Assignment of fundamental vibrations of a giant analogue of tetrathiafulvalene

167

Vibrational Spectroscopy, 6 (1994) 167-172 Elsevier Science Publishers B.V., Amsterdam

Assignment of fundamental vibrations of a giant analogue of tetrathiafulvalene M.E. Kozlov ’ and A. Graja Institute of Molecular Physics, Polish Academy of Sciences, 60-l 79 Poznari (Poland) (Received 11th March 1993)

Abstract The infrared and Raman spectra of powder samples of a new giant analogue of tetrathiafulvalene (TTF), in which two conjugated 1,4-dithiafulvalen-6-yl side arms are borne by a dihydro-TTF core (DDTF-DHTTF), are reported. An assignment of the fundamental vibrational modes is presented and discussed using the correlations between the data obtained and those for similar compounds; a comparison with our preliminary normal coordinate analysis is done. Kevwords: Infrared suectrometrv;_ Raman spectrometry; coordinate analysis; Tetrathiafulvalene

Intensive search for new highly conducting organic ion-radical salts and complexes has resulted in the synthesis of new electron donor and acceptor molecules. A vast majority of the most interesting salts, i.e. organic superconductors, are derivatives of tetrathiafulvalene (TTF), including such molecules as tetramethyltetrathiafulvalene (TMTTF), bis(ethylenedithiolo) (BEDT)-TTF, bis(ethylenedioxy) (BEDO)-TTF, or their selenium analogues, e.g. tetramethyltetraselenafulvalene (TMTSF) [l]. The search for interesting derivatives of ITF has recently led to the synthesis of a new giant analogue of TTF (Fig. 11, in which two conjugated 1,4-dithiafulvalen-6-yl side arms, later referred to as DDTF or wings, are borne by a dihydro-TTF core (DHITF) [2,3]; for the sake of brevity this donor will be called DDTF-DHTTF. The crystals of ion-radical salts of this donor with PF; , ClO; , Cu(SCN); and H,F; anions Correspondence to: A. Graja, Institute of Molecular Physics, Polish Academy of Sciences, 60-179 Poznan (Poland). * Permanent address: Institute of Semiconductors, Ukrainian Academy of Sciences, 252650 Kiev (Ukraine). 0924-2031/94/$07.00

Band assignment;

Charge transfer

complexes;

Normal

exhibit interesting electrical and optical properties [4,5]. In particular strong electron-molecular vibration (e-mv) couplings, typical of aB modes, have been observed, in spite of a formal lack of inversion center in the molecule [4]. The above observation confirms the hypothetical possibility that couplings occur also between electrons and b,, modes [6,7]. However, it is not possible to interpret spectral data of these ion-radical salts without a detailed analysis of vibrations of the neutral donor. Addition of new molecular frag-

Fig. 1. Internal coordinates of DDTF-DH’ITF.

0 1994 - Elsevier Science Publishers B.V. All rights reserved

ME. Kozlov and A. Graja /Vii. Spectrosc. 6 (1994) 167-I 72

168

ments to the TIF core leads to the formation of a new giant molecule. This calls for transformation of information referring to small molecules to expanded molecular systems. This paper presents new, more detailed information about infrared (IR) and Raman spectra of the DDTF-DHTTF molecule, enabling identification of absorption bands by comparing them to the bands of the molecules studied earlier and by comparison with our preliminary normal coordinate analysis. The results presented in this paper will in the future be used in the refinement of the force field of this donor; they are also expected to permit a more thorough examination of the proposed model of e-mv coupling in the (DDTFDHlTF)ClO, salt 141.

EXPERIMENTAL

DDTF-DHTI’F was obtained by means of the method described by Salle and coworkers [2,31, and purified by recrystallization from 1,1,2-trichloroethane. The IR powder spectra in the range 4000-400 cm-’ were measured for the compound in KBr pellets with a Perkin Elmer 1725 X Fourier transform (FT)-IR spectrometer. Spectra in the range 400-160 cm-’ were measured for

I

the compound in CsI pellets with a Bruker IFS 113 v IT-IR spectrometer. The Raman spectra were obtained using a near-infrared (NIR) FTRaman spectrometer interfaced with a Nicolet FT-Raman accessory. The deuterated-triglycine sulfate (DTGS) mid-IR (MIR) detector with a preamplifier cooled with liquid nitrogen has an noise equivalent power (NEP) of the order of lo-l5 W Hz-*/‘. An argon ion and a krypton ion laser were used as excitation sources with wavelengths ranging from 406 to 674 nm. However, a strong fluorescence covers and masks the DDTF-DHTTF spectra. By employing an Nd : YAG continuous-wave laser line at 1064 nm for excitation of the Rarnan spectra, fluorescence was virtually excluded. The Nd : YAG laser power used for the DDTF-DHTTF non-resonance study was up to 200 mW (at about 400 mW the sample was decomposed). The spectra were normalized for instrument response by dividing the Raman spectrum obtained for the sample by the spectrum of a whitelight source which has been normalized for source emittance. The Raman spectra were measured in a solid-state sample and verified by comparing them with the spectra of DDTF-DHTTF in solution (l,l,Ztrichloroethane or carbondisulfide). Both Raman and IR spectra presented in this paper were recorded at room temperature.

I

1000

500 FREQUENCY

Fig. 2. (a) Raman spectrum of DDTF-DDTTF measured in KBr/CsI pellets.xxxxx

(cm-‘)

molecule excited by laser light of 1064 nm and (b) infrared absorption spectrum

ME. Kozlov and A. Graja / Vi. Spectrosc. 6 (1994) 167-I 72 RESULTS

AND DISCUSSION

Unfortunately, the data concerning the x-ray structure of DDTF-DHTTF molecule in the neutral state are inaccessible. Thus, to perform a normal coordinate analysis of the donor we used structural data for DDTF-DHTTF+ in the (DDTF-DHTI’F)ClO, salt with a 1:l stoichiometry [8]. The data, being a good approximation of the structure of a neutral molecule, show that both dithiafulvenyl arms adopt the double S-truns conformation in the solid state, with the three S-heterocyclic moieties lying in the same plane [8,3]. It was also shown that the central molecular fragment is the only planar one, whereas the TABLE

Frequency (cm-‘) Raman

3109 VW 3090 sh

3090 w

2955 vw 2935 vw 2917 vw 1546 s 1508 vs

v(C;H)

b,v(C;H,)

al

a2 al al

bz 1445 m

al

bz 1325 s 1420 sh

al al

1

bz

a,,bz

1294 m 1273 m 1252 vw 1244 w, sh 1240 w

al bz et bz i a2

1130 m

1103 w 1102 w 1080 w 1021 vw

bz at

bz

144Ow

1143 w 1129 w

v(C;H)

at

a,, b, dC,H) 3062 w

1426 w 1414 w

All modes are both IR and Raman active with the exception of a2 species, which are only active in Raman spectroscopy. The Raman and IR spectra of the compound are presented in Fig. 2,

Frequency (cm-‘) Infrared

bz

3078 w 3062 m

w vs s w

I( C,,) = 30a, + l&z, + 16b, + 29b,

a

Assignment

Infrared

2916 1547 1512 1473

remaining atoms of the framework are slightly out-of-plane. In spite of a somewhat distorted shape of the molecule, our analysis of the vibrational spectrum was based on the assumption of C,, symmetry, as it was assumed for other TTF derivatives. In this case fundamental modes may be distributed among the symmetry species as follows:

1

Infrared and Raman spectra of DDTF-DHTTF

2955 vw

169

b, bz at 1 bz at

v(C,=C,) Y(C~ = C$> V(C, = C,) v(C, = C,) v(C*=C*) .(C: c_ = C:,r_ W;H,l

o(C,H) o(C;H,) w(C,* H)

t(C;H,)

Assignment Raman

992 w 970 vw

al

v(C’C’)

bz

W,C,S)

951 vw 915 vw 897 vw

a2

p(C;H2) V(C’S) + S(C’C’S)

866 vw 845m 801 s 801 s I 773m 759 m 743 vw 672 vw 648m 633 s 608 vw 579 s

al al7

481 vw 466 w

bz bz

S(C,C,C,*1

b2

bz al

bz al,

b2

4 673 vw 654 w

at at at. b2 b, at, ba

443 m 414 vw

b, at at bz at

203 vw

bz

468 vw 443w

158 w

S(C,C,S) s(c$c;s) V(c:s) + s(c:c:c,) v(C,S) v(C,S) v(C,S) p(C;H2) v(CfS) .cc: S) v(C$S) v(C,SI + v(Cc*S) v(C,S) V(c:s)

+ S(CfSC)

v(C,Sl s(c,sc;)

+ V(C,S)

S(C,W$) S wings

at

t(C; Hl t(C,H)

a Relative intensities: vs = very strong; s = strong; m = medium; w = weak; VW= very weak, sh = shoulder. have been used for group vibrations: v = stretching; 6 = bending; o = wagging; t = twisting; p = rocking.

The following

symbols

170

whereas the vibration band frequencies are collected in Table 1. The normal coordinate analysis was performed and bond lengths and angles were obtained by averaging appropriate values for symmetric bonds/angles in the (DDTF-DMTI’F)ClO, salt [8]. An initial set of force constants analogous to those previously reported for TI’F [9,1Ol and BEDT-TTF [ll] was used. The ethylenedithiol groups were analyzed using data of ref. 12. The calculations were made using the programs described in ref. 13. For the assignment of absorption bands to particular fundamental vibrations of the molecule, we used the spectra of similar molecules such as, e.g. ethylene trithiocarbonate (ETTC) [12], ethylene carbonate 1141, TTF [9,10] and BEDT-TTF [11,15], as well as compounds containing BEDTTTF fragments. Spectra of the compounds containing fragments of the studied donor and those used in its synthesis were additional sources of information. c--C stretches as well as C-H stretching and deformation vibrations were identified in the first place. Within the range of CH stretching modes two groups of bands appear (Fig. 2). The highfrequency part corresponds to the vibrations of the CH group, whereas the low-frequency one to the CH, group. On the basis of the data for TTF, one can assume that the highest-frequency bands at 3090 cm-’ (Raman) and 3109 cm- ’ (IR) correspond to in-phase CZH vibrations of a, and b, symmetry. The 3062 cm-’ (Raman) and 3062 cm-’ (IR) bands correspond to antiphase C$H stretches [lo]. Obviously, in this case, the 3078 cm-’ band corresponds to C,H vibrations of a, and b, type of symmetry. The analogy with BEDT-‘ITF [ll] indicates that in the lowfrequency region vibrations should be attributed as follows: 2955 cm-‘, b,; 2935 cm-’ (Raman), a,; 2917 cm-’ (Raman), a2 and 2916 cm-‘, b,. Four strong bands at 1546, 1508, 1445 and 1325 cm-’ are visible in the Raman spectrum (Fig. 2). In view of the fact that they are within the range of the stretching vibrations of the C=C group in the spectra of the corresponding TTF [9,10] and BEDT-TTF [11,15] molecules, they can unquestionably be ascribed to stretches of the

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Kozlov and A. Graja /Vii.

Spectrosc. 6 (1994) 167-I

72

double bonds in the central and peripheral fragments of the molecule, their symmetry being ui. The two bands at the highest frequencies appear also in the IR spectrum at 1547 and 1512 cm-‘. The change in dipole moment in case of the low-frequency vibration bands is inconsiderable, which accounts for the fact that they are not observed in the IR spectrum. According to the results of normal coordinate analysis, the stretches of the C, = C, bonds have the greatest share in the bands at the highest frequencies, whereas the stretches of C$ = C,* bonds contribute to those at the lower frequencies (Fig. 1). The above-mentioned bonds are located in the peripheral fragments of the molecule, at a considerable distance from the two-fold axis, which explains their remarkable contribution to the change in dipole moment. On the other hand, the long C:-C,* bonds (N 1.44 Al are characteristic of 1.5-fold rather than double bonds. Besides, the location close to the center of mass in the molecule clearly determines the lower frequency and low intensity of the C,*-Cz stretching vibration in the IR spectrum at 1325 cm-‘. Therefore, the remaining one of the four above-mentioned bands at 1445 cm-’ should evidently be attributed to vibrations of the central C,-C, bond. As we can notice, the frequencies of antisymmetrical 6, vibrations of C,-C, as well as C;-C$ are slightly lower than the corresponding a, modes. They can be observed in the IR spectrum at 1473 and 1440 cm-‘. The frequency range under consideration should also include CH, bending vibrations located a bit lower than the stretching vibrations of the C=C group. The data for ETTC [12] suggest that this group contains a weak band in the IR spectrum at 1426 cm-‘, as well as a more pronounced one at 1414 cm-’ of a, and b, symmetry, respectively (see Table 1). Another intensive band group in the proximity of 1250 cm- ’ is the result of CH wagging vibrations [11,12]. The comparison of IR spectra of the examined donor with those of its starting compounds, reveals that the most intense transition at 1294 cm-l, present only in the final compound, is brought about by C,H vibrations of a, and b, symmetry. The vibrations at 1273 and

M.E. Kozlov and A. Graja /Vii.

Spectrosc. 6 (1994) 167-172

1250 cm-r are close to those recorded for ETTC [12], and can be ascribed to the CLH, group vibration with a, and b, symmetry, respectively. Evidently, the lowest frequency in the region around 1240 cm-‘, close to that observed for ITF, can be ascribed to C$H vibrations in the peripheral areas of the molecule. It is clear from calculations that transition frequencies of a, and b, symmetry would be identical for the given modes. In an even lower frequency range of 1143-1080 cm-l are bands which can be related to CH twisting vibrations. A comparison with BEDTTTF spectra indicates that a very weak transition at 1143 cm-’ characterized by a2 symmetry, is connected with the CLH, group. Although one expects this transition to be forbidden in the IR spectrum, it appears there, owing to the broken selection rules. A distinct peak at 1130 cm-r in the Raman spectrum, as well as a maximum at 1129 cm-’ in the IR spectrum, are related to the CLH, twisting vibration of b, symmetry. By way of analogy to the 1102 cm-’ band in the Raman spectrum and the 1103 cm-’ band in the IR spectrum in case of ‘ITF [lo], the vibrations of the CZH peripheral groups can be related to the vibrations of a1 and b, symmetry. It has to be emphasized that the frequency for this type of vibrations in the C,H groups cannot be determined very precisely from the spectrum. Nevertheless, it can be assumed that the vibration having b, symmetry, is located at 1080 cm-’ and the one having a, symmetry at 1021 cm-‘; such a transition can be accounted for by a considerable contribution of the C,*C, bonds as well as by a distortion of adjacent angles. Finally, using the data from the literature [ll], we can ascribe very weak IR bands at 951 and 743 cm-’ to rocking vibrations of the CLH, group of u2 and b, symmetry, respectively. The vibrations of a, symmetry of the CLCL bonds in the ethylenedithiol part are generally characterized by insignificant intensity, and, like ETTC, are located in the proximity of 992 cm-‘. They contain a considerable contribution from CH, deformation. The remaining weak bands in the 1021-845 cm-l range are related to the deformations in

171

the SCC area (CZC, bonds). The deformations of the C,C,S angle characterized by b, symmetry, whose frequencies decrease markedly as the molecule becomes larger (as is the case for the TTF to BEDT-TTF transition), are responsible for the 970 cm-’ band. An analogous vibration of u1 symmetry can occur only in the peripheral wings of the molecule. It has a slightly lower frequency and in all likelihood it is located at 866 cm-‘. In the same range there should be a transition caused by S(C,*C,C,) and S(C,C,S) deformations of b, symmetry. It is likely to be the very weak band at 897 cm-‘. According to the data for BEDT-TI’F [ll], the band at 915 cm-’ can be ascribed to v(CLS) + S(CiC$ vibrations in the ETIC part and has b, symmetry. The two transitions of approximately the same frequencies, viz. 6(SCGCz) deformation in the wings with a contribution of 6(CH,) deformation, are characterized by b, and a, symmetry. The analogy to TTF [lo] suggests that they should be located in the proximity of 800 cm-‘. In our spectra this band lies at 845 cm-‘. According to the literature data on TTF and BEDT-TTF, a group of strong bands close to 770 cm-’ is expected to be related to CS vibrations in various parts of the examined molecule. In analogy to TTF, the bands may be interpreted as stretching C,*S with a considerable contribution of a deformation of adjacent angles, having b, symmetry (801 cm-‘). According to our calculations, the band corresponding to vibrations of a, symmetry is located at a considerably lower frequency, and can probably be related to a transition at 654 cm-’ in the Raman spectrum or at 648 cm-’ in the IR spectrum. In the proximity of 801 cm-’ there should also be a vibration of a, symmetry, corresponding to antiphase vibrations of C,S bonds in a TTF fragment. As in the case of TTF, a corresponding antisymmetrical b, vibration is located at 773 cm-‘, whereas the symmetrical al vibration of these bonds is at 468 cm-’ (Raman). The 759 cm-’ band corresponds to C,S vibrations of a, and b, symmetry; it is located at a slightly higher frequency than in the case of TTF [9,10]. A group of bands around 600 cm- ’ is also

172

connected with the vibrations of CS bonds, but their detailed interpretation is faced with difficulties. In the first place on the basis of the analogy to TTF, the strong IR band at 633 cm-‘, of a, and b, symmetry, can be attributed to the vibrations of the CZS bonds in the wings. A transition of a, symmetry in the ETI’C part of the molecule is expected to occur analogous to the E’ITC molecule; an IR band was observed there at 672 cm-* and a Raman band at 673 cm-‘. As it can be seen in our calculations, a similar antisymmetrical b, transition is located at a much lower frequency and it reveals a significant contribution of S(CLSC,> deformations. It is not unlikely that this is the very weak band at 481 cm-‘. If this is the case, then the Y(CS) vibration with b, symmetry in the ETI’C fragment would be located below 620 cm-‘, as it is observed for BEDT-TI’F [ll]. It may be the weak band at 608 cm-‘. A strong, single band at 579 cm-’ is attributed to symmetrical and antisymmetrical vibrations of the C,S bonds in the wings. The Raman band at 443 cm-’ as well as the band at the same frequency in the IR spectrum correspond to respective vibrations of a, and b, symmetries, caused mainly by C,SCz deformation, with a considerable contribution of Y(C_,,S>. Our calculations indicate that an analogous transition of a, symmetry, however, connected with the vibrations in the central part of the molecule, is located at a slightly lower frequency and corresponds most probably to a weak band at 414 cm-l. Low-frequency bands located below 400 cm-’ are characterized by very low intensities (Fig. 2). We only succeeded in recording weak bands at 203 cm-’ in the IR and at 158 cm-’ in the Raman spectrum. The above-mentioned bands can be attributed to deformations of the wings with b, and a, symmetry, respectively. The discussion performed by us and attribution of bands is simplified and may in the future be carried out more precisely. The basic difficulty lies in the fact that due to low symmetry of the molecule, the majority of vibrations are active in both IR and Raman spectroscopy. Moreover, the

ME. Kodov and A. Graja /I&.

Spectrosc. 6 (1994) 167-I 72

molecule is composed of three structurally similar fragments, whose vibrations are characterized by similar frequencies. Assignment of bands and preliminary normal coordinate analysis for the considered DDTFDHTT’F molecule will be employed in the improvement of the force field of the molecule, as well as in the analysis of electron-molecular vibration couplings in new conducting ion-radical salts of this donor. The research is being continued in our laboratory. We thank Professor Gorgues and Dr. Salle for synthesis of the donor molecule. M.E.K. wishes to thank the Institute of Molecular Physics in Poznan (Poland) for hospitality.

REFERENCES 1 A. Graja, Low-Dimensional Organic Conductors, World Scientific, Singapore/New Jersey/London/Hong Kong, 1992. 2 M. Salk& A. Gorgues, M. Jubault and Y. Gouriou, Synth. Met., 41-43 (1991) 2575. 3 M. Salle, M. Jubault, A. Gorgues, K. Boubekeur, M. FourmiguC, P. Batail and E. Canadell, Angew. Chem., in press. 4 A. Graja, V.M. Yartsev, C. Garrigou-Lagrange, M. Salle and A. Gorgues, Phys. Status Solidi (B), 174 (1992) 119. 5 A. Graja, M. Salle and A. Gorgues, Mol. Cryst. Liq. Cryst., 230 (1993) 95. 6 R. Bozio and C. Pecile, J. Phys. C, 13 (1980) 6205. 7 R. Swietlik, C. Garrigou-Lagrange, C. Sourisseau, G. Pages and P. Delhaes, J. Mater. Chem., 2 (1992) 857. 8 M. Salle, PhD Thesis, Angers, 1991. 9 R. Bozio, A. Girlando and C. Pecile, Chem. Phys. Lett., 52 (1977) 503. 10 R. Bozio, I. Zanon, A. Girlando and C. Pecile, J. Chem. Phys., 71 (1979) 2282. 11 M.E. Kozlov, K.I. Pokhodnia and AA. Yurchenko, Spectrochim. Acta, 43A (1987) 323. 12 G. Borch, L. Henriksen, P.H. Nielsen and P. Klaboe, Spectrochim. Acta, 29A (1973) 1109. 13 L.A. Gribov and V.A. Dement’ev, Simulation of Vibrational Spectra of Complex Compounds with a Computer, Nauka, Moscow, 1989. 14 B. Fortunato, P. Mirone and G. Fini, Spectrochim. Acta, 27A (1971) 1917. 15 M.E. Kozlov, K.I. Pokhodnia, A.A. Yurchenko and A.A. Artamonov, Preprint of Phys. Inst. Ukr. Acad. of Sci., N86/2, Kiev, 1986.