FT-IR and FT-Raman spectra of a series of oxidized a,a 0 -diethyl end-capped oligothienyls: a spectroscopic study of conjugational model defects

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1 Optical Materials 12 (1999) 321±325 FT-IR and FT-Raman spectra of a series of oxidized a,a 0 -diethyl end-capped oligothienyls: a spectroscopic study of conjugational model defects J. Casado a, S. Hotta b, V. Hernandez a, J.T. Lopez Navarrete a, * a Departamento de Quõmica Fõsica, Facultad de Ciencias, Universidad de Malaga, Malaga, Spain b Matsushita Research Institute, Advanced Materials Research Laboratory, , Higashimita Tama-ku, Kawasaki , Japan Abstract We present the results of an FT-IR and FT-Raman vibrational spectroscopic study carried out on a,a 0 -diethylquinquethiophene and a,a 0 -diethylsexithiophene upon doping with iodine vapours and nitrosyl salts. The wavenumbers and intensity dispersions observed for the doping induced Raman bands with the oxidation degree are utilized to elucidate the conjugational defect-type. The near-infrared Raman spectra indicate the existence of radical cations and dications as the major oxidized species in the I 2 -doped and NO-doped oligomers, respectively. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Oligothiophenes; FT-IR; FT-Raman; Radical-cations; Dications 1. Introduction Conjugated polymers have attracted considerable attention since the discovery of possible insulator±metal transitions following a chemical or electrochemical doping [1]. Many families of p- conjugated systems are currently being studied as models in search of precise relationships between the di erent chemical structures and their electronic and optical properties. In this regard, shortchain p-conjugated oligomers are long recognized as well-de ned chemical systems whose electronic structures can be easily controlled by varying simply their chain lengths [2]. * Corresponding author. Tel.: ; fax: ; teodomiro@uma.es Some X-ray di raction experiments have revealed that the oligothiophenes show a regular layered structure in both the neutral and oxidized forms, which lead to high anisotropic electronic responses [3±5]; thus, making these p-conjugated materials good candidates for molecular electronic applications. Alkyl end-capped oligothiophenes are particularly suited to investigate the electronic and vibrational structures of the charged defects because both the end a-positions are blocked with inert alkyl groups [6,7]. This a,a 0 -substitution prevents further polymerization upon doping at the terminal thiophene units, and the resulting doped species stand stable for a long time. It has also been shown that, contrary to the case of the unsubstituted oligothiophenes, large crystals are readily available for the a,a 0 -diethyl end-capped oligothiophenes through recrystallization from suitable solvents [3] /99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S ( 9 9 )

2 322 J. Casado et al. / Optical Materials 12 (1999) 321±325 Scheme 1. Chemical structures of the molecules studied in this work. Charged defects generated on the p-conjugated backbone either by chemical ionization or photoexcitation are characterized by the appearance in the vibrational spectrum of several bands with exceedingly large intensities. These bands are associated with the infrared and Raman active vibrations of a variety of self-localized excitations (i.e., polarons, bipolarons, p-dimers or others), whose spatial extensions along the chain in the case of a real polymer are, in principle, unkown. The usefulness of Raman spectroscopy in the characterization of conjugational defects in p- conjugated materials has been largely demonstrated by Furukawa et al. in the past few years [8,9]. The power and selectivity of the Raman characterization of doped conjugated polymers is enhanced by using exciting laser lines in resonance with the optical bands appearing in the NIR region. In this article we report on the changes of the FT-IR and FT-Raman spectra of a,a 0 -diethylquinquethiophene and a,a 0 -diethylsexithiophene (see Scheme 1 for chemical structures of the molecules studied in this work) during the chemical doping with dry iodine vapours and nitrosyl salts. The vibrational information is correlated to the spatial extension of the charged defects along the oligomeric chain. CH 3 CN/CH 2 Cl 2 /hexane (1:1:2) solution of NOBF 4 was dropped into the Raman tubes in which the pentamer (DEQqT) or the hexamer (DESxT) samples were placed. The doping process was performed under N 2 atmosphere, and using a dry box to ensure moisture free conditions of reaction. After reaction for 30 m, the solvent was evacuated and the tube sealed. Infrared absorption spectra were recorded with a Perkin Elmer Model 1760 X FT-IR spectrometer, and the Raman spectra of solid doped samples were collected with a Brucker Model FRA-106/S FT-Raman spectrometer, both at room temperature. 3. Results and discussion 3.1. Radical cations The FT-IR absorption spectra of DEQqT and DESxT doped with dry iodine vapours are reported in Figs. 1 and 2, respectively. A general raising of the absorption is observed on the higher wavenumber region of the spectra. This feature represents the tail of the absorbance of the p±p transition in the NIR region. By considering the evolution of the IR spectra of the oligomers from the neutral state (upper curve) to the oxidized state (lower curve), we rstly notice that band positions (wavenumbers) and relative intensities are greatly changed and that several new absorptions arise upon ionization. 2. Experimental The details of the synthesis and puri cation methods of the a,a 0 -diethyl end-capped oligothiophenes have been described elsewhere [7]. The low oxidation species were obtained, under dry atmosphere, by slow in situ sublimation of iodine at 50 C. The high oxidation species were obtained according to the following procedure. A 4 mm Fig. 1. Infrared spectra of a,a 0 -diethylquinquethiophene in neutral state (DEQqT) and oxidized with iodine (DEQqT ).

3 J. Casado et al. / Optical Materials 12 (1999) 321± Fig. 2. Infrared spectra of a,a 0 -diethylsexithiophene in neutral state (DESxT) and oxidized with iodine (DESxT ). A close look at the details shows that the generation of a positive polaron-type defect on the conjugated backbone is characterized by the appearance of at least four new rounded features centered around 1395, 1330, 1105 and 1015 cm 1, whose intensities readily increase with increasing doping level. On the other hand, we also observed a certain enhancement with increasing doping level for the bands at 1220±1200, 1170 and 1060±1050 cm 1. These three bands however are not new, and they were already present, almost at the same wavenumbers, in the FT-IR spectra of the neutral materials. The remaining absorptions seem to undergo only very small changes with respect to the neutral state [10,11]. The FT-Raman spectra of the neutral, radical cations and dications of DEQqT and DESxT are displayed in Figs. 3 and 4, respectively. The spectra of the radical cations are similar to each other, showing only very subtle changes with chain length. However, the two Raman spectra are rather di erent from those of the neutral compounds. The main spectral changes after doping are as follows: 1. The C ˆ C stretching region is characterized by the quick appearance of three strong Raman lines centered at about 1480, 1438 and 1420± 1415 cm 1. Their assignment is not however straightforward, and we could expect that the corresponding modes have a collective character, being assignable to mixtures of m(c ˆ C) ring and m(c±c) inter-ring vibrations (with a probable Fig. 3. FT-Raman spectra of a,a 0 -diethylquinquethiophene in neutral state (DEQqT), oxidized with iodine (DEQqT ) and oxidized with NOBF 4 (DEQqT 2 ). involvement of stretching vibrations of the ethyl end-caps). 2. A second observation is that the relative intensities of the Raman lines measured near 1511± 1503 and 1471±1470 cm 1 readily decrease as the chain length increases. Thus, we think that these modes should contain signi cant contributions from m(c ˆ C) ring vibrations mostly localized on the outer thiophene rings. 3. For bands below 1400 cm 1, the main spectral di erence between the pristine and oxidized molecules concerns the intensity of the bands at 1223±1217 and 1170±1150 cm 1, which undergo an appreciable enhancement upon ionization. Finally, we observe that the line at 1050 cm 1, which must be assigned to a totally symmetric CH in-plane bending vibration, is only slightly shifted with respect to the neutral state. Nonetheless, its intensity increase upon

4 324 J. Casado et al. / Optical Materials 12 (1999) 321±325 m(c ˆ C) ring modes in DEQqT 2 and DESxT 2 is a direct consequence of the softening experienced by the p-conjugated skeleton upon quinoidization of the molecule. Indeed, the transfer of Raman intensity from the band near 1440 cm 1 to that located at 1418 cm 1, in going from the radical-cations to the dications, seems to indicate that, for a given chain length, the molecular domain perturbed by the ionization is topologically more extended in the divalent ionized species than in the radical-cation one. 2. Vibrations such as 1220, 1156 and 1054 cm 1 (DEQqT 2 ) and 1217, 1150 and 1053 cm 1 (DESxT 2 ) are suspected also to be thoroughly mixed with the CC stretching modes, so that it would be more correct to emphasize their character of ring vibrations as well. 4. Conclusions Fig. 4. FT-Raman spectra of a,a 0 -diethylsexithiophene in neutral state (DESxT), oxidized with iodine (DESxT ) and oxidized with NOBF 4 (DESxT 2 ). ionization suggests that this mode should be largely coupled with m(cc) stretching vibrations Dications The chain length dependence of the Raman spectra of DEQqT 2 and DESxT 2 is very little, as occurred also for the radical cations. However we notice, by comparing the spectra of the two dicationic species with those of the neutral forms, that both relative intensities and band positions are signi cantly a ected upon ionization. A number of e ects can be deduced from the observation of the spectra: 1. The major Raman band of the dications, appearing at 1418 cm 1, is downshifted with respect to both the neutral and the radical cation species. The large red shift of the The spectroscopic data presented in this paper could be compared with the Raman spectra reported by Furukawa et al. some years ago for samples of doped polythiophene [9]. These authors suggested that the Raman spectrum of doped polythiophene arises probably from positive polarons or radical cations. In particular, the Raman spectrum collected by the Furukawa et al. with a laser excitation of 753 nm resembles our FT-Raman spectrum of DEQqT 2, while those recorded with an excitation of 1064 nm are almost identical to that of DESxT 2. The signi cance of these experimental ndings must be stressed since, although they are in contradiction with the nal conclusions reached by the group of Furukawa, they are however in full agreement with the accepted view that positive bipolarons of dications are the major species generated by doping in polythiophene. In addition, these results give us a rough idea about spatial extent of the bipolarons. Seeing that both DE- QqT 2 and DESxT 2 exhibit virtually the same spectra as doped polythiophene, the bipolaronic defects can su ciently be accommodated within a few thiophene rings.

5 J. Casado et al. / Optical Materials 12 (1999) 321± Acknowledgements This research was nancially supported by Direccion General de Ense~nanza Superior (DGES, MEC, Spain) through the research project PB96± We are also indebted to Junta de AndalucõÂa (Spain), funding for our research group (n FQM± 0159). The authors acknowledge the use of the scienti c instrumentation facilities of the Servicio Central de Apoyo a la Investigacion (SCAI) of the University of Malaga. JC is grateful to the MEC (Spain) for a personal grant. References [1] J.L. Bredas, R. Silbey, Conjugated Polymers: The Novel Science and Technology of Highly Conducting and Nonlinear Optically Active Materials, Kluwer Academic Publishers, Dordrecht, [2] D. Fichou, G. Horowitz, Y. Nishikitani, F. Garnier, Synth. Met. 28 (1989) C723. [3] S. Hotta, Handbook of Organic Conductive Molecules and Polymers, in: H.S. Nalwa (Ed.), Ch. 8, vol. 2, Conductive Polymers: Synthesis and Electrical Properties, Wiley, New York, [4] G. Horowitz, B. Bachet, A. Yassar, P. Lang, F. Demanze, J.L. Fave, F. Garnier, Chem. Mater. 7 (1995) [5] T. Siegrist, R.M. Fleming, R.C. Haddon, R.A. Laudise, A. Lovinger, H.E. Katz, P. Bridenbaugh, D.J. Davis, Mater. Res. 10 (1995) [6] P. Bauerle, Adv. Mater. 4 (1992) 102. [7] S. Hotta, K. Waragai, J. Mater. Chem. 1 (1991) 835. [8] A. Sakamoto, Y. Furukawa, M. Tasumi, J. Phys. Chem. 98 (1994) [9] (a) Y. Furukawa, N. Yokonuma, M. Tasumi, M. Kuroda, J. Nakayama, Mol. Cryst. Liq. Cryst. 256 (1994) 113; Chem. Phys. Lett. 255 (1996) 431. [10] V. Hernandez, J. Casado, F.J. Ramirez, G. Zotti, S. Hotta, J.T. Lopez Navarrete, J. Chem. Phys. 104 (1996) [11] V. Hernandez, J. Casado, F.J. Ramirez, G. Zotti, S. Hotta, J.T. Lopez Navarrete, Synth. Met. 76 (1996) 277.