FULL PAPERS DOI: 10.1002/asia.200900241

Phthalocyanine–C60 Fused Conjugates Exhibiting Molecular Orbital Interactions Depending on the Solvent Polarity Takamitsu Fukuda,[a] Naoaki Hashimoto,[a] Yasuyuki Araki,[b] Mohamed E. El-Khouly,[b] Osamu Ito,*[b] and Nagao Kobayashi*[a]

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Abstract: New covalently C60-connected zinc phthalocyanine (ZnPc) derivatives have been synthesized by utilizing successive cycloaddition reactions of C60 with a ZnPc derivative containing a pyridazine moiety employing Komatsus method in reaction of C60 with phthalazine. The UV/Vis absorption spectrum of the fused conjugate (5) shows red shifts from the corresponding absorption of ZnPc derivative (8), indicating interactions between the ZnPc and C60 moieties. The DFT calculations under non-polar medium predict that the HOMO and LUMO of 5

localize on the ZnPc moiety, whereas LUMO+1 localizes on the C60 moiety, which reasonably explain the magnetic circular dichroism (MCD) and absorption spectra in toluene. Electrochemical redox potentials of 5 in polar solvents indicate the first-oxidation potential arises from the ZnPc moiety, whereas the first reduction potential is associated with the C60 moiety, suggestKeywords: charge transfer · electronic structure · fullerenes · phthalocyanines · porphyrinoids

Introduction A variety of phthalocyanine (Pc)-based compounds have been developed in the past decade.[1–3] In particular, developments in synthetic methods and separation techniques enable us to expand the potential application of Pcs to various research fields in frontier science and technologies. Recent studies have demonstrated that properly designed conjugates of Pc and other functional components lead to photo and/or electrochemically unique functional molecules, that is, the properties of the conjugates can be different from the simple summation of those of the constituting units. One of the promising combinations of this kind is the Pc–C60 system[4–6] that can generate a stable charge-separate (CS) states after photo excitation of the Pc moiety. Since stable CS states are critical for mimicking the initial stage of natural photosynthesis, most of the previously reported conjugates have spatial linker components between the Pc and C60 units in order to achieve the stable CS states with high

[a] Dr. T. Fukuda, N. Hashimoto, Prof. N. Kobayashi Department of Chemistry Graduate School of Science Tohoku University, Sendai 980-8578 (Japan) Fax: (+ 81) 22-795-7719 E-mail: [email protected] [b] Dr. Y. Araki, Dr. M. E. El-Khouly, Prof. O. Ito Institute of Multidisciplinary Research for Advanced Materials Tohoku University Sendai 980-8577 (Japan) Fax: (+ 81) 22-217-5608 E-mail: [email protected]

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ing the LUMO localizes on the C60 moiety in polar solvent. This reversal of the LUMO is supported by the ZnPc-fluorescence quenching with a nearby C60 moiety in benzonitrile, which leads to the charge-separation via the excited singlet state of the ZnPc moiety. In toluene on the other hand, such a ZnPc-fluorescence quenching owing to the photoinduced charge separation is not observed as predicted by the DFT-calculated LUMO on the ZnPc moiety.

quantum yields.[4, 5] On the other hand, the ground state interactions between Pc and C60 are also scientifically essential in terms of novel Pc-based materials such as ambipolar field-effect transistors, optical limiting materials, molecular devices, and so forth.[6] This type of molecule was first reported in 1995, in which the importance of the electron deficiency and the ability to undergo multistage reduction of C60 was described.[7] Very recently, we have published the synthesis of tribenzotetraazachlorin (TBTAC)–C60 conjugates, and demonstrated that molecular orbital (MO) mixing between TBTAC and C60 can be tuned to on and off states by utilizing the electron donating/withdrawing properties of the peripheral substituents on the TBTAC moiety.[8] In these studies, the sufficiently close and symmetry-adapted arrangement of the TBTAC and C60 moieties enabled their porbitals to overlap, and as a result, strong MO interactions between the TBTAC and C60 moieties were realized for the first time. In the present paper, we report a new type of covalently fused C60-connected Pc derivatives, 4 (H2Pc) and 5 (ZnPc), obtained by utilizing successive cycloaddition reactions of C60 with a Pc derivative containing a pyridazine moiety (Scheme 1). In 2001, Komatsu et al. reported an interesting reaction of C60 with phthalazine, which gave the benzo derivative of the, so-called, open-cage fullerene with Cs symmetry.[9] The UV/Vis absorption spectrum of this compound resembles that of pristine C60, indicating that the original phybridization structure of C60 is almost kept. These findings prompted us to create new Pc–C60 fused conjugates, that is, 4 and 5 with the minimum distance in the almost same plane as shown in Scheme 1. We would like to emphasize

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structure, of which two carbon atoms at the open edge have sp3 hybridization, that is, the conjugation between the ZnPc and C60 moieties is disrupted formally at the linkage site, which implies that 5 is one of the donor–spacer–acceptor type molecules applicable to photoinduced electron transfer devices. The distance between the center of the C60 moiety and zinc atom is ca. 9.2 , which is

Scheme 1. Synthetic procedures for 4, 5 and 7, 8.

that compound 5 exhibits unusual electronic properties owing to the significant interactions between the ZnPc and C60 units. In addition, the DFT calculation also predicts that the LUMO of 5 localizes on the ZnPc moiety, whereas the upper LUMO (LUMO + 1) localizes on the C60 moiety in a non-polar medium. As a consequence, one-electron reduction by the alkali metal and electrochemistry gave both the radical anion centered on the ZnPc and C60 units, respectively, depending on the counter ion states in polar solvents, which has not been reported in any other porphyrin–C60 and Pc–C60 systems known to date. In addition, we have demonstrated that the observed decay pass after the photo-excitation depends on solvent polarity.

Results and Discussion Synthesis and Characterization Mixed condensation of 4,5-dicyanopyridazine (1) and 4-tertbutylphthalonitrile (2) in the presence of lithium alkoxide followed by chromatographic separations gave the metalfree Pc analogue (3) containing pyridazine-moiety. The initial [4+2] cycloaddition of 3 and C60 in 1-chloronaphthalene at 280 8C followed by the elimination of nitrogen and the intramolecular retro [2+2+2] reaction afforded the Pc–C60 conjugate (4).[9] Zinc was inserted to 4 by employing conventional metal insertion conditions. The tert-butyl groups increase the solubility of 4 and 5 to a greater variety of solvents. On the other hand, conjugates obtained from 4,5-bisbutoxyphthalonitrile cause aggregation, which is inappropriate for our spectroscopic studies. As a reference molecule, corresponding Pcs (tribenzotetraazaporphyrin TBTAP, 7 and 8) were synthesized similarly from 6 instead of 1.[10, 11] The open-cage C60 (9) was also synthesized according to the literature method[9] for the purpose of comparison. The conjugates were characterized by mass and 1H NMR spectroscopy to confirm the expected open-cage structure at the linkage site. The DFT-optimized structure of 5 (Figure 1) indicates that the center of the C60 unit is outside of the Pc square, but is almost on the same plane. The ZnPc and C60 are linked by the eight-membered ring affording an open-cage

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Figure 1. Top and side view of the DFT optimized structure of 5.

slightly longer than that of our TBTAC–C60 conjugate (ca. 8.0 ).[8] However, the distance between the proximate sp2 carbons is quite short (ca. 2.5 ) comparable to the TBTAC–C60 conjugates owing to the structural characteristics of 5, that is, the b-carbon of the C60-substituted pyrrole unit for 5 is sp2 hybridized, while that of TBTAC–C60 is sp3 hybridized. The structural considerations indicate that considerably strong MO interactions between the ZnPc and C60 can possibly be expected for the conjugate 5. The DFT calculations were performed to enhance the understanding of the observed interactions between ZnPc and C60 in detail. The tert-butyl groups were omitted to simplify the calculations. Figure 2 shows the partial MO energy diagram and frontier orbitals of 5 and 8 at the B3LYP/6-31G(d) level. Since the calculations did not take the effect of solvent into consideration, the obtained results correspond approximately to the molecule dissolved in a non-polar medium. The HOMO and LUMO of 5 localize on the ZnPc moiety, and the distribution patterns of the electron clouds are almost identical to those of 8. Unexpectedly, the LUMO of 5 is ZnPc-centered, although the ZnPc–C60 conjugates re-

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shown in Figure 3. The absorption spectrum of 8 is characterized by two split intense Q-bands at 666 and 633 nm in the 600–700 nm region, which are caused by the absence of

Figure 2. Partial MO energy diagram and frontier orbitals of 8 (left) and 5 (right) derived from the B3LYP/6-31G(d) level DFT calculations.

Figure 3. Absorption (bottom) and MCD (top) spectra of 5 (left) and 8 (right) in toluene.

ported hitherto had C60-centered LUMOs owing to the strong electron accepting nature of C60, to the best of our knowledge.[5] On the other hand, the LUMO+1 of 5 localizes mainly on the C60 moiety. The LUMO and LUMO+1 of 5 originate from the linear combinations of LUMOs of 8 and 9, although the interaction is weak enough to keep the electron clouds of the LUMOs of the components. The calculations further indicate that the LUMO+2 and LUMO+3 of 5 originate from the linear combinations of LUMO+1 of 8 and 9, leading to extensively delocalized MOs over the entire aromatic structure owing to the strong interaction between them. Almost all orbitals of 8 are slightly stabilized in energy when a C60 unit is fused to form 5, since the fused C60 unit functions as an electron withdrawing group for ZnPc as a result of its strong electron accepting nature, and vice versa. Table 1 summarizes the selected electronic excitation energies and the wave functions based on the eigenvectors predicted by the TDDFT calculations. In the case of 8, two transitions with the oscillator strength (f) greater than 0.1 were predicted at 586 and 557 nm, and the corresponding wave functions are dominated by the electronic transitions from the HOMO to LUMO and from the HOMO to LUMO+1, respectively. The observed absorption and magnetic circular dichroism (MCD) spectra of 8 in toluene are

one peripheral benzene ring from the ordinary ZnPc, reducing the symmetry of the aromatic structure of 8 from D4h to C2v. The negative and positive MCD signals on going from the longer to shorter wavelength (i.e., 664 and 630 nm, respectively, in Figure 3) are also typical of this type of compound.[12, 13] Similarly, the absorption spectrum of compound 5 in toluene was also predicted by the TDDFT calculations to have three electronic transitions at 612, 605, and 560 nm with the f value of 0.48, 0.01, and 0.31, respectively. The lower energy transition (612 nm) is dominated by transition from the HOMO to LUMO localizing on the ZnPc moiety; therefore, this corresponds to the observed longest absorption peak at 681 nm (Figure 3). The observed red-shift from 666 nm for 8 to 681 nm for 5 is mainly attributed to the larger stabilization of the LUMO than that of the HOMO by the interaction with C60 (Figure 2). Although not shown in Table 1, the HOMO to LUMO+1 transition is possible as the second lowest band. However, the transition moment corresponding to this band is expected to be extremely low, since the MO amplitudes localize mainly on the Pc and C60 moieties for the HOMO and LUMO+1, respectively. The weak second MCD peak with a negative sign at 653 nm can be ascribed to this transition, which is hidden in the broader absorption at 681 nm.[14–16] On the other hand, the calculated transitions at 605 and 560 nm of 5 correspond to the observed peaks at

Table 1. Calculated electronic excitation energies, oscillator strength and the related wave functions. comp.

energy [eV]

energy [nm]

f[a]

wave function[b]

5

2.03 2.05 2.22 2.12 2.23

612 605 560 586 557

0.48 0.01 0.31 0.28 0.39

0.58 j L H > 0.22 j L+4 H > 0.11 j L+2 H12 > +… 0.50 j L+2 H > 0.44 j L+3 H > +0.13 j L+2 H1 > +0.12 j L+3 H1 > +… 0.49 j L+3 H > +0.35 j L+2 H > +0.16 j L H12 > +0.14 j L H11 > +… 0.60 j L H > +0.20 j L+1 H5 > 0.11 j L+1 H4 > +… 0.61 j L+1 H > 0.20 j L H5 > +0.14 j L H2 > +…

!

!

!

!

!

!

!

!

! !

!

! !

! !

!

!

8

[a] Oscillator strength. [b] The wave functions based on the eigenvectors predicted by TDDFT. Eigenvectors greater than 0.10 are included.

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FULL PAPERS 639 and 622 nm in Figure 3. Since the LUMO+2 and LUMO+3 of 5 are delocalized (Figure 2), and the partition of the MO amplitude is almost equal, these transitions arise from the interaction of the ZnPc with C60 moieties. In Figure 3, the broad tail was observed at longer than 700 nm, which is known to arise from possible aggregation.[17] Interestingly, all these spectral characteristics of 5 were not observed for our previous TBTAC–C60 system, in which three absorption bands were observed in the Q-band region.[8a] Consequently, these observed MCD and absorption bands in the Q-band region of 5 can be explained by the MO diagram in Figure 2, which is in turn interpreted as an interacting composite molecule of ZnPc and C60. Figure 4 displays electrochemical data of 5, 8, and 9 as a reference in o-DCB containing 0.1 m TBAP as a supporting

Figure 4. Cyclic voltammograms of 5, 8, and 9 as a reference in o-DCB containing 0.1 mol dm3 TBAP. Sweep rate was 100 mV s1 unless otherwise stated.

moiety. It is obvious from Figure 4 that the redox couples of 5 are not the simple summation of those of 8 and 9. The first oxidation potential of 5 (0.64 V vs Ag/AgCl) is similar to that of 8 (0.56 V vs Ag/AgCl), indicating that the HOMO of 5 is localized on the ZnPc moiety, consistent with the HOMO calculated from the DFT calculations. The observed positive shift by ca. 80 mV compared to 8 suggests that the ZnPc moiety is stabilized under the influence of the C60 unit as predicted by the HOMO energy levels in Figure 2. The electrochemical reduction potentials indicate that the first reduction couple at 0.83 V of 5 in o-DCB containing the electrolytes is somewhat similar to that of 9 (0.66 V), and differs more markedly from the first reduction couple of 8 (1.07 V). The RDV shows the second reduction couple of 5 (1.24 V) is a two-electron process, indicating that the first reduction of the ZnPc and the second reduction of C60 sites take place at very similar potentials. These electrochemically obtained reduction potentials in polar media in the presence of electrolytes suggest the LUMO energy levels are not always consistent with those of the DFT calculations under the non-polar medium. Since previously reported ZnPc–C60 and ZnPor–C60 conjugates have C60-centered LUMO in polar and non-polar media, this kind of medium-sensitive LUMO is, therefore, one of the characteristics of 5 first observed in this study. From the Rehm–Weller equation, the energy of the charge-separated state of 5 could be evaluated to be 1.5 eV in o-DCB using the first oxidation potential (0.64 V of ZnPc) and the first reduction couple (0.83 V of C60), which implies the charge-separated state to be ZnPc· + C60·.[18] Since this energy of the charge-separated state is lower than the energy level of the lowest singlet excited state of ZnPc (1.80 eV, 1ZnPc*) as evaluated from the cross point of the normalized absorption and fluorescence spectra, it is possible to evaluate the free energy of the charge separation through 1ZnPc*, which could be evaluated to be 0.30 eV in o-DCB. This negative value increases with solvent polarity. The absorption spectra of the one-electron oxidized species of 5 could be obtained by bromine oxidation (Figure 5), which shows a practically identical absorption spectrum to the reported Pc cation radical,[19] supporting the DFT calcu-

electrolyte; the evaluated redox potential data are collected in Table 2. The redox potentials were determined on the basis of cyclic voltammetry (CV). Rotating disc voltammetry (RDV) was also performed in order to determine the number of electrons associating with each redox couple. Since ZnII does not undergo redox processes within this potential window in o-DCB for general Pcs, the observed processes are considered to arise from either the Pc-ring or C60

Table 2. Redox potentials/V vs Ag/AgCl in o-DCB containing 0.1 m TBAP. compd. 5 8 9

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2nd red

1st red

1st ox

0.83 1.07 0.66

0.64 0.56

1.55

1.24 1.35 1.03

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Figure 5. Absorption spectral development of 5 upon bromine oxidation in CHCl3.

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lated HOMO and electrochemical oxidation potential. Figure 6 exhibits spectroscopic development during the reduction of 5 (A) and 8 (B) in THF using a sodium mirror

Fluorescence and pico-second transient absorption spectra of 5 have also been recorded in order to understand its photodynamics. Figure 7 shows fluorescence spectra of 5 in tolu-

Figure 7. Steady state fluorescence spectra of 5 in toluene (solid line) and in benzonitrile (dashed line).

Figure 6. Absorption spectral development of 5 (A) and 8 (B) upon sodium reduction in THF.

technique. During the course of the reduction, the principal absorption bands in the 600–700 nm region lost their intensity, and new absorption bands with medium intensity appeared in the ca. 530–540, 610–620, and 920–930 nm regions for both 5 and 8. These spectral developments are similar to the absorption spectra reported for the radical anion of ZnPc.[20] These observations for 5 and 8 lend support that the unpaired electron localizes mainly on the ZnPc moiety to generate the ZnPc anion radical as an ion pair with a Na + -counter cation in THF. Although this is in agreement with the DFT calculated LUMO, it is not in agreement with the electrochemical reduction. In the case of the electrochemical reduction performed in the presence of TBAP in o-DCB, the radical anions usually exist as free ions. On the other hand, in the radical anion of 5, Na + -counter cation contacts with the anionic center in THF; nitrogen containing ZnPc may attract Na + to stabilize the ZnPc radical anion, rather than the all carbon C60 moiety. Finally, although the spectral envelope of the anion radicals of 5 and 8 became very similar to each other, the appearance of a weak shoulder at 1100 nm was found for 5. Since the C60 anion radical has the weaker absorption band in the 1000–1100 nm region, this 1100 nm shoulder suggests the movement of Na + to the C60 moiety to stabilize the C60 anion radical as a minor equilibrium.

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ene (solid line) and in benzonitrile (dashed line). In toluene, the fluorescence peak was observed at 685 nm with the fluorescence quantum yield of 0.028, indicating that the fluorescence of 5 is quenched relative to ZnPc(tert-butyl)4 (FF = 0.37 in benzene).[21] Changing the solvent from toluene to more polar benzonitrile, the overall quenching efficiency greatly increased (FF < 0.002). In addition, the lifetime of the fluorescence is shortened in benzonitrile (75 ps (92 %) and 2.1 ns (8 %)) compared to that in toluene (1.8 ns, Figure 8); the latter lifetime is almost the same as 8 (1.9 ns

Figure 8. Fluorescence decay profile of 5 in toluene (solid line) and benzonitrile (dashed line).

in toluene). The fluorescence quenching of the ZnPc intensity and lifetime of 5 suggests the generation of the chargeseparated state through 1ZnPc* after photo-excitation of 5 in benzonitrile. This implies that the energy level localizing on the C60 moiety is lower than that of the ZnPc moiety; that is, the LUMO is localized on the C60 moiety, whereas the LUMO+1 is localized on the ZnPc moiety, consistent with the electrochemically evaluated LUMO and LUMO+1. The pico-second transient absorption studies were therefore performed aiming to detect the charge separated states. After photo-excitation of 8, rapid increases of the transient absorption in the 400–600 nm region with absorption peak at 440 nm and shoulder at 520 nm were observed as shown

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FULL PAPERS in Figure 9 C. Since the absorption intensity reaches a maximum at 500 ps, this absorption can be attributed to 1ZnPc*. In the case of 5 in toluene (Figure 9 B), quite similar transi-

cal anion, although both species show quite similar absorption bands.[19, 22] In addition, an absorption appeared at 1000 nm due to the C60 radical anion, supporting the generation of the radical ion pair, ZnPc· + –C60·.[22] This implies that the LUMO localized on the C60 moiety is lower than the unoccupied MO of the ZnPc moiety (LUMO+1) of 5 in benzonitrile. The absorption peak of ZnPc· + at 930 nm begins to decay from 60 ps to 100 ps, giving the lifetime of ZnPc· + – C60· to be about 40 ps, which is short because of the close distance between the radical cation center and the radical anion center of ZnPc· + –C60·.

Conclusions

Figure 9. Pico-second transient absorption spectra of (A) 5 in benzonitrile, (B) 5 in toluene, and (C) 8 in toluene. The photoexcitation wavelength is 388 nm.

ent absorption bands to those of 8 were observed, generating only 1ZnPc*; however, the rise rate of 1ZnPc* of 5 was slower than that of 8, indicating that 1ZnPc* of 5 is generated indirectly. This implies that there is a photo-excited state higher than 1ZnPc*, although the upper ZnPc* cannot be considered as a higher photo excited state, since 8 did not show this kind of slow rise. Therefore, 1C60* is most conceivable; that is, the C60 moiety of 5 is directly photo-excited with laser light, generating 1C60*, from which singlet–singlet energy transfer occurs to the ZnPc moiety. The observed slight ZnPc-fluorescence quenching of 5 in toluene is attributed to a substituent effect. For 5 in benzonitrile (Figure 9 A), on the other hand, the absorption peak at 440 nm with a shoulder at 520 nm of 1ZnPc* appeared 10 ps after the laser light-pulse; 1ZnPc* decays quickly with a lifetime of 60–100 ps, in agreement with the fluorescence lifetime (75 ps) of 5 in benzonitrile. Concomitant with the fast decay of 1ZnPc*, a sharp transient absorption band appeared at ca. 930 nm reaching a maximal intensity at 60 ps, which can be ascribed to the ZnPc radical cation residue, but not the radi-

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In summary, we have succeeded in obtaining a novel ZnPc– C60 conjugate by employing a cycloaddition reaction of C60 with a ZnPc derivative containing a pyridazine moiety. In the conjugate 5, the ZnPc and C60 units are connected directly by covalent bonds, giving the fused ZnPc–C60 molecule. Thus, the distance between the ZnPc and C60 is minimized compared with previously reported Pc–C60 and Por– C60 conjugates. Spectroscopic elucidation with the aid of DFT calculations clarified that conjugate 5 shows significant MO interaction between the ZnPc and C60. Owing to the electronegative nature of C60, the proximate geometrical arrangement of the ZnPc and C60 units causes the stabilization of the LUMOs of the ZnPc moiety. As a consequence, the LUMO of 5 localizes on the ZnPc moiety in non-polar media, and shifts to the C60 moiety in polar media. Photophysical measurements indicate that conjugate 5 shows a very fast charge-separated process through 1ZnPc* in benzonitrile after photo excitations, as revealed by the picosecond fluorescence quenching and transient absorption measurements. In toluene, on the other hand, photoexcited 5 deactivates to the ground state, which is consistent with the DFT-calculated MO properties of 5. In this study, we have succeeded in demonstrating that the LUMO of the Pc– C60 conjugate 5 is sensitive to medium properties such as polarity. As a consequence, the LUMO of 5 localizes on the C60 moiety in polar solvent, while it localizes on the ZnPc moiety in non-polar solvent.

Experimental Section Measurements Electronic absorption spectra were recorded with Hitachi U-3410 and JASCO V-570 spectrophotometers. Absorption spectra of chemically reduced species were obtained by employing a conventional sodium-mirror technique in THF. MCD spectra were recorded with a JASCO J-725 spectrodichrometer equipped with a JASCO electromagnet, which produces magnetic fields of up to 1.09 T with both parallel and antiparallel fields. The MCD magnitudes were expressed in terms of molar ellipticity per tesla ([q]M/deg dm3 mol1 cm1 T1). Fluorescence spectral measurements were made with a Hitachi F-4500 spectrofluorometer. Fluorescence quantum yields (fF) were determined by using rhodamine B in EtOH (fF = 0.97) as the standard.[23] The picosecond time-resolved fluorescence spectra were measured using an argon-ion pumped Ti:sapphire

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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.[24] Picosecond transient absorption measurements were carried out using the SHG (388 nm) of a Ti:sapphire laser as the excitation source. The 1H NMR measurements were made using a JEOL GSX-400 spectrometer with CDCl3 or [D5]pyridine as the solvent. Mass spectra were recorded on Perspective Biosystem MALDI-TOF Mass Voyager DCE-S12 and Micromass LCT ESI-TOF MS spectrometers. CV and DPV measurements were recorded using a Hokuto Denko HZ5000 potentiostat under purified nitrogen in o-dichlorobenzene (o-DCB) solutions with tetrabutylammonium perchlorate (TBAP, 0.1 mol dm3) as a supporting electrolyte. Measurements were made with a glassy carbon electrode (area = 0.07 cm2), an Ag/AgCl reference electrode, and a Pt wire counter-electrode. The sweep rates were 100 mV s1 and 10 mV s1 for CV and DPV measurements, respectively. Rotating-disc voltammentry was conducted on Hokuto Denko HR-301 with a glassy carbon electrode at a rotation speed of 50 rpm. For spectroelectrochemical measurements, an optically transparent thin-layer electrode cell containing degassed sample solution was employed using a Pt minigrid as both working and counter electrodes and an Ag/AgCl wire as a reference electrode. Molecular Orbital Calculations The Gaussian 03 software package[25] was used to carry out DFT and TDDFT calculations using the B3LYP functional with 6-31G(d) basis sets putting molecules in the non-polar solvents. Synthesis Compound 3: Lithium (ca. 10 mg) was heated at 100 8C in 1-hexanol (ca. 2 mL) until all of the metal was dissolved. The bath temperature was set at 150 8C, and 4-tert-butylphthalonitrile (450 mg, 2.44 mmol) and 4,5-dicyanopyridazine[26, 27] (109 mg, 0.89 mmol) were added successively. The temperature was maintained with stirring for 30 min, and cooled to room temperature. One drop of acetic acid was added to the mixture. The solution was loaded to a short silica gel column, and hexanol and impurities flowed out using hexane as eluent. The green residue remaining on the silica gel was then eluted using CHCl3/methanol (20:1 v/v). The colored mixture was further chromatographed by using silica column (CHCl3/ methanol = 20:1 v/v), and the second green fraction was collected. Recrystallization from CHCl3/methanol gave the desired Pc containing pyridazine moiety as a green powder (160 mg), which was used for the subsequent synthesis of 4 and 5 without further purification, although this contains small amount of possible other derivatives, such as Pcs with two pyridazine units. MALDI-MS (m/z): 684 ([M + ] for 3). Compounds 4 and 5: Compound 3 (105 mg, 0.15 mmol) and C60 (315 mg, 0.44 mmol) were added to freshly distilled 1-chloronaphthalene (30 mL), and heated at 280 8C for 2.5 h. The mixture was chromatographed using a silica gel column and toluene/hexane = 1:4 (v/v) as eluent to remove 1chloronaphthalene and the unreacted C60, then the eluent was changed to CHCl3 to give a blue solid. Recrystallization from CHCl3/methanol afforded 21 mg of 4 (10 %). A mixture of 4 (21 mg, 0.015 mmol) and zinc acetate dihydrate (53 mg, 0.24 mmol) was dispersed in 1,2-dichloroethane/ethanol (2:1 v/v), and reacted at 100 8C for 1.5 h in the dark. The solvent was evaporated, and the crude products were imposed on an alumina column using first toluene to remove unreacted 4, and then toluene/methanol (20:1 v/v) as eluent to give 5 in about 30–40 % yield. The unreacted 4 was collected, and the reaction was performed in the same method fashion again. Finally, 5 was obtained almost quantitatively. 1

H NMR (400 MHz, CDCH3): for 4, d = 1.13–0.92 (m, 2 H; NH), 1.81– 1.91 (m, 27 H; tBu), 7.79–9.47 ppm (m, 11 H; ar and C60-H); MALDI-MS (m/z): 1376 ([M + ] for 4), ESI-MS (m/z): 1439 ([M + +1] for 5); elemental analysis calcd (%) for C102H40N8 (4): C 88.94, H 2.93, N 8.13; found: C 87.90, H 3.21, N 7.76. Compounds 7 and 8: Lithium (ca.10 mg) was heated at 100 8C in 1-hexanol (ca. 2 mL) until all of the metal was dissolved. The bath temperature was set at 150 8C, and a mixture of 4-tert-butylphthalonitrile (202 mg, 1.10 mmol) and fumalonitrile (60 mg, 0.77 mmol) was added. The reac-

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tion was continued at this temperature for 30 min. After cooling, a drop of acetic acid was added, and the mixture was loaded to a short silica gel column in order to remove hexanol and impurities using hexane as eluent. The green residue on the silica gel was eluted using CHCl3. The crude product was further purified using a silica gel column (CHCl3). The second fraction was collected, and recrystallized from CHCl3/methanol to give 7 (> 1 mg) as a blue solid. The zinc complex, 8, was synthesized in a similar manner to 5. 1H NMR (400 MHz, CDCH3): for 7, d = 0.83–0.72 (m, 2 H), 1.73–1.82 (m, 27 H), 7.50–9.25 ppm (m, 11 H); 1H NMR (400 MHz, CD5N): for 8, d = 1.62 (m), 7.34–9.86 ppm (m); MALDI-MS (m/z): 632 ([M + ] for 7), 694 ([M + ]for 8).

Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research on Innovative Areas (No. 20108007, “p-Space“) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. T.F. is grateful to the Exploratory Research Program for Young Scientists from Tohoku University. N.H. thanks the Sasakawa Scientific Research Grant from the Japan Science Society.

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