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2007, 111, 12500-12503 Published on Web 08/03/2007
Light-Induced Electron Transfer of a Supramolecular Bis(Zinc Porphyrin)-Fullerene Triad Constructed via a Diacetylamidopyridine/Uracil Hydrogen-Bonding Motif Suresh Gadde,† D.-M. Shafiqul Islam,‡,§ Channa A. Wijesinghe,† Navaneetha K. Subbaiyan,† Melvin E. Zandler,† Yasuyuki Araki,‡ Osamu Ito,*,‡ and Francis D’Souza*,† Department of Chemistry, Wichita State UniVersity, 1845 Fairmount, Wichita, Kansas 6726 0-0051, and Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Sendai 980-8577, Japan ReceiVed: May 21, 2007; In Final Form: July 3, 2007
Using a diacetylamidopyridine/uracil complementary hydrogen-bonding motif, a novel bis(zinc porphyrin)fullerene supramolecular triad is constructed and characterized. The geometry of the triad deduced from DFTMO studies revealed the presence of the “three-point” hydrogen bonding and that one of the porphyrin units of the dimer is closer to the fullerene entity. Picosecond time-resolved emission and nanosecond transient absorption techniques were employed, respectively, to evaluate the kinetics of electron transfer and to characterize the electron-transfer products. The positioning of the porphyrin entity with respect to the fullerene entity (near or far) seems to influence the kinetics of charge-separation and charge-recombination events, thus delineating the structural importance of the studied supramolecular triad in governing the electrontransfer rates.
Introduction Design and self-assembly via hydrogen bonding of fullerene derivatives into supramolecular architectures are of current interest as functional ensembles to build molecular machines and optoelectronic devices.1 Hydrogen bonds, offering a high degree of directionality and binding energies ranging between 5 and 120 kJ mol-1, are essential for molecular recognition and organization of molecules into three-dimensional structures.2 Additionally, stability of the self-assembly is achieved by introducing multiple hydrogen bonds and secondary supramolecular hydrophobic or electrostatic interactions. A few fullerene dimers3 and C60-based donor-acceptor conjugates4,5 using a hydrogen-bonding strategy have been constructed and studied. Some of these studies have demonstrated that the electronic communication in C60-based donor-acceptor ensembles connected through hydrogen bonds is at least as strong as that found in covalently linked systems,4j further strengthening the concept of hydrogen bonding as a viable method for the construction of functional materials for various applications. In the present study, by utilizing the familiar 2,6-diamidopyridine/uracil (DAPy/U) complementary hydrogen-bonding strategy,6 we have constructed a supramolecular bis(zinc porphyrin)-C60 triad ((ZnP)2/C60 in Figure 1). Here, the adopted DAPy/U provides a three-point hydrogen bonding, and the DAPy-bridged ZnP dimer affords porphyrin-fullerene units * To whom correspondence should be addressed. E-mail:
[email protected] (F.D.);
[email protected] (O.I.). † Wichita State University. ‡ Tohoku University. § Current address: Department of Chemistry, Jahangirnagar University, Dhaka, Bangladesh.
10.1021/jp073918m CCC: $37.00
within the interacting distances upon C60-uracil binding, a property that is not often seen in conjugates formed by a hydrogen-bonding strategy.1,4-5 The DAPy-bearing porphyrin dimer was synthesized by reacting two equivalents of 5-(4carboxyphenyl)-10,15,20-triphenylporphyrin with 2,6-diaminopyridine in the presence of a base followed by metalation of the free-base porphyrin to its zinc derivative (see Supporting Information for synthetic and experimental details). The formation of the supramolecular triad and the photochemical events were probed in a polar acetonitrile/o-dichlorobenzene solvent mixture (AN/DCB ) 6:4). UV-visible titrations involving the porphyrin dimer revealed a slightly diminished Soret band intensity without significant changes in either the Soret or visible band positions upon the addition of C60-uracil (see Supporting Information, Figure S1, for spectra). Under these conditions, only a small broadening of the ZnP bands was noticed, suggesting weak charge-transfer (CT)-type interactions between the zinc porphyrin and fullerene entities in the ground state. These results also suggest the supramolecular complex formation is mainly through DAPy/U complementary hydrogen bonding. DFT B3LYP/3-21G(*) calculations afforded the optimized structure, revealing the presence of DAPy/U complementary three-point hydrogen bonding (Figure 1). The H‚‚‚N hydrogen bond distances ranged between 1.75 and 1.91 Å for the DAPy/U unit, with a dihedral angle of 25.1°. The Zn-Zn distance for the (ZnP)2 unit was 20.7 Å. Additionally, one of the ZnP rings was found to be located within the van der Waals interacting distance of the C60 entity (edge-to-edge distance Ree ) 2.2 Å), and another was located at the far side (Ree ) 13.3 Å). The center-to-center distances (Rcc) were found to be 5.7 and 16.8 Å, respectively, for near-side- and far-side-located ZnP-C60 entities. © 2007 American Chemical Society
Letters
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Figure 1. Structure of the (ZnP)2/C60 supramolecular triad constructed via a diacetylamidopyridine/uracil complementary hydrogen-bonding motif. The optimized structure calculated by B3LYP/3-21G(*) methods (red ) O, blue ) N, gray ) C, and white ) H) is shown in the lower panel.
Cyclic voltammetric studies were performed to evaluate the redox potentials of the supramolecular triad. Within the potential window of DCB, (ZnP)2 revealed two one-electron oxidations and two one-electron reductions, while C60-uracil revealed three one-electron reductions. The first oxidation of (ZnP)2 was located at 0.21 V versus Fc/Fc+, while the first reduction of C60-uracil was located at -1.19 V versus Fc/Fc+. Upon formation of the (ZnP)2/C60 by the addition of C60-uracil to (ZnP)2, potential shifts of nearly 20 mV were observed for the C60 reductions (see Supporting Information, Figure S2, for cyclic voltammograms). The driving force for charge separation (-∆GCS) via the excited singlet state of (ZnP)2 (the lowest transition energy (E0,0) ) 2.05 eV) calculated according to the Rehm-Weller equation7 was found to be -0.81 eV in AN/ DCB. These results indicate the occurrence of an exothermic charge-separation (CS) process in the (ZnP)2/C60 supramolecule via the 1ZnP* moiety. Similarly, the free-energy change for charge recombination (CR, -∆GCR) of ZnP·+/C60·- was found to be -1.24 eV, from which a slow CR process is predicted from the Marcus theory; the highly exothermic CR process lies in the inverted region of the Marcus parabola because of the small reorganization energy of C60 (0.6-0.7 eV).8 Figure 2 shows the fluorescence quenching of the zinc porphyrin dimer by C60-uracil in AN/DCB (6:4 ratio). When the ZnP moiety was predominantly excited with the 550 nm light, the ZnP emission bands at 600 and 648 nm revealed quenching accompanied by a much less intense band at 716 nm, corresponding to C60-uracil emission, as a consequence of direct C60 excitation at 550 nm. The formation constant for the supramolecular triad, calculated by the Benesi-Hildebrand plot9 of fluorescence quenching (Figure 2 inset), was found to be 6.2 × 103 M-1. Job’s plot confirmed a 1:1 complexation between the (ZnP)2 and C60 entities (see Supporting Information, Figure S3, for the plot). The binding constant is close to that reported for DAPy/U hydrogen bonding in the literature.5 Further, the time dependences of the emission intensities of the (ZnP)2/C60 supramolecule were measured to evaluate the
Figure 2. Fluorescence spectral changes of (ZnP)2 (44 µM) in the presence of various amounts of C60-uracil ((0.1-1.5) × 10-4 M range from the upper to the lower, along with allows) in AN/DCB (6:4 v/v); λex ) 550 nm. Inset: The Benesi-Hildebrand plot; ∆I and I0 are the fluorescence intensities with and without C60-uracil.
fluorescence quenching rate constants, as shown in Figure 3. The fluorescence time profile of (ZnP)2 in AN/DCB (6:4 v/v) revealed a monoexponential decay with a lifetime of 1.99 ns. Upon forming the supramolecular triad by the addition of C60uracil, the time profile of the ZnP emission could be curvefitted satisfactorily to a triexponential function, which gave lifetimes (τf) of 0.07 (fraction ) 25%), 0.35 (fraction ) 25%), and 3.30 ns (fraction ) 50%). By assuming the quenching is predominantly due to CS in this polar solvent mixture, the CS rate constants (kCS)10 via the 1ZnP* moiety in the complex were evaluated to be 1.4 × 1010 s-1 from the shortest τf value (0.07
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Figure 3. Fluorescence decays (collected in the 600-660 nm range) of (ZnP)2 (0.05 mM) (i) in the absence and (ii) in the presence of C60uracil (0.05 mM) in an AN/DCB (6:4) mixture.
Letters constants originated from the far-side and near-side-located porphyrins. Interestingly, this seems to be the case for the present triad where ratios of 0.35/0.07 ) 5 for CS and 172/45 ) 4 for CR were obtained, supporting the self-consistency of the lifetimes on distances. The damping factor, evaluated by the exponential dependence of these rate parameters on Rcc, was found to be 0.15 Å-1. This value is smaller than the reported damping factors for through-bond ET via saturated bonds, suggesting through-space ET in the present supramolecular triad.12 In summary, the present study demonstrates successful utilization of a diamidopyridine/uracil hydrogen-bonding motif to construct a closely held bis(zinc porphyrin)-fullerene supramolecular triad. Both zinc porphyrin entities of the dimer seem to be involved in the electron-transfer process. The measured kCS and kCR values are found to depend on the positioning of the porphyrin entity with respect to the fullerene entity (near or far), thus delineating the structural importance of the studied supramolecular triad in controlling the electrontransfer rates. The calculated damping factor was suggestive of through-space ET in the present triad. These findings reveal potential application of the present supramolecular triad held by complementary hydrogen bonding in artificial photosynthetic systems. Acknowledgment. The authors are thankful to the National Science Foundation (Grant 0453464 to F.D.) and the donors of the Petroleum Research Fund administered by the American Chemical Society.
Figure 4. Transient absorption spectra observed by the excitation of the (ZnP)2/C60 supramolecule (0.05:0.04 mM) with 532 nm laser light in AN/DCB (6:4). Inset: Time profile of the 1020 nm peak; curve fitting gave two rate constants.
ns) and 2.8 × 109 s-1 from the modest τf value (0.35 ns). The two kCS values were attributed to the near-side- and far-sidelocated ZnP units with respect to the C60 entity of the supramolecular triad. The longest τf value (3.30 ns) can be attributed to unbound (ZnP)2. As shown in Figure 4, the nanosecond transient absorption spectra of the (ZnP)2/C60 supramolecule observed by the excitation of 532 nm laser light, which predominantly excites the ZnP moiety, confirmed the occurrence of CS in the supramolecular triad. That is, a transient absorption peak corresponding to the C60•- moiety at 1020 nm was observed in addition to the triplet absorption peaks of ZnP and C60 in the 700-900 nm range.11 Although the ZnP•+ band was expected to appear in the 600-670 nm region, this band was overlapped with the intense triplet absorption peaks. The kCR values evaluated by monitoring the biexponential decay of the C60•peak at 1020 nm (Figure 4 inset) yielded rate constants of 2.2 × 107 and 5.8 × 106 s-1, respectively. These two kCR values are attributed to the near-side- and far-side-positioned ZnP•+ entities with respect to the fullerene entity. The time profile after 500 ns can be ascribed to the tail of the triplet state absorption. These kCR values were 2 orders of magnitude smaller than the respective kCS values, suggesting stabilization of the CS state due to the small organization energy of the triad, affording possible application to an artificial photosynthetic model. The present supramolecular triad has given an opportunity to compare the distance-dependent electron coupling and its effect on the electron-transfer rate constants. Thus, one can predict the relative increase for electron-transfer (ET) time
Supporting Information Available: Experimental section including synthesis of the zinc porphyrin dimer, UV-vis spectral changes during C60-uracil titration of the porphyrin dimer, cyclic voltammograms of the triad, and Job’s plot constructed to evaluate the molecular stoichiometry. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) For a general review on hydrogen-bonding motifs in fullerene chemistry, see: Sa´nchez, L.; Martı´n, N.; Guldi, D. M. Angew. Chem., Int. Ed. 2005, 44, 5374. (2) (a) Rebek, J., Jr. Chem. Soc. ReV. 1996, 25, 255. (b) Diederich, F.; Gomez-Lopez, M. Chem. Soc. ReV. 1999, 28, 263. (c) Guldi, D. M. Chem. Commun. 2000, 321. (d) Sijbesma, R. P.; Meijer, E. W. Chem. Commun. 2003, 5. (e) D’Souza, F.; Ito, O. Coord. Chem. ReV. 2005, 249, 1410. (f) Sessler, J. L.; Lawrence, C. M.; Jayawickramarajah, J. Chem. Soc. ReV. 2006, 36, 314. (3) (a) Diederich, F.; Echegoyen, L.; Gomez-Lopez, M.; Kessinger, R.; Stoddart, J. F. J. Chem. Soc., Perkin Trans. 2 1999, 1577. (b) Rispens, M. T.; Sanchez, L.; Knol, J.; Hummelen, J. C. Chem. Commun. 2001, 161. (c) Gonzalez, J. J.; Gonzalez, S.; Priego, E.; Luo, C.; Guldi, D. M.; de Mendoza, J.; Martin, N. Chem. Commun. 2001, 163. (d) Schenning, A. P. H. J.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2001, 123, 409. (e) Sanchez, L.; Rispens, M. T.; Hummelen, J. C. Angew. Chem., Int. Ed. 2002, 41, 838. (4) (a) Guldi, D. M.; Ramey, J.; Martinez-Diaz, M. V.; de la Escosura, A.; Torres, T.; da Ros, T.; Prato, M. Chem. Commun. 2002, 2774. (b) Guldi, D. M.; Gouloumis, A.; Vazquez, P.; Torres, T. Chem. Commun. 2002, 2056. (c) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129. (d) Watanabe, N.; Kihara, N.; Forusho, Y.; Takata, T.; Araki, Y.; Ito, O. Angew. Chem., Int. Ed. 2003, 42, 681. (e) Solladie, N.; Walther, M. E.; Gross, M.; Duarte, T. M. F.; Bourgogne, C.; Nierengarten, J.-F. Chem. Commun. 2003, 2412. (f) D’Souza, F.; Chitta, R.; Gadde, S.; Zandler, M. E.; Sandanayaka, A. S. D.; Araki, Y.; Ito, O. Chem. Commun. 2005, 1279. (g) Sessler, J. L.; Jayawickramarajah, J.; Gouloumis, A.; Torres, T.; Guldi, D. M.; Maldonado, S.; Stevenson, K. J. Chem. Commun. 2005, 1892. (h) Peeters, E.; van Hal, P. A.; Knol, J.; Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C.; Janssen, J. J. Phys. Chem. B 2000, 104, 10174. (i) Segura, M.; Sa´nchez, L.; de Mendoza, J.; Martin, N.; Guldi, D. M. J. Am. Chem. Soc. 2003, 125, 1509.
Letters (j) Sanchez, L.; Sierra, M.; Martin, N.; Myles, A. J.; Dale, T. J.; Rebek, J., Jr.; Seitz, W.; Guldi, D. M. Angew. Chem., Int. Ed. 2006, 45, 4637. (5) (a) Shi, Z.; Li, Y.; Gong, H.; Liu, M.; Xiao, S.; Liu, H.; Li, H.; Xiao, S.; Zhu, D. Org. Lett. 2002, 4, 1179. (b) Neuteboom, E. E.; Meskers, S. C. J.; van Hal, P. A.; van Duren, J. K. J.; Meijer, E. W.; Janssen, R. A. J.; Dupin, H.; Pourtois, G.; Cornil, J.; Lazzaroni, R.; Bredas, J.-L.; Beljonne, E. J. Am. Chem. Soc. 2003, 125, 8625. (c) Liu, Y.; Xiao, S.; Li, H.; Li, Y.; Liu, H.; Lu, F.; Zhuang, J.; Zhu, D. J. Phys. Chem. B 2004, 108, 6256. (d) D’Souza, F.; Chitta, R.; Gadde, S.; Rogers, L. M.; Karr, P. A.; Zandler, M. E.; Sandanayaka, A. S. D.; Araki, Y.; Ito, O. Chem.sEur. J. 2007, 13, 916. (e) Torres, T.; Gouloumis, A.; Sanchez-Garcia, D.; Jayawickramarajah, J.; Seitz, W.; Guldi, D. M.; Sessler, J. L. Chem. Commun. 2007, 292.
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12503 (6) Berg, A.; Shuali, Z.; Someda, M. A.; Levanon, H. J. Am. Chem. Soc. 1999, 121, 7433. (7) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 7, 259. (8) Imahori, H.; Hagiwara, K.; Akiyama, T.; Akoi, M.; Taniguchi, S.; Okada, S.; Shirakawa, M.; Sakata, Y. Chem. Phys. Lett. 1996, 263, 545. (9) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703. (10) The kCS and ΦCS were calculated using kCS ) (1/τf)complex (1/τf0)free and ΦCS ) [(1/τf)complex - (1/τf0)free]/(1/τf)complex. (11) D’Souza, F.; Deviprasad, G. R.; El-Khouly, M. E.; Fujitsuka, F.; Ito, O. J. Am. Chem. Soc. 2001, 123, 5277. (12) Schlicke, B.; Belser, P.; De Cola, L.; Sabbioni, E.; Balzani, V. J. Am. Chem. Soc. 1999, 121, 4207.
