J. Phys. Chem. 1996, 100, 14661-14664
14661
Characterization of the Quinoid Structure for the 2,2′-Bithiophene and 2,2′,5′,2′′-Terthiophene Dications Carlos Alema´ n*,† and Luis Julia` ‡ Departament d’Enginyeria Quı´mica, E.T.S.I.I.B., UniVersitat Polite´ cnica de Catalunya, Diagonal 647, Barcelona 08028, Spain, and Departament de Materials Orga` nics Halogenats, Centre d’InVestigacio´ i DesenVolupament (CSIC), Jordi Girona 18-26, Barcelona 08034, Spain ReceiVed: February 8, 1996; In Final Form: May 31, 1996X
We report the results of a detailed ab initio study about the molecular structure of 2,2′-bithiophene and 2,2′,5′,2′′terthiophene dications. The molecular geometry and conformational behavior of the quinoid structure obtained for 2,2′-bithiophene dication are compared with the benzoid structure of noncharged 2,2′-bithiophene. Calculations on 2,2′,5′,2′′-terthiophene dication indicate that with the increasing number of thiophene rings the quinoid structure disappears whereas the benzoid structure gradually develops. Finally, the relative stabilies between the different minima found for the 2,2′-bithiophene dication (anti T syn) and the 2,2′,5′,2′′-terthiophene dication (anti-anti T syn-anti T syn-syn) are analyzed.
Introduction Among polyheterocycle conducting polymers, poly(thiophene)s are the more thoroughly investigated systems. Poly(thiophene)s belong to the class of electrically conducting polymers with a nondegenerate ground state and a possibility of nonlinear excitations such as polarons (radical cations or anions) and bipolarons (dications or dianions).1-3 Furthermore, poly(thiophene)s are particularly interesting because sulfur plays a crucial role in partially connecting the π-system of consecutive rings and allowing for delocalization of the electronic excitations.4,5 During the last years, we have carried out a systematic effort aimed to investigate the electronic and geometric properties of the poly(thiophene)s constituent oligomers.6-9 Thus, a detailed knowledgement of these oligomers can provide in many cases a better understanding of the poly(thiophene)s properties. The two-rings compound 2,2′-bithiophene, abbreviated Btp, may be envisaged as the most simple model of poly(thiophene). Its molecular structure has been experimentally investigated in both the gas phase10 and the solid state.11 In the gas phase, the two rings of Btp are not planar with a torsional angle of 146°, whereas a planar arrangement is found in the crystal with the two rings in the anti conformation. On the other hand, NMR spectroscopy experiments show that both the syn and anti conformations exist at room temperature, the energy difference between them being 0.2 kcal/mol.12 For larger oligomers as well as for their alkyl-substituted derivatives, the crystal structure data show an all-anti conformation,7,13-16 whereas in chloroform they adopt an anti-gauche conformation, which reduce the steric interactions.16,17 The conformational properties of Btp have been recently investigated by several authors using quantum mechanical calculations.9,18-22 In a very recent study,9 we found that the ab initio HF/6-31G(d) is the minimum level necessary to study the conformational preferences of oligothiophenes. Thus, ab initio HF calculations using basis sets without polarization functions are not able to represent the conformational preferences of Btp and overestimate the rotational barrier by 5090% with respect to that obtained at higher levels of theory. * All correspondence to this author. † Universitat Polite ´ cnica de Catalunya. ‡ Centre d’Investigacio ´ i Desenvolupament. X Abstract published in AdVance ACS Abstracts, August 1, 1996.
S0022-3654(96)00410-8 CCC: $12.00
On the other hand, the semiempirical AM123 method largely underestimates the barrier, whereas the MNDO24 method leads to an erroneously rotational profile. Consequently, a better understanding of small oligothiophenes can only be achieved from high level ab initio calculations. In this work we have extended the calculations to 2,2′bithiophene dication, abbreviated Btp(+2), which, owing to its intrinsic instability, is hard to study using experimental methods.25 This study permits us to make a rigorous comparison between the benzenoid structure of Btp and the quinoid structure of Btp(+2) (see Scheme 1). Furthermore, in order to obtain a more detailed description of the quinoid structure in bipolarons, we have also performed calculations on 2,2′,5′,2′′-terthiophene dication, abbreviated as Ttp(+2). Methods In all cases, the Btp system was treated at the RHF level of theory. Investigation of the minimum energy conformations of Btp(+2) was performed at both RHF and UHF levels, whereas minimum energy structures of Ttp(+2) were only obtained at the RHF level. All the computed equilibrium structures were optimized without any constraint. The standard 6-31G(d) (d functions on S and C atoms) basis set25 was used in all geometry optimizations. Comparison with previous results reveals that it is a suitable computational level for thiophene derivatives.9 Minimum energy structures for Btp and Btp(+2) were characterized as such by calculating and diagonalizing the Hessian matrix and ensuring that they do not have a negative value. Because of the size of the oligomer, minimum energy structures computed for Ttp(+2) were not characterized as such. Electron correlation effects were studied using single MP2 calculations26 with the 6-31G(d) basis set on the SCF optimized geometries. The rotational profiles of Btp and Btp(+2) were computed spanning the torsional angle between the planes of the two rings (θ) in steps of 30° (see Scheme 1). A flexible rotor approximation was used in the two cases, the molecular geometry of each point of the rotational profile being optimized at a fixed θ value. Rotational profile of Btp was investigated at the RHF level. However, rotation around double bonds cannot be represented by simple RHF calculations, therefore an UHF wave function has been used to investigate the rotational profile of Btp(+2). © 1996 American Chemical Society
14662 J. Phys. Chem., Vol. 100, No. 35, 1996
Alema´n and Julia`
SCHEME 1
TABLE 2: Optimized Geometries of Minimum Energy Conformations of 2,2′-Bithiophene (Btp) and 2,2′-Bithiophene Dication (Btp(+2)) Btp parameter
TABLE 1: Relative Energies (in kcal/mol) and Torsional Angles (θ, in degrees) of 2,2′-Bithiophene (Btp) and 2,2′-Bithiophene Dication (Btp(+2)) no.
Btp
Btp(+2)
global minimum barrier local minimum
0.0 (θ ) 147.9°) 1.7 (θ ) 90.0°) 0.7 (θ ) 42.2°)
0.0 (θ ) 180.0°) 35.5 (θ ) 90.0°) 0.7 (θ ) 0.0°)
All the calculations performed in this work were done with the Gaussian-92 program.27 All the computations were performed with a CONVEX C3480 of the Centre Europeu de Paralelisme de BArcelona (CEPBA).
distancea C2-C3 C3-C4 C4-C5 C5-S C2-Sb C2-C2′ C3-H C4-H C5-H anglea
