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Delocalization length, electronic properties and vibrational spectra of neutral α,α′ -dimethyl end-capped oligothiophenes
ELSEVIER
SyntheticMetals76 (1996) 277-280
Delocalization length, electronic properties and vibrational spectra of neutral a,a’-dimethyl end-capped oligothiophenes V. Hern6ndez a,J. Casado a,F.J. Ramirez a, G. Zotti b, Shu Hotta ‘, J.T. Ldpez Navarrete a7* b Istituto ’ Matsushita
a Departamento de @tt’tnica Fisica, Universidad de Mdlaga, 29071 Mdlaga, Spain di Polarografia ed Electrochirnica Preparativa, Consiglio Nazionale delle Ricerche, torso Stati Uniti 4, 35020 Padua, Italy Research Institute Tokyo, Inc., Advanced Materials Research Laboratovs 3-10-I Higashimita, Tama-ku, Kawasaki 214, Japan
Abstract We present FT-Raman and FT-IR spectra of neutral &,a’-dimethyl end-capped oligothiophenes (to six thiophene units) as solid samples and in solution. Gas-phase potential to internal rotation of a,cL’-dimethylbithiophene has been predicted by ab initio calculations at the RI-IF/ 6-3 lG** level, as a model for the theoretical study of the conformational flexibility of these molecules. Keywords:
Delocalizationlength;Electronicproperties;Vibrationalspectra;Oligothiophenes
1. Introduction Although conducting polymers with a defined composition can now be reproducibly prepared, long-chain polythiophenes have the complexity of the ‘real’ polymer such as solubility, structural defects and a broad distribution of the chain length [ 11. Semiconducting oligomers are currently being investigated as charge storage materials since their conjugation lengths can be exactly controlled. Nevertheless, nonsubstituted oligothiophenes suffer from a relatively large chemical instability because of the high activity of both terminal a-carbons. Alkyl substitution of these two positions has been proved as very effective in enhancing chemical stability while remaining highly electroactive [ 2,3], In the present paper we have investigated the vibrational FT-Raman and FT-IR spectra of neutral a,a’-dimethyloligothiophenes in both solid state and solution. The data are consistent with the existence of a chain-length-dependent r electron delocalization, and significant frequency dispersion with conjugation length was observed for some Raman or IR active vibrational modes. We also present here an ab initio theoretical analysis of the methyl-induced effects on the conformational properties of a,a’-dimethylbithiophene. 2. Experimental
and computational
methods
All calculations were performed using the GAUSSIAN-92 program [ 41 on a Convex 240 at the CICA Computer Center * Correspondingauthor.Tel.: + 34 52 132018;fax: + 34 52 132000;email: [email protected]. 0379-6779/96/$15.000 1996Elsevier ScienceS.A. All rights reserved
n = 0 @MBT), 1 @M’IT), 2 @MQtT), 3 (DMQqT), 4 @MSxT) Fig. 1. Chemicalstructureof theoligothiophenesstudiedin this work.
of Sevilla, Spain. Ground state geometries were optimized at the restricted Hartree-Fock level of theory. A 6-31G”” standard basis set was used [ 51. Calculations on a,cr’-dimethylbithiophene (DMBT) were performed assuming that the molecule belongs to the C,,l and C,, symmetry point groups in the anti and syn conformations, respectively. A C2 symmetry was imposed for twisted conformers. Gas-phase internal rotation barrier calculation was performed by fixing the torsional dihedral angle, 8, at the following selected values: 0 (syn), 45,90, 145, 180” (anti). Synthesis and purification of the ol,cr’-dimethyloligothiophenes as far as the hexamer (see Fig. 1 for chemical structures) have been described elsewhere [2,3]. IR spectra at room temperature between 4000 and 400 cm- ’ were recorded on a Perkin-Elmer 1760 X FT-IR spectrometer purged with dry Ar gas. The solid samples were registered in a KBr pellet. Solvents (carbon disulfide, carbon tetrachloride and 1,2dichloromethane) of analytical grade were supplied by Aldrich. Saturated solutions were prepared just before recording the spectra to avoid solvent evaporation. Raman spectra at room temperature between 1700 and 200 cm-’ were
V. Herna’ndez
218
et al. /Synthetic
Metals
76 (1996) 277-280
3
13.9-.
I
1sob.o
I
1500
I
1300
1400
I
I
I
I
I
I
1200
1100
1000
400
BOO
700
Wavenumber
I
I
600
wo
i
4130
.o
(cm*])
Fig. 2. FT-IR spectrum of DMBT between 1600 and 400 cm-‘.
recorded on a Bruker RFS 100 FT-Raman spectrometer. A 1064 nm exciting line, supplied by a Nd-YAG laser, was adjusted at 100 mW power.
3. Results and discussion 3.1. IR spectra
The IT-IR spectrum of solid DMBT can be observed in Fig. 2. Fig. 3 displays a comparison between the solid state and solution IR spectra of a,cu’-dimethylterthiophene (DMTT).
0
II 1450
11
1300
L
a
Wavenumber
I 1150
I1
1 loal
I
I
‘I 850
L
The appearance of the IR spectra of the solid samples is typical of very pure materials. Some bands show a large frequency dispersion with chain length. Particularly, an aromatic C=C antisymmetric stretching vibration around 1520 cm-’ shifts downwards by 28 cm-’ on going from the dimer to the hexamer. No IR absorption for the out-of-plane bending vibration of a C-H in the 01position has been observed near 690 cm-‘, thus indicating negligible misslinkings between adjacent thiophene rings. Concerning the solutions, their spectra also show chain length frequency dependence. In addition, some significant bands shift to higher frequencies with respect to those observed for the solid samples. Some other bands, that appear split for the solid samples due to a crystal effect (factor group splitting), are observed as single bands in solution (see 800 cm-’ region in Fig. 3). We emphasize that the aromatic C=C antisymmetric stretching vibration, measured around 1520 cm- ’ for the solids, appears as multiplets in solution, as can be observed in Fig. 4. While the IR spectra of solid samples agree with planar structures for all the oligomers studied here [ 6-81, the spectral changes observed for the solutions arise probably from at least one of the following concurrent effects: distortions from planarity and solvent-induced frequency shifts. 3.2. Ramanspectra
’
(cm- ‘)
Fig. 3. Comparison between the solid state (lower) and solution (upper) IR spectraof DMn. Solvents are dichloromethane (1600-1400 cm-‘) and carbon disulfide (1300-1000 and 900-700 cm-‘).
The Raman spectra of the oligothiophenes exhibit in common very few bands in spite of their complex structures. The existence of a strong electron-phonon coupling [ 91 and the
V. Herndndez
et al. /Synthetic
Metals
76 (1996)
277-280
219
cm-’ in DMSxT, and shows weak intensity dispersion. We think that this line arise from a totally symmetric C=C stretching largely involved in IT conjugation. The two bands around 1490 and 1450 cm-’ show frequency convergence with increasing chain length. In addition, the highest frequency band transfers a significant part of its intensity to the lowest one as the chain length is longer. The spectral patterns registered for the solutions are almost unchanged compared to those for the solid state. The main difference is a slight upshift for the band around 1490 cm-’ with respect to the corresponding solid state frequency. The correlative analysis of the IR and Raman spectra is consistent with a C,, symmetry for systems having an even number of thiophene units and a C,, symmetry when this number is odd. From the spectra of the solutions, no new selection rules can be deduced. The small spectral differences for the solutions can be rationalized since the Raman scattering in this class of materials is overwhelmingly determined by the fraction of chains whose energy gap is the smallest. Fig. 4. FT-IR spectra in the 1600-1450 cm-’ region of (a) DMTT, (b) DMQtT, (c) DMQqT and (d) DMSxT, in carbon disulfide solution.
near-resonance Raman conditions [ lo] explain this spectral simplicity. As an example, Fig. 5 displays the Raman spectrum of solid DMBT. For the solids, bands around 1545, 1490 and 1450 cm-’ are particularly sensitive to chain length. The line at 1560 cm-’ in DMBT ( 1546 cm- ’ in DMTT) shifts up to 1517
ii! 63500
I
I
I
I
I
3250
3000
2750
2500
2250
I
3.3. Theoretical results
Fig. 6 compares the theoretical gas-phase potentials to rotation of unsubstituted 2,2’-bithiohene [ 111 and of DMBT, evaluated using a standard ab initio RHF/6-31G”* method. Both potentials are similar and indicate a flat internal barrier to rotation (largest relative differences in energy are less than 1.9 kcal/mol). The main information is that a coplanar arrangement of the rings should be easily accessible in the solid state, although anti and syn twisted conformers could
I
2000 17513 Udvenumber cm-'
I
IWJ
I
I
I
I
I
1250
1000
750
500
2%
Fig. 5. FT-Raman spectrum of DMBT between 3500 and 100 cm-‘.
V. Herndndez et al. /SyntheticMetals 76 (1996) 277-280
280
indebted to Junta de Andalucia (Spain) for funding our research group (No. 6072). References
0
30
150
180
Toysional Angle (de;& Fig. 6. Torsional potential vs. inter-ring torsional angle of DMBT (0) and 2,2’-bithiophene (0). Results from ab initio full geometry optimizations at the RHF/6-31G** level of calculation.
also coexist in solution, when intermolecular switched off.
forces are
Acknowledgements The present studies are financially supported by Direcci6n General de Investigaci6n Cientifica y TCcnica (DGICYT, Spain) through the Research Project PB93-1244. We are also
[l] Proc. ICSM ‘90, Synth. Met., 4143 (1991); Proc. ICSM ‘92, Synth. Met., 55-57 (1993); Proc. ICSM ‘94, Synth. Met., 69-71 (199.5). [2] S. Hotta and K. Waragai, J. Mater. C/lent,, 1 (1991) 835. [3] S. Hotta and K. Waragai, J. Phys. Chea., 97 (1993) 7427. [4] M.J. Frisch, G.W. Trucks, M. Head-Gordon, P.M.W. Gill, M.W. Wong, B. Foresman, B.G. Johnson, H.B. Schlegel, M.A. Robb, ES. Replogle, R. Gomperts, J.L. Andres, K. Raghavachari, J.S. Binkley, C. Gonzalez, R.L. Martin, D.J. Fox, D,J. Defrees, J. Baker, J.J.P. Stewart and J.A. Pople, GAUSSIAN 92, Revision C.4, Gaussian, Inc., Pittsburgh, PA, 1992. [5] M.M.Francl, W.J. Pietro, W.J. Hehre, J.S.Binkley, MS. Gordon, D.J. Defrees and J.A. Pople, J. Chern. Phys., 77 (1982) 3654. [6] S. Hotta and H. Kobayashi, Synri~.Met., 66 (1994) 117. [7] V. Hemandez, J. Casado, F.J. Ramirez, G. Zotti, S. Hotta and J.T. Lopez Navarrete, J. Chetn. Phys., submitted for publication. [8] V. Hemandez, J. Casado, F.J. Ramirez, L.J. Alemany, S. Hotta and J.T. Lopez Navarrete, J. Phys. Chem., in press. [9] C. Castiglioni, J.T. L6pez Navarrete, G. Zerbi and M. Gussoni, Solid State Commun., 65 (1988) 625. [lo] CA. Albrecht, J. Chern. Phys., 34 (1961) 1476. [ 111 V. Hemandez and J.T. Lopez Navarrete, J. Ure~n. Phys., 101 (1994) 1369.
SyntheticMetals76 (1996) 277-280
Delocalization length, electronic properties and vibrational spectra of neutral a,a’-dimethyl end-capped oligothiophenes V. Hern6ndez a,J. Casado a,F.J. Ramirez a, G. Zotti b, Shu Hotta ‘, J.T. Ldpez Navarrete a7* b Istituto ’ Matsushita
a Departamento de @tt’tnica Fisica, Universidad de Mdlaga, 29071 Mdlaga, Spain di Polarografia ed Electrochirnica Preparativa, Consiglio Nazionale delle Ricerche, torso Stati Uniti 4, 35020 Padua, Italy Research Institute Tokyo, Inc., Advanced Materials Research Laboratovs 3-10-I Higashimita, Tama-ku, Kawasaki 214, Japan
Abstract We present FT-Raman and FT-IR spectra of neutral &,a’-dimethyl end-capped oligothiophenes (to six thiophene units) as solid samples and in solution. Gas-phase potential to internal rotation of a,cL’-dimethylbithiophene has been predicted by ab initio calculations at the RI-IF/ 6-3 lG** level, as a model for the theoretical study of the conformational flexibility of these molecules. Keywords:
Delocalizationlength;Electronicproperties;Vibrationalspectra;Oligothiophenes
1. Introduction Although conducting polymers with a defined composition can now be reproducibly prepared, long-chain polythiophenes have the complexity of the ‘real’ polymer such as solubility, structural defects and a broad distribution of the chain length [ 11. Semiconducting oligomers are currently being investigated as charge storage materials since their conjugation lengths can be exactly controlled. Nevertheless, nonsubstituted oligothiophenes suffer from a relatively large chemical instability because of the high activity of both terminal a-carbons. Alkyl substitution of these two positions has been proved as very effective in enhancing chemical stability while remaining highly electroactive [ 2,3], In the present paper we have investigated the vibrational FT-Raman and FT-IR spectra of neutral a,a’-dimethyloligothiophenes in both solid state and solution. The data are consistent with the existence of a chain-length-dependent r electron delocalization, and significant frequency dispersion with conjugation length was observed for some Raman or IR active vibrational modes. We also present here an ab initio theoretical analysis of the methyl-induced effects on the conformational properties of a,a’-dimethylbithiophene. 2. Experimental
and computational
methods
All calculations were performed using the GAUSSIAN-92 program [ 41 on a Convex 240 at the CICA Computer Center * Correspondingauthor.Tel.: + 34 52 132018;fax: + 34 52 132000;email: [email protected]. 0379-6779/96/$15.000 1996Elsevier ScienceS.A. All rights reserved
n = 0 @MBT), 1 @M’IT), 2 @MQtT), 3 (DMQqT), 4 @MSxT) Fig. 1. Chemicalstructureof theoligothiophenesstudiedin this work.
of Sevilla, Spain. Ground state geometries were optimized at the restricted Hartree-Fock level of theory. A 6-31G”” standard basis set was used [ 51. Calculations on a,cr’-dimethylbithiophene (DMBT) were performed assuming that the molecule belongs to the C,,l and C,, symmetry point groups in the anti and syn conformations, respectively. A C2 symmetry was imposed for twisted conformers. Gas-phase internal rotation barrier calculation was performed by fixing the torsional dihedral angle, 8, at the following selected values: 0 (syn), 45,90, 145, 180” (anti). Synthesis and purification of the ol,cr’-dimethyloligothiophenes as far as the hexamer (see Fig. 1 for chemical structures) have been described elsewhere [2,3]. IR spectra at room temperature between 4000 and 400 cm- ’ were recorded on a Perkin-Elmer 1760 X FT-IR spectrometer purged with dry Ar gas. The solid samples were registered in a KBr pellet. Solvents (carbon disulfide, carbon tetrachloride and 1,2dichloromethane) of analytical grade were supplied by Aldrich. Saturated solutions were prepared just before recording the spectra to avoid solvent evaporation. Raman spectra at room temperature between 1700 and 200 cm-’ were
V. Herna’ndez
218
et al. /Synthetic
Metals
76 (1996) 277-280
3
13.9-.
I
1sob.o
I
1500
I
1300
1400
I
I
I
I
I
I
1200
1100
1000
400
BOO
700
Wavenumber
I
I
600
wo
i
4130
.o
(cm*])
Fig. 2. FT-IR spectrum of DMBT between 1600 and 400 cm-‘.
recorded on a Bruker RFS 100 FT-Raman spectrometer. A 1064 nm exciting line, supplied by a Nd-YAG laser, was adjusted at 100 mW power.
3. Results and discussion 3.1. IR spectra
The IT-IR spectrum of solid DMBT can be observed in Fig. 2. Fig. 3 displays a comparison between the solid state and solution IR spectra of a,cu’-dimethylterthiophene (DMTT).
0
II 1450
11
1300
L
a
Wavenumber
I 1150
I1
1 loal
I
I
‘I 850
L
The appearance of the IR spectra of the solid samples is typical of very pure materials. Some bands show a large frequency dispersion with chain length. Particularly, an aromatic C=C antisymmetric stretching vibration around 1520 cm-’ shifts downwards by 28 cm-’ on going from the dimer to the hexamer. No IR absorption for the out-of-plane bending vibration of a C-H in the 01position has been observed near 690 cm-‘, thus indicating negligible misslinkings between adjacent thiophene rings. Concerning the solutions, their spectra also show chain length frequency dependence. In addition, some significant bands shift to higher frequencies with respect to those observed for the solid samples. Some other bands, that appear split for the solid samples due to a crystal effect (factor group splitting), are observed as single bands in solution (see 800 cm-’ region in Fig. 3). We emphasize that the aromatic C=C antisymmetric stretching vibration, measured around 1520 cm- ’ for the solids, appears as multiplets in solution, as can be observed in Fig. 4. While the IR spectra of solid samples agree with planar structures for all the oligomers studied here [ 6-81, the spectral changes observed for the solutions arise probably from at least one of the following concurrent effects: distortions from planarity and solvent-induced frequency shifts. 3.2. Ramanspectra
’
(cm- ‘)
Fig. 3. Comparison between the solid state (lower) and solution (upper) IR spectraof DMn. Solvents are dichloromethane (1600-1400 cm-‘) and carbon disulfide (1300-1000 and 900-700 cm-‘).
The Raman spectra of the oligothiophenes exhibit in common very few bands in spite of their complex structures. The existence of a strong electron-phonon coupling [ 91 and the
V. Herndndez
et al. /Synthetic
Metals
76 (1996)
277-280
219
cm-’ in DMSxT, and shows weak intensity dispersion. We think that this line arise from a totally symmetric C=C stretching largely involved in IT conjugation. The two bands around 1490 and 1450 cm-’ show frequency convergence with increasing chain length. In addition, the highest frequency band transfers a significant part of its intensity to the lowest one as the chain length is longer. The spectral patterns registered for the solutions are almost unchanged compared to those for the solid state. The main difference is a slight upshift for the band around 1490 cm-’ with respect to the corresponding solid state frequency. The correlative analysis of the IR and Raman spectra is consistent with a C,, symmetry for systems having an even number of thiophene units and a C,, symmetry when this number is odd. From the spectra of the solutions, no new selection rules can be deduced. The small spectral differences for the solutions can be rationalized since the Raman scattering in this class of materials is overwhelmingly determined by the fraction of chains whose energy gap is the smallest. Fig. 4. FT-IR spectra in the 1600-1450 cm-’ region of (a) DMTT, (b) DMQtT, (c) DMQqT and (d) DMSxT, in carbon disulfide solution.
near-resonance Raman conditions [ lo] explain this spectral simplicity. As an example, Fig. 5 displays the Raman spectrum of solid DMBT. For the solids, bands around 1545, 1490 and 1450 cm-’ are particularly sensitive to chain length. The line at 1560 cm-’ in DMBT ( 1546 cm- ’ in DMTT) shifts up to 1517
ii! 63500
I
I
I
I
I
3250
3000
2750
2500
2250
I
3.3. Theoretical results
Fig. 6 compares the theoretical gas-phase potentials to rotation of unsubstituted 2,2’-bithiohene [ 111 and of DMBT, evaluated using a standard ab initio RHF/6-31G”* method. Both potentials are similar and indicate a flat internal barrier to rotation (largest relative differences in energy are less than 1.9 kcal/mol). The main information is that a coplanar arrangement of the rings should be easily accessible in the solid state, although anti and syn twisted conformers could
I
2000 17513 Udvenumber cm-'
I
IWJ
I
I
I
I
I
1250
1000
750
500
2%
Fig. 5. FT-Raman spectrum of DMBT between 3500 and 100 cm-‘.
V. Herndndez et al. /SyntheticMetals 76 (1996) 277-280
280
indebted to Junta de Andalucia (Spain) for funding our research group (No. 6072). References
0
30
150
180
Toysional Angle (de;& Fig. 6. Torsional potential vs. inter-ring torsional angle of DMBT (0) and 2,2’-bithiophene (0). Results from ab initio full geometry optimizations at the RHF/6-31G** level of calculation.
also coexist in solution, when intermolecular switched off.
forces are
Acknowledgements The present studies are financially supported by Direcci6n General de Investigaci6n Cientifica y TCcnica (DGICYT, Spain) through the Research Project PB93-1244. We are also
[l] Proc. ICSM ‘90, Synth. Met., 4143 (1991); Proc. ICSM ‘92, Synth. Met., 55-57 (1993); Proc. ICSM ‘94, Synth. Met., 69-71 (199.5). [2] S. Hotta and K. Waragai, J. Mater. C/lent,, 1 (1991) 835. [3] S. Hotta and K. Waragai, J. Phys. Chea., 97 (1993) 7427. [4] M.J. Frisch, G.W. Trucks, M. Head-Gordon, P.M.W. Gill, M.W. Wong, B. Foresman, B.G. Johnson, H.B. Schlegel, M.A. Robb, ES. Replogle, R. Gomperts, J.L. Andres, K. Raghavachari, J.S. Binkley, C. Gonzalez, R.L. Martin, D.J. Fox, D,J. Defrees, J. Baker, J.J.P. Stewart and J.A. Pople, GAUSSIAN 92, Revision C.4, Gaussian, Inc., Pittsburgh, PA, 1992. [5] M.M.Francl, W.J. Pietro, W.J. Hehre, J.S.Binkley, MS. Gordon, D.J. Defrees and J.A. Pople, J. Chern. Phys., 77 (1982) 3654. [6] S. Hotta and H. Kobayashi, Synri~.Met., 66 (1994) 117. [7] V. Hemandez, J. Casado, F.J. Ramirez, G. Zotti, S. Hotta and J.T. Lopez Navarrete, J. Chetn. Phys., submitted for publication. [8] V. Hemandez, J. Casado, F.J. Ramirez, L.J. Alemany, S. Hotta and J.T. Lopez Navarrete, J. Phys. Chem., in press. [9] C. Castiglioni, J.T. L6pez Navarrete, G. Zerbi and M. Gussoni, Solid State Commun., 65 (1988) 625. [lo] CA. Albrecht, J. Chern. Phys., 34 (1961) 1476. [ 111 V. Hemandez and J.T. Lopez Navarrete, J. Ure~n. Phys., 101 (1994) 1369.