Structure and Degree of Charge Transfer of Simple and Complex

Structure and Degree of Charge Transfer of Simple and Complex Cyanine-TCNQ Anion Radical Salts Studied by Resonance Raman and Infrared Spectroscopy. S...
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J. Phys. Chem. 1995,99, 3618-3628

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Structure and Degree of Charge Transfer of Simple and Complex Cyanine-TCNQ Anion Radical Salts Studied by Resonance Raman and Infrared Spectroscopy Shin-ichi Terashita, Kazumi Nakatsu, and Yukihiro Ozaki* Department of Chemistry, Kwansei Gakuin University, Uegahara, Nishinomiya 662, Japan

Seishi Takagi Department of Physics, Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804, Japan Received: June 16, 1994; In Final Form: December 20, 1994@

The visible and near-infrared excited resonance Raman,infrared, and visible-near-infrared spectra have been measured for six cyanine-7,7,8,8-tetracyanoquinodimethane(TCNQ) anion radical salts. Among them three are insulating simple salts, [DMTzNC][TCNQ], [EMQSeC][TCNQ], and [DETOC][TCNQ] (DMTzNC = 3,3’-dimethyl-2,2’-thiazolinocyanine, EMQSeC = l-ethyl-3’-methyl-2,2’-quinoselenacyanine, DETOC = 3,3’diethyl-2,2’-thiaoxacyanine),and another three are moderately conducting complex salts, [DMTzNC]2[TCNQ]3, [EMQSeC][TCNQ]2, and [DETOC][TCNQ]2. The 1064-nm excited Raman spectra of both the simple and complex cyanine-TCNQ salts are resonance enhanced with a charge transfer transition from TCNQ- to TCNQ-. The simple salts give a spectrum almost identical to that of LPTCNQ-, suggesting the dominance of TCNQ- species in them. In contrast, the complex salts show bands attributed to TCNQO, TCNQ’l2-, and TCNQ-, indicating the coexistence of the species in them. Of particular note is that the spectral patterns of the complex salts are markedly different from each other. Therefore, it seems that the mechanism of the charge transfer differs from one compound to another. The 457.9-, 488.0-, and 514.5-nm excited Raman spectra of the cyanine-TCNQ salts, which are resonance enhanced with a n-n* transition of cyanine dyes, are dominated by their contributions. Comparisons of the spectra with those of iodine salts of the cyanines ( D M T z N C T , E M Q S e C T , D E T O P I - ) suggest that the geometric and electronic structure of the dyes are not perturbed upon complex formation with TCNQ. Although all the infrared spectra of the cyanine-TCNQ salts are characterized by appearances of Ag modes, the spectral patterns of the simple and complex salts can be differentiated by a glance. As in the case of the 1064-nm excited Raman spectra, the infrared spectra of the simple salts exhibit bands due to TCNQ- but do not show those assignable to other species of TCNQ. On the other hand, the infrared spectra of the complex salts are more complicated, showing the coexistence of TCNQ- and TCNQ’’2- species. The higher the conducting, the broader the Ag modes and the more remarkable the dips.

1. Introduction For the last two decades molecular complexes containing

7,7,8,8-tetracyanoquinodimethane(TCNQ) or its family as an electron acceptor have received keen interest because some of them show high cond~ctivity.’-~The TCNQ-based organic conductors can be classified into two groups according to the states of donor and acceptor: ionic charge transfer complexes and anion radical salts. TTF-TCNQ (‘ITF= tetrathiafuvalene) and [TEiDA]2[TCNQ]3 (TEDA = triethylenediamine) are their representative examples. In most cases an electron donor (D) and TCNQ (A) stack independently, making segregated columns (- *AAA* and --*DDD *). The TCNQ column plays the role of a conducting pathway, and thus TCNQ salts are regarded as examples of lowdimensional electron systems.’-5 In the case of the anion radical salts TCNQ can form a few kinds of anion radical salts for each cation with different stoichiometry such as 1:1, 1:2, and 2:3, and the electronic properties change largely with each cation and the stoichiometry. Although a great number of organic conductors based on the TCNQ family have been proposed and their interesting physical properties have been investigated in considerable detail, there ~~~

* Author to whom correspondence

~

~~

should be addressed. @Abstractpublished in Advance ACS Abstracrs, February 15, 1995.

are still a lot of things to do for establishing their structurefunction relationship. The most important point in the study of the structure-function relationship is to elucidate the relationship between the degree of charge transfer, i.e. formal charge (e),and crystal and molecular structure because plays a major role in the conduction me~hanism.l-~ In order to estimate and investigate the above relationship, X-ray crystallography,6-’0 ESR spectroscopy:.’ ‘-I4 and vibrational spectro~copy’~-~ have been employed extensively. X-ray crystallography is very powerful in investigating the crystal and molecular structure of the conductors, but it is not always easy to discuss their electronic structure from X-ray crystallographic studies. Also the X-ray analysis cannot monitor the fluctuation of electronic distribution in the charge transfer complexes. ESR is very useful to study the state of an unpaired electron and the magnetic properties of the organic conductors. Vibrational spectroscopy can provide the following information: (i) the degree of charge transfer; (ii) the type of stack of a charge transfer complex, segregated (-*-AAA-** and * *DDD .) or mixed (- ADADAD. (iii) the nature of the interaction between the molecules along a stack, whether a molecule has equal interactions with its neighbors or not; (iv) the electron-molecular vibration coupling constants. All the information provided by vibrational spectroscopy is of fundamental importance. However, the vibrational studies have a few problems.

0022-365419512099-3618$09.00/0 0 1995 American Chemical Society

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TABLE 1: Conductivity of the Complex Salts IS cm-l

[DMTzNC]z[TCNQ]3 [EMQSeCl[TCNQh [DETOC][TCNQ]2 From ref 9.

6x

2 x 10-3 5 x 10-5

monitor the structure of cyanine dyes and TCNQ species selectively by changing the excitation lines for Raman spectra because the dyes and TCNQ species show absorption maxima in different regions. This sort of systematic study may offer new insight into molecular design of new TCNQ-based organic conductors.

2. Experimental Section (c)

Figure 1. Structure of 3,3’-dimethyl-2,2’-thiazolinocyanine+(a, DMTzNC+), l-ethyl-3’-methyl-2,2’-quinoselenacyanine+ (b, EMQSeC+), and 3,3’-diethyl-2,2’-thiaoxacyaninef(c, DETOC+). One is that the charge transfer complexes including TCNQ are often decomposed by prolonged visible laser irradiation. Another is that no Raman spectra of the complexes have been obtained in resonance with the charge transfer transition. Yet another is that infrared spectra of the charge transfer complexes measured in KBr and Nujol do not always coincide. Moreover, vibrational spectroscopic studies on the TCNQ-based organic conductors so far reported have mainly dealt with rather simple compounds like M[TCNQ] (M = alkaline metal) and TCNQ with tertiary amines, and also systematic studies comparing “simple salts (1:l salts of TCNQ)” and “complex salts (2:3 and 1:2 salts of TCNQ)” have been very rare. In this paper we present systematic infrared and Raman studies of cyanine-TCNQ anion radical salts. Our purpose is to provide new insight into the relation between e and the molecular and electronic structure by using new technology, near-infrared Fourier transform (FT) Raman spectroscopy, which enables us to obtain Raman spectra of the salts in resonance with a charge transfer transition in the TCNQ column. One of the authors (Terashita) has developed studies on a series of 1:1, 1:2, and 2:3 TCNQ salts containing cyanine dye as a c a t i ~ n . ~ , ” - ’ ~The , ~ ~cyanine-TCNQ salts were first synthesized by Lupinski et aLZ6in 1967, and since that time the crystal structure and electronic properties of the salts have been investigated e x t e n s i ~ e l y . ’They ~ ~ ~are ~ ~very ~ ~ interesting because various kinds of cyanine compounds are available, and by changing the structure of cyanines, one can easily alter electronic properties of the complexes. Some of the cyanineTCNQ salts have been known to show quasi-two-dimensional electron systems at room temperature.”-14 We treat here three kinds of the simple salts, [DMTzNCI[TCNQ] (DMTzNC = 3,3’-dimethyl-2,2’-thiazolinocyanine), [EMQSeC][TCNQ] (EMQSeC = l-ethyl-3’-methyl-2,2’-quinoselenacyanine), and [DETOC][TCNQ] (DETOC = 3,3‘diethyL2,2’-thiaoxacyanine), and three kinds of complex salts [DMTzNC]2[TCNQ]3, [EMQSeC][TCNQ]z, and [DETOCI[TCNQ]z (see Figure 1). All the simple salts are insulating, while the complex ones are moderately conducting (Table 1). By measuring Raman, infrared, and visible-near-infrared spectra of this series of cyanine-TCNQ salts, we can investigate the relationship between the structure and function of the TCNQbased organic conductors systematically. Near-infrared excited Raman spectroscopy is less likely to cause photodecomposition in a sample. It is also possible to

TCNQ was obtained from Tokyo Kasei Kogyo Co., Ltd., and used after recrystallizing three times in an acetonitrile solution. DMTzNC+I-, EMQSeC+I-, DETOC+I- were purchased from Japanese Research Photosensitizing Dyes Co., Ltd., and employed without further purification. Simple and complex cyanine-TCNQ salts used were prepared according to the method reported by Klanderman and Hoesterey.28 Electric conductivity was measured on single crystals of the TCNQ salts at room temperature using conductive silver paste as ohmic electrodes. The near-infrared excited resonance Raman spectra of the cyanine-TCNQ salts in a solid state were obtained with a JEOL JRS-6500N FT-Raman spectrometer equipped with an InGaAs detector. The excitation wavelength at 1064 nm was provided by a CW Nd:YAG laser (Spectron SL301 1355), and the power at the sample position was typically 50 mW. All the data were collected at a spectral resolution of 4 cm-I, and generally several hundred scans were accumulated to ensure an acceptable signalto-noise ratio. For the Raman measurements the solid samples were put in a capillary tube, and then the Raman Scattering was collected in a backscattering configuration. The visible excited resonance Raman spectra were measured by use of a Raman system described elsewhere.29 For the visible excitations the samples were rubbed against a germanium plate and the laser beam was introduced to the plate under the 90” scattering geometry. The cyanine-TCNQ salts were, in general, easily photodecomposed with strong visible laser irradiation. Therefore, we used an unfocused laser beam of 100 mW. The IR spectra of the cyanine-TCNQ salts were measured with a JEOL JIR-6500N FT-IR spectrometer equipped with a MCT detector by using a micro-IR attachment (JEOL IR-MAU 110). The use of microinfrared spectroscopy enables us to measure nondestructively the infrared spectra of the salts. All the data were collected at a spectral resolution of 4 cm-I, and generally 500 scans were averaged to ensure an acceptable signal-to-noise ratio. As in the case of the visible excited Raman measurements, the samples were rubbed against a germanium plate for the IR measurements. Visible-near-infrared absorption spectra were recorded on a Shimadzu UV-3101pC spectrometer.

3. Results Simple Salts. Figure 2 shows UV-visible-near-infrared absorption spectra of TCNQ (a), DMTzNC+I- (b), EMQSeC+I(c), and DETOC+I- (d) in a solid state, and in Figure 3 are presented those of [DMTzNC][TCNQI (a), [EMQSeC][TCNQI (b), and [DETOC][TCNQ] (c) in a solid state. Bands observed

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Figure 2. UV-visible-near-infrared absorption spectra of TCNQ (a), DMTzNCT (b), EMQSeC+I- (c), and DETOPI- (d) in a solid state.

in the 300-600-nm region of the four spectra in Figure 2 are all assigned to n-n* transitions of the conjugated systems. Visible-near-infrared spectra of charge transfer complexes containing TCNQ have been extensively studied by several g r o u p ~ . ~ O In - ~ general, ~ solid alkaline salts of TCNQ such as Li-TCNQ give bands near 360 and 640 nm due to the TCNQ dimer and that near 1100 nm arising from the charge transfer transition (CTI). By referring to the band assignments for the simple charge transfer complexes of TCNQ, bands near 700 and 900 nm in parts a, b, and c of Figure 3 can be attributed to the TCNQ dimer and CTI absorption, respectively. Spectral features in the 300-500-nm region are rather complicated but include contributions from the TCNQ dimer and the cyanine cation. Figure 4A shows the 457.9-nm excited resonance Raman spectrum of EMQSeC+I- in a solid state, while Figure 4B exhibits the 1064-nm excited Raman spectra of TCNQ (a), DMTzNC+I- (b), EMQSeC+I- (c), and DETOC+I- (d) in a solid state. We tried to obtain visible excited resonance Raman spectra in DMTzNC+I- and D E T O C T , but we could not get them because of strong fluorescence. A Raman spectrum of TCNQ has been investigated in detail. According to previous ~tudies,~~ Raman - ~ ' bands at 2225, 1600,1454, and 1209 cm-l can be assigned to CEN stretching ( Y Z ) , C=C stretching (ring m),C% stretching (wing, ~ 4 )and , CH in-plane bending ( ~ 5 ) modes, respectively. Raman spectra of cyanine dyes are, in general, complicated irrespective of the excitation lines, so that it may happen that bands due to a cyanine dye overlap with those arising from TCNQ. The Raman spectra of cyanine dyes in the 1650- 1450cm-' region have been studied in some detai1;38,39we have

D 500 700 900 110013001500 2000 2500 300( WAVELENGTH/nm Figure 3. UV-visible-near-infrared absorption spectra of [DMTzNCI[TCNQ] (a), [EMQSeC][TCNQ] (b), and [DETOC][TCNQ] (c) in a

solid state. proposed the band assignments in that region on the basis of comparison between infrared and Raman spectra of a series of thiatri-, penta-, and heptamethine cyanine dyes. By referring to the above reference^,^^,^^ bands in the 1640-1600- and 1570- 1500-cm-' regions may be assigned to stretching modes of the central conjugated systems and that at 1583 cm-' to v8like modes of the phenyl rings, respectively. We do not try here to obtain assignments for the rest. In Figure 5A,B,C are exhibited the (a) 457.9-, (b) 488.0-, (c) 514.5, (d) 568.2-, (e) 647.1-, and (f) 1064-nm excited Raman spectra of [DMTzNC][TCNQ](A), [EMQSeC][TCNQ](B), and [DETOC][TCNQ](C) in a solid state. Of particular note in Figure 5 is that the 457.9-, 488.0-, and 514.5-nm excited Raman spectra of [EMQSeC][TCNQ] are very similar to the 457.9-nm excited resonance Raman spectrum of EMQSeC+I-, while the 568.2- 647.1- and 1 W n m excited Raman spectra of DMTzNCI[TCNQ], [EMQSeCI[TCNQ], and [DETOCI[TCNQ] are very close to the 1064-nm excited Raman spectrum of TCNQ. In this way by using the excitations in the 457.9-1064-nm region, it is possible to obtain selectively the resonance Raman spectra of cyanine and TCNQ chromophores, within cyanine-TCNQ charge transfer complexes. In other words, by changing the wavelength for excitation, the geometric conformation and electronic structure of both chromophores can be monitored separately. Bands at 1617,1522,1392,1356,1275,1221,1128, and 978 cm-I in parts a-c of Figure 5B may be ascribed to EMQSeC+ in the [EMQSeC][TCNQ] complex. These frequencies are almost identical to those of E M Q S e C T (Figure 4A,B(c)), indicating that both the geometric conformation and electronic structure of EMQSeC+ do not undergo a significant change upon the formation of the complex. The same conclusions may be

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1600

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Raman Shiftlcm"

0'2200'2000'1ci00'1600' 4

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Raman Shdc!?' Figure 4. (A, top) 457.9-nm excited resonance Raman spectrum of EMQSeC'I- in a solid state. (B, bottom) 1064-nm excited resonance Raman spectra of TCNQ (a), DMTzNCfI- (b), EMQSeC+I- (c), and DETOC+I- (d) in a solid state.

reached for DETOC+ because the band frequencies in the 457.9-, 488.0-, and 514.5-nm excited resonance Raman spectra of [DETOC][TCNQ] are very close to those of DETOC+ (Figure 4B(d)), respectively. As for DMTzNC' even the 457.9-, 488.0-, and 514.5-nm excited resonance Raman spectra of [DMTzNCI[TCNQ] are dominated by bands due to TCNQ, and therefore it is not so easy to discuss the structure of DMTzNC+ in the complex.

In contrast to the cyanine chromophores, it is very likely that the molecular and electronic structure of TCNQ change significantly upon the complex formation because the 647.1and 1064-nm excited resonance Raman spectra of [DMTzNCI[TCNQ], [EMQSeCl[TCNQ], and [DETOCI[TCNQ] are appreciably different from the Raman spectrum of TCNQ in terms of both the frequencies and relative intensities. It is notable that bands due to v2 and v4 modes show a large downward shift upon the complex formations. We shall discuss the cause of the band shifts later. Another interesting point in Figure 5 is that the 1064-nm excited resonance Raman spectra of [DMTZNCI[TCNQ], [EMQSeC][TCNQ], and [DETOC][TCNQ] are very close to each other. This observation indicates that the molecular and electronic structure of TCNQ change little among the three complexes. In Figure 6 are shown infrared spectra of TCNQ (a), DMTzNC+I- (b), E M Q S e C T (c), and DETOPI- (d) in a solid state. An infrared spectrum of TCNQ has also been well st~died;~~ bands - ~ ' at 2225, 1543, 1354, 1115, and 862 cm-' are due to C e N stretching (191, Y ~ o ) C=C , stretching (wing Y g l ) , C-C stretching (ring Y23), C-C stretching (m), and outof-plane bending ( ~ 5 0 )modes, respectively. As in the case of the Raman spectra, band assignments of the infrared spectra of DMTzNC+, EMQSeC+, and DETOC+ are not straightforward, but it seems that an intense band at 1551 cm-' in Figure 6b, those at 1618, 1564, and 1525 cm-' in Figure 6c, and those at 1637, 1599, 1579, and 1512 in Figure 6d are due to stretching modes of the central conjugated systems. The infrared band at 1551 cm-' of DMTzNC+I- does not have a counterpart in its Raman spectrum (Figure 4B(b)), while the Raman band at 1562 cm-I does not have a corresponding band in the infrared spectrum (Figure 6b). Therefore, the infrared band at 1551 cm-I and the Raman band at 1562 cm-' may be assigned to the modes having largely an antisymmetric and symmetric stretching character, respectively. The infrared bands at 1618 and 1525 cm-' of EMQSeCf, probably, correspond to Raman bands at 1615 and 1521 cm-' (Figure 4A), and those at 1599 and 1512 cm-' of DETOCf correspond to its Raman bands at 1601 and 1510 cm-I. Parts a-c of Figure 7 exhibit infrared spectra of [DMTzNCI[TCNQ], [EMQSeC][TCNQ], and [DETOC][TCNQ] in a solid state, respectively. The infrared spectra are more complicated than the corresponding Raman spectra and always consist of contributions from both the cyanine and TCNQ parts. In the spectrum of [DMTzNC][TCNQ] bands at 1547, 1423, 1379, 1292, and 1257 cm-' may be ascribed to the DMTzNC' chromophore. These frequencies of the DMTzNC+ chromophore are almost identical to those of DMTzNC+ in the solid state, suggesting that the molecular and electronic structure of DMTzNC+ change little upon the complex formation. The same conclusions can be reached also for EMQSeC+ and DETOC+. Infrared bands at 2175, 2150, 1585, 1504, 1360, 1178, and 833 cm-' in the spectrum of [DMTzNC][TCNQ] may be attributed to the TCNQ chromophore. Note that the frequencies and relative intensities of these bands are significantly different from those of the corresponding bands of TCNQ itself. These infrared bands are commonly observed in the spectra of the three kinds of simple salts, again indicating that the structure of the TCNQ chromophore is nearly unchanged among the three complexes. Complex Salts. Figure 8 shows UV-visible-near-infrared absorption spectra of [DMTzNC]z[TCNQ13 (a), [EMQSeCl[TCNQIz (b), and [DETOC][TCNQ]z (c) in a solid state. The spectra of the complex salts are readily distinguishable from those of the simple salts by a broad feature appearing in the

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Figure 7. Infrared spectra of [DMTzNC][TCNQ] (a), [EMQSeCI-

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1800 1600 1400 1200 lo00 8r WAVENUMBER/Cm- 1 Figure 6. Infrared spectra ofTCNQ (a),DMTzNC+I- (b),EMQSeC+I0 2200 2Ooo

(c), and DETOPI- (d) in a solid state. near-infrared to mid-infrared region. This band is assigned to a charge transfer transition from TCNQ- to TCNQO in the column ( C T Z ) . ~It~ has been well-known that the higher the conductivity, the lower the frequency of the CT2 band. In Figure 9 are exhibited the (a) 457.9-, (b) 488.0-, (c) 514.5, (d) 568.2-, (e) 647.1-, and (f) 1064-nm excited Raman spectra of [DMTzNCl2[TCNQ13 (A), [EMQSeC][TCNQ12 (B), and [DETOC][TCNQ];! (C) in a solid state. As in the case of the simple salts, the 457.9-, 488.0-, and 514.5-nm excited Raman spectra of [EMQSeCl [TCNQI;?and [DETOC][TCNQ];! consist of contributions mainly from the cyanine chromophores, whereas the 568.2-, 647.1- and 1064-nm excited spectra contain those largely from the TCNQ chromophore in the complexes. Comparisons of the Raman spectra of [EMQSeC][TCNQ]2 and [DETOC][TCNQ]2 with those of EMQSeC+ and DETOC+ reveal that the frequencies of bands due to the cyanine chromophores are almost unchanged upon complex formation. Accordingly, it is likely that the structure of the cyanine chromophores is nearly unperturbed in the complicated salts.

[TCNQ] (b), and [DETOC][TCNQ] (c) in a solid state. Bands marked with an asterisk are due to Ag modes. The structure of TCNQ undergoes a change upon the formation of the complex salts because spectral features in the v2 and v4 band regions show marked changes. It is also noted that in contrast to the simple salts the 1064-nm excited resonance Raman spectra of [DMTzNC]2[TCNQ]3, [EMQSeCI[TCNQ12, and [DETOC][TCNQ]2 are each specific. Infrared spectra of [DMTzNC]2[TCNQ13, [EMQSeCl [TCNQl2, and [DETOC][TCNQ]z in a solid state are presented in parts a-c of Figure 10, respectively. Particularly notable is that broad bands and dips are seen in Figure 10a,b. The appearances of these broad bands and dips are characteristic of moderately or highly conducting TCNQ charge transfer c o m p l e ~ e s . ~ Bands ~ , ~ ~due ~ , ~to~the TCNQ chromophore are identified at 2195,2187,2154, 1547, 1518, 1319, 1155, 1141, and 831 cm-' in Figure loa. Among them, those at 2154,1547, 1319, and 1155 cm-' are assignable to Ag We shall discuss the TCNQ bands in more detail in the Discussion.

4. Discussion Electrical Conductivity. Table 1 summarizes the electrical conductivity of the samples treated in this paper. Organic conductors can be grouped into three groups based on the electrical conductivity: insulating, intermediate, and highly conducting. All the simple salts of the cyanine-TCNQ system, so far investigated, are insulating, while its complex salts are intermediate or highly conducting. The highest conductivity observed was 10' S cm-' for [DEQC][TCNQ]z (DEQC = 1,l'diethyl-2,2'-quino~yanine).~~Of particular note in Table 1 is that [EMQSeC][TCNQ]2 shows an anisotropy for the conductivity. Until now, it has been found that [DEOCCl[TCNQI;!12

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complex salts displays their individuality. However, it may be possible to summarize their common basic structure as follows: 9,41.42 the crystal structure of the complex salts consists of a TCNQ column parallel to the a-axis or [loll direction and a dye column along the c-axis, these columns form TCNQ and dye layers parallel to the ac-plane, and these layers are repeated alternatively in the direction of the b-axis. The quasi-twodimensional anisotropy of the complex salts can be understood from the crystal structure described above. The X-ray crystallographic studies have been done for all the complex salts treated here?,41*42However, only the crystal structure of [DMTzNCIz[TCNQ]3 has been reported in the literature? In the case of [DMTzNC]2[TCNQ]3 both the cation and anion columns elongate in the same direction (a-axis). The conductivity of the complex salts changes largely from one compound to another, and its critical determinant is the degree of charge transfer in the TCNQ column. Therefore, it is very important to discuss the degree of charge transfer. Degree of Charge Transfer. The degree of charge transfer, i.e. the formal charge, e, plays a major role in the conduction mechanism, and there is a clear relationship between e and the conductivity.’ In an insulator such as the simple salts of the cyanine-TCNQ system the charge transfer is, in general, complete and 4 has the appropriate commensurate value (namely, 1 for the simple salts and 1/2,2/3, etc., for the complex ones). In such cases Coulombic forces constrain the electron mobility along the TCNQ column. In contrast, if the charge transfer is not complete, i.e. the average charge on a TCNQ molecule is less than the commensurate value, the constraint is 500 700 900 1100 1300 1500 2000 2500 300 removed and the salts become moderately or highly conductive. WAVELENGTHhm The degree of charge transfer in organic conductors can be Figure 8. W-visible-near-hfmred absorption spectra of [DMTzNC]2investigated by X-ray crystallographic data and infrared and [TCNQ]3 (a), [EMQSeC][TCNQ]2(b), and [DETOC][TCNQ]2 (c) in a Raman spectros~opy.’~-’~,~~-~~ In the case of X-ray data7-I0 solid state. the charge state of a TCNQ molecule can be estimated from the distribution of C-C bond lengths. In Table 2 are shown (DEOCC = 3,3’-diethyl-2,2’-oxacarbocyanine),[DEDMTzCIthe estimated charge states for the three complex molecules. [TCNQ]2 (DEDMTzC = 3,3’-diethyl-4,4’-dimethyl-2,2’-thia~aAs for infrared and Raman spectroscopy, it is well-known locyanine), [MEQSeC][TCNQ]2I4 (MEQSeC = l-methyl-3’that v21,~ 3 (CSN 0 stretching) infrared, v4 (C-C wing stretching) ethyl-2,2’quino~lenacyanine), and [DESeCl[TcNQ32’3.26@ESeC Raman, and ~ 3 1(C-C wing stretching) infrared frequencies = 3,3’-diethyL2,2’-selenacyanine) have similar anisotropy. depend on e.17-19,22-24 For example, the v4 band is observed Therefore, some of the complex salts have been considered to near 1388 and 1425 cm-’, respectively, for TCNQ- and be quasi-two-dimensional organic conductors at room temperTCNQIn- while the same mode arising from the neutral species ature. These quasi-two-dimensional organic conductors have is identified at 1453 cm-I. been characterized by a marked angular dependence of the ESR The v4 Raman bands are observed at 1452, 1416, and 1388 line width. cm-I, 1452, 1423, and 1389 cm-’, and 1454, 1421, and 1387 It is well-known that there is clear relation between the crystal cm-’ in the 1064-nm excited resonance Raman spectra of structure and electrical conductivity; the crystal structure of the [DMTzNC]2[TCNQ]3, [EMQSeCl[TCNQ12, and [DETOCIsimple salts is intrinsically different from that of the complex [TCNQ]2, respectively (Figure 9A,B,C). On the basis of salts, although in both cases donor (D) and acceptor (A) observations together with those in their Raman spectra excited molecules stack independently (so-called segregated system: with other laser lines, we estimated e, and the results are **.AAA.**or **.DDD.*.).9~40-42The crystal and molecular summarized in Table 3. By using a similar relationship for the structure of [DEDMTzC][TCNQ] and [DESeC][TCNQ] were v2 bands in the infrared spectra, we calculated as shown in studied by X-ray crystallography.40 In those simple salts the Table 4. The Raman data suggest that in the TCNQ column interaction between the two TCNQ molecules forming the dimer there are three kinds of species having a formal charge of -1, is strong, but that between the dimers is weak. Since cyanine -0.5, and 0, respectively. The infrared data support the molecules are large compared with other donor molecules, the existence of TCNQ’”- and TCNQ-, but the existence of TCNQO degree of segregation of the dimers may be larger in the cannot be confiied from the infrared data. Probably, the broad cyanine-TCNQ salts. In the simple salts all the TCNQ bands observed in the infrared spectra obscure the v2 band due molecules are ionized and have an unpaired electron (nto the TCNQO species. electron). These n-electrons are liable to be localized on the dimers and thus contribute little to the electrical conductivity. The agreement between the charge states estimated from the Although the crystal structures of [DMTzNC][TCNQ], [EMX-ray data and those from the infrared and Raman data is not excellent but good. The discrepancy between them may be QSeCI[TCNQ], and [DETOC][TCNQ] have not been reported yet, it is very likely that they are more or less similar to the understood by assuming that the fluctuations of charge distribucrystal structures of [DEDMTzl[TCNQI and [DESeC][TCNQ]. tions cannot be observed by X-ray analysis, but in the vibrational spectra they can be clearly seen. In contrast to the simple salts, the crystal structure of the

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m U

N*

24 0

2200 2Ooo

I-

, + , * , 1800 1600 1400 1200 lo00 WAVENUMBER/Cm- 1

b

Figure 10. Infrared spectra of [DMTzNC]2[TCNQ]3 (a), [EMQSeCI[TCNQ]2 (b), and [DETOC][TCNQ]2 (c) in a solid state. Bands marked with an asterisk are due to Ag modes. TABLE 2: Formal Charge of the Complex Salts Esdmated by X-ray Crystallomaphy

[DMTzNC]~[TCNQ]~ [EMQSeCI[TCNQh [DETOC][TCNQIz

0.6,O.g 0.3,0.6 0.2,0.7,0.9

TABLE 3: Formal Charge of the Complex Salts Estimated by a Raman Band Due to the C=C (Wing) Stretching Mode compound excitation [DMTzNC]2[EMQSeCI[DETOCIwavelengthlnm [TCNQ]3 [TCNQ12 [TCNQII 457.9 0,0.4 0, 0.5 488.0 0,0.6 0,0.7 514.5 0.5 0.6 1 0.6 568.2 0.6 0.5, 1 0.6 647.1 0.6 0, 0.5, 1 0,0.5,1 1064 0,0.6, 1 TABLE 4: Formal Charge of the Complex Salts Estimated by Infrared Bands Due to the CEN and C-C (Wing) Stretching Modes CN C--C wing _ _ _ _ _ _ ~ [DMTzNC]z[TCNQ]3 0.7,0.9 0.7, 1 [EMQSeCI[TCNQ12 0.6 0.8 [DETOC][TCNQ]z 0.7,O.g 1

Raman and Infrared Spectral Characteristics. Raman spectra of the TCNQ-based organic conductors have been studied in considerable detail, and the usefulness of Raman spectroscopic data has been well r e c o g n i ~ e d . ' ~ - However, ~~~,~4 most of the Raman studies have been carried out for rather simple compounds because complicated compounds such as

dye-TCNQ salts often encounter photodecomposition with the visible excitations and bands due to the dye and TCNQ overlap heavily. Also, some complexes showed severe fluorescence. The 1064-nm excitation solves the above three problems at the same time. By using this excitation, we could get the Raman spectrum of the TCNQ part selectively and nondestructively without any interference of fluorescence and bands due to the dyes. More interestingly, the 1064-nm excitation is close to a CTIband of the TCNQ column giving resonance Raman spectra of TCNQ species in the cyanine-TCNQ complexes. In contrast to the 1064-nm excitation, the 457.9-, 488.0-, and 5 14.5-nm excitations provide the Raman spectra characteristics of the cyanine dyes. In fact, the 457.9-, 488.0-, and 514.5-nm excited Raman spectra of [EMQSeC][TCNQ] and [EMQSeCl[TCNQIz and those of [DETOC][TCNQ] and [DETOCI[TCNQIz are very close to each other, respectively. The structure of the cyanine dyes can be investigated from the 457.9-, 488.0-, and 514.5-nm excited Raman and infrared spectra. All the data reveal that the vibrational frequencies of bands attributed to the cyanine dyes change little upon complex formation. Therefore, it may be concluded that the structure of the cyanine dyes does not undergo a significant change. Another important point in the Raman spectra is that the 647.1- and 1064-nm excited resonance Raman spectra of the simple salts are almost identical to each other. This observation seems to be very reasonable because the 647.1- and 1064-nm excited Raman spectra are resonance enhanced with the absorption due to the segregated dimer and charge transfer transition, respectively, and the charge transfer occurs mainly within the dimer. It is also noted that the 647.1- and 1064-nm excited resonance Raman spectra of the three simple complexes closely resemble each other. These results suggest that the structure of the dimers and the degree of charge transfer are very similar among the three compounds. In contrast to the simple salts the 1064-nm excited resonance Raman spectra of the three complex salts differ clearly from their 647.1-nm excited spectra; the latter are much more complicated. There is no band due to the TCNQ dimer in the 600-700-nm region, and therefore the 647.1-nm excited spectra are not resonance Raman enhanced with a band due to TCNQ groups but preresonance enhanced with the band arising from the cyanine groups. The 1064-nm excited resonance Raman spectra of the complex salts have both common and specific features. They commonly show three v2 and v4 bands. However, the relative intensities of the three v2 bands and those of the three v4 bands change largely from one compound to another. [DMTzNC]2[TCNQ]3 gives strongest v2 and v4 bands at 2189 and 1388 cm-", respectively, assignable to the TCNQ- species; [EMQSeC][TCNQIz, which is second in conductivity, provides them at 2206 and 1423 cm-I due to TCNQ'"-; and [DETOCI[TCNQIz, which is the worst, shows them at 2225 and 1454 cm-', ascribed to TCNQO. These observations give two interesting results. One is that the relative intensities of the v2 bands and those of the v4 bands alter in parallel. This result suggests that the v2 mode and the v4 mode are very useful to estimate the degree of charge transfer. Thus far, as for the Raman spectroscopy, only the v4 mode has been ~ ~ e d , ~ ~ * ~ but the present result strongly suggests the usefulness of the v2 mode. Probably, the charge transfer electrons are located in both the CEN and wing C-C bonds, ensuring the usefulness of the v.2 and v4 modes. Another interesting result is that the spectral pattern in the v2 and v4 regions changes with the conductivity. It seems, therefore, that the mechanism of CTI transition is different from

Cyanine-TCNQ Anion Radical Salts

J. Phys. Chem., Vol. 99,No. 11, 1995 3627

(1) The 457.9-, 488.0-, and 514.5-nm excited Raman spectra of the cyanine-TCNQ salts show the spectra of the cyanine part selectively, while their 568.2-, 647.1-, and 1064-nm excited Raman spectra are dominated by the contributions from the TCNQ part. Therefore, by changing the wavelength for the Raman excitation, the geometrical and electronic structure of both cyanine and TCNQ chromophores can be investigated separately. CO~UIIUI.~~ (2) The 647.1- and 1064-nm excited Raman spectra of the simple salts, which are resonance enhanced with the absorption The infrared spectra of the cyanine-TCNQ salts are chardue to the segregated dimer and charge transfer transition, acterized by the appearances of Ag modes. They are activated respectively, are almost identical to each other. This observation in the infrared spectra through a vibronic interaction mechanism with the expectation that in the simple that has been referred to as an “electron ~ s c i l l a t i o n ” . ~ ~ ~is ~in~ ~good ~ ~ ~agreement ~~~ salts the charge transfer occurs mainly within the dimer. Since the Ag modes are inactive in the infrared spectrum of (3) The 1064-nm excited resonance Raman spectra of the TCNQ, it is possible to identify them by comparing the spectra complex salts show three v2 and v4 bands, indicating that the of the cyanine-TCNQ salts with those of TCNQ and cyanine three complex salts investigated include TCNQO, TCNQ’”-, and dyes. In addition, the bands due to the Ag modes have the TCNQ- species. The relative intensities of the three bands in following characteristic^:^^,^^^^^^^ (i) the bands are much broader the v2 and v4 band regions change with the conductivity. It than usual infrared bands; (ii) the infrared frequencies of the seems, therefore, that the mechanism of CTI transition is Ag modes tend to be downshifted compared with the corredifferent from one to another. sponding Raman frequencies. We identified the Ag modes as (4) The relative intensities of the v2 bands change in parallel shown in Figures 7 and 10 (the bands with an asterisk are due with those of the v4 bands, showing that not only the v4 mode to the Ag modes) by comparing the spectra in Figures 7 and 10 but also the v2 mode is useful for monitoring the formal charge with those in Figure 6. The frequencies of the observed Ag of the complexes. The observation also suggests that the charge modes are close to those previously reported for TCNQ charge transfer electrons are located in both the C F N and wing C=C As expected, the frequencies are transfer bonds. lower than the corresponding Raman frequencies. (Compare ( 5 ) The infrared spectra of the cyanine-TCNQ salts are the Raman frequencies in Figure 5 with corresponding infrared characterized by the appearances of the Ag modes. The spectra frequencies in Figure 7. Also compare the Raman frequencies of the complex salts are much more complicated than those of in Figure 9 with the corresponding infrared frequencies in Figure the simple ones; the Ag modes become broader and the dips 10. All the Raman bands observed in the 1064-nm excited become more remarkable with increase in the conductivity. spectra of the complexes are assignable to the Ag modes.) (6) The structure of the cyanine dyes changes little upon The infrared spectra of the simple salts are clearly differenticomplex formation. ated from those of the complex salts. The former three are very close to each other except for the contributions from the cyanine Acknowledgment. We thank Prof. K. Ishii of Gakushuin dyes. The frequencies of infrared bands due to the TCNQ part University for illuminating discussions. are almost identical to each other and are located at the positions characteristic of TCNQ- species. On the other hand, the Supplementary Material Available: Table SIa,b listing infrared spectra of the complex molecules change from one crystallographic data of [DESeC][TCNQ] and [DEDMTzCIcomplex to another. Particularly notable for the spectra of [TCNQ], and Table SII, listing crystallographic data of [EMcomplex salts is the appearance of dips and broad bandwidths QSeC][TCNQ]z and [DETOC][TCNQ]2, Figure lS, showing the of the Ag modes. Such dips, which are examples of Fano crystal structure of [DESeC][TCNQ] and [DEDMTzC][TCNQ] interferences, arise because the infrared absorption falls inside viewed along the c-axis, Figure 2S, showing the structure of the conduction band.15943-45The dips clearly appear in the TCNQ in the crystal of [DESeC][TCNQ], Figure 3S, showing infrared spectra of [DMTzNC]2[TCNQ]3 and [EMQSeCIthe crystal structure of [EMQSeC][TCNQ];! and [DETOCI[TCNQ]2, for which the conduction band is seen in the infrared [TCNQ]2, and Figure 4S, showing the overlap between the region, as is evident from Figure 8. It has been generally TCNQ molecules in the crystal of [EMQSeCl[TCNQ12 (1 1 recognized that the charge transfer complexes with high pages). Ordering information is given on any current masthead conductivity show a CT2 band in the infrared region, giving page. dips in the infrared ~ p e c t r a . ’ ~The , ~ present ~~ results are in good agreement with this fact. References and Notes The broad bandwidths of the Ag modes probably arise from the vibronic ~ o u p l i n g . ~The ~ ,X-ray ~ ~ ~ crystallographic studies (11 Torrance. J. B. Ann. N . Y.Acad. Sci. 1978. 313. 210: Acc. Chem. R e s . ‘ l h 9 , 12, 79. of the three complexes suggest extensive electron delocalization (2) Jerome. D.: Schultz. H. J. Adv. Phvs. 1982. 31, 299; Mol. Crvst. for [DMTzNC]2[TCNQ]3 and [EMQSeC][TCNQ]2 and rather Liq. Cryst. 1985, 117, 121. narrower delocalization for [DETOC][TCNQ]2.9,42Therefore, (3) Miller, J. S., Ed. Extended Linear Chain Compounds; Plenum Press: New York, 1982; Vols. 1 and 2. the broadness of the Ag modes observed in Figure 10 is in good (4) Ferraro, J. R.; Williams, J. M. Introduction to Synthetic Electrical agreement with the results of X-ray crystallographic studies.

one to another. For [DMTzNC]z[TCNQ]3, TCNQ- species may play a key role in the transition. On the other hand, TCNQ112species contribute largely to the transition in [EMQSeCI[TCNQIz, for which the X-ray, infrared, and Raman data all support the dominance of TCNQIn- species. For [DETOCI[TCNQIz, TCNQO species may be a key contributor for the transition, although the X-ray crystallographic study shows that TCNQO species are somewhat out of the center of the TCNQ

5. Conclusion The present study has provided new insight into the structure and mechanism of TCNQ-based organic conductors by investigating resonance Raman and infrared spectra of the simple and complex cyanine-TCNQ anion radical salts. The following conclusions can be reached from the present study.

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