J O U R N A L O F C H E M I S T R Y Materials EVect of heteroaromatic spacers on the structure and electrical properties of cation radical salts of tetrathiafulvalene analogs Jean-Franc�ois Favard,a Pierre Fre`re,*a Ame�de�e Riou,a Amina Benahmed-Gasmi,b Alain Gorgues,a Michel Jubaulta and Jean Roncalia aInge�nierie Mole�culaire et Mate�riaux Organiques, UMR 6501, Universite� d’Angers, 49045 Angers, France bL aboratoire de Chimie Organique, Universite� d’Oran, Es-Senia, 31000, Alge�rie The X-ray crystal structure of single crystals of electrocrystallized cation radical salts of 2,5-bis(1,3-dithiol-2-ylidenemethyl)- thiophene and -furan 1 and 2 have been analysed.Both cation radicals adopt a syn conformation stabilized by strong intramolecular interactions. Bond length analysis reveals that the positive charge is delocalized over the whole molecule with enhanced delocalization for 2 + containing the less aromatic furan cycle. Whereas 2 + cation radicals are uniformly stacked along the c axis with interstack interactions, 1 + forms weakly interacting dimers stacked along the b axis.In agreement with an enhanced charge delocalization and increased dimensionality, 2 BF4 shows a conductivity ca.three orders of magnitude higher than 1 ClO4. Since the discovery of the conducting properties of charge- 2 ClO4 and 2 BF4, were grown whereas with the anion PF6- transfer complexes and cation radical salts (CRSs) of the p only powders were obtained. The 151 stoichiometry of 1 ClO4 donors of the tetrathiafulvalene (TTF) series,1 much eVort has and 2 BF4 was determined by X-ray crystallography.The been invested in the design of new p-donors by modification quality of the single crystals of 2 ClO4 has not allowed us to of the TTF framework.2 The main objective of this research is obtain a structural resolution but the unit cell indicates that the increased dimensionality and hence improved charge trans- it is isostructural with 2 BF4.port properties anticipated for the corresponding CRSs.3,4 A As shown in Figs. 1 and 2, both cation radicals adopt a syn recent trend in this area consists of the insertion of a conjugated conformation stabilized by two strong intramolecular interspacer between the two 1,3-dithiole rings in order to stabilize actions, S,S for 1 + and S,O for 2 +, between the the cation radical state by the enhanced delocalization of the heteroatom of the middle heterocycle and a S atom of each positive charge.3 Moreover, the decrease of the molecular 1,3-dithiole ring.The non-bonded lengths d1=3.11; d2=3.06 A ° charge density and the increase of p-interactions may allow for 1 + and d1=2.956; d2=2.926 A ° for 2 + are much shorter better intra- and inter-stack contacts between the donor mol- than the sum of the van der Waals radii (rwS=1.8 and ecules and hence an increased dimensionality for the corre- rwO=1.5 A ° ) but larger than a covalent single bond length sponding CRS.Based on these considerations, a new (SMS=2.04 and SMO=1.75 A ° ). superconducting CRS has recently been obtained from a fused Such 1,5-intramolecular interactions between thiophene or TTF with an ethylenic spacer.5 Linearly extended TTFs with furan rings and 1,3-dithiole rings have already been observed,7,8 a heterocyclic spacer have been synthesized almost simul- in particular for the neutral donor 1 which adopts the same taneously by four groups.6 However, the preparation and syn conformation.9 The similarity of the d1 and d2 lengths for characterization of the corresponding CRSs have not yet 1 (d1=3.11; d2=3.13 A ° ) and 1 + shows that oxidation does been reported.not significantly aVect the strength of these 1,5-intramolecular In this paper the preparation and X-ray diVraction structural stabilizing interactions. characterization of two CRSs with thiophene 1 and furan 2 In contrast, marked diVerences are observed between 1 and spacers are reported and the relationships between their struc- 1 + in the bond lengths along the p-conjugated path and in ture and electrical properties are discussed.particular for the exocyclic bonds x, x¾ and y, y¾ (Table 2). For the neutral molecule 1, the lengths of the bonds x, x¾ and y, y¾ are consistent with localized double and single bonds respectively while the diVerences between C(sp2)–C(sp2) bond lengths Dl and Dl¾ are close to the value expected for alternate single and double bonds (0.16 A ° ).Oxidation of 1 into 1 + produces a lengthening of bonds x and x¾ and a shortening of y and y¾, resulting in a decrease of Dl and Dl¾ and also in an inversion of the sign of Dl. The decrease in Dl and Dl¾ provides conclusive evidence for extensive delocalization of the positive charge Results and Discussion over the whole 1 + molecule (Scheme 1) while the inversion of Dl is consistent with an important contribution of the semi- Compounds 1 and 2 were synthesized using a known proquinoid geometry (B form in scheme 1) in the cation radical.cedure.6 As already demonstrated, the insertion of a hetero- Compared with 1 +, Dl and Dl¾ for the furan analog 2 + cyclic spacer into the TTF core results in a negative shift of decrease to ca. 0.01 and 0.02 A° . This bond length equalization the two oxidation potentials and in the decrease of their suggests that the lower aromatic resonance energy of the furan diVerence.6 Electrocrystallizations of 1 and 2 under galvanosring allows a better delocalization of the positive charge over tatic condition have been performed in the presence of ClO4-, the whole cation radical 2 + than for 1 +.This enhanced BF4- and PF6- as counter anions. The main results are presented in Table 1. Green single crystals, namely 1 ClO4, delocalization is further favored by the better planarity of 2 + J. Mater. Chem., 1998, 8(2), 363–366 363Table 1 Experimental conditions for galvanostic electrocrystallization on platinum wire (diameter: 0.5 mm; length: 1.5 cm) of donors 1 and 2.ACN: acetonitrile, EtOH: ethanol, THF: tetrahydrofuran donor 1 1 1 2 2 2 anion ClO4- BF4- PF6- ClO4- BF4- PF6- solvent ACN (10 ml ) ACN (10 ml ) ACN THF (20 ml ) THF(20 ml ) EtOH (20 ml ) EtOH (20 ml ) EtOH (10 ml ) EtOH (10 ml ) EtOH (10 ml ) current/mA 1 1 0.8 1 1 0.5 T /°C 0 0 2 5 5 5 results crystals small crystals green solution crystals crystals green solution and and powder powder Table 2 Exocyclic bond lengths in A ° of neutral molecule 19 and cation radicals 1 + and 2 + x x¾ y y¾ Dl=y-x Dl¾=y¾-x¾ 1 (X=S) 1.34(1) 1.28(1) 1.46(1) 1.45(1) 0.12 0.17 1 + (X=S) 1.43 (2) 1.37(2) 1.37(2) 1.40(2) -0.06 0.03 2 + (X=O) 1.379(5) 1.370(5) 1.390(4) 1.392(4) 0.01 0.02 Scheme 1 Fig. 1 ORTEP view of cation radical 1 + Fig. 2 ORTEP view of cation radical 2 + as shown by the decrease of the dihedral angles between the plane containing the middle heterocycle and those containing the 1,3-dithiole rings from 1.9° and 1.3° for 1 + to 1.12° and 0.24° for 2 +. The packing of 1 + ClO4- and 2 + BF4- is characterized by a head-to-tail overlap of radical cations (Figs. 3 and 4). In 1·ClO4, dimers of cation radicals stack along the b axis. As shown in Fig. 3(b), several S,S intermolecular contacts are observed in the centrosymmetric dimer. The strongest inter- Fig. 3 X-Ray crystal structure of 1 ClO4: (a) packing of the molecules and anions and (b) overlap mode of cation radicals 1 + in the dimer action occurs between the sulfur atoms of thiophene with a 364 J.Mater. Chem., 1998, 8(2), 363–366single crystal only gave values of 7×10-5, 4×10-2 and 10-2 S cm-1 for 1 ClO4, 2 ClO4 and 2 BF4 respectively. The isomorphous crystals 2 ClO4 and 2 BF4 have analogous conductivities whereas the ca. three orders of magnitude larger value obtained for 2 BF4 in comparison with 1 ClO4 is in fair agreement with the enhanced charge delocalization and increased dimensionality indicated by X-ray data.Conclusion The first X-ray structures of cation radical salts of extended tetrathiafulvalenes with a thiophene and furan spacer have been described. Both cation radicals adopt the synilized by strong intramolecular interactions, previously observed for the neutral state.Compared to 1 ClO4, the better electronic delocalization over the whole radical cation 2 + and above all the increased dimensionality of the crystal structure of 2 BF4 confer greater conductivity on this material. On the other hand, it is worth noting that the conductivity of 2 BF4 appears rather high for a CRS of 151 stoichiometry which should in principle behave as an insulator.This result suggests that the extension of charge delocalization allowed by the insertion of the heterocyclic spacer increases the probability of intermolecular charge transport. Work now in progress on more extended CRSs should permit us to confirm this hypothesis. Experimental Electrocrystallization Donors were dissolved in degassed solvent (Table 1) containing Fig. 4 X-Ray crystal structure of 2 BF4: (a) packing of the molecules the electrolyte and placed in the anode compartment of a and anions and (b) overlap mode of cation radicals 2 + 50 ml H-shaped electrocrystallization cell, separated from the cathode compartment by a porous glass frit. A constant current short distance d3=3.42 A ° . The sulfur atoms of the 1,3-dithiole was applied for 10–12 d.rings are in weaker contact with d4=3.61 and d5=3.76 A ° . Weak interactions are found between the dimers along the b X-Ray crystallography axis (3.81 A ° ) and no interstack contact is observed. In sharp contrast, for 2 BF4 the cation radicals are uniformly Single crystals were mounted on an Enraf-Nonius MACH3 diVractometer with a graphite monochromator and Mo–Ka stacked along the c axis with an overlap of the furan and 1,3- dithiole rings stabilized by regular S,S and weak S,O (l=0.71073 A ° ) radiation at T=294 K.Data collection was performed with the v/2h scan technique. intermolecular interactions with d6=3.70 and d7=3.35 A ° respectively [Fig. 4(b)]. Interstack contacts are observed with The structures were solved by direct methods (SIR) and refined by full-matrix least-squares techniques using MOLEN di=3.76 A ° [Fig. 4(a)]. Two-probe conductivity measurements performed on a software. Non-H atoms were refined anisotropically. The Table 3 Crystallographic data compound 1 ClO4 2 BF4 2 ClO4 symmetry monoclinic triclinic triclinic space group P21/n P19 P19 a/A ° 7.096(5) 7.964(2) 7.929(2) b/A° 25.193(7) 10.341(2) 10.391(2) c/A ° 9.765(4) 10.683(1) 10.820(3) a(°) 90 118.07(1) 117.97(2) b(°) 105.08(6) 100.99(1) 101.45(2) c(°) 90 94.12(2) 94.28(2) V /A ° 3 1685(2) 748.7(2) 757.1(4) Z 4 2 2 formula C12H8O4S5Cl C12H8OS4BF4 C12H8O5S4Cl M 411.97 383.26 395.90 Dc/g cm-3 1.62 1.70 1.74 F(000) 836 386 402 m/cm-1 8.31 6.47 7.98 hmin 2 2 hmax 28 30 h, k, l 0<h<9, 0<k<33, -12<l<12 0<h<11, -14<k<14, -15<l<15 data unique 4117 4634 data observed, I>3s(I ) 1229 3227 no.of variables 199 223 R 0.095 0.05 Rw 0.121 0.07 J. Mater. Chem., 1998, 8(2), 363–366 3656 (a) U. Scho� berl, J. Salbeck and J. Daub, Adv. Mater., 1992, 4, 41; hydrogen atoms were found by Fourier diVerence in 2 BF4 (b) A. S. Benahmed-Gasmi, P. Fre` re, B. Garrigues, A. Gorgues, and refined with fixed isotropic thermal parameters.In 1 ClO4, M. Jubault, R. Carlier and F. Texier, T etrahedron L ett., 1992, 33, the positions of hydrogen atoms were calculated using the 6457; (c) T. K. Hansen, M. V. Lakshmikantam, M. P. Cava, HYDRO program. Crystal data and experimental details are R. E. Niziurski-Mann, F. Jensen and J. Becher, J. Am. Chem. Soc., listed in Table 3 and data (excluding structure factors) for 1992, 114, 5035; (d) K.Takahashi, T. Nihira, M. Yoshifuji and K. Tomitani, Bull. Chem. Soc. Jpn., 1994, 66, 2330. 1 ClO4 and 2 BF4 have been deposited at the Cambridge 7 (a) Y. Yamashita, S. Tanaka, K. Imaeda, H. Inokuchi and M. Sano, Crystallographic Data Centre. (See Information for Authors, J. Org. Chem., 1992, 57, 5517; (b) Y. Misaki, T. Sasaki, T.Ohta, Issue No. 1; quote reference 1145/63 in any request to the H. Fujiwara and T. Yamabe, Adv. Mater., 1996, 8, 804; (c) A. Ohta CCDC). and Y. Yamashita, J. Chem. Soc., Chem. Commun., 1995, 557; (d) A. Ohta and Y. Yamashita, J. Chem. Soc., Chem. Commun., 1995, 1761; (e) A. Ohta and Y. 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