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A theoretical investigation of α,α′-dimethyl end-capped oligothiophenes: structures, vibrational spectra and conjugation defects
ELSEVIER
Synthetic Metals 89 ( 1997) 159-160
A theoretical investigation of a,a’-dimethyl end-capped oligothiophenes: structures, vibrational spectra and conjugation defects J. Casado, V. Hernhdez, Departammto
de t&iinica
Fisica,
F.J. Rarnfrez, J.T. L6pez Navarrete Universidad
de Milaga,
29071 Mdlaga,
Spain
Abstract Ab initio quantum-chemical methods are used in the study of the effect of ionization on the geometries and vibrationalspectraof a series of a,oc’-dimethylend-capped oligothiophenes (from dimerto hexamer).For thatpurposewe determinethegeometricstructures,vibrational frequencies, and infrared and Raman intensities of neutral, radical cationic and dicationic oligomers. These theoretical data, when combined with the results of the experimental studies, provide a complete picture of the vibrational structure of the neutral state of doped oligomers. 0 1997 Elsevier Science S.A. Kej~ords:
Ab initio quantum chemical methods and calculations; Polythiophene and derivatives
1. Introduction Conjugatedpolymers have attractedconsiderableattention since the discovery of possibleinsulator-to-metal transitions following chemical or electrochemicaldoping. In order to get a deeperinsight into the complex propertiesof the polymers, the study of oligomer analogshas emergedin the past few years asa very useful tool. Polythiophene and its oligomers have attracted much interestdue to their good environmental stability in both the neutral and doped states. The large amount of studiesdedicatedto thesecompoundshasopened the way to major developmentsin the fabrication of devices [ 11. Moreover, the ol,cy’-dimethyl substitution has recently proved very effective in enhancing chemical stability and crystallinity without damagingthe electronic responseof the resulting material [ 21. In this communication, the geometricandvibrationalproperties of a seriesof a,a’-dimethyl end-cappedoligothiophenesaretheoretically investigated in both neutral andoxidized states;the resultsare then comparedto the experimentaldata.
2. Methodology The methyl end-cappedoligothiophenes investigated in this work contain from 2 to 6 thiopheneunits (from the dimer to the hexamer). The geometry optimizations were carried out using wave functions of the Hartree-Fock type: restricted (RI-IF) for closed-shell systems (the neutral and the dicationic systems), spin unrestricted (UHF) and spin restricted 0379-6779/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved PUSO379-6779(97)03916-7
(ROHF) for open-shellsystems(the radical cation systems). The basisset usedin all calculationswas the 6-31G”” which is split valence and includes a setof d-polarization functions for heavy atomsand p-polarization functions for the hydrogens. We have initially limited our theoretical investigation to the all-anti planar conformations.
3. Results and discussion 3.1. Geometric stmctwes The 6-3 lG*” optimized geometricstructuresof the neutral oligomers show that all the inner thiophene rings present almost the samegeometry, which slightly differs from that found in the outer units due to the methyl-end substitution. Ab initio calculations are indicative of a degree of bondlength alternation of about 0.07 A in good correspondence with the experimental data. The 6-31G*” optimized geometry of the dicationic c~,ol’dimethyl end-cappedoligothiophenesindicatesthe formation of a positive bipolaron defect localized in the middle of the molecule and extending over the adjacent repeat units. The charged speciesare characterized by a reversal of the singledouble C-C bond pattern, the geometry relaxation process thus induces the appearanceof a strong quinoid character within the molecule. In the caseof the radical cations, there occurs the appearance of a positive polaron defect localized in the middle of the molecule.ROHF and UHF geometries(seeFig. 1 for the hexamer) of the radical cations exhibit weaker, structural
160
J. Crtsctdoet N/./Synthetic Metok 89 (1997) 159-160
1525
z
::
I487
1038
DMQtT
b
d Fig. 1. 6-31G”” optimized C-C bond lengths (in A) of the a,a’-dimethylsexithiophene in the neutral state (filled squares) and in the radical cationic state (open circles). ROHF method has been used for the open-shell system.
::
1700
1529
1500
1300
1035
DM-TT
1035
DMBT
1100
900
R-n
modifications than in the dications.We estimatethe polarons to extend over about two-three rings. As expected, the UHF and ROHP methodsgive comparablegeometriesfor all the radical cations calculated in this work. However, the ROHF formalism better reproducesthe localization of the charge carriers along the chain ascomparedto the UHF approach. 3.2. Vibrntionnl spectra ofnem-ol molecules We have used the ab initio 6-31G*” level of calculation to determine the infrared and Raman spectra of the neutral oligothiophenes.For comparisonpurposeswe have scaledall the calculated frequencies by a uniform factor of 0.9. All quoted scaledtheoretical resultsare thus the scaledvalues. We restrict our discussionto the most important Raman spectroscopic trends occurring upon chain elongation. In Fig. 2, the computed Ramanspectra of DMBT, DMTT and DMQtT are displayed. The spectra show very few bands despite the complex chemical structures,which exclusively originate from totally symmetric modes. As in the experimental spectra, for DMBT and DMTT the most important feature is the appearanceof only one intense band around 1530cm- ‘, this shifts downward by 10cm- I from the dimer to the tetramer (from 1492cm-’ to 1482 cm- ’ in the experimental spectra [ 31). For longer oligomers a second line around 1470cm- ’ becomesstrongerwith the molecularsize. As in the experimental Raman spectra,thesetwo lines show a convergent trend. The theoretical Raman spectra of the neutral moleculesshowthe four characteristic featurescommon to many other classesof oligothiophenes: 1615 1591 cm-’ (line A) weak, which showsdispersiontoward lower frequencieswhen increasingchain length; 1536-1525 cm-’ (line B), it is a common feature of the Raman spectra of aromatic andheteroaromaticsystems;1476 1487cm- ’ (line C) which is characteristicofa or o substitutedoligothiophenes and showsintensity enhancementwith increasingchain length; and 1035 cm-’ (line D), observed in the Raman spectrumfor all the ofgothiophenes. 3.3. Vibmtional spectra of oxidized
molecules
In Fig. 3 the computedinfrared spectraof the radical cation (ROHF/6-3 1G**) molecuIesare displayed. Comparingthe theoretical infrared spectrumof the neutral oligomersto that
shift
700
500
300
00
(cm”)
Fig. 2. Theoretical Raman spectra of neutral cc,a’-dirnethyl as calculated at the RHF/6-3 lG** level.
oligothiophenes
Dhfl-l-
=” ; 2! 9
.
DM
i
I
1700
lSO0
1300
1100 W*v+numbcr
900
700
500
B-i-
400
(cm”)
Fig. 3. Theoretical infrared spectra of a,o’-dimethyl cations as calculated at the ROHF/6-31G** level.
oligothiophene
radical
of its corresponding radical cations calculated in the same basisset, one immediately recognizes new intensive bands calculated at 1450 (m), 1404 (m), 1376-1290 (s), 1274 (vs), 1115 (s), 1039 (vs), 967 (m), 865 (m) cm-‘. The experimental infrared spectraof the I2 doped materialsshow [ 41 at leastsix new experimentalinfrared bandsat 1414(m), 1327 (s), 1162 (s), 1099 (s), 993 (s) and 946 (m) cm-‘, whoseintensitiesincreasewith increasingdopinglevel. Comparison between calculated and experimental data indicates the existence of a polaron-type defect in the iodine doped materials, as has been confirmed by the electronic spectra [51. Acknowledgements This researchwasfinancially supportedby Direccidn General de Investigacidn Cientifica y Tecnica (DGICYT, Spain) through the ResearchProject PB93- 1244.
References [l] J. Roncali, Chern. Rw., 92 (1992) 71 1. [2] S. Hotta and K. Waragai, J. Mater. Chern., I (1991) 535. [3] V. Hernandez, J. Casado, F.J. Ramirez. G. Zotti, S. Hottaand J.T. Lopez Navarrete, J. Chew. Bras., IO4 (. 1994) 9271. [4] J. Casado, F.J. Ramirez. S. Hotta, J.T. Lopez Navarrete and V. Hernandez, S~nth. Met.,54 ( 1997) 57 1. 151 S. Hotta and K. Waragai, J. Phys. Chem., 97 ( 1993) 7427.
Synthetic Metals 89 ( 1997) 159-160
A theoretical investigation of a,a’-dimethyl end-capped oligothiophenes: structures, vibrational spectra and conjugation defects J. Casado, V. Hernhdez, Departammto
de t&iinica
Fisica,
F.J. Rarnfrez, J.T. L6pez Navarrete Universidad
de Milaga,
29071 Mdlaga,
Spain
Abstract Ab initio quantum-chemical methods are used in the study of the effect of ionization on the geometries and vibrationalspectraof a series of a,oc’-dimethylend-capped oligothiophenes (from dimerto hexamer).For thatpurposewe determinethegeometricstructures,vibrational frequencies, and infrared and Raman intensities of neutral, radical cationic and dicationic oligomers. These theoretical data, when combined with the results of the experimental studies, provide a complete picture of the vibrational structure of the neutral state of doped oligomers. 0 1997 Elsevier Science S.A. Kej~ords:
Ab initio quantum chemical methods and calculations; Polythiophene and derivatives
1. Introduction Conjugatedpolymers have attractedconsiderableattention since the discovery of possibleinsulator-to-metal transitions following chemical or electrochemicaldoping. In order to get a deeperinsight into the complex propertiesof the polymers, the study of oligomer analogshas emergedin the past few years asa very useful tool. Polythiophene and its oligomers have attracted much interestdue to their good environmental stability in both the neutral and doped states. The large amount of studiesdedicatedto thesecompoundshasopened the way to major developmentsin the fabrication of devices [ 11. Moreover, the ol,cy’-dimethyl substitution has recently proved very effective in enhancing chemical stability and crystallinity without damagingthe electronic responseof the resulting material [ 21. In this communication, the geometricandvibrationalproperties of a seriesof a,a’-dimethyl end-cappedoligothiophenesaretheoretically investigated in both neutral andoxidized states;the resultsare then comparedto the experimentaldata.
2. Methodology The methyl end-cappedoligothiophenes investigated in this work contain from 2 to 6 thiopheneunits (from the dimer to the hexamer). The geometry optimizations were carried out using wave functions of the Hartree-Fock type: restricted (RI-IF) for closed-shell systems (the neutral and the dicationic systems), spin unrestricted (UHF) and spin restricted 0379-6779/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved PUSO379-6779(97)03916-7
(ROHF) for open-shellsystems(the radical cation systems). The basisset usedin all calculationswas the 6-31G”” which is split valence and includes a setof d-polarization functions for heavy atomsand p-polarization functions for the hydrogens. We have initially limited our theoretical investigation to the all-anti planar conformations.
3. Results and discussion 3.1. Geometric stmctwes The 6-3 lG*” optimized geometricstructuresof the neutral oligomers show that all the inner thiophene rings present almost the samegeometry, which slightly differs from that found in the outer units due to the methyl-end substitution. Ab initio calculations are indicative of a degree of bondlength alternation of about 0.07 A in good correspondence with the experimental data. The 6-31G*” optimized geometry of the dicationic c~,ol’dimethyl end-cappedoligothiophenesindicatesthe formation of a positive bipolaron defect localized in the middle of the molecule and extending over the adjacent repeat units. The charged speciesare characterized by a reversal of the singledouble C-C bond pattern, the geometry relaxation process thus induces the appearanceof a strong quinoid character within the molecule. In the caseof the radical cations, there occurs the appearance of a positive polaron defect localized in the middle of the molecule.ROHF and UHF geometries(seeFig. 1 for the hexamer) of the radical cations exhibit weaker, structural
160
J. Crtsctdoet N/./Synthetic Metok 89 (1997) 159-160
1525
z
::
I487
1038
DMQtT
b
d Fig. 1. 6-31G”” optimized C-C bond lengths (in A) of the a,a’-dimethylsexithiophene in the neutral state (filled squares) and in the radical cationic state (open circles). ROHF method has been used for the open-shell system.
::
1700
1529
1500
1300
1035
DM-TT
1035
DMBT
1100
900
R-n
modifications than in the dications.We estimatethe polarons to extend over about two-three rings. As expected, the UHF and ROHP methodsgive comparablegeometriesfor all the radical cations calculated in this work. However, the ROHF formalism better reproducesthe localization of the charge carriers along the chain ascomparedto the UHF approach. 3.2. Vibrntionnl spectra ofnem-ol molecules We have used the ab initio 6-31G*” level of calculation to determine the infrared and Raman spectra of the neutral oligothiophenes.For comparisonpurposeswe have scaledall the calculated frequencies by a uniform factor of 0.9. All quoted scaledtheoretical resultsare thus the scaledvalues. We restrict our discussionto the most important Raman spectroscopic trends occurring upon chain elongation. In Fig. 2, the computed Ramanspectra of DMBT, DMTT and DMQtT are displayed. The spectra show very few bands despite the complex chemical structures,which exclusively originate from totally symmetric modes. As in the experimental spectra, for DMBT and DMTT the most important feature is the appearanceof only one intense band around 1530cm- ‘, this shifts downward by 10cm- I from the dimer to the tetramer (from 1492cm-’ to 1482 cm- ’ in the experimental spectra [ 31). For longer oligomers a second line around 1470cm- ’ becomesstrongerwith the molecularsize. As in the experimental Raman spectra,thesetwo lines show a convergent trend. The theoretical Raman spectra of the neutral moleculesshowthe four characteristic featurescommon to many other classesof oligothiophenes: 1615 1591 cm-’ (line A) weak, which showsdispersiontoward lower frequencieswhen increasingchain length; 1536-1525 cm-’ (line B), it is a common feature of the Raman spectra of aromatic andheteroaromaticsystems;1476 1487cm- ’ (line C) which is characteristicofa or o substitutedoligothiophenes and showsintensity enhancementwith increasingchain length; and 1035 cm-’ (line D), observed in the Raman spectrumfor all the ofgothiophenes. 3.3. Vibmtional spectra of oxidized
molecules
In Fig. 3 the computedinfrared spectraof the radical cation (ROHF/6-3 1G**) molecuIesare displayed. Comparingthe theoretical infrared spectrumof the neutral oligomersto that
shift
700
500
300
00
(cm”)
Fig. 2. Theoretical Raman spectra of neutral cc,a’-dirnethyl as calculated at the RHF/6-3 lG** level.
oligothiophenes
Dhfl-l-
=” ; 2! 9
.
DM
i
I
1700
lSO0
1300
1100 W*v+numbcr
900
700
500
B-i-
400
(cm”)
Fig. 3. Theoretical infrared spectra of a,o’-dimethyl cations as calculated at the ROHF/6-31G** level.
oligothiophene
radical
of its corresponding radical cations calculated in the same basisset, one immediately recognizes new intensive bands calculated at 1450 (m), 1404 (m), 1376-1290 (s), 1274 (vs), 1115 (s), 1039 (vs), 967 (m), 865 (m) cm-‘. The experimental infrared spectraof the I2 doped materialsshow [ 41 at leastsix new experimentalinfrared bandsat 1414(m), 1327 (s), 1162 (s), 1099 (s), 993 (s) and 946 (m) cm-‘, whoseintensitiesincreasewith increasingdopinglevel. Comparison between calculated and experimental data indicates the existence of a polaron-type defect in the iodine doped materials, as has been confirmed by the electronic spectra [51. Acknowledgements This researchwasfinancially supportedby Direccidn General de Investigacidn Cientifica y Tecnica (DGICYT, Spain) through the ResearchProject PB93- 1244.
References [l] J. Roncali, Chern. Rw., 92 (1992) 71 1. [2] S. Hotta and K. Waragai, J. Mater. Chern., I (1991) 535. [3] V. Hernandez, J. Casado, F.J. Ramirez. G. Zotti, S. Hottaand J.T. Lopez Navarrete, J. Chew. Bras., IO4 (. 1994) 9271. [4] J. Casado, F.J. Ramirez. S. Hotta, J.T. Lopez Navarrete and V. Hernandez, S~nth. Met.,54 ( 1997) 57 1. 151 S. Hotta and K. Waragai, J. Phys. Chem., 97 ( 1993) 7427.