FULL PAPER DOI: 10.1002/chem.201002333

Mimicking Photosynthetic Antenna-Reaction-Center Complexes with a (Boron Dipyrromethene)3–Porphyrin–C60 Pentad Jian-Yong Liu,[a] Mohamed E. El-Khouly,[b, c] Shunichi Fukuzumi,*[b, d] and Dennis K. P. Ng*[a] Abstract: A highly efficient functional mimic of the photosynthetic antennareaction-center complexes has been designed and synthesized. The model contains a zinc(II) porphyrin (ZnP) core, which is connected to three boron dipyrromethene (BDP) units by click chemistry, and to a C60 moiety using the Prato procedure. The compound has been characterized using various spectroscopic methods. The intramolecular photoinduced processes of this pentad have also been studied in detail

with steady-state and time-resolved absorption and emission spectroscopic methods, both in polar benzonitrile and nonpolar toluene. The BDP units serve as the antennae, which upon excitation undergo singlet–singlet energy transfer to the porphyrin core. This is then followed by an efficient electron transfer Keywords: boron dipyrromethenes · electron transfer · fullerenes · photosynthesis · porphyrins

to the C60 moiety, resulting in the formation of the singlet charge-separated state (BDP)3–ZnPC + –C60C , which has a lifetime of 476 and 1000 ps in benzonitrile and toluene, respectively. Interestingly, a slow charge-recombination process (kTCR = 2.6  106 s 1) and a longlived triplet charge-separated state (tTCS = 385 ns) were detected in polar benzonitrile by nanosecond transient measurements.

Introduction [a] Dr. J.-Y. Liu,+ Prof. D. K. P. Ng Department of Chemistry and Center of Novel Functional Molecules The Chinese University of Hong Kong Shatin, N. T., Hong Kong (China) Fax: (+ 852) 2603-5057 E-mail: [email protected] [b] Dr. M. E. El-Khouly,+ Prof. S. Fukuzumi Department of Material and Life Science Graduate School of Engineering, Osaka University Suita, Osaka 565-0871 (Japan) Fax: (+ 81) 6-6879-7370668-797-370 E-mail: [email protected] [c] Dr. M. E. El-Khouly+ Department of Chemistry, Faculty of Science Kafr El-Sheikh University Kafr El-Sheikh 33516 (Egypt) [d] Prof. S. Fukuzumi Department of Bioinspired Science Ewha Womans University, Seoul 120-750 (Korea) [+] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201002333. This includes the normalized absorption spectra of 7, 9, and 10 in benzonitrile, excitation spectra of 9 and 10 in benzonitrile, nanosecond transient spectra of 8–10 in deaerated benzonitrile, and 1H and 13C{1H} NMR spectra of the new compounds.

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Mimicking photosynthetic functions by using synthetic model compounds is important for our understanding of bioACHTUNGREenergetic processes. There has been considerable interest in the development of bioinspired artificial photosynthetic systems.[1–3] A vast number of photo- and redox-active building blocks have been selected and assembled to mimic the primary events of natural photosynthesis, which include light harvesting, photoinduced multistep electron transfer, and catalysis. The studies are important not only for the photochemical conversion of solar energy into fuels,[1a, 4] but also for the construction of various optoelectronic devices.[5] Owing to their biological relevance and favorable properties, porphyrins have been widely used as key components in such models, both for energy-transfer and electron-transfer processes.[6–8] As another major class of functional materials, fullerene derivatives exhibit remarkable electron-accepting properties and low reorganization energies, rendering these compounds as ideal electron acceptors for incorporation into artificial reaction centers.[8, 9] As a consequence, a number of reaction-center mimics based on porphyrin–fullerene conjugates have been prepared and studied.[8] The charge-separation efficiencies of some of these systems are comparable to those found in natural systems. In addition,

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antenna functionality has also been introduced to reactioncenter mimics.[10] In these systems, singlet–singlet energy transfer occurs, followed by charge separation, thereby mimicking photosynthetic antenna-reaction-center complexes. Both covalent[10a–e] and supramolecular[10f,g] approaches have been adopted for the assembly of these components. Apart from porphyrins, boron dipyrromethenes (BDPs) are also of particular interest as light-harvesting units.[11] Generally, they exhibit large extinction coefficients, high fluorescence quantum yields, reasonably long excited singlet-state lifetimes, and good solubility and stability in many solvent systems.[12] More importantly, the BDP core can be readily modified to tailor the absorption and emission properties. The major absorptions of BDPs (  500 nm) are complementary to those of porphyrins (  430 and 500–600 nm). Therefore, the resulting conjugates absorb over a broad range in the visible region. For these conjugates, there is also good spectral overlap between the BDP (i.e., energy donor) emission and the porphyrin (i.e., energy acceptor) absorption. Both of these features are desirable for efficient intramolecular energy transfer. Hence, a combination of BDPs, porphyrins, and fullerenes is highly desirable for enhancing the light-harvesting efficiency throughout the solar spectrum, and for converting the harvested light into the high-energy state of charge separation by photoinduced electron transfer.[13] In fact, several studies have already demonstrated the excited-state interactions between light-harvesting BDP subunits and energy acceptors such as zinc(II) porphyrins,[13] perylenediimides,[14] subphthalocyanines,[15] and zinc(II) and silicon(IV) phthalocyanines.[9c, 16] A few BDP-containing supramolecular systems have been successfully constructed by coordinating C60 derivatives functionalized with either a pyridine or imidazole ligand to zinc(II) porphyrins or phthaloACHTUNGREcyanines.[9c, 13a,b] However, to the best of our knowledge, covalently-linked BDP–porphyrin–fullerene conjugates so far remain extremely rare.[13c] Herein we report the synthesis and photophysical properties of a novel pentad in which three BDP units serving as the antennae are linked covalently to a zinc(II) porphyrin– C60 conjugate. This pentad mimics the charge separation of the reaction center (Figure 1). Compared with the axially bound BDP–zinc(II)–porphyrin/phthalocyanine and C60 supramolecular systems,[9c, 13a,b] this covalent pentad has advantage that the photogenerated charge-separated states possess significant stability. The photoinduced energy- and electron-transfer processes of this pentad have been studied in detail by steady-state absorption and fluorescence spectroscopy, and using femtosecond and nanosecond laser photolysis techniques.

Results and Discussion Synthesis and characterization: Scheme 1 shows the synthetic route used to prepare this pentad. Mixed condensation of

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Figure 1. Structure of the pentad (BDP)3–ZnP–C60 and illustration of the photoinduced intramolecular processes involved.

4-(prop-2-ynyloxy)benzaldehyde (1) and 4-hydroxybenzaldehyde (in 3:1 mole ratio) with pyrrole gave the “3 + 1” porphyrin 2, which could be isolated from the reaction mixture by column chromatography. Treatment of this compound with ZnACHTUNGRE(OAc)2·2 H2O led to metalation, giving the zinc(II) porphyrin 3. This compound was then treated successively with 1,3-dibromopropane and 4-hydroxybenzaldehyde in the presence of K2CO3 in N,N-dimethylformamide (DMF), to afford the aldehyde 5 via the bromide 4. The porphyrin-conjugated fulleropyrrolidine 6 was prepared by the treatment of 5 with C60 and sarcosine according to the Prato procedure.[17] In the final step, click chemistry, which involves CuIcatalyzed 1,3-dipolar cycloaddition between azides and alkynes,[18] was employed to link up the porphyrin–fullerene and BDP components. The alkyne-substituted porphyrin–C60 conjugate 6 was treated with azido-BDP 7 in the presence of sodium ascorbate and CuSO4·5 H2O in a mixture of CHCl3, EtOH, and water (12:1:1 by volume) at room temperature, to give the pentad (BDP)3–ZnP–C60 (8). The reACHTUNGREaction was essentially completed in 24 h with a very good isolated yield (88 % after chromatographic purification). Similarly, the reference compound (BDP)3–ZnP (10) without the C60 moiety was prepared by treating 5,10,15,20-tetrakis[4-(prop-2-ynyloxy)phenyl]porphyrin (9) with three equivalents of azido-BDP 7 (Scheme 2). As another reference compound, the fulleropyrrolidine 11 was also prepared by the Prato reaction of C60 with 4-pyridinecarboxaldehyde.[19] All the new compounds were characterized using various spectroscopic methods, including 1H and 13C{1H} NMR spectroscopy, ESI or MALDITOF mass spectrometry, and accurate mass measurements.

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FULL PAPER

Scheme 1. Synthesis of the pentad (BDP)3–ZnP–C60 (8).

Steady-state absorption spectroscopy: To reveal the electronic interactions between the components in the pentad (BDP)3–ZnP–C60 (8), we recorded its ground-state absorption spectrum in benzonitrile, and compared it with the spectra of 7, 9, and 11, which served as the reference compounds. As shown in Figure 2, the pentad 8 exhibits five major absorption bands at 331, 434, 504, 566, and 607 nm, which are shifted at most by 2 nm compared to those of the reference compounds. This observation suggests that the chromophoric components in 8 do not have significant ground-state interactions. Similarly, the spectrum of the tetrad (BDP)3–ZnP (10) in benzonitrile overlaps well with those of 7 and 9 (Figure S1 in the Supporting Information), indicating that the BDP and porphyrin units in 10 are electronically decoupled in the ground state.

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Figure 2. Normalized absorption spectra of 7, 8, 9, and 11 in benzonitrile. The spectra of 7 and 8 were normalized at 504 nm, those of 8 and 9 were normalized at 434 nm, and those of 8 and 11 were normalized at 330 nm.

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Scheme 2. Synthesis of the reference compound (BDP)3–ZnP (10).

Steady-state fluorescence spectroscopy: The steady-state fluorescence spectra of compounds 7–10 in benzonitrile are shown in Figure 3. Upon excitation at 490 nm, at which only the BDP moieties have an absorption, the tetrad (BDP)3– ZnP (10) gave an emission band at 615 nm due to the zinc(II) porphyrin core. The BDP emission at  516 nm (as seen in the spectrum of BDP 7) was greatly reduced. The fluorescence quantum yield of 10 (Ff = 0.026) was found to be much smaller than that of BDP 7 (Ff = 0.35). Excitation of the reference compound 9 at the same position did not give the emission band at 615 nm. These observations clear-

Figure 3. Fluorescence spectra of BDP (7), (BDP)3–ZnP–C60 (8), ZnP (9), and (BDP)3–ZnP (10) in benzonitrile. lex = 490 nm (upper) or 550 nm (lower).

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ly indicate the occurrence of an efficient photoinduced energy transfer from the excited BDP units to the zinc(II) porphyrin core in 10. The excitation spectra of 9 and 10 were also recorded by monitoring the porphyrin emission (Figure S2 in the Supporting Information). The spectrum of 10 revealed an additional band at  500 nm, which was assigned to the BDP absorption. This result provided further evidence for the energy-transfer process in 10. Upon direct excitation at the zinc(II) porphyrin core of 10 at 550 nm, a strong emission band at 615 nm was also observed, the intensity of which was close to that of the reference compound ZnP 9 (Figure 3). The results suggest that the BDP moieties do not significantly quench the singlet excited state of porphyrin in 10 by energy- and/or electrontransfer processes. On the other hand, the fluorescence of the pentad (BDP)3–ZnP–C60 (8) was strongly quenched compared with that of the reference compound 10, both at 490 and 550 nm excitation (Figure 3). The fluorescence quantum yield of 8 (Ff = 0.006) was found to be much smaller than that of 10 (Ff = 0.026). The quenching pathways may involve energy transfer and/or electron transfer from the excited porphyrin to the C60 moiety. Energetically, these two pathways are feasible. However, as the fluorescence emission of C60 at  720 nm was not observed,[10e] the energy-transfer process can be excluded. Thus, photoinduced electron transfer may take place from the singlet excited state of the porphyrin core, which can be populated by excitation energy transfer from the peripheral BDP substituents to the covalently linked fullerene entity. Electrochemistry: To estimate the driving force for the electron-transfer processes, we studied the electrochemical properties of pentad 8 by differential pulse voltammetry in deACHTUNGREaerated benzonitrile. As shown in Figure 4, this compound exhibits its first and second oxidation processes at 0.72 and

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FULL PAPER Spectroscopic evidence for the radical ion pair formation in 8 was obtained from femtosecond transient absorption measurements in deaerated benzonitrile by using a 430 nm laser source, with which the porphyrin core was selectively excited. The time-resolved spectra at > 6 ps showed the characteristic absorption bands of the radical ion pair (BDP)3–ZnPC + –C60C (Figure 6).[21, 22] The characteristic ab-

Figure 4. Differential pulse voltammogram of (BDP)3–ZnP–C60 (8) in deaerated benzonitrile. Scan rate = 50 mV s 1.

1.20 V relative to the saturated calomel electrode (SCE); these oxidations are due to the porphyrin and BDP units, respectively. The reduction potential couples at 0.58, 0.98, and 1.45 V are assigned to the C60 moiety, whereas the potential couple at 1.18 V is due to the reduction process of the BDP units.[13c, 16] By using these electrochemical data and the Rehm–Weller equation, we estimated the free-energy change of charge separation (DGCS) in 8 (through changing from (BDP)3–1ZnP*–C60 to (BDP)3–ZnPC + –C60C in benzonitrile) to be 0.78 eV.[20] The corresponding value in toluene was also determined ( 0.43 eV) for comparison. The negative values show that this electron-transfer process is thermodynamically favorable, both in polar benzonitrile and nonpolar toluene. Furthermore, the charge separation via the triplet excited state of porphyrin (BDP)3–3ZnP*–C60 in benzonitrile (DGTCS  0.26 eV) seems feasible, whereas it is unfavorable in toluene due to energetic considerations. Femtosecond transient absorption measurements: Upon selective excitation at the BDP moieties at 470 nm, the femtosecond absorption spectrum of 10 in benzonitrile at 1 ps could be assigned to 1BDP*. However, the spectra at > 10 ps revealed an absorption band at  460 nm (Figure 5), which suggested the formation of 1ZnP*. The formation of 1ZnP* was clearly due to the excitation energy transfer from the three excited BDP moieties to the porphyrin core. The time profile of 1ZnP* showed a fast rise within the initial 400 ps, followed by a slow decay. From the rise profile, the rate of the singlet–singlet energy transfer was determined to be 2.7  1010 s 1.

Figure 5. Femtosecond absorption spectra of (BDP)3–ZnP (10) in benzonitrile (lex = 470 nm). The inset shows the time profile at 460 nm.

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Figure 6. Femtosecond absorption spectra of (BDP)3–ZnP–C60 (8) in deACHTUNGREaerated benzonitrile (lex = 430 nm). The inset shows the time profile at 1000 nm.

sorption band of the C60 radical anion in the near-IR region at  1000 nm was employed as a reliable probe to determine the rate constants for both charge-separation and charge-recombination processes. From the rise profile, the rate of charge separation (kSCS) was found to be 4.6  1011 s 1, indicating a fast charge-separation process. The rate of charge recombination (kSCR) was determined to be 2.1  109 s 1 from the decay profile. Based on this value, the lifetime of the charge-separated state (tSCS = 1/kSCR) was calculated as 476 ps, which is nearly twice as long as that of a related BDP–ZnP– C60 system.[13b] The femtosecond absorption spectra of 8 in deaerated toluene (Figure 7) showed similar features to those recorded in benzonitrile. kSCS was estimated to be 1.7  1011 s 1. From the decay profile of the C60 radical anion, kSCR and tSCS were found to be 1.0  109 s 1 and 1000 ps, respectively. The observation that the value of kSCR slightly increases along with the solvent polarity (or the lifetime of the charge-separated state is longer in a less polar solvent) can be understood if the charge-recombination process occurs in the Marcus inverted regions.[23–25] Photoinduced intramolecular events in microsecond time regions: It is essential to follow the dynamics in microsecond time regions by using nanosecond laser flash photolysis to determine the contribution of the triplet excited states to the electron-transfer processes. Upon ZnP excitation at 430 nm, the nanosecond transient spectra of the reference compound (BDP)3–ZnP (10) in deaerated benzonitrile exhibited an absorption band at  460 nm, and a number of broad absorptions in the region of 600–900 nm (Figure 8).

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Figure 7. Femtosecond absorption spectra of (BDP)3–ZnP–C60 (8) in deACHTUNGREaerated toluene (lex = 430 nm). The inset shows the time profile at 1000 nm.

Figure 8. Nanosecond transient spectra of (BDP)3–ZnP (10) in deaerated benzonitrile (0.04 mm) (lex = 430 nm). The inset shows the time profile at 460 nm.

These absorption bands are due to 3ZnP*, which was confirmed by comparison with the nanosecond transient spectra of 9 (Figure S3 in the Supporting Information). By fitting the decay of the absorption band at 460 nm with first-order kinetics, it was found that the 3ZnP* decayed to populate the ground state with a rate constant of 1.1  104 s 1. By selective excitation at the BDP units of 10 using a 500 nm laser source, the nanosecond transient spectra also showed the characteristic absorptions of 3ZnP*, but not of 3 BDP* (Figure S4 in the Supporting Information). This can be explained by considering that the initially formed 1BDP* decayed by singlet–singlet energy transfer to populate 1 ZnP* (as suggested from the steady-state emission and femtosecond spectroscopic studies), which relaxed to 3ZnP*. Finally, this triplet excited ZnP decayed to the ground state with a rate constant of 1.1  104 s 1. For the pentad (BDP)3–ZnP–C60 (8), the nanosecond transient spectra showed quite different features compared to those of 10. As shown in Figure 9, when the BDP units of 8 were excited at 490 nm, the transient spectra of 8 in deaerated benzonitrile revealed the absorption bands of 3ZnP*, which indicated the occurrence of intersystem crossing

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Figure 9. Nanosecond transient spectra of (BDP)3–ZnP–C60 (8) in deACHTUNGREaerated benzonitrile (0.05 mm) (lex = 490 nm). The inset shows the time profile at 1000 nm.

(ISC) from 1ZnP*. With the decay of 3ZnP*, the absorption bands at 650 and 1000 nm emerged, which suggested the generation of (BDP)3–ZnPC + –C60C via 3ZnP*. From the decay of the C60 radical-anion band at 1000 nm, the rate of charge recombination (kTCR) via 3ZnP* was found to be 2.6  106 s 1. Based on this value, the lifetime of the charge-separated state (tTCS = 1/kTCR) was estimated to be 385 ns, which is significantly longer than that of the reported BDP/zinc– phthalocyanine/C60 supramolecular triad (39.9 ns in toluene).[9c] The quantum yield of the triplet charge-separated state was found to be 0.27.[26] As the decay followed firstorder kinetics, this ruled out the possibility of intermolecular electron transfer. Similar spectral features were observed when the ZnP part of 8 was excited at 430 nm (Figure S5 in the Supporting Information). Energy diagrams: Figure 10 illustrates the energy-level diagrams summarizing the observed photoinduced intramolecular events of 8, in both benzonitrile and toluene. Some of the data were taken from the model compounds, and it is assumed that they are very similar to those of 8. In both solvents, 1BDP* decays to populate 1ZnP* through singlet–singlet energy transfer. The 1ZnP* then transfers an electron to the C60 unit to generate the charge-separated species (BDP)3–ZnPC + –C60C , which subsequently decays to the ground state with lifetimes of 476 and 1000 ps in benzonitrile and toluene, respectively. On the other hand, the charge separation from 3ZnP* to C60 was also observed in benzonitrile. The resulting (BDP)3–ZnPC + –C60C with a triplet spin character has a relatively long lifetime of 385 ns. Thus, for the pentad (BDP)3–ZnP–C60, the lifetimes of both the singlet and triplet radical ion pairs could be determined.

Conclusion We have prepared and characterized a novel pentad (BDP)3-ZnP-C60 (8), in which three BDP units are covalently linked to a ZnP–C60 conjugate. This pentad serves as an excellent model for artificial photosynthetic antenna-reACHTUNGREaction-center complexes. It exhibits a good spectral coverage

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FULL PAPER 9,[29] and 11[19] were prepared as described. All other solvents and reagents were of reagent grade and used as received. Chromatographic purifications were performed on silica gel (Macherey–Nagel, 70–230 mesh) and neutral alumina columns with the indicated eluents. Size-exclusion chromatography was carried out on Bio-Rad Bio-Beads S-X1 beads (200–400 mesh). 1

H and 13C{1H} NMR spectra were recorded on a Bruker AVANCE III 400 (1H, 400; 13C, 100.6 MHz) spectrometer in CDCl3 or [D6]DMSO. Spectra were referenced internally using the residual solvent (1H: CDCl3 (d = 7.26 ppm); [D6]DMSO (d = 2.50 ppm)] or solvent [13C: CDCl3 (d = 77.0 ppm); [D6]DMSO (d = 39.7 ppm)) resonances relative to SiMe4. MALDI-TOF mass spectra were taken on a Bruker Daltonics Autoflex MALDI-TOF mass spectrometer. ESI mass spectra were measured on a Thermo Finnigan MAT 95 XL mass spectrometer. Differential pulse voltammograms were measured on a BAS CV-50W voltammetric analyzer. A platinum disk electrode was used as the working electrode, and a platinum wire served as the counter electrode. SCE was used as the reference electrode. All measurements were carried out in deaerated benzonitrile containing 0.1 m [NBu4]ACHTUNGRE[ClO4] as the supporting electrolyte, at a scan rate of 50 mV s 1.

Figure 10. Energy level diagrams showing the photoinduced intramolecular events of (BDP)3–ZnP–C60 (8) in benzonitrile (upper) and toluene (lower).

in the visible region (300–700 nm), and undergoes a highly efficient energy migration from the BDP-based antennae to the ZnP core, followed by electron transfer to the C60 moiety to generate a charge-separated state (BDP)3–ZnPC + – C60C . From a mechanistic point of view, the advantage of this covalently linked pentad is that the lifetimes of both the singlet and triplet (BDP)3–ZnPC + –C60C could be determined. From the femtosecond transient absorption studies, the lifetimes of the singlet (BDP)3–ZnPC + –C60C were determined to be 476 and 1000 ps in benzonitrile and toluene, respectively. Interestingly, a relatively slow charge-recombination process (kTCR = 2.6  106 s 1) and a long-lived triplet (BDP)3–ZnPC + – C60C (tTCS = 385 ns) were detected in benzonitrile by nanosecond transient measurements. The formation of a relatively long-lived charge-separated state can be attributed to its triplet character, which makes the charge recombination back to the singlet ground state a forbidden process.

Experimental Section All the reactions were performed under an atmosphere of nitrogen. Pyrrole and propionic acid were distilled prior to use. Tetrahydrofuran (THF), toluene, and DMF were distilled from sodium benzophenone ketyl, sodium, and barium oxide, respectively. Toluene and benzonitrile used for photophysical measurements were of spectroscopic grade (Aldrich), and were used without prior purification. Compounds 1,[27] 7,[28]

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Femtosecond laser flash photolysis was conducted using a Clark-MXR 2010 laser system and an optical detection system provided by Ultrafast Systems (Helios). The source for the pump-and-probe pulses were derived from the fundamental output of the Clark laser system (775 nm, 1 mJ pulse 1, and fwhm = 150 fs) at a repetition rate of 1 kHz. A second harmonic generator introduced in the path of the laser beam provided 412 nm laser pulses for excitation. 95 % of the fundamental output of the laser was used to generate the second harmonic, and 5 % of the deflected output was used for white light generation. Prior to generating the probe continuum, the laser pulse was fed to a delay line that provided an experimental time window of 1.6 ns, with a maximum step resolution of 7 fs. The pump beam was attenuated at 5 mJ pulse 1 with a spot size of 2 mm diameter at the sample cell where it was merged with the white probe pulse in a close angle (< 108). The probe beam, after passing through the 2 mm sample cell, was focused on a 200 mm fiber optic cable which was connected to a CCD spectrograph (Ocean Optics, S2000-UV-vis for visible region and Horiba, CP-140 for NIR region) for recording the time-resolved spectra (450–800 and 800–1400 nm). Typically, 5000 excitation pulses were averaged to obtain the transient spectrum at a set delay time. The kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. Nanosecond time-resolved transient absorption measurements were carried out using the laser system provided by UNISOKU Co., Ltd. MeaACHTUNGREsurements of nanosecond transient absorption spectra were performed according to the following procedure. A deaerated solution containing the dyad was excited by a Panther OPO pumped by an Nd:YAG laser (Continuum, SLII-10, 4–6 ns fwhm) at l = 590 nm. The photodynamics were monitored by continuous exposure to a xenon lamp (150 W) as a probe light and a photomultiplier tube (Hamamatsu 2949) as a detector. TranACHTUNGREsient spectra were recorded using fresh solutions in each laser excitation. The solution was deoxygenated by purging with argon for 15 min prior to measurements. Porphyrin 2: Compound 1 (9.60 g, 0.06 mol) and 4-hydroxybenzaldehyde (2.44 g, 0.02 mol) were dissolved in propionic acid (300 mL) with stirring. The resulting solution was heated to reflux, then freshly distilled pyrrole (5.6 mL, 0.08 mol) was added dropwise. The mixture turned rapidly to a dark purple color. The mixture was stirred for 3 h under reflux conditions, cooled briefly, then concentrated to about 100 mL under reduced pressure. The mixture was precipitated by adding 200 mL of methanol. The precipitate was collected by filtration and washed with methanol (20 mL  2). The purple crude product was purified by column chromatography on neutral alumina using CHCl3/EtOH (v/v 10:1) as the eluent. The product was further purified by silica gel column chromatography using CHCl3 as the eluent (0.79 g, 5 %). 1H NMR ([D6]DMSO): d = 10.0 (br s, 1 H; OH), 8.82–8.92 (m, 8 H; pyrrole-H), 8.15 (d, J = 8.4 Hz, 6 H; ArH), 8.00 (d, J = 8.4 Hz, 2 H; ArH), 7.43 (d, J = 8.4 Hz, 6 H; ArH), 7.21 (d, J = 8.4 Hz, 2 H; ArH), 5.11 (d, J = 2.4 Hz, 6 H; CH2), 3.77 (t, J = 2.4 Hz, 3 H; alkyne-H), 2.91 ppm (s, 2 H; NH); 13C{1H} NMR

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([D6]DMSO): d = 157.7, 157.4, 135.7, 135.5, 134.4, 132.0, 131.5 (br s), 120.6, 119.7, 119.5, 114.1, 113.5, 79.6, 78.8, 55.9 ppm; MS (ESI): m/z (%): 793 (100) [M+H] + ; HRMS (ESI): m/z calcd for C53H37N4O4 [M+H] + : 793.2809; found: 793.2800. Porphyrin 3: A solution of ZnACHTUNGRE(OAc)2·2 H2O (1.10 g, 5.0 mmol) in methanol (15 mL) was added dropwise to a solution of porphyrin 2 (0.40 g, 0.5 mmol) in CHCl3 (50 mL). The mixture was heated under reflux for 3 h, and then the volatiles were removed under reduced pressure. The residue was subjected to chromatography on a silica gel column using CHCl3/MeOH (v/v 50:1) as the eluent. The product was obtained as a purple solid (0.42 g, 98 %). 1H NMR ([D6]DMSO): d = 9.95 (s, 1 H; OH), 8.87 (d, J = 4.8 Hz, 2 H; pyrrole-H), 8.78–8.85 (m, 6 H; pyrrole-H), 8.10 (d, J = 8.4 Hz, 6 H; ArH), 7.99 (d, J = 8.4 Hz, 2 H; ArH), 7.38 (d, J = 8.4 Hz, 6 H; ArH), 7.20 (d, J = 8.4 Hz, 2 H; ArH), 5.07 (d, J = 2.4 Hz, 6 H; CH2), 3.74 ppm (t, J = 2.4 Hz, 3 H; alkyne-H); 13C{1H} NMR ([D6]DMSO): d = 157.2, 157.1, 150.1, 149.8, 149.7, 136.0, 135.6, 135.5, 133.7, 132.0, 131.8, 131.7, 121.0, 120.0, 119.9, 113.9, 113.2, 79.8, 78.8, 56.0 ppm; MS (ESI): m/z (%): 854 (100) [M] + ; HRMS (ESI): m/z calcd for C53H34N4O4Zn [M] + : 854.1866; found: 854.1873. Porphyrin 4: Anhydrous potassium carbonate (0.55 g, 4.0 mmol) was added to a mixture of compound 3 (0.17 g, 0.2 mmol) and 1,3-dibromopropane (0.81 g, 4.0 mmol) in DMF (16 mL). The resulting mixture was stirred at room temperature overnight. The volatiles were then evaporated under reduced pressure, and the residue was mixed with CHCl3 (30 mL) and water (30 mL). The aqueous layer was separated and extracted with CHCl3 (30 mL  3). The combined organic fractions were dried over anhydrous MgSO4, and then filtered. The filtrate was collected and evaporated to dryness. The residue was purified by column chromatography on neutral alumina using CHCl3 as the eluent to give compound 4 as a purple solid (0.19 g, 97 %). 1H NMR (CDCl3): d = 8.96–9.01 (m, 8 H; pyrrole-H), 8.13 (d, J = 8.4 Hz, 6 H; ArH), 8.10 (d, J = 8.4 Hz, 2 H; ArH), 7.29 (d, J = 8.4 Hz, 6 H; ArH), 7.19 (d, J = 8.4 Hz, 2 H; ArH), 4.88 (d, J = 2.0 Hz, 6 H; CH2), 4.29 (t, J = 6.0 Hz, 2 H; CH2), 3.74 (t, J = 6.0 Hz, 2 H; CH2), 2.65 (t, J = 2.0 Hz, 3 H; alkyne-H), 2.46 ppm (quintet, J = 6.0 Hz, 2 H; CH2); 13C{1H} NMR (CDCl3): d = 158.2, 157.2, 150.5, 150.4, 136.1, 135.4, 135.3, 131.9, 120.7, 120.5, 112.9, 112.6, 78.7, 75.8, 65.5, 56.1, 32.5, 30.2 ppm; MS (ESI): m/z (%): 976 (100) [M] + ; HRMS (ESI): m/z calcd for C56H39BrN4O4Zn [M] + : 976.1417; found: 976.1406. Porphyrin 5: Anhydrous potassium carbonate (0.08 g, 0.6 mmol) was added to a mixture of compound 4 (0.20 g, 0.2 mmol) and 4-hydroxyACHTUNGREbenzaldehyde (0.07 g, 0.6 mmol) in DMF (30 mL). The resulting mixture was stirred at 80 8C for 10 h. The solvent was then evaporated under reduced pressure, and the residue was mixed with CHCl3 (40 mL) and water (40 mL). The aqueous layer was separated and extracted with CHCl3 (40 mL  3). The combined organic fractions were dried over anhydrous MgSO4, and then filtered. The filtrate was collected and evaporated to dryness. The residue was subjected to silica gel column chromatography using CHCl3 as the eluent, to give compound 5 as a purple solid (0.18 g, 89 %). 1H NMR (CDCl3): d = 9.82 (s, 1 H; CHO), 8.96 (d, J = 3.2 Hz, 8 H; pyrrole-H), 8.13 (d, J = 8.8 Hz, 6 H; ArH), 8.11 (d, J = 8.8 Hz, 2 H; ArH), 7.82 (d, J = 8.8 Hz, 2 H; ArH), 7.34 (d, J = 8.8 Hz, 6 H; ArH), 7.26 (d, J = 8.8 Hz, 2 H; ArH), 7.05 (d, J = 8.8 Hz, 2 H; ArH), 4.96 (d, J = 2.0 Hz, 6 H; CH2), 4.42 (t, J = 6.0 Hz, 2 H; CH2), 4.37 (t, J = 6.0 Hz, 2 H; CH2), 2.69 (t, J = 2.0 Hz, 3 H; alkyne-H), 2.46 ppm (quintet, J = 6.0 Hz, 2 H; CH2); 13C{1H} NMR (CDCl3): d = 190.7, 163.7, 158.1, 157.1, 150.3, 150.2, 136.3, 135.6, 135.4, 135.3, 131.8, 131.7, 129.7, 120.4, 120.2, 114.6, 112.8, 112.4, 78.7, 75.8, 64.8, 64.2, 56.0, 29.1 ppm; MS (ESI): m/z (%): 1016 (100) [M] + ; HRMS (ESI): m/z calcd for C63H44N4O6Zn [M] + : 1016.2547; found: 1016.2521. ZnP–C60 dyad 6: Aldehyde 5 (51 mg, 0.05 mmol) and sarcosine (69 mg, 0.78 mmol) were added to a solution of C60 (72 mg, 0.1 mmol) in toluene (50 mL). The mixture was heated under reflux for 8 h, then allowed to cool slowly to room temperature. The volatiles were then removed under reduced pressure. The residue was purified by silica gel column chromatography using toluene, and then toluene/CHCl3 (v/v 2:1), as the eluents. The product was obtained as a dark purple solid (48 mg, 54 %). 1H NMR (CDCl3): d = 8.98 (d, J = 4.8 Hz, 2 H; pyrrole-H), 8.94 (d, J = 4.4 Hz, 2 H; pyrrole-H), 8.86 (d, J = 4.4 Hz, 2 H; pyrrole-H), 8.82 (d, J = 4.8 Hz, 2 H;

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pyrrole-H), 8.18 (d, J = 8.4 Hz, 2 H; ArH), 8.07 (d, J = 7.6 Hz, 4 H; ArH), 7.78 (d, J = 8.4 Hz, 2 H; ArH), 7.56 (br s, 2 H; ArH), 7.41 (d, J = 8.8 Hz, 2 H; ArH), 7.35 (d, J = 8.8 Hz, 4 H; ArH), 7.15 (d, J = 8.4 Hz, 2 H; ArH), 7.00 (br d, J = 8.0 Hz, 2 H; ArH), 5.02 (d, J = 2.4 Hz, 2 H; CH2), 4.98 (d, J = 2.4 Hz, 4 H; CH2), 4.69 (d, J = 9.2 Hz, 1 H; CH), 4.59 (s, 1 H; CH), 4.56 (t, J = 5.4 Hz, 2 H; CH2), 4.28 (t, J = 5.4 Hz, 2 H; CH2), 3.94 (d, J = 9.2 Hz, 1 H; CH), 2.72 (t, J = 2.4 Hz, 1 H; alkyne-H), 2.70 (t, J = 2.4 Hz, 2 H; alkyne-H), 2.69 (s, 3 H; CH3), 2.40 ppm (quintet, J = 5.4 Hz, 2 H; CH2); HRMS (MALDI-TOF): m/z calcd for C125H49N5O5Zn [M] + : 1765.3049; found: 1765.3013. ACHTUNGRE(BDP)3–ZnP–C60 pentad 8: A mixture of compounds 6 (44 mg, 0.025 mmol) and 7 (110 mg, 0.27 mmol), CuSO4·5 H2O (6.8 mg, 0.027 mmol), and sodium ascorbate (10.8 mg, 0.054 mmol) in a 12:1:1 mixture of CHCl3, EtOH, and water (7 mL) was stirred at room temperature for 24 h. After removing the volatiles in vacuo, the residue was purified by silica gel column chromatography using CHCl3/methanol (100:1 v/v) as the eluent, followed by size-exclusion chromatography using THF as the eluent. The crude product was further purified by recrystallization from a mixture of CHCl3 and hexane (66 mg, 88 %). 1H NMR (CDCl3): d = 8.93 (d, J = 4.4 Hz, 2 H; pyrrole-H), 8.90 (d, J = 4.4 Hz, 2 H; pyrroleH), 8.82 (d, J = 4.4 Hz, 2 H; pyrrole-H), 8.79 (d, J = 4.4 Hz, 2 H; pyrroleH), 8.13 (d, J = 8.4 Hz, 2 H; ArH), 8.04 (d, J = 8.0 Hz, 4 H; ArH), 7.83 (s, 1 H; triazole-H), 7.80 (d, J = 8.4 Hz, 2 H; ArH), 7.77 (s, 2 H; triazole-H), 7.59 (br d, J = 7.2 Hz, 2 H; ArH), 7.31 (d, J = 8.4 Hz, 2 H; ArH), 7.20–7.28 (m, 4 H; ArH), 7.13–7.16 (m, 8 H; ArH), 6.96 (virtual t, J = 8.8 Hz, 8 H; ArH), 5.87 (s, 6 H; pyrrole-H), 5.21 (s, 2 H; CH2), 5.11 (s, 4 H; CH2), 4.67–4.76 (m, 7 H; CH2 and CH), 4.64 (s, 1 H; CH), 4.51 (t, J = 5.2 Hz, 2 H; CH2), 4.35–4.41 (m, 6 H; CH2), 4.27 (t, J = 5.2 Hz, 2 H; CH2), 4.00 (d, J = 9.6 Hz, 1 H; CH), 2.68 (s, 3 H; CH3), 2.47 (s, 18 H; CH3), 2.36–2.44 (m, 2 H; CH2), 1.36 ppm (s, 18 H; CH3); HRMS (MALDI-TOF): m/z calcd for C188H115B3F6N20O8Zn [M] + : 2992.8745; found: 2992.8711. ACHTUNGRE(BDP)3–ZnP tetrad 10: A mixture of compounds 7 (123 mg, 0.30 mmol) and 9 (89 mg, 0.10 mmol), CuSO4·5 H2O (7.5 mg, 0.03 mmol), and sodium ascorbate (11.9 mg, 0.06 mmol) in a 12:1:1 mixture of CHCl3, EtOH, and water (7 mL) was stirred at room temperature for 24 h. After removing the volatiles in vacuo, the residue was purified by silica gel column chromatography using CHCl3/methanol (100:1 v/v) as the eluent, followed by size-exclusion chromatography using THF as the eluent. The crude product was further purified by recrystallization from a mixture of CHCl3 and hexane (98 mg, 46 %). 1H NMR ([D6]DMSO): d = 8.77–8.82 (m, 8 H; pyrrole-H), 8.52 (s, 2 H; triazole-H), 8.51 (s, 1 H; triazole-H), 8.03–8.10 (m, 8 H; ArH), 7.41–7.47 (m, 6 H; ArH), 7.38 (d, J = 8.8 Hz, 2 H; ArH), 7.25 (d, J = 8.8 Hz, 6 H; ArH), 7.16 (d, J = 8.8 Hz, 6 H; ArH), 6.02 (s, 6 H; pyrrole-H), 5.45 (s, 4 H; CH2), 5.44 (s, 2 H; CH2), 5.08 (d, J = 2.4 Hz, 2 H; CH2), 4.92 (t, J = 4.8 Hz, 6 H; CH2), 4.55 (t, J = 4.8 Hz, 6 H; CH2), 3.75 (t, J = 2.4 Hz, 1 H; alkyne-H), 2.38 (s, 18 H; CH3), 1.33 ppm (s, 18 H; CH3); 13 C{1H} NMR ([D6]DMSO): d = 158.7, 157.8, 157.0, 154.8, 149.8, 143.1, 142.9, 142.1, 135.9, 135.5, 135.4, 131.7, 131.2, 129.4, 126.7, 125.6, 121.4, 120.1, 120.0, 115.5, 113.0, 79.7, 78.7, 66.5, 61.5, 55.9, 49.4, 14.4 ppm; MS (ESI): m/z (%): 2145 (100) [M + Na] + ; HRMS (ESI): m/z calcd for C119H102B3F6N19NaO7Zn [M+Na] + : 2144.7643; found: 2144.7628.

Acknowledgements This work was supported financially by a strategic investments scheme administrated by The Chinese University of Hong Kong, a Grant-in-Aid (No. 20108010), a Global COE program “the Global Education and Research Center for Bio-Environmental Chemistry” from the Japan Society of Promotion of Science (JSPS), and KOSEF/MEST through the WCU project (R31-2008-000-10010-0).

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