Photosynthetic electron transfer using fullerenes as novel acceptors

PERGAMON

Carbon 38 (2000) 1599–1605

Photosynthetic electron transfer using fullerenes as novel acceptors Hiroshi Imahori a , *, Koichi Tamaki a , Hiroko Yamada a , Koji Yamada a , Yoshiteru Sakata a , Yoshinobu Nishimura b , Iwao Yamazaki a,b , Mamoru Fujitsuka c , Osamu Ito a,c b

a The Institute of Scientific and Industrial Research, Osaka University, Mihoga-oka, Ibaraki, Osaka 567 -0047, Japan Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060 -8628, Japan c Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai 980 -5877, Japan

Abstract A variety of porphyrin-linked C 60 dyads and triads have been designed and synthesized to elucidate the special properties of fullerenes in electron transfer (ET). C 60 or naphthalenediimide with comparable reduction potentials was linked to a porphyrin with similar spacers. Accelerated photoinduced charge separation (CS) was observed in the former compared with the latter by picosecond fluorescence lifetime measurements. It may be explained by the small reorganization energy ( l) in C 60 compared with that in conventional planar aromatic acceptors, as we have already proposed. Porphyrin–pyromellitimide–C 60 triads have been prepared to mimic photosynthetic ET. Based on the fluorescence quenching experiments, it was concluded that the C 60 moiety accelerates the initial ET process via through-bond or enhances the direct through-space ET from the excited singlet state of the porphyrin. To optimize CS in triad models, a ferrocene–porphyrin–C 60 triad has been synthesized. The triad produced a long-lived, charge-separated state with a high quantum yield, compared with the previously reported similar conventional triads. These results clearly show that a combination of multistep ET strategy with small reorganization energies of fullerenes is a promising methodology toward the construction of a solar energy conversion system.  2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Fullerene

1. Introduction Molecule-based artificial photosynthesis is one of the most challenging fields in chemistry, since the pioneering works of Sakata and Mataga [1,2], Gust and Moore [3,4], and Wasielewski [5,6]. The fundamental concept is based on a multistep electron transfer (ET) strategy. When multistep ET takes place along an array of chromophores with a suitable redox gradient, a long-lived, charge-separated state with a high quantum yield is produced, which eventually leads to generation of chemical energy or products. Recently, fullerenes have been found to be excellent electron acceptors, because of the unique three dimensional delocalized p systems with a high symmetry *Corresponding author.

[7,8]. We have proposed that the peculiar effect of fullerenes in ET can be ascribed to the small reorganization energies ( l) of fullerenes [9–11]. It explains reasonably the acceleration effect of photoinduced charge separation (CS) as well as charge shift (CSH) and the deceleration effect of charge recombination (CR) in donor-linked fullerenes. This situation is quite similar to photosynthetic ET in proteins where each ET process is optimized by using the small reorganization energies of chromophores embedded in the protein matrix, in addition to tuning the free energy changes as well as the electronic couplings between the redox pair. Considering the similarity between the small l of fullerenes and of chromophores in photosynthetic proteins, the utilization of fullerenes in artificial photosynthesis is a promising methodology for the construction of a solar energy conversion system. Here we

0008-6223 / 00 / $ – see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 99 )00295-X

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report a new approach using fullerenes as acceptors toward artificial photosynthesis.

2. Discussion

2.1. Porphyrin-linked fullerene and naphthalenediimide We have already reported the comparison of ET rates between porphyrin–C 60 and –benzoquinone dyads with similar spacers [10]. Based on the fact that C 60 accelerates photoinduced CS and decelerates CR compared with benzoquinone, it was proposed that the reorganization energy of C 60 is smaller than that of benzoquinone. However, some deviation of the first reduction potentials of C 60 and benzoquinone did not allow us to evaluate accurately the difference of ET rates between the two systems. Bearing these in mind, we redesigned and synthesized model compounds 1 and 2 where a porphyrin is connected to C 60 and naphthalenediimide (NIm), respectively, with a relatively rigid spacer (Fig. 1). On the basis of CPK modeling, edge-to-edge (R ee ) (center-to-center ˚ (17.8 A) ˚ for 1 and 12.7 A ˚ (18.2 (R cc )) distances are 11.7 A ˚ for 2. Although free rotation around the spacer methylA) ene in 2 is possible, such rotation would not affect the CS and CR rates so much as reported in similar porphyrin– quinone linked systems [12].

Fig. 1. Structures of molecules 1–4.

Scheme 1. Reagents and conditions: (i) neopentyl glycol, ptoluenesulfonic acid, quantitative; (ii) KOH, THF / MeOH, 51%; (iii) 2-chloro-4,6-dimethoxy-1,3,5-triazine, N-methylmorpholine, THF; (iv) trifluoroacetic acid, H 2 SO 4 , 79% (two steps); (v) C 60 , N-methylglycine, toluene; (vi) Zn(OAc) 2 , 49% (two steps).

The synthesis of 1 and 2 was carried out as shown in Schemes 1 and 2. Formyl-protected benzoic acid 7 was prepared by the protection of the formyl group in 5, followed by saponification. Condensation of 7 with aminoporphyrin 8 and subsequent acid hydrolysis gave 9 in 79% yield. Porphyrin-linked C 60 1 was obtained by 1,3-dipolar cycloaddition using 9, C 60 , and N-methylglycine, followed by treatment with Zn(OAc) 2 [13]. Amino-protected benzoic acid 11 was synthesized by the protection of amino group in 10. Condensation of 11 with 8 and subsequent acid hydrolysis afforded 12 in 64% yield. Porphyrin-linked naphthalenediimide 2 was obtained by the cross-condensation of naphthalene-1,4,5,8-tetracarboxylic dianhydride with 12 and n-hexylamine, followed by treatment with Zn(OAc) 2 . Porphyrin and C 60 references 3 and 4 were also prepared [14,15]. All the structures were verified by spectroscopic analyses. The absorption spectra of 1 and 2 in THF are virtually the superposition of the spectra of the individual chromo-

Scheme 2. Reagents and conditions: (i) di-t-butyl dicarbonate, NaOH, t-BuOH / H 2 O, 49%; (ii) 8, 2-chloro-4,6-dimethoxy-1,3,5triazine, N-methylmorpholine, THF; (iii) trifluoroacetic acid, CH 2 Cl 2 , 64% (two steps); (iv) naphthalene-1,4,5,8-tetracarboxylic dianhydride, hexylamine, DMF; (v) Zn(OAc) 2 , 39% (two steps).

H. Imahori et al. / Carbon 38 (2000) 1599 – 1605

phores, indicating that there is no evidence for strong interaction between the chromophores in the ground state. Steady-state fluorescence spectra of 1–3 were taken in THF with the same concentration exciting at the Soret band where the porphyrin absorbs mainly. The emissions of 1 and 2 were reduced compared to that of 3 (relative intensities: 0.02 for 1 and 0.60 for 2), showing that the excited singlet state of the porphyrin ( 1 ZnP*) is quenched by the C 60 and the NIm, respectively. The emissions of 1 (550–750 nm) were observed only from the porphyrin, with no detectable emission from the C 60 (700–750 nm). Therefore, there is no evidence for the existence of singlet–singlet energy transfer (EN) from 1 ZnP* to C 60 . It is well established that in ZnP–C 60 dyads photoinduced CS occurs efficiently from 1 ZnP* to C 60 in solvents, at least, with moderate or high polarity [9], whereas in ZnP–NIm dyads photoinduced CS takes place from 1 ZnP* to NIm under similar conditions [16]. Based on the driving force for CS, photoinduced CS from 1 ZnP* to the attached acceptors is energetically feasible (Table 1). From these results, we can conclude that the CS is a main pathway for the emission quenching in polar solvents such as THF and DMF. The fluorescence lifetimes of 1–3 were measured by a picosecond single photon counting technique with excitation at 425 nm where the porphyrin absorbs mainly and monitoring at 605 nm where the fluorescence is due to only the porphyrin. The fluorescence decays of 1–3 in THF and DMF were analyzed to give one major component (Table 1). Based on the data, we can calculate the 21 CS rates, k CS (5t 21 2t ref ; tref 51.9 ns for 3 in THF and 1.8 ns for 3 in DMF). In THF and DMF k CS for 1 is faster than that for 2 by a factor of 65 and 37, respectively. The large acceleration of photoinduced CS in 1 is highly remarkable, considering that (i) both CS processes of 1 and 2 are in the normal region of Marcus parabola, (ii) the 2DGCS in 1 is quite similar to that in 2, and (iii) the number of the intervening bonds between the redox pair in

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1 and 2 is 13 and 12, respectively. The large acceleration effect does not seem to be a consequence of the difference in the electronic coupling between the redox pair. In contrast, the CR processes in 1 and 2 could not be compared, because of the inability to detect the characteristic band due to the NIm anion radical in 2 using picosecond time-resolved transient absorption spectroscopy [17]. This is because the CR rate is faster than for CS in 2, in addition to the weak quenching of 1 ZnP* by the NIm group [12]. The CS values for 1 and 2, respectively, are comparable in THF and DMF. A possible explanation for this effect comes from the fact that l and 2DGCS are functions of the solvent. An increase of the solvent polarity in the normal region would result in increases of both l and 2DGCS , so that the CS rates do not vary much. Accelerated CS observed in 1 may be ascribed to the small l in C 60 compared with those in typical twodimensional acceptors such as diimides and quinones which have a l of about 1 eV in solvents with moderate polarity such as CH 2 Cl 2 and THF. Calculation of solvent reorganization energies ( ls ) for 1 and 2 using the classical Marcus equation predicts that values of total l (5ls 1li ) are comparable (1.18 eV for 1 and 1.24 eV for 2 in THF), assuming the value of intramolecular reorganization energy ( li ) for typical porphyrin–quinone, 0.3 eV, is taken for both cases. Therefore, the l values obtained using the classical Marcus equation does not accommodate the observed large difference in ET. A plausible explanation for the small reorganization energy in C 60 is that the unit ?2 charge in C 60 is spread over the whole C 60 framework, while the NIm p system is planar and the charge in NIm?2 is concentrated heavily on the oxygens. Thus, the charge density of each atom in C ?2 60 is much smaller than that of NIm?2 , reducing the value of ls in C 60 . In addition, the rigid framework of C 60 remains unchanged in C ?2 60 , keeping the value of li small. Therefore, total l in C 60 would be small, compared with conventional planar acceptors. These results clearly support our previous conclusion.

2.2. Porphyrin–pyromellitimide–C60 triads Table 1 The Gibbs free energy changes for CS (2DGCS ), fluorescence lifetimes (t ), and ET rate constants for CS (k CS ) of 1 and 2 Compound

Solvent

2DGCS / eV a

t / ps

k CS / 10 8 s 21

1 2 1 2

THF THF DMF DMF

0.69 0.76 0.91 0.99

49 1200 72 1100

200 3.1 130 3.5

a

The free energy changes for CS were calculated by using the first oxidation potential of donor and the first reduction potential of acceptor in CH 2 Cl 2 and the corrected term for the ion solvation energies by Born equation for the other solvents. The radii of porphyrin, C 60 , and naphthalene diimide, and the center-to-center distances in 1 and 2 were estimated to be 5.0, 4.4, 4.1, 17.8, and ˚ respectively. 18.2 A,

As we and others have demonstrated, fullerenes as acceptors (A) accelerate photoinduced CS and retard CR in donor (D)-linked fullerenes. The peculiar effect of fullerenes in ET, which is reasonably explained by the small l, has inspired us to design fullerene-containing multicomponent systems as artificial photosynthetic models. There are several reports of triads and a pentad, such as D–S (sensitizer)–C 60 [18], S–S9–C 60 [19] and S–(C 60 ) n (n52, 4) [20–25] systems. Here we report new triad systems, porphyrin–pyromellitimide (Im)–C 60 triads 13a– c (Fig. 2). The energy gradients of each state in the triad are in the order of 1 ZnP*–Im–C 60 (2.06 eV for 13a). ZnP ?1 –Im?2 –C 60 (1.92 eV for 13a).ZnP ?1 –Im–C ?2 60 (1.84 eV for 13a) in dioxane. Therefore, 13 will display a sequential ET that is quite similar to primary ET events at

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H. Imahori et al. / Carbon 38 (2000) 1599 – 1605 Table 2 Relative fluorescence quantum yield a Compound

b

13a 13b c 13c b 14a b 14b c 14c b

Relative fluorescence quantum yield 1,4-dioxane

THF

0.06 0.19 0.04 0.08 0.41 0.06

0.02 0.11 0.03 0.04 0.15 0.03

a

Excited at Soret band under the same concentration. Versus 15a. c Versus 15b. b

Fig. 2. Structures of molecules 13–15.

the reaction center of photosynthesis. The characteristic intense absorption band due to the Im anion radical ( lmax 5720 nm) as well as the relatively rigid conformation will be helpful for analyzing the dynamics of ET. The synthesis of 13a–c is shown in Scheme 3. Crosscondensation of aminoporphyrin 8, pyromellitic dianhydride, and protected formyl aniline or aminomethyl derivative gave pyromellitimide-substituted porphyrin 16 and 19, respectively. Similarly, cross-condensation of aminomethylporphyrin 17, pyromellitic dianhydride, and protected formyl aniline afforded 18. The zincporphyrin– pyromellitimide–C 60 triads 13a–c were obtained in 33– 49% yield by 1,3-dipolar cycloadditions with 16, 18, and 19, N-methylglycine, and C 60 in toluene and subsequent treatment with Zn(OAc) 2 . C 60 -free dyads 14a–c as well as single-chromophore compounds 15a, 15b, and 4 were also

prepared. Their structures were verified by spectroscopic analyses. The absorption spectra of 13a–c in dioxane and THF are almost the superposition of the absorption spectra of the individual chromophores, giving no indication for strong interactions between these chromophores. The absorption due to the C 60 moiety is much weaker and broader than that of the porphyrin, whereas discernible absorption of the imide unit appears around 250–300 nm. Steady-state fluorescence spectra of 13–15 were taken in dioxane and THF with the same concentration exciting at the Soret band where the porphyrin absorbs mainly. The relative fluorescence intensities of 13 and 14 vs. 15 are summarized in Table 2. The fluorescence spectra of 13 and 14 are quenched more strongly than those of 15, implying that 1 ZnP* is quenched by the Im moiety. It should be noted here that the quenching efficiency of 13 is much larger than that of 14 both in dioxane and THF, indicating the additional involvement of C 60 in the deactivation pathway of 1 ZnP*. Preliminary results on photophysical properties of porphyrin–pyromellitimide–C 60 triad 13a has been already reported [26]. In spite of the existence of short

Scheme 3. Reagents and conditions: (i) pyromellitic dianhydride, 4-(5,5-dimethyl-1,3-dioxan-2-yl)aniline, DMF; (ii) trifluoroacetic acid, H 2 SO 4 , CHCl 3 , 19% for 16 and 13% for 18 (two steps); (iii) C 60 , N-methylglycine, toluene; (iv) Zn(OAc) 2 , 33% for 13a, 49% for 13b, and 47% for 13c (two steps); (v) pyromellitic dianhydride, 4-(5,5-dimethyl-1,3-dioxan-2-yl)aminomethylbenzene, DMF; (vi) trifluoroacetic acid, H 2 SO 4 , CHCl 3 , 25% (two steps).

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aromatic spacers between the chromophores, a sequential ET relay, that mimics the reaction center of photosynthesis and the resulting fairly long-lived CS state (1.3 ns in dioxane) with a moderate quantum yield (0.46 in dioxane), has been realized in 13a. Furthermore, the C 60 in 13a accelerates the quenching of 1 ZnP*, suggesting the existence of ‘through-bond’ or ‘through-space’ interaction between 1 ZnP* and C 60 . Insertion of the methylene spacer between the chromophores does not seem to affect the acceleration of the quenching of 1 ZnP* by C 60 in 13b and 13c. However, the detailed photodynamics of 13b and 13c must await transient absorption experiments, which are in progress.

2.3. Ferrocene–porphyrin–C60 triad Gust [18] and our group [26] reported the photodynamics of carotenoid (Car)-free base porphyrin (H 2 P)– C 60 and zincporphyrin (ZnP)–imide–C 60 triads, respectively. However, the overall quantum yields (0.17–0.46) and the lifetimes (1.3–170 ns) of the charge-separated state are not so satisfactory, compared with those of conventional triads, tetrads, and pentads [1–6,27]. Here we report a new triad system, ferrocene (Fc)–zincporphyrin–fullerene (C 60 ) triad 20 (Fig. 3). The energy gradients of each state in 20 are designed to be in the order of Fc– 1 ZnP*–C 60 ?2 (2.06 eV).Fc–ZnP ?1 –C ?2 (1.01 eV).Fc?1 –ZnP–C 60 60 (0.80 eV) in benzonitrile. Therefore, it will display a sequential ET within the molecule, as observed in the similar Car–H 2 P–C 60 triad [18]; Fc– 1 ZnP*–C 60 →Fc– ?1 ?2 ZnP ?1 –C ?2 –ZnP–C 60 . Since separation distance 60 →Fc between the chromophores as well as nature of the spacer are well-tuned in 20, it is expected that 20 produces a long-lived, charge-separated state with a high quantum yield. The synthetic route to 20 is shown in Scheme 4.

Fig. 3. Structures of molecules 20–24.

Scheme 4. Reagents and conditions: (i) H 2 SO 4 , 4-nitroaniline, 13%; (ii) H 2 , Pd / C, 65%; (iii) neopentyl glycol, TsOH, 95%; (iv) H 2 , Pd / C, 83%; (v) pyridine; (vi) TFA, H 2 SO 4 , 18% (two steps); (vii) N-methylglycine, C 60 , toluene; (viii) Zn(OAc) 2 , CHCl 3 , 14% (two steps).

Aminoferrocene 26 was prepared in two steps from ferrocene 25. Formyl-protected aniline 28 was synthesized in two steps from 4-nitrobenzaldehyde 27. Cross-condensation of porphyrin bis(acid chloride) 29 with 26 and 28 in benzene in the presence of pyridine, followed by acid hydrolysis, afforded ferrocene–porphyrin 30 in 18% yield. The triad 20 was obtained by 1,3-dipolar cycloaddition using 30, N-methylglycine, and C 60 in toluene and subsequent treatment with Zn(OAc) 2 in 14% yield. References 21–24 were also prepared [28]. Their structures were verified by spectroscopic analyses. The absorption spectrum of 20 in THF is virtually the superposition of the absorption spectra of 4, 23, and 24, indicating no evidence for strong interaction among the three chromophores in the ground state. Fluorescence spectra of 20 and 21 in THF are quenched strongly as compared with that of 23 when excited at the Soret band under the same concentration (relative intensity: 0.05 for 20 and 21). In contrast, the relative fluorescence intensity of 22 vs. 23 (0.37) is much larger, suggesting that quenching of 1 ZnP* by the attached C 60 is a dominant deactivation pathway in 20. Nanosecond time-resolved transient absorption spectra of 20 were measured in benzonitrile [29]. The typical examples are shown in Fig. 4. Immediately after excitation of 20 with 532 nm nanosecond pulse, where Fc, ZnP, and C 60 were excited in a relative absorption ratio (%) of 2:79:19, the characteristic bands due to C ?2 60 appeared

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H. Imahori et al. / Carbon 38 (2000) 1599 – 1605

Fig. 4. Transient absorption spectra of 20 (0.1 mM) at time delay of 250 ns (solid line with black circles) and of 2.5 ms (solid line with white circles) excited at 532 nm in deaerated benzonitrile.

around 700–1100 nm. In contrast, no appreciable absorption due to ZnP ?1 was observed. In our previous studies on a zincporphyrin–C 60 dyad with a similar spacer [9], the rate constants of CS (k CS1 ) from 1 P* to C 60 are 0.9–1.23 10 21 10 s with quantum yields of 0.94–0.98 in polar solvents such as THF and DMF, whereas the CR rate 9 21 10 21 (k CR1 ) changes from 2310 s in THF to .5310 s in DMF. Based on the results, it is concluded that the 1 initial photoinduced ET from ZnP* to C 60 occurs in 20, ?1 followed by the charge-shift (CSH) from Fc to ZnP to ?1 ?2 ?1 produce Fc –ZnP–C 60 . The lifetime (tCR2 ) of Fc – ?2 ZnP–C 60 in benzonitrile was obtained from the timeprofile of the absorbance at 980 nm under the O 2 -saturated 3 conditions to eliminate the contribution of ZnP* and / or 3 * . The value of tCR2 in benzonitrile is 7.5 ms. The C 60 overall quantum yield for the photoinduced CS based on ?2 total light absorbed by both the porphyrin and C 60 ([C 60 ]/ 1 1 * ])) equals 0.65 in benzonitrile. Consider([ ZnP*]1[ C 60 ing that the CSH is beyond the time resolution (10 ns) of our instrumentation, the rate constant of the charge-shift (k CSH ) is estimated to be 10 9 –10 11 s 21 . It is highly remarkable that both the quantum yield and the lifetime in 20 are much improved, compared with those of conventional triads [1–6,27] as well as those of the similar Car–H 2 P–C 60 triad (tCR2 5170 ns, F 50.14 in 2methyltetrahydrofuran) [18].

3. Conclusion Fullerenes have been found to be excellent acceptors by investigating photoinduced intramolecular ET in the porphyrin-linked C 60 molecules using time-resolved spectroscopic methods. The most intriguing characteristics of C 60 in ET are to accelerate photoinduced CS and to slow down CR. These results indicate that the ET properties of

fullerenes are related to the three-dimensional delocalized p systems with large size, spherical strained shape, and high symmetry. The porphyrin–imide–C 60 triad has been prepared to mimic the primary charge separation in the photosynthetic reaction center. It should be noted here that the C 60 moiety in the triad accelerates photoinduced CS via through-space or through-bond involving the imide moiety. The ferrocene–porphyrin–C 60 triad has been synthesized to optimize the stepwise CS properties. The present triad shows one of the highest balanced values both for the quantum yield and the lifetime of the final chargeseparated state, among the previously reported artificial photosynthetic triads. The more elaborate fullerene-containing systems such as tetrads and pentads will be a next synthetic target as artificial photosynthetic models which can demonstrate multistep ET to afford a long-lived charge-separated state in a high quantum yield. In addition, a combination of this strategy with molecular assembly such as self-assembled monolayers is promising for the construction of solar energy conversion systems [15,30– 37].

Acknowledgements We thank Dr. David Gosztola and Prof. Michael R. Wasielewski in Argonne National Laboratory for the measurements of picosecond time-resolved transient absorption spectra of 1 and 2. This work was supported by Grant-in-Aids for COE Research and Scientific Research on Priority Area of Electrochemistry of Ordered Interfaces and Creation of Delocalized Electronic Systems, from the Ministry of Education, Science, Sports and Culture, Japan. Y.S. thanks the Mitsubishi Foundation for financial support.

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