Linearly extended hybrid tetrathiafulvalene analogues with bridged dithienylethylene p-conjugating spacers Hugues Brisset,a Soazig Le Moustarder,a Philippe Blanchard,a Bertrand Illien,a Ame�de�e Riou,a Jesus Orduna,b Javier Garinb and Jean Roncali*a aInge�nierie Mole�culaire et Mate�riaux Organiques, CNRS UMR 6501, Universite� d’Angers, 2 Bd L avoisier, 49045 Angers, France bL aboratorio de Quimica Organica Universidad de Zaragoza, Zaragoza, Spain New linearly extended p-electron donors based on bridged dithienylethylene (DTE) end-capped with diversely substituted 1,3- dithiol-2-ylidene electron-releasing groups have been synthesized by Wittig–Horner olefination of appropriate aldehydes.Cyclic voltammetry shows that whereas the unbridged analogues are directly oxidized to the dication state through a two-electron transfer, rigidification of the DTE spacer leads to the splitting of the two-electron wave into two successive one-electron steps due to the decrease of the potential corresponding to the formation of the cation radical.This unusual electrochemical behaviour is interpreted with the help of theoretical calculations which suggest that these eVects are related to the enhanced electron delocalization resulting from the rigidification of the DTE spacer.This conclusion is supported by an X-ray diVraction structural analysis which reveals that in addition to a fully planar conformation stabilized by intramolecular S,S interactions, the bridging of the DTE spacer leads to a significant reduction of bond length alternation.Linear p-conjugated systems end-capped with 1,3-dithiol-2- X-ray structural analyses which indicate that the bridging of the DTE spacer produces a decrease in bond length alternation ylidene groups have recently emerged as a growing class of resulting in an enhanced p-electron delocalization. organic compounds with specific electronic properties.1 Interest in these systems was initially motivated by the increased dimensionality and hence improved electrical properties anticipated for the corresponding cation-radical salts, in the general frame of the chemistry of extended tetrathiafulvalene (TTF) analogues.2 However, more recent work has shown that these hybrid p-conjugated systems are also potentially interesting as small-bandgap molecular semiconductors3 or as building blocks for push–pull or push–push molecules for quadratic or cubic non-linear optics.4,5 Since these various applications are closely related to the delocalization of p-electrons, the design of a p-conjugating spacer group showing optimal electron transmission properties associated with good thermal and photochemical stability appears to be a priority.In this context oligoheteroarylenevinylene spacers have recently emerged as a good trade-oV between the eYcient but unstable polyalkenic systems6 and the more stable poly(hetero) aromatic ones which present excessive p-electron confinement.7 We have already reported the synthesis of linearly extended TTF analogues (LETTFs) based on oligoheteroarylenevinylenes end-capped with 1,3-dithiol-2-ylidene units.8 Whereas for S S S S S S R R R R S S S S S S R R R R a R = CO2Me b R = SMe c R = Prn 4a–c 1a–c LETTFs involving oligothiophenes, internal rotational disorder leads to rapid saturation of the eVective conjugation beyond a bithiophenic spacer, i.e.eight conjugated carbons,1e such a saturation has not been observed yet even for the Results and Discussion longest known LETTFs built around a tetrathienylenevinylene spacer (22 conjugated carbons).9 The synthesis of compounds 1a–c is depicted in Scheme 1.Recently, we have shown that the bridging of the thiophene Bridged dithienyethylene 3 has been prepared by McMurry rings with the central ethylene linkage of dithienyethylene coupling of 4,5-dihydro-6H-cyclopenta[b]thiophen-6-one11 as (DTE) produces a 0.40 eV decrease of the bandgap of the already described.10,12 Dilithiation of 3 using n-butyllithium resulting electrogenerated polymer.10 As a further step we followed by reaction with DMF aVorded the dicarbaldehyde report here the synthesis of new LETTFs built by grafting the 2 in 81% yield.The target compounds 1a–c were then obtained 1,3-dithiol-2-ylidene moiety to both ends of a bridged DTE in 45–50% yield by double Wittig–Horner olefination of 2 spacer 1a–c.Comparison of the electrochemical properties of using an appropriately substituted phosphonium salt 5a or these new p-donors with those of their non bridged analogues phosphonate anions derived from the dithiolium salts 6b,c.13 4a–c shows that rigidification of the spacer leads to an increase The cyclic voltammograms (CV) of 1a and 1c in methylene of p-donor ability with a stabilisation of the cation radical chloride are shown in Fig. 1. The CV of 1a exhibits two reversible one-electron oxidation waves with anodic peak state. These results are discussed in the light of theoretical and J. Mater. Chem., 1997, 7(10), 2027–2032 2027Table 1 Electrochemical data for compounds 1 and 4,b 10-4 M in 0.1 M Bu4NPF6/CH2Cl2.Scan rate 100 mVs-1, all potentials in V vs. SCE compound Epa1/V Epa2/V Epa2-Epa1/V 1a 0.49 0.65 0.16 1b 0.33 0.45 0.11 1c 0.22 0.31 0.09 4ab 0.59a 0.66 0.07 4bb 0.43 — 0.00 4cb 0.29 — 0.00 aShoulder. bFrom ref. 8(a). for each compound 1, decoalescence and hence the increase of Epa2-Epa1 results from the negative shift of Epa1 while Epa2 values remain similar to those of the unbridged analogues.This result implies that the bridging of the spacer leads to a S S R R S S H, PF6 – R R H P+Bu3,BF4 – S OHC S CHO S S 5a R = CO2Me 6b R = SMe c R = Prn 1. BunLi, THF 2. DMF 1a–c 5a, Et3N, CH3CN or 6b,c, 1. P(OMe)3, NaI 2.BunLi, THF 2 3 + Scheme 1 stabilization of the cation radical state. Previous work on LETTFs containing polyalkenic or oligoheteroarylenelvinylenes p-conjugating spacers has shown that the lengthening of the spacer group produces a negative shift potentials Epa1 and Epa2 at 0.49 and 0.65 V corresponding to of Epa1 and Epa2 together with a decrease of their diVerthe successive generation of the cation radical and dication.ence.1a,1e,8 Thus, beyond a certain conjugation length which As expected, replacement of the electron-withdrawing depends on the structure of the spacer, direct formation of the CO2Me by the electron-releasing SMe and especially n-propyl dication state through a two-electron transfer occurs.1a,8,9 In groups leads to a negative shift of Epa1 and Epa2 while the this context, the behaviour of compounds 1 appears as rather potential diVerence Epa2-Epa1 decreases from 0.16 to 0.09 V, surprising since, as far as we know, it is the first time that a indicating a reduction of the on-site coulombic repulsion decrease in oxidation potential is associated with an increase between positive charges in the dication [Fig. 1(b) and of Epa2-Epa1 and hence a stabilization of the cation radical Table 1]. The comparison of the CV data of compounds 1 instead of the dication.with those of their analogues built around a single thiophene Previous work has shown that the bridging of DTE leads ring1b reveals a ca. 0.20 V negative shift of the peak potentials to a negative shift of Epa1 and Epa2 from 1.10 and 1.40 to 0.72 thus confirming that the insertion of the bridged DTE spacer and 1.16 V, respectively while Epa2-Epa1 increases from 0.30 leads to a significant improvement of the p-donor ability.to 0.44 V.10 This increase in the HOMO level leads to a However, comparison of the CV data of compounds 1a–c 0.40 eV reduction of the HOMO–LUMO gap (DE) of the with those of 4a–c containing a classical DTE spacer shows molecule and of the bandgap of the resulting polymer. The that the bridging of the spacer produces two noticeable great similarity between these eVects and those observed for changes.Firstly, Epa1 undergoes a 70–100 mV negative shift 1a–c clearly shows that the bridging of the DTE spacer is the which is indicative of enhanced p-donor ability. However, origin of their unusual electrochemical properties. The smaller whereas compounds 4b and 4c are dized into their magnitude of the eVects observed here can be related to the dication state through a single step two-electron transfer,9 fact that the 1,3-dithiole moieties provide the major contririgidification of the DTE spacer leads to the splitting of this bution to the HOMO level in LETTFs.1a,8 two-electron wave into two successive one-electron oxidation In a recent joint X-ray diVraction and theoretical analysis steps. A closer examination of the data in Table 1 shows that of bridged DTEs, we have shown that the reduction of DE induced by the bridging of DTE results from a decrease of bond length alternation.12 As widely acknowledged, this parameter represents the main cause for the existence of a finite bandgap in linearly p-conjugated systems.14 In order to analyse the role of bond length alternation in the electronic properties of compounds 1, the structure of a single crystal of 1c has been investigated by X-ray diVraction.The ORTEP view in Fig. 2 shows that the molecule is non-centrosymmetric and adopts a fully planar geometry except for the n-propyl chains that lie outside the plane of the molecule. While the planarity of the conjugated DTE spacer is ensured by the bridge, both lateral parts of the molecule are rigidified by strong 1,5 intramolecular S,S interactions between the sulfur atom of the thiophene ring and a sulfur of the 1,3-dithiole moiety.The S1,S3 and S4,S5 distances (3.154 and 3.171 A ° respectively) are larger than a covalent SMS bond (2.04 A ° ) but shorter than twice the van der Waals radius of sulfur (3.60 A ° ).Such interactions have already been observed for related compounds.1e,8,15 The crystal structure of 1c involves columns of molecules along the [001] crystallographic direction (Fig. 3). The molecules centrosymmetrically related are stacked in a head-totail way along the c axis. Molecules within a column are equidistant with an average separation of 3.95 A ° .The contact distances between molecules in neighbouring columns are too Fig. 1 CVs of (a) 1a and (b) 1c (10-4 M in 0.1 M Bu4NPF6/CH2Cl2). Scan rate 100 mVs-1. large to take any interaction into account. 2028 J. Mater. Chem., 1997, 7(10), 2027–2032Table 2 Bond distances in the DTE conjugated path (see formulae for bond labelling) S S a b' c' d' e' b c d e bond DTEa 7b 1c e 1.351(8) 1.350(1) 1.36(2) d 1.44(1) 1.409(8) 1.41(2) c 1.40(1) 1.356(8) 1.37(2) b 1.457(7) 1.440(7) 1.44(2) a 1.309(8) 1.335(6) 1.33(2) b¾ 1.447(7) 1.440(7) 1.43(1) c¾ 1.40(1) 1.356(8) 1.34(2) Fig. 2 ORTEP view of 1c d¾ 1.44(1) 1.409(8) 1.40(2) e¾ 1.351(8) 1.350(1) 1.38(1) dr/A ° 0.096 0.078 0.064 aFrom ref. 16. bFrom ref. 12. Comparison of the bond distances in the DTE moiety for DTE,16 4,4¾-dibutyl-6,6¾-bi(4,5-dihydro-6H-cyclopenta[b]thienylidene) 7, a substituted analogue of 3,12 and 1c shows that the presence of the bridge produces a lengthening of the a, e and e¾ double bonds and a compression of the b, d, b¾ and d¾ single bonds (Table 2).An important consequence of these bond length modifi- cations is a significant decrease of bond length alternation expressed as the diVerence between the average length of single and double bonds (dr).Thus, as shown in Table 2, dr decreases from 0.096 A ° for DTE to 0.075 A °for 712 and 0.064 A ° for 1c. The smaller dr value obtained for 1c might be related to the further constraint imposed on the DTE moiety by the 1,5 intramolecular S,S interactions. Although the large uncertainties related to the rather limited quality of the 1c crystal mean that these data should be considered with care, the trend expressed here confirms that the reduction of bond length alternation already observed for 7 persists when this system is used as p-conjugating spacers in LETTFs.These results allow us to propose a possible explanation for the peculiar electrochemical behaviour of compounds 1.Oxidation of 1 and 4 to the cation radical state induces geometrical changes in the conjugated path, the thiophene rings adopt a quinoid structure while the central ethylene linkage acquires the character of an essentially single bond (Scheme 2). In this context, the changes in bond distances induced by the bridging of the DTE moiety and in particular the compression of bonds b, d, b¾, d¾ and the lengthening of bonds a and e contribute to prefigure the final geometry of the cation radical.Consequently, formation of the cation radical becomes easier and thus requires less energy, in agreement with the observed decrease of Epa1. On the other hand, since there is much less diVerence between the geometries of 1·+ and 4·+ than between their respective neutral forms, the bridging of the DTE spacer has much less influence on the energy required by the second oxidation step, which is consistent with for the quasi invariance of Epa2. In order to gain more information on this question, theoretical calculations have been performed for the neutral, cation radical and dication forms of compounds 1b and 4b.The geometry of neutral and dicationic species have been optimized using the PM3/RHF method and that of the cation radical with the PM3/UHF method.Table 3 lists the computed values Fig. 3 Crystal structure of 1c of the ionization potential (Ei), dipole moment (m), heat of J. Mater. Chem., 1997, 7(10), 2027–2032 2029Fig. 4 AO coeYcients in HOMO of (a) 4b and (b) 1b a strong localization of the positive charge in one 1,3-dithiole unit.In contrast, there is practically no change in the m value between 1b and 1b·+ which suggests that the positive charge is delocalized over the entire molecule. This result which agrees well with the reduced dr value of 1b appears consistent with the observed stabilization of the cation radical as indicated by CV data. S S S S S S R R R R S S S S S S R R R R S S S S S S R R R R • –e– +e– –e– +e– + + + Scheme 2 Conclusion To summarize, new conjugated p-donors incorporating bridged Table 3 Computed values of ionization potential (Ei), dipole moment DTE spacers have been synthesized.The combined eVects of (m), heat of formation (DHf) and degree of bond length alternation the bridge in the spacer and of 1,5-intramolecular S,S inter- (dr) for compounds 4b and 1b and their cationic species actions lead to a fully planar rigid structure with reduced bond length alternation.The geometrical modifications induced by compound Ei/eV m/D DHf/kcal mol-1 dr/A ° the rigidification of the DTE spacer result in the splitting of the two-electron oxidation wave observed for the unbridged systems into two successive one-electron steps with a concomi- 1b 7.82 0.1 177.9 0.062 1b·+ 0.2 331.7 -0.030 tant decrease of the first oxidation potential.This rather 1b++ 0.0 -0.098 unusual behaviour is attributed to a stabilization of the cation 4b 7.94 0.1 180.7 0.063 radical state associated with an enhanced delocalization of the 4b·+ 24 339.4 -0.018 positive charge. 4b++ 0.2 -0.113 Experimental Electrochemical experiments were carried out with a PAR 273 formation (DHf). The dr values used refer only to the DTE Potentiostat-Galvanostat in a three-electrode singlesystem.compartment cell equipped with platinum microelectrodes The Ei value for 1b is found 0.12 eV lower than for 4b; this of 7.85×10-3 cm2 area, a platinum wire counter electrode and diVerence exactly matches that found betwen Epa1 values a saturated calomel reference electrode (SCE).Solutions were (Table1), but this agreement might be fortuitous. The geodeaerated by nitrogen bubbling prior to each experiment which metries of 1b and 4b are very symmetrical and present a quasi was run under a nitrogen atmosphere. inversion centre leading to a dipole moment close to zero. The computed geometries do not provide evidence for the eVect of X-Ray structural analyses the bridge and show only a slight diVerence in dr (0.062 and 0.063 A ° for 1b and 4b respectively). The fact that the noticeable Crystal data for 1c.C34H40S6 MW 641.08, monoclinic, P21/c, Z=4, a=16.618(7), b=17.850(11), c=11.005(9) A ° , b= diVerence in dr indicated by X-ray data is not found here may reflect a limitation of the computing method used.The ca. 92.25(5)°, V=3262(6) A ° 3, l=0.71069 A ° . 12 kJ mol-1 decrease of DHf observed for 1b suggests that, as expected, the presence of the bridge enhances the stability of Data collection. Data collection by the zig-zag v scan technique, 2°H25°, tmax=40 s, range h, k, l (h 0,13; k 0,21; the molecule. As shown in Fig. 4, whereas for 4b the highest coeYcients of the HOMO are located on the 1,3-dithiole units, l -19,19), intensity controls without appreciable decay (0.2%) gives 6202 reflections from which 1279 were independent for 1b the distribution is more homogeneous which suggests a better delocalization of p-electrons over the whole molecule.with I>3s(I). The data of the cation radical show that in both cases dr becomes negative, in agreement with the expected inversion of Structure refinement.After Lorentz and polarisation corrections the structure was solved with direct methods (SIR) which bond alternation (see Scheme 2). Oxidation of 4b into 4b·+ leads to a large increase in the dipole moment, consistent with reveal all the non-hydrogen atoms. After isotropic and aniso- 2030 J.Mater. Chem., 1997, 7(10), 2027–2032tropic refinement of all the C and S atoms respectively, the 6.62 (s, 2H), 6.68 (s, 2H). MS m/z 657 (M·+), HRMS calc. for coordinates of H atoms were determined using the HYDRO C26H24S10 655.9085, found 655.9064. UV–VIS (CH2Cl2) program. The whole structure was refined by the full-matrix lmax/nm (log e) 481(4.84), 516 (4.82) least-squares techniques {use of F magnitude; Uij for S atoms, x, y, z and B fixed for H; 191 variables and 1276 observations, weighting w=1/s(F0)2=4(F0)2/ [s(I)2+(0.04 F02)2]} with the 2,2¾-Bis(4,5-dipropyl-1,3-dithiol-2-ylidenemethyl )-6,6¾-bi(4,5- resulting R=0.061, Rw=0.060.dihydro-6H-cyclopenta[b]thienylidene) 1c. This compound was prepared using the same procedure from dithiolium salt 6c Theoretical calculations (0.90 g (2.72 mmol), trimethyl phosphite (0.32 ml, 2.72 mmol), KI, (0.41 g, 2.72 mmol) and dicarbaldehyde 2 (0.20 g, The semi-empirical PM3 method17 has been parametrized to 0.67 mmol).After the usual work-up the product was purified reproduce gas-phase properties, i.e. geometry, dipole moment, by column chromatography (silica gel, CH2Cl2). Yield (47%), ionization potential and heat of formation.The PM3 method red powder, mp 175–180 °C. dH (CDCl3) 1.39–1.82 (m, 24H), was used in the framework of the HYPERCHEM 5.0 pack- 2.21–2.71 (m, 8H), 6.88 (s, 2H), 7.29 (s, 2H). dC (CDCl3) 13.51, age.18 The geometry of 1b, 4b and of their dication has been 13.63, 22.65, 29.66, 30.54, 35.27, 106.18, 118.47, 122.96, 127.55, optimized at the RHF level of theory and the geometry of 128.78, 128,92, 131.86. MS m/z 641 (M·+100), 455 (17%), 320 1b·+ and 4b·+ has been computed at the UHF level.All (21%), 111(34%). HRMS calc. for C34H40S6 640.1454, found optimized geometries are almost planar and have root-mean- 640.1442. UV–VIS (CH2Cl2) lmax/nm (log e) 484(4.83), square gradient values lower than 0.1 kcal mol-1A ° -1 (1 cal= 520(4.83). 4.184 J). Ionization potential values were obtained through Koopmans’ theorem.19 6,6¾-Bi(4,5-dihydro-6H-cyclopenta[b]thienylidene) 3. Com- References pound 3 was synthesized according to a previously described procedure.10,12 1 (a) T. Sugimoto, H. Awaji, I. Sugimoto, Y. Misaki, T. Kawase, S. Yoneda and Z. Yoshida, Chem. Mater., 1989, 1, 535; (b) A. Benahmed-Gasmi, P.Fre` re, B. Garrigues, A. Gorgues, 2,2¾-Diformyl-6,6¾-Bi(4,5-dihydro-6H-cyclopenta[b]thienyli- M. Jubault, R. Carlier and F. Texier, T etrahedron L ett., 1992, 33, dene 2. In a round-bottomed flask equipped with a dropping 6457; (c) T. K. Hansen, M. V. Lakshmikantam, M. P. Cava, funnel and nitrogen inlet was added 3 (0.1 g, 0.41 mmol) in R. E. Niziurski-Mann, F. Jensen and J. Becher, J.Am. Chem. Soc., 20 ml of dry tetrahydrofuran (THF). The mixture was cooled 1992, 114, 5035; (d) J. Roncali, M. GiVard, P. Fre`re, M. Jubault to 0 °C and BuLi (1.6 M in hexanes) (0.54 ml, 0.86 mmol) was and A. Gorgues, J. Chem. Soc., Chem. Commun. 1993, 689; (e) added dropwise. After 15 min stirring at 0 °C, anhydrous J. Roncali, L. Rasmussen, C. Thobie-Gautier, P. Fre` re, H.Brisset, dimethylformamide (DMF) (0.20 ml, 2.34 mmol) was added; M. Salle�, J. Becher, O. Simonsen, T. K. Hansen, A. Benahmed- Gasmi, J. Orduna, J. Garin, M. Jubault and A. Gorgues, Adv. the mixture was allowed to warm to room temp. and stirred Mater., 1994, 6, 841. for 30 min. Water was then added and the mixture extracted 2 (a) M. R. Bryce, J. Mater. Chem., 1995, 5, 1481; (b) M.Adam and with diethyl ether. The organic phase was dried over CaCl2. K. Mu� llen, Adv. Mater., 1994, 6, 439; (c) Y. Misaki, N. Higuchi, Removal of the solvent and column chromatography of the H. Fujiwara, T. Yamabe, T. Mori, H. Mori and S. Tanaka, Angew. residue (silica gel, CH2Cl2) gave 0.10 g (81%) of a red powder, Chem., Int. Ed. Engl., 1995, 34, 1222. mp 198–200 °C, dH (CDCl3) 3.00–3.15 (m, 8H), 7.79 (s, 2H), 3 H.Brisset, C. Thobie-Gautier, M. Jubault, A. Gorgues and J. Roncali, J. Chem. Soc., Chem. Commun., 1994, 1765. 9.86 (s, 2H). n/cm-1 (KBr) 1650 (CNO).MS m/z 300 (M.+ 100). 4 (a) V. P. Rao, K.-Y. Jen, K. Y. Wong and K. J. Drost T etrahedron L ett., 1993, 34, 1747; (b) U. Scho� bert, J. Salbeck and J. Daub, Adv. 2,2¾ - Bis[4,5 - bis(methoxycarbonyl) - 1,3 - dithiol-2 - ylidene - Mater., 1992, 4, 41; (c) K.-Y.Jen, V. P. Rao, K. Y. Wong and methyl]-6,6¾-bi(4,5-dihydro-6H-cyclopenta[b]thienylidene) 1a. K. J. Drost, J. Chem. Soc., Chem. Commun., 1993, 90. To a stirred solution of phosphonium salt 5a (1 g, 2 mmol) 5 (a)M. Sylla, J. Zaremba, R. Chevalier, G. Rivoire, A. Khanous and A. Gorgues, Synth. Met., 1993, 59, 111; (b) T.T. Nguyen, M. Salle�, and dialdehyde 2 (0.1 g, 0.33 mmol) in 20 ml of CH3CN, B. Sahraoui, M. Sylla, J. P. Bourdin, G. Rivoire and J. Zaremba, triethylamine (0.32 ml, 2.33 mmol) was added dropwise. After J.Modern Opt., 1995, 42, 2095. 2 h stirring at room temp., the solvent was removed by 6 (a) I. Cabrera, O. AlthoV, H.-T. Man and H. N. Yoon, Adv.Mater., evaporation and the residue purified by column chromatogra- 1994, 6, 43; (b) M.Blanchard-Desce, C. Runser, A. Fort, phy (silica gel, light petroleum–ethyl acetate 951) to give 67 mg M. Barzoukas, J. M. Lehn, V. Bloy and V. Alain, Chem. Phys., (30%) of a dark green powder with a metallic lustre, mp 1995, 199, 253; (c) H. Gibson and J. Pochan,Macromolecules, 1991, >280 °C, dH (CDCl3) 2.83–3.45, (m, 8H), 3.87, (s, 6H), 3.89 (s, 24, 4834. 7 (a) S. Gilmour, R. A. Montgomery, S. R. Marder, L.-T. Cheng, K.- 6H), 6.59 (s, 2H), 6.72 (s, 2H). MS (EI) m/z 704 (M.+ 100). Y. Jen, Y. Cai, J. W. Perry and L. R. Dalton, Chem. Mater., 1994, n/cm-1 (KBr) 1705 and 1737 (CNO). UV–VIS (CH2Cl2) 6, 1603; (b) V. Hernandez, C. Castiglioni, M. Del Zopo and lmax nm (log e) 469 (3.99), 501 (3.97). G. Zerbi, Phys.Rev. B, 1994, 50, 9815. 8 (a) E. Elandaloussi, P. Fre`re, J. Roncali, P. Richomme, M. Jubault 2,2¾ - Bis[4,5 - bis(methylthio) - 1,3 - dithiol -2 -ylidenemethyl] - and A. Gorgues, Adv.Mater., 1995, 7, 390; (b) A. Benahmed-Gasmi, P. Fre` re, E. Elandaloussi, J. Roncali, J. Orduna, J. Garin, 6,6¾-bi(4,5-dihydro-6H-cyclopenta[b]thienylidene) 1b. In a M. Jubault, A. Riou and A.Gorgues, Chem. Mater., 1996, 8, 2291; round-bottomed flask equipped with a dropping funnel and (d) E. Elandaloussi, P. Fre`re, A. Benahmed-Gasmi, A. Riou, nitrogen inlet were introduced dithiolium salt 6b (0.28 g, A. Gorgues and J. Roncali, J.Mater. Chem., 1996, 6, 1859. 1 mmol), trimethyl phosphite (0.12 ml, 1 mmol) and KI (0.15 g, 9 E. Elandaloussi, P. Fre` re and J. Roncali, T etrahedron L ett. 1996, 1 mmol) in 3 ml of acetonitrile. After 2 h stirring at room 37, 6121. temp., evaporation of the solvent and excess of trimethyl 10 J. Roncali, Thobie-Gautier, E. Elandaloussi and P. Fre`re, J. Chem. phosphite left the phosphonate a. Dry THF (3 ml) and Soc., Chem. Commun., 1994, 2249. 11 D. W. H. MacDowell, T. B. Patrick, B. K. Frame and D. L. Ellison, dicarbaldehyde 2 (0.08 g, 0.25 mmol) were then added and the J. Org. Chem., 1967, 32, 1227. mixture cooled to 0 °C. n-Butyllithium (0.63 ml, 2 mmol) (1.6 M 12 (a) H. Brisset, P. Blanchard, B. Illien, A. Riou and J. Roncali, Chem. in hexanes) was added dropwise, and the mixture stirred for Commun., 1997, 569 (b) P. Blanchard, H. Brisset, B. Illien, A. Riou 2 h at room temp. Upon addition of methanol a red precipitate and J. Roncali, J. Org. Chem., 1997, 62, 2401. was formed which is filtered, washed with methanol–diethyl 13 (a) K. Akiba, K. Ishikawa and N. Inasoto, Bull. Chem. Soc. Jpn., ether and dried. Yield 0.07 g (45%), mp 261–263 °C. dH (CDCl3) 1978, 51, 2674; (b) T. K. Hansen, M. V. Lakshmikantham, M. P. Cava, R. M. Metzger and J. Becher, J. Org. Chem., 1991, 56, 2.46 (s, 6H), 2.48 (s, 6H), 2.90–3.10 (m, 4H), 3.20–3.35 (m, 4H), J. Mater. Chem., 1997, 7(10), 2027–2032 20312720; (c) A. J. Moore, M. R. Bryce, D. T. Ando and 17 (a) J. J. P. Stewart, J. Comput. Chem., 1989, 10, 209; (b) J. J. P. Stewart, J. Comput. Chem., 1989, 10, 221. M. B. 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