Temperature-dependent infrared spectroscopic and crystallographic study of α,α[sup ʹ]-dimethyl...

JOURNAL OF CHEMICAL PHYSICS VOLUME 109, NUMBER 5 1 AUGUST 1998

Temperature-dependent infrared spectroscopic and crystallographic studyof a,a8-dimethyl oligothiophenes in neutral form

F. J. Ramıreza)

Departamento de Quı´mica Fısica, Facultad de Ciencias, Universidad de Ma´laga, Campus de Teatinos,29071-Malaga, Spain

M. A. G. ArandaDepartamento de Quı´mica Inorganica, Cristalografı´a y Mineralogı´a, Facultad de Ciencias,Universidad de Ma´laga, Campus de Teatinos, 29071-Ma´laga, Spain

V. Hernandez and J. CasadoDepartamento de Quı´mica Fısica, Facultad de Ciencias, Universidad de Ma´laga, Campus de Teatinos,29071-Malaga, Spain

S. HottaMatsushita Research Institute Tokyo, Inc. Advanced Materials Research Laboratory, 3-10-1 Higashimita,Tama-ku, Kawasaki 214-8501, Japan

J. T. Lopez NavarreteDepartamento de Quı´mica Fısica, Facultad de Ciencias, Universidad de Ma´laga, Campus de Teatinos,29071-Malaga, Spain

~Received 16 May 1997; accepted 28 April 1998!

We have studied structural and spectroscopic characteristics on a series ofa,a8-dimethylend-capped oligothiophenes~from two to six thiophene units! by varying temperature. Powder x-raydiffraction and infrared spectra at different temperatures between2170 and 150 °C were recorded.The diffractometric study shows that the thermal expansion coefficients of these materials are highlyanisotropic. The expansion in theab plane, which is perpendicular to the chain direction, is an orderof magnitude bigger than that along the chain direction. Corresponding to these structural features,the infrared spectra show interesting temperature dependence. For instance, the separation betweenadjacent components of the multiplets near 800 cm21, that are assigned to the C–H out-of-planebending vibrations, increases with decreasing temperatures. This behavior reflects the change in theintermolecular forces caused by the thermal expansion. Furthermore, the shifts in the C5Cstretching vibrations indicate that the extent of thep-electron delocalization increases withincreasing temperatures. The origin for this is briefly discussed. ©1998 American Institute ofPhysics.@S0021-9606~98!51929-0#

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INTRODUCTION

Actual research on the basic features of polyconjugaconducting polymers1 is to a large extent devoted to thstudy of selected oligomers in order to understand, withhelp of molecular models, several unrevealed questionsfrom this intriguing class of materials.2 Oligothiophenes,short conjugated chains in which thiophene units are linby their a positions, have recently been the focus of muattention. They are good models for studying charge storin conducting polymers.3–5 Their use as semiconductorselectronic devices has been proposed, recent results aapplications of these oligothiophenes as diodes6 and field-effect transistors7–10 making these compounds more attrative. Moreover, thiophene oligomers, mainly due to thpossible application as materials for nonlinear optics wparticular environmental stability,11 have been extensivelinvestigated by different experimental and theoretimethods.12

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Among the chemically modified oligothiophenes, whave found that those substituted with methyl groups ona carbons of both terminal thiophene rings show particulainteresting features.4,13–15 Nonsubstituted oligothiophenesuffer from chemical instability because of high chemicactivity of the terminal positions. The blocking on those wialkyl groups has proved very effective for enhancing checal stability and preventing further dimerization and pomerization processes while the resulting compounds reming highly electroactive.

Although polythiophenes and oligothiophenes arelated materials, with electronic and spectroscopic characistics that are also related because of the presence ofp-electron system extended along the backbone, their mphological features are considerably different. In the preously reported article by Ba¨ssler et al.,16 for example, theauthors concluded on the basis of the absorption spectrafor conducting polymers such as poly~phenylenevinylenes!and polythiophenes spatial extent of chromophores islonger than about ten monomer units. This means thatconducting polymers are severely entangled systems.

0 © 1998 American Institute of Physics

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1921J. Chem. Phys., Vol. 109, No. 5, 1 August 1998 Ramırez et al.

blue shift in the spectra upon heating the conducting pomers, which was exemplified by, e.g., Binhet al.17 can beinterpreted by augmented entanglement~coil structure! in thepolymer backbone chains with increasing temperatuHotta and Waragai,13 on the other hand, showed from x-radiffraction analysis that solid materials of the oligotiophenes are highly ordered systems, contrasted withconducting polymers.

Corresponding to these features different in morpholothe temperature-dependent spectroscopic features of ttwo systems of polymers and oligomers may also differ.particular, the study of the temperature-dependent vibratiospectra of polyconjugated molecules can provide us withdence for useful information such as conjugation lengcrystal packing, or intermolecular forces. During the layears our research group has paid attention to polycogated polymers with conductor properties: polythiophepolyfuran, poly~3-methylthiophene!, etc.18–21 More recently,we have focused our interest toward chain-length defimolecules~oligomers!, and these have been confirmedgood compounds.22–27

Prior to this work we have reported a series of papdetailing the vibrational and structural properties of neua,a8-dimethyl-oligothiophenes in solid state andsolution.28–30 In these works we have largely studied tconjugation length of the materials, and what changes apwhen they are dissolved in different solvents, by analyzthe FT-IR and FT-Raman spectra at room temperature. Hwe extend these efforts to the influence of temperaturesuch a properties. To do that, we have recorded the FTspectra of a series ofa,a8-dimethyl end-capped oligothiophenes~from two to six thiophene units, as is shownFig. 1! at different temperatures, ranging from2170 °C to150 °C. The spectra were analyzed on the basis of avibrational assignment previously proposed.28–30Informationabout crystal packing and intermolecular forces was derifrom the observed band splittings. In addition, some expmental frequencies, corresponding to characteristic vibtions of the oligomers, showed significant shifts, which weused to determine the influence of temperature on the cogation length of these molecules.

In parallel with the vibrational study, we have also rcorded crystallographic data at different temperatures forgomers having four, five, and six thiophene units~for whichthe conjugational effects are quite significant!. There are sev-eral polymorphic forms for dimethyl oligothiophenes. Thprincipal phase is monoclinic and it crystallizes in the C2space group.31 DMQtT, DMQqT, and DMST~see Fig. 1 forcompound names and their abbreviations as well aschemical structures! crystallize with approximately the sama, b, andb parameters, 5.98 Å, 7.89 Å, and 98.0°, resptively. The c parameters depending upon the chain lenwere 18.66, 22.34, and 25.96 Å for DMQtT, DMQqT, anDMST, respectively. Other structures have been repoorthorhombic ~e.g., Pbca, a57.71 Å, b55.94 Å,c536.03 Å! for DMQtT. This crystal structure has beefully determined from single crystal data.13


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EXPERIMENT

Details for synthesis and purification of the systemspicted in Fig. 1 were reported elsewhere.13 Combined ther-mogravimetric~TGA! and differential~DTA! thermal analy-ses were carried out in air on a Rigaku Thermoflex TG 81apparatus from room temperature at a heating rate of 10min with calcined Al2O3 as the reference. The powder paterns of the x-ray diffraction characterizations were collecin a Siemens D5000 diffractometer with (Q/2Q) Bragg–Brentano geometry by using graphite-monochromaCu Ka radiation. The diffractometer has two goniometeone optimized to collect high resolution patterns andother fully dedicated to record variable-temperature powpatterns. The high-temperature system is equipped witHTK 10 chamber to control the temperature between rotemperature and 1300 °C. A Pt foil was the sample holand the heating system. The patterns were collected~at thedesired temperature! between 2.5° and 32°~2Q!, in 0.04°steps, counting 1 s per step.

Alkyl-substituted oligothiophenes powder patterns shvery high preferential orientation. Hence, we have adapte

FIG. 1. Chemical structures and compound names together with theirbreviations of thea,a8-dimethyl oligothiophenes~from the dimer to thehexamer! studied in this work.


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1922 J. Chem. Phys., Vol. 109, No. 5, 1 August 1998 Ramırez et al.

FIG. 2. Room temperature x-ray powder diffraction patern for DMQtT as synthesized. The more intense nooverlapping peaks are indexed.

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standard method32,33 to reduce it as much as possible in tdiffractometric study. The sample was diluted with spherinanoparticles which can prevent the sample from becomvery oriented. We have used amorphous Cab-O-Sil M~from Fluka! that are spherical particles of silica with aaverage size of 30 nm. Approximately 2/1 Cab-O-Silsample volume ratio was employed. Cab-O-Sil is completamorphous and, hence, does not show any diffraction pe

Infrared spectra were recorded on a Perkin Elmer 176Fourier transform infrared spectrometer purged with drygon gas. A suitable variable temperature cell Specac21000 working on a Dewar principle and at a vacuum prsure of 0.5 Torr was used. It was equipped with a CoppConstanton thermocouple for temperature monitoring pposes, and a range from2190 °C to1250 °C ~83–523 K!can be regulated. Samples were always studied in potasbromide pellets. To increase the signal-to-noise ratio a mmum of 20 scans, with 1 cm21 spectral resolution each, weraccumulated in all the cases. The spectra were recordedwaiting for thermal equilibrium at the sample, which rquired 20 mins for every increment of 10 K.

RESULTS AND DISCUSSION

Powder x-ray diffraction

The powder pattern of a single phase of DMQtTshown in Fig. 2. The pattern has few nonoverlapped pewith Miller indices different from~001! which prevents theauto-indexing process. However, the patterns can be indein the monoclinic unit cells obtained from electron diffration data.31 As it can be seen in Fig. 2, the more intenpeaks are indexed as~001! indicating a high degree of preferential orientation. The room temperature unit cell valufor the three oligothiophenes are given in Table I.

In order to analyze the thermal expansion of these mterials it is necessary to minimize the preferential orientatto follow the thermal evolution of thea andb parameters. Asdescribed in the experimental section, the diffractomestudy was carried out for samples mixed with Cab-O-SThe variable-temperature powder patterns for DMQtT, DQqT, and DMST are shown in Fig. 3. The peaks have bindexed at the different temperatures, enabling us to de


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mine the unit cell values as reported in Table I. The zeshift errors due to the sample’s height on the holder hbeen taken into account in the indexing process oftemperature-variable patterns.

The unit cell values of the dimethyl oligothiophenes avery similar to those already reported in Ref. 31, althouthe a parameters seem to be slightly shorter. As previoushown,31 a, b, and b parameters are very similar for ththree materials. Only strong variations in thec parameter~along the chain direction! are observed in the three oligothiophenes. The relative intensity of the~110! and~020! peaksis higher when the materials are disoriented. This can be sby comparing Figs. 2 and 3~a!. The oriented pattern, Fig. 2for DMQtT has low relative intensity between~110! and~004! peaks, and~020! and~005! peaks. The disoriented paterns@Fig. 3~a!# have a higher-intensity ratio between~110!/~004! and~020!/~005!. This is essential if the thermal evolution of the unit cell has to be measured.

As clearly seen, the~001! peaks hardly move on heatingindicating that the thermal expansion along thec axis is verysmall. However, the evolution on heating of the~110! and~020! peaks is much more pronounced. This behavior is tycal of materials having anisotropic thermal expansion coficients. The expansion ratio in theab plane is more than tentimes bigger than that along thec axis, as deduced from th

TABLE I. Unit cell parameters for DMQtT, DMQqT, and DMST as function of temperature.a

Roomtemp. 50 °C 100 °C 150 °C 200 °C 240 °C

Rel.exp. %

DMQtTa/Å 5.62 5.66 5.67 5.70 ¯ ¯ 1.4b/Å 7.82 7.83 7.88 7.92 ¯ ¯ 1.3c/Å 18.55 18.56 18.56 18.57 ¯ ¯ 0.1DMQqTa/Å 5.63 5.64 5.66 5.69 5.74 ¯ 2.0b/Å 7.84 7.87 7.90 7.93 7.96 ¯ 1.5c/Å 22.19 22.19 22.19 22.17 22.15 ¯ 20.2DMSTa/Å 5.64 5.64 5.65 5.66 5.69 5.73 1.6b/Å 7.85 7.86 7.87 7.91 7.95 7.98 1.7c/Å 25.86 25.86 25.88 25.88 25.88 25.88 0.1

aThe deviations of theb monoclinic angle from 98.0° are negligible.


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FIG. 3. Variable-temperature powder patterns f~a! DMQtT, ~b! DMQqT, and~c! DMST. The more intensenonoverlapping peaks are indexed. Lines are drawn toderline the temperature evolutions of different diffractiopeaks.

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values of the unit cells reported in Table I. The relative epansion, at the highest recorded temperature, is determfrom the equation:~ucpht2ucprt!3100/~ucprt!, where ucp isthe unit cell parameter, and the subscripts ht and rt denthe highest temperature and room temperature, respectiIt is interesting to underline that DMQqT has the highepositive thermal expansion in theab plane. However, itshows negative thermal expansion along the chains, thatsay, the length of the chains slightly decreases with tempture.

The anisotropic thermal expansion may be understoofollows: the lattice vibrations in the oligothiophene mo


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ecules can be well approximated along the molecular loaxis by the harmonic oscillation in the parabolic potentfield produced by the delocalizedp electrons. This is, how-ever, not the case with lattice vibrations occurring perpdicular to the molecular long axis on account of subdivisiof the p-electron system in this direction. This leads to tanharmonic vibrations which cause large thermal expansIn light of the molecular arrangement of the oligomerchains in the unit cell, the thiophene rings of adjacent mecules are arranged in a laterally spreading herringboneray, where these molecules are somewhat diverted frorigorous face-to-face arrangement~for DMQtT, for instance,


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an angle ofca. 59° has been measured13!. Accordingly, thex-ray diffraction data presented in this work clearly confirthat the interchain spacing between molecules laterallyplayed on theab plane is increased largely with increasintemperatures. Concomitantly, strong interchain interacacts between these molecules, this interaction being strenened with decreasing temperatures.

The small thermal expansion along thec axis that themolecular long axis is almost parallel to, on the other hagives us a good opportunity to relate the infrared modes wthe p-electronic system encompassing a full moleculecause of its delocalization. This is because a change inlecular size or geometry such as conformation that is coupto thep-electronic state is most likely minimized.1 In otherwords, the effect of the temperature change upon thep-electronic system can be genuinely probed.

It is worthwhile mentioning the difference in morphoogy as a function of temperatures between the oligoiophenes in the present study and polythiophenes. Winoet al.34 reported temperature-dependent x-ray profilespoly~3-hexylthiophene!. Comparing Fig. 3 with Fig. 2 ofRef. 34, the oligothiophenes~in our present work! are rela-tively weakly temperature dependent, whereas thosepoly~3-hexylthiophene! are strongly temperature dependeCorrespondingly, the temperature-dependent infrared speof the oligothiophenes~see Figs. 4–8! show systematicchanges as a function of temperatures~Fig. 9!, even thoughthose recorded for poly~3-hexylthiophene!35 did not showsuch a change~see the next section!.

Infrared spectra

The infrared spectra ofa,a8-dimethyloligothiophenesfrom dimer to hexamer, are displayed in Figs. 4–8. Frequcies for the most significant bands are summarized in TaII–VI. The range studied was from2170 °C to 1150 °C~103 K to 423 K!; intermediate points at about170 ° C,0 °C, 250 °C, 2100 °C, and the room temperature~around120 °C! were analyzed in all the cases. DMBT and DMTexhibit the spectral profiles that dramatically change befthe maximum temperature (1150 °C) is reached~see Figs. 4and 5, respectively!. This is ascribed to melting of the materials; for the melting points of DMBT and DMTT as detemined from DTA are close to 50 °C and 90 °C, respectiveConcomitantly, the temperature-dependent x-ray diffractpatterns indicate the disappearances of the crystal ordethe same temperatures. For DMQtT, DMQqT, and DMSon the other hand, the melting points are not reached andspectra reversibly recover their profiles throughout thetemperature range examined.

The spectra displayed in the Figs. 4–8 exhibit rathsystematic trends which are common to all the oligomeFirst, the widths of the infrared bands decrease whentemperature decreases, as expected for crystalline sysAs a consequence, some bands seem to be split in thetemperature spectra. As a significant example, the bandtered at 462 cm21 in the spectra of DMST at room temperture @Fig. 8~b!#, that contains a shoulder at a close frequenappears as a doublet with peaks at 458 and 462 cm21 at2170 °C. This band has been assigned to an out-of-p


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thiophene ring vibration,g~ring!,28 and has been measurednear frequency for the rest of oligothiophenes studied. Inthe cases it is resolved into two peaks when loweringtemperature~see Tables II–VI!, except for the dimer, DMBT@Fig. 4~a!#, because in this molecule both thiophene ringssymmetrical, whereas in longer oligomers we can discrimnate at least two series of equivalent thiophene rings. In fDMBT cannot be properly considered as an oligomer

FIG. 4. Infrared spectra of DMBT at different temperatures betwe2170 °C and 150 °C. ~a! The 1700– 900 cm21 region; ~b! the900– 400 cm21 region.


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cause it only hasend rings, and as a consequence, conjugtion is not significant.36

Bands at 871 and 858 cm21 of DMBT at room tempera-ture were assigned to the antisymmetrical and symmetrcarbon-sulfur stretching modes,n~CS!, respectively.28 As oc-curred with theg~ring! band of this molecule, they do noresolve into peaks when the temperature decreases. Thihavior is also observed for then~CS! bands of the trimer and

FIG. 5. The infrared spectra of DMTT at different temperatures betw2170 °C and 150 °C. ~a! The 1700– 900 cm21 region; ~b! the900– 400 cm21 region.


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the pentamer; however, the low-temperature spectra oftetramer and hexamer show doublets. A deeper explanahas to be found regarding the symmetry of these molecuConsidering the series from DMTT to DMST, those havian even number of thiophene units belong to theC2h pointgroup, while those having an odd number of thiophene uareC2v . These facts have been supported previously bybrational spectroscopy28,29 and, in the case of DMQtT, con

nFIG. 6. The infrared spectra of DMQtT at different temperatures betw2170 °C and 150 °C. ~a! The 1700– 900 cm21 region; ~b! the900– 400 cm21 region.


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firmed also by x-ray measurements on single crystals.13 Be-cause lower symmetries increase the number of infraredRaman active modes, we can expect the spectra of the trand the pentamer to have more infrared-active frequenclose modes than the tetramer and the hexamer, giving risbroader bands whose components cannot be separatelytified at the low temperatures reached in the present wThe same behavior can be also observed for the strong inred absorptions around 800 cm21, corresponding to the out

FIG. 7. Infrared spectra of DMQqT at different temperatures betw2170 °C and 150 °C. ~a! The 1700– 900 cm21 region; ~b! the900– 400 cm21 region.


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of-plane carbon–hydrogen bending vibrations,g~CH!.A second fact related with the multiplets is that the se

ration between two components increases with decreatemperature, thus allowing peaks to be resolved and msured. Because we cannot ignore the influence of fagroup splittings in some infrared bands, additionally to tsymmetry effects, we expect these separations to be lawhen the temperature is lowered, as a consequence ointermolecular force increment. This is the case with t

nFIG. 8. The infrared spectra of DMST at different temperatures betw2170 °C and 150 °C. ~a! The 1700– 900 cm21 region; ~b! the900– 400 cm21 region.


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g~CH! vibrations in DMST, that appear as a completelysolved triplet with frequencies at 801, 796 and 786 cm21 at2170 °C, while at 150 °C these peaks are ill defined at 7791, and 786 cm21. For DMQtT this band is also composeof three peaks measured at 804, 798, and 788 cm21 at150 °C, and 796, 792, and 786 cm21 at 2170 °C. In thecarbon disulfide solution only two peaks were measuredthe g~CH! vibrations, as a consequence of the vanishingthe lattice forces, in agreement with the prior discussion29

The splittings of these band systems could be used to evate some intermolecular HH potentials between adjacenoligomeric chains.37,38

FIG. 9. A plot of the carbon–carbon stretching frequencies as a functiothe temperature for~a! DMQtT, ~b! DMQqT, and~c! DMST.

TABLE II. Frequencies (cm21) measured in the infrared spectra of DMBat different temperatures.

Assignments

Temperature~°C!

2170 2100 250 0 Roomta 70 150

na(C5C) 1534 1534 1534 1534 1531 1534 153ns(C5C) 1453 1453 1453 1453 1451 1451 144n~C–C! 1374 1374 1374 1374 1375 1378 137na(CS) 872 872 872 872 871 871 869ns(CS) 860 860 860 860 858 ¯ ¯

g~CH!788 788 788 788 786 788 788781 781 781 781 781 ¯ ¯

g ring 470 470 470 470 471 479 481


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The spectra of DMBT at low temperatures show sevesharp peaks close to the band assigned to the symmeC5C stretching mode,ns(C5C), around 1450 cm21; seeFig. 4~a!. This is also observed for DMTT and DMQqT iFigs. 5~a! and 7~a!, respectively, but the peaks have weakintensities. We have assigned these new bands to mebending vibrations,d(CH3), involving several rotational iso-mers. At room temperature these molecules exist as an elibrium mixture of two or more conformers, whose potentbarrier is easily exceeded by the environmental energy.30,39

When lowering the temperature, some of them are not egetically enough to overcome their potential barriers anda consequence, the population of some rotational isomeincreased. Since the intensity of a vibrational band is proptional to the population of the related state, we can expecobserve in the spectra at low temperatures new bands fod(CH3) vibrations that did not appear at higher temperaturMoreover, this fact has to be better observed in the shooligomers because the relative weight of the two metgroups increases when the chain length decreases, ascurred in our case, thus supporting this assignment.

Until now we have discussed a set of topics which aregeneral, described in the literature as expectedatemperature-dependent infrared study of crystalline sstructures. From now, we are going to describe a last syst

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TABLE III. Frequencies (cm21) measured in the infrared spectra of DMTat different temperatures.

Assignments

Temperature~°C!

2170 2100 250 0 Roomta 70 150

na(C5C) 1519 1519 1519 1519 1517 1516 152ns(C5C) 1445 1445 1445 1445 1444 1444 144n~C–C! 1371 1371 1371 1371 1372 1373 137na(CS) 872 872 872 872 870 870 865ns(CS) 854 854 854 854 853 853 852

803 803 803 803 802 ¯ ¯

g~CH! 797 797 796 795 795 794 789786 786 786 786 786 786 787

g ring 465 465 466 466 467 468 ¯

TABLE IV. Frequencies (cm21) measured in the infrared spectra oDMQtT at different temperatures.

Assignments

Temperature~°C!

2170 2100 250 0 Roomta 70 150

na(C5C) 1516 1516 1516 1515 1513 1512 151ns(C5C) 1447 1447 1447 1446 1445 1444 144n~C–C! 1369 1369 1369 1370 1371 ¯ ¯

na(CS)864 864 863 863 862 862 861861 861 861 860 860 860 ¯

ns(CS)846 846 846 845 844 843 842843 843 843 843 843 ¯ ¯

804 804 803 802 800 799 796g~CH! 798 798 797 796 794 793 792

788 788 788 788 787 786 786

g ring

465 465 465 465 465 465 464

462 463 463 462 465 ¯ ¯


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atic trend which is related with a specific feature of the poconjugated materials. It involves the carbon–carbon streing modes, both single and double bonds, which supply vuseful information about the conjugation length. As has bdiscussed in previous papers,28,29 the frequencies of then(C5C) modes shift downward and those of then~C–C!modes shift upward when the conjugation length increaThis fact is easily observed by comparing the frequenciesbands near 1500 and 1450 cm21 for the whole series of oli-gomers at room temperature, which were assigned ton(C5C) vibration, antisymmetrical, and symmetricarespectively.28 For both the bands, the frequencies decreon going from the dimer to the hexamer, as a consequencthe p-electron delocalization. This fact has been also sported by theoretical calculations in oligothiophenes1,3 andoligopyrroles,36–40 which indicate that observed dispersiofor the aforementioned mode must be originated fromincrease ofp-electron delocalization when the numberunits increases, other contributions being negligible. Thesults obtained from the temperature-dependent infrared stra for the larger oligomers are displayed in Fig. 9. As canseen, then(C5C) vibrations decrease their frequenciwhile the n~C–C! vibrations increase them on going fro2170 °C to 150 °C. Thus, for DMST the antisymmetrn(C5C) frequency goes from 1507 to 1501 cm21 and thesymmetricn(C5C) frequency goes from 1445 to 1440 cm21

in the aforementioned range of temperature, whereasn~C–C! goes from 1369 to 1373 cm21. For DMQqT thesefrequencies shift by26, 24, and 12 cm21, respectively,and in a similar way for the tetramer. The trimer and dimshow negligible shifts for the related bands.

At first glance, we must accept that theintermolecularforces ~i.e., the interchain interactions! weaken with raisedtemperature, because the oligomeric chains expand laterNonetheless, this change in the intermolecular forces caexplain the upshift in then~C–C! bands on one hand, and thdownshift in then(C5C) bands, on the other hand. Thefrequency shifts lead to the occurrence of the quinoid strture, which has to be considered in addition to the intermlecular force modifications. This is because in the quinstructure the C–C bonds are strengthened, while the C5C

TABLE V. Frequencies (cm21) measured in the infrared spectra of DMQqat different temperatures.

Assignments

Temperature~°C!

2170 2100 250 0 Roomta 70 150

na(C5C) 1512 1512 1512 1511 1509 1508 150ns(C5C) 1444 1444 1444 1443 1442 1441 144n~C–C! 1372 1372 1372 1373 1374 1374 137

na(CS)885 885 885 884 884 883 882882 882 ¯ ¯ ¯ ¯ ¯

ns(CS) 841 841 841 841 840 839 838801 801 801 799 797 794 794

g~CH! 795 795 795 794 792 791 790786 786 786 786 786 786 785

g ring

462 463 463 463 463 463 463

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bonds are weakened. This inference may be legitimized frthe point of view of thermodynamic equilibrium between tquinoid and benzenoid structures. In other words, the formplays an increasingly significant role with raised tempetures relative to the latter, because the quinoid counterpabelieved to possess higher energy than the benzenoid o41

Thus, a change in theintramolecular forcesis worthy ofbeing considered as the origin of the shifts in bothn~C–C!andn(C5C) modes.

CONCLUSIONS

The temperature-dependent x-ray diffraction and infred spectroscopy for a series of thea,a8-dimethyl end-capped oligothiophenes~from dimer to hexamer! have beenstudied covering a temperature range2170 °C to 150 °C.The x-ray diffraction studies demonstrate that the thermexpansion coefficients in theab plane are an order of magnitude larger than those along thec axis with which themolecular long axis is nearly parallel.

The infrared experiments show that several specbands exhibit splittings, frequency shifts, and change inband width depending on the temperature. These spefeatures have been successfully explained on the basiintermolecular forces, molecular symmetries, and analysithe populations of rotational levels and isomers. In partilar, the fact that theg~CH! mode exhibits a sharply resolvemultiplet at low temperatures is associated with the lathermal expansion in theab plane. This thermal expansiostrongly affects the intermolecular interaction thatstrengthened when the intermolecular spacing decreasesdecreasing temperatures, making the spectral line splitthe separation between the components of the multipletcrease. Then(C5C) andn~C–C! modes shift downward andupward with increasing temperatures, respectively. Thisbe interpreted by taking the quinoid structure into accoun

ACKNOWLEDGMENT

We acknowledge support by the Spanish MinistryEducation and Culture~Project No. PB96-0682!.

TABLE VI. Frequencies (cm21) measured in the infrared spectra of DMSat different temperatures.

Assignments

Temperature~°C!

2170 2100 250 0 Roomta 70 150

na(C5C) 1507 1507 1506 1506 1504 1503 150ns(C5C) 1445 1445 1445 1445 1443 1442 144n~C–C! 1369 1370 1370 1371 1371 1372 137

na(CS)877 877 877 877 876 875 873873 873 873 873 872 871 871

ns(CS) 840 840 840 840 838 837 835801 801 800 800 797 796 793

g~CH! 796 796 796 795 794 793 791789 789 788 788 786 786 786

g ring

462 462 462 462 462 462 462

458 458 459 459 ¯ ¯ ¯


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Temperature-dependent infrared spectroscopic and crystallographic study of α,α[sup ʹ]-dimethyl...

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