Oligoporphyrin arrays conjugated to [60] fullerene: Preparation, NMR analysis, and photophysical and electrochemical properties

Article abstract of DOI:10.1002/hlca.200590144

We report the synthesis and physical properties of novel fullerene - oligoporphyrin dyads. In these systems, the C-spheres are singly linked to the terminal tetrapyrrolic macrocycles of rod-like meso,meso-lmked or triply-linked oligoporphyrin arrays. Monofullerene-mono(Zn^II porphyrin) conjugate 3 was synthesized to establish a general protocol for the preparation of the target molecules (Scheme 1). The synthesis of the meso,meso-linked oligopophyrin-bisfullerene conjugates 4-6, extending in size up to 4.1 nm (6), was accomplished by functionalization (iodination followed by Suzuki cross-coupling) of the two free meso-positions in oligomers 21-23 (Schemes 2 and 3). The attractive interactions between a fullerene and a Zn^II porphyrin chromophore in these dyads was quantified as ΔG = -3.3 kcal mol^-1 by variable-temperature (VT) ¹H-NMR spectroscopy (Table 1). As a result of this interaction, the C-spheres adopt a close tangential orientation relative to the plane of the adjacent porphyrin nucleus, as was unambiguously established by ¹H- and ¹³C-NMR (Figs. 9 and 10), and UV/VIS spectroscopy (Figs. 13-15). The synthesis of triply-linked diporphyrin-bis[60]fullerene conjugate 8 was accomplished by Bingel cyclopropanation of bis-malonate 45 with two C₆₀ molecules (Scheme 5). Contrary to the meso,meso-linked systems 4-6, only a weak chromophoric interaction was observed for 8 by UV/VIS spectroscopy (Fig. 16 and Table 2), and the ¹H-NMR spectra did not provide any evidence for distinct orientational preferences of the C-spheres. Comprehensive steady-state and time-resolved UV/VIS absorption and emission studies demonstrated that the photophysical properties of 8 differ completely from those of 4-6 and the many other known porphyrin - fullerene dyads: photoexcitation of the methano[60]fullerene moieties results in quantitative sensitization of the lowest singlet level of the porphyrin tape, which is low-lying and very short lived. The meso,meso-linked oligoporphyrins exhibit ¹O₂ sensitization capability, whereas the triply-fused systems are unable to sensitize the formation of ¹O₂ because of the low energy content of their lowest excited states (Fig. 18). Electrochemical investigations (Table3, and Figs. 19 and 20) revealed that all oligoporphyrin arrays, with or without appended methano[60]fullerene moieties, have an exceptional multicharge storage capacity due to the large number of electrons that can be reversibly exchanged. Some of the Zn^II porphyrins prepared in this study form infinite, one-dimensional supramolecular networks in the solid state, in which the macrocycles interact with each other either through H-bonding or metal ion coordination (Figs. 6 and 7).

Full text of DOI:10.1002/hlca.200590144

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Oligoporphyrin Arrays Conjugated to [60]Fullerene: Preparation, NMR
Analysis, and Photophysical and Electrochemical Properties
by Davide Bonifazia), Gianluca Accorsic), Nicola Armaroli*c), Fayi Songb), Amit Palkarb),
Luis Echegoyen*b), Markus Scholla), Paul Seilera), Bernhard Jauna), and FranÁois Diederich*a)
a) Laboratorium f¸r Organische Chemie, ETH-Hˆnggerberg, CH-8093Z¸rich
(e-mail: diederich@org.chem.ethz.ch)
b) Department of Chemistry, Clemson University, 219 Hunter Laboratories, Clemson SC 29634, USA
(e-mail: luis@clemson.edu)
c
¡
) Istituto per la Sintesi Organica e la Fotoreattivita, Laboratorio di Fotochimica,
Consiglio Nazionale delle Ricerche, I-40129 Bologna (e-mail: armaroli@isof.cnr.it)
Dedicated to Professor Dr. Rolf Huisgen on the occasion of his 85th birthday
We report the synthesis and physical properties of novel fullerene oligoporphyrin dyads. In these systems,
the C-spheres are singly linked to the terminal tetrapyrrolic macrocycles of rod-like meso,meso-linked or triply-
linked oligoporphyrin arrays. Monofullerene mono(ZnII porphyrin) conjugate 3 was synthesized to establish a
general protocol for the preparation of the target molecules (Scheme 1). The synthesis of the meso,meso-linked
oligopophyrin bisfullerene conjugates 4 6, extending in size up to 4.1 nm (6), was accomplished by
functionalization (iodination followed by Suzuki cross-coupling) of the two free meso-positions in oligomers
21 23 (Schemes 2 and 3). The attractive interactions between a fullerene and a ZnII porphyrin chromophore in
these dyads was quantified as DG ˆ À3.3 kcal molÀ1 by variable-temperature (VT) 1H-NMR spectroscopy
(Table 1). As a result of this interaction, the C-spheres adopt a close tangential orientation relative to the plane
of the adjacent porphyrin nucleus, as was unambiguously established by 1H- and 13C-NMR (Figs. 9 and 10), and
UV/VIS spectroscopy (Figs. 13 15). The synthesis of triply-linked diporphyrin bis[60]fullerene conjugate 8
was accomplished by Bingel cyclopropanation of bis-malonate 45 with two C60 molecules (Scheme 5). Contrary
to the meso,meso-linked systems 4 6, only a weak chromophoric interaction was observed for 8 by UV/VIS
spectroscopy (Fig. 16 and Table 2), and the 1H-NMR spectra did not provide any evidence for distinct
orientational preferences of the C-spheres. Comprehensive steady-state and time-resolved UV/VIS absorption
and emission studies demonstrated that the photophysical properties of 8 differ completely from those of 4
6
and the many other known porphyrin fullerene dyads: photoexcitation of the methano[60]fullerene moieties
results in quantitative sensitization of the lowest singlet level of the porphyrin tape, which is low-lying and very
short lived. The meso,meso-linked oligoporphyrins exhibit 1O2 sensitization capability, whereas the triply-fused
systems are unable to sensitize the formation of 1O2 because of the low energy content of their lowest excited
states (Fig. 18). Electrochemical investigations (Table 3, and Figs. 19 and 20) revealed that all oligoporphyrin
arrays, with or without appended methano[60]fullerene moieties, have an exceptional multicharge storage
capacity due to the large number of electrons that can be reversibly exchanged. Some of the ZnII porphyrins
prepared in this study form infinite, one-dimensional supramolecular networks in the solid state, in which the
macrocycles interact with each other either through H-bonding or metal ion coordination (Figs. 6 and 7).
1. Introduction. The assembly of molecular chromophoric entities into multi-
component arrays may provide artificial systems capable of mimicking the basic
characteristics of photosynthesis, such as stepwise, photoinduced energy- and electron-
transfer processes. To generate such properties, it is essential to choose suitable
chromophoric fragments exhibiting specific electrochemical and spectroscopic proper-
¹ 2005 Verlag Helvetica Chimica Acta AG, Z¸rich

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ties and assemble them in a well-defined spatial arrangement [1]. With its strong
electron-accepting properties and remarkably small reorganization energy (ca. 0.23eV
[2]), C60 is one of the most popular chromophores that have been incorporated into
multicomponent molecular architectures [3]. Following the first reports on a fullerene-
containing donor acceptor dyad [4] and a fullerene porphyrin conjugate [5], a
myriad of fullerene porphyrin hybrids have been prepared and studied [6]. Our work
on photoactive, fullerene-containing donor acceptor dyads started with the prepara-
tion and photophysical investigation of CuI-complexed rotaxanes with fullerene
stoppers [7]. This early work was followed by the use of porphyrin tethers to
accomplish the regioselective trans-1 bisfunctionalization of C60 [8a].
Comprehensive investigations revealed that the photophysical and electrochemical
properties of conjugate 1 (Fig. 1), with two [60]fullerene moieties attached by single
linkers to the porphyrin macrocycle, were similar to those of 2 in which a single
fullerene is doubly bridged in a cyclophane-type fashion [8b]. Upon photoexcitation of
both dyads, the fullerene- and porphyrin-centered excited states are deactivated to a
low-lying charge-transfer (CT) state emitting in the near-infrared (NIR). The
spectroscopic observations suggested that a tight facing between fullerene and
porphyrin moieties does not require double cyclophane-type bridging, but can also
be established in singly-linked conjugates by taking advantage of attractive donor
acceptor interactions both in the ground and the excited state [8]. This was the starting
point for the preparation and spectroscopic characterization of the novel monopor-
phyrin 3 and the linear oligoporphyrin arrays 4 8 with one or two appended
[60]fullerene moieties (for preliminary communications on parts of this work, see
[9][10]). Two types of porphyrin arrays were considered in this investigation: in one
series, 4 7, the tetrapyrrolic macrocycles are singly linked to each other (meso,meso-
linked), whereas they are triply linked in conjugate 8. Although a large body of elegant
synthetic studies on the two types of oligoporphyrins has been published by Osuka and
co-workers [11 14], only a limited number of physical studies has been undertaken to
elucidate their electronic and photophysical characteristics [15 18]. In particular, their
chemical derivatization with other redox- and/or photoactive molecular species and
subsequent physical investigations remain largely unexplored [19].
Fig. 1. Original [60]fullerene porphyrin conjugates 1 and 2 reported by Diederich and co-workers [8]

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Here, we show that these multicomponent arrays prefer distinct conformations as a
result of strong intermolecular fullerene porphyrin interactions that could be
quantified by means of variable-temperature (VT) NMR measurements. The first full
electrochemical studies on triply-linked porphyrin dimers revealed that such com-
pounds are capable of undergoing as many as eight reversible electron-transfer
processes. Covalent conjugation with two fullerene moieties increases the number of
the electron-transfer processes to 15, which is unprecedented in non-dendritic
structures. Moreover, a comprehensive photophysical study showed that, despite the
exceptional electron-donating properties of triple-fused porphyrins, the low-lying and
very short-lived (4.5 ps) [10][15] singlet level offers an extremely competitive
deactivation pathway and thus acts as a sink for the higher-energy electronic states
of the covalently linked [60]fullerene moieties.
2. Results and Discussion.
2.1. Preparation of the [60]Fullerene Porphyrin
Conjugates. 2.1.1. Synthesis of Fullerene Porphyrin Dyad 3. A large number of
protocols for the synthesis of −asymmetrically× meso-substituted porphyrins has been
reported [20]. Mixed macrocyclizations of pyrroles [21] or meso-substituted dipyrryl-
methanes [22] with appropriate aromatic aldehydes afford tris- and tetrakis-meso-
substituted porphyrins, whereas other approaches take advantage of selective meso-
functionalization of preformed 5,15-disubstituted porphyrin scaffolds [23][24].
We opted for the latter variant to prepare the tris-meso-substituted precursors 9 and
10 on the way to conjugate 3 (Scheme 1). Thus, 5,15-diarylporphyrin 11 was readily
obtained by condensation of dipyrrylmethane 12 [25] with aldehyde 13 [26] (TFA,
CH2Cl2, for abbreviations, see the captions of Scheme 1 or Exper. Part), followed by
oxidation (DDQ, CH2Cl2). Metallation (Zn(OAc)2, MeOH) afforded ZnII porphyrin
14. For the introduction of the third meso-aryl ring by Pd-catalyzed cross-coupling
[24][27], 14 was brominated with NBS [28]; however, an unseparable mixture of meso-
and b-brominated porphyrin derivatives was obtained. In contrast, iodination (1 equiv.
I2, AgPF6, CHCl3/pyridine) selectively afforded mono-meso-iodoporphyrin 15 (63%)
besides only traces of diiodo derivative 16 [29]. Close monitoring of the reaction by
TLC (SiO2; cyclohexane/CH2Cl2 1:1) was necessary to prevent extensive decompo-
sition of the porphyrin substrate. Separation of 15 and 16 was achieved by repeated
column chromatography (SiO2; cyclohexane/CH2Cl2 1:1). Larger-scale reactions
afforded mixtures of 14, 15, and 16 from which the monoiodo derivative 15 was
isolated in yields ꢀ 40%.
In parallel, (t-Bu)Me2Si(TBDMS)-protected 17 was obtained in 95% yield by
reaction of benzyl alcohol 18 with (t-Bu)Me2SiCl (DMAP, THF). Boronate 19 was
subsequently formed by using 4,4,4',4',5,5,5',5'-octamethyl-2,2'-bi(1,3,2-dioxaborolane)
in the presence of [PdCl2(dppf)2] and AcOK. In view of its limited stability, it was used
in the next transformation without further purification (ca. 90% pure according to
1H-NMR analysis). Suzuki cross-coupling [30] between 15 and 19 ([Pd(PPh3)4],
Cs2CO3) afforded 5,10,15-trisubstituted porphyrin 10 in good yield (67%).
Removal of the (t-Bu)Me2Si protecting group with Bu4NF in THF (with a few drops
of H2O added) yielded alcohol 20. The deprotection was carefully monitored by TLC
(SiO2; cyclohexane/CH2Cl2 1:1) to avoid extensive decomposition of 10. Subsequent
conversion of 20 with ClCOCH2CO2Et in the presence of Et3N provided malonate-

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appended porphyrin 9. Bingel reaction of 9 with C60 (I2, DBU, PhMe) afforded the
desired dyad 3 as a brown solid in 45% yield. HR-FT-ICR-MALDI-TOF mass spectra
(matrix: DCTB) of 3 displayed the molecular ion as the only peak at m/z 1686.4048
‡
‡
(M , C120H62N4O4Zn ; calc. 1686.4057).
2.1.2. Synthesis of Bis[60]fullerene Oligoporphyrin Conjugates 4 6. Compounds
4 6 were prepared by the same synthetic route as described for 3. First, the meso,meso-
linked oligoporphyrin scaffolds with two, three, and four porphyrin units, 21 23,
respectively, were synthesized by oxidative coupling (AgPF6) of 14, according to
Osuka and co-workers (Scheme 2) [13]. Increasing the amount of AgPF6 from 0.5 to
0.8 equiv. improved the conversion of the starting porphyrin monomer.
Scheme 2. AgI-Promoted Oligomerization of Porphyrin 14
a) AgPF6 (0.8 equiv.), MeCN/CHCl3 1 :4, 258, 16 h; 44% (14); 25% (21); 11% (22); 7% (23).
A small dark-red crystal of dimer 21, suitable for X-ray diffraction, was obtained by
vapor diffusion of aqueous MeOH into a solution of 21 in CHCl3. The asymmetric unit
of the crystal structure contains one molecule of 21 and five MeOH molecules. The
molecular structure, depicted in Fig. 2,a, nicely reveals the nearly orthogonal arrange-
ment of the two porphyrins with an interplanar angle of ca. 848. Both ZnII ions deviate
by ca. 0.2 ä from the plane of the four surrounding pyrrolic N-atoms and, interestingly,
show two different coordination motifs. While Zn(2) is in contact with one MeOH
molecule (Zn(2) ¥¥¥ O(300) ˆ 2.25 ä) to give a penta-coordinated species, Zn(1) is in
contact with two MeOH molecules (Zn(1) ¥¥¥ O(200) ˆ 2.30 ä, Zn(1) ¥¥¥ O(500) ˆ

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Fig. 2. a) ORTEP Representation of porphyrin dimer 21 together with four MeOH molecules as determined by
X-ray-diffraction analysis. Arbitrary numbering. Atomic displacement parameters, obtained at 223K, are
drawn at the 30% probability level. Intermolecular contacts [ä]: O(200) ¥¥¥ Zn(1) ˆ 2.30; O(300) ¥¥¥ Zn(2) ˆ
2.25; O(500) ¥¥¥ Zn(1) ˆ 2.83; O(200) ¥¥¥ O(400) ˆ 2.92. The absolute values of the interplanar angles about the
C(porph)ÀC(aryl) bonds are 64.58 (C(12)ÀC(36)), 66.18 (C(24)ÀC(25)), 72.18 (C(64)ÀC(91)), 66.18
(C(76)ÀC(77)), and 83.68 (C(porph)ÀC(porph, C(18)ÀC(70)). The interplanar angles are based on the
least-square planes through the corresponding phenyl and porphyrin rings. A disordered MeOH molecule is not
shown. b) Relative arrangement of dimer 21 in the crystal packing clearly showing the p-p interactions between
the porphyrins. The t-Bu substituents on the phenyl moieties and the disordered MeOH molecules have been
omitted. c) Top view of the p-p interacting porphyrin pairs showing their relative orientation and offset. Some
substituents on the porphyrin rings have been omitted. Atom colors: blue N, red O, yellow Zn, gray C.

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2.83ä) leading to hexa-coordination. In addition, O(200) is connected to another
MeOH (O(200) ¥¥¥ O(400) ˆ 2.92 ä), while the remaining disordered MeOH is not
involved in any close contacts. The crystal packing (Fig. 2,b) shows an infinite network
in which each of the ZnII porphyrins displaying penta-coordination is involved in an
attractive p-p stacking interaction with an adjacent dimer. The p-systems of two
neighboring porphyrins are approximately parallel with an interplanar separation of ca.
3.37 3.66 ä. The distance between the two planes of N-atoms is close to 3.6 ä. One
porphyrin is shifted relative to its neighbor (parallel to the intramolecular axis C(58) ¥¥¥
C(70)) by ca. 3.45 ä (Fig. 2,c). It can be postulated that the presence of the bulky 3,5-
di(tert-butyl)phenyl substituents prevents a shorter interplanar distance and an optimal
porphyrin porphyrin arrangement in which the p-electrons of a pyrrole sit on top of
the metal center [31]. The fact that both ZnII porphyrins involved in the p-p interaction
are still coordinated to a MeOH molecule provides evidence for only a weak
electrostatic interaction between the positive charge on the Zn-atom (local charge on
Zn can be estimated to be ca. ‡ 0.4 eÀ [31]) in one porphyrin unit and the p-electrons
in the other one, which preserves the Lewis acidity of the metal centers [32].
Iodination of 21 23 (2 equiv. I2, AgPF6, CHCl3/pyridine) afforded, within 15 min,
diiodo derivatives 24 26 with complete selectivity for the meso-positions (> 70%
yield; Scheme 3). Suzuki cross-coupling of 24 26 with arylboronic ester 19 provided
the arylated porphyrins 27 29. Although the yields were good, some starting oligomers
21 23 and monosubstituted oligomers were obtained as side-products resulting from
reductive dehalogenation. While the purification of 27 proceeded smoothly by a single
column chromatography on SiO2, the separation of 28 and 29 from the undesired by-
products was unsuccessful, and the crude mixtures were directly used, without further
purification, in the next synthetic steps.
Cleavage of the (t-Bu)Me2Si protecting groups was performed in quantitative yield
with Bu4NF in THF, and the resulting diols 30 32 were easily purified by column
chromatography (SiO2; PhMe). Acylation with ClCOCH2CO2Et in CH2Cl2/Et3N 4 :1
provided bis-malonates 33 35 in yields of ca. 70%. Some demetallation of the ZnII
porphyrins was occasionally detected during the Suzuki cross-coupling and/or acylation
steps. In those cases, a remetallation of the tetrapyrrolic ligands with Zn(OAc)2 was
necessary. Cyclopropanation of C60 with 33 35 under modified Bingel conditions
afforded, after column chromatography (SiO2-H; PhMe), the targeted fullerene
porphyrin conjugates 4 6 (Scheme 3). The molecular mass of each compound was
unambiguously established by HR-FT-ICR-MALDI-MS (DCTB), which displayed as
prominent peak the molecular ion of each fullerene porphyrin conjugate: m/z
‡
‡
‡
3370.7940 (4; M , C240H122N8O8Zn2 ; calc. 3370.7963), 4117.1200 (5; M ,
‡
‡
‡
C288H172N12O8Zn3 ; calc. 4117.1290), and 4864.4307 (6; MH , C336H223N16O8Zn4 ; calc.
4864.4701). As a typical example, the spectrum of 6 is shown in Fig. 3. The structural
assignments of 4 6 were also supported by their H- and 13C-NMR spectra. All
1
fullerene porphyrin conjugates were found to be brown solids, displaying good
solubility in common organic solvents.
2.1.3. Synthesis of Mono[60]fullerene Diporphyrin Conjugate 7. The synthesis of 7
started with the iodination of meso,meso-linked diporphyrin 21 (1 equiv. I2, AgPF6) to
give the mono-iodo derivative, which was transformed into alcohol 36 by Suzuki cross-
coupling with 19 and deprotection (Scheme 4). Crude products of the iodination and

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Fig. 3. HR-FT-ICR-MALDI Mass spectrum of bis[60]fullerene oligoporphyrin conjugate 6 in the positive-ion
mode (matrix: DCTB, N2 laser: 337 nm).
desilylation reactions were used in the subsequent transformations, due to difficulties
with the purification. Acylation (36 ! 37) and Bingel addition afforded the desired Cs-
symmetric conjugate 7 which was fully characterized.
2.1.4. Synthesis of the Triply-Linked Diporphyrin C60 Conjugate 8. The intermedi-
ate 38 on the way to 8 was obtained following two routes (Scheme 5). In the first one,
Pd-catalyzed cross-coupling between 24 and phenylboronic ester 39 [33] afforded, after
chromatographic separation (SiO2; cyclohexane/CH2Cl2 1:1), compounds 40 (20%),
41 (69%), and 21 (11%). According to the protocol (DDQ, Sc(OTf)3, PhMe) reported
by Tsuda and Osuka for oxidative ring closure [11], biaryl-type dimer 41 was converted
into the triply-linked derivative 38 in almost quantitative yield. In the second route,
Suzuki cross-coupling between 15 and 39 gave, after column chromatography (SiO2;
cyclohexane/CH2Cl2 1:1), carbonitrile 42 (67%) and porphyrin 14 (23%; from
reductive dehalogenation). Homo-coupling of 42 under the above-mentioned
oxidative conditions provided, after several chromatographic purifications, the triply-
linked dimer 38 in 89% yield. While the first route afforded 38 in four steps starting
from 14 with an overall yield of 11%, the second route led to 38 in three steps in 39%
yield (from 14). The chemical structure of 38 was confirmed by HR-FT-ICR-MALDI
mass spectrometry, 1H-, 13C-, and DQF-COSY NMR spectroscopies.
Reduction of 38 with DIBAL-H at 08 gave dicarbaldehyde 43 in 94% yield
(Scheme 5). Subsequent reduction, again with DIBAL-H, afforded bis(benzyl alcohol)
44 (55%), which was transformed in 88% yield into bismalonate 45 (Scheme 5).
Modified Bingel cyclopropanation of C60 with 45 provided dyad 8 in 41% yield, after
filtration over a short plug (Al2O3; PhMe) and repeated precipitations from hexane,
followed by washings with hexane, MeOH, and Et2O. The compound is rather unstable
in concentrated solution.

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In the HR-FT-ICR-MALDI mass spectrum (matrix: DCTB) of conjugate 8, the
‡
prominent peak corresponds to the molecular ion at m/z 3371.7749 (M ,
‡
C
240H118N8O8Zn2 ; calc. 3371.7698). The 1H- and 13C-NMR, and UV/VIS spectra
further support the chemical structure of 8. Thus, the 13C-NMR spectrum of 8 displays
the following characteristic resonances: 163.57 and 163.42 ppm (2  CˆO), over-
lapping and broad signals in the range of 154.06 117.12 ppm (C(sp2) of fullerene and
diporphyrin), 70.86 ppm (fullerene C(sp3)-atom), 68.35 ppm (benzylic C(sp3)-atom),
52.74 ppm (methano bridge C-atom), and 63.46 and 14.22 ppm (ethoxy C(sp3)-atoms).
2.2. Supramolecular Networks in the Solid-State Structures of Monomeric Porphyr-
ins. Porphyrins 46 and 47 were prepared as controls for the planned physical studies,
according to synthetic strategies similar to those reported for 41 in Scheme 5 (46) and
for 20 in Scheme 1 (47). Both compounds as well as intermediate 42 show interesting
supramolecular network structures in the solid state.
Dark-red crystals of 42 and 46 were grown as solvates by slow diffusion of H2O into
solutions of the porphyrins in MeOH/CHCl3 5 :1. The ORTEP drawing of 42 is shown
in Fig. 4 (the crystal structure of 46 had been reported in [34]).
Compound 42 crystallizes in the monoclinic space group P21/n and 46 in the
othorhombic space group Pbca. In both molecules, the four pyrrolic N-atoms
coordinating to ZnII form a distorted square plane, and each ZnII ion is involved in a
short intermolecular contact to a neighboring MeOH molecule (Zn(1) ¥¥¥ O(61) ˆ
2.19 ä for 42 and Zn(1) ¥¥¥ O(69) ˆ 2.14 ä for 46). In both porphyrin complexes, the
penta-coordinated ZnII ions exhibit a square-pyramidal coordination geometry, with
the metal ion deviating from the mean plane of the four pyrrole N-atoms towards the
axial MeOH ligand by ca. 0.28 ä (42) and 0.25 ä (46), respectively. In 42, the average
ZnÀN distance is 2.07 ä, and the angles N(1)ÀZn(1)ÀN(17), N(11)ÀZn(1)ÀN(23)
are 165.28 and 163.28 (mean 164.28). In 46, the corresponding values are 2.06 ä, 164.88,
and 167.68 (mean 166.28). In the crystal packing, both compounds 42 and 46 are
arranged as infinite rod-like polymers in which the porphyrin units are connected to
each other through H-bonding interaction between a MeOH molecule coordinated to a
ZnII ion and a CꢁN group (Figs. 5 and 6). For porphyrin 42, the CꢁN ¥¥¥ O distance is
2.88 ä (N(46) ¥¥¥ O(61)) and the N ¥¥¥ HÀO angle 1638 (N(46) ¥¥¥ HÀO(61)), for 46 the
corresponding values are 2.86 ä (N(68) ¥¥¥ O(69)) and 1698 (N(68) ¥¥¥ HÀO(69)),
respectively. As a consequence of the intermolecular H-bonding networks, the
coordinative Zn ¥¥¥ O bonds are shorter than those reported for a number of penta-
coordinated porphyrinato ZnII complexes with a metal-ion-coordinated MeOH
molecule [35].

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Scheme 5. Synthesis of Triply-Fused Diporphyrin Fullerene Conjugate 8

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Scheme 5 (cont.)
a) 39, Cs2CO3, [Pd(PPh3)4], PhMe, 1008, 18 h; 20% (40); 69% (41). b) Sc(OTf)3, DDQ, PhMe, 1408, 30 min;
quant. c) 39, Cs2CO3, [Pd(PPh3)4], PhMe, 1008, 18 h; 67%. d) Sc(OTf)3, DDQ, PhMe, 1408, 30 min; 89%.
e) DIBAL-H, CH2Cl2, À 708 (2 h) ! 258 (18 h); 94%. f) DIBAL-H, CH2Cl2, À 708 (2 h) ! 258 (18 h); 55%.
g) ClCOCH2CO2Et, Et3N, CH2Cl2 1 :7, 08 (15 min) ! 258 (16 h); 88%. h) C60, I2, DBU, PhMe, 08 ! 258, 1 h;
41%. DDQ ˆ 2,3-Dichloro-5,6-dicyano-p-benzoquinone; DIBAL-H ˆ diisobutylaluminum hydride.
Small dark-red crystals of 47 were obtained by slow vapor diffusion of H2O into a
≈
solution of the porphyrin in MeOH. In the triclinic crystals (space group P1), there are
two independent molecules in the asymmetric unit (Fig. 7). While the porphyrin unit at
the center (with primed (') atoms) sits on a crystallographic center of symmetry, the
tetrapyrrolic macrocycles left and right are related by the center of symmetry. In
contrast to compounds 42 and 46, which form infinite one-dimensional chains via H-
bonded MeOH molecules, the self-assembly of porphyrin 47 is characterized by the

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Fig. 4. ORTEP Representation of porphyrin 42 with one MeOH molecule. A second solvent molecule (CHCl3)
in the crystal is omitted for clarity. Arbitrary numbering. Atomic displacement parameters obtained at 173K
are drawn at the 30% probability level. Intermolecular distance O(61) ¥¥¥ Zn(1) ˆ 2.19 ä. The absolute values of
the interplanar angles about the C(porph)ÀC(aryl) bonds are 60.48 (C(12)ÀC(47)), 84.28 (C(18)ÀC(39)), and
59.18 (C(24)ÀC(25)). The angles are based on the least-square planes through the corresponding phenyl and
porphyrin rings. Atom colors: blue N, red O, green Zn, white C.
coordination of the CH2OH residues to the metal centers of neighboring ZnII
porphyrins. Fig. 7 shows that Zn(1) is penta-coordinated due to an intermolecular
contact Zn(1) ¥¥¥ O(68') of 2.14 ä, while Zn(1'), involved in two symmetry-related
contacts Zn(1') ¥¥¥ O(46) of 2.47 ä, exhibits an octahedral coordination. As expected,
the Zn(1') ¥¥¥ O(46) distance is slightly larger than the distance Zn(1) ¥¥¥ O(68')
measured for the penta-coordinate Zn(1). The penta-coordinated Zn(1) ion is
displaced from the mean plane of the four pyrrolic N-atoms towards the coordinating
O(68')-atom by ca. 0.32 ä, and the angles N(1)ÀZn(1)ÀN(17) and
N(11)ÀZn(1)ÀN(23) are decreased to 162.78 and 163.68, respectively. Due to
symmetry, Zn(1') sits exactly in the plane of the four N-atoms. Notably, O(68') is H-
bonded to a MeOH molecule, as shown by the characteristic short intermolecular
O(71A) ¥¥¥ O(68') contact (2.72 ä).
2.3. NMR-Spectroscopic Conformational Analysis. In dyads 3 7, the C-spheres rest
atop the porphyrin plane. The tangential position of the fullerene moiety with respect
1
to the porphyrin ring was unambiguously established by H- and 13C-NMR spectros-
copy. This conformational preference is characterized by the non-equivalence of the
ortho and t-Bu H-atoms on the 3,5-di(tert-butyl)phenyl substituents, since phenyl
rotation, which exchanges the fullerene from one to the other porphyrin face, is slow on
the NMR time scale (Fig. 8). The geometrical preference is a consequence of the strong
attractive interaction between the two chromophores [8][9][36].

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Fig. 5. One-dimensional, MeOH-mediated supramolecular network of porphyrin 42 illustrating the short
intermolecular N ¥¥¥ O contacts (2.88 ä, dashed line) extending along the crystallographic b axis. The CHCl3
solvent molecules between two adjacent H-bonded columnar porphyrin arrays are also in close (CÀH ¥¥¥ O)
contact with the coordinated MeOH (C(100) ¥¥¥ O(61) ˆ 3.24 ä). The t-Bu substituents have been omitted.
Atom colors: blue N, red O, gray Zn, gray C, and green Cl.
In accordance with this reasoning, the 1H-NMR spectrum (500 MHz, CDCl3,
298 K) of 3 shows two triplets for the ortho H-atoms (Ha, Hb in Fig. 8) and two singlets
for the t-Bu H-atoms of its 3,5-di(tert-butyl)phenyl substituents. A 500-MHz
homonuclear DQF-COSY spectrum allowed the unambiguous assignment of the
resonances of these residues. The 13C-NMR (125 MHz, CDCl3, 298 K) spectrum
depicts, as expected, nine resonances for the C(sp3)-atoms and, due to some overlap, 52
out of the 56 expected resonances for the C(sp2)-atoms. Such signal pattern is in
agreement with the postulated Cs-symmetric conformation in which the two porphyrin
faces are non-equivalent.
Similarly, in 4 the two fullerenes also lie on a porphyrin plane but, as a consequence
of the orthogonal position of the two porphyrin planes, the dyad adopts a C2-symmetric
conformation (Fig. 9). The 1H-NMR (500 MHz, C2D2Cl4, 298 K) spectrum of 4
displays two and four triplets for HÀC(4) and HÀC(2), respectively, and four singlets
for the t-Bu H-atoms. The 13C-NMR (125 MHz, CDCl3, 298 K) spectrum displays 13
resonances for the C(sp3)-atoms and 79 expected resonances for the C(sp2)-atoms. The
1
500-MHz homonuclear DQF-COSY spectrum confirmed the assignment of the H
resonances. Analogous considerations are also valid for 6 which preferentially adopts a
C2-symmetric conformation with the two C-spheres nesting on the outer porphyrins. In
principle, such a conformation should be preferred by all oligomers of this type having
an even number of porphyrin units.

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Helvetica Chimica Acta Vol. 88 (2005)
1855
Fig. 7. Crystal structure of porphyrin 47 showing the intermolecular contacts between two independent molecules
of 47 and two MeOH molecules. The molecule at the center (with primed (') atoms) sits on an inversion center,
the molecules left and on the right are in general positions and related by the inversion center. Atomic
displacement parameters obtained at 203K are drawn at the 30% probability level. Intermolecular contacts
[ä]: O(68') ¥¥¥ Zn(1) ˆ 2.14; O(46) ¥¥¥ Zn(1') ˆ 2.46; O(68') ¥¥¥ O(71A) ˆ 2.72. The absolute values of the
interplanar angles about the C(porph)ÀC(aryl) bonds are 79.78 (C(12)ÀC(47)), 82.78 (C(6)ÀC(61)), 78.98
(C(24)ÀC(25)), 82.78 (C(18)ÀC(39)), 81.18 (C(6')ÀC(61')), and 81.68 (C(24')ÀC(30')). The angles are based
on the least-square planes through the corresponding phenyl and porphyrin rings. A disordered MeOH
molecule is not shown. Atom colors: blue N, red O, green Zn, white C.
Fig. 8. Schematic view of the face-to-face conformation adopted by the porphyrin fullerene conjugates (ke ˆ
rate constant of exchange)
Two conformers of 5 can be distinguished by NMR spectroscopy. The two C-
spheres can either be in a syn (C2v-symmetry) or an anti (C2h-symmetry) arrangement.
This hypothesis was confirmed by 1H- and 13C-NMR (CDCl3) analysis of conjugate 5 at
258, which revealed the presence of the two conformers in a 1:1 ratio (Fig. 10).
Whereas the two phenyl HÀC(4') protons are equivalent in the anti conformer, they
show non-equivalence in the syn-conformer. As expected, three triplets with relative
intensities 1:2 :1 are observed at 7.76, 7.67, and 7.58 ppm, respectively, for the HÀC(4')

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Fig. 10. Excerpts of the 500-MHz 1H-NMR spectrum of conjugate 5 (CDCl3, 298 K)

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Helvetica Chimica Acta Vol. 88 (2005)
protons in the 1:1 mixture of conformers. The same 1H-NMR pattern was also
observed for the t-BuÀC(3') protons. The latter are equivalent in the anti-conformer
(1.44 ppm), but split into two singlets in the syn-conformer (1.33 and 1.55 ppm).
Furthermore, the eight t-BuÀC(3) groups on the phenyl substituents of the outer
porphyrin ring form equivalent pairs in both conformers and the expected two singlets
1
are clearly observed (1.40 and 1.47 ppm) in the H-NMR spectrum (Fig. 10). Again,
both syn- and anti-conformations should also be adopted by higher oligomers of this
class with odd numbers of tetrapyrrolic macrocycles.
In sharp contrast, no evidence for a face-to-face interaction between the C-spheres
and the triply-fused porphyrins of 8 could be detected by NMR spectroscopy. In the
1H-NMR (500 MHz, CDCl3, 298 K) spectrum, the t-Bu H-atoms only display one
singlet, and in the 13C-NMR spectrum (125 MHz, CDCl3, 298 K), only two resonances
are attributed to the t-Bu groups (one to the quaternary C(sp3)-atom (34.87 ppm) and
one to the primary C(sp3)-atom (31.70 ppm)). This finding suggests that the
interchromophoric interactions in 8 are much weaker than in 4. As a consequence,
the conformation of 8 depicted in Scheme 5 is only one of many possible; conformers
with the C-spheres nesting on opposite faces of the triply-linked porphyrin dimer or
turned away from the macrocycle are equally probable.
1
By means of variable-temperature (VT) H-NMR measurements, two conforma-
tional motions were observed (Fig. 11): i) Rotation 1 around the single bond between
the terminal porphyrin rings and the 3,5-di(tert-butyl)phenyl moieties, which could be
monitored in all dyads 3 7 following the temperature-induced shifts of the HÀC(2)
and t-Bu proton resonances, and ii) Rotation 2 around the single bond between the
porphyrin and the meso-phenyl ring to which the fullerene moiety (or the silyl ether
residue in 27) is attached. This motion was monitored in dyad 4 by following the
temperature dependence of the resonances Hb(1) and Hb(2) (Fig. 11). The activation
parameters (DH=, DS=, and DG= at 298 K) correlated to these rotations were
subsequently determined and are reported in Table 1. Since the coalescence temper-
atures of the 1H resonances could not be reached due to boiling point limitations of the
used deuterated solvents, the activation parameters for the rotations in 3 and 4 were
Fig. 11. Rotatory motions observed for fullerene porphyrin
conjugates 3 7 by VT-NMR spectroscopy

Helvetica Chimica Acta Vol. 88 (2005)
1859
estimated using the method reported by Sandstrˆm [37], and applied to porphyrins by
Eaton and Eaton [38]. As an example, the Eyring plot obtained for 3 is shown in Fig. 12.
Very similar results were obtained for the two independently monitored resonances
of HÀC(2) and t-BuÀC(3). At 298 K, DG= for Rotation 1 in 3 was found to be ca.
19.2 kcal molÀ1, whereas the according values for Rotation 2 in 4 and 27 are ca. 21.4 and
18.1 kcal molÀ1, respectively.
Table 1. Rotatory Motions in Fullerene Porphyrin Conjugatesa)
Compound
Solvent
H-Atom
Rotationb)
DH=
[kcal ¥ molÀ1
DS=
[cal ¥ molÀ1 ¥ KÀ1
DG=298
[kcal ¥ molÀ1
]
]
]
3
C6D5CD3
(e ˆ 2.38)
HÀC(2)
1
18.5
À 1.6
18.9
t-BuÀC(3)
HÀC(2)
1
1
18.8
13.4
À 1.5
19.2
18.1
3
3
C2D2Cl4
(e ˆ 10.36)
À 15.8
t-BuÀC(3)
HÀC(2)
1
1
13.6
15.3
À 16.0
À 10.9
18.1
18.5
(D8)Dioxane
(e ˆ 2.25)
t-BuÀC(3)
t-BuÀC(3)
HÀC(4)
1
1
2
2
14.9
À 11.7
18.4
18.7
4
C6D5CD3
C6D5CD3
20.7
13.5
2.321.4
À 15.318.1
27
HbÀC(2)
a) Experimental uncertainty Æ 1.5 kcal molÀ1 (DH=) and Æ 3cal mol À1 KÀ1 (DS=). b) For the definition of the
rotations, see Fig. 11.
Fig. 12. Eyring plot and activation parameters from calculated rate constants (ke) for the phenyl rotation of
conjugate 3 in (D8)dioxane

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Assuming that there are no significant interactions between the (t-Bu)Me2Si groups
and the porphyrin rings in 27, we can conclude that the attractive interactions between
the fullerene and the tangential porphyrin in 4 increase the activation free enthalpy for
Rotation 2 by ca. 3.3 kcal molÀ1. In light of the flexibility of the malonate linker bearing
the fullerene moiety, it is reasonable to assume that this increase in DG=298 largely
reflects the magnitude of the ground-state interactions between the two chromophores
in C6D5CD3.
The good solubility of dyad 3 in a wide range of solvents allowed the study of the
rotary motion in other solvents such as (D8)dioxane and C2D2Cl4 (measurements
carried out in THF led to inaccurate results due to the limited accessible temperature
range). While DG=298 stays substantially unchanged (within the error range of the
measurement), DH= and DS= are strongly affected by the nature of the solvent. DH=
increases in the order C2D2Cl4 < (D8)dioxane < C6D5CD3, whereas DS= decreases in
the same order. At present, we do not have a good explanation for these observations.
2.4. Photophysical Analysis. 2.4.1. Steady-State UV/VIS Absorption Spectra
Analysis. The electronic absorption spectra of the meso,meso-linked bis[60]fuller-
ene oligoporphyrin arrays in PhMe at 298 Æ 2 K together with those of reference
compounds 14 and 21 23 are shown in Table 2 and Fig. 13. In the UV window, the
fullerene-centered absorption is stronger than that of the porphyrin, whereas in the
VIS-spectral region an opposite trend is observed. The spectra of the five conjugates
3 7 differ dramatically. In particular, a splitting of the Soret band (S2 state) due to
exciton coupling is observed for 4 7, relative to the parent monomer 3 (for UV/VIS
studies in the solid state, see [39]). Both bands show a progressive enhancement of the
molar absorption coefficient values (e) with increasing number of porphyrin moieties.
While the higher-energy Soret-type band negligibly shifts, the lower-energy feature
moves to higher wavelength upon elongation of the porphyrin backbone [18]. Band
Table 2. UV/VIS Data of Fullerene Porphyrin Conjugates 3 7 in Comparison with the Porphyrin Derivatives
14 and 21 23. Spectra recorded at 298 Æ 2 K in PhMe.
Compound
lmax/nm [eV] (e/mÀ1 cmÀ1
)
3
328 [3.78]
(41900)
421 [2.95]
(23300)
508 [2.44]
(3540)
546 [2.27]
(14300)
582 [2.13]
(2860)
682 [1.82]
(640)
4
331 [3.75]
(83300)
426 [2.91]
(123100)
420 [2.95]
(61100)
465 [2.67]
(143300)
481 [2.58]
(61300)
489 [2.54]
(286300)
458 [2.71]
(176100)
539 [2.30]
(11100)
451 [2.75]
(167800)
474 [2.62]
(234600)
485 [2.56]
(303100)
562 [2.21]
(36500)
568 [2.18]
(23200)
572 [2.17]
(121200)
558 [2.22]
(43400)
575 [2.16]
(1410)
554 [2.24]
(41500)
564 [2.20]
(72800)
569 [2.18]
(11600)
600 [2.21]
(6930)
682 [1.82]
(640)
682 [1.82]
(760)
682 [1.82]
(2160)
682 [1.82]
(760)
5
335 [3.70]
(102100)
333 [3.72]
(165000)
333 [3.72]
(70100)
309[4.01]
(8030)
309 [4.01]
(21900)
6
419 [2.96]
(256500)
418 [2.97]
(158700)
412 [3.01]
(237400)
415 [2.99]
(180400)
412 [3.01]
(279000)
413[3.00]
(346700)
7
594 [2.09]
(7680)
14
21
22
23
591 [2.10]
(4310)
600 [2.07]
(9020)
606 [2.05]
(16700)
309 [4.01]
(36100)
305 [4.07]
(48500)

Helvetica Chimica Acta Vol. 88 (2005)
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Fig. 13. UV/VIS Spectra of conjugates 3 (–), 4 (¥¥¥¥), 5 (–), 6 (- ¥ - ¥ -), and 7 (- ¥¥ - ¥¥ ) in PhMe at 298 K. The
arrow indicates the CS-state-centered absorption.
maxima shift from 558 (7), to 562 (4), to 568 (5), and to 572 nm (6). Similar red shifts
are also observed for the Q band above 540 nm (for a comprehensive photophysical
study of the bis([60]fullerene) porphyrin conjugates, see [40]).
Fig. 14 displays the absorption spectrum of conjugate 4 compared to the sum of the
spectra of its component units, taking both 21 and 27 as porphyrin and compound 48 as
fullerene reference fragments. In neither case, good overlapping is obtained, and this
indicates specific porphyrin fullerene interactions in the multicomponent system 4,
related to tight face-to-face vicinity between the two chromophores. This is also

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Fig. 14. UV/VIS Spectra of compounds 4 (–), 2  48 ‡ 21 (---), and 2  48 ‡ 27 (¥¥¥¥) recorded in PhMe at
298 K
signalled by the strong decrease and slight red shift of the higher energy Soret feature,
accompanied by the new absorption detected above 700 nm and attributed to low-
energy charge-transfer (CT) transitions [8][10]. The strong interchromophoric
interactions and the peculiarity of the specific porphyrin backbone are also exemplified
in the comparison depicted in Fig. 15. The experimental spectrum of meso,meso-linked
tetraporphyrin 6 bears no similarity with the profile obtained by summing a porphyrin
dimer (21, central core) and two terminal fullerene porphyrin dyads 3.
Fig. 16 depicts the UV/VIS spectra of bis[60]fullerene diporphyrin conjugates 4
and 8. Interestingly, while the higher-energy Soret-type band is centered almost at the
same wavelength in both dyads (426 and 423nm for 8 and 4, resp.), the lower-energy
band in 8 undergoes a significant bathochromic shift (ca. 100 nm) as compared to 4. The
exciton splitting energies are ca. 0.24 and 0.75 eV for 4 and 8, respectively.
2.3.2. Emission-Spectra Analysis of 8. At any excitation wavelength, 45 exhibits an
emission band in the NIR region (lmax ˆ 1080 nm) [10]. This is a mirror image of the Q-
band profile, and is unambiguously assigned to emission from the lowest singlet-excited
state. Upon selective excitation of the porphyrin chromophore (420 or 585 nm), the
1
NIR emission band is observed also for 8 (C60À (Zn ¥ PꢁP ¥ Zn)*ÀC60). This band is
shifted by 12 nm (lmax ˆ 1092 nm) relative to 45, in line with the absorption trend. Upon
excitation (ca. 80%) of the fullerene moiety of 8 at 330 nm, a strong quenching of the
fullerene fluorescence, relative to reference compound 48, is detected. Fullerene

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1863
Fig. 15. UV/VIS Spectra of a 2  3 ‡ 21 mixture (---) and conjugate 6 (–) recorded in PhMe at 298 K
quenching is accompanied by sensitization of the porphyrin fluorescence in the NIR
1
region. The population of the porphyrin singlet level (C60À (Zn ¥ PꢁP ¥ Zn)*ÀC60) is
quantitative and, by comparison with 45, no quenching of this excited state is detected
from fluorescence intensity measurements. The above results clearly indicate the
occurrence of photoinduced singlet energy transfer from the fullerene unit to the
1
1
*
porphyrin core ( C60À(Zn ¥ PꢁP ¥ Zn)ÀC60 ! C60À (Zn ¥ PꢁP ¥ Zn)*ÀC60), and that
electron transfer to the lowest-lying charge-separated state (C60À(Zn ¥ PꢁP ¥
.
.
Zn)ÀC60 ! C60À(Zn ¥ PꢁP ¥ Zn) ÀC60 À) is not competitive with ultrafast deactiva-
‡
1
tion of the porphyrin singlet (tF ˆ 4.5 ps) back to the ground state (C60À (Zn ¥ PꢁP ¥
Zn)*ÀC60 ! C60À(Zn ¥ PꢁP ¥ Zn)ÀC60; Fig. 17) [16].
The quenching factor (QF) of fullerene fluorescence relative to that for model
compound 48 is ca. 10, and a rate constant kEN of ca. 6 Â 109 sÀ1 can be estimated for the
energy transfer process from Eqn. 11):
kEN ˆ (QF À 1)/tF
(1)
where tF (1.6 ns) is the singlet lifetime of 48 [8].
1
)
This equation can be used to evaluate the quenching rate of a luminescent moiety and is obtained from
kQ ˆ 1/t À 1/t0, taking into account that F/F0 ˆ t/t0, where F (emission quantum yield) and t (excited-
state lifetime) refer to the quenched unit, and F and t refer to an unquenched reference model compound;
see [41].

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Fig. 16. UV/VIS Spectra of compounds 4 (---) and 8 (–) recorded in PhMe at 298 K
Formation of the fullerene triplet is ruled out by monitoring the NIR luminescence
of singlet O2 (1O2), a convenient marker for fullerene triplets (Fig. 18) [42]. The steady-
state VIS-NIR luminescence spectrum of 48 in air-equilibrated PhMe solution exhibits
1
the diagnostic O2 luminescence peak at 1270 nm, which is no longer observed after
removal of O2 from the solution. This treatment has no effect on the emission spectrum
1
of 8, confirming that no O2 (i.e., no fullerene triplet) is produced under fullerene
excitation at 330 nm. Notably, also the porphyrin reference compound 45 does not show
any 1O2 emission signal, at any excitation wavelength. Given the energy position of the
lowest singlet state of 45 (1.15 eV), it is likely that the corresponding triplet level is
lower in energy than that of the excited singlet state of molecular oxygen (1Dg(1O2) ˆ
0.98 eV), thus rendering thermodynamically forbidden the triplet-singlet energy
1
transfer sensitization process responsible for O2 generation [43].
Electronic delocalization in porphyrin tapes allows progressive lowering of the
electronic levels with increasing molecular length [11]. From the lack of 1O2
sensitization of the smallest (dimer) tape that has been observed here, one may
anticipate that all porphyrin tapes are unable to produce 1O2, unlike −regular× porphyrin
molecules, which are among the best and most widely investigated photosensitizers of
1
1O2 [43]. In this regard, we note that the relative O2 sensitization yield of porphyrin
monomer 14 and meso,meso-linked oligoporphyrins 21 23 in air-equilibrated PhMe
solutions turned out to be identical within the experimental error.

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Fig. 17. Energy-level diagram (PhMe) and intercomponent processes following photoexcitation of the
methano[60]fullerene residue of triply-fused diporphyrin fullerene conjugate 8 (C60À(Zn ¥ PꢁP ¥ Zn)ÀC60).
The lowest electronic excited states located on each moiety and the intramolecular charge-separated state are
reported. The excited-state energies localized on the fullerene and porphyrin units were calculated from
.
.
‡
absorption and luminescence spectra, except for that of C60À(Zn ¥ PꢁP ¥ Zn) ÀC6À0 which was estimated from
electrochemical data (Table 3).
2.4. Electrochemical Investigations. The redox characteristic of all new compounds
listed in Table 3 were studied by cyclic (CV) and differential pulse (DPV) voltammetry
in CH2Cl2 (‡0.1m Bu4NPF6) at 293 Æ 2 K. All potentials are referenced to the
‡
ferrocene/ferricinium (Fc/Fc ) couple, used as internal standard. Tetrakis(meso-
arylated) 46 displayed very similar electrochemical behavior to that of bis(meso-
arylated) 14. The typical DPV of 46 (Fig. 19, curve c) showed four redox peaks with a
potential difference of 0.29 V (Zn ¥ P1‡/Zn ¥ P2‡ À Zn ¥ P/Zn ¥ P1‡) and 0.38 V (Zn ¥ P1À/
Zn ¥ P2À À Zn ¥ P/Zn ¥ P1À) between the two oxidation and the two reduction steps,
respectively. Comparison of these electrochemical data with those of 14 revealed some
anodic shifts for both the two reduction and the first oxidation peaks. This can be
explained by the presence of two electron-withdrawing 3-cyanophenyl substituents at
positions 5 and 15 of the porphyrin macrocycle. Compound 41 (Fig. 19, curve b)
displayed very similar electrochemical behavior to 21, except for a small difference in
the reduction peak potentials. The biaryl-type dimer 41 showed two partially
overlapping reduction peaks at À 1.72 and À 1.83V with a difference of 0.11 V. The

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Fig. 18. Sensitized 1O2 luminescence spectra of compounds 48 (top) and 8 (bottom) in air-equilibrated (red) and
air-free (blue) solutions in PhMe. A ˆ 0.600 for all samples, lexc ˆ 330 nm. For compound 8, light absorption
partitioning between C60 and porphyrin moieties is 4 :1. The peak with a maximum at ca. 720 nm corresponds to
some residual signal fullerene-centered fluorescence (quenching factor of 10 relative to 48)
first one-eÀ oxidations of the two porphyrin moieties appeared as separate peaks at 0.33
and 0.47 V. Similarly, the second oxidation of both rings gives two couples as well, at
0.77 and 1.07 V. In comparison with compound 46, each peak is split into two and the
reduction peak potentials are negatively shifted by 40 110 mV (Table 3). The
1=2
1=2
potential gap between the first oxidation and reduction potentials (E À E
) in
ox;1
red;1
the CV is ca. 2.13V, which is almost identical to that of monomer 46 (2.15 V).
The electrochemical behavior of triply-linked porphyrin dimer 38 (Zn ¥ PꢁP ¥ Zn)
differs dramatically from those of monomeric porphyrin 46 and meso,meso-dimer 41
(Table 3). As shown in Fig. 19 (curve a), seven redox peaks in CH2Cl2 with identical
peak current were observed for the triply-linked porphyrin dimer 38. The first (Zn ¥
PꢁP ¥ Zn/Zn ¥ PꢁP ¥ Zn1À, À 0.97 V) and second (Zn ¥ PꢁP ¥ Zn1À/Zn ¥ P1ÀꢁP ¥ Zn1À,
À 1.23V) reduction peaks correspond to two one-e À processes, formally equivalent to
one-eÀ reductions for each porphyrin ring. Likewise, the first (Zn ¥ PꢁP ¥ Zn/Zn ¥ PꢁP ¥
Zn1‡, 0.08 V) and the second (Zn ¥ PꢁP ¥ Zn1‡/Zn ¥ P1‡ꢁP ¥ Zn1‡, 0.35 V) oxidation
peaks correspond each to a one-eÀ process, one per porphyrin ring. The third (Zn ¥
P1‡ꢁP ¥ Zn1‡/Zn ¥ P1‡ꢁP ¥ Zn2‡, 0.81 V) and fourth (Zn ¥ P1‡ꢁP ¥ Zn2‡/Zn ¥ P2‡ꢁP ¥
Zn2‡, 1.08 V) oxidation peaks represent the two second one-eÀ oxidation processes.
Relative to 46, the first one-eÀ reduction potential of 38 is anodically shifted by 0.71 V
(CH2Cl2), whereas the first one-eÀ oxidation potential is negatively shifted by 0.34 V
(CH2Cl2). Unfortunately, the fourth one-eÀ reduction step could not be identified in the

Helvetica Chimica Acta Vol. 88 (2005)
1867
Fig. 19. Differential pulse voltammogram of porphyrins 38 (a), 41 (b), and 46 (c) in CH2Cl2 at 293 K
cyclic voltammogram of 38 in CH2Cl2 because of the limited potential window which
did not permit a scan to potential values more negative than À 2.5 V.
CVand DPV measurements performed in THF, allowed the detection of the fourth
reduction peak for triply-linked dimer 38 (À2.56 V, THF; see Table 3), confirming that
each redox process of 46 splits into two processes in the case of 38. The reduction peaks
shifted negatively by 50 200 mV, while the oxidation peaks shifted positively by ca.
100 mV, and the peak-to-peak separations increased by ca. 20 50 mV, as compared to
those observed when the DPVs were performed in CH2Cl2.
To unambiguously confirm that each peak observed in the CV and DPV of 38
corresponds to a one-eÀ transfer process centered on the zinc porphyrin units, CV
measurements of 38 and 46 were performed at different concentrations. The current
intensity observed in the voltammogram of a 0.2 mm solution of 46 in CH2Cl2 was
found to be exactly twice as high as that of a 0.1 mm solution of 38. The normalized
peak current (peak current/concentration ratio) for 38 was, as expected, identical to
that of 46. It can be noted that the difference in potential between the first and second
reduction peaks for 38 (0.25 V, CH2Cl2) is almost identical to those between the first
and second (0.28 V, CH2Cl2), and the third and fourth oxidation peaks (0.27 V,
CH2Cl2). The potential difference between the first oxidation and the first reduction

1868
Helvetica Chimica Acta Vol. 88 (2005)
Table 3. Redox Potentials of Triply-Linked Porphyrin Derivatives 8, 38, and 45, and Reference Compounds 41, 45, and 46
‡
in Ar-Purged CH2Cl2. Tˆ 298 Æ 2 K, 0.1m Bu4NPF6 as supporting electrolyte; potentials vs. Fc/Fc .
Compound CVa) [V]
1=2
red;1
1=2
red;2
1=2
red;3
1=2
red;4
1=2
ox;1
1=2
ox;2
1=2
ox;3
1=2
E
E
E
E
E
E
E
E
ox;4
[DEpp
]
[DEpp
]
[DEpp
]
[DEpp
]
[DEpp
]
[DEpp
0.34 [68] 0.82 [80] 1.09 [70]
0.47 [75] 0.92 [110]
0.37 [60] 0.83 [62] 1.10 [64]
0.51 [62] 0.78 [128] 1.10 [60]
]
[DEpp
]
[DEpp]
8
À 0.99/ À 1.09 [60] À 1.40 [80] À 1.87 [108] À 2.29 [80] 0.03[60]
38c)
38
41
45
46
À 1.06 [80]
À 1.01 [62]
À 1.75 [60]
À 1.12 [62]
À 1.71 [68]
À 1.40 [90] À 2.29 [90] À 2.59 [110] 0.15 [68]
-
À 1.26 [60] À 2.18 [66]
À 1.86 [60] À 2.20 [80]
À 1.36 [60]
0.09 [60]
0.38 [62]
À 0.01 [60] 0.29 [60] 0.77 [50] 1.03[60]
À 2.09 [80]
0.44 [62]
0.74 [70]
DPVb) [V]
Epred;1
Epred;2
Epred;3
Epred;4
Epox;1
Epox;2
Epox;3
Epox;4
À
8
À 0.97/ À 1.06 (3eÀ) À 1.38 (3eÀ) À 1.84 (2eÀ) À 2.28 (3eÀ) À 0.01 (1eÀ) 0.3(1e
)
0.79 (1eÀ) 1.06 (1eÀ)
38c)
38
41
45
46
À 1.05 (1eÀ)
À 0.97 (1eÀ)
À 1.72 (1eÀ)
À 1.04 (1eÀ)
À 1.68 (1eÀ)
À 1.38 (1eÀ) À 2.26 (1eÀ) À 2.56 (1eÀ) 0.12 (1eÀ) 0.45 (1eÀ) 0.88 (1eÀ)
À 1.23(1e À) À 2.15 (1eÀ)
À 1.83(1e À) À 2.18 (1eÀ)
À 1.28 (1eÀ)
0.08 (1eÀ) 0.35 (1eÀ) 0.81 (1eÀ) 1.08 (1eÀ)
0.33 (1eÀ) 0.47 (1eÀ) 0.77 (1eÀ) 1.07 (1eÀ)
0.05 (1eÀ) 0.26 (1eÀ) 0.74 (1eÀ) 1.00 (1eÀ)
0.42 (1eÀ) 0.71 (1eÀ)
À 2.06 (1eÀ)
a) Scan rate: 0.1 mV sÀ1; E1/2 ˆ (Epa ‡ Epc)/2, where Epc and Epa are the cathodic and anodic peak potentials,
respectively; DEpp ˆ Epa Epc
.
b) Scan rate: 0.4 mV sÀ1, amplitude: 50 mV, pulse width: 0.05 sÀ1; Ep is the peak
potential. c) Data recorded in THF.
1=2
ox;1
1=2
(E
E
, CH2Cl2) decreases significantly upon changing from porphyrin 46
red;1
(2.15 V) and biaryl-type diporphyrin 41 (2.13V) to the planar porphyrin dimer 38
(1.10 V). This decrease of the electrochemical HOMO-LUMO gap is the result of the
extension of the p-conjugation between the porphyrin moieties. Hence, all differences
in the electrochemical behavior between 38, 41, and 46 can be explained in terms of
extension of the p-conjugation between the two fused ZnII tetrapyrrole rings.
Functionalization of the triply-linked porphyrin dimer with two methano[60]fuller-
ene moieties (8) introduces eight additional redox processes as shown in Fig. 20 (curves
a and b), thus leading to an electrochemical fingerprint with a total of fifteen electrons
per molecule in the investigated potential range (À2.5 to 1.25 V, CH2Cl2). The four
oxidation peaks correspond to the four one-eÀ oxidation steps centered on the
porphyrin units, the first oxidation (C60ÀZn ¥ PꢁP ¥ ZnÀC60/C60ÀZn ¥ PꢁP ¥ Zn1‡ÀC60)
peak being cathodically shifted by 60 mV relative to that of 38 (Zn ¥ PꢁP ¥ Zn/Zn ¥
PꢁP ¥ Zn1‡; Table 3). The two partially overlapping peaks at À 1.0 V correspond to the
1À
first fullerene- and porphyrin-centered reductions (C60ÀZn ¥ PꢁP ¥ ZnÀC60/C ÀZn ¥
60
PꢁP ¥ Zn1ÀÀC610À, a three-eÀ process). Similarly, the peak (C ÀZn ¥ PꢁP ¥ Zn1ÀÀC
/
1À
1À
60
60
2À
C
ÀZn ¥ P1ÀꢁP ¥ Zn1ÀÀC62À0 ) at À 1.38 V corresponds to the second fullerene- and
60
porphyrin-centered reductions (once more, a total of three eÀ are involved). The peak
at À 1.84 V is attributed to the third one-eÀ reduction (C ÀZn ¥ P1ÀꢁP ¥ Zn1ÀÀC
/
2À
2À
60
60
3À
C
ÀZn ¥ P1ÀꢁP ¥ Zn1ÀÀC63À0 , a two-eÀ process) of the two fullerene moieties. This
60
interpretation is based on the lower peak current when compared to those at À 1.0 and
À 1.4 V. The peak at À 2.28 V corresponds to the fourth one-eÀ reduction of the
fullerene moieties and to the third one-eÀ reduction (C ÀZn ¥ P1ÀꢁP ¥ Zn1ÀÀC
/
3À
3À
60
60
4À
C
ÀZn ¥ P1ÀꢁP ¥ Zn2ÀÀC640À, a three-eÀ process) of the diporphyrin units.
60