composition of donor : acceptor = 1 : 0.4, its upper bandsare not half-filled. Whereas all other k-type superconduc-tors have the composition of donor : acceptor = 2 : 1 andtherefore, half-filled upper bands. The deviation from half-filling makes the on-site Coulomb repulsion comparativelyunimportant; this stabilizes the metal state, but is disadvan-tageous with respect to superconductivity.[15] The same situ-ation has been found in (EOET±TTP)3AsF6,[5b] and (CH±TTP)(I3)0.31.[5c]

In summary, we have found a k-type organic metal basedon a newly prepared TTP derivative CPEO±TTP. To ourknowledge, there is no report on metallic k-type salt baseon ethylenedioxy-substituted TTF.[16] In contrast, CPEO±and EOET±TTP afford k-type salts retaining metallic con-ductivity down to low temperature. Therefore, combinationof TTP framework and ethylenedioxy substituent would besuitable for exploring new k-type metals. The preparationof radical-cation salts based on the other ethylenedioxysubstituted TTPs is actively in progress.

Experimental

CPEO±TTP: Red microcrystals; m.p. = 246±247 �C (decompose.); 1HNMR (270 MHz, CS2-[2H6]benzene) d 4.17 (s, 4H), 2.49±2.56 (m, 4H), 2.35±2.45 (m, 2H); I.R. (KBr) u (cm±1) 1653, 1452, 1168.

(CPEO±TTP)(SbF6)0.4: C15F2.4H10O2S8Sb0.4, M = 573.02, monoclinic,space group C2/c, a = 45.144(3), b = 7.984(4), c = 10.999(5) �. b = 99.72(2)�,V = 3907(2) �3, Z = 8, Dc = 2.235 g/cm3, Mo Ka radiation, l = 0.71069 �, m =23.06 cm±1, F(000) = 2576.00. The data were collected on a Rigaku AFC7Rdiffractometer equipped with graphite monochromated Mo Ka radiationusing the o±2y scan technique to a maximum 2y of 55�. The structure wassolved by direct methods and refined by full-matrix least squares analysis(anisotropic for non-hydrogen atoms) to R = 0.065, Rw = 0.070 for 1359 ob-served (I ³ 3s (I)) reflections from 4816 unique data. All calculations wereperformed using the teXsan crystallographic software package of MolecularStructure Corporation. Atomic coordinates, bond distances and angles, andthermal parameters have been deposited at the Cambridge CrystallographicData Center.

Received: March 17, 1998Final version: June 25, 1998

±[1] (a) M. Adam, K. Müllen, Adv. Mater. 1994, 6, 439; (b) M. R. Bryce, J.

Mater. Chem. 1995, 5, 1481; (c) T. Otsubo, Y. Aso , K. Takimiya, Adv.Mater. 1996, 8, 203.

[2] T. Mori, Y. Misaki, H. Fujiwara, T. Yamabe, H. Mori, S. Tanaka, Mol.Cryst. Liq. Cryst. 1996, 284, 271.

[3] Y. Misaki, N. Higuchi, H. Fujiwara, T. Yamabe, T. Mori, H. Mori, S.Tanaka, Angew. Chem. Int. Ed. Engl. 1995, 34, 1222.

[4] (a) Y. Misaki, H. Fujiwara, T. Yamabe, T. Mori, H. Mori, S. Tanaka,Chem. Lett. 1994, 1653; (b) T. Mori, Y. Misaki, H. Fujiwara, T. Yama-be, Bull. Chem. Soc. Jpn. 1994, 67, 2685.

[5] (a) T. Mori, H. Inokuchi, Y. Misaki, H. Nishikawa, T. Yamabe, H. Mo-ri, S. Tanaka, Chem. Lett. 1993, 733; (b) Y. Misaki, H. Nishikawa, K.Kawakami, T. Yamabe, T. Mori, H. Inokuchi, H. Mori, S. Tanaka,Chem. Lett. 1993, 2073; (c) Y. Misaki, T. Miura, M. Taniguchi, H. Fuji-wara, T. Yamabe, T. Mori, H. Mori, S. Tanaka, Adv. Mater. 1997, 9,714.

[6] (a) J. M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geiser,H. H. Wang, A. M. Kini, M. H. Whangbo, Organic Superconductors,Prentice Hall, Englewood Cliffs, NJ, 1992; (b) G. Saito, Phosphorus,Sulfur, and Silicon, 1992, 67, 345; (c) J. S. Zambounis, C. W. Mayer, K.Hauenstein, B. Hilti, W. Hofherr, J. Pfeiffer, M. Bürkle, G. Rihs, Adv.Mater. 1992, 4, 33; (d) J. A. Schlueter, U. Geiser, J. M. Williams, H. H.Wang, W. K. Kwok, J. A. Fendrich, K. D. Carlson, C. A. Achenbach,J. D. Dudek, D. Naumann, T. Roy, J. E. Schirber, W. R. Bayless, J.Chem. Soc. Chem. Commun. 1994, 1599.

[7] Y. Misaki, K. Kawakami, H. Fujiwara, T. Yamabe, T. Mori, H. Mori, S.Tanaka, Mol. Cryst. Liq. Cryst. 1997, 296, 77.

[8] A. M. Kini, T. Mori, U. Geiser, S. M. Budz, J. M. Williams, J. Chem.Soc. Chem. Commun. 1990, 647.

[9] Y. Misaki, T. Matsui, K. Kawakami, H. Nishikawa, T. Yamabe, M. Shi-ro, Chem. Lett. 1993, 1337.

[10] The final R value is lower than that of the 2 : 1 assumption (0.070 to0.065).

[11] (a) Y. Misaki, H. Nishikawa, T. Yamabe, T. Mori, H. Inokuchi, H. Mo-ri, S. Tanaka, Chem. Lett. 1993, 1341; (b) H. Fujiwara, Y. Misaki, M.Taniguchi, T. Yamabe, T. Kawamoto, T. Mori, H. Mori, S. Tanaka, J.Mater. Chem. 1998, 8, 1711.

[12] D. Jung, M. Evain, M. H. Whangbo, M. A. Beno, A. M. Kini, A. J.Schultz, J. M. Williams, P. J. Nigrey, Inorg. Chem. 1989, 28, 4516.

[13] T. Mori, A. Kobayashi, Y. Sasaki, H. Kobayashi, G. Saito, H. Inokuchi,Bull. Chem. Soc. Jpn. 1984, 57, 627.

[14] R. Kato, H. Kobayashi, A. Kobayashi, S. Moriyama, Y. Nishio, K. Kaji-ta, W. Sasaki, Chem. Lett. 1987, 459.

[15] K. Kanoda, Hyperfine Interact. 1997, 104, 235.[16] Preparation of a k-type salt based on bis(ethylenedioxy)-TTF has been

reported, however, it shows semiconductive temperature dependence;M. Fettouhi, L. Ouahab, D. Serhani, J. M. Fabre, L. Ducasse, J. Amiell,R. Canet, P. DelhaØs, J. Mater. Chem. 1993, 3, 1101.

Synthesis of Poly[p-(7-phenylene-7-(2¢,5¢-dihexyl-4-biphenylene))norbornane]:The First Soluble Polymer with AlternatingConjugation and Homoconjugation**

By Antonio García Martínez,* JosØ Osío Barcina,*Alvaro de Fresno Cerezo, Arnulf-Dieter Schlüter, andJörg Frahn

The design of materials for use in molecular-scale elec-tronic devices requires an understanding of the mechanismof information transfer by molecular connectors, spacers,

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Fig. 5. Energy band structure and Fermi surface of (CPEO±TTP)(SbF6)0.4.The intermolecular overlap integrals are p = 27.4, c = ±6.3, r = 7.6, b = 18.8 ´10±3.

±

[*] Prof. A. G. Martínez, Dr. J. O. Barcina, A. de F. CerezoDepartamento de Química Orgµnica IFacultad de Ciencias QuímicasUniversidad ComplutenseCiudad Universitaria, E-28040 Madrid (Spain)

Prof. A.-D. Schlüter, J. FrahnFreie Universität Berlin, Institut für Organische ChemieTakustrasse 3, D-14195 Berlin (Germany)

[**] We thank the DGICYT (Spain) for financial support of this work (PB-94-0274). A.F.C. thanks the ªComunidad Autonoma de Madridº for agrant. We are grateful to B. Karakaya for helpful discussions.

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or wires.[1] Connectors such as p-conjugated polymers andoligomers are receiving considerable attention.[1] Energytransfer can take place via two different mechanisms, theso-called Förster (dipole-dipole) mechanism[2] and Dex-ter's interaction.[3] In this second mechanism, the rate ofthe energy transfer strongly depends on the distance be-tween molecular orbitals involved in electron (or energy)transfer. Regarding the relative importance of the twomechanisms, the design of non-conjugated materials whosep-orbitals are close enough to allow energy transfer by theDexter mechanism is of great interest.

In the present paper we describe the synthesis of poly[p-(7-phenylene-7-(2¢,5¢-dihexyl-4-biphenylene))norbornane](5) (Scheme 1). This is the first soluble polymer that alter-nates aromatic conjugation and through-space p-orbitaloverlap (homoconjugation), based on the special character-istics of 7,7-diphenylnorbornane (2).[4] In the more stableconformation of this diphenylmethane derivative, the arylgroups are arranged in an apical cofacial fashion, forced bythe steric effect of the four exo-norbornylic C±H bonds.The Cipso±Cipso distance of 2.46 �, as revealed by X-rayscattering, is much shorter than the sum of the van derWaals radii of carbon (3.4 �). Consequently, a stronghomoconjugation exists between both aryl rings, as re-vealed by an intense absorption band in the UV spectrumat 228 nm (APK band),[4] which is not observed for confor-mationally unstable diphenylmethane derivatives.[5]

The synthesis of poly[p-(7-phenylene-7-(2¢,5¢-dihexyl-4-byphenylene))norbornane] (5) was carried out accordingto the procedure described in Scheme 1. In order to in-crease the solubility of the polymer, two unbranched hexylchains have been included in the p-terphenyl segment.[6]

7,7-diphenylnorbornane (2) was prepared following a mod-ified synthesis, using 7-hydroxy-7-phenylnorbornane (1) asthe starting material, instead of 7,7-bistrifliloxynorbor-nane.[4] Thus, by Friedel-Crafts reaction of 1 (preparedfrom 7-norbornanone) with benzene and trifluoromethane-sulfonic acid (HTfO), 2 was obtained in high yield (87 %).The diiodide 3 was prepared by reacting 2 with I2/AgTfO.[7]

Finally, polymer 5 was synthesized using the Pd-catalyzedSuzuki polycondensation method. This procedure allowsthe synthesis of structurally well defined, soluble, high mo-lecular weight poly(p-phenylene) (PPP) derivatives withmore than one hundred 1,4-phenylene rings.[6,8] Accord-ingly, polymer 5 was obtained in high yield (94 %) by reac-tion of the diiodide 3 with 2,5-dihexylbenzene-1,4-bisbo-ronic acid[6,9] catalyzed by Pd(PPh3)4. The same result wasobtained using Pd(p-tol3P)3 as the catalyst.

Polymer 5 is a white solid, soluble in common organicsolvents such as toluene or chloroform which is conduciveto allow a high degree of polymerization. Gel permeationchromatography (GPC) (Fig. 1) revealed the degree ofpolymerization and polydispersity (D): Pn = 18, Pw = 44,D = 2.4. The marked (*) peaks in the GPC curve could cor-respond to cyclic structures (PN ~7 and ~10). It has beenfound that dilute reaction conditions favor an increase inthese products. However, purification and subsequent anal-ysis of these compounds could not be achieved.

The molecular structure of 5 was confirmed by high-res-olution 1H and 13C NMR spectroscopy (Fig. 2). As ex-pected the 1H NMR spectrum shows three signals in thearomatic region at 7.44, 7.15, and 6.96 ppm with correct in-tegral ratios. This well resolved and shielded aromatic peakpattern is characteristic of the 7,7-diphenylnorbornane de-rivatives.[4] Small additional peaks in this region were as-signed to the terminal phenyl groups of the polymer. Sevensignals in the aromatic region of the 13C-NMR spectrum of5 are observed, proving that the phenyl rings are connectedto each other by an all-para-linkage. This high regiospecifi-city is one of the main advantages of the polymerizationprocedure employed here.[6] The remaining signals are con-sistent with the proposed structure 5.

The UV spectrum of 5 shows a broad absorption (245±322 nm) centered at 268 nm (e = 22100) (Fig. 3). The spec-tra of 7,7-diphenylnorbornane (2) and 2¢,5¢-dihexyl-p-ter-phenyl (6)[10] are also included for comparison. The absorp-tion band of polymer 5, which is the sum of the absorptionsof the conjugation band of the terphenyl segment and thehomoconjugation band of the 7,7-diphenylnorbornane sub-unit, shows bathochromic and hyperchromic shifts in con-trast to the corresponding band of 2¢,5¢-dihexyl-p-terphenyl(6). This effect results from the homoconjugation betweenthe phenyl groups of the 7,7-diphenylnorbornane unit of 5,which causes a delocalization of the p-electrons along thestructure of the polymer. The lmax of 5 and 6 are hypso-chromically shifted in comparison with the conjugationband of unsubstituted p-terphenyl (279 nm)[11] because thesteric hindrance of the alkyl chains in 5 and 6 causes a de-Scheme1.

crease in the conjugation of the aromatic rings. Therefore,the effect of the homoconjugation in the polymer is clearlyrevealed by the UV spectrum of 5. Similar effects are alsoclearly shown in the spectra of 7-(4-biphenyl)-7-phenylnor-bornane (7), 7,7-di-(4-biphenyl)norbornane (8), and biphe-nyl (9). Compounds 7 and 8 were prepared according toScheme 2. Figure 4 shows the effect of homoconjugationbetween the phenyl and biphenyl groups. In the case of 7this causes a bathochromic shift of both absorption bands.An even higher shift results for the biphenyl-biphenyl in-teraction that takes place in the case of 8.

In summary, in this paper we have described for the firsttime the synthesis of a p-extended soluble polymer with al-ternating conjugation and homoconjugation, capable of in-

tramolecular energy transfer by spatial p-orbital overlap.The homoconjugation between the phenyl groups in 5 wasrevealed by the UV spectrum. The solubility of this welldefined, all-para-linked polymer was due to the alkylchains of the terphenyl segment. Further work on the syn-thesis and properties of homoconjugated oligomers[12] andpolymers is presently under way.

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Fig. 1. GPC elution curve of 5 versus poly-styrene standard (Pn= 18, Pw= 44, D= 2.4).

Fig. 2. a) 1H NMR and b) 13C NMR spectra of polymer 5 in CDCl3.

Fig. 3. UV spectra of polymer 5 (CH2Cl2), 7,7-diphenylnorbornane (2)(MeOH), and 2¢,5¢-dihexyl-p-terphenyl (6) (MeOH).

Scheme2.

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Experimental

NMR spectra were recorded on Bruker AC-250, Bruker AC-500, andVarian XL-300 spectrometers operating at 250, 300, and 500 MHz for 1Hand 62.89, 75.41, and 125.77 MHz for 13C respectively. Chemical shifts aregiven in ppm relative to TMS. IR spectra were recorded on a Perkin Elmer781 spectrometer. Mass spectra were recorded on a GC-MS HP-5989(60 eV) mass spectrometer. For gas chromatography a Perkin Elmer 300chromatograph equipped with capillary OV-101 column was used. UV spec-tra were recorded using Cary 3-Bio and Perkin Elmer Lambda 3 spectrom-eters using methanol and dichloromethane as solvents. The GPC separa-tions were performed under the following conditions: active phase:Nucleosil 50-5, column: 30 ´ 250 mm, flow: 16 ml THF/min (referenced topolystyrene standard). UV-detection: 254 nm, room temperature. Flashchromatography was performed over Merck silica gel 60 (230±400 mesh).Melting points were measured using a Gallenkamp apparatus and are un-corrected. All reactions were carried out under a N2 atmosphere. Et2O,benzene, and toluene were distilled from deep-blue sodium/benzophenonesolutions. CH2Cl2 and CHCl3 were distilled from phosphorus pentoxide.7-norbornanone[13], 2,5-dihexylbenzene-1,4-bisboronic acid[9], and 2¢,5¢-di-hexyl-p-terphenyl[10] were prepared according to literature procedures.Starting materials and reagents obtained from commercial sources wereused without further purification.

7-Phenyl-7-norbornanol (1): A solution of 0.99 g (9 mmol) of 7-norbor-nanone[13] in 20 ml of Et2O were added toover a solution of 27 mmol ofphenylmagnesium bromide in 50 ml of Et2O. After refluxing for 3 h, 100 mlof saturated NH4Cl solution was added and the mixture extracted withEt2O (3 ´ 25 ml) and dried over MgSO4. After concentration at reducedpressure, the residue was purified by flash chromatography (hexane/Et2O10/1). 1.28 g (76 %) of 1[14] were obtained as a yellow oil. 1H NMR(250 MHz, CDCl3): d 7.50 (d, J (H,H) = 6.9 Hz, 2 H; Ph), 7.37 (t, J (H,H) =6.9 Hz, 2 H; Ph), 7.30 (t, J (H,H) = 6.9 Hz, 1 H; Ph), 2.40 (m, 2 H; bridge-head-H), 2.18 (m, 2 H; exo-H), 1.70 (s, 1 H; OH), 1.43 (m, 4 H; exo- and en-do-H), 1.22 (m, 2 H; endo-H); 13C NMR (62.89 MHz, CDCl3): d 142.3 (Cq),128.4 (2 CH), 127.4 (CH), 126.9 (2 CH), 87.6 (Cq), 41.8 (2 CH), 28.3(2 CH2), 27.2 (2 CH2); IR (Film): n = 3550, 3400, 3060, 3010, 2950, 2870,1600, 1050, 770, 700 cm±1; MS: m/z (%) = 188 (27) [M+], 171 (17), 133 (27),120 (16), 105 (100), 77 (36), 55 (39), 50 (14).

7,7-Diphenylnorbornane (2): To a solution of 0.80 g (5.3 mmol) of tri-fluoromethanesulfonic acid (HTfO) in 20 ml of benzene was slowly added1.00 g (5.3 mmol) of 7-phenyl-7-norbornanol (1) dissolved in 10 ml of ben-zene. After stirring for 2 h at room temperature, 50 ml of CH2Cl2 was addedand the reaction mixture washed with water (2 ´ 40 ml), saturated NaHCO3

solution (1 ´ 40 ml) and dried over MgSO4. The solvent was removed at re-duced pressure and the residue purified by flash chromatography (n-hex-ane). Recrystallization from hexane yielded 1.15 g (87 %) of 2[4]; M.p. 163±

165 �C; 1H NMR (250 MHz, CDCl3): d 7.41 (d, J (H,H) = 7.0 Hz, 4 H; Ph),7.20 (t, J (H,H) = 7.0 Hz, 4 H; Ph), 7.05 (t, J (H,H) = 7.0 Hz, 2 H; Ph), 3.08(m, 2 H, bridgehead-H), 1.64 (m, 4 H, exo-H), 1.32 (m, 4 H, endo-H); 13CNMR: d 146.0 (2 Cq), 128.2 (4 CH), 127.2 (4 CH), 125.3 (2 CH), 64.8 (Cq),41.6 (2 CH), 28.4 (4 CH2); IR (KBr): n = 3020, 2980, 2890, 1600, 1500, 1320,780, 760, 710 cm±1; UV/Vis (MeOH): l = 205 (23215), 228 (13365), 257 (sh),263 (sh), 271 (sh) nm; MS: m/z (%) = 248 (100) [M+], 205 (22), 194 (29), 167(31), 115 (28), 91 (34).

7,7-Di(4-iodophenyl)norbornane (3)[7]: To a suspension of 0.99 g(4.0 mmol) of 7,7-diphenylnorbornane (2) and 2.05 g (8.0 mmol) of silvertrifluoromethanesulfonate (AgTfO) in 20 ml of CHCl3, was added dropwisea solution of 2.03 g (8.0 mmol) of iodine dissolved in 50 ml of CHCl3 (untilthe color of the iodine persisted). After one day, the precipitated silver io-dide was filtered off, and the filtrate shaken with 20 ml of saturated NaH-CO3, 20 ml of 10 % NaS2O3 and dried over MgSO4. Evaporation of the sol-vent at reduced pressure and recrystallization of the residue from CHCl3yielded 1.89 g (95 %) of 3; M.p 302±304 �C; 1H NMR (300 MHz, CDCl3):d 7.54 (d, J (H,H) = 7.0 Hz, 4 H; Ph), 7.16 (d, J (H,H) = 7.0 Hz, 4 H; Ph),2.88 (m, 2 H, bridgehead-H), 1.61 (m, 4 H, exo-H), 1.35 (m, 4 H, endo-H);13C NMR: d 145.1 (2 Cq), 137.5 (4 CH), 129.2 (4 CH), 90.7 (2 Cq), 64.3 (Cq),41.6 (2 CH), 28.2 (4 CH2); IR (KBr): n = 3020, 2980, 2890, 1490, 1480, 1020,830, 820 cm±1; MS: m/z (%) = 500 (48) [M+], 373 (25), 318 (29), 246 (37), 217(100), 204 (60), 203 (49), 191 (57), 189 (76), 178 (48), 165 (97), 141 (49), 129(69), 128 (45), 127 (21), 116 (74), 101 (75), 91 (29), 77 (40).

Poly[p-(7-phenylene-7-(2¢,5¢-dihexyl-4-biphenylene))norbornane] (5): Toa two-phase system consisting of 60 ml aqueous 1 M Na2CO3 and 20 ml tol-uene was added 0.50 g (1.0 mmol) 7,7-di(4-iodophenyl)norbornane (3) and0.33 g (1.0 mmol) 2,5-dihexylbenzene-1,4-bisboronic acid (4)[9]. The result-ing mixture was degassed and kept under nitrogen. Then 0.012 g (1 mol %)of degassed Pd(Ph3P)4 was added and the mixture refluxed for three days.The phases were separated and the organic layer concentrated to 10 ml andadded dropwise into MeOH (150 ml). The precipitate formed was recov-ered by centrifugation and purified by several dissolution/reprecipitation cy-cles. Lyophilization using benzene afforded 0.46 g (94 %) of polymer 5 as awhite amorphous powder. 1H NMR (500 MHz, CDCl3): d 7.44 (m, 4 H; Ph),7.15 (m, 4 H; Ph), 6.96 (m, 2 H; Ph), 3.08 (m, 2 H; bridgehead-H), 2.43 (m,4 H; CH2Ph), 1.68 (m, 4 H; exo-H), 1.30 (m, 8 H; CH2-CH2Ph, endo-H), 1.00(m, 12 H; CH2), 0.70 (m, 6 H; CH3); 13C NMR (125.7 MHz, CDCl3): d 144.2(2 Cq), 140.7 (2 Cq), 138.8 (2 Cq), 137.5 (2 Cq), 130.8 (2 CH), 129.3 (4 CH),126.9 (4 CH), 64.3 (Cq), 41.9 (2 CH), 32.6 (2 CH2), 31.5 (4 CH2), 31.3(2 CH2), 29.2 (2 CH2), 28.5 (4 CH2), 22.4 (2 CH2), 14.0 (2 CH3); UV/vis(CH2Cl2): l = 268 (22100) nm.

7-(4-Biphenyl)-7-phenylnorbornane (7): To a solution of 0.80 g(5.3 mmol) of trifluoromethanesulfonic acid (HTfO) and 4.08 g(26.5 mmol) of biphenyl in 50 ml of CH2Cl2 was slowly added 1.00 g(5.3 mmol) of 7-phenyl-7-norbornanol (1) dissolved in 10 ml of CH2Cl2.After stirring for 2 h at room temperature, 50 ml of CH2Cl2 were added andthe reaction mixture washed with water (2 ´ 40 ml), saturated NaHCO3

solution (1 ´ 40 ml) and dried over MgSO4. The solvent was removed atreduced pressure and the residue purified by flash chromatography (n-hex-ane). Recrystallization from hexane yielded 1.72 g (60 %) of 7; M.p. 183±185 �C; 1H NMR (250 MHz, CDCl3): d 7.42 (m, 8 H; Ph), 7.23 (m, 5H; Ph),7.05 (t, J(H,H) = 6.9 Hz, 1 H; Ph), 3.15 (m, 2H; bridgehead-H), 1.65 (m, 4H;exo-H), 1.38 (m, 4H; endo-H); 13C NMR (62.89 MHz, CDCl3): d 145.9 (Cq),145.1 (Cq), 141.0 (Cq), 138.1 (Cq), 128.6 (2 CH), 128.3 (2 CH), 127.6 (4 CH),127.3 (2 CH), 127.0 (2 CH), 126.9 (CH), 125.4 (CH), 64.6 (Cq), 41.7 (2 CH),28.4 (2 CH2), 28.4 (2 CH2); IR(CCl4): n = 3060, 3020, 2960, 2940, 2880, 1600,1490, 1120, 700 cm±1; UV-Vis (MeOH): l = 205 (33938), 223 (sh),259 (17120) nm; MS: m/z (%) = 324 (100) [M+], 255 (11), 243 (21), 191 (11),167 (13), 115 (13), 91 (34).

7,7-Di(4-biphenyl)norbornane (8): Following the polycondensationmethod described previously, 0.50 g (1.0 mmol) 7,7-di-(4-iodophenyl)nor-bornane (3) was reacted with 0.12 g (1.0 mmol) phenylboronic acid and12 mg (1 mol %) Pd(Ph3P)4 for one day. The organic solvent was removedat reduced pressure and the residue purified by flash chromatography(n-hexane).After recrystallization from hexane, 0.33 g (83 %) of 6 was ob-tained; M.p. 197.1±199.2 �C; 1H NMR (300 MHz, CDCl3): d 7.48 (m, 12H;Ph), 7.37 (t, J(H,H) = 6.9 Hz, 4H; Ph), 7.27 (t, J(H,H) = 6.9 Hz, 2 H; Ph),3.14 (m, 2 H; bridgehead-H), 1.71 (m, 4 H, exo-H), 1.36 (m, 4 H; endo-H);13C NMR (75.4 MHz, CDCl3): d 145.0 (2 Cq), 140.9 (2 Cq), 138.2 (2 Cq),128.6 (4 CH), 127.7(2 CH), 127.1 (4 CH), 126.9 (8 CH), 64.4 (Cq), 41.8(2 CH), 28.5 (4 CH2); IR (KBr): n = 3010, 2960, 2860, 1590, 1480, 830, 740,690 cm±1; UV-Vis (MeOH): l = 208 (16780), 260 (sh), 268 (13278) nm;MS: m/z (%) = 400 (100) [M+], 331 (26), 205 (17), 191 (30), 167 (69), 165(30), 60 (34).

Received: June 2, 1998

Fig. 4. UV spectra (MeOH) of 7-(4-biphenyl)-7-phenylnorbornane (7), 7,7-di(4-biphenyl)norbornane (8), and biphenyl (9).

±[1] a) J. Roncali, Chem. Rev. 1997, 97, 173; b) W. J. Feast, J. Tsibouklis,

K. L. Pouwer, L. Groenendaal, E. W. Meijer, Polymer 1996, 37, 5017;c) An Introduction to Molecular Electronics (Eds: M. C. Petty, M. R.Bryce, D. Bloor), Edward Arnold, London, 1995; d) J. M. Tour, Adv.Mater. 1994, 6, 190; e) U. Scherf, K. Müllen, Synthesis 1992, 23; f) F. Ef-fenberger, H. Schlosser, P. Bäuerle, S. Maier, H. Port, H. Wolf, Angew.Chem. Int. Ed. Engl. 1998, 27, 281; g) J. M. Lehn, Proc. Nat. Acad. Sci.1986, 83, 5355; h) Handbook of Conducting Polymers, (Ed. T. A. Skot-heim), Marcel Dekker, New York, 1986.

[2] T. Förster, Discuss. Faraday Soc. 1959, 27, 7.[3] D. L. Dexter, J. Chem. Phys. 1953, 21, 836.[4] a) A. G. Martínez, J. O. Barcina, A. F. Cerezo, R. G. Rivas, J. Am.

Chem. Soc. 1998, 120, 673; b) A. G. Martínez, J. O. Barcina, A. Albert,F. H. Cano, L. R. Subramanian, Tetrahedron Lett. 1993, 34, 6753.

[5] a) T. Strassner, Can. J. Chem. 1997, 75, 1011; b) M. Feigel, J. Mol.Struct. (Theochem) 1996, 366, 83; c) W. Weissensteiner, Monatsh.Chem. 1992, 123, 1135; d) J. C. Barnes, J. D. Paton, J. R. Damewood,K. Mislow, J. Org. Chem. 1981, 46, 4975; e) D. Gust, K. Mislow, J. Am.Chem. Soc. 1973, 95, 1535; f) G. Montaudo, P. Finocchiaro, J. Am.Chem. Soc. 1972, 94, 6745.

[6] a) M. Rehahn, A.-D. Schlüter, G. Wegner, Makromol. Chem. 1990,191, 1991; b) A.-D. Schlüter, G. Wegner, Acta Polym. 1993, 44, 59.

[7] Y. Kobayashi, I. Kumadaki, T. Yoshida, J. Chem. Res. 1977, 215.[8] N. Miyaura, T. Yanagi, A. Suzuki, Synth. Commun. 1981, 11, 513.[9] M. Rehahn, A.-D. Schlüter, W. J. Feast, Synthesis 1988, 386.

[10] P. Galda, M. Rehahn, Synthesis 1996, 614.[11] Heinz-Helmut Perkampus, UV-Vis. Atlas of Organic Compounds, 2nd

ed., VCH, Weinheim, 1992.[12] Oligomers with stacked aromatic rings, both homoconjugated and

non-homoconjugated, are described in: a) S. Mataka, Y. Mitoma, T.Thiemann, T. Sawada, M. Taniguchi, M. Kobuchi, M. Tashiro, Tetrahe-dron 1997, 53, 3015; b) S. Breidenbach, S. Ohren, F. Vögtle, Chem.Eur. J. 1996, 2, 832; c) M. Kuroda, J. Nakayama, M. Hoshino, N. Furus-ho, T. Kawata, S. Ohba, Tetrahedron 1993, 49, 3735; d) W. Grimme,H. T. Kämmerling, J. Lex, R. Gleiter, J. Heinze, M. Dietrich, Angew.Chem. Int. Ed. Engl. 1991, 30, 205; e) K. Yamaguchi, G. Matsumura,H. Kagechika, I. Azumaya, Y. Ito, A. Itai, K. Shudo, J. Am. Chem.Soc. 1991, 113, 5474; f) T. Otsubo, T. Kohda, S. Misumi, Bull. Chem.Soc. Jp 1980, 53, 512; g) M. Nakazaki, K. Yamamoto, S. Tanaka, H. Ka-metani, J. Org. Chem. 1977, 42, 287.

[13] a) P. G. Gassman, P. G. Pape, J. Org. Chem. 1964, 29, 160. b) P. G.Gassman, J. L. Marshall, Org. Synth. 1968, 48, 68.

[14] H. Tanida, T. Tsushima, J. Am. Chem. Soc. 1970, 92, 3397.

Clusters of C60 Molecules**

By Doroteo Mendoza,* Gonzalo Gonzalez, andRoberto Escudero

After the discovery of the method for production of largequantities of C60,[1] the structural and optical properties ofthis material have been studied in the form of single crys-tals, thin films, or in solution in a variety of liquid sol-vents.[2] The commonly accepted crystalline structure ofsolid C60 consists of a fcc lattice at room temperature thatundergoes a structural transition to a sc phase at^249 K.[3] Nevertheless, other studies indicate that, de-pending on the experimental conditions in which solid C60

has been obtained, a mixture of fcc and hcp phases may co-exist at room temperature.[4±6] The optical absorption spec-tra in the UV±vis range are very similar among the differ-ent forms in which C60 has been studied, namely in solidform and in solution.[7] This implies that the intermolecularinteractions are weak, of the van der Waals type, and thatsolid C60 is considered as a molecular crystal.[8] Generallyspeaking, the four main peaks in the absorption spectra, lo-cated in the range of 2±6 eV, remain the same with smallred shifts in energy and broadening in the peaks of solidC60 compared with those in the molecules.[7] On the otherhand, C60 has also been studied in the form of clusters inthe range of 13±55 molecules, being the intermediate clus-ters in a set of ªmagicº numbers.[9] One of the main conclu-sions of that work is that, in these small clusters, icosahe-dral symmetry is preferred. Greater aggregates of C60

molecules, in the range of 100±140 nm, have also beensynthesized via aerosol routes.[10] In this case, the clustersof C60 molecules are spheroidal and polycrystalline. To ourknowledge, no optical properties have been reported forthese kinds of systems.

In the present work we report on the synthesis and char-acterization of clusters of C60 molecules. The most intrigu-ing finding is that the optical absorption spectrum in theUV±vis range of the material obtained is different to thatusually observed in C60 thin films. The results presentedhere might be useful in future optical applications or forcatalytic purposes, such as have been proposed for similarnanophase fullerene particles obtained by a different meth-od than that reported here.[10]

Figure 1a shows a TEM micrograph of one of the sam-ples. It can be observed that the sample consists of isolatedspheroidal clusters which, based on statistical analysis ofthe size distribution, have a mean diameter of 180 nm. Fig-ure 1b shows the electron diffraction pattern associatedwith the cluster system, which indicates its nanocrystallinenature. Simple analysis of the diameter of some of the mostprominent diffraction rings gives the following interplanarspacing: 8.19, 5.01, 4.29, 3.21, and 2.9 �. These interplanarspacings can be attributed to either a fcc (with a lattice pa-rameter of a ^14.1 �) or a hcp (with lattice parameters a^10 �, and c ^16.4 �) crystalline structures.[4±6] With thepresent data we are not able to determine which of thecrystalline structures might correspond to the clusters ofC60 molecules, and further analysis will be necessary in thisrespect. Nevertheless, it is certain that these interplanardistances do not belong to graphite or any other possibleresidual phases resulting during the preparation process.

In Figure 2 the optical absorption spectrum in the UV±vis range of the cluster system is shown (solid line). Forcomparison the spectrum corresponding to a film obtainedunder high vacuum conditions is also presented (dashedline). Note the typical absorption spectrum for this film,and note for the cluster system a red shift of the main ab-sorption bands. It is worth mentioning the large enhance-ment of the ratio of the absorbance at low energies (2 eV)

Adv. Mater. 1999, 11, No. 1 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/0101-0031 $ 17.50+.50/0 31

Communications

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[*] Dr. D. Mendoza, Dr. G. Gonzalez, Dr. R. EscuderoInstituto de Investigaciones en MaterialesUniversidad Nacional Autónoma de MØxicoApdo. Postal 70-360, MØxico, D.F. 04510 (Mexico)

[**] We thank M.A Canseco for the UV±Vis and IR measurements. Thiswork was partially supported by grants CONACYT-G0017E andDGAPA-IN105597