Self-Assembled Supramolecular FerroceneâFullerene Dyads and

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Self-Assembled Supramolecular Ferrocene-Fullerene Dyads and Triad: Formation and Photoinduced Electron Transfer Yasuyuki Araki,*,† Raghu Chitta,‡ Atula S. D. Sandanayaka,† Kevin Langenwalter,‡ Suresh Gadde,‡ Melvin E. Zandler,‡ Osamu Ito,† and Francis D’Souza*,‡ Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Sendai, 980-8577, Japan, and Department of Chemistry, Wichita State UniVersity, 1845 Fairmount, Wichita, Kansas 67260-0051 ReceiVed: September 25, 2007; In Final Form: NoVember 12, 2007

Supramolecular ferrocene-fullerene constructs in which the donor, ferrocene linked to a benzo-18-crown-6 entity (Fc-crown), was self-assembled with the acceptor, fullerene bearing one or two alkyl ammonium ions (NH3+-C60), yielding dyads or a triad, respectively. The newly formed conjugates were characterized by spectroscopic (fluorescence, electospray ionization-mass, and 1H NMR) and electrochemical methods. The adopted crown ether-alkyl ammonium ion binding strategy resulting in stable donor-acceptor conjugates was also supported by the computational studies performed at the DFT B3LYP/3-21G(*) level in addition to the binding constants obtained from fluorescence quenching studies. The experimentally calculated freeenergy changes indicated exothermic light-induced charge-separation process. Accordingly, efficient photoinduced charge-separation processes were confirmed by the combination of the time-resolved fluorescence and nanosecond transient absorption spectral measurements. The rates of charge recombination were found to be 2-3 orders of magnitude lower, yielding radical ion-pairs, Fc+-crown/NH3+-C60•- with lifetimes in the 10-240 ns range. Generally, by increasing the donor-acceptor distance, a decrease in both kCS and kCR was observed for the supramolecular ferrocene-fullerene dyads; that is, the lifetimes of Fc+-crown/NH3+-C60•changed from 10 to 165 ns. However, for the triad, involving two ferrocene donors of varying donoracceptor distances, the kCR originating from the far-side located ferrocene was found to be 240 ns while the kCR from the near-side located ferrocene was faster than the time duration of the nanosecond laser pulse (6 ns).

Introduction In recent years, supramolecular photochemistry has steadily enlarged the spectrum of interest toward light-driven molecular machines, molecular switches and sensors, and molecular electronics, thus reaching the crossroad between chemistry, biology, and information technology.1-4 The design of supramolecular logic elements, revealing complex photophysical processes such as energy/electron transfer, is considered to be important for the development of molecular-scale photonic devices,5-7 including optical switches.8 In this context, the excited-state properties of fullerene derivatives in the molecular and supramolecular systems have attracted interest as photosynthetic reaction center models, as well as components for the development of photovoltaic devices.9-10 Consequently, various covalently linked C60-based donor-acceptor systems involving electron-mediating and hole-transfer reagents have been extensively studied to establish efficient charge separation via the excited singlet state of fullerene or the donor molecule, resulting in long lifetimes of the charge-separated states due to slow charge recombination process.11-15 Among the different models, the dyads involving ferrocene and fullerene entities represent a class of compounds for investigation of electron-transfer processes. Covalently linked with different type of spacers, viz., pyrrolidine,16a-d pyrazole,16e,f * Corresponding authors. E-mail: (Y.A.) [email protected]; (F.D.) [email protected]. † Tohoku University. ‡ Wichita State University.

cumulene,17 acetylene,18 fused and substituted cyclopropane,19 and compact dyad with ferrocene directly linked to azafullerene20 have been reported. A few studies also have utilized π-π interacting21 and rotaxane22 type binding approaches. Photophysical studies on some of these ferrocene-fullerene dyads have shown that electron transfer occurs from ferrocene to the fullerene singlet excited state with efficiencies depending on the geometry and the nature of the spacer.16,18,22 Crown ether-alkyl ammonium ion binding is one of the robust self-assembly approaches with binding energy ranging between 50-200 kJ/mol.23 Several fullerene and carbon nanotube containing donor-acceptor supramolecular systems have been built using this concept.24 In the present study, we have extended this approach of crown ether-alkyl ammonium complexation to build ferrocene-fullerene donor-acceptor dyads and a triad, and demonstrate the occurrence of photoinduced charge separation in these conjugates. The structures of the investigated donor and acceptor entities are shown in Scheme 1. The donor ferrocene entity was functionalized to have a benzo-18-crown-6 receptor site (1; Fc-crown), while fullerene was functionalized to have one or two alkyl ammonium ions of varying chain length (2-5; NH3+-C60) via fulleropyrrolidine synthetic approach. Self-assembled conjugates (Fc-crown/NH3+C60) were formed by mixing stoichiometric amounts of donor and acceptor entities in polar benzonitrile solvent. As a result of the different alkyl chain length on fullerene entities, it has been possible to probe the effect of the donor-acceptor distance

10.1021/jp077699g CCC: $40.75 © 2008 American Chemical Society Published on Web 01/18/2008

Ferrocene-Fullerene Dyads and Triad

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SCHEME 1: Structures of the Ferrocene and Fullerene Derivatives Utilized to Form Supramolecular Dyads and Triad in the Present Study

Figure 1. (a) Absorption spectral changes observed during titration of 2 (6 × 10-5 M) with 1 (0.2 eq. each addition) in benzonitrile. (b) Fluorescence spectrum of 2 (6 × 10-5 M) on increasing addition of 1 (0.2 eq. each addition) in benzonitrile. λex ) 480 nm. The inset in panel b shows Benesi-Hildebrand plot constructed for determination of binding constant.

on the kinetics of light-induced charge separation and recombination in self-assembled supramolecular conjugates. Results and Discussion UV-vis Studies. Figure 1a shows the absorption spectral changes observed for 2 on increasing addition of 1 in a benzonitrile solution. All of the fulleropyrrolidine derivatives revealed an absorption band around 324 nm and a sharp peak 431 nm, while the functionalized ferrocene 1 revealed a broad band around 442 nm. During the titration of 2 by 1, superposition of the spectra was observed without additional absorption bands. These results suggest absence of a direct ground-state interaction between the supramolecular fullerene and ferrocene entities in the dyads and triad, although the complexation through crown ether-ammonium ion binding was revealed by the fluorescence quenching and other spectroscopic techniques as discussed below. Fluorescence Emission Studies. The photochemical behavior of the ferrocene-fullerene dyads and triad was investigated, initially by using steady-state fluorescence measurements. The emission spectrum of the alkyl ammonium ion-appended fulleropyrrolidines was found to be similar to that of simple fulleropyrrolidine with an emission band around 720 nm.24 The fluorescence intensities of fullerenes 2-5 were found to be quenched upon complexing ferrocene 1 even at relatively low

concentrations (∼10-4-10-5 M) (Figure 1b), indicating the occurrence of excited-state events within the ferrocene-fullerene conjugates but not collisional events between the components. This observation supports the crown ether-ammonium ion binding in 1:2 as equilibrium. By using the emission data, the binding constants (K) for the formation of self-assembled dyads were obtained by constructing Benesi-Hildebrand plots (Figure 1b inset),25 and the data are listed in Table 1. The magnitude of the binding constants varied between 5 × 103 and 104 M-1 indicating stable self-assembly process in polar benzonitrile. The binding constant for 1:2, where the fullerene had a short alkyl ammonium chain, was found to be higher than that of 1:3 or 1:4. In the case of 1 binding to 5, a 1:1 composition of the complex was established under low concentration of 1, and subsequent formation of a 2:1 complex was possible under higher concentrations of 1, resulting in an overall binding constant of ∼106 M-2 for 12:5 formation. 1H NMR Studies. The ability of crown ether-appended ferrocene 1 to form a supramolecular complex with the fullerene-appended alkyl ammonium ion 2 was further supported by 1H NMR binding studies carried out in CDCl3/CD3OD (1:1 v/v) solvent mixture at room temperature. As shown in Figure 2, the addition of increasing amounts of 2 revealed complexation-induced changes in the chemical shifts of the ferrocenecrown ether proton resonances. These changes are particularly visible for the crown ether and the aromatic protons of the benzo-18-crown-6 moiety. Deshielding of the crown ether protons near to the phenyl ring, primarily located at 3.94-3.98 ppm (red), 4.17-4.19 ppm (magenta), and 4.20-4.23 ppm (green) up to 0.15 ppm were observed indicating the formation of crown ether-ammonium ion complex. The complexation was further supported by the chemical shift in the phenyl protons of benzo-crown ether located at 7.4 up to 7.7 ppm (blue). These complexation-induced changes were observed only up to the

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TABLE 1: Binding Constant (K), Computed Geometric Parameter (RCt-Ct), and Electrochemical Oxidation (Eox) and Reduction (Ered) Potentials, Free Energy of Radical Ion Pair (∆GRIP) and Charge Separation (∆GCS) of Ferrocene-Fullerene Supramolecular Conjugates conjugate

Ka

RCt-Ct, Åb

Eoxc/V

Eredd/V

-∆GRIP/eV

-∆GCS/eV

1:2 1:3 1:4 12:5

1.45 × 104 M-1 4.45 × 103 M-1 9.10 × 103 M-1 8.64 × 106 M-2

19.1 20.6 21.8 15.2 21.8

0.13 0.13 0.12 0.13

-1.05 -1.05 -1.06 -1.06

1.15 1.15 1.17 1.18

0.60 0.60 0.58 0.57

a From fluorescence quenching, using Benesi-Hildebrand plots. bDistance from Fe of ferrocene to center of fullerene spheroid. cFirst oxidation potential corresponding to ferrocene oxidation in benzonitrile, 0.1 M (n-Bu)4NClO4. dFirst reduction potential corresponding to fullerene reduction in benzonitrile, 0.1 M (n-Bu)4NClO4.

Figure 2. 1H NMR spectra of 1 on increasing addition of 2 (0.2 eq. each addition except for the top two where 0.4 eq. was added) in CDCl3/ CD3OD (1:1 v/v) at room temperature.

addition of one equivalent of 2. Further addition of the fulleropyrrolidine induced only slight changes in the chemical shifts confirming the formation of 1:1 complex (see Figure 2). Additionally, during the complexation studies no changes in the chemical shifts of ferrocene moiety was observed suggesting absence of direct ground-state interactions between the ferrocene and fullerene entities. ESI-Mass Spectral Characterization of the Dyads and Triad. Owing to the presence of strong crown ether-alkyl ammonium ion interactions, it was possible to characterize the self-assembled dyads and triad in the gas phase by electrospray ionization (ESI)-mass spectral analysis. Unlike other mass spectroscopic methods, ESI-mass spectrum allows pre-existing ions in solution to be transferred to the gas phase without fragmentation. This soft-ionization technique, developed by Fenn and co-workers,26 is routinely applied to characterize noncovalent assemblies. Recently, Nierengarten et al.27e successfully utilized this technique to characterized porphyrin-fullerene conjugates held by crown ether-alkyl ammonium ion complexation. Earlier, we also utilized this technique to characterize porphyrin-fullerene dyads held by metal-ligand axial coor-

dination.28 Figure 3 shows the positive ESI-mass spectra of dyad, 1:2 obtained from a 1:1 mixture of the individual species, and triad, 12:5, obtained from 2:1 mixing of the individual species, respectively, under mild conditions (low Ve). While the molecular ion peak corresponding to the dyad was prominent for 1:2 at 1422.4, the corresponding molecular ion peak for the triad, 12:5 at 1025 (doubly charged) was also accompanied by a peak at 1007 indicating appreciable fragmentation under the experimental conditions. The ESI-mass spectra for dyads 1:3 and 1:4 gave similar results; that is, under mild conditions a predominant signal corresponding to the supramolecular dyads was observed. The occurrence of molecular ion peak with limited fragmentation demonstrates the existence of the stable supramolecular complex in solution. Density Functional Theory (DFT) Calculations. To gain insights into the supramolecular geometry and to probe possible existence of stable supramolecular complexation, computational studies were performed using DFT at the B3LYP/3-21G(*) level. The B3LYP/3-21G(*) methods have recently been successfully used to predict the geometry and electronic structure of molecular and self-assembled supramolecular dyads and triads.29


Figure 3. ESI-mass spectra of (a) dyad, 1:2 and (b) triad, 12:5 in CH2Cl2/CH3OH matrix.

Here, both the ferrocene-crown ether 1 and functionalized fullerenes 2-5 were optimized to a stationary point on the Born-Oppenheimer potential energy surface and allowed to self-assemble via crown ether-ammonium ion complexation. The structures for the supramolecular dyads, 1:2, and 1:4, as well as the triad, 12:5, are shown in Figure 4. In the former dyads, the ferrocene moiety and fullerene sphere are off-site, which revealed absence of direct ground-state interaction between the ferrocene and fullerene entities. The center-to-center distances (RCt-Ct), calculated as the distance between Fe of ferrocene and center of fullerene spheroid, are listed in Table 1. These RCt-Ct values ranged between 19.1 and 21.8 Å and increased along the series: 1:2 < 1:3 < 1:4. However, in the case of the triad 12:5 one of the ferrocene units (attached with phenol moiety) was closer to the fullerene entity (RCt-Ct ) 15.2 Å) but far away from causing any direct interactions. As shown in Figure 4b for representative 1:2, the highest occupied molecular orbital (HOMO) was located on the ferrocene, and the lowest unoccupied molecular orbital (LUMO) was on the fullerene entity suggesting the formation of Fc+-crown-NH3+C60•- ion-pair by charge-separation process. No orbital coefficient was found on the crown ether-ammonium ion binding site of the conjugates, indicating their role as linker in the selfassembly process without altering the primary donor-acceptor characteristics of the ferrocene and fullerene entities. Electrochemical Studies and Electron-Transfer Driving Forces. Electrochemical studies using the cyclic voltammetric technique were performed to evaluate the redox potentials of the dyads and the triad, which allowed evaluating the energetics of electron-transfer processes. The crown ether-appended ferrocene 1 was titrated with various amounts of functionalized fullerenes 2-5 to probe the effect of inclusion on the redox potentials of both the ferrocene and fullerene entities. Figure 5 shows representative cyclic voltammograms obtained for dyad 1:2 (obtained by equimolar addition of 1 and 2) in benzonitrile containing 0.1 M (n-C4H9)4NClO4; the redox potential data are given in Table 1. During the anodic scanning of the potential, an one-electron oxidation as judged from their peak-to-peak separation, ∆Epp, and the cathodic-to-anodic peak current ratios30 corresponding to the oxidation of the ferrocene unit, was

J. Phys. Chem. C, Vol. 112, No. 6, 2008 2225 observed. The half-wave potential for this process, as a result of functionalization, was anodically shifted by 0.12-0.13 V compared to ferrocene used as an internal standard. During the cathodic scanning of the potential, two one-electron reversible reductions corresponding to fulleropyrrolidine were observed. Scanning the potential beyond the second reduction revealed additional fullerene-based reductions; however, they were not fully reversible. The energy levels of the charge-separated states (∆GRIP) were evaluated using the Weller-type approach31 utilizing the redox potentials, center-to-center distance, and dielectric constant of the solvent as listed in Table 1. By comparing these energy levels of the charge-separated states with the energy levels of the singlet excited-state of fullerene (1.75 eV), the driving forces (∆GCS) for charge separation from the fullerene to the singlet excited fullerene were evaluated to be ca. -0.6 eV. The generations of Fc+-crown/NH3+-C60•- were found to be exothermic via the singlet excited-state fullerene in benzonitrile for all of the dyads and the triad. Picosecond Time-Resolved Emission Studies. The timeresolved emission spectra of the self-assembled conjugates tracked those of steady-state spectra shown in Figure 1b. Figure 6 shows the emission decay time profiles of the alkyl ammonium ion-appended fullerenes 2-5 in the absence and presence of crown-ether-appended ferrocene 1 in benzonitrile. The fullerenes 2-5 revealed a mono exponential decay with a lifetime around 1390 ps. Upon forming the dyads and the triad by complexing 1, fast fluorescence decays were observed; each time profile could be curve-fitted to a biexponential function involving a major short lifetime (140-220 ps) and a minor longer lifetime (1400 ps), as listed in Table 2. The fluorescence quenching could be safely attributed to charge separation from the ferrocene entity to the singlet excited fullerene; thus, the rates (kSCS) and quantum yields (ΦSCS) of charge separation were evaluated in the usual manner (see footnote of Table 2 for relevant equations) as summarized in Table 2. The kCS values are in the order of 109 s-1 with the ΦSCS values ranging between 0.84 and 0.90, which are similar in magnitude compared to the covalently linked fullerene-ferrocene dyads.16 These results suggest occurrence of efficient charge separation in the studied supramolecular dyads and the triad. Nanosecond Transient Absorption Studies. Further studies involving the nanosecond transient absorption technique were performed to identify the electron-transfer products and monitor kinetics of charge recombination in the dyads and the triad. The transient absorption spectrum of 2-5 after 532 nm laser irradiation in the absence of 1 exhibited an absorption peak at 720 nm corresponding to the excited triplet state.16 Upon formation of the ferrocene-fullerene dyads and triad, the transient absorption spectra showed a peak at 1020 nm corresponding to the formation of fulleropyrrolidine anion radical (Figure 7).16 Although no new peak corresponding to the formation of ferrocenium cation was observed for reasons of weak absorbance, these results indicated the formation of Fc+crown/NH3+-C60•- in the studied supramolecular conjugates. In these conjugates, the 720 nm band was also observed until 100 ns after the pulsed laser light irradiation, suggesting a part of the 1C60* moiety relaxes down to 3C60* in a fraction of (1ΦSCS); however, the decay of the 720 nm band was quite faster than the usually decay of the 3C60* moiety to the ground state, indicating fast triplet energy transfer takes place to low lying triplet state of the ferrocene moiety.32 To calculate the rate of charge recombination, kCR, the decay of the fulleropyrrolidine anion radical peak at 1020 nm was


Araki et al.

Figure 4. DFT B3LYP/3-21G(*) optimized (a) structure of the dyad 1:2, (b) HOMO and LUMO of the dyad 1:2, (c) structure of the dyad 1:3, and (d) structure of the triad, 12:5.

Figure 5. Cyclic voltammograms of the dyad 1:2 obtained by mixing equimolar concentrations of 1 and 2 (0.03 mM) in benzonitrile containing 0.1 M (n-C4H9)4NClO4. Scan rate ) 100 mV s-1.

monitored, which followed first-order decay kinetics. The kCR values thus evaluated are given in Table 2, where the kCR values are 2-3 orders of magnitude smaller than kCS, which could easily be rationalized based on the low reorganization energy of fullerenes in electron-transfer reactions; that is, on adopting the reported reorganization energy of fullerene as 0.6 V,33 the charge-separation process (-∆GCS ) 0.6 eV) locates at the top region of the Marcus parabola, whereas the charge recombination process (-∆GCR ) 1.2 eV) belongs to the deep inverted region. Generally, by increasing the donor-acceptor distance, a decrease in both kCS and kCR was observed (Table 2). By using the kCR values thus evaluated, the lifetimes of the radical ion pair, τRIP were found to be 130-165 ns for 1:3 and 1:4 dyads, clearly demonstrating charge stabilization in the supramolecular dyads and the triad. For 1:2, τRIP was as short as 10 ns owing to the shortest RCt-Ct between Fc+ and C60•-. On the other hand, for 12:5 having both short and long RCt-Ct, τRIP was the longest (240 ns); probably this τRIP could be due to the chargerecombination process between the far-side located Fc+ and C60•- (RCt-Ct ) 21.8 Å). This also implies that the charge

recombination between the Fc+ located on the near-side of the C60•- (RCt-Ct ) 15.2 Å) may be as fast as nanosecond-laser light pulse. Recently, we demonstrated such an example of the role of donor-acceptor distance on the kCS and kCR in a bisporphyrin-fullerene supramolecular triad held by diacetylamido-pyridine/uracil complimentary hydrogen bonding.34 Summary. A series of supramolecular dyads and a triad bearing ferrocene and fullerene entities were designed and studied using various physicochemical techniques. The adopted crown ether-alkyl ammonium ion complexation binding strategy resulted in well-defined conjugates. The supramolecular integrity of the newly formed conjugates was established from optical absorption and emission, ESI-mass, 1H NMR, computational, and electrochemical methods. The photoinduced processes are summarized in an energy level diagram, schematically depicted as Figure 8, on the basis of electrochemical and spectroscopic data as listed in Table 1. The charge-separation processes via 1C60* moiety were monitored by quenching of the fullerene fluorescence. The measured rates of charge separation and the quantum yields revealed occurrence of efficient light-induced exothermic charge separation process. The rates of charge recombination monitored by nanosecond transient absorption spectral studies were found to be 2-3 orders of magnitude lower than the rates of charge separation suggesting charge stabilization in these conjugates, which can be reasonably interpreted by the Marcus parabola combining with the energy diagram in Figure 8. Generally, by increasing the donor-acceptor distance, decreases in both kCS and kCR were observed for the investigated supramolecular conjugates. In the assembled supramolecular triad involving two ferrocene donors of varying donor-acceptor distances, the kCR value for the farside located ferrocene was found to be smaller than that from the near-side located ferrocene. Compared with the porphyrinfullerene conjugates connected with crown ether-alkyl am-


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Figure 6. Fluorescence decays collected at the 720 nm range of (a) 1:2 (0.12:0.10 mM), (b) 1:3 (0.12:0.10 mM), (c) 1:4 (0.12:0.10 mM), and (d) 12:5 (0.22:0.20 mM) in benzonitrile. λex ) 400 nm. The curve in black color in each figure indicates decay of the corresponding fullerene derivative in the absence of ferrocene 1.

TABLE 2: Fluorescence Lifetime (τF,dyad), Charge-Separation Rate Constant (kSCS), Charge-Separation Quantum Yield (ΦSCS) via the Excited Singlet State of Fullerene, Charge-Recombination Rate Constant (kCR), and Lifetime of the Radical Ion-Pair (τRIP) for the Investigated Ferrocene-Fullerene Conjugates in Benzonitrile conjugate τF,dyad/ps (%)a 1:2 1:3 1:4 12:5

135 (90%) 208 (84%) 220 (78%) 145 (91%)c

kSCS/s-1b

ΦSCSb

kCR/s-1

τRIP/ns

6.69 × 4.09 × 109 3.83 × 109 6.18 × 109

0.90 0.85 0.84 0.89

9.74 × 107 6.05 × 106 7.55 × 106 4.11 × 106

10 165 132 243

109

a The lifetime of the fullerene derivatives, 2-5, were 1390 ps. bkSCS ) (1/τF,dyad) - (1/τF,ref), ΦSCS ) [(1/τF,dyad) - (1/τF,ref) ]/(1/τF,dyad).

monium cation complexation, reported in our previous studies,27b,c,d the τRIP values of ferrocene-fullerene conjugates are longer, indicating excellence of these new donor-acceptor conjugates to probe light-induced electron-transfer events. Experimental Section Chemicals. Buckminsterfullerene, C60 (+99.95%) was from SES Research, (Houston, TX). All the reagents were from Aldrich Chemicals (Milwaukee, WI), while the bulk solvents utilized in the syntheses were from Fischer Chemicals. Tetrabutylammonium perchloride, n-Bu4NClO4, used in electrochemical studies was from Fluka Chemicals. Synthesis of fullerenes 2-4 is given elsewhere.27g Synthesis of Ferrocenyl Amido Benzo-[18-crown-6] (1). Ferrocene carboxylic acid (176 mg, 0.77 mmol) and 4′aminobenzo-18-crown-6 (250 mg, 0.77 mmol) were dissolved in 50 mL of dry CH2Cl2. Then 1,3-dicyclohexycarbodiimide (190 mg, 0.92 mmol) and 4-(dimethylamino) pyridine (31 mg, 0.25 mmol) were added, and the reaction mixture was stirred for 28 h. Then the solvent was evaporated under reduced

pressure, and the crude compound was purified on silica gel column using CHCl3/ethylacetate (10:90 v/v) as eluent. Subsequent column chromatography on silica gel using CHCl3/ methanol (98:2 v/v) yielded the title compound as the yellow band. Evaporation of the solvent yielded the desired compound as the orange powder. Yield: 208 mg (51%). 1H NMR (CDCl ): δ (in ppm): 7.52-7.49 (m, 1H, phenyl 3 H), 7.37 (s, 1H, phenyl H), 6.94-6.89 (m, 1H, -NH), 6.876.84 (m, 1H, phenyl H), 4.80-4.76 (m, 2H, ferrocenyl H), 4.45-4.40 (m, 2H, ferrocenyl H), 4.25 (s, 5H, ferrocenyl H), 4.24-4.19 (m, 2H, crownethylene H), 4.20-4.15 (m, 2H, crownethylene H), 3.97-3.91 (m, 4H, crownethylene H), 3.813.69 (m, 12H, crownethylene H). ESI mass (in CH2Cl2): calcd 539.41; found 540.1 (100%) [M+]. Synthesis of 5. To a solution of C60 (100 mg, 0.138 mmol) in toluene, H-Lys-(Boc)-OH (68 mg, 0.27mmol) and 4-(2-Bocamino) ethoxy benzaldehyde (110 mg, 0.415 mmol) were added and refluxed for 4 h. The solvent was evaporated under vacuum, and the crude product was purified on silica gel by using toluene/ ethyl acetate (75:25 v/v) as eluent. Yield: 80 mg (50%). Next, to a dichloromethane solution (5 mL) of N-Boc protected 5 (50 mg, 0.05 mmol), trifluoroacetic acid (3 mL) was added, and the mixture was stirred for 3 h. The solvent and excess acid was removed under vacuum, and the solid product was washed with toluene several times to remove any unreacted N-Boc protected 5 and to yield the desired compound 5 as brown solid. Yield: 50 mg (92%). 1H NMR (CD OD): δ (in ppm): 7.71 (d, 2H, phenyl H), 3 6.95 (d, 2H, phenyl H), 5.75 (s, 1H, pyrrolidine H), 4.75 (d, 2H, -CH2-), 4.1 (t, 1H, pyrrolidine H), 4.0 (d, 2H, -(OCH2)-), 3.51 (m, 2H, -(O-CH2)-), 3.01-1.94 (m, m, m, 6H, -(CH2)3-).


Araki et al.

Figure 7. Nanosecond transient absorption spectra of (a) 1:2 (0.12:0.10 mM), (b) 1:3 (0.12:0.10 mM), (c) 1:4 (0.12:0.10 mM), and (d) 12:5 (0.22:0.20 mM) in benzonitrile after 532 nm laser irradiation. Inset: absorption time profile of the 1020 nm band.

Figure 8. Schematic energy level diagram showing the photoinduced processes.

Instrumentation. The UV-vis spectral measurements were carried out with a Shimadzu Model 1600 UV-vis spectrophotometer. The steady-state fluorescence emission was monitored by using a Varian Eclipse spectrometer. A right angle detection method was used. The 1H NMR studies were carried out on a Varian 400 MHz spectrometer. Tetramethylsilane was used as an internal standard. Cyclic voltammograms were recorded on an EG&G PARSTAT electrochemical analyzer using a three electrode system in benzonitrile containing 0.1 M n-Bu4NClO4 as the supporting electrolyte. A platinum button electrode was

used as the working electrode. A platinum wire served as the counter electrode and Ag/AgCl was used as the reference electrode. Ferrocene/ferrocenium (Fc/Fc+) redox couple was used as an internal standard. All the solutions were purged prior to spectral measurements using argon gas. The ESI-mass spectral analyses of the self-assembled complexes were performed by using a Fennigan LCQ-Deca mass spectrometer. The starting compounds and the conjugates (about 0.1 mM concentration) were prepared in freshly distilled CH3Cl/CH3OH (1:1 v/v) solvent mixture. The computational calculations were performed by the DFT B3LYP/3-21G(*) method with the GAUSSIAN 03 software package35 on various PCs. The frontier HOMO and LUMO were generated using GaussView program. Time-Resolved Emission and Transient Absorption Measurements. The picosecond time-resolved fluorescence spectra were measured using an argon-ion pumped Ti:sapphire laser (Tsunami; pulse width ) 2 ps) and a streak scope (Hamamatsu Photonics; response time ) 10 ps). The details of the experimental setup are described elsewhere.36 Nanosecond transient absorption measurements were carried out using the SHG (532 nm) of an Nd:YAG laser (Spectra Physics, Quanta-Ray GCR130, full width at half-maximum 6 ns) as excitation source. For the transient absorption spectra in the near-IR region (6001600 nm), the monitoring light from a pulsed Xe lamp was detected with a Ge-avalanche photodiode (Hamamatsu Photonics, B2834).36 Acknowledgment. The authors are thankful to N. K. Subbaiyan for help in cyclic voltammetry measurements. This

Ferrocene-Fullerene Dyads and Triad work is supported by the National Science Foundation (Grant 0453464 to F.D.), the donors of the Petroleum Research Fund administered by the American Chemical Society, and Grantsin-Aid for Scientific Research on Primary Area (417) from the Ministry of Education, Science, Sport, and Culture of Japan. References and Notes (1) (a) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39, 3349-3391. (b) Biological Aspects of Fullerenes; Wilson, S. R., Kadish, K., Ruoff, R., Eds.; John-Wiley: New York, 2000. (2) (a) Keefe, M. H.; Benkstein, K. D.; Hupp, J. T. Coord. Chem. Rev. 2000, 205, 201-228. (b) Kamat, P. V. J. Phys. Chem. C 2007, 111, 28342860. (3) De Silva, A. P.; McClenaghan, N.; McCoy, C. P.; Balzani, V. Logic Gates. In Electron Transfer in Chemistry; Wiley-VCH: Weiheim, 2001; Vol. 5, pp 156-185. (4) Luo, Y.; Collier, C. P.; Jeppesen, J. O.; Nielsen, K. A.; Delonno, E.; Ho, G.; Perkins, J.; Tseng, H. 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