- Email: [email protected]
Spectral properties of the TCNQ anion radical salt (N-Me-2,5-(Me)2-Pz)(TCNQ)2
Available online at www.sciencedirect.com
Synthetic Metals 158 (2008) 246–250
Spectral properties of the TCNQ anion radical salt (N-Me-2,5-(Me)2-Pz)(TCNQ)2 B. Barszcz a,∗ , A. Graja a , D.V. Ziolkovskiy b , V.A. Starodub c a
Institute of Molecular Physics, Polish Academy of Science, ul. Smoluchowskiego 17, 60-179 Pozna´n, Poland b V. Karazin Kharkiv National University, 61077 Kharkiv, Ukraine c Institute of Chemistry, Jan Kochanowski University, ul. Ch˛ eci´nska 5, 25-020 Kielce, Poland Received 19 November 2007; accepted 18 January 2008 Available online 4 March 2008
Abstract We present the optical conductivity, infrared- and Raman-active charge sensitive phonon modes of the new 7,7 ,8,8 -tetracyanoquinodimethane (TCNQ) anion radical salt containing N-methyl-2,5-dimethyl-pirazinium cations (N-Me-2,5-(Me)2 -Pz)+ . Comparison of the calculated frequencies of the totally symmetric TCNQ modes with corresponding vibrations in the (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 salt is given. An attribution of the main bands observed in the spectra of the salt is offered and discussed. The charges estimation with both, structural and spectroscopic methods is in good agreement with each other and confirms almost full charge transfer from cation to TCNQ species. © 2008 Elsevier B.V. All rights reserved. Keywords: TCNQ anion radical salt; Pirazinium-derived cation; IR and Raman spectra; Optical conductivity
1. Introduction Extreme interest in organic low-dimensional conductors observed in the 1970s of the past century born such achievements as experimental verification of theoretically anticipated extraordinary behaviors of one-dimensional solids [1,2], phase transitions specific for low-dimensional molecular materials [1–4], and first organic superconductors [5,6]. However, contrary to expectations [7,8], the low-dimensional organic conductors do not meet broad applications. Even discovery of melting conducting organic materials based on anion radical salts of 7,7 ,8,8 -tetracyanoquinodimethane (TCNQ) [9,10] do not give enormous interest in organic conductors. Renewed and huge interest in organic TCNQ-based low-dimensional system is observed recently. One of the most important reasons of this interest are new chances for applications of organic semiconductors and synthetic metals as molecular diodes, biosensors, photochromic systems, light-emitting devices, controllable nanoribbons, and specific homopolymers.
∗
Corresponding author. E-mail address: [email protected] (B. Barszcz).
0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.01.010
The studies of organic conductors have mainly been carried out in the solid state, while not so much investigations have done in the melt state. In order to develop and better detect this state of low-dimensional systems one can investigate molecular conductors with low melting point. Some TCNQ anion radical salts with the partial charge transfer (CT) states, which are known as highly electronic-conductive materials, are suitable for such studies [11–13]. In general, asymmetric cations, such as quinolinium, imidazolium or pyrazinium derivatives give TCNQ anion radical salts with relatively low melting points [13,14]. In these salts the electronic conductivity extremely increases by melting. Such compounds may be employed in the development of a new type electrolyte metaloxide capacitors [15,16]. Several complex TCNQ anion radical salts with Nalkylpyrazinium cations have been recently synthesized and characterized [17–20]. In such salts certain additional interactions via non-alkylated nitrogen atom of pyrazine between TCNQ anion-radical stacks and cations are possible, and may lead for this class of TCNQ salts to the appearance of unusual structures. All these compounds have layered structure where conducting anion radical layers formed by TCNQ alternate with non-conducting cation layers. However, the anion layers consist of TCNQ anions of various organizations (two or three
B. Barszcz et al. / Synthetic Metals 158 (2008) 246–250
247
Scheme 1. Chemical formulas of the components of investigated anion radical salt: N-Me-2,5-(Me)2 -Pz+ cation and TCNQ molecule.
nonequivalent and differently oriented TCNQ species) and alternate with cations layers along b- or c-axes. The main goal of this work is spectral characterization of the anion radical (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 salt containing Nmethyl-2,5-dimethyl-pirazinium cations (N-Me-2,5-(Me)2 -Pz)+ and TCNQ anions. 2. Experimental As it was described by Kazheva et al. [19] the TCNQ anion radical salt (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 (chemical formulas of the components are presented in Scheme 1) was prepared according to reactions: MeI + 2, 5-(Me)2 -Pz → (N-Me-2, 5-(Me)2 -Pz)I
Fig. 1. Experimental Raman spectrum of (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 single crystals and calculated spectra of the components: (N-Me-2,5-(Me)2 -Pz)+ cation and TCNQ0 molecule. Calculations were carried out using B3LYP hybrid functional and 6-311++G(d,p) basis set. Arrows represent assignment of bands. Intensities of the calculated spectra were changed for clarity.
was made by visual inspection of the individual modes using the GaussView program. The normal modes assignment was performed based on the experimental and theoretical data. 3. Results and discussion
1.5(N-Me-2, 5-(Me)2 -Pz)I + 2TCNQ → (N-Me-2, 5-(Me)2 -Pz)(TCNQ)2 + 0.5(N-Me-2, 5-(Me)2 Pz)(I3 ) Precipitate of the salt was filtered and washed with diethyl ether and acetone mixture. In order to grow single crystals, recrystallization from acetonitrile and acetone was performed. Black-violet needle crystals with length up to 25 mm has been obtained. The room temperature Raman spectra of the (N-Me-2,5(Me)2 -Pz)(TCNQ)2 single crystals were collected with a Horiba Jobin Yvon LabRam HR800 spectrometer with excitation beam (λex = 633 nm) from the He–Ne laser. The power of the beam at the sample was less than 1 mW to avoid damages of the sample. The infrared reflection spectra of the samples were recorded in the frequency range from 400 to 7500 cm−1 , at room temperature. These spectra were collected for two polarizations of the infrared beam: parallel to the stacking axis of TCNQ and perpendicular to it. The maximum of reflected energy is observed for the direction of TCNQ stacks. These investigations were performed with FT-IR Bruker Equinox 55 spectrometer equipped with Hyperion 1000 microscope. The ab initio calculations of normal modes frequencies of the TCNQ neutral molecule and (N-Me-2,5-(Me)2 -Pz)+ cation were performed using the Gaussian 03 program [21]. Calculations were carried out on an isolated molecule using the density functional theory (DFT) with Becke’s three parameter exchange functional combined with the Lee–Yang–Parr correlation functional (B3LYP). The 6-311++G(d,p) basis set was used for optimizations and calculations. The mode description
The experimental Raman spectrum of the investigated crystal together with theoretical Raman spectra of TCNQ molecule and (N-Me-2,5-(Me)2 -Pz)+ cation are presented in Fig. 1. Usually, in the spectra of TCNQ salts one can observe mainly the bands related to the vibrations of TCNQ molecule. In the case of (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 salt we have similar situation. Its Raman spectra are dominated by totally symmetric (Ag ) modes of the TCNQ. However, we observe some bands related to the cation. Table 1 summarizes the positions of the Ag modes of TCNQ observed in the salt and calculated by us for neutral TCNQ molecule. The first of them (ν1 (Ag ) mode) is symmetrical stretching of C–H bonds. In appropriate spectral range we observe three well resolved but not very intensive bands at 3074, 3062, and 2936 cm−1 . The band at 3062 cm−1 is probably due to the ν1 (Ag ) mode (because of the intensity). The band at 3074 cm−1 could be assign to the C–H stretching vibration of TCNQ with B3g symmetry. The reason of such assignment is that this band is present in the calculated spectrum at 3190 cm−1 . The band at 2936 cm−1 can be assigned to the C–H stretching vibrations of the (N-Me-2,5-(Me)2 -Pz)+ cation. This spectral region is shown in Fig. 2; where the optical conductivity spectrum together with the Raman spectrum are presented. In the optical conductivity spectrum we observed the mentioned Ag mode at 3053 cm−1 and the B3g mode at 3068 cm−1 . These modes are activated by electron-molecular vibration (EMV) coupling. The coupling is typical for Ag modes but also possible for modes with other symmetry. If the dips in the IR absorption (in our case—optical conductivity) spectrum are located at almost the same positions as the bands in the Raman spectrum, the EMV coupling is not linear and there is a possibility to activate not
248
B. Barszcz et al. / Synthetic Metals 158 (2008) 246–250
Table 1 Totally symmetric vibrations of the neutral TCNQ molecule and corresponding bands observed in the spectra of (Me-2,5-di-Pz)(TCNQ)2 Symmetry species (Ag )
1 2 3 4 5 6 7 8 9 10
TCNQ0 calculateda
TCNQ0 literature [23]
TCNQ−1 literature [27]
(Me-2,5-di-Pz)(TCNQ)2
ν
A
ν
ν
Raman
IRb
3204 2325 1665 1489 1231 971 724 615 335 120
152.2 4408.8 3875.7 6101.3 1023.6 123.6 132.8 54.7 91.2 43.9
3048 2229 1602 1454 1207 948 711 602 334 144
2220 1610 1396 1184 974 729 620 347 143
3062 2206 1600 1422 1188 967 720 605 331 145
3053 2149 1553 1284 1094 952 686 592 – –
˚ 4 /amu) (A
All frequencies in cm−1 . a D symmetry point group, B3LYP/6-311++G(d,p), theoretical IR intensities for A modes are 0. 2h g b Positions in the IR spectrum were taken from the optical conductivity spectra.
fully symmetric modes (e.g. B3g ) [22]. It explains appearance of the second band near the ν1 (Ag ) mode in the conductivity spectrum. The small band at 2855 cm−1 in optical conductivity spectrum can be assigned to ν(CH3 ) mode of the cation. From the other side of view another assignment of the band at 3074 cm−1 in the Raman spectra is also possible. X-ray analysis shows that the salt has a layered structure, where cation layers alternate along the c-axis with the layers consisting of TCNQ anion-radicals (Fig. 3) [19]. Anions form stacks along the a-axis, where they form pairs characterized by two ways of overlapping (Fig. 4). Within the pairs the molecules are eclipsed in a higher degree than between the pairs. That is why we can speak about dimerization of TCNQ in stacks, despite the fact that interplanar distances within and between the pairs are practically the ˚ respectively. So, if TCNQ species are same—3.24 and 3.27 A,
Fig. 2. Comparison of the bands positions in the Raman and optical conductivity spectra of (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 single crystals in the region of ν1 (Ag ) mode of TCNQ molecule. The polarization of the optical conductivity spectra corresponds to the direction of TCNQ stacks.
dimerized, splitting of the active bands is possible and second band near the ν1 (Ag ) mode can be treated as splitting component of ν1 (Ag ). In such case we should observe this splitting for all other modes. The strongest band in the Raman spectrum located at 2206 cm−1 is due to ν2 (Ag ) mode. This band has a second component at 2213 cm−1 which is most probably related to the mode with a B3g symmetry located at 2311 cm−1 in the calculated spectrum. Additional, less intensive bands at 2183 and 2156 cm−1 are probably due to the ν2 (Ag ) mode disturbed by different crystal environment of TCNQ acceptor. The ν2 (Ag ) mode involves the external C N bonds and for that reason it is sensitive to the neighborhood of the molecule. Near the band corresponding to ν3 (Ag ) mode (1600 cm−1 in the experimental Raman spectrum) we observe two bands at 1623 and 1573 cm−1 . Comparing the positions of the bands with the calculated ones of
Fig. 3. Layers of (N-Me-2,5-(Me)2 -Pz)+ cations and TCNQ− anions in (N-Me2,5-di-Pz)(TCNQ)2 structure. Dashed lines show short contacts between atoms (distances shorter than sum of Van der Waals radii).
B. Barszcz et al. / Synthetic Metals 158 (2008) 246–250
Fig. 4. Stacks of TCNQ− anions in the organic, conducting layer. Inclination of some TCNQ species is seen. Dashed lines show short contacts between atoms (distances shorter than sum of Van der Waals radii).
(N-Me-2,5-(Me)2 -Pz)+ cation, we suggest that these bands are due to the vibrations of the cation (mainly in-plane C–N stretching together with C–N–C bending, calculated wavenumbers are 1641 and 1588 cm−1 , respectively). It is well known that the position of the Ag modes depends on the average charge on the molecule. In our case the position of the ν3 (Ag ) mode is almost identical as for neutral TCNQ (1602 cm−1 [23]) while the position of the ν2 (Ag ) mode is exactly the same as for TCNQ− anion (2206 cm−1 [24]). However, we do not observe the splitting of the Raman bands characteristic for the situation when in the crystal structure molecules with a different charge occur. This lead to the conclusion that the ν2 (Ag ) and ν3 (Ag ) modes are not the best indicators of the average charge on the TCNQ molecule. The most convenient way to estimate an average charge on TCNQ molecule is to use ν4 (Ag ) mode. This mode is related to the stretching of C C bonds and this is why its position is very sensitive to average charge located on the molecule [25]. In our case the band position at 1422 cm−1 corresponds to the average charge of −0.51e on each TCNQ molecule. The shift between positions of the bands in the Raman and conductivity spectra is also larger for ν4 (Ag ) than for previous modes (see Fig. 5). The band at 1379 cm−1 is probably due to the vibration of CH3 group of the cation—see the calculated spectrum in Fig. 1 at 1419 cm−1 (it is kind of scissoring mode, but for CH3 not CH2 group). Two bands at 1306 and 1261 cm−1 can be also assigned to the cation (C–CH3 stretching together with ring deformation modes at 1294 and 1270 cm−1 ) or one of them is due to C C–H bending mode of the TCNQ molecule (B3g symmetry, calculated wavenumber 1340 cm−1 ). In the case of ν5 (Ag ) mode we observe similar situation as for ν1 (Ag ) mode; there are two Raman lines at 1188 and 1166 cm−1 . The first one is the ν5 (Ag ) mode and the second one is a TCNQ normal mode of B3g symmetry located at 1202 cm−1 in the calculated spectrum. Both these modes are activated by EMV coupling and visible in the conductivity spectrum at 1094 and 1078 cm−1 , respectively. The band at 967 cm−1 is related to ν6 (Ag ) mode of TCNQ molecule.
249
Fig. 5. Optical conductivity spectra of (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 single crystals obtained by the Kramers–Kr¨onig analysis of the reflection data recorded for the direction parallel to the TCNQ stacks. For comparison, the calculated spectra of the components: (N-Me-2,5-(Me)2 -Pz)+ cation and TCNQ0 molecule are also given. Calculations were carried out using B3LYP hybrid functional and 6-311++G(d,p) basis set. Intensities of the calculated spectra were changed for clarity.
The position of this band is between the positions typical for neutral (948 cm−1 ) and charged (978 cm−1 ) TCNQ. It can suggest that the estimation of the average charge from the position of ν4 (Ag ) mode is correct, but in this case it is not so obvious because we observe also the additional band at 956 cm−1 in the experimental Raman spectrum. What is really interesting, that in this spectral region we can find also two bands in the optical conductivity spectrum at 963 and 952 cm−1 . Anyway, the assignment of these two features in the IR is not certain. The next band in the Raman spectrum, located at 926 cm−1 , is probably related to the in-plane C N–C bending and C–N stretching vibration of the cation observed at 942 cm−1 in the calculated spectrum. The distinct band at 787 cm−1 is also due to the cation vibration (inplane ring deformation). The ν7 (Ag ) mode of TCNQ is observed at 720 cm−1 and this position is again between the positions for neutral (711 cm−1 ) and charged (725 cm−1 ) species. This feature is also very intensive in the IR spectrum. The ν8 (Ag ) is very weak in the Raman spectrum but well visible in the IR. However, the theoretical Raman activity of this mode is relatively low (see Table 1)—it explains low band intensity. On the other hand, the ν10 (Ag ) mode has smaller activity but an adequate band is very intensive in the experiment. This can be explain by the fact that the ν10 (Ag ) mode has relatively low wavenumber. The intensities of the bands in low frequency region of the Raman spectra can be strongly disturbed by the neighborhood of the Rayleigh peak. Of course, we should also remember that the calculations are approximate (the environment of the molecule is neglected, the vibrations are harmonic, and so on). In the experimental Raman spectrum of the salt there are still a few undiscussed features. The bands at 561 and 510 cm−1 are related to the normal modes of the cation (in-plane ring deformations) located at exactly the same wavenumbers in the calculated spectrum. We are not sure about the origin of the band at 227 cm−1 but there is a possibility that it is also due to the cation vibrations—in this region there are a few normal modes with very low theoretical activity
250
B. Barszcz et al. / Synthetic Metals 158 (2008) 246–250
(mainly out-of-plane deformations). As it was mentioned earlier, the normal modes of the TCNQ are dominating in the spectra. Despite of that we were able to observe some bands not related to it in the Raman spectrum. The strong EMV coupling causes that in the conductivity spectrum (Fig. 5) we observe almost only the Ag modes activated by the coupling to electrons. They are strong, broad and cover almost any other, IR active vibrations. The exception could be the band at 835 cm−1 which is related to the IR active mode of TCNQ molecule (out of plane bending of C–H bonds, calculated wavenumber 873 cm−1 ). Anyway, we do not observe any band that can be certainly assigned to the vibrations of the cation in the optical conductivity spectrum. The fact, that pseudo dimers of TCNQ contain equally charged and crystallographically equivalent species suggest, that the band splitting should be absent or scarcely detectable. This is in good agreement with very low intensity of additional components of the splitted bands in the Raman spectra, if it is observed in general. Using HOSE approach [26] and experimental bond length in TCNQ molecule we estimated the charge on anions as about −0.44 e. Both, structural and spectroscopic methods of the charge determination are approximate (with the error of a few percent) and neglects effects given by neighboring molecules. So we can state that both our charge estimations (−0.51 and −0.44) are in good agreement with each other and confirm almost full charge transfer from cation to TCNQ species. On the other hand it suggests an equal charge distribution between them. 4. Conclusions For the first time, the radical anion salt (N-Me-2,5-(Me)2 Pz)TCNQ2 has been characterized using infrared reflection and Raman spectroscopy. Comparing the experimental data with results of DFT calculations the assignment of observed bands has been proposed. Moreover, the average charge on the TCNQ molecule has been estimated from the spectroscopic and crystallographic data. Acknowledgments One of us (D.V. Ziolkovskiy) is grateful to J´ozef Mianowski Found, a Foundation for the Promotion of Sciences and the Foundation for Polish Science for support. This work was partially supported by the Ministry of Science and Higher Education (Warsaw) as the research project in the years 2007–2008. References [1] D. J´erome, H.J. Schulz, Adv. Phys. 31 (1982) 299. [2] L.G. Caron, in: J.-P. Farges (Ed.), Organic Conductors. Fundamentals and Applications, Marcel Dekker, Inc., New York/Basel/Hong Kong, 1994.
[3] R.E. Peierls, Quantum Theory of Solids, Oxford Press, London, 1955. [4] N.F. Mott, Metal-Insulator Transitions, Taylor and Francis, London, 1974. [5] D. J´erome, A. Mazaud, M. Ribault, K. Bechgaard, J. Phys. Lett. 41 (1980) L-95. [6] T. Ishiguro, K. Yamaji, G. Saito, Organic Superconductors, Springer, Berlin, 1998. [7] F.L. Carter (Ed.), Molecular Electronic Devices, Marcel Dekker, Inc., New York/Basel, 1982. [8] A. Graja, Low-Dimensional Organic Conductors, Word Scientific, Singapore/New Jersey/London/Hong Kong, 1992. [9] M. Tanaka, F. Urano, M. Nakabata, Japan Patent 60-139832 (1987). [10] V.A. Starodub, V.V. Barabashova, Ye.M. Gluzman et al., USSR Patent 1,389,226 (1987). [11] S. Niwa, Synth. Met. 18 (1987) 665. [12] V.A. Starodub, Ye.M. Gluzman, K.I. Pokhodnya, M.Ya. Valakh, Theor. Exp. Chem. 29 (1993) 240. [13] K. Nishimura, G. Saito, Synth. Met. 153 (2005) 385. [14] V.A. Starodub, Ye.M. Gluzman, Yu.A. Kaftanowa, I. Olejniczak, Theor. Exp. Chem. 33 (1997) 111. [15] V.A. Starodub, Ye.M. Gluzman, K.V. Krikunov et al., USSR Patent 1,696,428 (1991). [16] Sanyo Electric Co., US Patent No. 458,055 (1986). [17] D.V. Ziolkovskiy, A.V. Kravchenko, V.A. Starodub, O.N. Kazheva, A.V. Khotkevich, Funct. Mater. 12 (2005) 577. [18] D.V. Ziolkovskiy, O.N. Kazheva, G.V. Shilov, O.A. Dyachenko, A.V. Kravchenko, A.V. Khotkevich, V.A. Starodub, Funct. Mater. 13 (2006) 119. [19] O.N. Kazheva, D.V. Ziolkovskiy, G.G. Alexandrov, O.A. Dyachenko, V.A. Starodub, A.V. Khotkevich, G.Y. Vasilets, Funct. Mater. 13 (2006) 692. [20] O.N. Kazheva, D.V. Ziolkovskiy, G.G. Alexandrov, A.N. Chekhlov, O.A. Dyachenko, V.A. Starodub, A.V. Khotkevich, Synth. Met. 156 (2006) 1010. [21] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision B. 05, Gaussian, Inc., Pittsburgh, PA, 2003. [22] A.D. Bandrauk, K.D. Truong, C. Carlone, S. Jandl, K. Ishii, J. Phys. Chem. 89 (1985) 434. [23] A. Girlando, C. Pecile, Spectrochim. Acta 29 (1973) 1859. [24] R. Bozio, C. Pecile, in: L. Alc´acer (Ed.), The Physics and Chemistry of Low Dimensional Solids, D. Reidel Publishing Company, 1980, pp. 165– 185. [25] S. Matsuzaki, R. Kuwata, K. Toyoda, Solid State Commun. 33 (1980) 403. [26] T.M. Krygowski, R. Anulewicz, Acta Crystallogr. B 39 (1983) 732. [27] M.S. Khatkale, J.P. Devlin, J. Chem. Phys. 70 (4) (1979) 1851–1859.
Synthetic Metals 158 (2008) 246–250
Spectral properties of the TCNQ anion radical salt (N-Me-2,5-(Me)2-Pz)(TCNQ)2 B. Barszcz a,∗ , A. Graja a , D.V. Ziolkovskiy b , V.A. Starodub c a
Institute of Molecular Physics, Polish Academy of Science, ul. Smoluchowskiego 17, 60-179 Pozna´n, Poland b V. Karazin Kharkiv National University, 61077 Kharkiv, Ukraine c Institute of Chemistry, Jan Kochanowski University, ul. Ch˛ eci´nska 5, 25-020 Kielce, Poland Received 19 November 2007; accepted 18 January 2008 Available online 4 March 2008
Abstract We present the optical conductivity, infrared- and Raman-active charge sensitive phonon modes of the new 7,7 ,8,8 -tetracyanoquinodimethane (TCNQ) anion radical salt containing N-methyl-2,5-dimethyl-pirazinium cations (N-Me-2,5-(Me)2 -Pz)+ . Comparison of the calculated frequencies of the totally symmetric TCNQ modes with corresponding vibrations in the (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 salt is given. An attribution of the main bands observed in the spectra of the salt is offered and discussed. The charges estimation with both, structural and spectroscopic methods is in good agreement with each other and confirms almost full charge transfer from cation to TCNQ species. © 2008 Elsevier B.V. All rights reserved. Keywords: TCNQ anion radical salt; Pirazinium-derived cation; IR and Raman spectra; Optical conductivity
1. Introduction Extreme interest in organic low-dimensional conductors observed in the 1970s of the past century born such achievements as experimental verification of theoretically anticipated extraordinary behaviors of one-dimensional solids [1,2], phase transitions specific for low-dimensional molecular materials [1–4], and first organic superconductors [5,6]. However, contrary to expectations [7,8], the low-dimensional organic conductors do not meet broad applications. Even discovery of melting conducting organic materials based on anion radical salts of 7,7 ,8,8 -tetracyanoquinodimethane (TCNQ) [9,10] do not give enormous interest in organic conductors. Renewed and huge interest in organic TCNQ-based low-dimensional system is observed recently. One of the most important reasons of this interest are new chances for applications of organic semiconductors and synthetic metals as molecular diodes, biosensors, photochromic systems, light-emitting devices, controllable nanoribbons, and specific homopolymers.
∗
Corresponding author. E-mail address: [email protected] (B. Barszcz).
0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.01.010
The studies of organic conductors have mainly been carried out in the solid state, while not so much investigations have done in the melt state. In order to develop and better detect this state of low-dimensional systems one can investigate molecular conductors with low melting point. Some TCNQ anion radical salts with the partial charge transfer (CT) states, which are known as highly electronic-conductive materials, are suitable for such studies [11–13]. In general, asymmetric cations, such as quinolinium, imidazolium or pyrazinium derivatives give TCNQ anion radical salts with relatively low melting points [13,14]. In these salts the electronic conductivity extremely increases by melting. Such compounds may be employed in the development of a new type electrolyte metaloxide capacitors [15,16]. Several complex TCNQ anion radical salts with Nalkylpyrazinium cations have been recently synthesized and characterized [17–20]. In such salts certain additional interactions via non-alkylated nitrogen atom of pyrazine between TCNQ anion-radical stacks and cations are possible, and may lead for this class of TCNQ salts to the appearance of unusual structures. All these compounds have layered structure where conducting anion radical layers formed by TCNQ alternate with non-conducting cation layers. However, the anion layers consist of TCNQ anions of various organizations (two or three
B. Barszcz et al. / Synthetic Metals 158 (2008) 246–250
247
Scheme 1. Chemical formulas of the components of investigated anion radical salt: N-Me-2,5-(Me)2 -Pz+ cation and TCNQ molecule.
nonequivalent and differently oriented TCNQ species) and alternate with cations layers along b- or c-axes. The main goal of this work is spectral characterization of the anion radical (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 salt containing Nmethyl-2,5-dimethyl-pirazinium cations (N-Me-2,5-(Me)2 -Pz)+ and TCNQ anions. 2. Experimental As it was described by Kazheva et al. [19] the TCNQ anion radical salt (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 (chemical formulas of the components are presented in Scheme 1) was prepared according to reactions: MeI + 2, 5-(Me)2 -Pz → (N-Me-2, 5-(Me)2 -Pz)I
Fig. 1. Experimental Raman spectrum of (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 single crystals and calculated spectra of the components: (N-Me-2,5-(Me)2 -Pz)+ cation and TCNQ0 molecule. Calculations were carried out using B3LYP hybrid functional and 6-311++G(d,p) basis set. Arrows represent assignment of bands. Intensities of the calculated spectra were changed for clarity.
was made by visual inspection of the individual modes using the GaussView program. The normal modes assignment was performed based on the experimental and theoretical data. 3. Results and discussion
1.5(N-Me-2, 5-(Me)2 -Pz)I + 2TCNQ → (N-Me-2, 5-(Me)2 -Pz)(TCNQ)2 + 0.5(N-Me-2, 5-(Me)2 Pz)(I3 ) Precipitate of the salt was filtered and washed with diethyl ether and acetone mixture. In order to grow single crystals, recrystallization from acetonitrile and acetone was performed. Black-violet needle crystals with length up to 25 mm has been obtained. The room temperature Raman spectra of the (N-Me-2,5(Me)2 -Pz)(TCNQ)2 single crystals were collected with a Horiba Jobin Yvon LabRam HR800 spectrometer with excitation beam (λex = 633 nm) from the He–Ne laser. The power of the beam at the sample was less than 1 mW to avoid damages of the sample. The infrared reflection spectra of the samples were recorded in the frequency range from 400 to 7500 cm−1 , at room temperature. These spectra were collected for two polarizations of the infrared beam: parallel to the stacking axis of TCNQ and perpendicular to it. The maximum of reflected energy is observed for the direction of TCNQ stacks. These investigations were performed with FT-IR Bruker Equinox 55 spectrometer equipped with Hyperion 1000 microscope. The ab initio calculations of normal modes frequencies of the TCNQ neutral molecule and (N-Me-2,5-(Me)2 -Pz)+ cation were performed using the Gaussian 03 program [21]. Calculations were carried out on an isolated molecule using the density functional theory (DFT) with Becke’s three parameter exchange functional combined with the Lee–Yang–Parr correlation functional (B3LYP). The 6-311++G(d,p) basis set was used for optimizations and calculations. The mode description
The experimental Raman spectrum of the investigated crystal together with theoretical Raman spectra of TCNQ molecule and (N-Me-2,5-(Me)2 -Pz)+ cation are presented in Fig. 1. Usually, in the spectra of TCNQ salts one can observe mainly the bands related to the vibrations of TCNQ molecule. In the case of (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 salt we have similar situation. Its Raman spectra are dominated by totally symmetric (Ag ) modes of the TCNQ. However, we observe some bands related to the cation. Table 1 summarizes the positions of the Ag modes of TCNQ observed in the salt and calculated by us for neutral TCNQ molecule. The first of them (ν1 (Ag ) mode) is symmetrical stretching of C–H bonds. In appropriate spectral range we observe three well resolved but not very intensive bands at 3074, 3062, and 2936 cm−1 . The band at 3062 cm−1 is probably due to the ν1 (Ag ) mode (because of the intensity). The band at 3074 cm−1 could be assign to the C–H stretching vibration of TCNQ with B3g symmetry. The reason of such assignment is that this band is present in the calculated spectrum at 3190 cm−1 . The band at 2936 cm−1 can be assigned to the C–H stretching vibrations of the (N-Me-2,5-(Me)2 -Pz)+ cation. This spectral region is shown in Fig. 2; where the optical conductivity spectrum together with the Raman spectrum are presented. In the optical conductivity spectrum we observed the mentioned Ag mode at 3053 cm−1 and the B3g mode at 3068 cm−1 . These modes are activated by electron-molecular vibration (EMV) coupling. The coupling is typical for Ag modes but also possible for modes with other symmetry. If the dips in the IR absorption (in our case—optical conductivity) spectrum are located at almost the same positions as the bands in the Raman spectrum, the EMV coupling is not linear and there is a possibility to activate not
248
B. Barszcz et al. / Synthetic Metals 158 (2008) 246–250
Table 1 Totally symmetric vibrations of the neutral TCNQ molecule and corresponding bands observed in the spectra of (Me-2,5-di-Pz)(TCNQ)2 Symmetry species (Ag )
1 2 3 4 5 6 7 8 9 10
TCNQ0 calculateda
TCNQ0 literature [23]
TCNQ−1 literature [27]
(Me-2,5-di-Pz)(TCNQ)2
ν
A
ν
ν
Raman
IRb
3204 2325 1665 1489 1231 971 724 615 335 120
152.2 4408.8 3875.7 6101.3 1023.6 123.6 132.8 54.7 91.2 43.9
3048 2229 1602 1454 1207 948 711 602 334 144
2220 1610 1396 1184 974 729 620 347 143
3062 2206 1600 1422 1188 967 720 605 331 145
3053 2149 1553 1284 1094 952 686 592 – –
˚ 4 /amu) (A
All frequencies in cm−1 . a D symmetry point group, B3LYP/6-311++G(d,p), theoretical IR intensities for A modes are 0. 2h g b Positions in the IR spectrum were taken from the optical conductivity spectra.
fully symmetric modes (e.g. B3g ) [22]. It explains appearance of the second band near the ν1 (Ag ) mode in the conductivity spectrum. The small band at 2855 cm−1 in optical conductivity spectrum can be assigned to ν(CH3 ) mode of the cation. From the other side of view another assignment of the band at 3074 cm−1 in the Raman spectra is also possible. X-ray analysis shows that the salt has a layered structure, where cation layers alternate along the c-axis with the layers consisting of TCNQ anion-radicals (Fig. 3) [19]. Anions form stacks along the a-axis, where they form pairs characterized by two ways of overlapping (Fig. 4). Within the pairs the molecules are eclipsed in a higher degree than between the pairs. That is why we can speak about dimerization of TCNQ in stacks, despite the fact that interplanar distances within and between the pairs are practically the ˚ respectively. So, if TCNQ species are same—3.24 and 3.27 A,
Fig. 2. Comparison of the bands positions in the Raman and optical conductivity spectra of (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 single crystals in the region of ν1 (Ag ) mode of TCNQ molecule. The polarization of the optical conductivity spectra corresponds to the direction of TCNQ stacks.
dimerized, splitting of the active bands is possible and second band near the ν1 (Ag ) mode can be treated as splitting component of ν1 (Ag ). In such case we should observe this splitting for all other modes. The strongest band in the Raman spectrum located at 2206 cm−1 is due to ν2 (Ag ) mode. This band has a second component at 2213 cm−1 which is most probably related to the mode with a B3g symmetry located at 2311 cm−1 in the calculated spectrum. Additional, less intensive bands at 2183 and 2156 cm−1 are probably due to the ν2 (Ag ) mode disturbed by different crystal environment of TCNQ acceptor. The ν2 (Ag ) mode involves the external C N bonds and for that reason it is sensitive to the neighborhood of the molecule. Near the band corresponding to ν3 (Ag ) mode (1600 cm−1 in the experimental Raman spectrum) we observe two bands at 1623 and 1573 cm−1 . Comparing the positions of the bands with the calculated ones of
Fig. 3. Layers of (N-Me-2,5-(Me)2 -Pz)+ cations and TCNQ− anions in (N-Me2,5-di-Pz)(TCNQ)2 structure. Dashed lines show short contacts between atoms (distances shorter than sum of Van der Waals radii).
B. Barszcz et al. / Synthetic Metals 158 (2008) 246–250
Fig. 4. Stacks of TCNQ− anions in the organic, conducting layer. Inclination of some TCNQ species is seen. Dashed lines show short contacts between atoms (distances shorter than sum of Van der Waals radii).
(N-Me-2,5-(Me)2 -Pz)+ cation, we suggest that these bands are due to the vibrations of the cation (mainly in-plane C–N stretching together with C–N–C bending, calculated wavenumbers are 1641 and 1588 cm−1 , respectively). It is well known that the position of the Ag modes depends on the average charge on the molecule. In our case the position of the ν3 (Ag ) mode is almost identical as for neutral TCNQ (1602 cm−1 [23]) while the position of the ν2 (Ag ) mode is exactly the same as for TCNQ− anion (2206 cm−1 [24]). However, we do not observe the splitting of the Raman bands characteristic for the situation when in the crystal structure molecules with a different charge occur. This lead to the conclusion that the ν2 (Ag ) and ν3 (Ag ) modes are not the best indicators of the average charge on the TCNQ molecule. The most convenient way to estimate an average charge on TCNQ molecule is to use ν4 (Ag ) mode. This mode is related to the stretching of C C bonds and this is why its position is very sensitive to average charge located on the molecule [25]. In our case the band position at 1422 cm−1 corresponds to the average charge of −0.51e on each TCNQ molecule. The shift between positions of the bands in the Raman and conductivity spectra is also larger for ν4 (Ag ) than for previous modes (see Fig. 5). The band at 1379 cm−1 is probably due to the vibration of CH3 group of the cation—see the calculated spectrum in Fig. 1 at 1419 cm−1 (it is kind of scissoring mode, but for CH3 not CH2 group). Two bands at 1306 and 1261 cm−1 can be also assigned to the cation (C–CH3 stretching together with ring deformation modes at 1294 and 1270 cm−1 ) or one of them is due to C C–H bending mode of the TCNQ molecule (B3g symmetry, calculated wavenumber 1340 cm−1 ). In the case of ν5 (Ag ) mode we observe similar situation as for ν1 (Ag ) mode; there are two Raman lines at 1188 and 1166 cm−1 . The first one is the ν5 (Ag ) mode and the second one is a TCNQ normal mode of B3g symmetry located at 1202 cm−1 in the calculated spectrum. Both these modes are activated by EMV coupling and visible in the conductivity spectrum at 1094 and 1078 cm−1 , respectively. The band at 967 cm−1 is related to ν6 (Ag ) mode of TCNQ molecule.
249
Fig. 5. Optical conductivity spectra of (N-Me-2,5-(Me)2 -Pz)(TCNQ)2 single crystals obtained by the Kramers–Kr¨onig analysis of the reflection data recorded for the direction parallel to the TCNQ stacks. For comparison, the calculated spectra of the components: (N-Me-2,5-(Me)2 -Pz)+ cation and TCNQ0 molecule are also given. Calculations were carried out using B3LYP hybrid functional and 6-311++G(d,p) basis set. Intensities of the calculated spectra were changed for clarity.
The position of this band is between the positions typical for neutral (948 cm−1 ) and charged (978 cm−1 ) TCNQ. It can suggest that the estimation of the average charge from the position of ν4 (Ag ) mode is correct, but in this case it is not so obvious because we observe also the additional band at 956 cm−1 in the experimental Raman spectrum. What is really interesting, that in this spectral region we can find also two bands in the optical conductivity spectrum at 963 and 952 cm−1 . Anyway, the assignment of these two features in the IR is not certain. The next band in the Raman spectrum, located at 926 cm−1 , is probably related to the in-plane C N–C bending and C–N stretching vibration of the cation observed at 942 cm−1 in the calculated spectrum. The distinct band at 787 cm−1 is also due to the cation vibration (inplane ring deformation). The ν7 (Ag ) mode of TCNQ is observed at 720 cm−1 and this position is again between the positions for neutral (711 cm−1 ) and charged (725 cm−1 ) species. This feature is also very intensive in the IR spectrum. The ν8 (Ag ) is very weak in the Raman spectrum but well visible in the IR. However, the theoretical Raman activity of this mode is relatively low (see Table 1)—it explains low band intensity. On the other hand, the ν10 (Ag ) mode has smaller activity but an adequate band is very intensive in the experiment. This can be explain by the fact that the ν10 (Ag ) mode has relatively low wavenumber. The intensities of the bands in low frequency region of the Raman spectra can be strongly disturbed by the neighborhood of the Rayleigh peak. Of course, we should also remember that the calculations are approximate (the environment of the molecule is neglected, the vibrations are harmonic, and so on). In the experimental Raman spectrum of the salt there are still a few undiscussed features. The bands at 561 and 510 cm−1 are related to the normal modes of the cation (in-plane ring deformations) located at exactly the same wavenumbers in the calculated spectrum. We are not sure about the origin of the band at 227 cm−1 but there is a possibility that it is also due to the cation vibrations—in this region there are a few normal modes with very low theoretical activity
250
B. Barszcz et al. / Synthetic Metals 158 (2008) 246–250
(mainly out-of-plane deformations). As it was mentioned earlier, the normal modes of the TCNQ are dominating in the spectra. Despite of that we were able to observe some bands not related to it in the Raman spectrum. The strong EMV coupling causes that in the conductivity spectrum (Fig. 5) we observe almost only the Ag modes activated by the coupling to electrons. They are strong, broad and cover almost any other, IR active vibrations. The exception could be the band at 835 cm−1 which is related to the IR active mode of TCNQ molecule (out of plane bending of C–H bonds, calculated wavenumber 873 cm−1 ). Anyway, we do not observe any band that can be certainly assigned to the vibrations of the cation in the optical conductivity spectrum. The fact, that pseudo dimers of TCNQ contain equally charged and crystallographically equivalent species suggest, that the band splitting should be absent or scarcely detectable. This is in good agreement with very low intensity of additional components of the splitted bands in the Raman spectra, if it is observed in general. Using HOSE approach [26] and experimental bond length in TCNQ molecule we estimated the charge on anions as about −0.44 e. Both, structural and spectroscopic methods of the charge determination are approximate (with the error of a few percent) and neglects effects given by neighboring molecules. So we can state that both our charge estimations (−0.51 and −0.44) are in good agreement with each other and confirm almost full charge transfer from cation to TCNQ species. On the other hand it suggests an equal charge distribution between them. 4. Conclusions For the first time, the radical anion salt (N-Me-2,5-(Me)2 Pz)TCNQ2 has been characterized using infrared reflection and Raman spectroscopy. Comparing the experimental data with results of DFT calculations the assignment of observed bands has been proposed. Moreover, the average charge on the TCNQ molecule has been estimated from the spectroscopic and crystallographic data. Acknowledgments One of us (D.V. Ziolkovskiy) is grateful to J´ozef Mianowski Found, a Foundation for the Promotion of Sciences and the Foundation for Polish Science for support. This work was partially supported by the Ministry of Science and Higher Education (Warsaw) as the research project in the years 2007–2008. References [1] D. J´erome, H.J. Schulz, Adv. Phys. 31 (1982) 299. [2] L.G. Caron, in: J.-P. Farges (Ed.), Organic Conductors. Fundamentals and Applications, Marcel Dekker, Inc., New York/Basel/Hong Kong, 1994.
[3] R.E. Peierls, Quantum Theory of Solids, Oxford Press, London, 1955. [4] N.F. Mott, Metal-Insulator Transitions, Taylor and Francis, London, 1974. [5] D. J´erome, A. Mazaud, M. Ribault, K. Bechgaard, J. Phys. Lett. 41 (1980) L-95. [6] T. Ishiguro, K. Yamaji, G. Saito, Organic Superconductors, Springer, Berlin, 1998. [7] F.L. Carter (Ed.), Molecular Electronic Devices, Marcel Dekker, Inc., New York/Basel, 1982. [8] A. Graja, Low-Dimensional Organic Conductors, Word Scientific, Singapore/New Jersey/London/Hong Kong, 1992. [9] M. Tanaka, F. Urano, M. Nakabata, Japan Patent 60-139832 (1987). [10] V.A. Starodub, V.V. Barabashova, Ye.M. Gluzman et al., USSR Patent 1,389,226 (1987). [11] S. Niwa, Synth. Met. 18 (1987) 665. [12] V.A. Starodub, Ye.M. Gluzman, K.I. Pokhodnya, M.Ya. Valakh, Theor. Exp. Chem. 29 (1993) 240. [13] K. Nishimura, G. Saito, Synth. Met. 153 (2005) 385. [14] V.A. Starodub, Ye.M. Gluzman, Yu.A. Kaftanowa, I. Olejniczak, Theor. Exp. Chem. 33 (1997) 111. [15] V.A. Starodub, Ye.M. Gluzman, K.V. Krikunov et al., USSR Patent 1,696,428 (1991). [16] Sanyo Electric Co., US Patent No. 458,055 (1986). [17] D.V. Ziolkovskiy, A.V. Kravchenko, V.A. Starodub, O.N. Kazheva, A.V. Khotkevich, Funct. Mater. 12 (2005) 577. [18] D.V. Ziolkovskiy, O.N. Kazheva, G.V. Shilov, O.A. Dyachenko, A.V. Kravchenko, A.V. Khotkevich, V.A. Starodub, Funct. Mater. 13 (2006) 119. [19] O.N. Kazheva, D.V. Ziolkovskiy, G.G. Alexandrov, O.A. Dyachenko, V.A. Starodub, A.V. Khotkevich, G.Y. Vasilets, Funct. Mater. 13 (2006) 692. [20] O.N. Kazheva, D.V. Ziolkovskiy, G.G. Alexandrov, A.N. Chekhlov, O.A. Dyachenko, V.A. Starodub, A.V. Khotkevich, Synth. Met. 156 (2006) 1010. [21] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision B. 05, Gaussian, Inc., Pittsburgh, PA, 2003. [22] A.D. Bandrauk, K.D. Truong, C. Carlone, S. Jandl, K. Ishii, J. Phys. Chem. 89 (1985) 434. [23] A. Girlando, C. Pecile, Spectrochim. Acta 29 (1973) 1859. [24] R. Bozio, C. Pecile, in: L. Alc´acer (Ed.), The Physics and Chemistry of Low Dimensional Solids, D. Reidel Publishing Company, 1980, pp. 165– 185. [25] S. Matsuzaki, R. Kuwata, K. Toyoda, Solid State Commun. 33 (1980) 403. [26] T.M. Krygowski, R. Anulewicz, Acta Crystallogr. B 39 (1983) 732. [27] M.S. Khatkale, J.P. Devlin, J. Chem. Phys. 70 (4) (1979) 1851–1859.