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Synth Met 43 3501

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Synth Met 43 3501
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  Synthetic Metals, 41--43 (1991) 3501-3504 3501CONFORMATIONAL AND VIBRATIONALPOLY (3-METHY LTHI OPHENE) PROPERTI ES OFV. Hern~ndez, J.T. L6pez Navarrete and J.l. Marcos.Departamento de Qufmica-Ffsica. Facultad de Ciencias. Universidad de M~laga.29071 Mfilaga. Spain.ABSTRACTIn this work, semiempirical MNDO and AM1 methods have been used to evaluate thedifferences in stability between some isomers of oligomers of 3-methylthiophene taken asmodel molecules of the polymer. Electronic and vibrational properties are also evaluated forthe oligomers and the polymer.INTRODUCTIONRecently a great number of heteroaromatic molecules such as pyrrole and thiophenehave been electrochemically polymerized to form doped conducting polymers. Thesemolecules can be substituted to give materials with good stability while maintaining highlevels of conductivity. Specially versatile are polymers based on 3-alkylthiophenes becauseof their solubility in common organic solvents [1].In the present work, we partially report our calculations on the geometry, totalenergy, electronic and vibrational properties of some oligomers of 3-methylthiophene(3-MeTh). The semiempirical methods MNDO [2] and AM1 [3] (using MNDO parametersfor the S atom) have been used in our calculations. Electronic properties ofpoly(3-methylthiophene) (P3-MeTh) have been also evaluated assuming that the polymersare one-dimensional perfect crystals and using the band theory at MNDO level.RESULTS AND DISCUSSIONa) ConformationsFirst of all, the semiempirical MNDO and AM1 methods have been used to evaluatethe conformational stability of the dimer (3-MeTh)2 and tetramer (3-MeTh)4. In all cases,both possible linkages, Head-Head (H-H) and Head-Tail (H-T), between monomeric unitshave been taken into account. Two strategies have been used in the energy minimization[4]:0379-6779/91/$3.50 © Elsevier Sequoia/Printed in The Netherlands  3502 i) A full geometry optimization including also the torsional angles between theheteroaromatic rings. These results can be compared with the experimental data of themolecules in the gas phase or in solution state [4,5].ii) An optimization carried out constraining the molecule to be anti-coplanar inorder to mimic the solid state when we assume that the molecules have this conformation[6,7]. In this case a local minimum is found for the energy.The differences in stability between the isomers of (3-MeTh)2 and (3-MeTh)4 areshown in Table 1 and 2 respectively. The most important conclusions from these data areas follows a)within semiempirical methods, the most favorable linkages betweenmonomeric units are always the H-T, b) the very important deviations from the coplanarityobserved for the fully optimized geometries are due essentially to the repulsive effectbetween the S atoms and the methyl groups.Table 1.AM1MNDODifferences in stability between some isomers of (3-MeTh)2.(All values of torsional angle from syn-coDlanarity).(3-MeTh)2 AE (cal/mol) 0 (degrees)H-T; minimum 0.0 32.8H-T; anti-coplanarH-H; minimumH-H; anti-coplanarH-T; minimumH-T; anti-coplanarH-H; minimumH-H; anti-coplanar1091.9 180933.9 61.83373.3 1800.0 92.52957.3 1801225.3 96.87341.5 180Table 2.AM1MNDODifferences in stability between some isomers of (3-MeTh)4. (All values oftorsional angles from syn-coplanarity).(3-MeTh)4H-T; minimumH-T; anti-coplanarH-H; minimumH-H; anti-coplanarH-T; minimumH-T; anti-coplanarH-H; minimumH-T; anti-coplanarAE (cal/mol) 01 (degrees) 02 (degrees) 03 (degrees)0.0 32.0 33.2 32.63044.9 180 180 1801273.6 62.4 163.7 88.05778.4 180 180 1800. 0 93.5 91.6 95.98596.3 180 180 1801399.9 94.2 113.3 91.514098.0 180 180 180Further understanding of the changes in the properties of the polymer in solid and solutionstates comes from the study of the evolution of the electronic properties as functions of thetorsional angles between the rings [4]. Ionization potential (Ip) and electron affinity (EA)values indicate the easiness of p-type and n-type doping, respectively. As reported in Fig. 1the changes of the Ip and EA about the coplanar conformations of (3-MeTh)2 are small. Ifwe extend this trend to the polymer this means that even if P3-MeTh is slightly distorted insolid or solution states it can still be doped.  9,6 9949,2 .= 9,08,8 Fig.1. H-H 0 (3MeTh)2• (3MeTh):'~ u u u u u 0 30 60 90 120 150 180torsional angle (degrees) 1,03503 0,8 '0,6 '0,4 '0,2 ' H-H0 (3MeTh) 2 o 8,6 0,0 J , , , ,0 30 60 90 120 150torsional angle (degrees)180Dependences of Ip and EA with torsional angle in (Th)2, (3-MeTh)2H-H and(3-MeTh)2 H-T b) Electronic propertiesThe energy band structures of the one-dimensional periodic neutral PTh and(P3-MeTh) H-T have been calculated using the MNDO crystal orbital method. A fullgeometry optimization has been carried out for the two polymers with the only constraint ofkeeping the molecule anti-coplanar. The most important electronic properties of thepolymers are shown in Table 3. From these data one can conclude that the methylderivative presents slightly better electronic properties for the conduction than the PTh.Table 3. Calculated electronic properties (in eV) for PTh and (P3-MeTh)H-T .PTh (P3-MeTh)H-TIp 8.04 8.01Eg 6.33 6.31EA 1.71 1.70c) Vibrational propertiesStarting with the MNDO optimized geometries of the anti-coplanar oligomers we havecalculated Pulay group valence force constants from which normal modes and frequenciesare derived. We report in Table 4 some force constants which refer to the skeletalstretchings of the tetramers (Th)4, (3-MeTh)4H-H and (3-MeTh)4H-T. The same tableincludes the corresponding values of Ip, Eg and Effective Conjugation Force ConstantsF~I [8] for every molecule. From Table 4 one can derive two important results: i) the skeletalC-C bonds attached to the methyl groups are weakened with respect to the corresponding  3504 in (Th)4, ii) the general effect of the methyl groups in the short polymer chains in anincrease of ~ electrons delocalization with respect to (Th)4. v 5~j~ v 13~5 Table 4.Comparison of some relevants Pulay MNDO force constants and EffectiveConjugation Force Constants calculated for (Th)4, (3-MeTh)4 HH and(3-MeTh)4H-T.Force constants (Th)4 (3-MeTh)4H'H (3-MeTh)4H'T8 8. 0807 8. 0937 7. 99309 9. 5920 9. 8054 9. 663510 7. 6068 7. 4094 7. 423811 9. 5986 9. 4464 9. 54909/10 1.1589 1. 1295 1. 12158/9 0. 8786 0. 8769 0.93178/10 -0. 0973 -0. 0904 -0. 10258/11 -0. 1229 -0. 1257 -0. 14116/10 -0. 0170 -0. 0167 -0. 0163F~t 6. 6817 6. 5578 6. 5826ACKNOWLEDGEMENTSWe are grateful to Prof. G. Zerbi, and his group of Politecnico di Milano, for the manyhelpful discussion. One of us (VII) is grateful to the MEC (Spain) for a grant. The work wascarried out with the financial support of the DGICYT (Project PB88/0529).REFERENCF~1. a) R.L. Elsenbaumer, K.Y. Jen and R. Oboodi, Svnth. Metals.15 (1986).169;b) S. Hotta, S.D.D.V. Rughooputh, A.J. Heeger and F. Wudl, Macromolecules. 20(1987) 212.2. M.J.S. Dewar and W. Thiel, J. Am. Chem. Soc.. 99 (1977) 4899,4907.3. M.J.S. Dewar, E.G. Zoebisch. E.I. Healy and J.J.P. Sterwart, J. Am. Chem. Soc.. 107(1985) 3902.4. J.T. LSpez Navarrete, B. Tian and G. Zerbi, Svnth. Metals. (in press).5. V. Hern~indez, J.T. LSpez Navarrete and J.I. Marcos, J. Mol. Struct.. 219 (1990) 397.6. a) J.T. I~pez Navarrete and G. Zerbi, ~alJ~(1989) C15;b) J.T. LSpez Navarrete and G. Zerbi, ~(in press)7. a) B. Tian and G. Zerbi, Svnth. Metals,28 (1989) C1;b) B. Tian and G. Zerbi~[~~ (1990) 3886, 3892.
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