemistry, biology and material science as it helps to rationalize materials properties such as interfacial properties, wetting, adhesion and adsorption. Quantitative relation between changes in enthalpy (ΔH) and frequency shift (Δν) during the acid-base complexation is particularly important. We investigate ΔH and Δν of twenty-five complexes of acids (methanol, ethanol, propanol, butanol and phenol) with bases (benzene, pyridine, DMSO, Et2O and THF) in CCl4 using intermolecular perturbation theory calculations. ΔH and Δν of complexes of all alcohols with bases except benzene fall in the range from -14 kJ mol-1 to -30 kJ mol-1 and 215 cm-1 to 523 cm-1, respectively. Smaller values of ΔH (-2 kJ mol-1 to -6 kJ mol-1) and Δν (23 cm-1 to 70 cm-1) are estimated for benzene. Linear correlations are found between theoretical and experimental values of ΔH as well as Δν. For all the studied complexes, ΔH varies linearly (R2 ≥ 0.97) with Δν concurrent with the Badger–Bauer rule yielding the average slope and intercept of 0.053(± 0.002) kJ mol-1 cm and 2.15(± 0.56) kJ mol-1, respectively.

Effect of Fluorination on Electronic Properties of Polythienothiophene-co-benzodithiophenes and Their Fullerene Complexes

Fluorination of conjugated polymers is a popular way of designing new electron donors for the bulk heterojunction (BHJ) based organic solar cells (OSCs). However, not all fluorinated polymers observed experimentally enhance the power conversion efficiency of OSCs, and the fundamental understanding of the effect of fluorination is not yet fully uncovered. Herein, we report the effect of fluorine substitution on the electronic properties of polythienothiophene-co-benzodithiophenes as well as their complexes with fullerene, using density functional theory (DFT) and time-dependent DFT methods at the molecular level. Systematic computations of energy gaps (E(g)(opt) and E(g)(hl)), ionization potentials (IP), electron affinities (EA), molecular electrostatic potential (MEP) surfaces, and dipole moments (μ) are carried out for these systems. We found that the fluorination of the thienothiophene unit favors lower E(g)(opt), E(g)(hl), IP, and EA as well as stronger μ compared to the fluorination of the benzodithiophene unit, suggesting that efficient exciton dissociation and charge carriers formation may take place efficiently for the former case. These results support recent experimental findings that the performance of polythienothiophene-co-benzodithiophene-based organic solar cells enhances when thienothiophene unit is fluorinated. The present results highlight that more efficient conjugated polymers for OSC can be designed if the gap engineering is carried out by focusing on the low IP, low EA, and high dipole moment.

Torsion–Inversion Tunneling Patterns in the CH-Stretch Vibrationally Excited States of the G12 Family of Molecules Including Methylamine

Two torsion-inversion tunneling models (models I and II) are reported for the CH-stretch vibrationally excited states in the G12 family of molecules. The torsion and inversion tunneling parameters, h(2v) and h(3v), respectively, are combined with low-order coupling terms involving the CH-stretch vibrations. Model I is a group theoretical treatment starting from the symmetric rotor methyl CH-stretch vibrations; model II is an internal coordinate model including the local-local CH-stretch coupling. Each model yields predicted torsion-inversion tunneling patterns of the four symmetry species, A, B, E1, and E2, in the CH-stretch excited states. Although the predicted tunneling patterns for the symmetric CH-stretch excited state are the same as for the ground state, inverted tunneling patterns are predicted for the asymmetric CH-stretches. The qualitative tunneling patterns predicted are independent of the model type and of the particular coupling terms considered. In model I, the magnitudes of the tunneling splittings in the two asymmetric CH-stretch excited states are equal to half of that in the ground state, but in model II, they differ when the tunneling rate is fast. The model predictions are compared across the series of molecules methanol, methylamine, 2-methylmalonaldehyde, and 5-methyltropolone and to the available experimental data.

Structural Dependence of Electronic Properties in A-A-D-A-A-Type Organic Solar Cell Material

Small conjugated molecules (SCMs) are promising candidates for organic photovoltaic (OPV) devices because of their structural simplicity, well control over synthetic reproducibility, and low purification cost. However, industrial development of SCM-based OPV devices requires improving their performance, which in turn relies on the fundamental understanding of structural dependence of electronic properties of SCMs. Herein, we report the structural and electronic properties of the BCNDTS molecule as a model system for acceptor-acceptor-donor-acceptor-acceptor (A-A-D-A-A) type SCMs, using density functional theory (DFT) and time-dependent DFT methods. Systematic calculations of two-dimensional potential energy surfaces, molecular electrostatic potential surfaces, ground state frontier molecular orbital energies, and the vertical excitation energies are performed. We found that the lowest energy conformation of the BCNDTS molecule is planar. The planar conformation favors the lowest ground state and the excited state energies as well as the strongest oscillator strength. The present results suggest that SCMs containing central dithienosilole cores connected with 2,1,3-benzothiadiazole groups have potential to be an efficient electron donor for OPV devices.

Vibrational conical intersections in CH3SH: implications for spectroscopy and dynamics in the CH stretch region

The adiabatic separation in methyl mercaptan of the high-frequency asymmetric CH stretch vibrations from the lowfrequency torsional (γ) and CSH bend (ρ) coordinates yields a set of 7 vibrational conical intersections (CIs). The three CIs in the staggered conformation at ρ = 79◦ are close to the global minimum energy geometries (ρe = 83.3◦), accounting for the observed near-degeneracy of the two asymmetric CH stretch vibrations. The vibrational frequencies were computed at the CCSD(T)/aug-cc-pVTZ level. A new high-order Exe Jahn-Teller model, which involves a spherical harmonic expansion in ρ and γ, fits the calculated electronic and vibrational energies over the whole range of γ and for ρ between 0◦ and 100◦ to within a standard deviation of 0.2 cm−1. The pattern of the CIs contrasts with that in methanol where the CIs occur only in the eclipsed conformation near the top of the torsional barrier. An examination of three alternative diabatization schemes for the two molecules points to rather different nuclear dynamics. In CH3SH crossings between the upper and lower adiabatic surfaces are predicted to occur predominantly with motion along the CSH bending coordinate; whereas in CH3OH, such crossings are predicted to occur predominantly with torsional motion.

VIBRATIONAL JAHN-TELLER EFFECT IN NON-DEGENERATE ELECTRONIC STATES

The Jahn-Teller theorem a states that “All non-linear nuclear configurations are therefore unstable for an orbitally degenerate electronic state.” In 1982, Kellman b realized that the Jahn-Teller theorem also applies to nonlinear molecular species in non-degenerate electronic states when there are high-frequency vibrations that are degenerate at a symmetrical reference geometry. When those high frequencies can be considered as adiabatic functions of degenerate low-frequency coordinates, there is a spontaneous Jahn-Teller distortion that lifts the degeneracy of the high-frequency vibrations. Kellman applied the vibrational Jahn-Teller (vJT) concept to the Van der Waals dimer (SF6)2. In this talk, the vJT concept is applied to E ⊗ e systems that are small bound molecules in non-degenerate electronic states. The first case considered in systems for which the global minimum of the electronic potential has C3v symmetry.For such systems, including (C6H6)Cr(CO)3 and CH3CN, the vJT effect leads to a significant splitting of the degenerate highfrequency vibrations (CH or CO stretches), but the spontaneous vJT distortion is exceptionally small. The second case in systems for which the global minimum of the electronic potential is substantially distorted from the C3v reference geometry. For the second case systems, including CH3OH and CH3SH, the vJT splitting of the degenerate CH stretches is much larger, on the order of several 10Äôs of cm−1). For both cases, there is the symmetry-required vibrational conical intersection at the C3v reference geometry. For the second case systems, there are additional symmetry-allowed vibrational conical intersections far from the C3v geometry but energetically accessible to the molecule at thermal energies. For both cases, the vibrationally adiabatic surfaces, including the multiple conical intersections, are well described by modest extensions to a high-order Hamiltonian that was developed for the electronic Jahn-Teller problem.c