Photoelectrochemistry of LangmuirBlodgett Films of the Endohedral

ReceiVed: January 24, 2001; In Final Form: May 28, 2001. Monolayer and multilayer Langmuir-Blodgett (LB) films of the endohedral metallofullerene Dy@C...
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9406

J. Phys. Chem. B 2001, 105, 9406-9412

Photoelectrochemistry of Langmuir-Blodgett Films of the Endohedral Metallofullerene Dy@C82 on ITO Electrodes Shangfeng Yang and Shihe Yang* Department of Chemistry, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ReceiVed: January 24, 2001; In Final Form: May 28, 2001

Monolayer and multilayer Langmuir-Blodgett (LB) films of the endohedral metallofullerene Dy@C82 mixed with arachidic acid (Dy@C82/AA) were deposited onto ITO substrates from the N2/water interface. The photoelectrochemical response of the Dy@C82/AA LB films on ITO was investigated. A stable anodic photocurrent was observed, and the effects of bias voltage, light intensity, electron donor, number of layers, and O2 were studied in detail. The results indicate that electrons flow from the electrolyte through the LB film to the ITO electrode. Dy@C82/AA and C60/AA films are compared in terms of the film formation and photoelectric response. Under appropriate conditions, the Dy@C82/AA LB films exhibit a higher photocurrent quantum yield than the C60/AA films on ITO electrodes.

Introduction The unique graphitic shell structure of the fullerenes gives rise to very interesting photophysical, photoelectric, and photoelectrochemical properties.1-8 Photoelectrochemical studies of C60 films on metal electrode surfaces and C60 single crystals have shown that C60 displays distinct n-type semiconductor character.9 The utilization of a photoelectrochemical solid/liquid junction in place of the solid-state photovoltaic junction has been found to improve the photocurrent response.10 Both C60 and C70 can be embedded within a lipid membrane and act as efficient photosensitizers for electron transfer from a donor and mediators for electron transport across the membrane.11 Photoelectrochemical studies have also been carried out on Langmuir-Blodgett (LB) films of fullerene C60 and its derivatives.12 Efficient photodriven electron transfer occurred at the interface between the fullerene-modified electrodes and the electrolytes, and the quantum yield for photocurrent generation based on the monolayer of C60 and its derivatives on ITO surfaces reached 1.5-8% under favorable conditions.12 During the past few years, success in preparing macroscopic quantities of endohedral metallofullerenes has made it possible to characterize their structures and physical properties.13-17 Electrochemistry studies revealed that the monometallofullerenes M@C82 (M ) lanthanide elements) can be successively reduced at relatively low reduction potentials, and this strong electronaccepting ability may lead to potential applications such as photoelectric energy conversion.18 In fact, the first reduction potentials of M@C82 are even more positive than that of C60, and consequently, the metallofullerenes possess a greater electron accepting ability than C60. Such a good electron accepting ability makes the photoelectric properties of M@C82 particularly interesting. However, to our knowledge, no investigation has been reported so far of the photoelectrochemical behavior of endohedral metallfullerenes, which also belong to the fullerene family but have metal atoms trapped inside the cage. This might be due to the difficulty of obtaining a sufficient * Corresponding author. E-mail: [email protected].

amount of pure metallofullerene samples and the paucity of data on the photoexcited states of these species. Recent work in our laboratory has studied the LB film formation behavior of Dy@C82 for the first time.19 Similar to empty fullerenes,20 LB films of the endohedral metallofullerene tend to aggregate, are sensitive to vibration, and are difficult to transfer onto solid substrates due to the high hydrophobicity of the rigid ball molecules. Nevertheless, high-quality LB films can still be prepared on a layer-by-layer basis. In particular, the metallofullerene Dy@C82 can form stable mixed LB films with long-chain fatty acids analogous to the empty fullerenes C60 and C70.20,21 Furthermore, the films thus formed are easily transferred onto solid substrates with a uniform transfer ratio. This paper reports our preparation of the mixed LB films of Dy@C82 mixed with arachidic acid (Dy@C82/AA) and our investigation of the photoelectrochemical behavior of the LB films on ITO electrodes. In principle, other long-chain organic acids can also be used as matrices, and we selected AA for illustration. The dependence of the generated photocurrent on such factors as bias voltage, light intensity, electron donor, the number of metallofullerene layers, and O2 has been studied. Some differences in photoelectric response between Dy@C82 and C60 have been identified, and a possible mechanism for the photoinduced electron transfer is discussed. Experimental Section Materials. High-purity Dy@C82 and C60 (>99.0%) were prepared in our laboratory by a combination of the standard DC arc-discharge method and an isolation method described previously.22-24 The raw soot from the arc-discharge was subjected to Soxhlet extraction using N,N-dimethylformamide (DMF) as the solvent, followed by HPLC separation using a 5PYE column with toluene being the mobile phase. The collected toluene solutions of Dy@C82 and C60 in toluene were concentrated and repurified with the same HPLC column just before spreading onto the water surface. The identity and purity of the sample was verified by methane DCI negative ion mass spectrometry.

10.1021/jp010250h CCC: $20.00 © 2001 American Chemical Society Published on Web 09/07/2001

Endohedral Metallofullerene Dy@C82 Arachidic acid (AA) was purchased from Acros Organics Co. KCl (Riedel-De Haen AG) and was used as the electrolyte for photoelectrochemical experiments. L(+)-Ascorbic acid (AsA) (Acros Organics Co.) was used as an electron donor. Deionized water which had been purified by passing through an EASYpure compact ultrapure water system (Barnstead Co., U.S.A.) was used in all the experiments reported in this paper. Preparation of LB Films. A Langmuir minitrough (Applied Imaging, UK) was employed for forming metallofullerene (fullerene) Langmuir films. Before experiments, N2 was kept flowing for several hours to minimize the presence of O2 in the system. The N2/water interface was thoroughly cleaned by complete barrier movement to ensure that the maximum surface pressure difference was less than 0.2 mN m-1 upon compression. The subphase was ultrapure water (20 ( 1 °C, pH ) 6.50, F ) 18.3 MΩ cm). The initial concentration of the Dy@C82 solution in toluene (2.44 × 10-5 mol dm-3) was determined by weighing the vacuum-dried solid from a known volume of solution using a precision balance (Autobalance Model AD-6). The molar ratios of Dy@C82:AA and C60:AA (in toluene) were both 1:4 with a concentration of 9.86 × 10-6 mol dm-3 for Dy@C82 and 1.04 × 10-5 mol dm-3 for C60, respectively. A known volume of the Dy@C82/AA solution (2-3 mL) in toluene was carefully spread onto the surface of the clean water subphase (505 cm2) dropwise using a 0.5 mL syringe. This procedure lasted for over 1 h. After the solvent had evaporated completely (>30 min), the metallofullerene Langmuir films at the N2/water interface were compressed at a barrier speed of 1 cm/min, and the surface pressure-area (π-A) isotherm was recorded. The monolayer or multilayer LB films of metallofullerenes (or fullerenes) were deposited from the N2/water interface by vertical dipping onto a transparent indium-tin oxide (ITO) glass substrate (10 × 15 mm, Delta Technologies Ltd., Stillwater, U.S.A.) or quartz plate (10 × 15 mm, Electronic Space Products International, U.S.A.). The substrate was hydrophilically treated beforehand by refluxing in 2-propanol for 24 h.25 The LB film was deposited at a dipping rate of ∼0.9 mm/min under a constant surface pressure of 15-20 mN m-1. High-purity N2 (purity > 99.99%) was kept in a state of stationary flow above the water subphase at a throughput of ∼50 mL/min. UV-vis absorption spectra were recorded in a Milton Roy spectrometer (Spectronic 3000). Photoelectrochemical Measurements. The photoelectrochemical measurements were carried out with a model 600 electrochemical analyzer from CH Instruments Inc., U.S.A. A 500 W mercury-xenon lamp (Hamamatsu, Japan) was used as the light source. The beam intensity of the light was calibrated with a power meter (Newport Co., U.S.A.). The collimated white-light beam passed through an FS-3 filter (OD ) 0.05, Newport Co., U.S.A.), which was used to eliminate wavelengths 99.9% for most of the experiments and > 99.99% for the experiment on the effect of O2) for ∼15 min. A three-electrode configuration was used for the cell. The metallofullerene (or fullerene) LB film on ITO was used as the working electrode. The counter electrode was a Pt wire, and the reference electrode was Ag/ AgCl. The electrolyte was an aqueous solution of KCl with a concentration of 0.1 mol dm-3. More than eight metallofullerene (or fullerene) films on ITO electrodes were measured under each

J. Phys. Chem. B, Vol. 105, No. 39, 2001 9407

Figure 1. Surface pressure-area (π-A) isotherms of Dy@C82/AA (IA), Dy@C82 (IB), C60/AA (IIA), and C60 (IIB) at the N2/water interface, pH ) 6.5. The barrier compression speed is 2 cm min-1. IA: Dy@C82:AA ) 1:4 (molar ratio), 9.86 × 10-6 mol dm-3 (Dy@C82). IB: 2.44 × 10-5 mol dm-3. IIA: C60:AA ) 1:4 (molar ratio), 1.04 × 10-5 mol dm-3 (C60). IIB: 3.25 × 10-5 mol dm-3. The inset in I illustrates a possible structure of the mixture Langmuir film. The elongated balls are the metallofullerenes, and the zigzag chains are AA molecules.

experimental condition to ensure the reproducibility of the photocurrent data. Results and Discussion Formation of Langmuir-Blodgett Films. A typical π-A isotherm for Dy@C82/AA at the interface between N2 and ultrapure water is shown in Figure 1 (IA). Expansion and recompression of a previously compressed film revealed little hysteresis in the isotherm, and no visible patchy domains on the subphase were observed. No sign of film collapse was observed even when surface pressure reached ∼65 mN m-1. The marked regions in the isotherms, a-b (a′-b′), b-c (b′-c′), and c-d (c′-d′), are assigned to gas, liquid, and solid phases, respectively. The region d-e (d′-e′) is believed to be in another solid phase. The LB film deposition was carried out in the low-pressure solid-phase region. The average limiting area per molecule was estimated to be ∼27 Å2/molecule according to the π-A isotherm in the low-pressure solid-phase region. It is well-known that arachidic acid (AA) has a limiting area of 20 Å2/molecule. Taking into account the molecular area of AA (20 Å2), the molecular area of Dy@C82 measured in Dy@C82/AA should be 55 Å2. This value is smaller than the minimum cross section of such molecules (e.g., ∼120 Å2 for Y@C82 based on an X-ray diffraction measurement 26). In comparison with the π-A isotherm of pure Dy@C82 shown in Figure 1 (IB) and described in our previous paper,19 the mixed Langmuir film of Dy@C82/AA can sustain a surface pressure about 15 mN m-1 higher than that of the pure Dy@C82 film. This suggests that the mixed metallofullerene Langmuir film becomes less rigid, and the intermetallofullerene interactions are much weaker than those in pure metallofullerene Langmuir films. In addition, the limiting area per molecule for a pure Dy@C82 film is estimated to be ∼42 Å2 (Figure 1 (IB)). This

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Figure 2. UV-vis absorption spectra of the Dy@C82/AA LB films on the ITO substrates as a function of the number of layers (solid lines). The spectrum of the Dy@C82/AA mixture dissolved in toluene is also shown as a reference (dotted line). A, 7 layers; B, 5 layers; C, 3 layers; D, 1 layer; E, Dy@C82/AA mixture in toluene (Dy@C82: 9.86 × 10-6 mol dm-3). Inset: The absorbance at 406 nm as a function of the number of layers.

value is smaller than that estimated for a Dy@C82/AA film and much smaller than the minimum cross section of free Dy@C82. The π-A isotherms of C60/AA and pure C60 are shown in Figure 1 (IIA and IIB) for comparison with those of the metallofullerene films described above. The average limiting area per molecule for C60/AA is ∼23 Å2, and this translates to a molecular area of 35 Å2 for C60 in the mixed Langmuir film if we assume the molecular area of AA to be 20 Å2. For comparison, the molecular area of C60 in the pure C60 Langmuir film is ∼28 Å2 according to our measurement (Figure 1 (IIB)). However, these values of limiting area per molecule for C60 are both much smaller than the cross section of free C60 (100 Å2) estimated from X-ray powder diffraction data,27 just as in the case of Dy@C82 described above. The small limiting area per molecule measured for both Dy@C82 and C60 suggests that the carbon cages are not arranged in a perfect monolayer but alternate in height (multilayerlike) within the enclosures of the AA molecules (inset of Figure 1I). This explanation has been proposed for a C60/AA (1:1) Langmuir film at an air/water interface.12a,20,21 As described previously,19 the substantial dipole moment in Dy@C82 results in strong electrostatic interactions among metallofullerene molecules. This may explain why there are apparent differences between pure Dy@C82 and C60 in the shape and slope of the π-A isotherms in the solid-state region and why the Langmuir film of Dy@C82 showed a poorer arrangement than that of C60 (Figure 1 (IB and IIB)). However, the quality and stability of the Langmuir film of Dy@C82 was found to be significantly improved after mixing with AA molecules in terms of the increased surface pressure which the film can stand, the limiting molecular area, and the isotherm slope of the solid phase. The monolayer films of both Dy@C82/AA and C60/AA were easily transferred onto a hydrophilic ITO substrate or quartz plate by vertical dipping (upstroke) with a transfer ratio of ∼1. Multilayers of Dy@C82/AA were also deposited on ITO electrodes in Y-type, but the transfer ratio of the downstrokes (0.2-0.5) is much smaller than that of the upstrokes (∼1). Figure 2 shows the UV-vis absorption spectra of the Dy@C82/ AA LB films on ITO as a function of the number of layers up to 7. Good linearity between the absorbance at 406 nm and the number of layers indicates that reasonable film quality was obtained. Compared with the absorption spectrum of a mixture

Yang and Yang

Figure 3. Representative photocurrent trace vs time obtained from a Dy@C82/AA monolayer-modified ITO electrode (0.20 V bias voltage, 0.1 mol dm-3 KCl electrolyte solution, light intensity ) 37.99 mW cm-2). The arrows indicate the light on-off cycles.

of Dy@C82 and AA in toluene, the overall rising feature of the spectra toward UV is quite similar. This implies that the integrity of the metallofullerenes may be preserved upon film formation. Photoelectric Response of the Dy@C82/AA and C60/AA Monolayers on ITO Electrodes. Shown in Figure 3 is a typical photocurrent response curve for a Dy@C82/AA monolayer on an ITO electrode at a bias voltage of 0.20 V. The net anodic photocurrent was ∼58 nA/cm2, with little attenuation for the successive light on-off cycles. Under the same conditions, the C60/AA monolayer exhibited an anodic photocurrent of ∼50 nA/cm2. To this extent, Dy@C82/AA and C60/AA appear to have a similar photoelectric response. As a reference, the photoelectric responses of the bare ITO electrode and the ITO electrode coated with arachidic acid were also investigated under the same measurement conditions to estimate their contributions to the photocurrent of the mixed fullerene/AA monolayer LB films. A filter was always used in order to remove the short-wavelength component of the white-light source (λ < 330 nm), which is effective in the photoexcitation of the ITO substrate. In the case of the bare ITO electrode, the photocurrent was found to be quite low. Even at high bias voltages (e.g., 0.40 V), the anodic photocurrent of was below 5 nA/cm2. For the ITO electrode with a deposited layer of arachidic acid, the anodic photocurrent ranged from 8 to 15 nA/cm2 (at a bias voltage of 0 to 0.40 V), and this is again about 1 order of magnitude smaller than the photocurrent generated by the ITO with a fullerene/AA LB film. This, therefore, confirmed that the anodic photocurrents were indeed the result mainly of the fullerene molecules Dy@C82 and C60. A photocurrent action spectrum for a Dy@C82/AA monolayer on ITO is shown in Figure 4 along with its absorption spectrum. Because absorption of ITO increases sharply below 340 nm, the photocurrent action spectrum was recorded at wavelengths > 400 nm. Obviously, the photocurrent spectrum coincides well with the absorption spectrum, suggesting again that Dy@C82 is responsible for the photocurrent generation. At 400 nm excitation (1.92 × 1014 photons cm-2 s-1), an anodic photocurrent of 22.1 nA/cm2 was obtained with a Dy@C82/AA monolayer under zero bias. The external quantum yield can be calculated according to Φ ) Nel/Nph ) (Iph/q)/(Fabs)λ.28 (Nel, electron number of the generated photocurrent; Nph, photon number absorbed by the monolayer; Iph, photocurrent (nA cm-2); (Fabs)λ ) Fiλ(1 - 10-Aλ), where Fiλ is the incident photon flux (photons cm-2 s-1) and Aλ is the monolayer absorbance). Knowing that Aλ is 0.8% (Figure 4), we obtain a photocurrent

Endohedral Metallofullerene Dy@C82

Figure 4. Action spectrum (solid squares) of the anodic photocurrent (no bias voltage, 0.1 mol dm-3 KCl electrolyte solution) and absorption (solid line) spectrum for a monolayer of Dy@C82/AA on ITO. The photocurrents are normalized to the incident light intenisty at different wavelengths. The error bars were calculated based on 3-5 independent measurements.

Figure 5. Photocurrent as a function of electrode potential for a monolayer LB film of Dy@C82/AA on an ITO electrode (0.1 mol dm-3 KCl electrolyte solution, light intensity ) 37.99 mW cm-2). The inset shows the linear region from -0.10 to 0.20 V where the photocurrent was proportional to the bias voltage.

quantum yield of 3.9%. This value is somewhat higher than that estimated for the C60/AA monolayer (3.6%). Effects of Bias Voltage and Light Intensity. The effect of bias voltage was investigated to understand the electron transfer between the ITO electrode and the LB film. Figure 5 plots the measured photocurrent against the bias voltage over the range from -0.10 to 0.40 V versus Ag/AgCl. The curve appears to consist of two regions. For the portion from -0.10 to 0.20 V, a good linear fit was obtained with a slope of 146.4 nA cm-2 V-1, which represents the electric resistance of the LB film. The fact that the anodic photocurrent increased with increasing positive bias of the ITO electrode indicates that the electrons flow from the electrolyte through the LB film to the ITO electrode. In this linear region (i.e., J ∝ V), volume conductivity dominates, obeying Ohm’s law, and the injection of excess carriers is negligible. However, the situation changed in the highbias region of the curve (0.20-0.40 V), where the slope increased significantly. This may be because the injected carrier density exceeded the volume generated carrier density, and therefore, the contacts became superohmic, and space-charge effects became important (Jsc ∝ V2/t3, where t is the thickness of film).25 Likewise, C60/AA monolayers on ITO exhibited a similar I-V curve, but with a slightly smaller photocurrent response (not shown here).

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Figure 6. Dependence of photocurrent on light intensity for monolayer LB films of Dy@C82/AA (0.1 mol dm-3 KCl electrolyte solution). A: 0.40 V bias voltage, R2 ) 0.9931, k ) 3.770. B: 0.20 V bias voltage, R2 ) 0.9887, k ) 1.865. C: 0.10 V bias voltage, R2 ) 0.9832, k ) 1.279. D: no bias voltage, R2 ) 0.9664, k ) 0.8975. Here R2 is the deviation coefficient, and k is the slope.

Figure 7. Influence of the ascorbic acid (AsA) concentration on the photocurrent from monolayer LB films of Dy@C82/AA (A) and C60/ AA (B) on ITO electrodes (0.1 mol dm-3 KCl electrolyte solution, no bias voltage, light intensity ) 37.99 mW cm-2). The inset shows a representative photocurrent trace obtained from a Dy@C82/AA monolayer-modified ITO electrode with the same light intensity (no bias voltage, 15 mmol dm-3 AsA).

The photocurrent response was found to be a linear function of the light intensity at different bias voltages, as shown in Figure 6. The anodic photocurrent increased with increasing light intensity, indicating an increasing population of charge carriers due to the more extensive excitation at higher light intensities. Furthermore, the slope increases as the bias voltage increases, suggesting that the quantum yield Φ increased with increasing bias voltage due presumably to the increased carrier collection efficiency.28 The linearity between the measured photocurrent and light intensity indicates a unimolecular recombination process of separated charges.29 In the range of photon fluxes used in these experiments, no sign of saturation was observed. Effect of Electron Donor. The effect of an electron donor (AsA) on the photoelectric response was investigated, and the result is shown in Figure 7 for monolayers of both Dy@C82/ AA and C60/AA without bias voltage. The anodic photocurrent increased with increasing AsA concentration in the electrolyte solution, and a maximum photoelectric response was reached at an AsA concentration of ∼15 mmol dm-3 for Dy@C82/AA and ∼10 mmol dm-3 for C60/AA. The enhancement of the

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Yang and Yang

Figure 8. Dependence of the photocurrent on the number of layers for LB films of Dy@C82/AA on ITO electrodes (0.1 mol dm-3 KCl electrolyte solution, zero bias, light intensity ) 37.99 mW cm-2). A, 15 mmol dm-3 AsA; B, no AsA. The inset shows a representative photocurrent trace obtained with a Dy@C82/AA multilayer-modified ITO electrode and the same light intensity (5 layers, 0.20 V bias voltage, no AsA).

anodic photocurrent as a result of AsA addition verifies the inference made above that the electrons flow from the electrolyte, through the fullerene/AA monolayer, to the ITO substrate. The maximum anodic photocurrent for a Dy@C82/AA monolayer with AsA in the electrolyte solution was ∼70 nA/cm2, which is about 3 times higher than that using the pure 0.1 mol dm-3 KCl electrolyte solution. Under the same bias condition (i.e., 0 V bias), however, the maximum anodic photocurrent for the C60/AA monolayer with AsA in the electrolyte solution was only about 1.5 times higher than that for the pure electrolyte solution. This demonstrates that more effective electron transfer and, consequently, higher quantum efficiency for photocurrent generation can be obtained with the Dy@C82/AA monolayer in the presence of appropriate reducing agents. At the same time, due to the presence of the reducing agent AsA, the dark current increased significantly, and the photocurrent attenuated continuously over time. This was probably due to the consumption of photoactive molecules through electron-transfer reactions. Therefore, a balance has to be struck between high photocurrent quantum yield and good stability of the film in the choice of an appropriate reducing agent. Effect of Film Thickness. The ability to construct the mixed fullerene LB films in a layer-by-layer fashion facilitated the investigation of the effect of the LB film thickness on the photocurrent response. Figure 8 presents such data for Dy@C82/ AA LB films. Because of the way the Dy@C82/AA LB films were prepared, only odd number of layers could be deposited onto the hydrophilic ITO substrates. The anodic photocurrent increased initially with an increasing number of layers, as would be expected due to the increase in photoactive molecules, until a maximum was reached at about 7 layers. The increase was more obvious when the reducing agent AsA was present, presumably due to the enhanced effectiveness of electron transfer. Finally, as shown in the inset of Figure 8, the response time of the multilayer film for the photocurrent to reach the saturation value was much longer than that of a monolayer film (see Figure 2). This is probably because it takes a longer time to complete the electron transfer across multilayers of Dy@C82/ AA from the electrolyte to the ITO electrode. Two opposing factors should be considered in order to understand the thickness effects revealed in Figure 8. One is the increase in the number of photoactive molecules, which would be expected to enhance

Figure 9. Effect of O2 on the photocurrent responses of the C60/AA (I) and Dy@C82/AA (II) monolayers on ITO electrodes. Photoelectrochemical conditions: 0.1 mol dm-3 KCl electrolyte solution, light intenisty ) 37.99 mW cm-2, and no bias voltage. I-IIA: under N2 atmosphere (solid line). I-IIB: in air (dotted line). The arrows indicate the light on-off cycles.

the anodic photocurrent. The other is the concomitant increase in the film’s electrical resistance and in space-charge effects, which would tend to decrease the photocurrent response. Effect of O2. It is known that O2 quenches the triplet state of C60 as the O2 is raised from the triplet ground state to the singlet excited state.1a,4a,c,11b This was also what was observed for the C60/AA monolayer on ITO. As shown in Figure 9 (I), the anodic photocurrent of the C60/AA monolayer on ITO was suppressed by at least 27.3% in the presence of O2. An immediate interesting question is how O2 affects the photocurrent response of a Dy@C82/AA monolayer on an ITO electrode. This is shown in Figure 9 (II). It appears that the effect of O2 on the photocurrent response of a Dy@C82/AA monolayer on ITO is somewhat smaller: the photocurrent decreased by about 16.8% in the presence of O2. Although the electronically excited states of metallofullerenes M@C82 are much less well-known than those of C60, the existence of a triplet state of Dy@C82 in this spectral region seems unlikely due to the presence of an unpaired electron in the highest occupied molecular orbital (HOMO) of the carbon cage (if the spin exchange between Dy3+ and C823is neglected). Therefore, O2 is much less likely to quench the excited state of Dy@C82. Mechanism of Photoinduced Electron Transfer. The results mentioned above show that the photocurrent response is quite similar for Dy@C82/AA and C60/AA LB films on ITO electrodes. As discussed below, however, the photocurrent generation mechanism seems to be somewhat different. Because the photocurrent generation mechanism of C60 has been described extensively, in the following, we will focus our discussion on the mechanism of photocurrent generation for the Dy@C82/AA LB films. Since photoexcitation of bare ITO and of arachidic acid is negligible under these experimental conditions, only the photoexcitation of Dy@C82 is considered to contribute to the photocurrent response we observed. In the case of C60,1-3 photoabsorption primarily leads to the formation of the singlet excited state, which usually undergoes rapid and quantitative

Endohedral Metallofullerene Dy@C82

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SCHEME 1: Schematic Representation of the Electron Transfer Processes for Anodic Photocurrent Generation

anodic photocurrent quantum yield is expected for a metallofullerene LB film on an ITO electrode, without taking into account the differences in film thickness and optical absorbance. Conclusion

intersystem crossing to the triplet state. The energy levels of Dy@C82 are not yet well understood, but its excited states are likely to be more complicated because it has an unpaired electron in the HOMO of the carbon cage C82. The first reduction potential and the first oxidation potential of Dy@C82 have been measured to be -0.367 V (or 0.213 V vs SCE) and 0.132 V versus Fc+/Fc (or 0.712 V vs SCE), respectively.18g The HOMO-LUMO gap of Dy@C82 can be estimated to be ∼0.8 eV from its absorption spectrum and scanning tunneling spectroscopic measurement.30,31 With this information and available data from literature,12e,32 a schematic energy level diagram for the ITO/Dy@C82/AsA system is constructed and shown in Scheme 1. It should be mentioned that the reduction and oxidation potentials of the metallofullerenes are based on the solution data, and they may be somewhat different when the metallofullerenes are in the LB film. Anyway, Scheme 1 simply serves to illustrate the photoinduced electron-transfer processes involved. Note that the reduction potential E1/2(Dy@C82/Dy@C82-) is even more positive than the conduction band (CB) potential of ITO, and therefore, electron injection to the CB of ITO from Dy@C82seems unlikely. However, the fact that we did observe the anodic photocurrent suggests an efficient electron transfer from the excited Dy@C82* to the CB of ITO. Specifically, when Dy@C82 on the ITO electrode is excited sufficiently above the HOMOLUMO gap, the injection of the excited electron to the CB of ITO appears to compete favorably with de-excitation processes, leading to the formation of Dy@C82+ (see Scheme 1). This electron injection is followed by electron transfer to Dy@C82+ from a donor such as AsA (E0AsA+/AsA ) -0.21 V vs SCE), which completes the external circuit. Because of the lower reduction potential of AsA than H2O (E0O2/H2O ) 0.66 V vs SCE at pH ) 5.60), for instance, an enhancement of the anodic photocurrent is obtained. The overall process of anodic photocurrent generation can be described as follows:

Dy@C82 + hν f Dy@C82*

(1)

Dy@C82* + CBITO f Dy@C82+ + e-(CBITO)

(2)

Dy@C82+ + AsA f Dy@C82 + AsA+

(3)

Because Dy@C82 is both a better electron donor and a better electron acceptor than C60, it is understandable that the Dy@C82 is a good electron-transfer mediator. Consequently, a higher

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