Covalently linked heterofullerene–porphyrin conjugates; new model systems for long-lived intramolecular charge separation

Tetrahedron 62 (2006) 1923–1927

Covalently linked heterofullerene–porphyrin conjugates; new model systems for long-lived intramolecular charge separation Frank Hauke,a Stefan Atalick,b,c Dirk M. Guldib,* and Andreas Hirscha,* a

Institut fu¨r Organische Chemie, Henkestrasse 42, D-91054 Erlangen, Germany Institut fu¨r Physikalische Chemie, Egerlandstr 3, D-91058 Erlangen, Germany c Radiation Laboratory, Notre Dame, IN 46556, USA

b

Received 3 November 2004; revised 23 April 2005; accepted 25 April 2005 Available online 28 November 2005

Abstract—Attaching tetraphenyl porphyrins, with peripheral acetyl or malonate groups, to C59N leads to the first covalently linked heterofullerene–porphyrin conjugates that exhibit long-lived intramolecular charge separation. q 2005 Elsevier Ltd. All rights reserved.

One of the most intensively investigated areas of fullerene chemistry concerns the physico-chemical properties of fullerene derivatives covalently tethered to one or more photoactive electron donors.1 In particular, porphyrins are electron donor/chromophores ideally suited for devising integrated, multicomponent model systems to transmit and process solar energy. A significant setback—when using fullerenes as electron-acceptor units—is the reduced electron affinity stemming from most functionalization reactions.2 Incorporating a heteroatom into the all carbon-framework of C60 enhances its electron affinity and, thereby, overcomes some of the deficiencies encountered in conventional derivatization.3 The core functionalization of C59N, on the other hand, is more demanding than that of the parent [60]fullerene.4 This is based on the fact that addition reactions to the Cs symmetrical C59N framework may lead to up to 16 regioisomeric monoaddition products. However, the monoazaheterofullerene exhibits a highly regioselective functionalization pattern, which can exclusively be found with (C59N)2 as starting material. The thermal cleavage of the inter dimer bond and subsequent oxidation of the formed C59N radical yields the highly reactive C 59N C cation. C 59N C undergo electrophilic substitution reactions with aromatics or enolizable coumpounds.5 In such monomeric C59N derivatives the addend is connected to the heterofullerene Keywords: Fullerenes; Porphyrins; Fluorescence quenching. * Corresponding authors. Fax: C49 9131 8526864; e-mail addresses: [email protected]; [email protected] 0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2005.04.078

core by a sp3 carbon atom adjacent to the nitrogen atom. We have reported earlier on the formation of the supramolecular heterofullerene–porphyrin dyad 1.6 In that system we used the functionalization technique mentioned above to connect an anchor group (acetylpyridine) to the C59N framework. By an axial coordination of the pyridine nitrogen atom to the zinc atom of a tetraphenyl zinc porphyrin the dyad 1 was generated. Depending on the solvent either photoinduced singlet–singlet energy transfer or electron-transfer was observed for that supramolecular assembly of two redoxactive components. In the present contribution, we wish to report on the first examples of covalently linked heterofullerene–porphyrin conjugates as novel artificial light harvesting antenna and reaction center mimics. First, we synthesised functional porphyrins that carry suitable linking groups for the attachment to the C59N core. Malonate functionalised tetraphenylporphyrin 2 and acetyl porphyrin derivative 3 (Fig. 1) are good target compounds, since they can be connected to the heterofullerene framework via the C-atom a to the carbonyl groups (Scheme 1). The reaction of these two porphyrin derivatives (2 and 3) with an equimolar amount of (C59N)2 and 30 equiv of p-TsOH at 150 8C in o-dichlorobenzene (ODCB) in a constant stream of air afforded dyads 4 and 5 (Fig. 2) in moderate yields. 4 and 5 were purified by flash chromatography (silica gel, toluene) and were isolated in high purity (greater than 99%, determined by HPLC). The structural characterisation of 2–5 was carried out by 1H, 13 C NMR, UV/vis and FT-IR spectroscopy as well as by

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Figure 1. Tetraphenylporphyrins with malonate and acetyl groups.

downfield shift of the signals, which are asscociated with the methylene group in 2 and the methyl group in 3 by about 1.67 and 2.58 ppm, respectively. This effect is due to connecting the electron withdrawing C59N to porphyrins 2 and 3. The 13C NMR spectra allow assigning the symmetry of the heterofullerene–porphyrin dyads. Dyad 4, for example, exhibits C1 symmetry, due to the newly introduced asymmetric C-atom within the malonate group. Therefore, a set of 58 signals for the cage carbon atoms can be detected in the 13C NMR spectrum.

Scheme 1. Structure of dyad 1.

FAB mass spectrometry.7 The UV/vis spectra of the C59N– porphyrin dyads 4 and 5 are, in first approximation, the superposition of the spectra of the C59N core3,4 and the respective porphyrin with the characteristic absorption bands at lZ257, 320 nm of C59N and the Soret-Band and the four Q-bands of the porphyrins. The 1H spectra of the dyads 4 and 5 are closely related to the spectra of the porphyrins 2 and 3. Worth mentioning is the

Figure 2. The two first covalently linked C59N–porphyrin dyads 4 and 5.

Dyad 5 on the other hand exhibits Cs symmetry. The C-atoms of the heterofullerene framework, of the porphyrin core and of the four benzene rings resonate in the region between dZ117–155. When comparing the signals of dyad 5 with those of porphyrin 3, 30 signals, which are attributed to the resonances of the C59N core, can be sorted out. 28 signals have double intensity and two signals have single intensity, as they are located at the mirror plane of the C59N core. The characteristic resonance of the sp3 -carbon atom of the C 59N cage, which is also located at the mirror plane, occurs at dZ78.40. The carbonyl group of the ketone functionality resonates at dZ194.78 and the resonances of the a-methylene group are found at dZ49.96. Finally, the signals at dZ34.49 and 31.56 correspond to the carbon atoms of the three t-butyl groups.

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toluene o-dichlorobenzene benzonitrile

10

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6

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Figure 3. Fluorescence spectra of dyad 4 in different solvents (see labels for assignment) with matching absorption at the 427 nm excitation wavelength—OD427 nmZ0.5. 8 7 toluene

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o-dichlorobenzene

3 2 benzonitrile

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Figure 4. Fluorescence spectra of dyad 5 in different solvents (see labels for assignment) with matching absorption at the 427 nm excitation wavelength—OD427 nmZ0.5.

In complementary transient absorption studies we probed the fate of the H2P fluorescence quenching and looked into product identification. Pumping light into the H2P’s ground state in 4 and 5 with short laser pulses leads to the

0.05 0.04 0.03 ∆OD/ a.u.

Heterofullerene–porphyrin dyads 4 and 5 exhibit interesting photophysical properties. The porphyrin fluorescence—a sensitive probe for the magnitude of intramolecular electron transfer activation—is independent on the excitation (i.e., 414 nm Soret-band excitation or 520 nm Q-band excitation) quenched in both conjugates 4 and 5—see Figures 3 and 4. Fluorescence quantum yields are on the order of w10K3— relative to 0.10 for H2P. In 4, where the spacer connecting H2P and C59N allows for configurational flexibility, the solvent dependence on the fluorescence, that is, from toluene to THF and benzonitrile gives rise to invariant quenching (i.e., w2!10K3). This suggests a solvent assisted through-space electron transfer mechanism. On the contrary, in 5 the data (i.e., toluene: 1.5!10K3; benzonitrile: 0.4!10K3) is in line with a through-bond mechanism.8

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0.02 0.01 0 -0.01 -0.02 400

500

600 Wavelength / nm

700

800

Figure 5. Differential absorption spectrum (visible and near-infrared) obtained upon femtosecond flash photolysis (382 nm) of w1.0!10K5 M solutions of 5 in nitrogen saturated THF with a several time delay between K50 and 200 ps at room temperature. The spectrum corresponds to the changes that are associated with the transformation of the H2P singlet excited state to the radical pair spectrum, H2P%C–C59N%K.

population of its singlet excited state, 1*H2P (1.9 eV). In 4 and 5 the lifetime of this intermediate state (!0.4 ns) is shorter than in a H2P reference (10 ns), since the lowest vibrational state of the singlet excited state undergoes exothermic electron transfer, generating the final (H2P%C)– (C59N)%K state—see, for example, Figure 5. Interestingly, the singlet–singlet deactivation in 4 and 5 gives rise to trends, which resemble the fluorescence quenching: in 4 the singlet lifetimes (w0.35 ns) are invariant with respect to the solvent polarity, while for 5 shorter lifetimes were noted in polar benzonitrile (0.08 ns) relative to less polar THF (0.19 ns). Figure 6 illustrates the corresponding timeabsorption profiles for 5. Spectral characteristics of the charge-separated state comprise transient absorption in the visible and near-infrared with characteristic maxima in the 600–700 nm region (i.e., H2P%C) and at 1010 nm (i.e., C59N%K), respectively. In oxygen-free solutions, the decays of the H2P%C/C59N%K radical ion pair transient absorption for 4 and 5 was best fitted by first-order kinetics. For all dyads, the higher the solvent polarity, the shorter the lifetime of the radical ion pair state. For example, in 5 the radical pair lifetimes were 260 and 155 ns in THF and benzonitrile, respectively—see Figure 7. Slightly larger were the lifetime values of 4 (i.e., THF: 445 ns; benzonitrile: 362 ns). As far as the thermodynamics are concerned, the larger the solvent polarity the less negative is the free energy (KDG8) associated with the charge recombination.1f A decrease of KDG8 and faster charge recombination kinetics is typical of Marcus inverted region, where the electron transfer rates start to decrease with increasing free energy change.9 In summary, on the basis of available experimental results the proposed ensembles promise to be valuable to solar energy conversion and photovoltaics, specifically to novel chemical and light driven systems. Relative to previous systems, in which several arenes10 or C6011 itself were linked to the C59N core, the choice of H2P—as an excited state electron donor—is crucial to activate intramolecular charge separation. Particular important is the finding that

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Figure 6. Time-absorption profile at 450 nm (upper figure) and 440 nm (lower figure) following the femtosecond flash photolysis (382 nm) of w1.0!10K5 M solutions of 5 in nitrogen saturated THF and benzonitrile, respectively.

0.15

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Time / ns Figure 7. Time-absorption profile at 1010 nm following the nanosecond flash photolysis (532 nm) of w1.0!10K5 M solutions of 5 in nitrogen saturated THF.

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C59N-based donor–acceptor ensembles exhibit small reorganization energies, a phenomenon typically encountered in C60-based systems.1 This assists in placing the energy wasting charge recombination dynamics into the Marcus inverted region and, in turn, to slow them down. Acknowledgements We thank the Deutsche Forschungsgemeinschaft (DFG), the Graduiertenkolleg ‘Homogener und heterogener Elektronentransfer’, and SFB 583 (Redoxaktive Metallkomplexe— Reaktivita¨tssteuerung durch molekulare Architekturen) for financial support. Part of this work was supported by the Office of Basic Energy Sciences of the US Department of Energy (NDRL No. 4650).

References and notes 1. (a) Imahori, H.; Sakata, Y. Adv. Mater. 1997, 9, 537. (b) Prato, M. J. Mater. Chem. 1997, 7, 1097. (c) Martin, N.; Sanchez, L.; Illescas, B.; Perez, I. Chem. Rev. 1998, 98, 2527. (d) Imahori, H.; Sakata, Y. Eur. J. Org. Chem. 1999, 2445. (e) Guldi, D. M. Chem. Commun. 2000, 321. (f) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695. (g) Reed, C. A.; Bolskar, R. D. Chem. Rev. 2000, 100, 1075. (h) Gust, D.; Moore, T. A.; Moore, A. L. J. Photochem. Photobiol., B 2000, 58, 63. (i) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (j) Guldi, D. M. Chem. Soc. Rev. 2002, 31, 22. (k) Armaroli, N. Photochem. Photobiol. Sci. 2003, 2, 73. (l) Imahori, H.; Mori, Y.; Matano, Y. J. Photochem. Photobiol., C 2003, 4, 51. 2. Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593. 3. (a) Gruss, A.; Dinse, K.-P.; Hirsch, A.; Nuber, B.; Reuther, U. J. Am. Chem. Soc. 1997, 119, 8728. (b) Bellavia-Lund, C.; Keshavarz, M.; Collins, T.; Wudl, F. J. Am. Chem. Soc. 1997, 119, 8101. (c) Hummelen, J. C.; Bellavia-Lund, C.; Wudl, F. Top. Curr. Chem. 1999, 199, 93. (d) Nuber, B.; Hirsch, A. Acc. Chem. Res. 1999, 32, 795. 4. Hauke, F.; Hirsch, A. Chem. Commun. 2001, 1316. 5. (a) Nuber, B.; Hirsch, A. Chem. Commun. 1998, 406. (b) Hauke, F.; Hirsch, A. Chem. Commun. 1999, 2199. (c) Hauke, F.; Hirsch, A. Tetrahedron 2001, 57, 3697. 6. Hauke, F.; Swartz, A.; Guldi, D. M.; Hirsch, A. J. Mater. Chem. 2002, 12, 2088. 7. Selected data for compound 4: n (KBr) (cmK1)Z3052, 2920, 2850, 1741, 1637, 1595, 1574, 1551, 1471, 1438, 1348, 1316, 1287, 1215, 1176, 1155, 1071, 1031, 1000, 964, 799, 727, 700, 658, 579, 556, 524, 480, 469, 440 and 417; lmax (CH2Cl2) (nm)

8.

9. 10. 11.

1927

257, 324, 402, 423, 517, 551, 591 and 648; 1H NMR d (400 MHz, CS2/CDCl3)Z8.74 (m, 8H, bH), 8.26 (d, 1H, ar), 8.19 (m, 3H, ar), 8.00 (d, 2H, ar), 7.94 (m, 1H, ar), 7.74 (m, 10H, ar), 7.47 (m, 1H, ar), 7.31 (m, 1H, ar), 5.32 (s, 1H, –CH–), 4.89 (dt, 2H, –CH2–), 4.46 (t, 2H, –CH2–), 4.01 (s, 3H, –CH3), K3.02 (s, 2H, –NH); 13C NMR d (100 MHz, CS2/CDCl3) 165.33/165.13 (each 1C, –C]O), 156.15, 154.19, 154.08, 146.08, 145.90, 145.79, 145.64, 145.60, 145.55, 145.52, 145.45, 145.32, 145.06, 144.94, 144.82, 144.71, 144.63, 144.60, 144.44, 144.34, 143.84, 143.58, 143.47, 143.34, 143.20, 143.06, 142.93, 142.88, 142.50, 142.26, 141.86, 141.76, 141.72, 141.67, 141.61, 141.50, 140.60, 140.48, 140.26, 140.11, 139.79, 139.71, 139.41, 139.30, 137.87, 136.04, 134.30, 134.27, 134.17, 133.97, 133.90, 127.40, 127.33, 127.23, 126.52, 126.41, 123.68, 123.58, 121.00, 119.96, 119.80, 119.73, 118.84, 114.55, 78.79 (1C, C-sp31), 65.83/64.27 (each 1C, –CH2–), 61.34 (1C, –CH–), 52.63 (1C, –CH3); m/z (FAB) 1496 [M]C and 722 [C59N]C. Selected data for 5: n (KBr) (cmK1) 2956, 2925, 2901, 2863, 1684, 1598, 1560, 1500, 1457, 1421, 1397, 1361, 1264, 1185, 1106, 1023, 983, 966, 927, 877, 846, 798, 729, 580, 552, 523 and 420; lmax (CH2Cl2) (nm) 257, 322, 399, 420, 517, 552, 595 and 648; 1H NMR ( (400 MHz, CS2/ CDCl3) 8.91 (d, 3JZ4.9 Hz, 2H, bH), 8.83 (s, 4H, (H), 8.55 (d, 3 JZ8.2 Hz, 2H, bH), 8.10 (m, 8H, ar), 7.75 (m, 8H, ar), 5.44 (s, 2H, –CH2–), 1.64 (s, 9H, –CH3), 1.61 (s, 18H, –CH3), K2.87 (s, 2H, –NH); 13C NMR ( (100 MHz, CS2/CDCl3) 194.78 (1C, –C]O), 154.67 (2C, C59N), 150.09 (3C, ar), 148.31 (1C, C59N), 148.22 (2C, C59N), 147.35 (1C, ar), 146.93 (2C, C59N), 146.56 (2C, C59N), 146.40 (2C, C59N), 146.28 (2C, C59N), 146.08 (2C, C59N), 145.87 (2C, C59N), 145.70 (2C, C59N), 145.34 (2C, C59N), 145.30 (1C, C59N), 145.19 (2C, C59N), 144.49 (2C, C59N), 144.25 (2C, C59N), 143.78 (2C, C59N), 143.53 (2C, C59N), 143.34 (2C, C59N), 142.38 (2C, C59N), 142.22 (2C, C59N), 141.39 (2C, C59N), 141.06 (2C, C59N), 140.67 (2C, C59N), 140.51 (2C, C59N), 140.42 (2C, C59N), 140.13 (2C, C59N), 138.92/138.88 (1C/2C, ar), 138.76 (2C, C59N), 136.90 (2C, C59N), 135.52 (1C, ar), 135.14 (2C, ar), 134.27 (6C, ar), 134.09 (2C, C59N), 126.82 (2C, ar), 124.29 (2C, C59N), 123.57 (6C, ar), 120.77 (1C, porph), 120.39 (2C, porph), 117.45 (1C, porph), 78.40 (1C, C-sp3), 49.96 (1C, –CH2–), 34.49 (3C, –CR3), 31.56 (9C, –CH3); m/z (FAB) 1547 [M]C, 1490 [M]CK[tBu], 722 [C59N]C. Only for 4 we found fluorescence lifetimes that fall within our experimental detection range of larger than 100 ns. The lifetime values were in toluene (0.33 ns), THF (0.33 ns) and benzonitrile (0.30 ns). Marcus, R. A. Angew. Chem., Int. Ed., Engl. 1993, 32, 11. Hauke, F.; Atalick, S.; Guldi, D. M.; Mack, J.; Scott, L.; Hirsch, A. Chem. Commun. 2004, 766. Hauke, F.; Herranz, M. A.; Echegoyen, L.; Guldi, D. M.; Hirsch, A. Chem. Commun. 2004, 600.