J O U R N A L O F C H E M I S T R Y Materials Communication New electron acceptors containing thieno[3,4-b]pyrazine units Kazuharu Suzuki, Masaaki Tomura and Yoshiro Yamashita* Department of Structural Molecular Science, T he Graduate University for Advanced Studies and Institute forMolecular Science,Myodaiji, Okazaki 444-8585, Japan Reaction of 6a with tetracyanoethylene oxide (TCNEO) in refluxing 1,2-dibromoethane aVorded 4a and 5a in 6 and 5% Tetracyano-p-quinodimethane (TCNQ) analogues containing the title heterocyclic units were synthesized, and the interesting yields, respectively.The reaction in the presence of Cu powder gave 4a in 15% yield along with a trace amount of 5a. Reaction crystal structures of the neutral compounds and the chargetransfer complexes with TTF were revealed by X-ray analysis.of 6b with TCNEO without Cu gave 5b in 30% yield along with a trace amount of 4b. The reaction in the presence of Cu gave 4b and 5b in 24 and 6% yields, respectively. Such a reaction of TCNEO with dibromo compounds was used in the synthesis of 1–3. In these cases, the yields were higher than those of 4a,b, but no dimerization products such as 5 were Electron acceptors containing sulfur atoms are of interest as obtained.The low yields of 4 may be attributed to the lability components for organic conductors since the intermolecular of the pyrazine ring to TCNEO.6 The mechanism of formation interactions leading to multi-dimensionality can be expected of 5 is still unclear. to result from interheteroatom contacts.1 With this in mind, The absorption maxima of 5 are red-shifted compared to the TCNQ analogue 1 containing a thiophene unit was prethose of 4 as shown in Table 1.The PM3 calculations7 show pared.2 Furthermore, p-extended compounds 23 and 34 were the smaller HOMO–LUMO gap of 5a compared to those of prepared to decrease on-site Coulombic repulsion. 4a and TCNQ, where the LUMO energies of TCNQ, 4a and Benzothiophene–TCNQ 2 is a weak acceptor due to the fused 5a are -3.06, -2.85 and -2.84 respectively, and the HOMO benzene ring.3 In order to overcome this disadvantage, we energies of TCNQ, 4a and 5a are -9.58, -9.72 and -9.01 eV, have now replaced the benzene ring by electron-withdrawing respectively.It is noteworthy here that the LUMO level pyrazine rings to give 4.We have also prepared new acceptors becomes a little higher with increasing p-extension. Actually 5 which are pyrazine fused derivatives of p-extended acceptor the first reduction potentials of 5 are lower than those of 4 as 3. The new acceptors 4 and 5 are expected to be planar shown in Table 1, indicating that compounds 5 are weaker molecules due to the absence of peri-hydrogen atoms, and the electron acceptors than 4.The same trend is observed in the nitrogen atoms of the pyrazines may be involved in the system of 1 and 3.3 The diVerences between the first and second interheteroatom contacts. reduction potentials (DE) decrease in 5 compared to 4, indicat- The acceptors 4 and 5 were prepared from 5,7-dibromoing the decrease in on-site Coulombic repulsion in 5 due to thieno[3,4-b]pyrazines 6 which were obtained by bromination the extended p-conjugation.with NBS of the corresponding thieno[3,4-b]pyrazines.5 N N S R R NC NC S N N R R CN CN S CN NC NC CN 1 S CN NC NC CN 2 S NC NC 3 S CN CN N N S R R CN NC NC CN N N S R R Br Br 4a R = H 4b R = C6H5 5a R = H 5b R = C6H5 6a R = H 6b R = C6H5 Fig. 1 Crystal structure of 4a (orthorhombic crystal) *E-mail: yoshiro@ims.ac.jp J.Mater. Chem., 1998, 8(5), 1117–1119 1117Table 1 The absorption maxima and reduction potentials of acceptors heteroatom contact was observed. On the other hand, in the monoclinic crystal there exist three crystallographically indeacceptor lmax/nma Ered/Vb DE/V pendent molecules and three kinds of short S,N contacts (3.15, 3.26 and 3.28 A ° ) between the S atoms of the thiophene 4a 403 -0.04, -0.56 0.52 rings and the N atoms of the pyrazine ones, which are shorter 4b 402 -0.10, -0.59 0.49 5a 512 -0.22, -0.47 0.25 than the sum of the van der Waals distance (3.35 A ° ).The net 5b 531 -0.28, -0.52 0.24 atomic charges of 4a calculated by the PM3 method show TCNQ 401 +0.22, -0.35 0.57 that the sulfur and nitrogen atoms are positively and negatively charged, respectively, suggesting an electrostatic interaction aIn CH2Cl2 .b0.1 mol dm-3 Bu4NClO4 in CH2Cl2 , Pt electrode, scan rate 100 mV S-1, V vs. saturated calomel electrode (SCE). leading to the S,N contacts. These interactions result in an interesting molecular network as shown in Fig. 2, where a helical structure is constructed. Reaction of the acceptor 4a with an equal amount of TTF in acetonitrile aVorded two kinds of charge-transfer complexes with TTF [a 152 complex (green plates) and a 352 complex (green needles)] in the same batch, which could be separated on the basis of the diVerence in the crystal form.The bond lengths of the TTF and acceptor molecules in the complexes are comparable to those of neutral ones.8 The nitrile stretching frequencies for the 152 and 352 complexes were observed at 2225.4 and 2224.0 cm-1, respectively, which are almost the same as that for the neutral 4a (2224.2 cm-1).These facts Fig. 3 Crystal structure of (TTF) (4a)2 Fig. 2 Crystal structure of 4a (monoclinic crystal ) The single crystals of 4a were obtained as two crystal forms, i.e. orthorhombic (yellow cubes) and monoclinic forms (yellow needles).† In the orthorhombic crystal (Fig. 1) no short inter- †Crystal data for 4a: C12H2N6S, M=262.25, orthorhombic, space group P212121 , Z=4, a=10.517(5), b=14.683(5), c=7.628(5) A ° , V= 1177(1) A ° 3, Dc=1.479 g cm-3. The final R value was 0.037 for 405 reflections with I>3s(I). 4a: C12H2N6S, M=262.25, monoclinic, space group P21/c, Z=12, a=12.636(2), b=15.086(3), c=18.350(5) A° , b= 93.57(1)°, V=3491(1) A ° 3, Dc=1.497 g cm-3.The final R value was 0.048 for 2508 reflections with I>3s(I). (TTF) (4a)2: C30H8N12S6 , M=728.83, triclinic, space group P19, Z=1, a=7.3720(6), b= 7.5992(5), c=14.3903(7) A ° , a=96.607(5), b=98.741(5), c=99.304(6)°, V=778.22(10 A ° 3, Dc=1.555 g cm-3. The final R value was 0.053 for 2313 reflections with I>3s(I).(TTF) (4a)2: C42H16N12S14 , M= 1137.51, triclinic, space group P19, Z=1, a=7.4174(3), b=11.6811(5), c=13.9003(8) A ° , a=95.824(4), b=93.879(4), c=98.471(4)°, V= 1180.90(10) A ° 3, Dc=1.599 g cm-3. The final R value was 0.056 for 4246 reflections with I>3s(I). Full crystallographic details, excluding structure factors, have been deposited at the Cambridge Crystallographic Data Centre (CCDC).See Information for Author, J. Mater. Chem., 1998, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference Fig. 4 Crystal structure of (TTF) (4a)2 number 1145/90. 1118 J. Mater. Chem., 1998, 8(5), 1117–1119suggest that the component molecules are almost non-ionic. References Therefore, the conductivities are low [152 complex; 5×10-9 1 F.Ogura and T. Otsubo, Handbook of Organic Conductive S cm-1 as a compressed pellet, 352 complex; 5×10-4 S cm-1 Molecules and Polymers, ed. H. S. Nalwa, John Wiley & Sons Ltd, as a single crystal (Ea=0.11 eV)]. However, the crystal struc- 1997, vol. 1, ch. 4, pp. 229–248; N. Martý�n, J. L. Segura and tures are interesting as shown in Fig. 3 and 4.† In the 152 C. Seoane, J.Mater. Chem., 1997, 7, 1661. 2 S. Gronowitz and B. Uppstro�m, Acta Chem. Scand., 1974, B28, 981; complex the TTF molecule bridges two acceptor molecules. N. F. Haley, J. Chem. Soc., Chem. Commun., 1979, 1030. To the best of our knowledge, this type of molecular overlap- 3 K. Yui, Y. Aso, T. Otsubo and F. Ogura, J. Chem.Soc., Chem. ping has not been observed in the charge-transfer complexes. Commun., 1987, 1816. In the crystal there are short S,N contacts (3.15 and 3.35 A ° ) 4 D. Lorcy, K. D. Robinson, Y. Okuda, J. L. Atwood and M. P. Cava, between the S atoms of the TTF and the N atoms of the CN J. Chem. Soc., Chem. Commun., 1993, 345. group, which may result in the unique crystal structure. In the 5 F.Outurquin a. Paulmier, Bull. Soc. Chim. Fr. II, 1983, 159. 6 G. Matsubayashi, Y. Sakamoto and T. Tanaka, J. Chem. Soc., 352 complex a mixed stacking is formed and the S,N contacts Perkin T rans. 2, 1985, 947. (3.03 and 3.26 A ° ) are observed between the columns as shown 7 J. J. P. Stewart, J. Comput. Chem., 1989, 10, 209, 221. in Fig. 4. Another TTF molecule is located at the position 8 D. A. Clemente and A. Marzotto, J.Mater. Chem., 1996, 6, 941. orthogonal to the column. These results suggest that TCNQ analogues containing polarizable heterocycles are promising Communication 8/01558I; Received 24th February, 1998 electron acceptors to give organic conductors with multidimensional structures by the interheteroatom contacts. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan. J. Mater. Chem., 1998, 8(5), 1117–1119 1119