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Infrared and Raman studies of TTM-TTP and TSM-TTP charge-transfer salts
Journal of Molecular Structure 704 (2004) 89–93 www.elsevier.com/locate/molstruc
Infrared and Raman studies of TTM-TTP and TSM-TTP charge-transfer salts R. S´wietlika,b,*, K. Yakushib, K. Yamamotob, T. Kawamotoc, T. Moric a
Polish Academy of Sciences, Institute of Molecular Physics, ul. Mariana Smoluchowskiego 17, Poznan´ 60-179, Poland b Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan c Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 17 November 2003; accepted 14 January 2004 Available online 10 May 2004
Abstract We measured room temperature infrared and Raman spectra of the neutral TTM-TTP molecule and four conducting charge-transfer salts: (TTM-TTP)I3, (TTM-TTP)AuI2, (TTM-TTP)(I3)5/3 and (TSM-TTP)(I3)5/3. The vibrational bands related to the CyC stretching modes were analysed. The frequencies of CyC stretching modes observed both in infrared and Raman spectra depend linearly upon the charge on TTMTTP (or TSM-TTP) molecules. q 2004 Elsevier B.V. All rights reserved. Keywords: Organic conductors; TTM-TTP and TSM-TTP salts; IR and Raman spectroscopy; CyC stretching vibrations
1. Introduction The bis-fused TTF (tetrathiafulvalene) molecule and its derivatives are good electron donors for synthesis of new conducting ion-radical salts with interesting physical properties [1]. One of these derivatives, TTMTTP [2,5-bis(4,5-bis(methylthio)-1,3-dithiol-2-ylidene)1,3,4,6-tetrathia-pentalene] (Fig. 1), yields mostly quasi-one-dimensional semiconductors, because four S-CH3 groups attached to the bis-fused TTF skeleton separate efficiently neighbouring TTM-TTP stacks [2]. Nevertheless, some TTM-TTP salts exhibit metallic properties. The charge-transfer salt (TTM-TTP)I3 was reported as the first organic metal with 1:1 stoichiometry and a highly onedimensional half-filled energy band (TTM-TTP has a charge þ 1) [3,4]. It was an important discovery since, usually the 1:1 salts are semiconductors, owing to large values of the onsite Coulomb interaction energies, which localise electrons. On the other hand, the salt (TTM-TTP)AuI2 also has 1:1 composition, but it shows semiconducting behaviour [5]. The salt (TTM-TTP)(I3)5/3 is an organic metal with even higher * Corresponding author. Address: Polish Academy of Sciences, Institute of Molecular Physics, ul. Mariana Smoluchowskiego 17, Poznan´ 60-179, Poland. Tel.: þ 48-61-8695165; fax: þ48-61-8684524. E-mail address: [email protected] (R. S´wietlik). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.01.050
oxidation degree of TTM-TTP molecules (þ 5/3) [6,7]. Analogous compound based on TSM-TTP donor (Fig. 1), namely (TSM-TTP)(I3)5/3, also shows metallic properties [8,9] but a TSM-TTP metal of 1:1 stoichiometry was not obtained. It is well known that frequencies of vibrational modes of a molecule are dependent on average charge residing on this molecule, therefore, vibrational spectroscopy was often used for determination of the charge distribution in organic conductors [10 –13]. For TTF derivatives especially sensitive for charge are modes related to the CyC stretching vibrations. In particular, it was shown that the frequencies of CyC modes depend linearly (or nearly linearly) on charge density for conducting charge-transfer salts based on BEDT-TTF [10,11,13] and BEDO-TTF [12] donors. On increasing the average oxidation state of BEDT-TTF (or BEDO-TTF), the frequencies of various CyC bands shift towards lower frequency on average about 95 and 60 cm21 (or 110 and 75 cm21) per charge þ 1e, respectively. The molecular charge was usually determined by using Raman spectroscopy, but also IR active vibrations can be very useful. It is important that in organic conductors, the frequencies of totally symmetric, CyC Raman active modes can be disturbed not only by environmental (crystal field) effects but also by coupling with electrons [13].
R. S´wietlik et al. / Journal of Molecular Structure 704 (2004) 89–93
90
Fig. 1. Molecular structures of TTM-TTP and TSM-TTP.
The coupling with electrons is not expected for nonsymmetric CyC modes which are IR active. The skeleton of TTM-TTP and TSM-TTP molecules is formed by the bis-fused TTF, i.e. the BDT-TTP molecule [2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathia-pentalene]. The fundamental in-plane modes of BDT-TTP were assigned in Ref. [14] and these data can be used for discussion of the CyC stretching vibrations of TTM-TTP and TSM-TTP. There are three types of non-equivalent CyC bonds in these molecules: one inner-ring bond, two CyC bridges and two outer-ring bonds (Fig. 1). Spectroscopic studies showed that within the region of CyC stretching, the neutral BDT-TTP has three Raman active modes of ag symmetry and two IR active modes of b1u symmetry: n2(ag) ¼ 1555 cm21, n3(ag) ¼ 1525 cm21, n4(ag) ¼ 1504 cm21, n21(b1u) ¼ 1550 cm21, n22(b1u) ¼ 1518 cm21 [14]. Various CyC modes are significantly mixed in neutral BDT-TTP but they are rather separated in BDT-TTPþ cations. Therefore, for BDT-TTPþ cations: the n2(ag) and n3(ag) are mainly assigned to the in-phase CyC stretching of outer-rings and bridges, respectively; the n4(ag) is assigned to the CyC inner-ring stretching; and the n21(b1u) and n22(b1u) are assigned to the out-ofphase CyC stretching of outer rings and bridges, respectively. Calculations of the neutral BDT-TTP0 molecule and its radical cation BDT-TTPþ have shown that all the CyC modes exhibit large frequency downshift when the molecule is oxidised [14]. Evidently, these modes should also be strongly sensitive to ionisation degree in the case of TTM-TTP and TSM-TTP molecules. We studied the IR and Raman spectra of selected TTM-TTP and TSM-TTP salts to examine modifications of their vibrational spectra due to increase of the donor oxidation state. The following compounds were measured: neutral TTM-TTP0 molecules; salts (TTM-TTP)I3 and (TTM-TTP)AuI2 containing TTM-TTPþ1 ions; as well as salts (TTM-TTP)(I 3) 3/5 and (TSM-TTP)(I 3) 3/5 with TTM-TTP þ5/3 and TSM-TTPþ5/3 ions, respectively. Our attention was mainly focused on vibrational bands related to the CyC stretching vibrations to examine their modifications upon increasing molecular charge.
2. Experimental Single crystals of the studied charge-transfer salts were prepared by electrocrystallisation by methods previously described in the following Refs: [3] for (TTM-TTP)I3, [5]
for (TTM-TTP)AuI2, [6] for (TTM-TTP)(I3)5/3 and [8] for (TSM-TTP)(I3)5/3. The IR reflectance spectra were measured using a FT-IR spectrometer Nicolet Magna 760, equipped with an IR microscope Spectratech IR, in the frequency region from 600 to 10,000 cm21. If possible, the spectra were recorded from various crystal faces in two perpendicular polarisations, i.e. the electrical vector of the polarised IR beam was either parallel or perpendicular to TTM-TTP (or TSM-TTP) stacking direction. However, in the analysis we focused our attention on the spectra obtained for polarisation perpendicular to the stacks, since the IR active CyC modes were mainly observed for this polarisation. Additionally, we studied the IR absorption spectra of TTM-TTP neutral molecules by using the standard KBr pellet technique. The optical conductivity spectra were determined by Kramers – Kronig analysis of the reflectance data. The low-frequency data were extrapolated to zero frequency assuming a constant reflectance. The high-frequency data were extended up to about 30,000 cm21 on basis of the available reflectance spectra from Refs. [5,15,16] and above a constant reflectance was assumed. The Raman spectra were studied using the backscattering geometry on a Renishaw Ramascope System 1000 composed of a notch filter, single monochromator and CCD detector cooled by thermoelectric device. For excitation we used mainly He –Ne laser (l ¼ 632:6 nm). To avoid the sample overheating, the laser power was strongly reduced (usually below about 250 mW) Each spectrum was collected at room temperature with about 30 – 60 min accumulation.
3. Results and discussion The reflectance spectra of the (TSM-TTP)(I3)5/3 salt at room temperature are shown in Fig. 2. The spectrum obtained from the crystal face (001) for polarisation Eka is very similar to that one reported in Ref. [16]. Unfortunately, for this crystal face we could not obtain any good spectrum for perpendicular polarisation ðE ’ aÞ because of interference effects in too thin crystals (similar effect was also mentioned in Ref. [16]). However, the IR active CyC modes cannot be observed by measuring reflectance from this crystal face since the TSM-TTP long axis is nearly perpendicular to it. These CyC bands can be found in reflectance spectra taken from another crystal face, for example the face (010), and we succeeded to measure such spectrum: it is shown in the lower panel of Fig. 2 and the optical conductivity data in Fig. 3. Analogously, by investigating the reflectance from various crystal faces of the salt (TTM-TTP)(I3)5/3 we were able to record bands attributed to the IR active CyC modes of TTM-TTP molecule. Our spectra of (TTM-TTP)I3 and (TTM-TTP)AuI2 salts were very similar to those reported in Refs. [15,5], respectively. The spectra within the CyC stretching region are displayed in Fig. 4. In the absorption spectrum of neutral
R. S´wietlik et al. / Journal of Molecular Structure 704 (2004) 89–93
Fig. 2. Polarised reflectance spectra of (TSM-TTP)(I3)5/3 salt obtained from the crystal face (001) (upper panel) and the (010) face (lower panel).
TTM-TTP, the doublet at 1491 and 1507 cm21 is assigned to n22(b1u) mode and the band at 1547 cm21 is most probably due to n21(b1u) mode. The two doublets at 1417, 1426 cm21 and at 1307, 1312 cm21 we assign to the asymmetric and symmetric bending mode of the methyl group in S-CH 3, respectively. In S-CH3 group, the asymmetric bending is usually observed at 1414 – 1440 cm21 and symmetric bending usually at 1290 –1330 cm21 [17]. On increasing the ionisation degree of TTM-TTP molecule ðrÞ; the frequency of n22(b1u) band decreases nearly linearly by about 98 cm21 per charge þ 1e (Table 1 and Fig. 6). Unfortunately, we could not assign
91
Fig. 4. Optical conductivity data of the studied salts for polarisation perpendicular to the stacking direction and IR absorption spectrum of the neutral TTM-TTP molecule dispersed in KBr pellet within the region of CyC stretching vibrations (r is the ionisation degree of molecules).
unambiguously the n21(b1u) band in the reflectance spectra of the salts. The line at 1300 cm21 for (TTM-TTP)(I3)5/3 and at 1270 cm21 for (TSM-TTP)(I3)5/3 are assigned to the symmetric bending vibration of methyl groups in S-CH3 and Se-CH3, respectively. The appropriate asymmetric bending mode is observed at about 1417 cm21 for (TTM-TTP)I3 and (TTM-TTP)AuI2 but is not clearly seen in the spectra of (TTM-TTP)(I3)5/3 and (TSM-TTP)(I3)5/3. The Raman data within the CyC stretching region are displayed in Fig. 5. For each salt we observe three Raman lines related to n2(ag), n3(ag) and n4(ag) modes of TTM-TTP (or TSM-TTP), respectively. For neutral TTM-TTP molecule, the n2(ag) and n4(ag) bands show a doublet structure, probably due to the crystal field effect. When Table 1 Positions of vibrational bands assigned to the CyC stretching modes in IR and Raman spectra of TTM-TTP and TSM-TTP salts Compound
TTM-TTP
Fig. 3. Optical conductivity spectra of (TSM-TTP)(I3)5/3 as obtained by Kramers– Kronig analysis of the reflectance data for the crystal face (010).
(TTM-TTP)I3 (TTM-TTP)AuI2 (TTM-TTP)(I3)5/3 (TSM-TTP)(I3)5/3
Charge r (þ e)
0 0 1 1 5/3 5/3
Frequency (cm21)
n22(b1u)
n2(ag)
n3(ag)
n4(ag)
1507 1491 1395 1403 1337 1332
1561 1558 1486 1486 1454 1452
1512
1493 1487 1420 1424 1381 1386
1453 1455 1420 1419
92
R. S´wietlik et al. / Journal of Molecular Structure 704 (2004) 89–93
Fig. 5. Raman spectra of the studied compounds within the region of CyC stretching (r is the ionisation degree of molecules).
the charge on donor molecules increases, all the CyC Raman bands shift nearly linearly towards lower frequencies by about 64 cm21 for n2(ag), 56 cm21 for n3(ag) and 63 cm 21 for n4 (a g), when charge grows by þ 1e.
The frequencies of various CyC modes are collected in Table 1 and their dependence upon the charge is shown in Fig. 6. Our spectroscopic data are to be compared with the analogous frequency shifts for BDT-TTP molecule. The calculation of normal modes of BDT-TTP0 and BDT-TTPþ shows that the Raman active modes decrease their frequencies by: 30 cm21 for n2(ag), 70 cm21 for n3(ag) and 99 cm21 for n4(ag) mode; whereas the IR active modes shift down by: 25 cm21 for n21(b1u) and 126 cm21 for n22(b1u) mode [14]. Our experimental study shows that for TTM-TTP (or TSM-TTP) molecule all the ag modes show approximately the same low-frequency shift (about 60 cm21) when the molecule is oxidised. It is possible that this surprising discrepancy is yielded by the calculation method used for BDT-TTP. On the other hand, the shift of n22(b1u) mode is also the largest in our compounds, though lower than that one calculated for BDT-TTP. In conclusion, our investigations have shown that the frequencies of CyC stretching modes of TTM-TTP (or TSM-TTP) molecules depend linearly on the charge on these molecules. Therefore, both IR and Raman spectroscopy can be used as easy and accurate method for determination of charge on TTM-TTP (or TSM-TTP) in their complexes as well as for observation of charge ordering effects inside conducting TTM-TTP (or TSMTTP) stacks. The size of the studied molecules is larger than other TTF-derivatives (e.g. BEDT-TTF or BEDO-TTF), therefore, the positions of CyC stretching bands are less sensitive on molecular charge.
References
Fig. 6. Dependence of the IR frequencies of n22(b1u) mode (upper panel) and the Raman frequencies of n2(ag) n3(ag) and n4(ag) modes (lower panel) versus the charge on donor molecules.
[1] T. Mori, Y. Misaki, H. Nishikawa, K. Kawakami, T. Yamabe, H. Mori, S. Tanaka, Synth. Met. 70 (1995) 1179. [2] Y. Misaki, H. Nashikawa, T. Yamabe, T. Mori, H. Mori, S. Tanaka, Synth. Met. 70 (1995) 1153. [3] T. Mori, H. Inokuchi, Y. Misaki, T. Yamabe, H. Mori, S. Tanaka, Bull. Chem. Soc. Jpn 67 (1994) 661. [4] T. Mori, T. Kawamoto, J. Yamaura, T. Enoki, Y. Misaki, T. Yamabe, H. Mori, S. Tanaka, Phys. Rev. Lett. 79 (1997) 1702. [5] T. Kawamoto, T. Mori, T. Yamamoto, H. Tajima, Y. Misaki, K. Tanaka, J. Phys. Soc. Jpn 60 (2000) 4066. [6] T. Mori, Y. Misaki, T. Yamabe, Bull. Chem. Soc. Jpn 70 (1997) 1809. [7] T. Kawamoto, T. Mori, Y. Misaki, K. Tanaka, H. Mori, S. Tanaka, Synth. Met. 103 (1999) 1829. [8] T. Mori, T. Kawamoto, Y. Misaki, K. Tanaka, Bull. Chem. Soc. Jpn 71 (1998) 1321. [9] T. Kawamoto, T. Mori, Y. Misaki, K. Tanaka, H. Mori, S. Tanaka, Physica C 299 (1998) 36. [10] J. Moldenhower, Ch. Horn, K.I. Pokhodnia, D. Schweitzer, I. Heinen, H.J. Keller, Synth. Met. 60 (1993) 31. [11] H.H. Wang, J.R. Ferraro, J.M. Williams, U. Geiser, J. Schluter, J. Chem. Soc. Chem. Commun. (1994) (1994) 1893. [12] O. Drozdova, H. Yamochi, K. Yakushi, M. Uruichi, S. Horiuchi, G. Saito, J. Am. Chem. Soc. 122 (2000) 4436.
R. S´wietlik et al. / Journal of Molecular Structure 704 (2004) 89–93 [13] K. Yamamoto, K. Yakushi, K. Miyagawa, K. Kanoda, A. Kawamoto, Phys. Rev. B65 (2002) 085110. [14] J. Ouyang, K. Yakushi, T. Kinoshita, N. Nanbu, M. Aoyagi, Y. Misaki, K. Tanaka, Spectrochim. Acta A58 (2002) 1643. [15] H. Tajima, M. Arifuku, T. Ohta, T. Mori, Y. Misaki, T. Yamabe, H. Mori, S. Tanaka, Synth. Met. 71 (1995) 1951.
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[16] T. Kawamoto, M. Ashizawa, M. Aragaki, T. Mori, T. Yamamoto, H. Tajima, H. Kitagawa, T. Mitani, Y. Misaki, K. Tanaka, Phys. Rev. B60 (1999) 4635. [17] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, 1990.
Infrared and Raman studies of TTM-TTP and TSM-TTP charge-transfer salts R. S´wietlika,b,*, K. Yakushib, K. Yamamotob, T. Kawamotoc, T. Moric a
Polish Academy of Sciences, Institute of Molecular Physics, ul. Mariana Smoluchowskiego 17, Poznan´ 60-179, Poland b Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan c Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 17 November 2003; accepted 14 January 2004 Available online 10 May 2004
Abstract We measured room temperature infrared and Raman spectra of the neutral TTM-TTP molecule and four conducting charge-transfer salts: (TTM-TTP)I3, (TTM-TTP)AuI2, (TTM-TTP)(I3)5/3 and (TSM-TTP)(I3)5/3. The vibrational bands related to the CyC stretching modes were analysed. The frequencies of CyC stretching modes observed both in infrared and Raman spectra depend linearly upon the charge on TTMTTP (or TSM-TTP) molecules. q 2004 Elsevier B.V. All rights reserved. Keywords: Organic conductors; TTM-TTP and TSM-TTP salts; IR and Raman spectroscopy; CyC stretching vibrations
1. Introduction The bis-fused TTF (tetrathiafulvalene) molecule and its derivatives are good electron donors for synthesis of new conducting ion-radical salts with interesting physical properties [1]. One of these derivatives, TTMTTP [2,5-bis(4,5-bis(methylthio)-1,3-dithiol-2-ylidene)1,3,4,6-tetrathia-pentalene] (Fig. 1), yields mostly quasi-one-dimensional semiconductors, because four S-CH3 groups attached to the bis-fused TTF skeleton separate efficiently neighbouring TTM-TTP stacks [2]. Nevertheless, some TTM-TTP salts exhibit metallic properties. The charge-transfer salt (TTM-TTP)I3 was reported as the first organic metal with 1:1 stoichiometry and a highly onedimensional half-filled energy band (TTM-TTP has a charge þ 1) [3,4]. It was an important discovery since, usually the 1:1 salts are semiconductors, owing to large values of the onsite Coulomb interaction energies, which localise electrons. On the other hand, the salt (TTM-TTP)AuI2 also has 1:1 composition, but it shows semiconducting behaviour [5]. The salt (TTM-TTP)(I3)5/3 is an organic metal with even higher * Corresponding author. Address: Polish Academy of Sciences, Institute of Molecular Physics, ul. Mariana Smoluchowskiego 17, Poznan´ 60-179, Poland. Tel.: þ 48-61-8695165; fax: þ48-61-8684524. E-mail address: [email protected] (R. S´wietlik). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.01.050
oxidation degree of TTM-TTP molecules (þ 5/3) [6,7]. Analogous compound based on TSM-TTP donor (Fig. 1), namely (TSM-TTP)(I3)5/3, also shows metallic properties [8,9] but a TSM-TTP metal of 1:1 stoichiometry was not obtained. It is well known that frequencies of vibrational modes of a molecule are dependent on average charge residing on this molecule, therefore, vibrational spectroscopy was often used for determination of the charge distribution in organic conductors [10 –13]. For TTF derivatives especially sensitive for charge are modes related to the CyC stretching vibrations. In particular, it was shown that the frequencies of CyC modes depend linearly (or nearly linearly) on charge density for conducting charge-transfer salts based on BEDT-TTF [10,11,13] and BEDO-TTF [12] donors. On increasing the average oxidation state of BEDT-TTF (or BEDO-TTF), the frequencies of various CyC bands shift towards lower frequency on average about 95 and 60 cm21 (or 110 and 75 cm21) per charge þ 1e, respectively. The molecular charge was usually determined by using Raman spectroscopy, but also IR active vibrations can be very useful. It is important that in organic conductors, the frequencies of totally symmetric, CyC Raman active modes can be disturbed not only by environmental (crystal field) effects but also by coupling with electrons [13].
R. S´wietlik et al. / Journal of Molecular Structure 704 (2004) 89–93
90
Fig. 1. Molecular structures of TTM-TTP and TSM-TTP.
The coupling with electrons is not expected for nonsymmetric CyC modes which are IR active. The skeleton of TTM-TTP and TSM-TTP molecules is formed by the bis-fused TTF, i.e. the BDT-TTP molecule [2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathia-pentalene]. The fundamental in-plane modes of BDT-TTP were assigned in Ref. [14] and these data can be used for discussion of the CyC stretching vibrations of TTM-TTP and TSM-TTP. There are three types of non-equivalent CyC bonds in these molecules: one inner-ring bond, two CyC bridges and two outer-ring bonds (Fig. 1). Spectroscopic studies showed that within the region of CyC stretching, the neutral BDT-TTP has three Raman active modes of ag symmetry and two IR active modes of b1u symmetry: n2(ag) ¼ 1555 cm21, n3(ag) ¼ 1525 cm21, n4(ag) ¼ 1504 cm21, n21(b1u) ¼ 1550 cm21, n22(b1u) ¼ 1518 cm21 [14]. Various CyC modes are significantly mixed in neutral BDT-TTP but they are rather separated in BDT-TTPþ cations. Therefore, for BDT-TTPþ cations: the n2(ag) and n3(ag) are mainly assigned to the in-phase CyC stretching of outer-rings and bridges, respectively; the n4(ag) is assigned to the CyC inner-ring stretching; and the n21(b1u) and n22(b1u) are assigned to the out-ofphase CyC stretching of outer rings and bridges, respectively. Calculations of the neutral BDT-TTP0 molecule and its radical cation BDT-TTPþ have shown that all the CyC modes exhibit large frequency downshift when the molecule is oxidised [14]. Evidently, these modes should also be strongly sensitive to ionisation degree in the case of TTM-TTP and TSM-TTP molecules. We studied the IR and Raman spectra of selected TTM-TTP and TSM-TTP salts to examine modifications of their vibrational spectra due to increase of the donor oxidation state. The following compounds were measured: neutral TTM-TTP0 molecules; salts (TTM-TTP)I3 and (TTM-TTP)AuI2 containing TTM-TTPþ1 ions; as well as salts (TTM-TTP)(I 3) 3/5 and (TSM-TTP)(I 3) 3/5 with TTM-TTP þ5/3 and TSM-TTPþ5/3 ions, respectively. Our attention was mainly focused on vibrational bands related to the CyC stretching vibrations to examine their modifications upon increasing molecular charge.
2. Experimental Single crystals of the studied charge-transfer salts were prepared by electrocrystallisation by methods previously described in the following Refs: [3] for (TTM-TTP)I3, [5]
for (TTM-TTP)AuI2, [6] for (TTM-TTP)(I3)5/3 and [8] for (TSM-TTP)(I3)5/3. The IR reflectance spectra were measured using a FT-IR spectrometer Nicolet Magna 760, equipped with an IR microscope Spectratech IR, in the frequency region from 600 to 10,000 cm21. If possible, the spectra were recorded from various crystal faces in two perpendicular polarisations, i.e. the electrical vector of the polarised IR beam was either parallel or perpendicular to TTM-TTP (or TSM-TTP) stacking direction. However, in the analysis we focused our attention on the spectra obtained for polarisation perpendicular to the stacks, since the IR active CyC modes were mainly observed for this polarisation. Additionally, we studied the IR absorption spectra of TTM-TTP neutral molecules by using the standard KBr pellet technique. The optical conductivity spectra were determined by Kramers – Kronig analysis of the reflectance data. The low-frequency data were extrapolated to zero frequency assuming a constant reflectance. The high-frequency data were extended up to about 30,000 cm21 on basis of the available reflectance spectra from Refs. [5,15,16] and above a constant reflectance was assumed. The Raman spectra were studied using the backscattering geometry on a Renishaw Ramascope System 1000 composed of a notch filter, single monochromator and CCD detector cooled by thermoelectric device. For excitation we used mainly He –Ne laser (l ¼ 632:6 nm). To avoid the sample overheating, the laser power was strongly reduced (usually below about 250 mW) Each spectrum was collected at room temperature with about 30 – 60 min accumulation.
3. Results and discussion The reflectance spectra of the (TSM-TTP)(I3)5/3 salt at room temperature are shown in Fig. 2. The spectrum obtained from the crystal face (001) for polarisation Eka is very similar to that one reported in Ref. [16]. Unfortunately, for this crystal face we could not obtain any good spectrum for perpendicular polarisation ðE ’ aÞ because of interference effects in too thin crystals (similar effect was also mentioned in Ref. [16]). However, the IR active CyC modes cannot be observed by measuring reflectance from this crystal face since the TSM-TTP long axis is nearly perpendicular to it. These CyC bands can be found in reflectance spectra taken from another crystal face, for example the face (010), and we succeeded to measure such spectrum: it is shown in the lower panel of Fig. 2 and the optical conductivity data in Fig. 3. Analogously, by investigating the reflectance from various crystal faces of the salt (TTM-TTP)(I3)5/3 we were able to record bands attributed to the IR active CyC modes of TTM-TTP molecule. Our spectra of (TTM-TTP)I3 and (TTM-TTP)AuI2 salts were very similar to those reported in Refs. [15,5], respectively. The spectra within the CyC stretching region are displayed in Fig. 4. In the absorption spectrum of neutral
R. S´wietlik et al. / Journal of Molecular Structure 704 (2004) 89–93
Fig. 2. Polarised reflectance spectra of (TSM-TTP)(I3)5/3 salt obtained from the crystal face (001) (upper panel) and the (010) face (lower panel).
TTM-TTP, the doublet at 1491 and 1507 cm21 is assigned to n22(b1u) mode and the band at 1547 cm21 is most probably due to n21(b1u) mode. The two doublets at 1417, 1426 cm21 and at 1307, 1312 cm21 we assign to the asymmetric and symmetric bending mode of the methyl group in S-CH 3, respectively. In S-CH3 group, the asymmetric bending is usually observed at 1414 – 1440 cm21 and symmetric bending usually at 1290 –1330 cm21 [17]. On increasing the ionisation degree of TTM-TTP molecule ðrÞ; the frequency of n22(b1u) band decreases nearly linearly by about 98 cm21 per charge þ 1e (Table 1 and Fig. 6). Unfortunately, we could not assign
91
Fig. 4. Optical conductivity data of the studied salts for polarisation perpendicular to the stacking direction and IR absorption spectrum of the neutral TTM-TTP molecule dispersed in KBr pellet within the region of CyC stretching vibrations (r is the ionisation degree of molecules).
unambiguously the n21(b1u) band in the reflectance spectra of the salts. The line at 1300 cm21 for (TTM-TTP)(I3)5/3 and at 1270 cm21 for (TSM-TTP)(I3)5/3 are assigned to the symmetric bending vibration of methyl groups in S-CH3 and Se-CH3, respectively. The appropriate asymmetric bending mode is observed at about 1417 cm21 for (TTM-TTP)I3 and (TTM-TTP)AuI2 but is not clearly seen in the spectra of (TTM-TTP)(I3)5/3 and (TSM-TTP)(I3)5/3. The Raman data within the CyC stretching region are displayed in Fig. 5. For each salt we observe three Raman lines related to n2(ag), n3(ag) and n4(ag) modes of TTM-TTP (or TSM-TTP), respectively. For neutral TTM-TTP molecule, the n2(ag) and n4(ag) bands show a doublet structure, probably due to the crystal field effect. When Table 1 Positions of vibrational bands assigned to the CyC stretching modes in IR and Raman spectra of TTM-TTP and TSM-TTP salts Compound
TTM-TTP
Fig. 3. Optical conductivity spectra of (TSM-TTP)(I3)5/3 as obtained by Kramers– Kronig analysis of the reflectance data for the crystal face (010).
(TTM-TTP)I3 (TTM-TTP)AuI2 (TTM-TTP)(I3)5/3 (TSM-TTP)(I3)5/3
Charge r (þ e)
0 0 1 1 5/3 5/3
Frequency (cm21)
n22(b1u)
n2(ag)
n3(ag)
n4(ag)
1507 1491 1395 1403 1337 1332
1561 1558 1486 1486 1454 1452
1512
1493 1487 1420 1424 1381 1386
1453 1455 1420 1419
92
R. S´wietlik et al. / Journal of Molecular Structure 704 (2004) 89–93
Fig. 5. Raman spectra of the studied compounds within the region of CyC stretching (r is the ionisation degree of molecules).
the charge on donor molecules increases, all the CyC Raman bands shift nearly linearly towards lower frequencies by about 64 cm21 for n2(ag), 56 cm21 for n3(ag) and 63 cm 21 for n4 (a g), when charge grows by þ 1e.
The frequencies of various CyC modes are collected in Table 1 and their dependence upon the charge is shown in Fig. 6. Our spectroscopic data are to be compared with the analogous frequency shifts for BDT-TTP molecule. The calculation of normal modes of BDT-TTP0 and BDT-TTPþ shows that the Raman active modes decrease their frequencies by: 30 cm21 for n2(ag), 70 cm21 for n3(ag) and 99 cm21 for n4(ag) mode; whereas the IR active modes shift down by: 25 cm21 for n21(b1u) and 126 cm21 for n22(b1u) mode [14]. Our experimental study shows that for TTM-TTP (or TSM-TTP) molecule all the ag modes show approximately the same low-frequency shift (about 60 cm21) when the molecule is oxidised. It is possible that this surprising discrepancy is yielded by the calculation method used for BDT-TTP. On the other hand, the shift of n22(b1u) mode is also the largest in our compounds, though lower than that one calculated for BDT-TTP. In conclusion, our investigations have shown that the frequencies of CyC stretching modes of TTM-TTP (or TSM-TTP) molecules depend linearly on the charge on these molecules. Therefore, both IR and Raman spectroscopy can be used as easy and accurate method for determination of charge on TTM-TTP (or TSM-TTP) in their complexes as well as for observation of charge ordering effects inside conducting TTM-TTP (or TSMTTP) stacks. The size of the studied molecules is larger than other TTF-derivatives (e.g. BEDT-TTF or BEDO-TTF), therefore, the positions of CyC stretching bands are less sensitive on molecular charge.
References
Fig. 6. Dependence of the IR frequencies of n22(b1u) mode (upper panel) and the Raman frequencies of n2(ag) n3(ag) and n4(ag) modes (lower panel) versus the charge on donor molecules.
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