Oligo(furan=2,5=diylvinylene)s as n-conjugating spacers in linearly extended hybrid tetrathiafulvalene analogues El Hadj Elandaloussi," Pierre Fr&re,*" Amina Benahmed-Gasmi,".b AmCdCe Riou," Alain Gorguesa and Jean Roncali**" "Inginierie Moliculaire et Matiriaux Organiques, CNRS EP 66, Universiti d 'Angers, 2 Bd Lavoisier, 49045 Angers, France bLaboratoire de Chimie Organique, Universitd d 'Oran ES-SENIA, 31 000, Algirie The synthesis of a new series of push-push systems based on furan-2,5-diylvinylene oligomers end-capped with 1,3-dithiol-2- ylidene electron-releasing groups is described. 'H NMR spectroscopy and X-ray diffraction reveal that the molecules adopt a planar conformation stabilized by strong intramolecular interactions. Electronic absorption spectroscopy indicates that the effective conjugation length increases steadily with the length of the n-conjugated system without any evidence of saturation. Analysis of electrochemical behaviour by cyclic voltammetry shows that, whereas all compounds are easily oxidized into stable dications, with extension of the conjugation length the oxidation process progressively evolves from two successive one-electron steps to a single step two-electron transfer. Linear n-conjugated oligomers end-capped with 1,3-dithiol-2- ylidene units are the subject of much current interest.'-7 While initially developed as precursors of conducting cation-radical salts in the general frame of spatially extended tetrathiafulva- lene analogue^,^,^ recent years have witnessed a wide diversifi- cation of the potential applications of these systems as small bandgap molecular semiconductors," or as building blocks in push-pull or push-push molecules for quadratic or cubic nonlinear optics.''-18 From this viewpoint, it is clear that the properties of the final molecule will be determined largely by the nature of the n-conjugating spacer group.During the past few years, a great variety of n-conjugating structures have capped with 1,3-dithiol-2-ylidene electron-releasing groups (F,V,a-d). In the short-hand notation used, F, refer to the number of furan rings in the spacer and V, to the number of double bonds, while a-d refer to the R substituents of the 1,3-dithiole ring. The geometry of these compounds has been characterized by 'H NMR spectroscopy and X-ray diffraction and the relationships between their structure and their optical and electrochemical properties are discussed.been developed including acetylenic,12 ethyleni~'*~.~~-~~ and (hetero-) aromatic groups.2-6i10,1'~'6-20 Although polyalkenic spacers present high electron trans- mission effi~iency,~~,'~*~~ their lack of thermal and photochem- ical stability has been pointed out as a severe drawback.13v21 On the other hand, the efficiency of the more stable aromatic- and acetylenic-based systems appears limited by excessive charge c~nfinement.'~.~~ In this context oligo(aryleneviny1-ene)s, and in particular thiophene-2,5-diylvinyleneoligomers (nTVs), have progressively emerged as a good trade-off between the efficiency of polyethylenic systems and the stability of the polyaromatic one^.^^,^' We have recently reported the synthesis of push-push sys-tems based on nTVs end-capped with 1,3-dithiol-2-ylidene moieties.22 We have shown that, contrary to their analogues involving oligothiophenes (nTs), which are subject to interan- nular rotational disorder,' the lengthening of the n-conjugating spacer in nTVs-based systems does not lead to saturation of the effective conjugation.In addition to the synthesis of longer nTVs-based systems,23 an interesting development of this class of linearly .n-conjugated systems could involve replacement of the nTVs spacer by furan-2,5-diylvinylene oligomers (nFVs). This is interesting for two reasons.First, the smaller aromatic resonance energy of furan (0.70eV us. 1.26 for thi~phene)~~ will decrease the overall aromatic character of the spacer, which could allow a better electron delocalization over the whole n-conjugated system. Secondly, as already underlined, furaldehyde and furfuryl alcohol are much less expensive starting materials than their thiophene analogues and represent possible non-food appli- cations for agricultural product^.^' As a first step in this direction, we report here the synthesis of a new series of push-push systems based on nFVs end- Fa,b,d R a R = CaMe b R=SMe c R=Pr d R=H Results and Discussion Compounds Fa,b,d were prepared as previously described2 and diethyl 2-furylmethylphosphonate 2 was obtained in 68 % yield from the corresponding alcohol 1 via the bromomethyl derivative.26 Compounds F,V, were synthesized using the strategy shown in Scheme 1. E-1,2-Bis(2-furyl)ethene4 was obtained by McMurry coupling27 of furaldehyde 3 in 72% J.Muter. Chem., 1996, 6(12), 1859-1863 1859 1 2 &CHO0 111_ %L%cHo\I \I 3 4 5 6 7R=H 8R=CHO1 VII or VIIIv3:1 FSV2k Scheme 1 Reagents 1, PPh3, Br,-Et,O, 11, HPO(OEt),, NaH-THF, 111, TiCl,, Zn, iv, POCl,, DMF-DCE, v, BuLi, TMEDA, DMF-THF- hexane, vi, 2, Bu'OK, THF, vii, W, Et,N-THF, viii, P, BuLi-THF yield Formylation of 4 followed by Wittig-Horner olefination with 2 afforded compound 7 in 46% overall yield Dicarbaldehyde 6 was obtained in 46% yield by reaction of DMF with the dilithiated derivative of 4 prepared in the presence of N,N,N',N'-tetramethyldiaminoethane (TMEDA) Vilsmeier bisformylation of 7 using a large excess of reagent (4 equiv) gave the dicarbaldehyde 8 in 71% yield The trans configuration of 8 is clearly confirmed by the 3J 15 Hz coupling of the 'H NMR spectral signal of the ethenic protons Finally, Wittig or Wittig-Horner bis-alkenation of dicarbaldehydes 6 or 8 with Akibas' reagents W, or Pb,28-30afforded compounds F,Vla-c and F,V,a-c in 65-90% yields Compound F2Vld was obtained in 20% yield by thermal demethoxycarbonylation of F,V,a in hexamethylphosphoramide (HMPA) in the pres- ence of LiBr.H,O (Scheme 2) ' All F,V, compounds were obtained as single isomers The 'H NMR spectral signals of the ethenic protons of compounds F3V2,,c give an AB system with a 16 Hz 3J coupling which confirms the trans configuration, whereas for F,V, compounds a singlet was observed X-Ray structure determination of F,V,d confirms the trans configuration of the ethenic linkage, while the 134" angle between the plane of the furan nng and that of the dithiole Scheme 2 Reagents 1, HMPA, LiBr.H,O 1860 J Mater Chern, 1996, 6(12), 1859-1863 Fig.1 ORTEP view of F,Vld, H atoms omitted, ellipsoids drawn at 2 5% probability level ring shows that the molecule adopts a planar conformation (Fig 1) Furthermore, the crystal structure reveals that each end of the molecule adopts a syn conformation stabilized by strong S 0 intramolecular interactions The non-bonded contact [d=2 88(1)A] is shorter thay the sum of S and 0 van der Waals radii (18.+ 15 =3 3 A), but larger than a covalent S-0 bond (1 75 A) Such interactions between the sulfur atoms of a 1,3-dithiole ring and the oxygen or sulfur atom of an adjacent heterocycle have already been observed Table 1 lists the main electronic absorption data for F,V, compounds For each n value, the absorption maximum (Amax) exhibits a marked dependence on the nature of the substituent grafted at the 1,3-dithiole ring (R) Thus, for F2Vld, substitution of hydrogen by the electron-withdrawing C0,Me group to give F,Vla produces a blue shift of A,,, from 470 to 460nm, whereas introduction of electron releasing groups such as SMe (F,V,b) or propyl (F,Vlc) leads to the opposite effect, with a bathochromic shift of A,,, to 476 and 477nm, respectively On the other hand, Fig 2 and the data in Table 1 show that, independently of R, the lengthening of the spacer group produces as expected a red shift of A,,, Thus for compounds b, insertion of one (F,V,b) or two (F,V,b) FV units leads to a red shift of La,from 441 to 476 nm and 502 nm, respectively These results indicate that, in contrast to nTs-based systems,' the use of nFVs conjugating spacers does not lead to saturation of the effective conjugation, in agreement with the results obtained for their nTVs analogues 22 The cyclic voltammetry data of F,V, are listed in Table 2 As expected, the electrochemical behaviour depends on R and on the length of the nFV spacer group Thus, for F,V, compounds, replacing the electron-withdrawing C02Me group with the electron-releasing propyl group produces a consider- able negative shift of the first anodic peak potential (Epal)from 0 48 to 0 18 V Unlike what might be expected from optical data, replacement of hydrogen by SMe in the F1 and F2Vl series induces a 100-170 mV positive shift of Epal Such behaviour is commonly observed in the TTF series where alkylsulfanyl groups decrease n-donor ability 32 Table1 UV-VIS spectroscopic data for compounds Fa, b, d and F,V,a-d in CH,Cl, compound Laxlnm 402,425 421,441 411,437 460 476 470 477 501 508 340 380 420 460 500 540 580 Alnm Fig.2 UV-VIS spectra of (a) F,V,b and (b) F3V2bin CH2C1, Although insertion of an additional FV unit into F, com-pounds leads to a negative shift of Epal,the magnitude of this shift depends strongly on R. Thus, whereas for R =COzMe E,, shifts by 0.16 V from 0.64 to 0.48 V, this negative shift, which is only of 0.08 V for R=SMe, vanishes for R=H, and Fd and F,V1d show the same Epalat 0.23 V. A further extension of the length of the FV spacer does not lead to a further decrease of Epa,but, in contrast, to a 10-20 mV positive shift. Such behaviour, already observed for the parent compounds containing ethenic' or nTVsZ2 spacers suggests, in agreement with previous theoretical conclusions, that in these extended n-conjugated systems, the major contribution to the HOMO level arises from the 173-dithiole ring, while the length of the spacer essentially affects the LUMO level.Attempts to confirm this conclusion by recording the reduction potential of the compounds have until now remained unsuccessful, and perhaps further theoretical work could contribute to a more detailed understanding of this point. The CV of Fa exhibits two reversible one-electron oxidation steps corresponding to the successive generation of the cation radical and dication [Fig. 3(u)]. The difference between the two anodic peak potentials (Epa2-Epal) decreases from 220 mV for Fa to 170 and 80 mV for respectively Fb and Fd (Table 2), indicating that the increase of the electron-releasing effect of R leads to a reduction of the coulombic repulsion between positive charges in the dication.Extension of the conjugation length leads to a significant decrease of Epa2-Epal and, as shown by the CV of F2V,b [Fig. 3(b)],the two successive one-electron oxidation steps draw closer together, with a first anodic shoulder at 0.48 V followed by a main reversible oxidation wave peaking at 0.57 V. Although the proximity of these two waves does not allow a direct determi- nation of Epa2-Epal,measurement of the peak width at half maximum (PWHM) and application of the Nicholson and Myers treatment33 gave a value for Epa2-Epal of 110 mV. Whereas the CV of all other F,V, compounds display only a single reversible oxidation wave, determination of the PWHM for the F2Vl series gives values of 60 mV for F2V,b and 45 mV for F2Vlc and d (Table 2).These values, slightly larger than the 28.5 mV expected for an ideal two-electron transfer, suggest that the oxidation process involves two very close one- electron steps (Epa2 -Epal =10 mV) rather than a true two- electron transfer. In contrast, the 30 mV PWHM and zero value of Epaz-Epal obtained for the longest F3V2 series unequivocally show that these compounds are directly oxidized into their dicationic state through a single-step two-electron transfer. Furthermore, as shown in Fig. 3(c), this two-electron wave is followed by a reversible one-electron oxidation peak at 0.96 V and by a less reversible peak at 1.42 V. While the limited potential window of the used electrolytic medium does not allow a definitive assignment of this last peak to the formation of the tetracation, these results clearly show that the tricationic state of F,V2 compounds can be reached at rather moderate potential (cu.1.0 V us. SCE). Conclusion New linearly extended push-push molecules built around oligo(f~ran-2~5-diylvinylene)conjugating spacers have been synthesized. These compounds adopt a trans planar geometry and an increase in the length of the spacer produces the expected red shift of absorption without evidence for satu- ration. Extension of the conjugation length leads to an increase of the number of accessible redox states, allowing the direct formation of stable dications at low potentials.The analysis of the third order non-linear optical properties of these com- pounds is now under way and will be reported in future publications. Experimental Electrochemical experiments were carried out with a PAR 273 potentiostat-galvanostat in three-electrode single compartment cells. All potentials are referenced to the saturated calomel electrode (SCE). Cyclic voltammetry was performed using Pt microelectrodes of 7.85 x cm2 area and a Pt wire counter electrode. Oxidation potentials were determined from dichloro- methane solutions (HPLC grade) containing M substrate and lo-' M tetrabutylammonium hexafluorophosphate (TBHP) (Fluka puriss). Solutions were deaerated by nitrogen bubbling prior to each experiment which was run under a nitrogen atmosphere.Diethyl 2-furylmethylphosphonate 2 was obtained from 2- furylmethanol 1 using a known pr0cedu1-e.~~ E-1,2-Bis( 2-fury- 1)ethene 4 was prepared by McMurry coupling of furaldehyde. Compound 5 was prepared by formylation of 4 and purified by column chromatography (silica gel, CH,Cl,) giving yellow crystals (53%), mp 75°C; MS (EI) m/z 188 (M'; 100%); 6, [270 MHz, (CD3),SO] 9.53 (s, lH, CHO), 7.77 (s, lH, furan- Table 2 Electrochemical data for FVn" 0.64 0.86 220 0.40 0.57 170 0.23 0.31 80 0.48' 0.57 110 0.32 60 40' 1.35 0.23 45 1oc 1.30 0.18 45 10' 1.35 0.33 30 0 1.1 1.42 0.25 30 0 0.96 1.40 "Samples, M in 0.1 M Bu4NPF6-CH2C1,. Scan rate 100mV s-'. 'Shoulder. "Calculated by Myers and Shain's method (ref.33). J. Mater. Chem., 1996,6( 12), 1859-1863 1861 I I 000 025 050 075 1.00 0.00 0.25 EN Fig. 3 Cyclic voltammograms of (a)Fa, (b) F,V,b and (c) F,V,c Samples 100mV s-' H), 7 56 (d, lH, 3J 3 8 Hz, furan-H), 7 17 (d, lH, 3J 16 0 Hz, =CH), 6 89 (d, lH, 3J 160 Hz, =CH), 6 86 (d, lH, 3J 3 8 Hz, furan-H), 6 75 (d, lH, 3J 3 3 Hz, furan-H), 6 59 (dd, lH, 3J 3 8 and 3J 3 3 Hz, furan-H), v,,,(KBr)/cm-' 1660 cm-I (C=O), 1611 (C=C) E-1,2-Bis( 5-formyl-2-fury1)ethene 6 Butyllithium (1 6 M in hexane, 48 mmol) was added dropwise to a stirred solution of 4 (20 mmol) and TMEDA (48 mmol) in 100 ml THF-hexane (1 1) at room temperature The mix- ture was refluxed for 30 min, cooled to 40 "C and DMF (10 ml, 130 mmol) was added dropwise After 30 min stirring at ambi- ent temperature, the mixture was hydrolysed with 05~ HCl and extracted with CH2C1, The organic phase was washed with water and dried over MgS04 After removal of the solvent, the solid residue was washed with CC14 and recrys- tallized from chloroform giving 2 06 g (47%) of brown powder, mp 244"C, MS (EI) m/z 216 (M+, loo%), 6H[(CD3)2S0] 9 59 (s, 2H, CHO), 7 60 (d, 2H, 3J 3 8 Hz, furan-H), 7 26 (s, 2H, =CH), 7 03 (d, 2H, 3J 3 8 Hz, furan-H), 6c[(CD3)2SO] 178 0, 156 4,151 9, 118 2,114 0,108 4, vmX (KBr)/cm-l 1665 (C=O) E,E-2,5-Bis [ 242-furyl )vinyl] furan 7 To a stirred solution of 5 (094 g, 5 mmol) and diethyl 2- furylmethylphosphonate 2 (1 1 g, 6 mmol) in 30 ml dry THF, ButOK (0 73 g, 6 5 mmol) was added portionwise under N, After 40min stirring at room temperature the mixture was poured into 30ml water and extracted with CH2C12 The organic phase was washed with water, dried (MgS04) and evaporated under reduced pressure Column chromatography (silica gel, CH,C12) and recrystallization from hexane gave 1 1 g (87%) of brown powder, mp 104"C, MS (EI) m/z 252 (M+, loo%), hH (CDC13) 7 42 (d, 2H, 3J 1 65 Hz, furan-H), 6 94 (d, 2H, 3J 16 0 Hz, =CH), 6 81 (d, 2H, 3J 16 0 Hz, =CH), 6 45 (dd, 2H, 3J 3 3 and 3J 1 65 Hz, furan-H), 6 39 (d, 2H, 3J 3 3 Hz, furan-H), 6 36 (s, 2H, furan-H), G,(CDCl,) 1530, 152 6, 1422, 1148, 1144, 111 8, 111 6, 1089 1862 J Muter Chem, 1996, 6(12), 1859-1863 I I I 11 I 0.00 0.50 1.00 1.50 0.50 0.75 M in 0 1 M tert-butyl hydroperoxide (TBHP)-CH,Cl, Scan rate E,E-2,5-Bis [2-(5-formyl-2-fury1 )vinyl J furan 8 To a stirred solution of 7 (0 504 g, 2 mmol) and DMF (0 77 ml, 10 mmol) in 20 ml 1,2-dichloroethane, POC13 (30 ml, 48 mmol) was added dropwise and the mixture was refluxed for 3 h The mixture was cooled, hydrolysed with 3 M aqueous NaOAc and extracted with CH2Cl, The organic phase was washed with water and dried over MgSO, After removal of the solvent, the solid residue was recrystallized from chloroform to provide 0 44 g, (71%) of red powder, mp 183 "C, MS (EI) m/z 308 (M+, loo%), SH(CDC13) 7 24 (d, 2H, 3J 3 8 Hz, furan-H), 7 10 (d, 2H, 3J 16OHz, =CH), 690 (d, 2H, 3J 160Hz, =CH), 653 (d, 2H, '.I38Hz, furan-H), 651 (s, 2H, furan-H), h,(CDC13) 176 8, 158 1, 152 8, 151 7, 123 9, 119 3, 114 8, 113 9, 111 4, vmaX (KBr)/cm-' 1671 (C=O), 1620 (C=C) Compounds F,V,a-d and F,V,b,c Compounds F,V, and F3VZwere prepared by Wittig or Wittig- Horner reaction at 0°C with the corresponding ylide or phosphonate anions using a literature procedure 22 1,2-Bis(5-[ 4,sbis (methox ycarbon yl)-2H- 1,3-dithiol-2- ylidenemethyl]-2-furyl)ethene F,V,a Recrystallized from CHC13 (65%), brown powder, mp 255 "C (dec ), MS (EI) m/z 620 (M', 100%) (Found C, 5034, H, 325 Calc for C2&I2001oS4 C, 50 32, H, 3 22), GH[(CD3)2S0] 6 77 (s, 2H, =CH), 6 71 (s, 2H, S2C=CH), 6 69 (d, 2H, 'J 3 6 Hz, furan- H), 6 31 (d, 2H, 3J 3 6 Hz, furan-H), 3 87-3 86 (2s, 12H, Me0,C) 1,2-Bis(5-[ 4,5-bis(methylsuIfanyl)-2H-1,3-dithiol-2-ylidenemethyl]-2-furyl)ethene F2Vlb Recrystallized from CHC1, (go%), red-orange powder, mp 156°C (dec), MS (EI) m/z 572 (M+, loo%), hH[(CD3),SO] 6 75 (s, 2H, =CH), 6 70 (s, 2H, S,C=CH), 6 68 (d, 2H, 3J 3 5 Hz, furan-H), 6 34 (d, 2H, 3J 3 5 Hz, furan-H), 2 49-2 45 (2s, 12H, SMe) 1,ZBis[5-( 4,5-dipropyl-2H-l,3-dithiol-2-ylidenemethyl)-2-furyllethene FzVlc.Recrystallized from CHC1, (50%), red powder, mp 139°C (dec.); MS (EI) m/z 556 (M'; 100%); ~H[(CD~)~SO]6.70 (s, 2H, =CH), 6.66 (s, 2H, S,C=CH), 6.62 (d, 2H, 3.5 Hz, furan-H), 6.30 (d, 2H, 3.5 Hz, furan-H), 2.44-2.34 (m, 8H, 4 CH,), 1.67-1.51 (m, 8H, 4 CH,), 1.03-0.92 (m, 12H, 4 CH,). 2,5-Bis (2-{5-[4,5-bis(methylsulfanyl)-2H-1,3-dithiol-2-ylidenemethyll-2-fury1 )vinyl )furan F3V2b.Recrystallized from CHC1, (88%), red powder, mp 170°C (dec.); MS (FAB') m/z 664 (M+; 100%) (Found: C, 50.38; H, 3.58. Calc. for C2sH2403Ss: c, 50.61; H, 3.64); dH(CDC13) 6.87 (d, 2H, 15.7 Hz, =CH), 6.75 (d, 2H, 15.7 Hz, =CH), 6.40-6.37 (m, 4H, furan-H), 6.35 (s, 2H, S2C=CH), 6.16 (d, 2H, 3.3Hz, furan-H), 2.49-2.45 (2s, 12H, 4 MeS). 2,5-Bis{2-[ 5-( 4,5-dipropyl-2H-l,3-dithiol-2-ylidenemethyl)-2-furylI viny1)furan F3V2c. Chromatographed (silica gel, CH,Cl,) (76%), red powder, mp 118°C (dec.); MS (FAB+) m/z 648 (M'; 100%) (Found: C, 66.62; H, 6.63. Calc. for C36H4003S4:C, 66.65; H, 6.22); G,(CDCl,) 6.85 (d, 2H, 15.7 Hz, =CH), 6.74 (d, 2H, 15.7 Hz, =CH), 6.39 (d, 2H, 3.3 Hz, furan-H), 6.35-6.34 (2s, 4H, 2 furan-H and 2 S,C=CH), 6.13 (d, 2H, 3.3 Hz, furan-H), 2.44-2.34 (m, 8H, 4 CH,), 1.67-1.51 (m, 8H, 4 CH2), 1.03-0.92 (m, 12H, 4 CH,).1 ,2-Bis [5-(2H-1,3-dithiol-2-ylidenemethyl)-2-furyl]ethene FzVld. A solution of F,V,a (0.34 g) and LiBr (1.14 g) in HMPA (15 ml) was warmed for 1h at 90 "C, then for 20 min at 120 "C. The mixture was poured into 60 ml water and extracted with CH,Cl,. After drying over MgSO,, the solvent was evaporated under reduced pressure and the residual oil was purified by column chromatography (silica gel, CH2Cl,) to provide a red solid that was recrystallized from CHCl, (42%), mp 224°C (dec.); MS (EI) m/z 388 (M'; 100%); i5H[(CD3)2S0] 6.81-6.78 (m, 4H, SCH=), 6.74 (s, 2H, =CH), 6.55 (s, 2H, S2C=CH), 6.62 (d, 2H, 3J 3.5 Hz, furan-H), 6.26 (d, 2H, 3.5 Hz, furan-H). X-Ray diffraction Crystals of F,V,d were obtained from CHC1, solution. A single crystal (0.8 x 0.5 x 0.1) was mounted on an Enraf-Nonius CAD4-MACH3 diffractometer with graphite monochromator and Mo-Ka radiation at T=294 K.Unit cell parameters were known from a least-square analysis of 25 reflections. Crystal data. C18H12S402bM =388.55, orbhorhombic, Pb~u, U= 12.158 (6), b=7.939 (6) A, c 18.803 (5) A, I/= 1815 (4) A,, 2=4, F(000)=800, ,~=3.442cm-~,D,=1.42$)g~m-~, A= 0.71069 A. Data collection. The data collection was by the zig-zag 428 scan technique, 2.0 <8<30, t,,, =40 s, range HKL (0<H <11; 0 <K <17; -26 <L <26), intensity controls without appreci- able decay (0.2%) gives 5354 reflections from which 674 independent were measured with I >3a(I), Rint=2.0%.Structure refinement. After Lorentz and polarization correc- tions the structure was solved with Direct Methods (SIR) which reveal all the non-hydrogen atoms. After anisotropic refinement, the hydrogen atoms were found from hydro pro- grams. The whole structure was refined by the full-matrix least-square techniques (use of F magnitude; x, y, z, Pij for S, C and 0 atoms; x, y, z for H; 127 variables and 674 obser- vations; o=l/c~(F~)~=[a2(I)+(0.04 F02)2]-1/2) with the resulting R =0.066, Rw =0.071. Atomic coordinates, thermal parameters, and bond lengths and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC). See Information for Authors, J.Mater. Chem., 1996, Issue 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 1145/12. References 1 T. Sugimoto, H. Awaji, I. Sugimoto, Y. Misaki, T. Kawase, S. Yoneda and Z. Yoshida, Chem. Mater., 1989,1,535. 2 A. Benahmed-Gasmi, P. Frere, B. Garrigues, A. Gorgues, M. Jubault, R. Carlier and F. Texier, Tetrahedron Lett., 1992, 33, 6457. 3 T. K. Hansen, M. V. Lakshmikantam, M. P. Cava, R. E. Niziurski- Mann, F. Jensen and J. Becher, J. Am. Chem. SOC., 1992,114,5035. 4 J. Roncali, M. Giffard, P. Frere, M. Jubault and A. Gorgues, J. Chem. SOC., Chem. Commun., 1993,689. 5 J. Roncali, L. Rasmussen, C. Thobie-Gautier, P. Frere, H. Brisset, M. Sallt, J. Becher, 0. Simonsen, T.K. Hansen, A. Benahmed- Gasmi, J. Orduna, J. Garin, M. Jubault and A. Gorgues, Adu. Mater., 1994,6, 841. 6 K. Takahashi, T. Nihira, M. Yoshifuji and K. Tomitani, Bull. Chem. SOC. Jpn., 1994,66,2330. 7 Y. Misaki, N. Higuchi, H. Fujiwara, T. Yamabe, T. Mori, H. Mori and S. Tanaka, Angew. Chem., Int. Ed. Engl., 1995,34, 1222. 8 M. R. Bryce, J. Mater. Chem., 1995,5, 1481. 9 M. Adam and K. Miillen, Adu. Mater., 1994,6,439. 10 H. Brisset, C. Thobie-Gautier, M. Jubault, A. Gorgues and J. Roncali, J. Chem. SOC., Chem. Commun., 1994,1765. 11 H. E. Katz, K. D. Singer, J. E. Sohn, C. W. Dirk, L. A. King and H. M. Gordon, J. Am. Chem. SOC., 1987,109,6561. 12 M. Sylla, J. Zaremba, R. Chevalier, G. Rivoire, A. Khanous and A. Gorgues, Synth.Met., 1993,59, 11 1. 13 I. Cabrera, 0. Althoff, H-T. Man and H. N. Yoon, Adu. Mater., 1994,6,43. 14 T. T. Nguyen, M. Salk, B. Sahraoui, M. Sylla, J. P. Bourdin, G. Rivoire and J. Zaremba, J. Mod. Opt., 1995,42,2095. 15 T. T. Nguyen, Y. Gouriou, M. Salk, P. Frere, M. Jubault, A. Gorgues, L. Toupet and A. Riou, Bull. Soc. Chim. Fr., 1996, 133, 301. 16 V. P. Rao, A. K-Y. Jen, K. Y. Wong and K. J. Drost, Tetrahedron Lett., 1993,34, 1747. 17 V. P. Rao, A. K-Y. Jen, K. Y. Wong and K. J. Drost, J. Chem. SOC., Chem. Commun., 1993,1118. 18 V. P. Rao, Y. M. Cai and A. K-Y. Jen, J. Chem. Soc., Chem. Commun., 1994,1689. 19 S. Gilmour, R. A. Montgomery, S. R. Marder, L-T. Cheng, A. K-Y. Jen, Y. Cai, J. W. Perry and L. R. Dalton, Chem. Mater., 1994,6, 1603. 20 M. Blanchard-Desce, C. Runser, A. Fort, M. Barzoukas, J. M. Lehn, V. Bloy and V. Alain, Chem. Phys., 1995,199,253. 21 H. Gibson and J. Pochan, Macromolecules, 1991,24,4834. 22 E. Elandaloussi, P. Frkre, J. Roncali, P. Richomme, M. Jubault and A. Gorgues, Adv. Mater., 1995,7,390. 23 E. Elandaloussi, P. Frere and J. Roncali, Tetrahedron Lett., in the press. 24 J. March, Advanced Organic Chemistry, Wiley, New York, 1985, p. 42. 25 U. Schobert, J. Salbeck and J. Daub, Adu. Mater., 1992, 4,41. 26 H. Seeboth and S. Andreae, Z. Chem., 1977,16,399. 27 J. E. McMurry, Chem. Rev., 1989,89,1513. 28 K. Akiba, K. Ishikawa and N. Inasoto, Bull. Chem. SOC. Jpn., 1978, 51,2674. 29 T. K. Hansen, M. V. Lakshmikantham, M. P. Cava, R. M. Metzger and J. Becher, J. Org. Chem., 1991,56,2720. 30 A. J. Moore, M. R. Bryce, D. T. Ando and M. B. Hursthouse, J. Chem. SOC., Chem. Commun., 1991,320. 31 T. K. Hansen, M. R. Bryce, J. A. K. Howard and D. S. Yffit, J. Org. Chem., 1994,59,5324. 32 G. Schukat, A. M. Richter and E. Fanghanel, Sulfur Rep., 1987, 7,155. 33 R. L. Myers and I. Shain, Anal. Chem., 1969,41,480. Paper 6/042185; Received 17th June, 1996 J. Muter. Chem., 1996, 6( 12), 1859-1863 1863