Twisted, Two-Faced Porphyrins as Hosts for Bispyridyl Fullerenes

J. Phys. Chem. C 2008, 112, 10559–10572

10559

Twisted, Two-Faced Porphyrins as Hosts for Bispyridyl Fullerenes: Construction and Photophysical Properties Yongshu Xie,†,¶ Jonathan P. Hill,*,† Amy Lea Schumacher,‡ Atula S. D. Sandanayaka,§ Yasuyuki Araki,§ Paul A. Karr,‡ Jan Labuta,3 Francis D’Souza,*,‡ Osamu Ito,*,§ Christopher E. Anson,# Annie K. Powell,# and Katsuhiko Ariga† Supermolecules Group, WPI-Center for Materials Nanoarchitectonics, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, Department of Chemistry, Wichita State UniVersity, 1845 Fairmount, Wichita, Kansas 67260-0051, Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Aoba-ku, Sendai, 980-8577 Japan, Faculty of Mathematics and Physics, Charles UniVersity, V HolesoVickach 2, 180 00 Prague 8, Czech Republic, and Institut fu¨r Anorganische Chemie, UniVersita¨t Karlsruhe, Engesserstrasse 15, Karlsruhe, Germany D-76128 ReceiVed: April 2, 2008

A new class of multichromophoric host compounds capable of binding guest species through two-point coordinative interaction is reported. The hosts contain appended zinc porphyrin (ZnP) moieties connected covalently by acetylene linkages to a central oxoporphyrinogen (OxP) unit through its central macrocyclic nitrogen atoms. Orientation of the ZnP groups was controlled to some extent by variation of the substitution pattern at the OxP N-substituent. Up to four porphyrin appendages are accommodated, and two cofacial bis-porphyrin interaction sites can be created in orthogonal geometry within the same molecule. The multiporphyrin hosts, termed “twisted, two-faced porphyrins”, abbreviated as OxP-(MPx)n where M ) 2H or Zn, n ) 2 or 4, and x ) meta (m) or para (p), and their complexes with a bis-4-pyridyl-substituted fullerene derivative were investigated using spectroscopic, electrochemical, and photochemical methods. Porphyrin fluorescence emission of the OxP-(ZnPx)n compounds is quenched substantially compared to that of pristine ZnP with this quenching being more significant for OxP-(ZnPx)2 than for the OxP-(ZnPx)4. The electrochemically determined highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO-LUMO) gap for the OxP-(ZnPx)n series is ∼1.65 eV. For the OxP-(ZnPx)n series, charge separation from the singlet excited ZnP to OxP occurs, while, for their complexes with the bis-pyridyl fullerene derivative, charge separation appears to occur predominantly from the singlet excited porphyrin to fullerene. Introduction Porphyrins and fullerenes are two of the most well-studied supramolecular components because of their well-defined electrochemical and photophysical properties.1,2 Their importance is reinforced by the complementarity of the aforementioned properties of these two families of compounds, and is bolstered by their now well-developed synthetic chemistries.3 The chemistry of porphyrins and fullerenes is further enhanced by the natural affinity between the tetrapyrrole macrocycle and the curved surface of the fullerene, which has resulted in a variety of attractive supramolecular systems.4 Of course, one of the main sources of interest for materials containing these two species has been the possibility of generating photoinduced * To whom correspondence should be addressed. (J.P.H.) Supermolecules Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba 3050044, Japan. Phone: +81-29-851-3354. Fax: +81-29-860-4706. E-mail: [email protected]. (F.D.) Department of Chemistry, Wichita State University, 1845 Fairmount, Wichita, Kansas, 67260-0051. E-mail: [email protected]. (O.I.) Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai, 980-8577 Japan. E-mail: [email protected]. † National Institute for Materials Science. ‡ Wichita State University. § Tohoku University. 3 Charles University. # Universita ¨ t Karlsruhe. ¶ Present Address: Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, P. R. China.

charge-separated complexes for comparison with biologically occurring photosynthetic processes, and this has become a major subject of investigation.5 In particular, charge-separated states with long lifetimes have been achieved in systems bearing multiple redox- and photoactive entities.6 These systems have illustrated the power of synthetic and supramolecular chemistries in the realization of complex photochemical activity.7 Various means have been suggested for optimizing interaction between porphyrins and fullerenes. These have included the “jaws”8 or “pacman”9 porphyrins and their binding of fullerene species through π-π stacking interactions. Generally, construction has been achieved using linkers that permit or enforce a cofacial orientation of porphyrins suitable for a double porphyrin-fullerene interaction. Linkers such as ortho- or metasubstituted phenyl groups,10 naphthyl11 or anthracenyl,12 and metal coordination,13 among others,14 have been employed. However, more complex linkers including calixarenes15 or even double linkers16,17 have been introduced in order to modulate fullerene binding and influence energy transfer and electron transfer processes. In previous work,18 we chose to combine “jaws” porphyrintype geometry with an electron-deficient oxoporphyrinogen (OxP), a tetrapyrrolic linking unit. Since this linking unit possesses four sites for substitution, two at each face of the tetrapyrrole, two porphyrins may be positioned on each of the opposing faces of the molecule so that two cofacial bis-porphyrin interaction sites can be created in orthogonal geometry in the

10.1021/jp8028209 CCC: $40.75  2008 American Chemical Society Published on Web 06/21/2008

10560 J. Phys. Chem. C, Vol. 112, No. 28, 2008

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SCHEME 1: Structures of Twisted, Two-Faced Porphyrins

same molecule (in Scheme 1, OxP-(ZnPp)2). In the present study, we have extended this contorted Janus-type arrangement of binding sites, leading us to term these new compounds “twisted, two-faced porphyrins”, abbreviated as OxP-(MPx)n where M ) 2H or Zn, x ) meta (m) or para (p), and n ) 2 or 4 (Scheme 1). In a recent study,19 we constructed donor-acceptor supramolecular triads and heptads by coordinating a monoimidazole-appended fullerene to the zinc porphyrin (ZnP) entities ofOxP-(ZnPp)2).Inthepresentstudy,furthernoveldonor-acceptor supramolecular tetrads and heptads have been constructed by coordinating a bis-4-pyridyl-appended fullerene (py2C60) to the ZnP entities of OxP-(ZnPx)n, as shown in Scheme 2. The bisfunctionalized fullerene is expected to yield stable complexes with defined distance and orientation of the resulting polyads as a result of the “two-point” binding approach. The resulting supramolecular systems exhibit different topology so that interesting redox and photophysical properties can be expected. Additionally, since the linking unit, OxP, is electron deficient (it is in fact a π-extended quinone) it should not be innocent in energetic and electron transfer processes occurring within the polychromophoric systems and should compete with guest fullerenes in any such processes that might occur. Finally, in seeking to mimic naturally occurring electron transfer and

energetic processes, the structure of the multichromophores has been often essentially one-dimensional in configuration, in line with an electrochemical gradient between electron donor and ultimate electron acceptor. In this work, we sought not only to study the corresponding properties but also to determine whether a “nonlinear” spatial arrangement of the different chromophores would lead to effects related to competition, for instance, between acceptors in energetic or electron transfer processes. Results and Discussion Synthesis. Eight compounds were prepared based on the 5,10,15,20-tetrakis(3,5-di-t-butyl-4-oxocyclohexadien-2,5ylidene)porphyrinogen skeleton (OxP) with porphyrins substituted at the macrocyclic nitrogen atoms through 4-ethynylbenzyl linkages as illustrated in Scheme 1. The conformations of the N-benzylated derivatives of 3,5-di-t-butyl-4-oxocyclohexadien2,5-ylideneporphyrinogen have been previously assigned by X-ray structure determinations20 and suggested the possibility of cofacial substitution of nitrogen atoms N21 and N23 with subsequent N-alkylation at the opposing face on N22 and N24. Porphyrins substituted at the 3-position (meta, m) and at the 4-position (para, p) of the N-substituent benzyl group are employed. Thus, compounds OxP-(MPm)2 and OxP-(MPp)2

Twisted, Two-Faced Porphyrin Hosts for Fullerenes

J. Phys. Chem. C, Vol. 112, No. 28, 2008 10561

SCHEME 2: Supramolecular Tetrad and Heptad Assembliesa

a

The structure of ZnP is highlighted in red. An equivalent compound was used as the reference species in this work.

(M ) 2H or Zn) contain two porphyrin units representing a single binding site. Compounds OxP-(MPm)4 and OxP-(MPp)4 (M ) 2H or Zn) possess four porphyrins, two each on opposing faces of the tetrapyrrole linker, representing two bis-porphyrin binding sites. It should be noted that disubstituted compounds contain an additional binding site at the unsubstituted nitrogen atoms. This site is capable of binding a guest through hydrogen bonding.20 Additionally, the porphyrin bears 3,5-(isopentyloxy)phenyl groups at 10,20 positions for solubility. Other tetraN-alkylated derivatives of 3,5-di-t-butyl-4-oxocyclohexadien2,5-ylideneporphyrinogen have exhibited surprisingly poor solubilities especially where the N-substituent is a planar aromatic group.21 The synthetic pathway is shown in Scheme 3. Initial Nalkylation of OxP22 using either 3- or 4-benzyl bromide gave the N21,N23-disubstituted and N21,N22,N23,N24-tetrasubstituted compounds bearing 3- or 4-bromobenzyl groups, respectively, in moderate to high yield. Chromatographic separation of the di- and tetra-N-substituted compounds is required, and the crystal structure of the tetrakis(4-bromobenzyl) derivative is shown in Figure S1 (Supporting Information) in order to illustrate the isomeric identity of the tetrasubstituted compound. The disubstituted compound contains both N-alkyl groups on the same face of OxP at opposing nitrogen atoms, as has been demonstrated previously.18,20 Ethynylation of each of these bromo-substituted compounds under Sonogashira reaction conditions using Pd(0) and tetramethylsilane (TMS)-acetylene gave the corresponding trimethylsilylacetylene-substituted compounds. Subsequent desilylation by the addition of 1.4 equiv of a 1.0 M tetra-n-butylammonium fluoride in tetrahydrofuran (THF) to solutions of the TMS-protected compounds in THF was followed by Pd(0) cross-coupling with the appropriate 5-bromo-10,20-bis[3,5-bis(3-methylbutoxy)phenyl]porphyrin, giv-

ing the desired compounds OxP-(MPx)2 and OxP-(MPx)4 (M ) 2H). Porphyrin moieties were subsequently metallated using zinc(II) acetate in refluxing chloroform/methanol. The Pd(0)catalyzed cross-coupling reaction was unsuccessful if Pd(PPh3)2Cl2 and CuI were employed, but proceeded in reasonable yields of coupled products if the milder catalytic system of triphenylarsine and tris(dibenzylideneacetone)dipalladium (ø) was used.23 The bis(4-pyridyl)-substituted fullerene24 and the 5,15-substituted porphyrin25 used in this work were prepared following literature procedures. It should be noted that the bis(4pyridyl)-substituted fullerene compound was not subject to isomer separation prior to complexation with the porphyrin hosts. One minor problem was encountered during the first Pd(0)catalyzed Sonogashira coupling to the TMS-acetylene compounds. The OxP framework could be reduced at its mesopositions, leading to macrocyclic tetrapyrroles containing both oxidized and reduced meso-substituents. This occurs presumably by the Pd(0)-catalyzed hydrogenation of the quinones to phenol groups and leads to compounds as shown in Scheme 3. Products from up to two reductions could be isolated and characterized with triply reduced species detectable by mass spectrometry. These compounds are unstable in both solution and solid state, reverting slowly to the parent N-substituted OxP. Triply reduced compounds are stable for several days in solution, while the doubly then singly reduced analogues are increasingly stable so that singly reduced compounds are stable at room temperature in solution for several weeks. According to an analysis of NMR data, the doubly reduced compound appears to be isomerically pure, with reduction at 5,15 positions (see Figure S2 for 1H NMR spectra of the reduced compounds). Reduction at the meso-positions of tetraphenylporphyrins has been reported


Xie et al.

SCHEME 3: Synthesis of Compounds OxP-(MPx)n and By-Products from the Reduction of the OxP Framework during the Coupling Reaction between N-Bromobenzyl-oxoporphyrinogens and TMS-Acetylenea

a (i) 3(4)-Bromobenzyl bromide, K2CO3, ethanol, reflux, 18 h; (ii) PdCl2(PPh3)2, CuI, trimethylsilylacetylene, THF, TEA, 80°C, 18 h; (iii) TnBAF, THF, rt, 2 h; (iv) 5-Bromo-10,20-bis[3,5-bis(3-methylbutoxy)phenyl]porphyrin, Pd2(dba)3, Ph3As, THF, 4 d; (v) Zn(OAc)2 · 2H2O, NaOAc, CHCl3/ CH3OH, 6 h.

previously by Callot et al., who described isomerism and solidstate structures of the N-alkyl phlorins.26 Optical Absorbance and Steady-State Fluorescence Emission Studies. The optical absorption spectra of the newly synthesized multimodular systems, OxP-(ZnPx)n (n ) 2 or 4; x ) m or p), contain absorption bands corresponding to both ZnP and OxP entities. The Soret band of ZnP is located at 439 nm, irrespective of the substitution through meta- or para-phenyl ring positions (Figure 1a). The OxP band is located at ∼510 nm and coincides with the visible band(s) of the porphyrins. The Soret band of the porphyrin components is red-shifted (15-18 nm) compared to that of the more usually used tetraarylporphyrins, and this shift can be attributed to the differing porphyrin substitution pattern, the most significant effect being due to the acetylenic linkage extending the porphyrin macrocyclic π-conjugation. Control experiments performed using the starting porphyrin derivatives (prior to linking OxP) confirmed this observation. The spectra shown in Figure 1a reveal that the relative intensities of the porphyrin Soret band and the OxP band correspond closely with the number of porphyrin units. Thus, the relative intensity of the OxP band in the OxP-(ZnP)2 series is almost double that observed in the OxP-(ZnP)4 series. Importantly, the peak positions do not vary appreciably, indicating a lack of intra- or intermolecular interactions between the chromophores. From the spectrum shown in Figure 1a it is also notable that the combination of porphyrins and OxP used to construct the OxP-(ZnPx)n molecules results in a substantial absorbance across the visible wavelengths of the optical spectrum. OxP and its N-substituted derivatives are weakly fluorescent compounds emitting in the 725 nm region. Figure 1b shows the fluorescence spectra of OxP-(ZnP)n (n ) 2 or 4) observed with λex ) 440 nm, which excites the ZnP moiety (95%) and, to a small extent, the OxP moiety (ca. 5%). Interestingly, as shown in Figure 1b for OxP-(ZnP)n (n ) 2 or 4), the emission

Figure 1. (a) Optical absorption spectra and (b) fluorescence emission spectra of (i) OxP-(ZnPm)2, (black) (ii) OxP-(ZnPp)2, (green) (iii) OxP-(ZnPm)4 (blue), and (iv) OxP-(ZnPp)4 (red), in o-dichlorobenzene; the fluorescence spectra were measured with λex ) 440 nm. The concentrations were 20 µM in (i) and (ii), and 10 µM in (iii) and (iv).

bands corresponding to ZnP at 612 and 667 nm and OxP at 725 nm are resolved. The ZnP emission bands are located at



Figure 2. B3LYP/3-21(G(*)-optimized structure of (a,b) OxP-(ZnPp)2 and (c) OxP-(ZnPp)4. Figures d and e represent the HOMO and LUMO of OxP-(ZnPp)2.

612 and 667 nm, while the OxP emission band is at 725 nm. The red-shifted emission bands of ZnP compared to the mesotetraarylporphyrin derivatives (10-15 nm) is due to the single ethynyl meso-substituent and the resulting extension of π-conjugation in the constituent porphyrin macrocycles. The porphyrin emission of the OxP-(ZnP)n series of compounds was found to be quenched substantially compared to that of pristine ZnP; indeed, the fluorescence intensity of predominantly excited ZnP was similar to that for the OxP due to the minor excitation with 440 nm light. As shown in Figure 1b, quenching of ZnPfluorescence was more significant in the case of the OxP-(ZnP)2 compounds than for the OxP-(ZnP)4 series because of the greater multiplicity of fluorescent ZnP versus only one OxP quencher in each molecule. In order to verify whether the quenching of ZnP-emission and the presence of OxP emission are due to energy transfer occurring from singlet excited ZnP to OxP, control experiments were performed using di- and tetraN-benzyl-substituted OxP under similar experimental conditions (i.e., same concentration and excitation wavelength). Under the conditions for direct excitation of OxP, the emission intensity of OxP contained in the OxP-(ZnP)n derivatives was similar to that observed for OxP itself. These observations indicate that the OxP emission illustrated in Figure 1b is due to direct excitation of OxP, suggesting that energy transfer from 1ZnP* to OxP is a minor process (Vide supra); rather, charge-separation from 1ZnP* to OxP is more probable.18c Computational Studies using B3LYP/3-21G(*) Methods. Although crystals of several of the present compounds could be grown, they were not suitable for crystallographic analysis. Consequently, we turned to computational methods27 to obtain

the energy-minimized structures of the OxP-(ZnPx)n derivatives. The B3LYP/3-21G(*)-optimized structures of OxP-(ZnPp)2 and OxP-(ZnPp)4 are shown in Figure 2 with the key geometry parameters summarized in Table 1. An important geometric characteristics is the face-to-face orientation of the two ZnP planes, compared with the previously reported one without acetylene bonds between the porphyrin unit and the benzyl unit where the two ZnP plains were almost in the same plain.18c,19 The disubstituted derivatives, OxP-(ZnPm)2 and OxP-(ZnPp)2, contained similar orientations of the porphyrin groups relative to the OxP core in spite of the structural changes. For OxP-(ZnPp)2 (see Figure 2a,b) the porphyrin geometry follows that expected from the orientation of the benzyl N-substituents’ 4-positions, while, in OxP-(ZnPm)2, a twisting at the benzylic methylene group, presumably in response to steric constraints, causes the porphyrin groups to have an orientation similar to that in OxP-(ZnPp)2. However, a consequence of the twisting at the benzylic methylene groups in OxP-(ZnPm)2 is a significantly shorter Zn-Zn distance (Zn-Zn(OxP-(ZnPm)2) ) 17.4 Å, Zn-Zn(OxP-(ZnPp)2) ) 22.9 Å), which could influence the relative guest binding properties of the complexes. For OxP-(ZnPm)4 and OxP-(ZnPp)4, similar situations exist at opposing faces of the OxP core, making for an attractive tetrahedral orientation of the four porphyrin units (see Figure 2c). It is perhaps unsurprising that the porphyrin groups adopt a tetrahedral conformation to provide maximum separation of the N-substituents (Table 1). For OxP-(ZnPm)4, there exists the possibility of intramolecular stacking between porphyrin moieties attached at the same side of the macrocycle, but its energy-minimized structure also indicates a preference for a


Xie et al.

TABLE 1: B3LYP/3-21G(*)-Optimized Geometric Parameters of the Investigated Molecular and Supramolecular Assemblies angle, °

distancea, Å

a

compound

OxP-ZnPa

OxP-(ZnPm)2 OxP-(ZnPp)2 OxP-(ZnPm)4 OxP-(ZnPp)4 OxP-(ZnPm)2:py2C60 OxP-(ZnPp)2:py2C60 OxP-(ZnPm)4:(py2C60)2 OxP-(ZnPp)4:(py2C60)2

12.1 14.5 11.9 14.5 13.0 13.8 12.6 14.0

ZnP-C60

10.6 9.6 9.4 10.1

OxP-C60

ZnP-OxP-ZnP

ZnP-C60-ZnP

17.4 18.4 17.5 18.6

93 105 78 95 68 64 67 62

93 95 91 94

Average distance when there are two moieties, which are usually within ( 0.5 Å.

tetrahedral arrangement of the porphyrin groups. This is probably related to the phenyl groups at the 10,20-positions of the porphyrin since these groups might prevent a mutual close approach. This is despite several points of rotational flexibility, at the benzyl methylene or acetylene, which could favor an interporphyrin interaction. One other important factor regarding these structures is the distance between chromophores, which will affect energy and electron transfer processes within the polyads. OxP-(ZnPp)2 and OxP-(ZnPp)4, with porphyrins substituted at the 4-position of the benzyl N-substituents, have distances of around 14.2 Å between the Zn atom and the center of the OxP macrocycle, while the computed structures of OxP-(ZnPm)2 and OxP-(ZnPm)4, with porphyrins substituted at the 3-position, have distances of about 12.1 Å. The frontier highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for OxP-(ZnPp)2 are shown in Figure 2, panels d and e, respectively. The first two HOMOs are located on the ZnP entities, while the first two LUMOs are located on the OxP entity. These observations indicate the electron-deficient nature of OxP, and the electrochemical results discussed below indeed concur with this prediction.18c Electrochemical Studies Cyclic and differential pulse voltammetry studies were performed to evaluate the redox potentials of the individual entities and estimate the free-energy change associated with photoinduced electron transfer. As mentioned earlier, the OxP macrocycle possesses extended π-conjugation with four hemiquinone substituents at its periphery, so that it is expected to undergo facile oxidation and reduction processes. Furthermore, the redox-active N-substituents are expected to influence the redox potentials of the OxP core in addition to exhibiting their own redox processes. Generally, increasing the number (n) of N-substituents from 0 to 4 results in reversible reductions and anodic shifting of the oxidation potentials of the OxP macrocycle, and, because of this, complex voltammograms were expected for these polychromophoric compounds.18 However, an accurate analysis of the site of electron transfer corresponding to the individual redox entities has been possible in the present study by comparison of the voltammograms of the OxP-(ZnPx)n compounds with those of ZnP, and the bis- and tetra-N-benzylsubstituted porphyrinogens (abbreviated as OxP-(bz)2 and OxP-(bz)4). The N-benzyl substituents on the latter control compounds are redox inactive. Figures 3 and 4 show cyclic voltammograms of the bis- and tetra-N-substituted OxP derivatives with the redox data summarized in Table 2. The first oxidation and first reduction processes of OxP-(bz)2 are located at 0.48 and -1.38 V, respectively, versus Fc/Fc+ (Figure 3), while those of OxP-(bz)4

Figure 3. Cyclic voltammograms of (a) OxP-(bz)2, (b) OxP-(ZnPp)2, and (c) py2C60 in Ar-saturated o-dichlorobenzene containing 0.1 M (nBu4N)ClO4. Scan rate ) 0.1 V/s. The asterisk (*) represents a ferrocene redox couple added as an internal standard, and the vertical dashed lines show OxP-based redox processes. The different colored lines show the scanned potential range to check the reversibility individually.

are located at 0.73 and -1.34 V, respectively, versus Fc/Fc+ (Figure 4). Increasing multiplicity of N-substituents leads to an anodic shift in the potential of the oxidation processes. Nsubstitution at the OxP macrocycle by ZnP entities leads to the appearance of additional redox processes corresponding to these entities.18c,19 The peak currents correspond to the ZnP multiplicity so that currents corresponding to porphyrin redox processes in OxP-(ZnP)2 are double those for the OxP redox processes, while for OxP-(ZnP)4 they are quadrupled. The first oxidation involving ZnP is located at 0.29 V, irrespective of the number or mode of attachment (meta or para). Similarly, irrespective of the nature of N-substituents, the first reduction always involves the electron deficient OxP macrocycle. Spectroelectrochemical studies confirm this observation (see Figure S3). That is, during the first reduction of compounds in the OxP-(ZnPx)n series, peaks corresponding to the reduction of OxP were observed. The more facile reduction of the OxP entity (close to quinones) in the OxP-(ZnPx)n series suggests that it should act as an electron acceptor.18c,19 The HOMO-LUMO gap, measured as the potential difference between the first oxidation of the donor ZnP entity and the acceptor OxP entity, was found to be ∼1.65 eV for the OxP-(ZnPx)n series. The free-energy calculations were performed using the Weller


Figure 4. Cyclic voltammograms of (a) OxP-(bz)4 and (b) OxP-(ZnPp)4 in Ar-saturated o-dichlorobenzene containing 0.1 M (nBu4N)ClO4. Scan rate ) 0.1 V/s. The asterisk (*) represents a ferrocene redox couple added as an internal standard, and the vertical dashed lines show OxP-based redox processes. The different colored lines show the scanned potential range to check the reversibility individually.

approach28 to assess the feasibility of electron transfer from the singlet excited ZnP to OxP, and the data obtained are listed in Table 2. Supramolecular Donor-Acceptor Assembly Formation using Bis-pyridyl Fullerenes. The unique topology of the present OxP-(ZnPx)n (n ) 2 or 4) molecules with a V-alignment of the oppositely positioned porphyrin entities (see Figure 2b,c) provided us with an opportunity to build supramolecular donor-acceptor conjugates by adopting the well-established “two-point” coordinative binding approach using a fullerene functionalized with two pyridine coordinating ligands (py2C60), as shown in Scheme 2. The V-shape made by two ZnP entities and the oppositely positioned pyridine groups of the fullerene were together expected to form a 2:1 type complex with high binding constants due to the resulting cooperative binding effect. Figure 5a shows the spectral changes observed during the titration of py2C60, which has a main absorption band at 320 nm and quite weak bands in the visible region, to a solution of OxP-(ZnPp)2 in o-dichlorobenzene. The spectral changes include a red shift of both the ZnP Soret band (from 439 to 444 nm) and visible bands due to both ZnP and Oxp accompanied by isosbestic points at 422, 548, 593, and 608 nm, suggesting the existence of one equilibrium process in solution. Interestingly, during the titration, the band at 511 nm corresponding to OxP underwent no changes in the position of the absorption maximum, indicating a lack of interaction between the OxP and py2C60 entities. Job’s plot constructed using the spectral data yielded 1:1 complexation, confirming formation of the OxP-(ZnPp)2:py2C60 tetrad (Scheme 2). Similar spectral changes were observed during the complexation of py2C60 with other OxP-(ZnP)2 molecular systems. However, for the OxP-(ZnP)4 series, 2 equiv of py2C60 are involved in the complexation, resulting in the formation of heptads of the type shown in Scheme 2 (see Figure S4 for representative spectral changes and Job’s plot). The formation constants calculated by construction of Benesi-Hildebrand plots were K ∼ 2 × 105 M-1 for 1:1 complex formation in o-dichlorobenzene, as listed in Table 2. The higher magnitude compared to the K values reported earlier for single-point bound

J. Phys. Chem. C, Vol. 112, No. 28, 2008 10565 complexes (K ∼ 1 × 104 M-1 for 1:1 complex formation)19,29 indicates higher stability as a result of the “two-point” binding approach. To investigate the geometry of the supramolecular donoracceptor assemblies, computational studies using B3LYP/321G(*) were performed. Figure 6 shows representative examples of the tetrad and heptad assemblies, while the key geometric parameters are listed in Table 1. As predicted from the binding constant values, the computed structures revealed the “twopoint” binding as their least energy structures. In the supramolecular tetrad, the Zn-Zn distance between the ZnP:pyC60py: ZnP fragment was ∼14.2 Å, closer by 3-5 Å compared to the Zn-Zn distances prior to C60py2 binding. The center-to-center distance between ZnP and fullerene was found to be around 10 Å and was shorter by ∼3 Å compared to the center-to-center distance between ZnP and OxP entities. Similar dimensions were found in the case of the heptad. That is, closer ZnP:pyC60py: ZnP and ZnP-C60 distances were noted. It is noteworthy that, in the heptad, the two fullerenes are located as far as 37 Å apart. It also proved possible to generate the frontier orbitals for the supramolecular donor-acceptor complexes of these substantially sized systems! In all cases, the HOMO was located on one of the ZnP groups, while the LUMO was located on the fullerene (see Figure 6c,d for representative frontier orbitals) so that initial formation of ZnP•+. and C60•- during photoinduced electron transfer reactions was envisioned. Further, voltammetric analysis was extended to the supramolecular tetrads and heptads. The first three reductions of fullerene, py2C60, were found at -1.12, -1.50, and -2.04 V versus Fc/Fc+ (see Figure 3c for voltammogram). The addition of OxP-(ZnPp)n to form the supramolecular complexes did not change the redox potentials of the fullerene entity significantly, although ZnP oxidation revealed a small (∼20 mV) cathodic shift upon axial coordination. The electrochemically evaluated HOMO(ZnP)-LUMO(py2C60) gap for the tetrad and heptad was found to be ∼1.40 V, which is smaller by 250 mV than the HOMO-LUMO gap of ZnP-OxP donor-acceptor dyads. Table 2 contains the free-energy for electron transfer via the singlet excited states of the C60 entity. Steady-state fluorescence spectroscopy of OxP-(ZnPp)n upon binding to py2C60 was also investigated. As shown in Figure 5b and Figure S4c and observed by the predominant excitation of the ZnP, the already quenched fluorescence peaks of the ZnP and OxP entities of OxP-(ZnPp)2 and OxP-(ZnPp)4 undergo additional quenching upon binding of py2C60, indicating that additional photochemical events occur in the tetrads and heptads. The additional fluorescence quenching of ZnP with py2C60 can be attributed to the exothermic charge-separation from 1ZnP* to py2C60, which was also observed in the case for the earlier reported single-point bound complexes.19 On the other hand, fluorescence quenching of OxP by fullerene guest was not observed in the earlier reported single-point bound complexes.19 The results in the present study probably support the “pacman” geometry of the two-point bound complexes shown in Figure 6. Since the charge-separation from 1OxP* to C60 is endothermic (last column in Table 2), this quenching could be attributed to excited-state energy transfer. In order to understand the fluorescence quenching mechanism in the present molecular and supramolecular conjugates, systematic photophysical studies involving time-resolved emission and transient absorption techniques were performed, and the results are discussed below.


Xie et al.

TABLE 2: Formation Constant of Supramolecular Complex, Electrochemical Redox Potentials (E, V vs Fc/Fc), and Free-Energy Changes for Charge-Separation (-∆GCS) for the Investigated Molecular/Supramolecular Systems in o-Dichlorobenzene a

compound

OxP-(bz)2 OxP-(ZnPm)2 OxP-(ZnPp)2 OxP-(bz)4 OxP-(ZnPm)4 OxP-(ZnPp)4 py2C60

b

K

1.71 × 105 M-1 1.86 × 105 M-1 5.61 × 105 M-2 5.37 × 105 M-2

first EOx (ZnP) /V

first ERed (C60) /V

0.48 0.28 0.29 0.73 0.29 0.29

-1.38 -1.37 -1.35 -1.34 -1.37 -1.35 -1.12

-∆GCSc (OxP-1ZnP*) /eV

-∆GCSc (1ZnP*-C60) /eV

-∆GCSc (ZnP-1C60*) /eV

-∆GCSc (1OxP*-C60) /eV

0.20 0.21

0.48 0.49

0.07 0.08

-0.06 -0.05

0.19 0.21

0.47 0.49

0.02 0.02

-0.31 -0.30

a See Scheme 1 for structures. b From optical absorption data using Benesi-Hildebrand plots. c -∆GCS ) ∆E0-0 - ∆GRIP, where ∆E0-0 is the energy of the lowest excited states (2.04 eV for 1ZnP*, 1.80 eV for 1OxP*, and 1.72 eV for 1C60*, and), and ∆GRIP ) Eox - Ered + ∆GS, and ∆Gs ) (e2/(4π0))[(1/(2R+) + 1/(2R-) - 1/R(D - A))/S - (1/(2R+) + 1/(2R-))/R] where 0 and R refer to the vacuum permittivity and dielectric constant of o-dichlorobenzene; Eox and Ered are in V. The geometric parameters are used as listed in Table 1. The first oxidation and first reduction processes of OxP-(bz)2 are located at 0.48 and -1.38 V vs Fc/Fc+; those of OxP-(bz)4 are located at 0.73 and -1.34 V vs Fc/ Fc+.

Figure 5. (a) UV-visible spectral changes observed during the titration of py2C60 (2 µM each addition) with OxP-(ZnPp)2 in o-dichlorobenzene. The inset shows the Benesi-Hildebrand plot at 445 nm constructed to evaluate the binding constant. Ao is the intensity observed in the absence of py2C60 and ∆A is the change in absorption upon the addition of py2C60. (b) Fluorescence quenching of OxP-(ZnPp)2 upon py2C60 binding (2 µM each addition) in o-dichlorobenzene. λex ) 440 nm.

Time-Resolved Spectral Studies OxP-(ZnPx)n (n ) 2 or 4, x ) m or p) Multimodular Systems. Steady-state fluorescence studies revealed efficient quenching of the ZnP emission in the triads and pentads. Control experiments were suggestive of a lack of energy transfer from the singlet excited porphyrin to OxP (Vide infra). Further, freeenergy calculations were performed using the Weller approach28a to assess the feasibility of electron transfer from the singlet excited porphyrin to OxP, and the data obtained are listed in Table 2. Such calculations are suggestive of photoinduced

electron transfer from singlet excited ZnP to OxP in the OxP-(ZnP)n series of compounds.18c,19 Figure 7a and Figure S5a show the time-profile of emission decay of OxP-(ZnPm)2 and OxP-(ZnPm)4 observed by 410nm picosecond laser light, which excites both OxP and ZnP moieties in ratios of 1:2 and 1:4, respectively, in o-dichlorobenzene. Similar decay curves were obtained for the investigated OxP-(ZnPp)n series of compounds. The rapid decay monitored separately for ZnP emission (Figure 7a, black) revealed substantial quenching by OxP, in agreement with the steadystate emission behavior. All of the decay curves could be fitted satisfactorily to a monoexponential decay function. The kCS values were estimated under the assumption that the decay is due to charge separation via the excited singlet state of the ZnP moiety, and are listed in Table 3. The magnitudes of kCS and the quantum yield, ΦCS suggest an efficient chargeseparation process. Further nanosecond transient absorption studies were performed to characterize the electron transfer products. The one-electron reduced products of bis and tetra N-substituted OxP exhibit bands in the visible-near-infrared (NIR) region. For bis N-substituted OxP derivatives, these bands are located at 575 and 739 nm, respectively, while for tetra N-substituted OxP derivatives, these bands are located at 722 and 916 nm, respectively (see Figure S3) as has been revealed by spectroelectrochemical studies. Hence, in addition to the porphyrin cation radical band in the 600-650 nm region, transient absorption peaks corresponding to the reduced OxP in the NIR region are expected immediately following light excitation. The transient absorption spectra of OxP-(ZnPm)2 and OxP-(ZnPm)4 shown in Figure 7b and Figure S5b indeed revealed such bands with depletion of the original bands of OxP-(ZnPm)n by the nanosecond laser light at 532 nm, which excites both components (ca. 2:1 and ca. 1:1, respectively). However, an accurate data analysis was difficult because of the overlap of the strong absorptions of the triplet states of ZnP and OxP entities in these wavelength regions. Supramolecular Tetrads and Heptads Bearing OxP, ZnP, and C60 Entities. Figure 8a,b shows the ZnP-emission decay profiles upon coordination of 1 and 2 equiv of py2C60 to OxP-(ZnPp)2 and OxP-(ZnPp)4, (for tetrad and heptad formation), respectively, measured using the 410-nm laser light, which excites the three components in almost the same fraction. Similar results were obtained for meta derivatives, OxP-(ZnPm)2 and OxP-(ZnPm)4 (see Figure S6). The emission decay of ZnP entities revealed additional rapid emission quenching upon coordination of the fullerene entity, which is in agreement with



Figure 6. B3LYP/3-21G(*) optimized structure of the (a) OxP-(ZnPp)2:py2C60 tetrad, (b) OxP-(ZnPp)4:(py2C60)2 heptad, (c) HOMO, and (d) LUMO of OxP-(ZnPp)2:py2C60.

the steady-state emission behavior shown in Figure 5b and Figure S4c. Under these conditions, the emission decay could be fitted to a biexponential function. By using the short-lived component, the kCS values were evaluated as listed in Table 3. The magnitudes of kCS indicate that the charge-separation process for OxP-(ZnPp)2:py2C60 and OxP-(ZnPp)4:py2C60 is much more efficient than that observed for OxP-(ZnP)2 and OxP-(ZnP)4 prior to fullerene coordination. Furthermore, these values are larger than those reported for single-point bound complexes.19 To further understand the observed efficient charge-separation quenching in the tetrad and heptad compared to their precursor donor-acceptor triads and pentads, free-energy calculations were carried out using electrochemical and emission data according to Weller’s approach.28 As summarized in Table 2, the ∆GCS values for the charge-separation from the excited singlet state of ZnP (1ZnP*) to fullerene-coordinated tetrad and heptad were found to be around -0.72 eV, and are more negative than the values of ca. -0.48 eV for the chargeseparation from (1ZnP*) to OxP in OxP-(ZnP)2 and OxP-(ZnP)4, suggesting the occurrence of a much more exothermic electron transfer process in the supramolecular systems. The higher exothermicity and proximity of the ZnP-C60 entities seems to play an important role in causing efficient charge separation from 1ZnP* to the coordinated fullerene in these supramolecular donor-acceptor conjugates. Further, characterization of the electron transfer products and evaluation of the extent of charge stabilization in the supramolecular conjugates were performed using nanosecond transient absorption spectral studies. As shown in Figure 9 and Figure

S7, both the tetrads and heptads revealed characteristic bands corresponding to the fulleropyrrolidine anion radical at 1020 nm, as an ultimate proof of the charge-separation quenching process in the supramolecular donor-acceptor systems. It may be mentioned here that, when supramolecular complexes using single-point (singlet axial coordination) were adopted on a different set of OxP-ZnP compounds,19 the transient absorption spectral features were not very clear. This demonstrates the importance of the “two-point” binding strategy, which not only provides higher stability of the resulting complexes but also results in quality spectral data. The 1020 nm band is located sufficiently far from the OxP, ZnP, and fullerene triplet absorption, thus giving us an opportunity to monitor the kinetics of charge recombination, kCR. The values of kCR thus observed are given in Table 4 along with the lifetime of the radical ionpair calculated as the reciprocal of kCR. The lifetimes of the radical ion-pair range between 75-175 ns, indicating charge stabilization in the supramolecular tetrads and heptads. Compared with the lifetimes of the radical ion-pairs for “singlepoint” bound systems (110 ns for a single C60 coordinated complex and 120 ns for a bis-C60 coordinated complex),19 the lifetimes for the present “two-point” bound complexes were slightly longer (174 and 163 ns, respectively). These observations suggest that the conformational fixation in the “two-point” bound complexes prolongs the lifetimes by prohibiting any intramolecular interactions that might result from the flexible conformation of the single-point bound complexes. Energy Level Diagram. The energy level diagram for the different photochemical and photophysical events occurring in the supramolecular tetrad is shown in Figure 10; a similar


Xie et al.

Figure 8. Time-profiles of ZnP-fluorescence decays of (a) OxP-(ZnPp)2: py2C60 and (b) OxP-(ZnPp)4:(py2C60)2 in 550-650 nm in o-dichlorobenzene; λex ) 410 nm.

Figure 7. (a) Time-profile of ZnP-fluorescence decay of OxP-(ZnPm)2 in 550-650 nm in o-dichlorobenzene, λex ) 410 nm. (b) Nanosecond transient absorption spectra of 0.10 mM OxP-(ZnPm)2 observed after 532 nm laser irradiation at 0.1 µs (b) and 1.0 µs (O) in Ar-saturated o-dichlorobenzene. Inset: absorption-time profiles.

TABLE 3: Fluorescence Lifetimes,a (τf), Charge-Separation Rate Constant (kCS) and Quantum Yield (ΦCS) of the Investigated Molecular/Supramolecular Compounds in o-Dichlorobenzene compound

τf/ps 1ZnP*

kCSa/s-1 via 1ZnP*

ΦCS via 1ZnP*

OxP-(ZnPm)2 OxP-(ZnPp)2 OxP-(ZnPm)4 OxP-(ZnPp)4 OxP-(ZnPm)2:py2C60 OxP-(ZnPp)2:py2C60 OxP-(ZnPm)4:(py2C60)2 OxP-(ZnPp)4:(py2C60)2

70 71 60 62 56 60 40 55

1.4 × 1010 1.4 × 1010 1.6 × 1010 1.6 × 1010 1.7 × 1010 1.6 × 1010 2.4 × 1010 1.7 × 1010

0.96 0.96 0.97 0.97 0.97 0.97 0.98 0.97

a Lifetime (τf) of the reference compound; ZnP was evaluated to be 1900 ps in o-dichlorobenzene. The kCS and ΦCS were calculated from the following equations ((τf)ref ) 1.9 ns for ZnP): kCS ) (1/τf)sample - (1/τf)ref; ΦCS ) [(1/τf)sample - (1/τf)ref ]/(1/τf)sample.

assemblies seem complex, it has been possible to slice up the different photochemical events by systematic studies involving control experiments. The energy levels of the charge separated states in Figure 10 were evaluated from their spectral and redox data. In the absence of coordinated fullerene, charge separation from the singlet excited ZnP to OxP occurs, whose rate is referred as to kCSI. Upon coordination of fullerene using the “two-point” binding approach, charge separation appears to occur predominantly from the singlet excited porphyrin to fullerene (kCSII), as a result of energetic and proximity effects (the ZnP-C60 distance is shorter than the ZnP-OxP distance). Although the competing energy- and electron-transfer processes shown by the dashed arrows are possible from any of the three photoactive entities under light excitation, such processes are considered less likely. Furthermore, intramolecular electron transfer from OxP•- to C60 can be considered to be inefficient as a result of the long distance (∼18 Å, Table 1) between them. From the decay profiles in Figure 9, the charge recombination rate constants were evaluated (kCRII). Such data are indicative of charge stabilization in the supramolecular assemblies, which can be attributed to the inherent properties of fullerene owing to its low-energy of reorganization in electron-transfer reactions.30 This feature also negates the participation of the OxP unit in electron transfer processes so that it could be said that the charge separation between porphyrin and the OxP moiety is effectively switched off by introduction of the bis-pyridylsubstituted fullerene guest. Despite this, the OxP group remains an active chromophore within the supramolecular complexes, being involved in intercomponent energetic processes. Summary

diagram could be envisioned for the heptad. Although the photochemical events of the supramolecular donor-acceptor

In summary, we have synthesized a new class of multichromophoric host compounds, the twisted two-faced porphyrin



Figure 10. Energy level diagram showing the different photochemical events for the OxP-(ZnPp)2:py2C60 tetrad. The solid arrows show major photochemical events, while the dashed arrows show minor photochemical events.

computational studies. Charge transfer occurring in the noncomplexed OxP-(ZnPx)n host molecules could be switched to efficient photoinduced electron transfer processes from singlet excited porphyrin to fullerene upon formation of the novel supramolecular donor-acceptor complexes and the subsequent transient absorption spectral studies indicate charge stabilization. The present study nicely demonstrates the construction of welldefined supramolecular systems having up to seven photo- and redox-active entities and their applicability in light energy harvesting. In addition, the docking reaction performed by the fullerene (or other electron-deficient guest) at the biporphyrin binding site(s) in these compounds and the resulting modulation of charge transfer could be of utility as a switching mechanism in molecular electronics, and we are currently investigating this idea. Figure 9. Nanosecond transient absorption spectra of (a) 0.10 mM OxP-(ZnPp)2 with 0.11 mM py2C60, and (b) 0.05 mM OxP-(ZnPp)4 with 0.11 mM py2C60 observed by 532 nm laser irradiation at 0.1 µs (b) and 1.0 µs (O) in Ar-saturated o-dichlorobenzene. Inset: absorption-time profiles at 1000 nm.

TABLE 4: Charge Recombination Rate-Constant, and the Lifetime of the Radical Ion Pair of the Investigated Supramolecular Compounds in o-Dichlorobenzene compound

kCR/s-1 (C60•-)

τRIP/ns (C60•-)

OxP-(ZnPm)2:py2C60 OxP-(ZnPp)2:py2C60 OxP-(ZnPm)4:(py2C60)2 OxP-(ZnPp)4:(py2C60)2

5.7 × 106 6.1 × 106 1.1 × 107 1.6 × 107

174 163 92 64

receptors, which have porphyrins in a conformation appropriate for binding of guest species through the well-established “twopoint” binding motif. Up to four porphyrin appendages are accommodated on an electron-deficient OxP through N-substitution, letting up to two cofacial bis-porphyrin coordinating sites for host fullerene. Novel supramolecular tetrads and heptads with defined geometry and orientation were formed using the present strategy. The geometry and electronic structures were established using various spectroscopic, electrochemical, and

Experimental Section General. Solvents and reagents were obtained from Aldrich Chemical Co., Fischer Chemical Co., Wako Chemical Co., Tokyo Kasei Chemical Co., or Kanto Chemical Co. NMR spectra were measured from CD2Cl2 or CDCl3 solutions of the samples using a JEOL AL300BX spectrometer, with TMS as an internal standard. Electronic absorption spectra were measured using a Shimadzu UV-3600 UV/vis/NIR spectrophotometer. Mass spectra were measured using a Shimadzu-Kratos Axima CFR+ MALDI-TOF mass spectrometer with dithranol as matrix. Electrochemistry was performed using a threeelectrode system. A platinum button electrode was used as the working electrode. A platinum wire served as the counter electrode, and an Ag/AgCl electrode was used as the reference. Solutions were purged prior to electrochemical measurements using argon gas. All experiments were carried out at 25 ( 1 °C. 5,15-bis[3,5-bis(3-methylbutoxy)phenyl]porphine was prepared by acid-catalyzed condensation of 3,5-bis(3-methylbutoxy)benzaldehyde with dipyrromethane.25 Ox[T(DtBHP)P] (OxP) was prepared as previously described.18 Syntheses of OxP-(MP)n (M ) 2H and Zn, n ) 2 or 4) Compounds. The synthetic route from 1 to 5 and 9 and then to OxP-(ZnPp)2 and the other final products in Scheme 3 is

10570 J. Phys. Chem. C, Vol. 112, No. 28, 2008 described in this section; other synthetic procedures are described in the Supporting Information. N21,N23-Bis(3-bromobenzyl)-5,10,15,20-tetrakis(3,5-di-tbutyl-4-oxocyclohexadien-2,5-yli deneporphyrinogen, 1. In a 2-neck flask, a mixture of Ox[T(DtBHP)P] (0.55 g, 0.49 mmol), 3-bromobenzyl bromide (1.47 g, 5.88 mmol), K2CO3 (3.9 g, 28 mmol), and EtOH (80 mL) was refluxed under N2 with vigorous stirring for 12 h and then allowed to cool to room temperature. CH2Cl2 (160 mL) was added, and the resulting mixture was washed with H2O (3 × 160 mL), and dried over anhydrous Na2SO4. The solvent was removed under reduced pressure, and the residue was chromatographed on silica gel with gradient elution with 40% ∼ 75% dichloromethane-hexane giving 2 (95 mg, yield: 11%) and 1 (470 mg, yield 65%). 1H NMR (300 MHz, CD2Cl2, 25 °C): δ ) 9.33 (2 H, s, NH), 7.64 (4 H, d, 3J ) 2.43 Hz), 7.28 (2 H, d, 3J ) 7.80 Hz), 7.06-7.03 (6 H, m), 7.01-6.98 (6 H, m), 6.72 (2 H, d, 3J ) 7.32 Hz), 6.60 (4 H, s), 4.45 (4 H, s, benzylic-H), 1.33 [36 H, s, -C(CH3)3], 1.22 [36 H, s, -C(CH3)3] ppm. MALDI-TOF-MS (dithranol): 1466.05 [(M + H)+]; calcd. average mass for C90H104Br2N4O4 ) 1465.65. N21,N23-Bis(3-trimethylsilylethynylbenzyl)-5,10,15,20-tetrakis(3,5-di-t-butyl-4-oxocyclohe xadien-2,5-ylidene)porphyrinogen, 5. 1 (250 mg, 0.17 mmol), CuI (12.5 mg, 0.0656 mmol), and PdCl2(PPh3)2 (25 mg, 0.036 mmol) were combined in a Schlenk tube and degassed, then trimethylsilylacetylene (165 µL, 1.17 mmol) and piperidine (10 mL) were added by syringe. The solution was heated under N2 at 70 °C for 20 h. After cooling, 30 mL CH2Cl2 was added, and the dark brown solution was washed with saturated aqueous NH4Cl (3 × 50 mL) and dried over anhydrous Na2SO4. The solvent was removed under reduced pressure, and the residue was chromatographed on silica gel eluting with 65% CH2Cl2-hexane, giving 5 (145 mg, yield: 56%). 1H NMR (300 MHz, CD2Cl2, 25 °C): δ ) 9.54 (2 H, s, NH), 7.61 (4 H, d, 3J ) 2.19 Hz), 7.24 (2 H, d, 3J ) 9.00 Hz), 7.13-7.07 (6 H, m), 6.93 (4 H, d, 3J ) 2.19 Hz), 6.84 (2 H, s), 6.68 (2 H, d, 3J ) 8.04 Hz), 6.60 (4 H, s), 4.50 (4 H, s, benzylic-H), 1.36 [36 H, s, -C(CH3)3], 1.23 [36 H, s, -C(CH3)3], 0.15 [18 H, s, (CH3)3Si] ppm. MALDI-TOF-MS (dithranol): 1500.32 [(M + 2H)+]; calcd average mass for C100H120N4O4Si2 ) 1498.22. N21,N23-Bis(3-ethynylbenzyl)-5,10,15,20-tetrakis(3,5-di-tbutyl-4-oxocyclohexadien-2,5-ylidene)porphyrinogen, 9. Bu4NF (400 µL, 1.0 M, 0.4 mmol) was added to a solution of 5 (135 mg, 0.090 mmol) in THF (40 mL). After stirring for 2 h, the solvent was removed under reduced pressure. The residue was dissolved in CH2Cl2 (30 mL), washed with water (3 × 50 mL), and dried over anhydrous Na2SO4 followed by chromatography on silica gel eluting with 90% dichloromethanehexane, giving 9 (119 mg, yield 98%). 1H NMR (300 MHz, CD2Cl2, 25 °C): δ ) 9.23 (2 H, s, NH), 7.64 (4 H, d, 3J ) 2.43 Hz), 7.26 (2 H, d, 3J ) 7.8 0 Hz), 7.14-7.07 (6 H, m), 6.97-6.91 (6 H, m), 6.77 (2 H, d, 3J ) 7.80 Hz), 6.61 (4 H, s), 4.48 (4 H, s, benzylic-H), 3.03 (2 H, s, acetylene-H), 1.33 [36 H, s, -C(CH3)3], 1.23 [36 H, s, -C(CH3)3] ppm. MALDI-TOFMS (dithranol): 1355.94 [(M + 2H)+]; calcd average mass for C94H104N4O4 ) 1353.86. OxP-(2HPm)2. 9 (64 mg, 0.047 mmol), 5-bromo-10,20bis[3,5-bis(3-methylbutoxy)phenyl]porphyrin (115 mg, 0.130 mmol), AsPh3 (57 mg, 0.19 mmol), and Pd2(dba)3 (22 mg, 0.024 mmol) were combined in a Schlenk tube and degassed, then dry THF (25 mL) and triethylamine (2.3 mL) were added. The mixture was stirred at room temperature under N2 with light excluded for 4 days, then the solvents were removed under

Xie et al. reduced pressure. The residue was purified first by normal chromatography on silica gel (eluent: 85% dichloromethanehexane) and then by size exclusion chromatography (BioBeads SX-1, CH2Cl2) to afford OxP-(2HPm)2 (80 mg, yield: 57%). 1H NMR (300 MHz, CD Cl , 25 °C): 10.08 (2 H, s, meso-H), 2 2 9.64 (4 H, s, pyrrole-H), 9.47 (2 H, s, NH), 9.24 (4 H, d, 3J ) 4.14 Hz, pyrrole-H), 9.02-8.98 (8 H, m, pyrrole-H), 7.91 (2 H, d, 3J ) 7.29 Hz), 7.74 (4 H, s), 7.65 (2 H, s), 7.44 (2 H, t, 3J ) 7.65 Hz), 7.32 (8 H, s), 7.25 (8 H, d, 3J ) 5.49 Hz), 6.97 (2 H, d, 3J ) 7.02 Hz), 6.85 (4 H, s), 6.72 (4 H, s), 4.86 (4 H, s, benzylic-H), 4.13 (16 H, t, 3J ) 6.45 Hz, OCH2), 1.88 - 1.68 [24 H, m, OCH2CH(CH3)2], 1.25 [36 H, s, -C(CH3)3], 1.22 [36 H, s, -C(CH3)3], 0.93 [48 H, d, J ) 6.57 Hz, CH(CH3)2], -2.89 (4H, s, NH) ppm. UV/vis (CH2Cl2): λmax (ε/104) ) 275 (6.71), 341 (6.49), 431 (64.9), 497 (sh, 14.1), 520 (14.9), 566 (16.6), 594 (5.92), 659 (1.80). MALDI-TOF-MS (dithranol): 2965.60 [(M + 2H)+]; calcd average mass for C198H224N12O12 ) 2963.97. OxP-(ZnPm)2. A solution of Zn(OAc)2 · 2H2O (44 mg, 0.2 mmol) and NaOAc (16.5 mg, 0.2 mmol) in methanol (20 mL) was added with stirring to a solution of 1 (30 mg, 0.01 mmol) in CHCl3 (20 mL), and the solution was refluxed for 6 h. The solvents were then evaporated, and the residue was chromatographed on silica gel eluting with CH2Cl2 containing 1% Et3N. The main product (pink band) was collected, and the solvents were evaporated and then purified by chromatography over basic alumina, eluting first with dichloromethane then 0.5% methanoldichloromethane affording OxP-(ZnPm)2 (25 mg, yield: 80%).1H NMR (300 MHz, CD2Cl2, 25 °C): 10.03 (2 H, s, NH), 9.87 (2 H, s, meso-H), 9.73 (4 H, d, 3J ) 4.38 Hz, pyrrole-H), 9.11 (4 h, d, 3J ) 4.38 Hz, pyrrole-H), 8.98 (4 H, d, 3J ) 4.38 Hz, pyrrole-H), 8.75 (4 H, d, 3J ) 4.38 Hz, pyrrole-H), 7.93 (2 H, d, 3J ) 7.56 Hz), 7.80 (2 H, s), 7.68 (s, 4 H), 7.48 (2 H, t, 3J ) 7.56 Hz), 7.38 (4 H, s), 7.17 (4 H, s), 6.81-6.72 (14 H, m), 6.01 (4 H, s), 4.97 (4 H, s, benzylic-H), 3.59-3.47 (16 H, m, OCH2), 1.66-1.23 [60 H, m, OCH2CH(CH3)2, C(CH3)3], 1.16 [36 H, s, C(CH3)3], 0.83 [48 H, d, 3J ) 6.33 Hz, CH(CH3)2] ppm. UV/vis (CH2Cl2): λmax (ε/104) ) 276 (5.79), 315 (5.75), 435 (64.6), 509 (12.3), 559 (10.8), 600 (6.32). MALDI-TOFMS (dithranol): 3091.96 [(M + H)+]; calcd average mass for C198H220N12O12Zn2 ) 3090.72. OxP-(ZnPm)4. A solution of Zn(OAc)2 · 2H2O (53 mg, 0.24 mmol) and NaOAc (20 mg, 0.24 mmol) in methanol (20 mL) was added with stirring to a solution of 2 (29 mg, 0.006 mmol) in CHCl3 (20 mL) then worked up by a procedure similar to that for OxP-(ZnPm)2 affording OxP-(ZnPm)4 (27 mg, yield: 88%). 1H NMR (300 MHz, CD2Cl2, 25 °C): 9.63 (8 H, d, 3J ) 4.38 Hz, pyrrole-H), 9.57 (4 H, s, meso-H), 8.96 (8 H, d, 3J ) 4.38 Hz, pyrrole-H), 8.86 (8 H, d, 3J ) 4.62 Hz, pyrrole-H), 8.67 (8 H, d, 3J ) 4.38 Hz, pyrrole-H), 7.99 (4 H, d, 3J ) 7.56 Hz), 7.84 (4 H, s), 7.61 (8 H, s), 7.50 (4 H, t, 3J ) 7.68 Hz), 7.13 (8 H, s), 6.84 (16 H, s), 6.64 (4 H, d, J ) 7.53 Hz), 6.14 (8 H, s), 5.13 (8 H, s, benzylic-H), 3.69-3.57 (32 H, m, OCH2), 1.71-1.62 (16 H, m, CH), 1.51-1.43 (32 H, m, OCH2CH2CH), 1.23 [72 H, s, C(CH3)3], 0.85 [96 H, d, J ) 6.60 Hz, CH(CH3)2] ppm. UV/vis (CH2Cl2): λmax (ε/104) ) 282 (9.55), 312 (10.5), 435 (111), 500 (14.2), 561 (10.6), 607 (6.96). MALDI-TOFMS (dithranol): 5053.18 [(M + 2H)+]; calcd average mass for C320H348N20O20Zn4 ) 5055.87. OxP-(ZnPp)2. A solution of Zn(OAc)2 · 2H2O (30 mg, 0.14 mmol) and NaOAc (11.5 mg, 0.14 mmol) in methanol (20 mL) was added with stirring to a solution of 3 (20.5 mg, 0.0069 mmol) in CHCl3 (20 mL) then worked up by a procedure similar to that for OxP-(ZnPm)2 affording OxP-(ZnPp)2 (18 mg, yield:

Twisted, Two-Faced Porphyrin Hosts for Fullerenes 84%). 1H NMR (300 MHz, CD2Cl2, 25 °C): 10.21 (2 H, s, mesoH), 9.93 (2 H, s, NH), 9.78 (4 H, d, 3J ) 4.62 Hz, pyrrole-H), 9.36 (4 H, d, 3J ) 4.38 hz, pyrrole-H), 9.17-9.12 (8 H, m, pyrrole-H), 7.88 (4 H, d, 3J ) 7.80 Hz), 7.76 (4 H, s), 7.37 (8 H, d, 3J ) 1.71 Hz), 7.26 (4 H, s), 7.15-7.10 (8 H, m), 6.90 (4 H, s), 6.72 (4 H, s), 4.77 (4 H, s, benzylic-H), 4.18 (16 H, t, 3J ) 6.60 Hz, OCH2), 1.94-1.77 (24 H, m, OCH2CH2CH), 1.40 [36 H, s, C(CH3)3], 1.36 [36 H, s, C(CH3)3], 1.00 [48 H, d, J ) 6.57 Hz, CH(CH3)2] ppm. UV/vis (CH2Cl2): λmax (ε/104) ) 281 (5.87), 312 (5.91), 435 (88.1), 513 (14.0), 557 (13.1), 600 (8.72). MALDI-TOF-MS (dithranol): 3091.56 [(M + H)+]; calcd average mass for C198H220N12O12Zn2 ) 3090.72. OxP-(ZnPp)4. A solution of Zn(OAc)2 · 2H2O (40 mg, 0.18 mmol), and NaOAc (15 mg, 0.18 mmol) in methanol (20 mL) was added with stirring to a solution of 4 (22 mg, 0.0046 mmol) in CHCl3 (20 mL) then worked up by a procedure similar to that for OxP-(ZnPm)2 affording OxP-(ZnPp)4 (22 mg, yield: 95%). 1H NMR (300 MHz, CD2Cl2, 25 °C): 10.23 (4 H, s, mesoH), 9.83 (8 H, d, 3J ) 4.65 Hz, pyrrole-H), 9.37 (8 H, d, 3J ) 4.38 Hz, pyrrole-H), 9.19-9.13 (16 H, m, pyrrole-H), 7.99 (8 H, d, 3J ) 8.04 Hz), 7.58 (8 H, s), 7.38 (16 H, d, 3J ) 1.95 Hz), 7.14-7.07 (16 H, m), 6.90 (8 H, s), 4.91 (8 H, s, benzylicH), 4.18 (32 H, t, 3J ) 6.57 Hz, OCH2), 1.92-1.74 [48 H, m, CH2CH(CH3)2], 1.43 [72 H, s, C(CH3)3], 0.99 [96 H, d, 3J ) 6.58 Hz, CH(CH3)2] ppm. UV/vis (CH2Cl2): λmax (ε/104) ) 282 (9.75), 311 (10.9), 436 (181), 502 (17.5), 558 (14.2), 601 (9.98). MALDI-TOF-MS (dithranol): 5054.85 [(M + H)+]; calcd average mass for C320H348N20O20Zn4 ) 5055.87. Time-Resolved Emission and Transient Absorption Measurements. The picosecond time-resolved fluorescence spectra were measured using an argon-ion pumped Ti:sapphire laser (Tsunami; pulse width ) 2 ps) and a streak scope (Hamamatsu Photonics; response time ) 10 ps). The details of the experimental setup are described elsewhere.31 Nanosecond transient absorption measurements were carried out using the SHG (532 nm) of an Nd:YAG laser (Spectra Physics, Quanta-Ray GCR130, fwhm 6 ns) as the excitation source. For the transient absorption spectra in the NIR region (600-1600 nm), the monitoring light from a pulsed Xe lamp was detected with a Ge-avalanche photodiode (Hamamatsu Photonics, B2834).31 Acknowledgment. This work was supported by the World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan. This work was supported by the National Science Foundation (Grant 0453464 to F.D.), the donors of the Petroleum Research Fund administered by the American Chemical Society, and Grants-in-Aid for Scientific Research on Primary Area (417 (to O.I. and Y.A.)) and a Priority Area “Super-Hierarchical Structures” (to J.P.H. and K.A.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Supporting Information Available: Cif file for the crystal structure of 4, and additional spectral and photophysical data on OxP-(ZnPx)2 and OxP-(ZnPx)4 (x ) m or p) and their fullerido-bis-pyridyl complexes. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) The Porphyrin Handbook; Kadish, K. M., Smith K. M., Guilard, R. Eds.; Academic Press: New York, 2000; Vol. 8. (2) (a) Fullerenes: Chemistry and Reactions; Hirsch, A., Brettreich, M., Eds.; Wiley VCH: Weinheim, 2004. (b) Fullerenes: Principles and Applications; De La Puente, F. L., Nierengarten, J.-F., Eds.; Royal Society of Chemistry: London, 2007. (c) Sotzmann, A.; Mattay, J. In CRC-Handbook

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