HETEROCYCLES

Paper | Regular issue | Vol. 83, No. 9, 2011, pp. 2115-2126
Received, 8th June, 2011, Accepted, 1st July, 2011, Published online, 13th July, 2011.
DOI: 10.3987/COM-11-12277

■ Synthesis and Properties of a New Donor-Acceptor Diad Composed of DT-TTF and Dicyanomethylidene Group

Ken-ichi Nakamura, Takashi Shirahata, Hisakazu Miyamoto, and Yohji Misaki*

Department of Applied Chemistry, Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime, Japan

Abstract

Several derivatives of a new donor-acceptor diad 1 composed of DT-TTF and dicyanomethylidene group were synthesized. The IR spectra of 1 demonstrated considerable contribution of a polarized structure. Electronic spectra and molecular orbital calculation suggested that the absorption maximum of 1 was due to NHOMO-LUMO transition. Cyclic voltammetry revealed that the compound 1 exhibited three-stage of oxidation and one-stage of reduction processes. The bis(methylthio)-substitudted derivative 1a exhibited conductivity of 10-6 S cm-1 in spite of a single-component material.

INTRODUCTION
Tetrathiafulvalene (TTF) is a two-stage redox system, which forms one and two aromatic 1,3-dithiolylium cationic structures in the one- and two-electrons oxidized states. TTF and its derivatives have played a leading role for the development of conductive molecular organic materials, because their high planarity in the oxidized states has advantage for the formation of molecular stacks and resulting wide bands.1 On the other hand, TTF derivatives with strongly electron-withdrawing site have received considerable attention as candidates for electronic materials such as single-component molecular conductors, non-linear optics, field-effect transistors and so on.2,3 As for development of highly conducting single-component materials, decrease in HOMO-LUMO gap is an indispensable requirement. The strategies for reducing HOMO-LUMO gap are as follows; (i) realization of effective intermolecular overlaps to form a wide band,3-5 (ii) introduction of electron-accepting unit to lower the LUMO.4-6 Kobayashi and co-workers reported that a metal complex, Ni(tmdt)2 which has TTF dithiolene as a ligand, showed high electrical conductivity of 400 S cm-1 at room temperature and exhibited metallic conducting behavior down to 0.6 K.4 Stable metallic behavior of Ni(tmdt)2 is derived from the presence of electron-donating TTF moiety and electron-accepting nickel bis(dithiolene) moiety as well as three-dimensional electronic structure of the Ni(tmdt)2 crystal.4 As for purely organic molecular materials which does not contain any metal atom, the highest conductivity of 3.7 x 10-3 S cm-1 was observed in a single crystal of BTQBT, which includes thiadiazole rings as acceptor units in an quinoid-extended TTF molecule.5 In the development of new purely organic single component molecular conductors, 1,3-dithiolo[4,5-d]-TTF (DT-TTF)7,8 is a promising donor unit, because DT-TTF derivatives have many molecular conductors showing metallic conductivity down to low temperature (≤ 4.2 K).8 In this paper, we report the synthesis and properties of new donor-acceptor (D-A) diads 1a−c, which have DT-TTF moiety as a donor unit.

RESULTS AND DISCUSSION
The synthesis of new D-A diads was carried out according to Scheme 1. Phosphonates with TTF core (2a, b)9 were treated with thiophene dicarboaldehyde derivatives (3A, B) in the presence of LDA to give the mono adducts 4a−c in 66−89% yields. In this reaction, an excess amount (1.5 equiv. mol) of 3 was used so as to avoid the formation of the

bis-adducts. The reaction of 4a−c with an excess of malononitrile in the presence of ammonium acetate and acetic acid in refluxing toluene10 gave the target molecules 1a−c in 86−95% yields. In the IR spectra, all the derivatives of 1 exhibited the cyano stretching band at 2210-2211 cm-1, which is lower by 14-15 cm-1 than that of TCNQ (2225 cm-1), and is higher by 27 cm-1 than that of TCNQ-• (2183 cm-1). This result indicates considerable contribution of a polarized structure as shown in Figure 1, in other words, the molecule 1 is situated in an intramolecular charge-transfer (ICT) state. The degree of charge-transfer was estimated to be about 0.3 on the basis of comparison of the wavenumber of the cyano stretching absorption band according to the Chapell’s equation.11

Figure 2 shows electronic spectra of 1a and 1c measured in dicholoromethane together with their related compounds. Both compounds exhibited three absorption maxima at 568, 460 and 322 nm for 1a, 584, 489 and 324 nm for 1c, respectively. The absorption maximum at the longest wavelength region of 1c slightly shifted by 10 nm to the longer wavelength region than 1a. This result suggests that introduction of electron-donating methoxy groups might not be effective for enhancement of the occupied molecular orbitals. The absorption maximum at the longest wavelength region of 1a was longer by 175 nm than that of 512 due to the presence of strongly electron-withdrawing dicyanomethylidene moiety. In contrast, the red-shift of the absorption maximum at the longest wavelength region of 1a was only 21 nm compared to 610 in spite of the presence of strongly electron-donating TTF moiety.
The molecular orbital calculation of unsubstituted derivative 1A and its related compounds 5A and 6A was carried out based on the density functional theory (DFT) using B3LYP/6−31+G method.13 Figure 3 shows an optimized structure of 1A. The 1,3-dithiole ring fused with the TTF moiety, thiophene ring, and the dicyanomethylidene moiety have high planarity, indicating effective conjugation between the donor and the acceptor moieties. However, the DT-TTF moiety is a little bent with a dihedral angle of 21.3°. As shown in Figure 4, the HOMO of 1A mainly distributed on the DT-TTF moiety. On the other hand, the second highest occupied orbital (NHOMO) spread over the whole molecule, although the molecular orbital coefficients of the sulfur atoms in the 1,3-dithiole ring fused with 6A was smaller than those of the others. The LUMO distributed on only the moiety of 6A, while the TTF moiety mainly contributed to the second lowest

unoccupied molecular orbital (NLUMO). The shape of the HOMO, NHOMO, and NLUMO in the DT-TTF unit is close to those of 5A. In contrast, NHOMO and LUMO of 1A in the unit of 6A resembled to HOMO and LUMO of 6A. The energy levels of the HOMO and LUMO of 1A were -5.141 and -2.856 eV, respectively (see Table 1). The energy gap of HOMO-LUMO in 6A (2.720 eV) is more close to the NHOMO-LUMO gap in 1A (2.802 eV) than HOMO-LUMO gap in 1A (2.285 eV).

The absorption maximum at the longest wavelength region of 1a was comparable to that of 6 as mentioned above. Considering the shape of the molecular orbitals and their energy difference obtained from the molecular orbital calculation, it is suggested that this absorption might be attributed to NHOMO-LUMO transition and that the HOMO-LUMO transition is forbidden.
Electrochemical properties of 1 were investigated by cyclic voltammetry. Figure 5 shows deconvoluted cyclic voltammograms of 1b in benzonitrile and THF. All the derivatives exhibited three pairs of one-electron oxidation processes in both benzonitrile and THF. On the other hand, an irreversible reduction process was observed only in THF. The redox potentials of 1 are summarized in Table 2. All the oxidation potentials of 1b were higher than those of 5,

indicating that the oxidized species of 1b were destabilized owing to electron withdrawing effect of the dicyanomethylidene moiety. The E3ox of 1a shifted by 0.15 V to high potential region than 5, while first and second ones shifted by only 0.03 V. The third oxidation process was more affected by the dicyanomethylidene moiety than the first and second ones, suggesting that the positive charges formed by the first and second oxidations mainly distributed in the TTF moiety (see Scheme 2). The 1,3-dithiol-2-ylidene unit fused with TTF, which is located more closely to the dicyanomethylidene moiety, mainly contributed to the third oxidation process (see also Scheme 2). On the other hand, the reduction potential of 1a shifted by 0.04 V to positive voltage region compared with that of 6. This result suggested the radical anion of 1a might be destabilized owing to the presence of electron-donating TTF unit. The E3ox value of 1c was lower by 0.07 V than that of 1a. This result can be explained by contribution of a resonance structure in which a positive charge delocalized in a methoxy group as shown in Figure 6.
pectroelectrochemistry of 1a was investigated in order to elucidate the electronic structures of the oxidized species. Figure 7 shows the UV-vis-NIR spectra of 1a, 5, and their oxidized species in benzonitrile. All the oxidized species

exhibited new absorption bands in the longer wavelength region at 935-1220 nm. These bands of oxidized species for 1a blue-shifted to the shorter wavelength region, as the oxidation state became higher. Similar spectra were obtained for the oxidized species of 5 except for 52+ in which a new shoulder band appeared at 1770 nm. The absorption bands at the longest wavelength region of 1a and 5 in mono- and dicationic states were probably due to be ICT from the 1,3-dithiol-2-ylidene unit fused with TTF to the oxidized TTF moiety. The ICT bands of 1a+• and 1a2+ significantly blue-shifted compared with those of 5, because the donating ability of the 1,3-dithiole-2-ylidene unit of 1a is weakened owing to the presence of electron-withdrawing the dicyanomethilidene moiety compared with 5.

The electrical conductivity of 1a was measured on a compressed pellet using two-probe technique. It exhibited relatively high conductivity of 8.5 x 10-6 S cm-1 at room temperature in spite of a single-component material. On the other hand, the compound 5, which has no dicyanomethylidene moiety, showed low conductivity of σrt = 1.6 x 10-9 S cm-1 on a compressed pellet. The conductivity of 6 was too low to be measured probably owing to little effective intermolecular overlap derived from small π-conjugation of the donor moiety.

SUMMARY
In summary, we have succeeded in the synthesis of new donor-acceptor diads 1a-c. Infrared and electronic spectra suggested that the molecules 1a-c were situated in an ICT state from 1,3-dithiol-2-ylidene fused TTF moiety to dicyanomethylidene moiety, and the degree of CT was estimated to be about 0.3 from wavenumber of cyano stretching absorption band. The molecular orbital calculations based on DFT theory suggested the contribution of NHOMO-LUMO transition to the absorption band at the longest wavelength region in the electronic spectra of 1. Cyclic voltammetry and spectroelecrochemistry indicated that positive charges in the mono- and dicationic states mainly distributed in the TTF moiety, and that ICT occurred from the 1,3-dithiol-2-ylidene unit fused with TTF moiety to oxidized TTF moiety. The compound 1a exhibited relatively high electrical conductivity of 8.5 x 10-6 S cm-1 on a compressed pellet at room temperature in spite of a single-component material, while 5 and 6 showed lower conductivity than 1a. The results obtained in this work bring much expectation that donor-acceptor diads possessing DT-TTF donor unit are a promising candidate to develop highly conducting purely organic single-component conductors.

EXPERIMENTAL
General Methods
1H NMR spectra were recorded on JEOL NM-SCM270 spectrometer, operating at 270 MHz. Spectra are reported (in δ) with referenced to Me4Si. CDCl3 was used as solvent. MS spectra were determined on a SHIMADZU QP5050A Spectrometer, Applied Biosystem MALDI TOFMS Voyager-DETM PRO, and JEOL JMS-LX2000. Electronic spectra were recorded on Perkin Elmer Japan LAMBDA 750. Melting points were determined with a Yanaco MP-J3. The gaussian03 program package was used to all the optimized calculations. Cyclic voltammetry was performed by using a ALS/chi 617B Electrochemical analyzer. The cell for the cyclic voltammetry consisted of a Pt disk working electrode, a Pt wire counter electrode, and an Ag/AgNO3 reference electrode. The measurements were carried out in benzonitrile or THF solution containing 0.1 M Bu4N+PF6- as a supporting electrolyte. All redox potentials were measured against Ag/Ag+ and converted to vs. Fc/Fc+. Spectroelectrochemistry was performed on a sample placed within a quartz cell with a path length of 1 mm. Pt gauze (100 mesh) was used as an optically transparent electrode. The reference and auxiliary electrodes were Ag and Pt wire respectively. UV-Vis-NIR spectra were acquired by using the same instrument as that described above. A fixed-potential electrolysis was performed by using the same electrochemical analyzer as that employed for the cyclic voltammetry measurement.

Typical Procedure for the Preparation of 4
4a: To a solution of 2a (0.15 g, 0.30 mmol) and 2,5-thiophenedicarboxaldehyde (3A) (63 mg, 0.45 mmol) in THF (3 mL), a 0.5 M THF solution of LDA was slowly added at -78 °C under argon atmosphere. After the mixture was warmed up at 0 °C, MeOH was added. The resultant precipitate was filtered off, and washed with MeOH. The precipitate was purified by column chromatography on silica gel using CH2Cl2 elution to afford 4a (98 mg, 0.20 mmol) in 66% yield; purple solid; mp 172-173 °C; IR (KBr) 2915, 1630, 1561, 1427, 1397, 1277, 1054, 765 cm-1; 1H NMR(270 MHz, CDCl­3) δ 9.86 (s, 1H), 7.68 (d, 1H), 6.93 (d, 1H), 6.88 (s, 1H), 6.85 (s, 1H), 2.45 (s, 3H), 2.43 (s, 3H); MS (MALDI) m/z = 494 [M+]; Anal. Calcd for C15H10OS9: C, 36.41; H, 2.04. Found: C, 36.44; H, 2.04.

4b: The same manner as for 4a was adopted for the synthesis of 4b and a purple solid of 4b was obtained in 78%; mp 117-119 °C; IR (KBr) 2916, 2788, 2372, 2345, 1653, 1560, 1543, 1497, 1429, 1221, 1051 cm-1; 1H NMR (270 MHz, CDCl­3) δ 7.75 (s, 1H), 7.56 (d, 1H), 6.88 (d, 1H), 6.85 (s, 1H), 2.72-2.79 (m, 4H), 1.2-1.6 (m, 20H), 0.82 (t, 6H); MS (MALDI) m/z = 634 [M+]; Anal. Calcd for C25H30OS9: C, 47.28; H, 4.76. Found: C, 47.11; H, 4.60.

4c: The same manner as for 4a was adopted for the synthesis of 4c and a purple solid of 4c was obtained in 89%; mp 124-125 °C; IR (KBr) 2917, 2345, 1637, 1470, 1428, 1398, 1279, 1056 cm-1; 1H NMR (270 MHz, CDCl­3) δ 9.98 (s, 1H), 6.88 (s, 1H), 4.13 (s, 3H), 3.84 (s, 3H), 2.44 (s, 3H), 2.43 (s, 3H); MS (MALDI) m/z = 554 [M+]; Anal. Calcd for C17H14O3S9: C, 36.80; H, 2.54. Found: C, 37.01; H, 2.51.

Typical Procedure for the Preparation of 1
1a: Acetic acid (0.3 mL) was added to a solution of 4a (62 mg, 0.13 mmol), malononitrile (42 mg, 0.64 mmol), and ammonium acetate (224 mg, 2.90 mmol) in toluene (16 mL) under argon atmosphere. The reaction mixture was stirred for 2 h under reflux. After the reaction mixture was cooled down to room temperature, the precipitate was collected by filtration and washed with MeOH. The precipitate was column chromatographed on silica gel eluted with CH2Cl2 to afford 1a (62 mg, 0.11 mmol) in 95% yield; a black solid; mp 228-230 °C; IR (KBr) 2211, 1570, 1536, 1501, 1422, 1349, 1282, 1062 cm-1; 1H NMR (270 MHz, CDCl­3) δ 7.71 (s, 1H), 7.63 (d, 1H), 6.93 (d, 1H), 6.86 (s, 1H), 2.45 (s, 6H); MS (MALDI) m/z = 542.14 [M+]; Anal. Calcd for C18H10N2S9: C, 39.82; H, 1.86; N, 5.16. Found: C, 39.80; H, 2.07; N, 4.88.

1b: The same manner as for 1a was adopted for the synthesis of 1b and a black solid was obtained in 86%; mp 158-159 °C; IR (KBr) 2957, 2921, 2850, 2210, 1570, 1534, 1261, 1068 cm-1; 1H NMR (270 MHz, CDCl­3) δ 7.75 (s, 1H), 7.56 (d, 1H), 6.88 (d, 1H), 6.85 (s, 1H), 2.72-2.79 (m, 4H), 1.2-1.6 (m, 20H), 0.82 (t, 6H); MS (MALDI) m/z = 682.55 [M+], HRMS (LDI): calcd for C28H30N2S9: 681.9895. Found: 981.9907.

1c: The same manner as for 1a was adopted for the synthesis of 1c and a black solid was obtained in 87%; mp 157-158 °C; IR (KBr) 2915, 2210, 1536, 1469, 1399, 1292, 1166, 1057 cm-1; 1H NMR (270 MHz, CDCl­3) δ 7.71 (s, 1H), 7.64 (s, 1H), 6.95 (s, 1H), 6.93 (s, 1H), 2.45 (s, 6H); MS (MALDI) m/z = 602 [M+], HRMS (LDI): calcd for C20H14N2O2S9: 601.8542. Found: 601.8525.

ACKNOWLEDGEMENTS
This work was partially supported by a Grant-in-Aid for Scientific Research (Nos. 18GS0208, 20110006, 21750148 and 23550155) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and Japan Society for the Promotion of Science.

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12. The compound 5 was newly synthesized by the reaction of 2a with 2-thiophenecraboaldehyde in the presence of LDA in THF at -78 °C. a purple solid; mp 177-178 °C; IR (KBr) 2917, 2375, 1571, 1545, 1421, 1312, 683, 671 cm-1; 1H NMR (270 MHz, CDCl3) δ 7.75 (s, 1H), 7.56 (s, 1H), 6.88 (s, 1H), 6.85 (s, 1H), 2.76 (t, 4H), 1.2-1.6 (m, 20H), 0.82 (t, 6H); MS MS (MALDI) m/z = 466 [M+]; Anal. Calcd for C14H10S9: C, 36.02; H, 2.16. Found: C, 36.27; H, 2.22.
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