Implementing a tripodal relay station in a phthalocyanine-[60]fullerene conjugate

Article abstract of DOI:10.1039/b716322c

A tripodal architecture 1 - based on a tetraphenylmethane core bearing three phthalocyanine arms and a fullerene arm - has been realized en route towards a novel electron donor-acceptor conjugate that reveals long range electron transfer activity. The Royal Society of Chemistry.

Full text of DOI:10.1039/b716322c

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Implementing a tripodal relay station in a phthalocyanine–
[60]fullerene conjugate†‡
a
b
a
b
Maurizio Quintiliani, Axel Kahnt, Purificacion Vazquez, Dirk M. Guldi and Tomas Torres
a
*
*
´
´
´
Received 23rd October 2007, Accepted 11th December 2007
First published as an Advance Article on the web 8th January 2008
DOI: 10.1039/b716322c
A tripodal architecture 1—based on a tetraphenylmethane core bearing three phthalocyanine arms and
a fullerene arm—has been realized en route towards a novel electron donor–acceptor conjugate that
reveals long range electron transfer activity.
liquid crystals,5 and entered the market as scanning probe micro-
scopy (SPM) tips.6 Remarkably, Otsubo et al. have demonstrated
the important role of a tripodal rigid anchor in preparing well-
organized SAMs of [60]fullerene-linked oligothiophenes onto
gold electrodes to construct highly efficient photovoltaic cells.7
Phthalocyanines8 (Pcs), on one hand, emerged as a class of
molecular building blocks that suggest their implementation as
light harvesting antennae in light energy conversion systems.
Particularly important are their very high extinction coefficients
around 700 nm. The extraordinary electron acceptor properties
of fullerenes, on the other hand, have led to noteworthy utiliza-
tions of such building blocks in areas of light induced electron
Introduction
Photoinduced electron transfer is an essential step in the conver-
sion of solar energy into chemical energy in photosystems I and
II, and is also frequently used by chemists to build complex
molecules from simple precursors.1 During this process, light
absorption generates molecules in excited electronic states that
are susceptible to accepting or donating charges.
One of the most important focuses of chemistry in recent
decades has been the construction of molecular and supra-
molecular based artificial solar energy harvesting systems that
have the ability to absorb light from the sun and convert it
into useful and storable forms. The classical approach is based
on the development of electron donor–acceptor conjugates using
well defined molecular spacers. Long-lived radical ion pair states
have been achieved, in some cases, by incorporating a third,
a fourth or even a fifth component—donor and/or acceptor—
to realize multicomponent arrays such as triads, tetrads, pentads,
etc. This research is driven by the important incentives to govern
factors such as the nature, electronic coupling or structural
arrangement of the components. A better understanding of these
factors will help to design molecular systems mimicking the high
efficiency of solar energy conversion in natural photosynthesis,
that is, efficient formation of radical ion pairs with superior
properties.
In this context, a tetrahedral molecular building block may
represent a valuable design strategy for achieving multifunctional
donor–acceptor ensembles. Over the years, derivatives of tetra-
phenylmethane and tetraphenylsilane have played a significant
role as molecular subunits. Such tetrahedral building blocks
have been successfully incorporated into supramolecular
networks,2 nanoscale structures,3 optoelectronic materials,4 and
a
´
´
´
Departamento de Quımica Organica, Universidad Autonoma de Madrid,
Cantoblanco, Madrid, 28049. E-mail: tomas.torres@uam.es; Fax: (+)34
914973966
bDepartment of Chemistry and Pharmacy & Interdisciplinary Center for
Molecular
Materials,
Friedrich-Alexander-Universita¨t
Erlangen-
Nu¨rnberg, Erlangen, 91058, Germany. E-mail: dirk.guldi@chemie.
uni-erlangen.de; Fax: (+)49 9131 8528307
† This paper is part of a Journal of Materials Chemistry theme issue on
carbon nanostructures.
‡ Electronic supplementary information (ESI) available: NMR, IR and
mass spectra of 1, 2 and 4; fluorescence time profile of 1, differential
absorption spectra of 1; singlet oxygen phosphorescence spectra of 1
and 2. See DOI: 10.1039/b716322c
Fig. 1 Tripodal phthalocyanine systems 1 and 2.
1542 | J. Mater. Chem., 2008, 18, 1542–1546
This journal is ª The Royal Society of Chemistry 2008

View Article Online
transfer chemistry and solar energy conversion.9 To this end,
several examples of phthalocyanine–fullerene ensembles have
been synthesized for studying photoinduced electron transfer
and applications in photovoltaic devices.10 No doubt, some
notable results have already been obtained—still more effort is
needed, especially to achieve higher and better conversion
efficiencies.
This motivated us to pursue a tetrahedral arrangement, which
combines three phthalocyanines and one fullerene, as a unique
molecular mimic for natural photosynthesis. In this article we
describe the synthesis and photophysical properties of a tripodal
architecture 1—based on a tetraphenylmethane core bearing
three phthalocyanine arms and a fullerene arm (Fig. 1)—and
the corresponding reference system 2.
Fig. 2 Absorption spectra of 1 (dashed line) and 2 (solid line) in THF.
SX-1)ꢀ in THF as eluent. Tripod 1 was finally obtained in 22%
yield by 1,3-dipolar cycloaddition reaction of 2 with C60 in the
presence of sarcosine and toluene as solvent.
Results and discussion
Synthesis
All the compounds were characterized by mass spectrometry,
1H-NMR, UV-Vis and FT-IR spectroscopy. The 1H-NMR
spectra of tripods 1 and 2 show the typical broad multiplets in
the aromatic region, between 9.0 and 7.0 ppm, characteristic of
The synthetic strategy towards the tripodal system 1 consisted
of the preparation of the precursor formyl-phthalocyanine 2,
followed by 1,3-dipolar cycloaddition reaction with C60 and
sarcosine (Scheme 1).
tert-butyl substituted phthalocyanines. The presence of
a
Tetraphenylmethane derivative 4 was synthesized by means of
Sonogashira cross-coupling reaction of 4-ethynylbenzaldehyde11
over one of the iodophenyl moieties of 312,6c in the presence of
Et3N, THF and a catalytic amount of [Pd(PPh3)2Cl2] and CuI
in 29% yield. Tri-tert-butyl-ethynylphthalocyaninatozinc(II)
(5)13 was then subjected to a threefold cross-coupling reaction
over 4 to give precursor formyl tripod 2. In order to minimize
the formation of the bisphthalocyanine derivative from 5 via
oxidative homocoupling, the reaction was carried out by using
tris(dibenzylidenacetone)dipalladium(0) ([Pd2(dba)3]) as palla-
dium catalyst and triphenylarsine as ligand. In spite of this, the
Pc dimer butadynyl-bridged bisphthalocyaninato was formed
at the very first steps to generate the active catalytic species
[Pd(0)]. To avoid the formation of an undesired mixture of Pc
adducts, a strong excess of phthalocyanine 5 (10 equivalents)
was necessary, yielding 74% of 2 after column chromatography
on silica gel (hexane–THF, 2 : 1) followed by an additional
purification by gel permeation chromatography (Bio Beads
complex mixture of regioisomers renders the assignment of the
peaks in this region of the spectra difficult. Additionally, the
characteristic aldehyde signal appeared in the spectrum of
formyl-Pc 2, as well as signals corresponding to the pyrrolidine
ring in the case of 1. Concerning MS, molecular ion peaks were
observed, while a peak corresponding to the [M ꢀ C60]+ fragment
was present in the spectrum of 1.
The UV-Vis spectra of tripods 1 and 2 in THF—see Fig. 2—
reveal that both compounds exhibit in toluene, anisole, THF
and benzonitrile a set of Q-bands. In particular, a strong
maximum develops around 686 nm, which is flanked by a minor
maximum around 673 nm and a shoulder around 620 nm. The
Soret band, on the other hand, has a maximum at 348 nm. An
additional band is seen at 330 nm in 1, which reflects a fullerene
centered transition.14
Photophysical studies
First insight into donor–acceptor interactions between ZnPc and
C60 came from fluorescence assays. To ensure comparable
excitation of the ZnPc chromophores, solutions of 1 and 2
were adjusted to exhibit absorbances of around 0.05 at the 610
nm excitation wavelength. Moreover, keeping the absorption
low avoids inner filter effects, namely, artificial impact of the
emission. In the monitored range between 600 and 850 nm 2—
as a reference system—emits strongly with a maximum at 698
nm. From the absorption and fluorescence features we derive
an excited state energy of 1.8 eV. Surprisingly the fluorescence
quantum yield is 0.056 in DMF, which is appreciably smaller
than established, for example, for a ZnPc reference (0.3).
Likewise, the fluorescence lifetime, which was determined in
time-resolved measurements, is 1.19 ns, shorter than in a ZnPc
reference (3.1 ns) (ESI‡ Fig. S11). When turning to 1, the fluores-
cence quantum yield further drops to 0.029 (Fig. 3).
Scheme 1 Synthesis of phthalocyanine–fullerene tripod 1. (i) 4-ethynyl-
benzaldehyde, [Pd(PPh3)2Cl2], CuI, Et3N, THF, rt, 24 h (29%); (ii)
Pd2(dba)3, AsPh3, Et3N, THF, rt, 48 h (74%); (iii) C60, sarcosine, toluene,
reflux, 24 h (22%).
Transient absorption spectroscopy by means of femtosecond
laser photolysis helped to provide evidence for a charge transfer
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 1542–1546 | 1543

View Article Online
time scale of our instrumental detection (i.e., up to 3000 ps). A
multi-wavelength analysis gives rise a singlet excited state life-
time of about 680 ps. The product of this intersystem crossing
is a triplet excited state, for which a broad transient with a
notable maximum at 500 nm is registered. To determine the
quantum yield of triplet excited state formation, we turned to
singlet oxygen sensitization. In fact, for 2 a value of 0.44 was
determined in DMF.
When probing donor–acceptor system 1 the ZnPc singlet
excited state characteristics (vide supra)—despite the presence
of the electron accepting C60—were registered upon photoexcita-
tion. The ZnPc singlet excited state features in 1 decay much
faster than the slow intersystem crossing dynamics, which were
noted for 2. Singlet lifetimes of 96 ps and 50 ps were measured
in benzonitrile and DMF, respectively, from fitting the 1200
nm time absorption profiles. Within this decay the formation
of a new transient species evolves. Distinct maxima were noted
at 520, 840 and 1000 nm. The earlier maxima, that is, at 520
and 840 nm, are attributes of the one-electron oxidized ZnPc
radical cation. The latter maximum at 1000 nm matches the
absorption of the one-electron reduced fullerene radical anion.
In summary, transient absorption measurements testify to the
formation of an intramolecularly formed radical ion pair state.x
The quantum yield of charge separation assumes values in
benzonitrile that are typically around 0.28. Please note that in
less polar anisole and THF (ESI‡ Fig. S12) neither kinetic nor
spectroscopic evidence was gathered that would indicate charge
separation.
Fig. 3 Fluorescence spectra of the tripod reference 2 (solid line) and the
donor–acceptor conjugate 1 (dotted line) in DMF upon 610 nm photo-
excitation.
Both features of the radical ion pair state—ZnPc (i.e., 520 and
840 nm) and C60 (i.e., 1000 nm)—decay on the time scale of our
femtosecond experiments. The impact that the solvent polarity
exerts on the decay of the radical ion pair state is reflected in
the time profiles of the ZnPc radical cation absorption at 845
nm—see Fig. 5. The lifetime of the radical ion pair state is
somewhat longer in less polar benzonitrile (1000 ps) than in
more polar DMF (650 ps). Concomitantly with the decay of
the transient radical ion pair state a new band with a maximum
at 500 nm grows in. This transient absorption corresponds to the
ZnPc triplet excited state.15 We consider two pathways for the
triplet formation: firstly, indirect formation through a deactiva-
tion of the radical ion state and, secondly, direct formation
through a competition between intersystem crossing and charge
separation. The triplet quantum yields were again estimated by
following the singlet oxygen formation: 0.21 (THF), and 0.12
(DMF) (ESI‡ Fig. S13).
Fig. 4 Upper part: differential absorption spectra (visible and near-
infrared) obtained upon femtosecond flash photolysis (387 nm) of 2 in
argon saturated THF with several time delays (1 ps: closed squares;
100 ps: open squares; 2900 ps: crosses) at room temperature. Lower
part: time–absorption profiles of the spectra shown above at 500 (closed
squares) and 800 nm (open squares), monitoring the intersystem crossing.
In summary, a tripodal architecture 1 based on a tetraphenyl-
methane core bearing three phthalocyanine arms and a fullerene
arm has been prepared. The compound reveals long range
electron transfer activity. Transient absorption spectroscopy by
means of femtosecond laser photolysis provided spectroscopic
evidence for a charge transfer mechanism between photoexcited
ZnPc and C60. When relating the radical ion pair state lifetime of
1000 ps (in benzonitrile) to previously assayed ZnPc–C60 donor–
acceptor conjugates, we note in general a significant stabiliza-
tion. As a leading example the para-cyclophane-vinylene bridged
mechanism between photoexcited ZnPc and C60. Fig. 4 gathers
the transient absorption changes that correlate with the ZnPc
singlet excited state in 2. An instantaneously formed broad
transient maximum develops at 490 nm. Bleaching, on the other
hand, is seen between 620 nm and 685 nm—a spectral range that
is dominated by the ground state absorption of ZnPc. This
transient decays rather slowly via intersystem crossing on the
x Electrochemical measurements—by means of differential pulse
voltammetry—revealed the first C60 reduction at ꢀ0.78 ꢁ 0.1 V and
the first ZnPc oxidation at +0.68 ꢁ 0.1 V versus Fc/Fc+).
1544 | J. Mater. Chem., 2008, 18, 1542–1546
This journal is ª The Royal Society of Chemistry 2008

View Article Online
8.7, 6H); dC(75.5 MHz; CDCl3; Me4Si) 191.4 (CHO), 145.6,
145.1, 137.1, 135.4, 132.8, 132.6, 132.1, 131.3, 130.7, 120.5,
92.4, 92.2 and 63.9 (central C); m/z (FAB; m-NBA) 826.9 [M]+,
701.0 [M ꢀ I]+ and 622.9 [M ꢀ C6H5I]+.
Phthalocyanine tripod 2. A solution of tri-tert-butylethynyl-
phthalocyaninatozinc(II) (5)13 (54 mg, 0.07 mmol), 4 (6.1 mg,
0.007 mmol), Pd2(dba)3 (4 mg, 0.004 mmol), AsPh3 (4 mg,
0.015 mmol) and triethylamine (0.5 mL) in dry THF (6 mL)
was stirred at room temperature under argon during 2 days.
The solvent was evaporated and the residual solid was extracted
with CHCl3, dried over Na2SO4 and purified first by silica gel
column chromatography (hexane–THF 2 : 1) and then by Bio-
Beadsꢀ size-exclusion column chromatography (THF). The
green solid obtained was washed with MeOH and hexane, and
dried under reduced pressure affording 2 (14 mg, 74%.) in the
ꢂ
form of a mixture of regioisomers. Mp > 300 C; lmax (THF)/
nm (log e) 686 (5.5), 673 (5.5) and 348 (5.3); nmax(KBr)/cmꢀ1
2966, 2926, 2872, 1705 (CO), 1610, 1489, 1462, 1394, 1367,
1327, 1286, 1259, 1192, 1151, 1097, 1057, 930, 835, 754 and
700; dH(500 MHz; THF-d8; Me4Si) 10.1 (s, 1H; CHO), 9.7–7.0
(br, 56H; arom H) and 1.9 (br m, 81H; CMe3); m/z (MALDI;
dithranol) 2753.0 [M]+; m/z (HR-MALDI; dithranol)
2746.9263 (C172H138N24OZn3 requires 2746.9354).
Phthalocyanine–C60 tripod 1. A dry toluene solution (50 mL)
of C60 (23.5 mg, 0.03 mmol), N-methylglycine (4.0 mg, 0.05
mmol) and 2 (30 mg, 0.01 mmol) was heated to reflux under
argon atmosphere for 24 h. The solution was then cooled to
room temperature and concentrated under vacuum until ca. 10
mL. The resulting mixture was poured onto a silica gel column
and eluted with toluene–AcOEt 95 : 5, passing to toluene–AcOEt
8 : 1. The green solid obtained was washed with acetone,
methanol and hexane, leading to 1 (8 mg, 22%.) in the form of
a mixture of regioisomers. Mp > 250 ꢂC; lmax (THF)/nm (log
e) 686 (5.57), 673 (5.54) and 348 (5.39); nmax(KBr)/cmꢀ1 2966,
1614, 1493, 1398, 1333, 1265, 1153, 1099, 1059, 922, 839, 750,
702 and 677; dH(500 MHz; THF-d8; Me4Si) 9.8–7.0 (br, 56H,
arom H), 5.49, 5.45 and 4.83 (br, 3H; pyrrolidine H), 3.46 (s,
3H; N-Me) and 1.9 (br, 81H; CMe3); m/z: (MALDI; dithranol)
3500.8 [M + H]+, 2779.8 [M ꢀ C60]+; m/z (HR-MALDI;
dithranol) 3494.9866 (C234H143N25Zn3 requires 3494.9860).
Fig. 5 Upper part: differential absorption spectra (visible and near-
infrared) obtained upon femtosecond flash photolysis (670 nm) of 1 in
argon saturated benzonitrile with several time delays (1 ps: closed
squares; 102 ps: open squares; 2892 ps: crosses) at room temperature.
Lower part: time–absorption profiles of the spectra shown above at
845 nm, monitoring the charge separation and charge recombination in
DMF (open squares) and benzonitrile (closed squares).
analogue should be considered, for which a lifetime of 330 ps has
been determined.15 We are currently investigating related
multinuclear Pc–fullerene-based systems in which both covalent
and supramolecular motifs are involved en route towards novel
electron donor–acceptor conjugates.
Experimental
Synthesis
Photophysics
4-{2-[4-Tris(4-iodophenyl)phenyl]ethynyl}benzaldehyde (4). A
solution of tetrakis(4-iodophenyl)methane (3) (400 mg, 0.48
mmol), 4-ethynylbenzaldehyde11 (60 mg, 0.46 mmol),
Pd(PPh3)2Cl2 (10 mg, 0.014 mmol), CuI (4 mg, 0.02 mmol),
and triethylamine (0.5 mL) in dry THF (6 mL) was stirred at
room temperature under argon for 24 h. After this period the
solvent was removed under reduced pressure and the residue
triturated with hexane. The precipitate was filtered off and puri-
fied by silica gel column chromatography (hexane–dioxane 4 : 1).
Compound 4 (117 mg, 29%.) was obtained as a white solid; mp >
250 ꢂC (Found: C, 49.05; H, 2.7. C34H21I3O requires C, 49.4; H,
2.6%); nmax(KBr)/cmꢀ1 2222, 1705 (CO), 1597, 1479, 1398, 1308,
1207, 1167, 1011, 810, 756 and 716; dH(300 MHz; CDCl3; Me4Si)
10.02 (s, 1H; CHO), 7.86 (d, J 8.5, 2H), 7.66 (d, J 8.5, 2H), 7.59
(d, J 8.7, 6H), 7.44 (d, J 8.5, 2H), 7.16 (d, J 8.5, 2H) and 6.91 (d, J
Optical absorptions were measured with a Cary 5000 UV-vis-
NIR spectrometer (Varian). Such measurements, with the
exception of the femtosecond photolysis experiments, were
carried out by using quartz cells with a 1 cm optical path length.
Steady-state fluorescence measurements were performed by
using a Fluoromax 3 (Horiba Jobin Yvon). The time resolved
fluorescence and steady state singlet oxygen phosphorescence
measurements were performed using a Fluorolog TCSPC
(Horiba Jobin Yvon). The solvents were always of spectroscopic
grade. The femtosecond transient absorption measurements
were carried out with a CPA-2001 femtosecond laser (Clark
MXR). The excitation wavelength was generated with an
NOPA (Clark MXR). The laser output energy was 240 nJ for
1 and 300 nJ for 2.
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 1542–1546 | 1545

View Article Online
8 (a) G. de la Torre, C. G. Claessens and T. Torres, Chem. Commun.,
2007, 2000; (b) Phthalocyanines: Properties and Applications, ed.
C. C. Leznoff and A. B. P. Lever, VCH, Weinheim, 1989, 1993, 1996,
vol. 1–4; (c) The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith
and R. Guilard, Academic Press, San Diego, 2003, vol. 15–20.
9 (a) H. Imahori, K. Tamaki, D. M. Guldi, C. Luo, M. Fujitsuka,
O. Ito, Y. Sakata and S. Fukuzumi, J. Am. Chem. Soc., 2001, 123,
2607; (b) H. Imahori, D. M. Guldi, K. Tamaki, Y. Yoshida,
C. Luo, Y. Sakata and S. Fukuzumi, J. Am. Chem. Soc., 2001, 123,
6617; (c) H. Imahori, H. Yamada, D. M. Guldi, Y. Endo,
A. Shimomura, S. Kundu, K. Yamada, T. Okada, Y. Sakata and
S. Fukuzumi, Angew. Chem., Int. Ed., 2002, 41, 2344; (d)
D. M. Guldi, H. Imahori, K. Tamaki, Y. Kashiwagi, H. Yamada,
Y. Sakata and S. Fukuzumi, J. Phys. Chem., 2004, 108, 541; (e)
Acknowledgements
This work has been supported by the Spanish MEC (CTQ-2005-
08933-BQU and Consolider Ingenio 2010 CDS2007-00010
NANOCIENCIA MOLECULAR), the Comunidad de Madrid
(S-0505/PPQ/000225), the European Union (STREP, 516982,
HETEROMOLMAT), the ESF-MEC (project SOHYD), and
by the Deutsche Forschungsgemeinschaft through SFB583,
DFG (GU 517/4-1), FCI, and the Office of Basic Energy Sciences
of the U.S. Department of Energy.
´
N. Martın, Chem. Commun., 2006, 2093; (f) A. Hirsch and
References
M. Brettreich, Fullerenes: Chemistry and Reactions, Wiley-VCH,
Weinheim, 2004; (g) D. M. Guldi and N. Mart´ın, Fullerenes: From
Synthesis to Optoelectronic Properties, Series: Developments in
Fullerene Science, Kluwer Academic, Dordrecht, 2002; (h) Fullerenes:
Principles and Applications, ed. F. Langa and J.-F. Nierengarten,
Royal Society of Chemistry, Cambridge, 2007.
1 (a) Photoinduced Electron Transfer, ed. M. A. Fox and M. Chanon,
Elsevier, Amsterdam, 1988; (b) V. Balzani and F. Scandola, Supramo-
lecular Photochemistry, Ellis Horwood, Chichester, 1991, pp. 161–196
and 355–394; (c) S. Speiser, Chem. Rev., 1996, 96, 1953–1976; (d)
J. G. Calbert, Photochemistry, Wiley, New York, 1996; (e) D. Gust,
Nature, 1997, 386, 21–22; (f) Electron Transfer in Chemistry,
ed. V. Balzani, Wiley-VCH, Weinheim, 2001 vol. I–V;
M. N. Paddon-Row, in Stimulating Concepts in Chemistry,
ed. F. Vo¨gtle, J. F. Stoddart and M. Shibasaki, Wiley-VCH,
Weinheim, 2000.
2 (a) M. Simard, D. Su and J. D. Wuest, J. Am. Chem. Soc., 1991, 113,
4696; (b) X. Li and B. C. Gibb, Supramol. Chem., 2003, 15, 495; (c)
J.-H. Fournier, T. Maris, J. D. Wuest, W. Guo and E. Galoppini,
´
J. Am. Chem. Soc., 2003, 125, 1002; (d) D. Laliberte, T. Maris and
J. D. Wuest, Can. J. Chem., 2004, 82, 386.
´
10 (a) A. Sastre, A. Gouloumis, P. Vazquez, T. Torres, V. Doan,
B. J. Schwartz, F. Wudl, L. Echegoyen and J. Rivera, Org. Lett.,
´
1999, 1, 1807; (b) D. M. Guldi, A. Gouloumis, P. Vazquez and
T. Torres, Chem. Commun., 2002, 2056; (c) M. A. Loi,
H. Neugebauer, P. Denk, C. J. Brabec, N. S. Sariciftci,
´
A. Gouloumis, P. Vazquez and T. Torres, J. Mater. Chem., 2003, 13,
´
´
700; (d) D. M. Guldi, J. Ramey, M. V. Martınez-Dıaz,
A. delaEscosura, T. Torres, T. DaRosandM. Prato, Chem. Commun.,
2002, 2774; (e) M. V. Mart´ınez-D´ıaz, N. S. Fender, M. S. Rodr´ıguez-
´
Morgade, M. Gomez-Lopez, F. Diederich, L. Echegoyen,
J. F. Stoddart and T. Torres, J. Mater. Chem., 2002, 12, 2095; (f)
´
D. M. Guldi, I. Zilbermann, A. Gouloumis, P. Vazquez and
T. Torres, J. Phys. Chem. B, 2004, 108, 18485; (g) A. de la Escosura,
M. V. Martınez-Dıaz, D. M. Guldi and T. Torres, J. Am. Chem.
Soc., 2006, 128, 4112; (h) M. E. El-Khouly, O. Ito, P. M. Smith and
F. D’Souza, J. Photochem. Photobiol., C, 2004, 5, 79; (i)
´
3 (a) L. M. Wilson and A. C. Griffin, J. Mater. Chem., 1993, 3, 991; (b)
O. Mongin and A. Gossauer, Tetrahedron, 1997, 53, 6835; (c)
U.-M. Wiesler and K. Mullen, Chem. Commun., 1999, 2293; (d)
¨
S. Sengupta and S. K. Sadhukhan, Tetrahedron Lett., 1999, 40,
´
´
9157; (e) G. M. Hubner, G. Nachtsheim, Q. Y. Li, C. Seel and
¨
F. Vo¨gtle, Angew. Chem., Int. Ed., 2000, 39, 1269; (f)
E. C. Constable, O. Eich, C. E. Housecroft and D. C. Rees, Inorg.
Chim. Acta, 2000, 300–302, 158; (g) J. P. Novak and
D. L. Feldheim, J. Am. Chem. Soc., 2000, 122, 3979; (h)
E. C. Constable, O. Eich, D. Fenske, C. E. Houseeroft and
L. A. Johnston, Chem.–Eur. J., 2000, 6, 4364.
4 (a) N. Armaroli, V. Balzani, J.-P. Collin, P. Gavin˜a, J.-P. Sauvage
and B. Ventura, J. Am. Chem. Soc., 1999, 121, 4397; (b) S. Wang,
W. J. Oldham, R. A. Hudack and G. C. Bazan, J. Am. Chem. Soc.,
2000, 122, 5695; (c) T. J. Zimmermann, O. Freundel, R. Gompper
´
D. M. Guldi, A. Gouloumis, P. Vazquez, T. Torres, V. Georgakilas
and M. Prato, J. Am. Chem. Soc., 2005, 127, 5811; (j) J. L. Sessler,
J. Jayawickramarajah, A. Gouloumis, T. Torres, D. M. Guldi,
S. Maldonado and K. J. Stevenson, Chem. Commun., 2005, 1892; (k)
B. Ballesteros, G. de la Torre, T. Torres, G. L. Hug,
G. M. A. Rahman and D. M. Guldi, Tetrahedron, 2006, 62, 2097; (l)
T. Torres, A. Gouloumis, D. Sanchez-Garcıa, J. Jayawickramarajah,
W. Seitz, D. M. Guldi and J. L. Sessler, Chem. Commun., 2007, 292;
(m) B. Ballesteros, G. de la Torre, C. Ehli, G. M. A. Rahman,
´
´
F. Agullo-Rueda, D. M. Guldi and T. Torres, J. Am. Chem. Soc.,
and T. J. J. Muller, Eur. J. Org. Chem., 2000, 3305; (d) H. Zhao,
¨
2007, 129, 5061; (n) M. E. El-Khouly, O. Ito, P. M. Smith and
F. D’Souza, J. Photochem. Photobiol., C, 2004, 5, 79; (o) F. D’Souza
and O. Ito, Coord. Chem. Rev., 2005, 249, 1410.
C. Tanjutco and S. Thayumanavan, Tetrahedron Lett., 2001, 42,
4421.
5 A. Pegenau, T. Hegmann, C. Tschierske and S. Diele, Chem.–Eur. J.,
1999, 5, 1643.
6 (a) Y. Yao and J. M. Tour, J. Org. Chem., 1999, 64, 1968; (b) H. Jian
and J. M. Tour, J. Org. Chem., 2003, 68, 5091; (c) Q. Li,
A. V. Rukavishnikov, P. A. Petukhov, T. O. Zaikova and
J. F. W. Keana, Org. Lett., 2002, 4, 3631.
7 D. Hirayama, K. Takimiya, Y. Aso, T. Otsubo, T. Hasobe,
H. Yamada, H. Imahori, S. Fukuzumi and Y. Sakata, J. Am.
Chem. Soc., 2002, 124, 532.
11 (a) W. B. Austin, N. Bilow, W. J. Kelleghan and K. S. Y. Lau, J. Org.
Chem., 1981, 46, 2280; (b) M. Ravikanth, J. P. Strachan, F. Li and
J. S. Lindsey, Tetrahedron, 1998, 54, 7721.
12 D. Su and F. M. Menger, Tetrahedron Lett., 1997, 38, 1485.
´
13 E. M. Maya, P. Vazquez and T. Torres, Chem.–Eur. J., 1999, 5, 2004.
14 D. M. Guldi and M. Prato, Acc. Chem. Res., 2000, 33, 695.
´
15 A. Kahnt, D. M. Guldi, A. de la Escosura, M. V. Martınez-Dıaz and
T. Torres, J. Mater. Chem., 2008, 18, 77.
1546 | J. Mater. Chem., 2008, 18, 1542–1546
This journal is ª The Royal Society of Chemistry 2008