Free-base tetraarylporphyrin covalently linked to [60]fullerene through ethynylfluorene spacer

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Journal of Porphyrins and PhthalocyaninesJ. Porphyrins Phthalocyanines 2011; 15: 1231–1238

DOI: 10.1142/S1088424611004257

Published at http://www.worldscinet.com/jpp/

Copyright © 2011 World Scientific Publishing Company

INTRODUCTION

The photophysical and electrochemical properties of covalently linked arrays of porphyrins are of great inter-est in a variety of biomimetic [1–3], supramolecular [4, 5] and materials areas [6–9]. On the other hand, fullerene C60 has proved to behave as an ideal acceptor due to their unique chemical and electronic properties [10]. Actually, C60 has shown to reversibly accept up to six electrons in solution, with a first reduction potential resembling that of the substituted p-benzoquinone deriv-atives acting as electron acceptors in the photosynthetic reaction center. Interestingly, in photoinduced electron transfer processes, fullerenes accelerate the charge sepa-ration and slow down the charge recombination, thus stabilizing the charge separated state in donor–acceptor (D–A) dyads in comparison with other related systems bearing traditional electron acceptors such as quinones and imides [11]. Porphyrins and fullerenes have been covalently and supramolecularly connected through a

variety of different π-conjugated systems in order to facil-itate the electronic communication between both electro-active species upon light irradiation [12]. Furthermore, these π-conjugated systems are useful to study novel concepts for modern electronic and photovoltaic applica-tions, which require rapid and efficient charge transport over long distances [13]. In this regard, Wasielewski and coworkers have incorporated different oligomers, such as p-phenylenevinylene [14], p-phenylene [15] and fluo-rene [16], to bridge phenothiazine and perylene-3,4:9,10-bis(carboximide) as donors and acceptors, respectively. Here, the oligomeric π-conjugated bridges show a switch in the electron transfer mechanism from superexchange to hopping at longer bridge lengths.

Aimed by the above results, we have joined efforts to obtain novel electroactive organic molecules endowed with π-conjugated cores [17]. Previously, we have reported some fluorene-based molecular wires [18]. One important feature in our systems is the incorporation of vinylene spacers into the oligofluorene molecular wires. Interestingly, experimental and theoretical results reveal that the vinyl spacer improves the charge-transfer fea-tures significantly to give a very low attenuation factor (β) value of 0.075 ± 0.001 Å-1. Based on these results, in this work we present the synthesis, electronic and

Free-base tetraarylporphyrin covalently linked to [60]fullerene through ethynylfluorene spacer

Carlos A. Echeverrya, Alexis Tigrerosa, Alejandro Ortiza, Braulio Insuasty*a and Nazario Martín*b

a Departamento de Química, Facultad de Ciencias Naturales y Exactas, Universidad del Valle, A.A. 25360 Cali, Colombia b Departamento de Química Orgánica, Facultad de Química, Universidad Complutense, 28040 Madrid, Spain

Dedicated to Professor Karl M. Kadish on the occasion of his 65th birthday

Received 30 July 2011Accepted 25 August 2011

ABSTRACT: The synthesis, structural and electronic characterization of novel electroactive systems based on porphyrin-fullerene in which the chromophores are linked by an ethynylfluorene spacer unit is reported. Sonogashira couplings have been used in short and efficient sequences to give access to these new molecules on a practical scale. The absorption studies, voltamperometric measurements and theoretical calculations at DFT level reveal the push-pull behavior for these systems.

KEYWORDS: porphyrins, fullerene, ethynylfluorene, electron transfer, electroactive.

*Correspondence to: Braulio Insuasty, email: [email protected], tel: +57 2-3393248, fax: +57 2-3393248 and Nazario Martín, email: [email protected], tel: +34 91-3944227, fax: +34 91-3944103

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http://dx.doi.org/10.1142/S1088424611004257

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Copyright © 2011 World Scientific Publishing Company J. Porphyrins Phthalocyanines 2011; 15: 1232–1238

1232 C.A. ECHEVERRY ET AL.

electrochemical characterization of a novel electroactive molecule (10) in which the donor (free-base porphyrin) and acceptor ([60]fullerene) moieties are covalently con-nected through a soluble linker of ethynylfluorene.

EXPERIMENTAL

Materials and methods

Commercially available starting materials, reagents and solvents were used as supplied. Solvents used in photochemical measurements were of spectroscopic grade. TLC analyses were performed on Merck TLC-plates aluminium silica gel 60 F254, melting points were determined in a Buchi Melting Point Apparatus and were uncorrected. The 1H and 13C NMR spectra were run on a Bruker AVANCE 400 spectrometer operating at 400 MHz and 100 MHz respectively, using chloroform d1 as solvent and tetramethylsilane as internal standard. The mass spectra were scanned on a Shimadzu MS-QP 2010 spectrometer and operating at 70 eV. UV-vis spectra were recorded in a Shimadzu 1700 spectrometer. Electro-chemical measurements were performed on an autolab PGStat 30 equipment using a three-electrode configura-tion system. The measurements were carried out using a o-dichlorobenzene/CH3CN 4/1 solution 0.1 M in tet-rabutylammoniun perchlorate. A glassy carbon electrode (3 mm diameter) was used as the working electrode, and a platinum wire and an Ag/AgNO3 electrode were employed as the counter and the reference electrodes, respectively. Both the counter and the reference elec-trodes were directly immersed in the electrolyte solution. The surface of the working electrode was polished with commercial alumina prior to use. Solutions were deaer-ated by bubbling argon for a few minutes prior to each voltammetric measurement. Unless otherwise specified the scan rate was 100 mV.s-1.

For the preparation of compounds 2 and 3, we fol-lowed a synthetic methodology reported by Promarak et al. [19].

Preparation of 7-iodo-9,9-dioctyl-9H-fluorene-2- carbaldehyde (4). A solution of the 2,7-diiodo-9,9-dioctyl fluorene (6.09 g, 9.48 mmol) in dry diethyl ether (50 mL) at -78 °C was added dropwise n-BuLi in hexane solution (4.98 mL, 9.95 mmol) under argon atmosphere. After the mixture was stirred at -78 °C for 1 h, anhy-drous DMF (0.73 mL, 9.48 mmol) was added into the mixture. After it was slowly warm up to room tempera-ture and was stirred over night. A solution of HCl 2M (50 mL) was added and the mixture was stirred for 2 h. The aqueous layer was extracted with diethyl ether, and the combined organic part was washed with brine and dried over Na2SO4. After removal of the solvents under reduced pressure, the residue was purified by column chromatography (silica gel, hexane/dichloromethane, 2/1) to afford 4 as a yellow oil (4.26 g, 80%). 1H NMR

(400 MHz, CDCl3): δ, ppm 0.58 (m, 4H), 0.80 (t, J = 14.1 Hz, 6H), 1.05 (m, 20H), 1.99 (m, 4H), 7.52–7.87 (m, 6H), 10.07 (s, 1H). 13C NMR (100 MHz, CDCl3): δ, ppm 191.6, 153.9, 150.4, 146.0, 138.7, 135.8, 135.2, 131.8, 130.0, 122.0, 119.6, 94.4, 55.0, 39.6, 31.2, 29.3, 28.7, 23.1, 22.1, 13.6. MS-EI: m/z 544 [M]+.

Preparation of 9,9-dioctyl-7-[(trimethylsilyl)ethynyl]- 9H-fluorene-2-carbaldehyde (5). The compound 4 (544.0 mg, 1.0 mmol), Pd(PPh3)Cl2 (35.1 mg, 0.05 mmol), CuI (19.0 mg, 0.1 mmol) and PPh3 (26.2 mg, 0.1 mmol) were dissolved in THF anhydrous (30 mL) and degassed with argon. To the reaction flask was added triethyla-mine (151.8 mg, 1.5 mmol) and trimethylsilylacetylene (150.3 mg, 1.5 mmol). The reaction was stirred at reflux for 12 h under argon atmosphere. The mixture was poured into aqueous NH4Cl solution and washed with a solution of HCl (10%). After aqueous layer was extracted with chloroform and the combined organic part was washed with brine and dried over Na2SO4. After removal of the solvents under reduced pressure, the residue was purified by column chromatography (silica gel, hexane/dichloro-methane, 2/1) to afford 5 as yellow oil (370 mg, 72%). 1H NMR (400 MHz, CDCl3): δ, ppm 0.27 (s, 9H), 0.60 (m, 4H), 0.82 (t, J = 7.20 Hz, 6H), 1.14 (m, 20H), 2.01 (m, 4H), 7.48–7.52 (m, 2H), 7.71 (d, J = 7.80 Hz, 1H), 7.85 (m, 3H), 10.07 (s, 1H). 13C NMR (100 MHz, CDCl3): δ, ppm 192.1, 152.0, 151.8, 146.6, 139.9, 135.6, 131.4, 130.4, 126.4, 123.2, 123.1, 120.7, 120.3, 105.7, 95.1, 55.4, 40.1, 31.7, 31.9, 29.8, 29.1, 29.1, 23.6, 22.5, -0.04. MS-EI: m/z 514 [M]+.

Preparation of 7-ethynyl-9,9-dioctyl-9H-fluorene-2- carbaldehyde (6). To a solution of 5 (116.5 mg, 0.23 mmol) in THF/MeOH (4/1) was added potassium car-bonate (62.5 mg, 0.45 mmol) and it was stirred for 2 h to room temperature. The mixture was washed with brine and dried over Na2SO4. After removal of the sol-vents under reduced pressure, the residue was purified by column chromatography (silica gel, hexane/dichloro-methane, 1/1) to afford 6 as yellow oil (75 mg, 74%). 1H NMR (400 MHz, CDCl3): δ, ppm 0.58 (m, 4H), 0.82 (t, J = 6.60 Hz, 6H), 1.16 (m, 20H), 2.01 (m, 4H), 3.19 (s, 1H, ≡CH), 7.51(s, 1H), 7.52 (d, J = 7.70 Hz, 1H), 7.73 (d, J = 7.70 Hz, 1H), 7.80–7.87 (m, 3H), 10.08 (s, 1H). 13C NMR (100 MHz, CDCl3): δ, ppm 192.1, 152.1, 151.8, 140.2, 138.0, 135.7, 131.4, 130.4, 126.7, 123.2, 122.1, 120.8, 120.4, 100.0, 84.2, 55.4, 40.1, 31.7, 29.8, 29.1, 29.1, 23.7, 22.5, 14.0. MS-EI: m/z 442 [M]+.

Preparation of dyad (7). To a solution of fullerene C60 (158.5 mg, 0.22 mmol) in chlorobenzene (25 mL), N-octylglycine (61.8 mg, 0.33 mmol) and compound 6 (50.0 mg, 0.11 mmol) were added. The reaction was stir-red at reflux with a Dean-stark trap for 8 h. After removal of the solvent under reduced pressure, the residue was purified by column chromatography (silica gel, CS2) to afford 7 as a black solid (49.5 mg, 43%); mp > 350 °C. 1H NMR (400 MHz, CDCl3): δ, ppm 0.85–1.66 (m, 43H), 1.95 (m, 6H), 2.64–328 (m, 2H), 3.13 (s, 1H, ≡CH),

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FREE-BASE TETRAARYLPORPHYRIN COVALENTLY LINKED TO [60]FULLERENE 1233

4.18 (d, J = 8.10, 1H), 5.14 (d, J = 8.10 Hz, 1H), 5.16 (s, 1H), 7.26–7.47 (m, 3H, H-Fl), 7.61–8.00 (m, 3H, H-Fl). 13C NMR (100 MHz, CDCl3): δ, ppm 156.5, 154.2, 153.7, 153.6, 150.8, 147.3, 146.7, 146.5, 146.3, 146.2, 146.1, 145.9, 145.7, 145.6, 145.5, 145.3, 145.2, 145.1, 144.7, 144.5, 144.4, 144.3, 143.1, 143.0, 142.7, 142.6, 142.5, 142.4, 142.3, 142.1, 142.0, 141.9, 141.7, 141.6, 141.5, 141.5, 140.6, 140.2, 140.1, 139.8, 139.3, 136.9, 136.7, 136.6, 135.8, 135.7, 131.1, 126.4, 123.5, 120.3, 119.6, 99.9, 84.7, 82.6, 68.9, 66.9, 55.1, 53.3, 40.5, 31.9, 31.8, 30.2, 29.8, 29.6, 29.5, 29.4, 28.4, 27.9, 24.0, 23.8, 22.7, 22.6, 14.2, 14.1. MS-ESI: m/z 1047 [M+ + 1].

Preparation of free-base meso-ethynyltetraaryl-porphyrin (8). To a solution of pyrrole (0.149 g, 2.22 mmol), 3,5-di-t-butylbenzaldehyde (0.620 g, 2.84 mmol) and 4-(trimethylsilyl)ethynylbenzaldehyde (0.150 g, 0.74 mmol) in dry chloroform, 0.18 mL de BF3–OEt2 was added and the solution was refluxed and stirred by 90 min. Then, 450 mg of p-chloranile was added and left stirring at this temperature for 90 min. The solvent was removed under vacuum and the product was puri-fied by column chromatography (silica gel, chloroform-hexane 1/3) to afford 8 as purple solid (92.9 mg, 12%). 1H MNR (400 MHz, CDCl3): δ, ppm -2.67 (s, 2H, NH), 0.41 (s, 9H, (CH3)3–Si), 1.46 (s, 18H, t-Bu), 1.56 (s, 36H, t-Bu), 7.83 (m, 3H, Hp-Ar), 7.90 (d, J = 8.28 Hz, 2H, Ho-Ar), 8.11 (m, 6H, Ho-Armeso), 8.21 (d, J = 8.28 Hz, 2H, Hm-Ar), 8.24 (d, J = 4.77 Hz, 2H, HPyrrole), 8.93 (d, J = 4.77 Hz, 2H, HPyrrole), 8.94 (m, 4H, HPyrrole).

13C NMR (100 MHz, CDCl3): δ, ppm 148.7 142.9, 141.2, 134.3, 130.3, 129.8, 129.7, 122.4, 121.7, 121.5, 121.0, 118.6, 105.2, 95.3, 31.7 (t-Bu), 30.4 (t-Bu), 26.9 ((CH3)3Si). MS-ESI: m/z 1047 [M+ + 1]. This compound (92.0 mg, 0.09 mmol) was treated with K2CO3 (60 mg) in metha-nol (5.0 mL) and THF (5.0 mL) at 25 °C for 2 h, under argon. The solvent was removed under vacuum and the residue was dissolved in dichloromethane (20 mL). This solution was washed once with an aqueous saturated solution of NaHCO3 and once with water, before being dried over anhydrous Na2SO4 and the solvent evapora-ted under vacuum. The purple residue was purified by column chromatography using dichloromethane-hexane (1/1) for elution to give (67 mg, 80%) of the title com-pound. 1H MNR (400 MHz, CDCl3): δ, ppm -2.65 (s, 2H, NH), 1.42 (s, 18H, t-Bu), 1.53 (s, 36H, t-Bu), 3.31 (s, 1H, ≡CH), 7.80 (m, 3H, Hp-Ar), 7.90 (d, J = 8.28 Hz, 2H, Ho-Ar), 8.15 (m, 6H, Ho-Armeso), 8.20 (d, J = 8.28 Hz, 2H, Hm-Ar), 8.28 (d, J = 4.77 Hz, 2H, HPyrrole), 8.92 (d, J = 4.77 Hz, 2H, HPyrrole), 8.96 (m, 4H, HPyrrole).

13C NMR (100 MHz, CDCl3): δ, ppm 148.9 143.1, 141.4, 134.1, 130.1, 129.8, 129.4, 122.4, 121.9, 121.3, 121.2, 118.9, 105.7, 96.7 (≡CH), 31.8 (t-Bu), 26.6 (t-Bu). MS-ESI: m/z 976 [M+ + 1].

Preparation of dyad (9). A solution of 8 (80.0 mg, 0.081 mmol), 4 (0.073 mmol, 40.0 mg), Pd2(dba)3, (5.0 mg, 0.005 mmol), AsPh3 (10.0 mg, 0.033 mmol) and anhy-drous THF (25.0 mL) was heated to reflux under argon

atmosphere for 24 h. The solvent was removed under reduced pressure and the crude material was carefully chromatographed over silica using hexane and dichloro-methane 3/1 as eluents (63 mg, 62%); mp 260–263 °C. 1H NMR (400 MHz, CDCl3): δ, ppm -2.68 (s, 2H, NH), 0.66 (m, 4H, CH2), 0.84 (t, J = 7.30 Hz, 6H, CH3), 1.17 (m, 20H, CH2), 1.52 (s, 18H, t-Bu), 1.53 (s, 36H, t-Bu), 2.09 (t, J = 8.26 Hz, 4H, CH2-Fl), 7.71 (m, 2H, H-Fl), 7.80 (m, 3H, Ho-Ar), 7.83 (m, 1H, H-Fl), 7.88 (m, 2H, H-Fl), 7.92 (m, 1H, H-Fl), 7.97 (d, J = 8.28 Hz, 2H, Ho-Ar), 8.08 (d, J = 1.75 Hz, 2H, Hm-Ar), 8.09 (d, J = 2.01 Hz, 4H, Hm-Ar), 8.26 (d, J = 8.28 Hz, 2H, Hm-Ar), 8.87 (d, J = 4.76 Hz, 2H, HPyrrol), 8.91 (m, 6H, HPyrrol), 10.09 (s, 1H, CHO). 13C NMR (100 MHz, CDCl3): δ, ppm 197.2 (CHO), 152.3, 150.9, 147.8, 144.2, 144.1, 141.9, 140.0, 134.0, 133.3, 132.3, 131.0, 130.9, 130.8, 130.6, 129.9, 128.5, 122.9, 122.2, 122.0, 121.7, 121.0, 120.0, 118.8, 117.7, 104.7, 95.0, 95.0, 67.6, 55.7, 41.2, 32.0 (t-Bu), 31.8 (t-Bu), 29.7, 29.3, 25.5, 23.7, 22.0, 14.8. MS-ESI: m/z 1391 [M+ + 1].

Preparation of triad (10). A solution of C60 (20.7 mg, 0.029 mmol), H2P-CHO (20.0 mg, 0.014 mmol) and N-octylglycine (8.1 mg, 0.043 mmol) was refluxed in toluene (30 mL) for 6 h. The solvent was removed under reduced pressure and the crude material was carefully chromatographed over silica using carbon disulfide and toluene 5/1 as eluents (13 mg, 45%); mp 292–294 °C. 1H NMR (400 MHz, CDCl3): δ, ppm -2.69 (s, 2H, NH), 0.85 (m, 9H, CH2), 0.93 (t, J = 6.90 Hz, 6H, CH3), 1.26 (m, 20H, CH2), 1.35 (m, 8H, CH2), 1.52 (s, 18H, t-Bu), 1.53 (s, 36H, t-Bu), 1.97 (m, 6H, CH2), 2.56 (m, 2H, CH2-N), 3.92 (d, J = 8.00 Hz, 1H, CH), 4.90 (s, 1H, CH), 4.93 (d, J = 8.00 Hz, 1H, CH), 7.44 (m, 1H, H-Fl), 7.61 (m, 2H, H-Fl), 7.65 (d, J = 1.25 Hz, 1H, H-Fl), 7.72 (m, 2H, H-Fl), 7.80 (m, 3H, Hp-Ar), 7.92 (d, J = 8.03 Hz, 2H, Ho-Ar), 8.08 (m, 6H, Hm-Ar), 8.19 (d, J = 8.04 Hz, 2H, Hm-Ar) 8.84 (d, J = 4.77 Hz, 2H, HPyrrol), 8.89 (m, 6H, HPyrrol). 13C NMR (100 MHz, CDCl3): δ, ppm 156.0, 151.1, 149.5, 148.7, 148.7, 146.7, 146.3, 145.9, 145.7, 145.6, 145.5, 145.4, 145.3, 145.2, 145.1, 145.0, 144.9, 144.8, 144.7, 144.4, 144.3, 144.2, 144.1, 143.9, 143.8, 143.5, 142.5, 142.4, 141.8, 141.7, 141.7, 141.6, 141.5, 141.4, 141.3, 141.3, 141.2, 141.2, 141.2, 141.0, 140.9, 140.8, 140.7, 139.4, 138.9, 136.1, 134.9, 134.5, 130.8, 129.8, 129.8, 129.7, 126.0, 122.7, 121.7, 121.6, 121.0, 118.8, 118.7, 91.6, 82.5, 77.2, 68.5, 66.7, 55.2, 35.1, 35.1, 32.0, 31.9, 31.9, 31.9 (CH3 t-Bu), 31.8 (CH3 t-Bu), 29.9, 29.8, 29.7, 29.6, 29.5, 29.4, 28.5, 26.9, 22.8, 22.7, 22.6, 14.2, 14.1. MS-ESI: m/z 2021 [M+ + 1].

RESULTS AND DISCUSSION

We first explored the synthesis of compounds 4 and 6 by a routine procedure in a multistep methodology (Scheme 1). Palladium (0) promoted cross-coupling with trimethylsilylacetylene provided good quantities of com-pound 5 easily separable by column chromatography due

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to the presence of the octyl chains. Stepwise deprotection of the TMS group was feasible by strict control of the reaction time and the separation of 6 was again easy.

Compounds 5 and 6 were fully characterized by spec-troscopic techniques; the 1H NMR of 6 showed the char-acteristic signal of the formyl proton at δ = 10.08 and the acetylene proton at δ = 3.19. All spectroscopic details are gathered in the experimental section.

Our next target became the preparation of the refer-ence dyad 7 by means of a 1,3-dipolar cycloaddition of compound 6 and N-octylglycine to fullerene C60 in refluxing toluene (Scheme 2). Compound 7 was used as a reference for studying the electronic behavior of the covalent assemble between the C60 unit and ethynylfluo-rene system.

UV-vis spectra for compounds 6, 7 and C60 were mea-sured in toluene as solvent. Interestingly, the absorption bands of 7, is a linear combination of the spectra of com-pound 6 and C60 (Fig. 1), which indicates that there is not significant electronic communication between both units in the ground-state.

In the subsequent step, novel donor–bridge–acceptor system with C60 and tetraphenylporphyrin as photo- and redox-active components covalently linked by an ethy-nylfluorene bridge was prepared.

As a first attempt, it was carried out the Hagihara-Sonogashira cross-coupling reaction between compound 7 and a free-base meso-iodotetraarylporphyrin. How-ever, this synthetic approach was discarded due to the low yield of product obtained. Therefore, we decided to obtain the free-base meso-ethynyltetraarylporphyrin (P)

8 by means of the synthetic method-ology reported by Lindsey [20]. Then, porphyrin 8 was subjected to Hagihara-Sonogashira cross-coupling reaction with 4 to obtain the dyad (π)-bridge 9. Finally, compound 9 underwent a 1,3-dipolar cycloaddition reaction with N-octylglycine and C60 to form the triad 10 in relatively good yield (Scheme 3).

Compounds 8, 9 and 10 were iso-lated by column chromatography in silica gel and fully characterized by standard spectroscopic techniques. The 1H NMR spectrum of 8 shows signals at around δ = 8.24–8.94 for the pyrrol protons of the porphyrin core, and the signal of the acetylene proton at δ = 3.31. For compound 9 the signal for the acetylene proton disappears and the signal for the formyl proton can be seen at δ = 10.09. The signals of the fluorene moiety are observed in the aromatic region around δ = 7.71–7.92. The 1H NMR spectrum of compound 10 shows the characteristic pyrrolidine signature between δ = 3.92 and

I I

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4 (80%)

C8H17C8H17

O

TMS

5 (72%)

C8H17C8H17

O

6 (74%)

(i) (ii)

(iii)

(iv)(v)

Scheme 1. Reagents and conditions: (i) I2/KIO3, CH3CO2H/H2SO4 20%, 80 °C; (ii) NaOH(aq)/TBAB, C8H17Br/toluene, 60 °C; (iii) 1. BuLi/DMF, -78 °C to rt 2. HCl/H2O; (iv) Pd(PPh3)2Cl2, PPh3/CuI, trimethylsilylacetylene, THF/Et3N, Ar/reflux; (v) K2CO3, THF/MeOH, rt

CHO

C8H17 C8H17

+ C60(i)

NOct

C8H17 C8H17

7 (43%)6

Scheme 2. Reagents and conditions: (i) N-octylglycine/toluene, reflux

Fig. 1. Absorption spectra for compounds 6, 7 and C60 recorded in toluene

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4.93 as two doublets (AB system) and one singlet at δ = 4.90 ppm.

The electronic characterization of triad 10 was carried out by recording several absorption spectra (Fig. 2). In Fig. 2a, the absorption bands of 10 are observed as a linear combination of the spectra of com-pounds 6, 8 and C60, with a best contribu-tion of the porphyrin core. Additionally, a solvatochromic study shows a slightly hypsochromic shift of the absorption Q-band of the porphyrin in solvents with different polarity (Fig. 2b); in toluene λQ = 516.5 nm; THF λQ = 515.0 nm; hexane λQ = 514.5 nm; methylene dichloride λQ = 517.5 nm and ethyl acetate λQ = 514.5 nm.

The emission spectra for compounds 6 and 10 were recorded in toluene and were carried out by exciting tetraaryl porphyrin at 548 nm of the Q-band. The fluorescence maxima are found red-shifted at 656 nm

Scheme 3. Reagents and conditions: (i) 1. BF3.OEt2/CHCl3, rt; 2. p-chloranyl, reflux. (ii) K2CO3, THF/MeOH, rt; (iii) Pd(PPh3)2Cl2, PPh3/CuI, THF/Et3N, Ar/reflux; (iv) C60, N-octylglycine/toluene, reflux

NH

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I

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N

NH N

HN

Ar

Ar

Ar

C8H17C8H17

O

9 (62%)

(iv)

N

Oct

C8H17C8H17

N

NHN

HN

Ar

Ar

Ar

10 (45%)

1112

13

Ar =

Fig. 2. (a) Absorption spectra for compounds 6, 10 and C60 recorded in toluene; (b) Inset: absorption spectra for triad 10 recorded in different solvents

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for 6 and 650 nm for 10 compared with the respective absortion Q-band. Such spectra provides first insights into the excited state deactivation pathways; the quench-ing of the fluorescence of 10 compared with 6 is due to the link of the π-conjutated moiety to C60 in 10. This fact favors the intramolecular electron-transfer and reduces the emission processes. Emission spectra were also mea-sured for 10 in different solvents, and a fluorescence quenching with increasing of the dielectric constant of such solvents was observed (Fig. 3).

The electrochemical features of the electroactive compounds 7 and 10 were studied by cyclic voltamme-try using glassy carbon as working electrode; Ag/Ag+ as reference electrode; Pt as counter-electrode; 0.1 M Bu4NClO4, a scan rate of 100 mV/s at room temperature and Fc/Fc+. Compound 7 shows the reduction profile of three quasireversible C60 reduction waves at potential val-ues for the first reduction wave at -0.78 V, the second and the third waves appearing at -1.20 V and -1.67 V, respectively. As expected, the reduction potential values for 7 are cathodically shifted in comparison with those of pristine C60, which has been accounted for by the satura-tion of a double bond of the C60 sphere which raises the LUMO energy level. The electrochemistry of 10 shows the first reductive couple of the C60 unit at -0.79 V, which is in good agreement with values typically observed for C60 derivatives, the other reduction waves for the fuller-ene core of 10 appearing at -1.31 V and -1.60 V. The first and second one-electron oxidation waves of the porphy-rin center occurs at +0.85 and +1.13 V, which is slightly more negative than that found for pristine tetraphenyl-porphyrins [21]. Similar redox potentials reveal, in agree-ment with the data stemming from the electronic spectra, only weak interactions between the redox chromophores in their ground state (Table 1). Additionally, a reduction potential at -1.56 V can be reasonably assigned to the

first reduction process for the porphyrin unit, which is in agreement with the values found for the pristine free-base porphyrin.

Theoretical calculations at DFT/B3LYP level were carried out in order to gain a better understanding of the electronic structure for triad 10 and underpinning their experimentally observed push-pull character. The geom-etry of 10 was optimized using a 3-21G basis set in the Gaussian 0.3 program [22]. It is interesting to note the quasi-planar ethynylfluorene-C60 union, which shows a dihedral angle of only 1.8º. In addition, it can be seen a quasi-planar organization of the bridge between the two redox units (P-C60). This planar geometry would facili-tate the electronic communication along the molecule through the ethynyl-fluorene wire (Fig. 4a).

In frontier orbital analysis, it is evident that the HOMO is mainly localized in the electron–donor moi-ety with a small bridge contribution. As expected, the

Fig. 3. Emission spectra for compounds 6 and 10 recorded in toluene.; Inset: emission spectra for triad 10 recorded in different solvents

Table 1. Redox potentials (V) in o-dichlorobenzene/MeCN (4/1)a

Compound E1/23

ox E1/22

ox E1/21red E1/2

2red E1/2

3red E1/2

4red

H2P b +1.18 +0.89 — — -1.43 -1.75

C60 — — -0.72 -1.12 — -1.60

7 — — -0.78 -1.20 — -1.67

10 +1.13 +0.85 -0.79 -1.31 -1.56 -1.60

a GCE (glassy carbon) as working electrode, Ag/AgNO3 as ref-erence electrode, Bu4NClO4 (0.1 M) as supporting electrolyte, and o-DCB/CH3CN 4:1 (v/v) as solvent. Scan rate 100 mV.s-1. b Data collected from literature [21].

Fig. 4. (a) Minimum-energy conformation calculated for 10 with energy in atomic units at the B3LYP/3-21G level. (b) Elec-tron density contours (0.03 eBohr-3) calculated for the HOMO (left) and LUMO (right) of 10

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LUMO is fully localized in the electron–acceptor unit of C60 (Fig. 4b). Furthermore, the HOMO–LUMO energy gap resulted to be of 4.86 eV. The last finding shows a better molecular wire behavior for compound 10 accord-ing with the calculated (AO) composition.

These results are in agreement with the previous experimental UV-vis and CV findings, and the frontier orbital behavior of the HOMO and LUMO in the triad 10 support the electronic communication between the electroactive species, namely free porphyrin and [60]fullerene in this new triad and, therefore, its interest to be tested as a molecular wire.

In summary, it has been shown that the ethynyl-fluorene spacer is a good system for electron-transfer communication between electroactive organic molecu-lar systems with potential applications in fields such as molecular electronics and solar-electric energy transfor-mation. These results pave the way for further works in donor–acceptor systems containing more sophisticated ethynylfluorene-based bridges of different lengths. Fur-thermore, a further photophysical study will confirm the experimental evidences now reported in the excited state. Work is currently under progress in this way.

Acknowledgements

This work has been supported by COLCIENCIAS (Colombia), the Universidad del Valle (Colombia). Finan-cial support by the MEC of Spain (projects CTQ2008-0795/BQU and Consolider-Ingenio 2010C-07-25200), the CAM (MADRISOLAR-2 project S2009/PPQ-1533) and EU (FUNMOLS FP7-212942-1) are acknowledged.

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Documents

Free-base tetraarylporphyrin covalently linked to [60]fullerene through ethynylfluorene spacer