Ab Initio Study of Torsional Potentials in 2,2'-BITHIOPHENE and 3,4'- and 3,3'-DIMETHYL-2,2'-BITHIOPHENE as Models of the Backbone Flexibility in Poly Thiophene and POLY(3-METHYLTHIOPHENE) (original) (raw)
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A conformational study of ethyl-substituted bithiophenes. Semi-empirical versus ab initio methods
Synthetic Metals, 1998
Semi-empirical (AM 1, PM3) and ab initio calculations (STO-3G, 3-21G*) are employed to obtain the equilibrium optimized geometries and the torsional potential surfaces of 2,2'-bithiophene as well as its 3,4'-and 3,3'-ethyl-substituted derivatives. For the unsubstituted molecule, ab initio calculations have also been performed at the HF/6-31G* level. The geometries were completely optimized along the torsional potential curves to account for the molecular relaxation, yielding a physically meaningful picture of the nonrigid rotation. The results given by each theoretical method are compared and discussed. It is found that ethyl substitution causes rather small changes in the thiophene ring structure. Contrary to these results, ethylation dramatically influences the overall shape of the torsional potentials, leading to a large tilt from planarity. The barrier against planarity is found much higher for the 3,3'-ethyl derivative. It is also observed that the steric hindrance created by ethyl groups is much higher than that induced by methyl substituents.
Macromolecular Theory and Simulations, 1997
A molecular mechanics computational procedure, previously used for the refinement and the analysis of several crystalline polymers, was applied to investigate the crystal structures of the tetramer (T4) and hexamer (T6) of thiophene, as well as the crystal structure of polythiophene (PT). Simultaneous minimization of intra- and intermolecular energies of the T4 and T6 structures, obtained by Rietveld analysis of powder X-ray diffraction profiles, leads to molecular conformations showing smaller deviations from the ring co-planarity than the original models. For both oligomers the calculations confirm that the molecular centre of inversion is not a crystallographic centre of symmetry, as also revealed by X-ray diffraction of the T6 single crystal. This surprising effect appears to arise from intermolecular interactions between the terminal residues, hence is not relevant with respect to the PT polymer structure. The small energy cost for constraining the molecules at the crystallographic centre of symmetry is in agreement with experimental findings that reveal the existence of polymorphs for both T4 and T6. The calculations on the T6 single crystal were used to upgrade the MM2-like force field, which was then used to determine the minimum-energy model of the monoclinic crystal structure of polythiophene.
Poly(alkylthiophenes): chain conformation and thermochromism
Synthetic Metals, 1992
The role of conformational disorder in the electronic structure of alkyl-substituted polythiophene is investigated. Thermally-induced twisting of thiophene rings out of the main conjugation plane is assumed. AM1 geometry optimizations were carried out to obtain the molecular torsion potential curves for substituted bithiophenes. Total torsion potentials acting on interring rotations are assumed to come from two contributions, the molecular potential and a phenomenological 'solid state potential' that accounts for inter-chain interactions. It is found that the molecular potential for rotations does not depend on the length of the alkyl group but only on the regiochemistry of substitution. A long, disordered thiophene chain is built to be representative of a classical probability distribution of torsion angles. The electronic structure associated with valence and conduction u bands is calculated within VEH pseudopotential theory and the NFC technique. The dependence of the optical gap on temperature is obtained and is in agreement with experiment.
Structural studies of polythiophenes
Synthetic Metals, 1996
A theoretical investigation of the electronic structures of the oligomers of thiophene (T) and their derivatives, namely, 2-methylthiophene (2MT), 3-methylthiophene (3MT), 2-cyanothiophene (2CT) and 3-cyanothiophene (3CT), are presented. The most stable forms of the monomer, dimer, trimer and tetramers are obtained by the semi-empirical quantum mechanical methodology using AM1 parametrization. All possible binding sites are investigated in order to understand the bonding in polythiophenes. The 3-substituted molecules form dimers which lie nearly at tmns-planar positions but 2CT and 2MT favor cis conformations. The activation energies between these structures and local minima change from 0.2 to 1.8 kcal/mol. Methyl-and cyano-substituted polythiophenes favor linear growth mechanisms. The branching requires 3-6 kcal/mol per substitution. Polythiophenes seem to form fairly flexible chains as understood from the relatively low rotational barriers.
The Journal of Chemical Physics, 1997
A set of supramolecular cage-structuresspherophanes-was studied at the density functional B3LYP level. Full geometrical structure optimisations were made with 6-31G and 6-31G(d) basis sets followed by frequency calculations, and electronic energies were evaluated at B3LYP/6-31++G(d,p). Three different symmetries were considered: C1, Ci, and Oh. It was found that the bonds between the benzene rings are very long to allow π-electron delocalisation between them. These spherophanes show portal openings of 2.596 Å in Spher1, 4.000 Å in Meth2, 3.659 Å in Oxa3, and 4.412 Å in Thia4. From the point of view of potential host-guest interaction studies, it should also be noted that the atoms nearest to the centre of the cavities are carbons bonded to X groups. These supramolecules seem to exhibit relatively large gap HOMO−LUMO: 2.89 eV(Spher1), 5.26 eV(Meth2), 5.73 eV(Oxa3), and 4.82 eV(Thia4). The calculated ΔH°f (298.15 K) values at B3LYP/6-31G(d) are (in kcal mol −1) 750.98, 229.78, −10.97, and 482.49 for Spher1, Meth2, Oxa3, and Thia4, respectively. Using homodesmotic reactions, relative to Spher1, the sphero-phanes Meth2, Oxa3, and Thia4 are less strained by −399.13 kcal mol −1 , −390.40 kcal mol −1 , and −411.38 kcal mol −1 , respectively. Their infrared and 13 C NMR calculated spectra are reported.
Effects of geometrical disorder on the electronic structure of alkyl-substituted polythiophenes
Synthetic Metals, 1991
In this work, the role of structure disorder on the alkyl-substituted polythiophene electronic spectrum is investigated. A series of AM1 calculations is carried out to estimate the total energy as a function of torsion angle of alkyl-thiophene dimers. The dependence of torsion energy on the radical length and substitution position are studied. The calculated torsion barriers are used to simulate the out-of-plane disordered twisting of thiophene rings within a simple statistical model. Electronic structure calculations on long disordered thiophene chains are performed by the combination of Valence Effective Hamiltonian method and the Negative Factor Counting technique. Thermally induced disorder on alkyl-substituted polythiophenes is shown to explain irreversible changes on infrared spectrum of the polymer upon heating and cooling cycles. INTRODUCTION Functionalization of polythiophene has been proposed as a method for the modification and control of the polymer physical and chemical properties [i]. The preparation of alkyl-substituted polythiophenes has recently been reported in the literature [2-4]. The presence of long aliphatic tails connected to the conjugated chain has shown to increase polymer solubility in commo~ orjanic solventes -high molecular weight chains are then obtained -while the polymer crystallinity is improved, with smaller effects on conductivity [5]. The possibility of obtaining soluble conjugated polymers represents not only a very important step for polymer processability, but it allows one chain properties to be accessed, and it gives rise to new phenomena, as solvatochromism and thermochromism [2, 5-6]. We note that the VEH method underestimates ~ -~* band gape, which is of 1.605 e V in our calculation and is experimentally estimated to be 2.0 -2.1 eV [6]. For the first set of data (H = 1.5 kcal), the gap rapialy increases up to ss room temperature and then saturates. This is in fact the correct behavior [6], but the raise in optical gap starts at room temperature and saturates at about 450K for long alkyl radicals. This is an indication that our solid state barrier should be even higher to correctly describe the Tbermocharomic Transition. An important feature of the disordered chain is the possibility of large torsion angles to occur, though in small amounts. On heating, large angles are more and more populated. As the torsion potential curves are often non-symmetrical around 90 ° , some of these large angles could freeze, on cooling, in the symmetry broken syn conformation (twist angle of 180 °) instead of returning to the initial anti conformation. No effects would be seen in the optical absorption spectrum, since differences in geometries of ~ and anti conformations are negligible [14], while infrared absorption would be affect by the symmetry breaking. This could be an explanation for the non-reversibility of the infrared spectrum on heating and cooling cycles [5].
Difficulties of density functional theory in predicting the torsional potential of 2,2?-bithiophene
International Journal of Quantum Chemistry, 1998
The internal rotation of 2,2Ј-bithiophene was investigated within the Ž . density functional theory DFT approach. Fully optimized DFT torsional potentials are Ž . compared with Møller᎐Plesset MP2 results which predict a fourfold potential with s-cis-and s-trans-gauche minima. DFT calculations fail in describing the energetics of the internal rotation because they favor planar vs. perpendicular conformers. Gradientcorrected functionals provide torsional potentials where the gauche minima have almost vanished and the s-cis l s-trans interconversion barriers are twice as high as the barriers obtained at the MP2 level. The use of local functionals augments the shortcomings of the DFT approach. The gauche minima completely disappear and the rotational barriers are now about three times higher than the MP2 barriers. As an efficient computational approach, we suggest having geometries optimized at the DFT level and conformational energies evaluated via single-point MP2 calculations. The fitting of MP2rrDFT energies to truncated Fourier expansions allows one to predict the torsional angles and the relative energies of the critical points of the rotational potential with an accuracy similar to that afforded by MP2 calculations including full geometry optimization.
Molecular Modeling of Crystalline Alkylthiophene Oligomers and Polymers
The Journal of Physical Chemistry B, 2010
We present the results of a thorough molecular modeling study of several alkylthiophene-based oligomers and polymers. In particular, we consider two polymers whose limit-ordered crystal structures have been recently reported by our group, on the basis of powder X-ray data analysis: poly(3-(S)-2-methylbutylthiophene) (P3MBT) and form I′ of poly(3-butylthiophene) (P3BT). We first describe the development of a series general purpose force fields for the simulation of these and related systems. The force fields incorporate the results of ab initio calculations of the bond torsion energies of selected oligomers and differ in the set of atomic charges used to represent the electrostatic interactions. We then present the results of an extensive validation of these force fields, by means of molecular mechanics (MM) energy minimizations and molecular dynamics (MD) simulations of the crystal structures of these oligomers and polymers. While our "best" force field does not outperform the others on each of the investigated systems, it provides a balanced description of their overall structure and energetics. Finally, our MM minimizations and MD simulations confirm that the reported crystal structures of P3MBT and P3BT are stable and correspond to well-defined energetic minima. The roomtemperature MD simulations reveal a certain degree of side-chain disorder, even in our virtually defect-free polymer crystal models.