Synthesis, characterization and optical response of polyene-cored stilbenoid dendrimers

Tetrahedron 61 (2005) 395–400

Synthesis, characterization and optical response of polyene-cored stilbenoid dendrimers Antonio R. Cano-Marı´n, Enrique Dı´ez-Barra and Julia´n Rodrı´guez-Lo´pez* A´rea de Quı´mica Orga´nica, Facultad de Quı´mica, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain Received 8 September 2004; revised 26 October 2004; accepted 28 October 2004 Available online 13 November 2004 Dedicated to Dr. Juan Carlos del Amo, deceased on 11 March 2004 in Madrid attacks.

Abstract—The synthesis of first- and second-generation dendrimers bearing phenylenevinylene chromophores within the dendritic branches (stilbenoid dendrimers) and polyenes (3 and 5 double bonds) as cores is described. A preliminary study of the optical properties of the resulting compounds was conducted by UV/vis and fluorescence spectroscopy. q 2004 Elsevier Ltd. All rights reserved.

1. Introduction In recent years organic molecules with high photoluminescence efficiencies have been the focus of intensive research effort. Such compounds have been considered as advanced materials for electronic and photonic applications.1 For this reason, it is important to devise efficient methods for the synthesis of novel fluorophores that are amenable to further chemical functionalization or modification, which in turn can be elaborated to obtain materials with tuneable optoelectronic properties. Monodisperse dendritic materials have emerged as attractive candidates for photonic applications. There has been a substantial body of work published over the past decade regarding the synthesis of new dendrimeric structures and the study of such systems in the development of new applications.2 Dendrimers with polyconjugated branches represent an important group within this class of material. These compounds are interesting because of their electrical, optical, nonlinear optical, electroluminescent and photophysical properties. For example, such compounds have been used successfully both as charge transporting3 and light-emitting materials.4 Moreover, dendritic structures have been shown to be efficient synthetic light-harvesting antenna molecules.5 Herein we describe the synthesis and characterization of Keywords: Dendrimers; Stilbenoid dendrimers; Polyphenylenevinylene chromophores; Polyenes. * Corresponding author. Tel.: C34 926295300; fax: C34 926295318; e-mail: [email protected] 0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2004.10.083

new first- and second-generation polyene-cored dendrimer architectures containing phenylenevinylene chromophores within the dendritic branches (stilbenoid dendrimers). We also present the preliminary studies on the photophysical properties (UV/vis and fluorescence measurements) of these luminescent compounds. 2. Results and discussion Figure 1 shows the structures of the synthesized first- (1, 2) and second-generation (3, 4) dendrimers. The dendritic branches are known polyphenylenevinylene (stilbenoid) chromophores while polyenes are used as dendrimer cores for the first time. We chose the 3,5-di-tert-butylphenyl group as the surface functionality in order to impart solubility and to achieve short wavelength absorption and emission. Indeed, THF, chloroform and dichloromethane were found to be very good solvents for all the dendrons and dendrimers prepared. On the other hand, polyene systems of different lengths allow control of the absorption as well as the colour of the light emission, although it is known that after six double bonds the red shift trend in polyenes shows a decrease. 2.1. Synthesis The preparation of branches (in this case the phenylenevinylene chromophores) with a wide variety of peripheral groups is well established.6 We therefore focused our efforts on developing a versatile methodology to build the cores from the appropriate focal points. An aldehyde group at the focus of the dendron was used to achieve this goal by

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of polyenals could be prepared from these dendrons (and their extended counterparts) by successive chain extensions. The efficient use of Wittig8 and HWE9 reactions for this purpose has been widely reported. However, in our case, the use of reagents such as the commercially available triphenylphosphoranylideneacetaldehyde and diethyl phosphonoacetaldehyde diethyl acetal, as well as 1,3-dioxan-2-ylmethyltributyl phosphorane, proved unsuccessful. All other attempts based on these reagents failed. In an effort to overcome this drawback we decided to perform a Heck coupling reaction with acrolein on the first and second generation iodo-focused dendrons 10 and 11, which were easily prepared by HWE reactions of 1-iodo-3,5-bis (diethoxyphosphorylmethyl)benzene with 3,5-di-tert-butylbenzaldehyde and dendron 5, respectively. In this way, the corresponding vinylogous compounds 6 and 8 were obtained in 70 and 68% yield, respectively. In all cases the E-isomers were the only products (Scheme 2). With dendrons 5–8 in hand, we embarked on building the desired dendrimers by a new HWE reaction at room temperature with diphosphonate 9 in KButO/THF. The 1H NMR spectra of the crude products showed some impurities that arose, at least in part, from geometrical isomers. However, after careful chromatography over silica, the desired dendrimers were isolated in good yields. Figure 1. Structure of the prepared polyene-cored stilbenoid dendrimers.

reaction with the diphosphonate derivative 9 to form the polyene-cored dendrimers by a Horner–Wadsworth– Emmons (HWE) reaction (Scheme 1).7 The synthesis of aldehyde-focused dendrons has been reported previously by Burn and Samuel.4a Initially, it was expected that a number

Scheme 1. Synthesis of polyene-cored dendrimers 1–4. Reagents and conditions: (i) (EtO)2(O)P–CH2–CH]CH–CH2–P(O)(OEt)2 (9), KtBuO/ THF, rt.

All new compounds were characterized using various analytical techniques. MS and NMR experiments proved very useful to confirm the structures of the compounds (see Section 4). With their high degree of symmetry, the assignment of the dendrimer structures by 1H and 13C NMR was relatively straightforward. The trans stereochemistry for the double bonds was unequivocally established on the basis of the coupling constant of the vinylic protons in the 1H NMR spectra (JZ15–16 Hz).10 HRMS analyses of first-generation dendrimers gave the expected molecular ions. The MALDI-TOF technique proved to be very useful for the identification of the higher structures. All

Scheme 2. Synthesis of vinylogous compounds 6 and 8. Reagents and conditions: (i) acrolein, trans-di(m-acetato)bis[o-(di-o-tolylphosphino)benzyl]dipalladium (II), 2,6-di-tert-butylcresol, anhydrous N,N-dimethylacetamide, sodium bicarbonate, 130 8C.

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the spectra registered for the higher generations showed peaks matching the calculated molecular weights. 2.2. Optical properties of dendrimers 1–4 The optical properties of the dendrimers were investigated by UV/vis and PL spectroscopy on CH2Cl2 solutions of the compounds at room temperature. Figure 2 shows the UV/vis spectra of compounds 1–4 and the main data obtained are listed in Table 1. Owing to the meta arrangement through which the dendrons

Figure 3. PL spectra of 1 and 2 in CH2Cl2 (cZ10K7 M). 1; lexc: (a) 324.0, (b) 372.5 and (c) 393.0 nm. 2; lexc: (a) 320.5 and (b) 373.5 nm.

process). However, it is not possible to affirm this phenomenon in this case. The spectra also contain a shoulder at 470 nm, indicating that some energy transfer could have taken place from the stilbene units to the polyene core. Excitation at higher wavelengths (372–393 nm) resulted in a decay in the intensity of the bands in the range 400–450 nm.

Figure 2. Absorption spectra of 1–4 in CH2Cl2 (cZ10

K6

M).

Table 1. UV/vis and photoluminescence (PL) data for dendrimers 1–4 Compound 1 2 3 4

UV/vis lmax. (nm) 324.0, 372.5 320.5, 373.0 313.5, 391.0, 412.5, 438.5 319.5, 392.0, 414.5, 439.5

3 (MK1 cmK1) 99,100, 77,700 300,100, 80,100 82,400, 75,900, 105,900, 91,100 351,300, 87,700, 106,100, 88,600

PL lmax. (nm) 421, 444 423, 445 424, 447, 513, 550, 589 422, 513, 550, 587

are linked, in all cases the observed absorption spectra consist of a superposition of the absorptions due to the different chromophores, stilbenoid branches and polyene cores. The strength of the absorption due to the core (above 370 nm) relative to the peak at 320 nm (associated with stilbene units) decreases for higher generations because of the increase in the number of stilbene units from 4 to 12. The main difference between the spectra of the dendrimers is the bathochromic shift observed for the bands due to the core when five double bonds are present. This shift allows the absorptions due to each chromophore to be clearly differentiated and will be useful to analyse the fluorescence behaviour when branches and cores are irradiated. Figure 3 shows the fluorescence spectra of compounds 1 and 2. In both cases the spectra are similar. Excitation at 320–4 nm, that is, at the absorption maxima of the stilbene units, resulted in emission bands at 422 and 444 nm, which are typical of stilbenoid compounds.11 It has been reported4c for similar stilbenoid dendrimers that the absence of emission below 400 nm indicates an efficient energy transfer from the branches to the core (a light-harvesting

The fluorescence spectra of compounds 3 and 4 (Fig. 4) are even more illustrative. As indicated above, in these dendrimers it is possible to irradiate selectively the branches and the core because of their well-resolved absorption bands. In both cases, excitation at 313–9, that is, at the absorption maxima of the stilbene units, again resulted in fluorescence in the region 400–450 nm together with small peaks at 513, 550 and 589 nm (emission from the core).12 This can be indicative of some energy transfer, however, these data give only a qualitative idea on the behaviour of these dendrimers, while more quantitative information is necessary in order to discuss the occurrence (or lack of occurrence) of energy transfer between the different chromophoric groups. Excitation at higher wavelengths resulted, as expected, in a decay in the intensity of the emission due to the stilbene units. When the samples were

Figure 4. PL spectra of 3 and 4 in CH2Cl2 (cZ10K6 M). 3; lexc: (a) 313.5, (b) 391.0, (c) 412.5 and (d) 438.5 nm. 4; lexc: (a) 319.5, (b) 392.0, (c) 414.5 and (d) 439.5 nm.

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irradiated at 439 nm, only fluorescence from the core was observed. 3. Conclusions A Heck reaction with acrolein followed by a HWE reaction with diphosphonate 9 allowed the synthesis of first- and second-generation dendrimers having phenylenevinylene (stilbenoid) chromophores as the branches and polyenes (3 and 5 double bonds) as the cores. The meta-substitution pattern causes all chromophores to be independent. The PL spectra of the resulting materials show that excitation of the branches results in fluorescence in the region 400–450 nm, which is typical of stilbenoid compounds, together with emission from the core. More quantitative information is necessary in order to discuss the occurrence (or lack of occurrence) of energy transfer between the different chromophoric groups. Complementary experiments probing this effect are now in progress in our laboratories. 4. Experimental 4.1. General In air- and moisture-sensitive reactions all glassware was oven-dried and cooled under Ar. All reagents were used as received and without further purification—except acrolein, which was distilled prior to use. THF was refluxed over sodium/benzophenone ketyl and distilled under a positive pressure of dry argon immediately prior to use. CCl4 was ˚ ). Column distilled and stored over molecular sieves (4 A chromatography was carried out with Merck silica gel for flash columns (230–400 mesh). NMR spectra were recorded in CDCl3 on a Varian Inova-500 instrument with TMS or the solvent carbon signal as the standards. IR spectra were recorded on a Nicolet 550 spectrophotometer (FT-IR). UV/vis spectra were recorded in CH2Cl2 on a Jasco V-530 spectrophotometer using standard 1 cm quartz UV cells. Fluorescence spectra were recorded on a Jasco FP-750 spectrofluorimeter. Mass spectrometry and elemental analyses were performed at the Universidad Auto´noma de Madrid (Servicio Interdepartamental de Investigacio´n, S. I. D. I.). Spectra matrices: 3-nitrobenzyl alcohol (HRMS, LSIMS) and dithranol, a-cyano-4-hydroxycinnamic acid (compound 10) and 2,5-dihydroxybenzoic acid (compound 6) (MALDI-TOF). Poly(ethylene glycol) was used for internal calibration. 3,5-Di-tert-butylbenzaldehyde,13 trans-di(macetato)bis[o-(di-o-tolylphosphino)benzyl]dipalladium(II)14 and dendrons 5 and 74a were prepared according to literature procedures. 1-Iodo-3,5-bis-(diethoxyphosphorylmethyl)benzene and diphosphonate 9 were prepared by Arbuzov reaction of 1,3-bis(bromomethyl)-5-iodobenzene15 and trans-1,4-dibromo-2-butene, respectively, with triethyl phosphite following a standard methodology. 4.1.1. Compound 10. To a stirred solution of 1-iodo-3,5bis(diethoxyphosphorylmethyl)benzene (1.3 g, 2.58 mmol) and 3,5-di-tert-butylbenzaldehyde (1.13 g, 5.16 mmol) in anhydrous THF (20 mL) under argon was added potassium

tert-butoxide (1.74 g, 15.48 mmol) in small portions. The coloured mixture was stirred at room temperature for 4 h. After hydrolysis with water, the mixture was extracted with CH2Cl2 (!3). The combined organic layers were successively washed with water and brine, and then dried (MgSO4). After filtration and evaporation of the solvent, the crude product was triturated thoroughly with hot EtOH to give 1.26 g (77%) of the desired compound as a white solid. 1H NMR (CDCl3, 500 MHz): d 1.37 (s, 36H, 12! CH3), 7.01 (A of ABq, 2H, JZ16.5 Hz, 2!CH]), 7.19 (B of ABq, 2H, JZ16.5 Hz, 2!CH]), 7.38 (s, 6H, arom.), 7.61 (br s, 1H, arom.), 7.77 (d, 2H, JZ1.5 Hz, arom.). 13C NMR and DEPT (CDCl3, 125 MHz): d 151.1 (C), 139.9 (C), 136.0 (C), 133.8 (CH), 131.2 (CH), 126.2 (CH), 124.0 (CH), 122.5 (CH), 121.0 (CH), 95.2 (C–I), 34.9 (C), 31.4 (CH3). MS (MALDI) m/z 632.3 (MC). HRMS Calcd for C38H49I 632.2879. Found: 632.2878. 4.1.2. Compound 11. The synthetic procedure used was similar to that described for 10. Starting from 1-iodo-3,5bis(diethoxyphosphorylmethyl)benzene (471 mg, 0.93 mmol) and dendron 5 (995 mg, 1.86 mmol) the desired compound was obtained as a white solid (871 mg, 74%). 1H NMR (CDCl3, 500 MHz): d 1.39 (s, 72H, 24!CH3), 7.16 (A of ABq, 2H, JZ16.5 Hz, 2!CH]), 7.17 (A of ABq, 4H, JZ 16.5 Hz, 4!CH]), 7.23 (B of ABq, 2H, JZ16.5 Hz, 2! CH]), 7.28 (B of ABq, 4H, JZ16.5 Hz, 4!CH]), 7.39 (t, 4H, JZ1.5 Hz, arom.), 7.43 (d, 8H, JZ1.5 Hz, arom.), 7.61 (br s, 4H, arom.), 7.64 (br s, 2H, arom.), 7.68 (br s, 1H, arom.), 7.82 (br s, 2H, arom.). 13C NMR and DEPT (CDCl3, 125 MHz): d 151.1 (C), 139.7 (C), 138.4 (C), 137.4 (C), 136.4 (C), 134.4 (CH), 130.4 (CH), 130.1 (CH), 127.5 (CH), 127.3 (CH), 124.4 (CH), 124.1 (CH), 123.8 (CH), 122.3 (CH), 120.9 (CH), 95.2 (C), 34.9 (C), 31.5 (CH3). MS (MALDI) m/z 1265.9 (MC). HRMS Calcd for C86H105I 1264.7261. Found: 1264.7260. 4.1.3. Compound 6. A mixture of dendron 10 (600 mg, 0.95 mmol), acrolein (70 mg, 1.2 mmol), anhydrous sodium carbonate (127 mg, 1.20 mmol), trans-di(m-acetato)bis[o(di-o-tolyl-phosphino)benzyl]dipalladium(II) (catalytic amount), 2,6-di-tert-butylcresol (105 mg, 0.47 mmol) and anhydrous N,N-dimethylacetamide (6 mL) was deoxygenated thoroughly by stirring under oil-pump vacuum followed by purging with argon several times. The mixture was then heated under argon at 130 8C for 8 h. After cooling, ether and hydrochloric acid (1.5 M) were added carefully. The organic layer was washed with water (!5) and dried (MgSO4). After filtration and evaporation of the solvent, the crude product was triturated thoroughly with hexanes. Recrystallization from CHCl3/hexanes gave 375 mg (70%) of the desired compound as a pale yellow solid. 1H NMR (CDCl3, 500 MHz): d 1.38 (s, 36H, 12! CH3), 6.84 (dd, 1H, JZ7.5, 16.0 Hz, CH]), 7.14 (A of ABq, 2H, JZ16.0 Hz, 2!CH]), 7.27 (B of ABq, 2H, JZ 16.0 Hz, 2!CH]), 7.40 (t, 2H, JZ1.5 Hz, arom.), 7.41 (d, 4H, JZ1.5 Hz, arom.), 7.54 (d, 1H, JZ16.0 Hz, CH]), 7.62 (d, 2H, JZ1.0 Hz, arom.), 7.77 (br s, 1H, arom.), 9.76 (d, 1H, JZ7.5 Hz, CH]O). 13C NMR and DEPT (CDCl3, 125 MHz): d 193.7 (CHO), 152.7 (CH), 151.2 (C), 138.8 (C), 136.0 (C), 134.7 (C), 131.2 (CH), 129.0 (CH), 127.0 (CH), 126.7 (CH), 125.2 (CH), 122.6 (CH), 121.0 (CH), 34.9 (C), 31.5 (CH3). IR (KBr): n 1683 (C]O) cmK1. MS

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(MALDI) m/z 560.4 (MC). HRMS Calcd for C41H52O 560.4018. Found: 560.4006. 4.1.4. Compound 8. The synthetic procedure used was similar to that described for 6. Starting from dendron 11 (300 mg, 0.24 mmol) and acrolein (20 mg, 0.36 mmol) the desired compound was obtained as a pale yellow solid (195 mg, 68%). 1H NMR (CDCl3, 500 MHz): d 1.39 (s, 72H, 24!CH3), 6.86 (dd, 1H, JZ7.5, 16.0 Hz, CH]), 7.18 (A of ABq, 4H, JZ16.5 Hz, 4!CH]), 7.30 (s, 4H, 4! CH]), 7.30 (B of ABq, 4H, JZ16.5 Hz, 4!CH]), 7.39 (t, 4H, JZ1.5 Hz, arom.), 7.44 (d, 8H, JZ1.5 Hz, arom.), 7.57 (d, 1H, JZ16.0 Hz, CH]), 7.65 (s, 4H, arom.), 7.66 (br s, 4H, arom.), 7.84 (br s, 1H, arom.), 9.79 (d, 1H, JZ7.5 Hz, CH]O). 13C NMR and DEPT (CDCl3, 125 MHz): d 193.6 (CHO), 152.4 (CH), 151.1 (C), 138.7 (C), 138.4 (C), 137.4 (C), 136.3 (C), 134.9 (C), 130.4 (CH), 130.2 (CH), 129.2 (CH), 127.8 (CH), 127.5 (CH), 127.0 (CH), 125.7 (CH), 124.4 (CH), 123.8 (CH), 122.3 (CH), 120.9 (CH), 34.9 (C), 31.5 (CH3). IR (KBr): n 1681 (C]O) cmK1. MS (MALDI) m/z 1193.9 (MC). HRMS Calcd for C89H108O 1192.8400. Found: 1192.8435. 4.1.5. Compound 1. To a stirred solution of diphosphonate 9 (154 mg, 0.47 mmol) and dendritic aldehyde 5 (503 mg, 0.94 mmol) in anhydrous THF (20 mL) under argon was added potassium tert-butoxide (316 mg, 2.82 mmol) in small portions. The coloured mixture was stirred at room temperature for 4 h. After hydrolysis with water, the mixture was extracted with CH2Cl2 (!3). The combined organic layers were successively washed with water and brine, and then dried (MgSO4). After filtration and evaporation of the solvent, the crude product was purified by column chromatography (silica gel, hexanes/EtAcO, 9.5:0.5) and recrystallization from CHCl3/EtOH to give the desired compound as a pale yellow solid (352 mg, 69%). 1H NMR (CDCl3, 500 MHz): d 1.39 (s, 72H, 24!CH3), 6.63 (dd, 2H, JZ7.0, 3.0 Hz, 2!CH]), 6.68 (d, 2H, JZ 15.5 Hz, 2!CH]), 7.03 (ddd, 2H, JZ15.5, 7.0, 3.0 Hz, 2!CH]), 7.14 (A of ABq, 4H, JZ16.0 Hz, 4!CH]), 7.25 (B of ABq, 4H, JZ16.0 Hz, 4!CH]), 7.38 (t, 4H, JZ1.5 Hz, arom.), 7.42 (d, 8H, JZ1.5 Hz, arom.), 7.50 (d, 4H, JZ1.5 Hz, arom.), 7.60 (t, 2H, JZ1.5 Hz, arom.). 13C NMR and DEPT (CDCl3, 125 MHz): d 151.1 (C), 138.2 (C), 138.0 (C), 136.4 (C), 133.8 (CH), 132.6 (CH), 130.2 (CH), 129.6 (CH), 127.6 (CH), 123.8 (CH), 123.6 (CH), 122.2 (CH), 120.9 (CH), 34.9 (C), 31.5 (CH3). MS (LSIMS) m/z 1088.5 (MC). HRMS Calcd for C82H104 1088.8138. Found: 1088.8140. Anal. Calcd for C82H104: C, 90.38; H, 9.62. Found: C, 89.98; H, 9.79. The synthetic procedure used for 2, 3 and 4 was similar to that described for 1. 4.1.6. Compound 2. Yellow solid. Yield: 65%. 1H NMR (CDCl3, 500 MHz): d 1.38 (s, 72H, 24!CH3), 6.42–6.52 (m, 6H, 6!CH]), 6.64 (d, 2H, JZ15.5 Hz, 2!CH]), 6.96–7.02 (m, 2H, JZ15.5 Hz, 2!CH]), 7.13 (A of ABq, 4H, JZ16.5 Hz, 4!CH]), 7.24 (B of ABq, 4H, JZ 16.5 Hz, 4!CH]), 7.38 (t, 4H, JZ2.0 Hz, arom.), 7.41 (d, 8H, JZ2.0 Hz, arom.), 7.47 (br s, 4H, arom.), 7.58 (br s, 2H, arom.). 13C NMR and DEPT (CDCl3, 125 MHz): d 151.1 (C), 138.2 (C), 138.1 (C), 136.4 (C), 133.8 (CH), 133.6

399

(CH), 132.5 (CH), 130.2 (CH), 129.7 (CH), 127.7 (CH), 123.7 (CH), 123.6 (CH), 122.2 (CH), 120.9 (CH), 34.9 (C), 31.5 (CH3). MS (LSIMS) m/z 1141.7 (MC). HRMS Calcd for C86H108 1140.8451. Found: 1140.8474. 4.1.7. Compound 3. Pale yellow solid. Yield: 60%. 1H NMR (CDCl3, 500 MHz): d 1.40 (s, 144H, 48!CH3), 6.68 (dd, 2H, JZ7.0, 3.0 Hz, 2!CH]), 6.73 (d, 2H, JZ 15.0 Hz, 2!CH]), 7.05–7.12 (m, 2H, 2!CH]), 7.19 (A of ABq, 8H, JZ16.0 Hz, 8!CH]), 7.30 (s, 8H, 8! CH]), 7.31 (B of ABq, 8H, JZ16.0 Hz, 8!CH]), 7.36 (br s, 2H, arom.), 7.40 (t, 8H, JZ1.5 Hz, arom.), 7.45 (d, 16H, JZ1.5 Hz, arom.) 7.57 (br s, 4H, arom.), 7.66 (br s, 8H, arom.), 7.67 (br s, 4H, arom.). 13C NMR and DEPT (CDCl3, 125 MHz): d 151.1 (C), 138.3 (C), 138.2 (C), 138.1 (C), 137.8 (C), 136.4 (C), 133.9 (CH), 132.6 (CH), 130.3 (CH), 129.8 (CH), 129.1 (CH), 128.7 (CH), 127.7 (CH), 124.1 (CH), 124.1 (CH), 123.9 (CH), 123.7 (CH), 122.3 (CH), 120.9 (CH), 34.9 (C), 31.5 (CH3). MS (MALDI) m/z 2355.5 (MC). Anal. Calcd for C178H216 C, 90.76; H, 9.62. Found: C, 90.41; H, 9.75. 4.1.8. Compound 4. Yellow solid. Yield: 50%. 1H NMR (CDCl3, 500 MHz): d 1.40 (s, 144H, 48!CH3), 6.46–6.56 (m, 6H, 6!CH]), 6.67 (d, 2H, JZ16.0 Hz, 2!CH]), 6.99–7.05 (m, 2H, JZ15.5 Hz, 2!CH]), 7.18 (A of ABq, 8H, JZ16.5 Hz, 8!CH]), 7.28 (s, 8H, 8!CH]), 7.30 (B of ABq, 8H, JZ16.5 Hz, 8!CH]), 7.36 (br s, 2H, arom.), 7.39 (t, 8H, JZ1.5 Hz, arom.), 7.44 (d, 16H, JZ1.5 Hz, arom.), 7.53 (br s, 4H, arom.), 7.64 (br s, 8H, arom.), 7.65 (br s, 4H, arom.). 13C NMR and DEPT (CDCl3, 125 MHz): d 151.1 (C), 138.3 (C), 138.2 (C), 138.1 (C), 137.8 (C), 136.4 (C), 134.0 (CH), 133.7 (CH), 133.6 (CH), 132.3 (CH), 130.3 (CH), 129.9 (CH), 129.2 (CH), 128.8 (CH), 127.7 (CH), 124.1 (CH), 124.0 (CH), 123.8 (CH), 123.7 (CH), 122.3 (CH), 120.9 (CH), 34.9 (C), 31.5 (CH3). MS (MALDI) m/z 2407.7 (MC).

Acknowledgements Financial support from the Spanish DGI (BQU2002-01327) and the Junta de Comunidades de Castilla-La Mancha (GC02-013) is gratefully acknowledged.

References and notes 1. (a) Seminario, J. M.; Tour, J. M. In Molecular ElectronicsScience and Technology; Aviran, A., Ratner, M., Eds.; New York Academy of Science: New York, 1998. (b) Electronic Materials: The Oligomer Approach; Mu¨llen, K., Wegner, G., Eds.; Wiley-VCH: New York, 1998. 2. (a) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendrimers and Dendrons; Wiley-VCH: Weinheim, 2001. (b) Dendrimers and Other Dendritic Polymers; Fre´chet, J. M. J., Tomalia, D. A., Eds.; Wiley: Chichester, 2001. (c) Hawker, C. J.; Wooley, K. L. In Newkome, G. R., Ed.; Advances in Dendritic Macromolecules; Jai: Greenwich, 1995; Vol. 2. (d) Vo¨gtle, F., Ed.; Dendrimers in Top. Curr. Chem.; Springer: Berlin, 1998; Vol. 197. (e) Bosman, A. W.; Janssen, H. M.; Meijer, E. W.

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