Synthesis, spectroscopic and nonlinear optical properties of multiple [60]fullerene-oligo(p-phenylene ethynylene) hybrids

Article abstract of DOI:10.1002/chem.200401198

A series of multiple [60]fullerene terminated oligo(p-phenylene ethynylene) (OPE) hybrid compounds has been synthesized through a newly developed in situ ethynylation method. Structural and magnetic shielding properties of the highly unsaturated carbon-ri

Full text of DOI:10.1002/chem.200401198

FULL PAPER
Synthesis, Spectroscopic and Nonlinear Optical Properties of Multiple
[60]Fullerene–Oligo(p-phenylene ethynylene) Hybrids
Yuming Zhao,[a] Yasuhiro Shirai,[b] Aaron D. Slepkov,[c] Long Cheng,[b]
Lawrence B. Alemany,[b] Takashi Sasaki,[b] Frank A. Hegmann,*[c] and James M. Tour*[b]
Abstract:
A
series of multiple
groups are connected via p-conjugated
OPE frameworks, they present diminu-
tive electronic interactions in the
ground state, and the electronic behav-
ior of the [60]fullerene cages are only
affected by the OPE backbones
through modest inductive effects. Inter-
estingly, sizable third-order nonlinear
optical (NLO) responses (g) and en-
hanced two-photon absorption (TPA)
cross-sections (s(2)) were determined
for the multifullerene–OPE hybrid 31
relative to its OPE precursor from dif-
ferential optical Kerr effect (DOKE)
experiments. Such enhanced NLO per-
formance is presumably due to the oc-
currence of periconjugation and/or
charge transfer effects in the excited
state. In addition, comparatively strong
excited-state absorption was observed
and characterized for OPE pentamer
12. Thus, the use of such fullerene-de-
rivatized conjugated oligomers aids the
quest for molecules with large third-
order NLO and TPA properties.
[60]fullerene terminated oligo(p-phen-
ylene ethynylene) (OPE) hybrid com-
pounds has been synthesized through a
newly developed in situ ethynylation
method. Structural and magnetic
shielding properties of the highly unsa-
turated carbon-rich C60 and OPE scaf-
folds were characterized by 1D and 2D
NMR spectroscopic analyses. Electron-
ic interactions between the [60]full-
erenes and the OPE backbones were
investigated by UV/Vis spectroscopic
and cyclic voltammetry (CV) experi-
ments. Our studies clearly show that al-
though the multiple [60]fullerene
Keywords:
conjugation
·
electrochemistry · fullerenes · NMR
spectroscopy · nonlinear optics
Introduction
There is an active search for novel conjugated organic mate-
rials capable of functioning as key components in a wide
range of electronic and photonic devices.[1–3] Toward this
end, a myriad of p-conjugated systems have been synthe-
sized and investigated in the past two decades. Extensive re-
search attention has been focused on p-conjugated oligom-
er/polymer systems such as oligo(enyne)s,[4,5] polyynes,[6] oli-
go(arylene ethynylene)s,[7] oligo(arylene vinylene)s,[8,9] and
oligo(thiophene)s,[10–12] owing to their diverse and tunable
electronic and photonic properties, as well as the flexibility
and ease with which these organic materials are handled.
Such research has considerably widened the applicability of
organic-based materials with respect to the fields of molecu-
lar electronics,[13,14] opto-electronics,[15,16] and organic nonlin-
ear optics,[17–19] to name but a few. The derivatization of con-
jugated oligomers with various electro- and photo-active
functional groups has emerged as an important approach to
the design and discovery of useful materials for molecular
electronic and photonic devices. In particular, molecules
with highly delocalized and polarizable p-electron density
[a] Prof. Dr. Y. Zhao
Department of Chemistry
Center for Nanoscale Science and Technology
Rice University, MS-222, 6100 Main St.
Houston, TX 77005 (USA)
Current address:
Department of Chemistry, Memorial University of Newfoundland
St. Johnꢀs, Newfoundland, A1B 3X7 (Canada)
[b] Y. Shirai, Dr. L. Cheng, Dr. L. B. Alemany, T. Sasaki,
Prof. Dr. J. M. Tour
Department of Chemistry
Center for Nanoscale Science and Technology
Rice University, MS-222, 6100 Main St.
Houston, TX 77005 (USA)
Fax : (+1)713-348-6250
E-mail: tour@rice.edu
[c] A. D. Slepkov, Prof. Dr. F. A. Hegmann
Department of Physics, University of Alberta
Edmonton, AB T6G2J1 (Canada)
Fax : (+1)780-492-0714
E-mail: hegmann@phys.ualberta.ca
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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are among the most intensively studied species for such pur-
poses.
conjugated oligomers.[58–65] As such, the molecules would
likely show through-space rather than through-bond interac-
tions between each electronically active unit. Although ex-
tensively used, these methods show limitations in terms of
strengthening fullerene-oligomer interactions and forming
regularly patterned molecular assemblies. Multiple-fullerene
containing molecules synthesized by the ethynylation reac-
tion,[66–68] however, are rare and are far from being thor-
oughly addressed in the literature. Unlike other types of
fullerene derivatives, the organic substituents are closely
linked to the fullerene cages via an acetylenic bond in a
nearly linear geometry, with only one sp3 carbon bridging
the conjugated path on the fullerene and its pendant groups.
In this case, the fullerene p-electrons may possibly commu-
nicate with the alkynyl p-electrons in a through-space p-or-
bital overlapping mechanism, namely “periconjuga-
tion.”[42,69–71] It is likely that such structural features would
give rise to enhanced electronic interactions between fuller-
enes and conjugated functionalities, thereby providing a uni-
formly shaped molecular building block (tecton) for the con-
struction of various molecular machinery and nano-architec-
tures, as well as supramolecular assemblies.
Of great interest is the Buckminster [60]fullerene—an ap-
pealing carbon allotrope with an aesthetic football-shaped
molecular structure, unique three-dimensional p-electron
delocalization pattern, and small dimensions (ca. 0.7 nm in
diameter).[20,21] Since its discovery in the late 1980s,
[60]fullerene (henceforth denoted fullerene) and its deriva-
tives have been found to exhibit many extraordinary elec-
tronic and photonic properties, such as the unique electro-
chemical and optical absorption features in the excited
state.[22,23] Accordingly, fullerenes provide a useful platform
for the design of new materials in the fields of artificial pho-
tosynthetic mimics,[24] long-range intramolecular energy/
charge transfer,[25,26] efficient photon-energy conversion,[27–29]
photoinduced long-lived charge-separation species,[30] optical
devices based on second- and third-order nonlinear optical
(NLO) behavior, multiple photon absorption processes,[31–33]
and optical limiters.[34] However, the direct application of
pristine fullerene in device fabrication has been limited by
its poor solubility in most solvents, and in particular polar-
organic solvents. Different approaches have been explored
to overcome this obstacle, and much effort has been invest-
ed in blending fullerenes into various solid matrices of poly-
mers and inorganic composites,[35,36] as well as non-covalent-
ly encapsulating fullerene molecules into soluble host mole-
cules such as [n]calixarenes[37,38] and cyclodextrins.[39] Cova-
lent functionalization of fullerene,[40,41] on the other hand, is
attracting wide interest as it can greatly alter the physical
and chemical properties of fullerene to readily achieve de-
sired processabilities and performance. Clearly, new synthet-
ic methodologies for fullerene derivatization are continuous-
ly important for synthetic organic chemists.
Our group is interested in integrating multiple fullerenes
in various p-conjugated oligomers[29] so as to develop new
electronic and photonic properties. In principle, such hybri-
dized fullerene–oligomer compounds would feature not only
the intrinsic properties of individual fullerenes and conjugat-
ed oligomers, but in some circumstances new behavior and
functions arising from the mutual interactions between full-
erenes and oligomers.[42] The synthesis of rigid conjugated
oligomeric structures containing more than one fullerene
moiety is a challenging task. Synthetically, the low solubility
together with the moderate-to-low yields of multiple fuller-
ene derivatives makes it extremely difficult to develop suita-
ble reaction protocols. In addition, the isolation of products
using conventional chromatographic means tends to be
problematic for the same reasons. Currently there is a lack
of reliable and facile synthetic methodologies for making
the multiple-fullerene–oligomer compounds. Reported full-
erene derivatization methods, including 1,3-dipolar and [4+
2]-cycloaddition reactions, yield the fullerene derivatives pri-
marily in the forms of fullerenepyrollidines,[43–48] methano-
fullerenes,[49–55] and fullerene-acenes.[56,57] Through these re-
actions, fullerenes and organic substituents are usually
bonded to give irregular and distorted molecular shapes,
with more than one sp3 carbon between the fullerenes and
In our previous work,[72] we have described an efficient
and facile synthesis of a series of multiple-fullerene termi-
nated conjugated oligo(p-phenylene ethynylene)s (OPEs)
via a novel in situ ethynylation methodology. Herein, along
with complete synthetic details, we primarily highlight the
electronic and optical characterization of these materials.
First, we describe the synthesis of p-conjugated OPE mole-
cules and their fullerene derivatives. Next, the structural and
magnetic shielding properties of the multifullerene–OPE hy-
brids are elucidated with 1D and 2D NMR spectroscopic
analyses. Then, the electronic absorption behavior and elec-
trochemical redox properties characterized by UV/Vis spec-
troscopy and cyclic voltammetry (CV) are presented. Final-
ly, we report on the molecular third-order hyperpolarizabili-
ties (g), two-photon absorption (TPA) cross section (s(2)),
and excited-state absorption of selected OPE molecules 12,
14 and fullerene–OPE hybrid 31 as studied with the differ-
ential optical Kerr effect (DOKE) technique.[73]
Results and Discussion
Synthesis of OPE precursors: The synthesis of a series of
linearly conjugated phenylene acetylene oligomers is out-
lined in Schemes 1 and 2. The essential building block 3 for
the elongation of OPE chains was prepared via alkylation
and iodination of 1,4-dihydroquinone (1). The incorporation
of the decyloxy group is necessary to gain sufficient solubili-
ty for both the long-chain OPEs and their fullerene deriva-
tives. In addition, the polar nature of the alkyloxy groups
turns out to be very beneficial to subsequent column chro-
matographic separations. Next, monoiodide 4 was obtained
from cross coupling 0.7 equiv of trimethysilylacetylene
(TMSA) with 3 under Sonogashira conditions. Compound 4
was then cross-coupled with triisopropylacetylene (TIPSA)
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Fullerene–Phenylene Hybrids
FULL PAPER
under palladium catalysis to yield unsymmetrically protected
phenyl acetylene monomer 5. Selective desilylation of the
TMS group in the presence of K2CO3 gave 6, which was
converted to dimeric phenyl acetylenes 7 and 8 after anoth-
er iteration of cross-coupling/desilylation.
pure products 11 and 12 through this approach. Alternative-
ly, a convergent route (as laid out in Scheme 2) was carried
out, and, indeed, it gave satisfactory yields for trimer 11 and
pentamer 12 pure enough for NMR spectroscopic character-
ization.
In a similar manner, a series
of centrosymmetric octupolar-
shaped OPE molecules 14–16
was prepared as shown in
Scheme 3. 1,3,5-Triiodobenzene
(13) was used as the central
core to couple with dimer 8, af-
fording trigonal-shaped hepta-
meric phenylene acetylene 14.
Removal of the TIPS groups
using
tetrabutylammonium
fluoride (TBAF) followed by
silica column separation gave
terminal alkyne 15. For a con-
cise synthesis, compound 14
was desilyated and then cou-
pled with compound 4 to afford
decameric phenylene acetylene
16. Interestingly, in this reac-
Scheme 1. Synthesis of building blocks for long-chain OPEs.
tion, oxidative homocoupling
was not as significant as those
that occurred in the divergent
Monomeric phenyl acetylenes 9 and 10 were readily syn-
thesized by cross-coupling and K2CO3 desilylation reactions
(Scheme 2). For the synthesis of longer OPEs, trimer 11 and
pentamer 12, our preliminary efforts using a divergent elon-
gation route were not satisfactory. A significant amount of
oxidative homocoupling byproducts persistently formed
even with vigorous exclusion of oxygen from the reaction
mixture. These by-products had similar Rf values on silica
gel column chromatography as the products, and as a result
of this, it was extremely difficult to obtain spectroscopically
synthesis of linearly shaped OPEs (see Scheme 2). This is
probably due to the increased steric hindrance in the rigid,
two-dimensional OPE structure.
In addition to the aforementioned OPEs, we have synthe-
sized another series of OPE molecules in which the terminal
ethynylene groups are oriented at the ortho-position relative
to the central phenylene ethynylene backbones. Schemes 4
and 5 outline the synthetic details of the ortho-ethynyl ter-
minated OPEs. In Scheme 4, the building blocks 22 and 23
were readily prepared from 1,2-dihydroquinone (17) via a
Scheme 2. Iterative synthesis of OPEs via palladium-catalyzed coupling reactions.
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F. A. Hegmann, J. M. Tour et al.
Scheme 3. Synthesis of trigonal shaped OPE oligomers.
mono-terminal alkynes with
fullerene, whereas the fullerene
addition to multiple terminal
alkyne species failed to yield
the desired multiple fullerene-
alkyne adducts. To solve this
problem, we have devised an
alternative in situ ethynylation
methodology.[72] The essence of
the new ethynylation method is
to add the base, lithium hexam-
ethyldisilazide
slowly into
(LHMDS),
well-sonicated
a
mixture of fullerene and termi-
nal alkynes in THF. In this way,
LHMDS can generate the lithi-
um acetylide species in situ,
which then quickly undergoes
Scheme 4. Synthesis of building blocks for ortho-ethynylated OPEs.
the fullerene addition reaction
to form the desired ethynylated
fullerene compounds. The reac-
synthetic approach similar to that for the previous OPE
molecules (see Scheme 1).
tion proceeds smoothly at ambient temperature under an
inert gas environment, and completes within 1 h in general.
Careful removal of oxygen and moisture is essential to a
successful reaction. After adding LHMDS, the mixture of
fullerides develops a deep green color that is an indication
that the reaction has occurred. Quenching the reaction with
excess trifluoroacetic acid (TFA) then gives the desired
ethynylated fullerene product. So far, in our hands, only
LHMDS leads to satisfactory results, whereas many other
bases such as tBuOK, nBuLi, tBuLi, LDA, and LTMP have
not produced any positive results.
With these building blocks, we further extended the OPEs
to pentamer 25 and trigonal-shaped heptamer 27, in a con-
vergent way, by coupling dimer 23 with 1,4-diiodobenzene
(24) and 1,3,5-tribromobenzene (26) respectively. Again,
since the complex molecular structures appear to increase
the steric hindrance, the formation of oxidative homocou-
pling byproducts was diminished dramatically in these cross
coupling reactions.
Synthesis of multi-[60]fullerene–OPE hybrids: With the
OPE precursors in hand, we then made efforts to bond full-
erene cages at the termini of each OPE framework. Initially,
we followed the ethynylation procedure as described in the
literature,[38,74–77] in which lithium acetylides were first gener-
ated by nBuLi or tBuLi and then transferred into a slurry of
fullerene in THF under an inert atmosphere. However, this
method was found to work only for the ethynylation of
To shed more light on the reaction mechanistically, a
blank test was carried out in which LHMDS was added into
a slurry of fullerene in THF (for experimental details see
the Supporting Information). The reaction slowly turned
green, similar to that observed in fullerene–ethynylation re-
actions. In the meantime, the solubility of the fullerene
slurry appeared to somewhat improve. After quenching with
TFA, the reaction mixture was separated into two fractions
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Fullerene–Phenylene Hybrids
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Scheme 5. Synthesis of ortho-ethynylated OPE oligmers.
by passing it through a short silica plug. Pristine fullerene
was recovered in greater than 90% yield as the first frac-
tion, while, in the second fraction, a trace amount of a
quent column purification of fullerene products would be
problematic. The synthetic details and yields for the fuller-
ene–OPE hybrids 28–35 are listed in Scheme 6.
1
brownish solid was obtained and characterized by H NMR
1
and MALDI-TOF mass spectrometry. In the H NMR spec-
Spectrometric and NMR spectroscopic characterization: Af-
firmative evidence on the formation of high molecular
weight OPEs and their multiple fullerene adducts was ob-
tained from MALDI-TOF mass spectrometric analysis. The
molecular ion peaks for alkyloxylated OPE precursors can
be readily observed with dithranol as the matrix and the in-
strument operating in the linear mode. For the fullerene–
OPE hybrids, in some cases, using the dithranol matrix re-
sulted in poor molecular ion signals. Alternatively, by using
sulfur as the matrix improved the spectrum quality. Molecu-
lar ion peaks, together with signals due to fragmentation,
were clearly observed in all of the mass spectra of new com-
pounds. As an example, the molecular ion peak of com-
pound 33 is observed at m/z 6021, which represents the
highest molecular weight of multi-fullerene species we have
made to date.
trum, resonance peaks at 6.96 ppm were observed, which
suggests the presence of protons directly attached to fuller-
ene cages. However, the resonance pattern shows that it is a
mixture of several compounds rather than one pure product.
Therefore, it is difficult to deduce further structural informa-
1
tion from the H NMR spectrum. The MALDI-TOF mass
spectrum clearly shows several ion peaks at m/z 738, 760,
782, and 1001, which are probably from the fullerene ad-
ducts formed in the blank test reaction. Possibly, the peaks
at m/z 738 and 782 are due to the adduct ions of
[C60+NH4]+ and [C60+Si2H6]+, whereas the other two are
unidentifiable. According to these data, it is reasoned that
the base LHMDS might interact with fullerene to solvate
and stabilize the reaction intermediates, which in turn facili-
tates the fullerene addition reaction.
Through this in situ ethynylation approach, we were able
to synthesize the multiple-fullerene terminated OPEs direct-
ly from respective OPE precursors. Although it is advanta-
geous to use purified terminal alkynyl OPE precursors to
produce higher yields and easier purification, we have found
that in some cases the silyl-deprotected OPEs could be di-
rectly used in the fullerene ethynylation, after a brief aque-
ous workup, without significantly lowering the yields. How-
ever, it is worth pointing out that if the amount of impurities
in the deprotection reaction is high, the mixture should be
purified before carrying it forward. Otherwise, the subse-
1D and 2D NMR spectroscopic analyses provide detailed
and crucial information establishing the molecular structures
of the OPE precursors and their fullerene derivatives. Fortu-
nately, the fullerene derivatives 28–35 are sufficiently solu-
ble for NMR analysis; the solubilizing alkyloxy chains signif-
icantly interrupt the intermolecular aggregation between
fullerenes. However, the signal overlap in the aromatic and
1
alkyl regions somewhat limits spectral assignments. H and
13C NMR data for the 2,5-dialkyloxy-p-phenylene ethyny-
lene moiety have been reported for numerous small com-
pounds and polymers,[78a–m] but detailed assignments for sim-
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F. A. Hegmann, J. M. Tour et al.
Scheme 6. Synthesis of multi-[60]fullerene-OPE hybrids by an in situ ethynylation method.
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Fullerene–Phenylene Hybrids
FULL PAPER
ilar but symmetrically inequivalent sites in such oligomers
apparently have not been reported. Of particular relevance
are the NMR data reported for trimers similar to 11 and 30
and pentamers similar to 12 and 31 but differing in the
length of the alkyloxy side chains and the nature of the end
groups.[78e,i]
all 13 long-range couplings up to five bonds from the fuller-
ene proton to the fullerene sp2 carbons. In comparison, no
more than five such couplings have previously been report-
ed in other Cs symmetry C60HR species bearing one proton
and one adjacent functional group.[78n,o]
In the 13C NMR spectra of some of the longer OPE com-
pounds, aromatic C-H or C-O signals sometimes overlap,
but the alkynyl carbon signals remain resolved. Compound
34 is remarkable in that six different signals are observed
for the fullerene proton and five types of aromatic protons,
and all of the OPE backbone carbon signals are resolved.
Detailed chemical shift assignments for 30, 34, and other
compounds could be made through a combination of 1D
(1H, 13C with and without 1H decoupling, and DEPT-135
13C) and 2D (1H,1H COSY, 1H,13C HSQC, and 1H,13C
HMBC) NMR experiments, as described in detail in the
Supporting Information.
To better understand the magnetic shielding properties of
the OPE precursors and fullerene derivatives, a thorough
NMR study was conducted on these compounds.
A
500 MHz spectrometer can easily resolve the proton and
carbon resonances from the OPE backbone of compounds
as complex as 11, where, for example, three aromatic proton
signals, nine aromatic carbon signals, four alkynyl carbon
signals, and three -OCH2- carbon signals are clearly ob-
served. In the corresponding fullerene derivative 30, two of
the aromatic C-H carbon signals overlap, but remarkably, all
1
30 fullerene sp2 signals are resolved. The H and 13C signal
assignments for 30 are shown in Figure 1 and listed in
Table 1. The aromatic carbons bonded to Hb and Hd give
Electronic absorption properties: The electronic absorption
characteristics of OPE oligomers and their fullerene deriva-
tives were studied by UV/Vis spectroscopy in o-dichloroben-
zene solutions. In the normalized UV/Vis spectra of linear
fullerene–OPEs 28–31 (Figure 2a), a bathochromic shift of
absorption maxima, lmax, is observed. A similar shift is seen
in the UV/Vis spectra of their OPE precursors 7, 9, 11, and
12 (Figure 2b), which suggests that the bathochromic shift in
the fullerene–OPEs is mainly due to the increasing p conju-
gation path in the OPE backbones. By extrapolation, these
shifts are predicted to reach saturation by the heptamer or
octamer stage, which is in agreement with previous studies
of OPEs.[78p] There is a significant increase in the absorption
intensity in the high-energy absorption region (ca. 315 nm)
for fullerene compounds 28–31 in comparison with their
counterpart OPEsꢀ spectra and that of pristine C60. It is rea-
sonable to believe that this enhancement is due to the con-
tribution from fullerene groups; most likely due to an elec-
tronic transition from the orbitals of OPE to those of the
fullerene cages. Although electrochemistry results suggest
that inductive effects might be present in these fullerene de-
rivatives (see below), such effects cannot be clearly con-
firmed by the low-energy absorption band in the UV/Vis
spectrum since the characteristic hump of the fullerene cage
(ca. 432 nm), along with its tail, hinders confirmative analy-
sis.[77] Nevertheless, in the region above 450 nm of the spec-
trum, distinctively enhanced absorption tails are observed in
a consistent pattern for fullerene–OPE hybrids 28–31. These
absorption bands differ from the UV/Vis patterns obtained
by simply summing the UV/Vis curves of pristine fullerene
and respective OPE oligomers (see Supporting Informa-
tion). Likely, these absorptions are due to the relatively
weak electronic interactions between fullerene cages and
OPE backbones. Despite being connected by a conjugated
OPE bridge, an absorption band from intramolecular elec-
tronic interaction between the fullerene groups is not detect-
ed.
Figure 1. Assignments (Table 1) of 1H and 13C NMR resonances for com-
pound 30 based on long-range proton-carbon couplings, 2D NMR experi-
ments, and calculated 13C shifts for C60H(CꢀCH).[77] The assignments for
C5, C6, and C7 are tentative.
1
Table 1. 13C and H NMR Assignments.
Assignment
Resonance (ppm)
C1
C2
C3
C4
C5
C6
C7
C8
55.43
61.93
151.46
151.34
136.07
135.08
142.09
97.47
80.56
115.17
154.33
116.71
112.78
91.66
92.18
114.31
7.14
C9
C10
C11
C12
C13
C14
C15
C16
Ha
Hb
7.21
Hc
7.05
Hd
6.98
overlapping signals at d 117.10, while the aryloxy carbon sig-
nals at d 153.38 and 153.39 cannot be assigned to specific
The UV/Vis spectra of the octupolar fullerene-derivatives
32 and 33 are shown in Figure 3a. There is a substantial red-
1
sites. In addition, a H-coupled 13C spectrum of 30 displays
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F. A. Hegmann, J. M. Tour et al.
Figure 3. UV/Vis spectra of a) multi-[60]fullerene-OPEs 32–35 in compar-
ison with b) OPE precursors 14, 16, 25, and 27.
Figure 2. UV/Vis spectra of a) linear bis(fullerenyl)-OPEs 28–31 in com-
parison with b) OPE precursors 7, 9, 11, and 12.
shift of the lmax value by about 10 nm from compounds 32
to 33, which is comparable to the shift observed in the spec-
tra of their OPE precursors 14 and 16. This comparison sug-
gests that the red-shift is primarily due to the extension of
the p delocalization path in the OPE moieties. Again, the
long tails from 450 to 600 nm that are characteristic absorp-
tions of fullerene moieties are seen in the UV curves of 32
and 33. The ortho-terminal-alkynylated fullerene–OPE spe-
cies 34 and 35 show similar UV/Vis profiles (Figure 3a) with
respect to their precursors 25 and 27 (Figure 3b). A slight
shoulder at 430 nm in the UV profile of 35 is consistent with
the functionalized fullerene absorption peaks observed in
the spectra of 28 and 29.
between fullerene moieties on the CV time scale—a conclu-
sion that is consistent with the earlier report on acetylene-
connected dumbbell-type fullerenes.[66–68] Detailed redox po-
tential data of 28–33 and 35 are summarized in Table 2. No
well-defined reversible redox waves were observed in the
positive potential region (selected CV spectra in Supporting
Information). The three well-defined redox waves (I, II, and
III in Figure 4) observed for all listed compounds shift
toward more negative potentials relative to pristine fuller-
ene. This is because of the predominantly inductive elec-
tronic interactions from the electron-rich OPE moiety and/
or the decrease of p delocalization on the fullerene cage
due to the introduction of two sp3 carbon atoms. For the
linear multi-fullerene–OPEs 28–31, the formal potentials of
the first redox wave I shift cathodically with the increasing
chain length of the OPE. This observation strongly suggests
the existence of electronic interplays between fullerenes and
OPE moieties, and is further consistent with the UV/Vis re-
sults. The cyclic voltammogram of ortho-alkynylated com-
pound 35 shows features similar to the other fullerene deriv-
atives, except that the second and third formal potentials of
35 are the least negative among them (Table 2). Likely, this
is due to the enhanced inductive effects since the fullerenes
and OPEs are oriented closer to each other in 35.
Cyclic voltammetry studies of [60]fullerene–OPE hybrids:
The electrochemical behavior of multiple fullerene–OPE hy-
brids 28–33, and 35, in solutions of o-dichlorobenzene at
room temperature with Bu4NBF4 as the electrolyte, were
characterized by cyclic voltammetry (CV). The most pro-
nounced feature of the CV profiles for the para-oriented
fullerene–OPEs 28–33 is the close resemblance to that of
pristine fullerene. Figure 4 shows the selected cyclic voltam-
mograms in comparison with pristine fullerene. Although
there are possible electronic interactions between the fuller-
ene moieties, only three couples of reversible redox waves
are observed in the accessible potential window of the sol-
vent under the present conditions for compounds 28–33 and
35. These results indicate that there is no electronic interaction
Third-order optical nonlinearities and two-photon absorp-
tion properties: Third-order nonlinear optical (NLO) prop-
erties of organic systems are at the forefront of photonics
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Chem. Eur. J. 2005, 11, 3643 – 3658

Fullerene–Phenylene Hybrids
FULL PAPER
erenes are appealing candidates for NLO applications.[92,93]
NLO research on pristine C60 has proved disappointing, as
unfunctionalized fullerenes display miniscule hyperpolariza-
bilites and poor solubility.[94–96] Functionalized fullerenes,
however, have been shown to yield large g and s(2)
values.[33,97–99] By bonding fullerenes to conjugated oligomers
such as OPEs, the electronic characteristics of both compo-
nents are altered, thus modifying the overall NLO proper-
ties. Therefore, it is of particular interest to shed light on the
fullerene-controlled third-order NLO behavior as well as re-
spective structure–property relationships. From a geometry
standpoint, the OPE symmetries investigated in this work
are also of particular interest for NLO research, with the ef-
fects of both extended conjugation and polar/multipolar
symmetries on the NLO properties of molecular systems
being investigated herein.[100–110] In the OPE-based com-
pounds that we have studied, both quadrupolar (linearly p-
conjugated C60–p-C60) and octupolar (cross-conjugated) sys-
tems with similarly conjugated backbones are investigated.
For third-order NLO measurements, we used a differen-
tial optical Kerr effect (DOKE) detection technique, de-
scribed elsewhere.[73] In brief, DOKE is an ultrafast pump-
probe technique that uses an intense linearly-polarized
(pump) pulse to induce nonlinearities in the sample that are
consequently probed by a weaker, degenerate, circular-po-
larized (probe) pulse. By varying the input polarization of
the pump pulse with respect to the analyzed polarizations of
the probe pulse, we can obtain the time-resolved real (bire-
fringent) and imaginary (nonlinear absorption) components
of the sampleꢀs third-order optical nonlinearities. Our pulses
are generated by an amplified Ti/sapphire laser producing
800 nm, 90 fs pulses at a 1 kHz repetition rate.
Figure 4. Cyclic voltammograms of C60, and 28, 32 and 35 in 0.1m
Bu4NBF4/o-dichlorobenzene at a scan rate of 0.01 Vsꢁ1 at room tempera-
ture.
Table 2. Results of the cyclic voltammetry measurements.
Formal potential[a], E = [V]
1
2
Compound
I
II
III
C60
28
29
30
31
32
33
35
ꢁ0.832
ꢁ0.901
ꢁ0.905
ꢁ0.906
ꢁ0.909
ꢁ0.918
ꢁ0.906
ꢁ0.906
ꢁ1.227
ꢁ1.281
ꢁ1.288
ꢁ1.286
ꢁ1.292
ꢁ1.304
ꢁ1.299
ꢁ1.274
ꢁ1.691
ꢁ1.813
ꢁ1.814
ꢁ1.813
ꢁ1.814
ꢁ1.831
ꢁ1.846
ꢁ1.807
[a] Formal potentials are calculated as averages of oxidation and reduc-
tion potentials. Potentials are given in Volts (ꢂ0.003 V) versus a non-
aqueous reference electrode, Ag/AgNO3. The formal potential of 2 mm
ferrocene in 0.1m Bu4NBF4 and acetonitrile is 70 mV versus this Ag/
AgNO3 reference electrode.
Preliminary efforts to investigate short fullerene-OPE
chains 28, 29, and pristine C60 failed due to their low solubil-
ity in organic solvents. The inability to resolve nonlinearities
of dilute C60 solutions is consistent with prior findings that
pristine fullerenes display extremely small nonlinear-
ites.[94–96,111] On the other hand, fullerenes bonded to long-
chain OPEs should be more soluble and were primarily tar-
geted in our study. Compounds 31 and 32 were selected as
potential candidates for NLO investigation, since they rep-
resent not only multiple-fullerene derivatives but also two
different types of molecular dimensionalities, that is, linearly
p-conjugated (quadrupolar) and cross-conjugated (octupo-
lar) structures respectively. To determine the effects of full-
erene terminal groups in governing the third-order NLO
properties, the corresponding OPE compounds 12 and 14
were also investigated. For NLO studies, all samples were
analyzed in CH2Cl2 solutions. Both compounds 31 and 32
are weakly soluble in CH2Cl2. Whereas a dilute solution of
31 (<2 mm)[112] gave detectable signals, 32 was not soluble
enough for accurate measurements, with a saturated solution
(ca. 0.5m) yielding an unresolvable response. Although CS2
is a better solvent for fullerene compounds, CH2Cl2 displays
both smaller and faster nonlinearites that allow for the ex-
traction of small solute-derived signals with far better reso-
lution.
research for technological applications. Finding high-trans-
parency materials with appropriately large ultrafast hyper-
polarizabilites, g, is desired for all-optical switching applica-
tions.[79] Moreover, the imaginary component of third-order
susceptibilities determines the molecular two-photon ab-
sorption (TPA) cross-section, s(2) an optical property poten-
tially useful for optical power limiting,[80,81] upconverted
lasing,[82] two-photon fluorescence spectroscopy,[83] three-di-
mensional optical memory and microfabrication,[84–88] as well
as photodynamic therapy.[89] In general, TPA refers to a mo-
lecular process in which the simultaneous absorption of two
photons promotes an electron to an energy level at the pho-
tons sum-energy.[90] The efficiency of this process is charac-
terized by the TPA cross section s(2)(w), which is related to
Im(g) by Equation (1):[91]
sð2Þ ¼ ðð8p2 hw2Þ=ðn 2c 2ÞÞL 4Imgðꢁw,w,w,ꢁwÞ
ð1Þ
where h is Planck constant, n is the refractive index, c is the
speed of light, w is the frequency of the light, and L is the
local field factor.
Possessing a dense network of delocalized p electrons, as
well as high-transparency in the near- and mid-infrared, full-
Chem. Eur. J. 2005, 11, 3643 – 3658
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3651

F. A. Hegmann, J. M. Tour et al.
In Table 3, g and s(2) values for the OPE sample series are
listed. All g values are presented with respect to a reference
value of 5.2ꢂ10ꢁ37 esu for THF.[73] s(2) Values are presented
linear conjugation is interrupted by the meta-linked central
phenyl ring (i.e., cross conjugation), whereas the quadrupo-
lar molecule is fully conjugated along its length.[107] On the
other hand, other groups have synthesized octupolar mole-
cules with an amide core that has been shown to allow some
degree of conjugation through the center.[107,118,119] More-
over, the superiority of the g values of 12 over 14 further
suggests that the degree of p delocalization plays a more
crucial role than the (multi)polar symmetry in controlling
the NLO behavior of the molecules synthesized here.
In addition to instantaneous two-photon absorption, the
TIPS-terminated OPEs also display excited-state absorption
(ESA).[91,120] The DOKE layout is only sensitive to changes
in the probe beam. TPA is recorded when one photon is ab-
sorbed from each of the pump and probe beams. This can
only occur when the beams are overlapped temporally. For
excited-state absorption, two photons are simultaneously ab-
sorbed from the intense pump beam to a high-lying two-
photon state. After relaxation to a lower-lying excited state,
a photon may be absorbed from the probe beam. This proc-
ess will depend on the relaxation dynamics from the two-
photon and lower excited-state as well as the transition
probability of the excited state from which the subsequent
absorption is to take place.[91,120]
with respect to
a reference (DOKE) value of 380ꢂ
10ꢁ50 cm4 sꢁ1 moleculeꢁ1 of MPPBT,[113] a diacetylene-based
chromophore used as a reference for all our nonlinear-ab-
sorption scans. Fullerene-terminated sample 31 displays the
largest molecular hyperpolarizability of g = 10ꢂ10ꢁ34 esu.
This is a relatively large nonlinearity for a conjugated mole-
cule in solution, investigated nonresonantly by femtosecond
Kerr spectroscopy, and compares well with other ultrafast
third-order NLO studies of fullerene derivatives.[94,98] Of
compounds 12, 14 and 31, compound 31 also has the largest
TPA cross-section, with s(2) =140ꢂ10ꢁ50 cm4 sꢁ1 moleculeꢁ1.
This value is large for a non-polymeric compound studied
with femtosecond pulses, and displays a value that compares
well with the only other fullerene-derivative TPA results re-
ported in the literature, studied with nanosecond
pulses.[82,97,103,114]
Table 3. Molecular third-order hyperpolarizabilities g, TPA cross sections
s
(2), and low-energy absorption peak wavelengths for compounds 12, 14,
and 31. (s(2) values are relative to 380ꢂ10ꢁ50 cm4 sꢁ1 moleculeꢁ1 of
MPPBT in DMSO).
Figure 5 presents the time-resolved nonlinear absorption
responses of 12, 14, 31, and MPPBT. CH2Cl2, lacking TPA
only, displays a very small transient grating signal typical of
coherent coupling (not shown).[121] Whereas OPE com-
pounds 12 and 14 both display a measure of excited-state
absorption, the fullerene-terminated 31 shows only instanta-
neous TPA. When the nonlinear absorption of MPPBT is
shown for comparison, it becomes clear that the excited-
state dynamics of both the quadrupolar OPE 12 and octupo-
lar OPE 14 are quite similar (Figure 5b). This is somewhat
surprising as the electronic levels in these systems are ex-
pected to be closely governed by the transition dipoles and
geometry. For example, the lowest lying energy state in the
centrosymmetric 12 is expected to be two-photon forbidden,
while having a higher-energy state that is one-photon forbid-
den but two-photon allowed. On the other hand, the non-
centrosymmetric 14 is expected to have both a one- and
two-photon allowed first excited state.[105,107] Because the
band-gap wavelength (lmax =415–430 nm) of these samples
Compound
Concentration
lmax
g
s(2)
[mm]
[nm]
[10ꢁ34 esu]
[10ꢁ50 cm4 sꢁ1
]
12
14
31
20
20
2
430
415
425
5.7ꢂ0.4
4.4ꢂ0.2
10ꢂ3
65ꢂ10
28ꢂ8
140ꢂ40
Aside from absolute NLO coefficient values (the compar-
ison of which can be misleading[115,116]) we find several inter-
esting trends in our data: Fullerene-terminated OPE 31
shows a g value that is nearly twice that of its OPE precur-
sor 12. Likewise, a two-fold enhancement in s(2) is observed,
when comparing compound 31 with 12. A similar doubling
of the two-photon coefficients of conjugated chromophores
upon attaching a terminal fullerene is reported by Chiang
et al.[97] Considering that pristine fullerenes display negligi-
ble nonlinearities, it is possible that this enhancement of g
and s(2) for 31 compared with 12 is due to periconjugation
effects and/or charge transfer from OPE to fullerene in the
excited state. In previous works, Chiang et al., as well as
others, attribute the sizable TPA response of functionalized
fullerenes to charge transfer from the conjugated backbone
to the terminal fullerene that acts as an electron accept-
or.[33,94,97,98,111,117] This explanation is consistent with our
NLO and UV/Vis results. That same study, however, found
that an octupolar conjugated backbone geometry displays
larger s(2) values than a linear conjugated quadrupolar con-
jugated backbone. In contrast, we find that the quadrupolar
12 displays a larger TPA cross-section than the octupolar 14.
This is, perhaps, not surprising when we consider that al-
though the through-centre arms of these compounds have
the same number of OPE units, in the octupolar geometry,
is longer than the two-photon wavelength (1= llaser
=
2
400 nm), it is possible that the TPA is occurring to a high-
lying excited state. Accordingly, from the UV/Vis spectra
and time-resolved ESA dynamics, it is reasonable to suggest
that it is the OPE chromophore units themselves, rather
than the multipolar geometry, that regulates the excited-
state behavior of samples 12 and 14. The fact that the OPE
precursor 12 displays excited-state absorption while the full-
erene-terminated compound 31 does not, is further consis-
tent with a charge-transfer mechanism, in which relaxations
from the two-photon to the ground state take place without
significant population of the intermediate excited state that
is observed in 12 and 14.[97] However, in the absence of two-
photon induced fluorescence spectroscopy data[90] it is diffi-
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Chem. Eur. J. 2005, 11, 3643 – 3658

Fullerene–Phenylene Hybrids
FULL PAPER
backbone and terminal fullerenes is taking place. Other
groups have attributed similar findings to the formation of a
charge-transfer species in which excited electrons pass from
the conjugated backbone to the terminal fullerene. This pro-
posed mechanism is further supported by the comparisons
of the excited-state absorption dynamics of compounds 12,
14 and 31. Whereas both of the pure OPE frameworks dis-
play similar excited-state absorption, end-capping with full-
erene (in 31) yields a compound that only displays instanta-
neous two-photon absorption. Finally, the fullerene-termi-
nated OPE 31 displays sizable g- and s(2)-values that are
consistent with other C60 derivatives reported in the litera-
ture. Thus, our findings are potentially instructive to guiding
the molecular design and engineering of organic-based elec-
tronic and nonlinear optical materials.
Figure 5. Time-resolved (molecular) nonlinear absorption response of 31,
12, 14, and MPPBT. The MPPBT response has been scaled down by 50%
for ease of comparison. a) Two-photon absorption is seen on-peak (t=
0 ps) for all samples, while longer time scales represent excited-state ab-
sorption (ESA). Note that ESA is absent in the fullerene-terminated
OPE 31. b) (Log-linear graph) Excited-state absorption decays, showing
that samples 12 and 14 have strikingly similar relaxation dynamics.
Experimental Section
General: All reactions were performed under an atmosphere of nitrogen
unless stated otherwise. Precursors 2–15 and multiple [60]fullerene deriv-
atives 28–32 were available from previous studies (see Supporting Infor-
mation).[72] Precursors 18 and 19 were prepared according to literature
procedures.[122] Reagent grade diethyl ether and tetrahydrofuran (THF)
were distilled from sodium/benzophenone. Triethylamine (TEA) was dis-
tilled over CaH2. Fullerene (99.5+ % pure) was purchased from MTR
Ltd. and used as received. LHMDS (1m solution in THF) and TBAF (1m
solution in THF) were obtained from Aldrich. Flash column chromatog-
raphy was performed using 230–400 mesh silica gel from EM Science.
Thin layer chromatography was performed using glass plates pre-coated
cult to conclusively map out the electronic states in these
samples.
Conclusion
1
with silica gel 40 F254 purchased from EM Science. H and 13C NMR spec-
We have developed a new in situ alkynylation methodology
that allows fullerene to be functionalized with a variety of
terminal alkynes in satisfactory yields. The protocol is par-
ticularly useful for generating various multi-fullerene con-
taining compounds, and thus widens the synthetic capability
in the design and preparation of new fullerene derivatives.
In this context, we have synthesized a number of multiple-
fullerene OPE hybrid molecules. The high purity of the
products obtained enables the detailed characterization of
the NMR features of the functionalized fullerene cages as
well as the carbon-rich OPE backbones by 1D and 2D
NMR studies. Electronic properties of these new fullerene-
OPE derivatives have been probed by UV/Vis spectroscopic
analysis, which suggests the existence of electronic interac-
tions between the fullerene and OPE components of the
molecule. This is further supported by electrochemical stud-
ies using the CV method. The observation that the formal
potentials of the first redox wave consistently shift toward
the negative direction as the OPE chain length increases is
an important piece of evidence for the interplays between
the fullerenes and OPE moieties. Furthermore, we investi-
gated the third-order NLO and nonlinear-absorption proper-
ties of fullerene derivative 31 and OPE precursors 12 and 14
by using a DOKE detection technique. The measured g and
s(2) values for fullerene derivative 31 are approximately
double those of its OPE precursor 12, whereas pristine C60
nonlinearities are negligible. This provides further evidence
that a synergistic interaction between the conjugated OPE
tra were recorded on Bruker Avance-400 and 500 spectrometers. 2D-
NMR experiments (1H,1H COSY, 1H,13C HSQC, and 1H,13C HMBC)
were conducted on the Bruker 500 spectrometer. FT-IR spectra were
measured on a Perkin–Elmer 1600 series instrument. Electron impact
mass spectra were recorded on a Finnigan MAT95 mass spectrometer
and MALDI-TOF mass spectrum was performed on a Bruker BiFlex-III
MALDI mass instrument. Melting points were measured on a Mel-Temp
instrument (uncorrected). UV/Vis spectra were recorded on a Shimadzu
UV-3101PC spectrometer. Ultra-sonicated fullerene slurry in THF was
prepared in a general ultrasonic cleaner.
General procedure for the coupling of a terminal alkyne with an aryl
halide using a palladium-catalyzed cross-coupling (Sonogashira) protocol:
The aryl halide, the terminal alkyne, PdCl2(PPh3)2 (ca. 5 mol% per aryl
halide), and CuI (ca. 10 mol% per aryl halide) were added to an oven-
dried round bottom flask equipped with a magnetic stir bar. A solvent
system of TEA and/or THF was added depending on the substrates.
Upon completion of the reaction, it was quenched with a saturated so-
lution of NH4Cl. The organic layer was then diluted with hexanes, diethyl
ether or CH2Cl2, and washed with water or saturated NH4Cl (1ꢂ). The
combined aqueous layers were extracted with hexanes, diethyl ether or
CH2Cl2 (2ꢂ). The combined organic layers were dried over MgSO4 and
the solvent was removed in vacuo to afford the crude product, which was
purified by column chromatography (silica gel). Eluents and other slight
modifications are described below for each compound.
General procedure for the addition of C60 to terminal alkynes using
LHMDS: The terminal alkyne and C60 (2 equiv per terminal alkyne H)
was added to an oven-dried round bottom flask equipped with a magnet-
ic stirrer. After adding THF, the flask was sonicated for at least 3 h. To
the greenish-brown suspension formed after sonication was added
LHMDS drop-wise at room temperature. As the reaction progressed, the
mixture turned into a deep greenish-black solution. During the addition
of the LHMDS, small aliquots from the reaction were extracted and
quenched with trifluoroacetic acid (TFA), dried, and re-dissolved in CS2
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3653

F. A. Hegmann, J. M. Tour et al.
for TLC analysis (developed in a mixture of CH2Cl2 and hexanes). Com-
pletion of the reaction was confirmed by the disappearance of the start-
ing materials. The reaction was usually complete within 0.5–1.5 h. Upon
completion of the reaction, it was quenched with TFA to give a brownish
slurry. Excess TFA and solvent were then removed in vacuo to afford a
crude product that was purified by flash column chromatography (silica
gel). Eluents and other slight modifications are described below for each
compound.
105.3, 93.3, 82.8, 79.6, 69.4 (2ꢂ), 32.1, 29.8, 29.78, 29.6, 29.3, 29.26, 26.2,
26.1, 22.9, 19.0, 14.3, 11.6 ppm; FTIR (CH2Cl2 cast): n˜ = 3314, 2931,
2856, 2149, 2108 cmꢁ1; HRMS (EI): m/z: calcd for C39H66O2Si: 594.4832;
found: 594.4833 [M]+.
Compound 23: See the general procedure for the Pd/Cu coupling reac-
tion. The materials used were 22 (0.85 g, 1.43 mmol),
4 (0.84 g,
1.37 mmol), [PdCl2(PPh3)2] (0.032 g, 0.046 mmol), CuI (0.013 g,
0.070 mmol), and TEA (17 mL) at 508C overnight. The residue was fil-
tered through a plug of silica gel, and subjected to the next reaction with
K2CO3 (0.33 g, 2.39 mmol) and THF/MeOH (1:1, 34 mL). The reaction
mixture was stirred for 3 h at room temperature and then quenched with
satd NH4Cl and diluted with hexanes. The organic layer was washed with
water (1ꢂ), dried over MgSO4, and the solvent was removed in vacuo.
The residue was purified by flash column chromatography with 20%
CH2Cl2 in hexanes to give product 23 (1.13 g, 82%) as an orange waxy
solid. 1H NMR (400 MHz, CDCl3): d=6.98 (s, 1H), 6.96 (s, 1H), 6.95 (s,
1H), 6.94 (s, 1H), 4.01–3.96 (m, 8H), 3.34 (s, 1H), 1.84–1.80 (m, 8H),
1.48–1.46 (m, 8H), 1.35–1.24 (m, 48H), 1.12 (s, 21H), 0.91–0.86 ppm (m,
12H); 13C NMR (100 MHz, CDCl3): d=154.3, 153.8, 149.4, 149.3, 119.3,
118.7, 118.0, 117.1, 117.0, 116.7, 115.5, 112.4, 105.9, 94.4, 93.0, 87.9, 82.3,
80.3, 69.9, 69.8, 69.5, 69.4, 32.1, 29.9, 29.84, 29.81, 29.78, 29.64, 29.57,
29.45, 29.37, 29.33, 26.2, 26.1, 22.9, 19.0, 14.3, 11.6 ppm; FTIR (CH2Cl2
cast): n˜ = 3313, 2923, 2853, 2147 cmꢁ1; MALDI-TOF MS (dithranol as
the matrix): m/z: calcd for C67H110O4Si: 1007; found: 1007 [M]+.
Compound 16: TBAF (0.32 mL, 0.32 mmol) was added to a solution of
14 (328 mg, 0.106 mmol) in THF (5 mL). After stirring at RT for 5 min,
the reaction was quenched with H2O and extracted with diethyl ether.
After a brief aqueous workup, the solvents were removed in vacuo. The
resulted desilylated product was coupled with 4 (260 mg, 0.424 mmol)
under catalysis of [PdCl2(PPh3)2] (12 mg, 0.017 mmol) and CuI (6 mg,
0.03 mmol) in Et3N (40 mL) following the general coupling procedure.
After reaction the residue was purified by silica column chromatography
with 50–60% CH2Cl2 in hexanes to afford 16 (337 mg, 78%) as a yellow
1
solid. H NMR (500 MHz, CDCl3): d=7.64 (s, 3H), 7.03 (s, 3H), 7.013 (s,
3H), 7.008 (s, 6H), 6.96 (s, 3H), 6.95 (s, 3H), 4.03 (m, 24H), 3.97 (m,
12H), 1.86 (m, 36H), 1.51 (m, 36H), 1.41–1.16 (m, 216H), 0.93–0.78 (m,
54H), 0.27 ppm (s, 27H); 13C NMR (125 MHz, CDCl3): d=154.4, 154.0,
153.72, 153.71, 153.68, 153.5, 134.1, 124.4, 117.6, 117.5, 117.4, 117.2, 115.0,
114.8, 114.53, 114.47, 113.9, 113.7, 101.4, 100.3, 93.5, 91.9, 91.8, 91.7, 87.4,
70.0, 69.9, 69.6, 32.13, 32.11, 31.97, 29.92, 29.87, 29.84, 29.80, 29.71, 29.69,
29.67, 29.59, 29.57, 29.52, 20.39, 26.34, 26.29, 26.25, 26.21, 26.18, 23.1,
22.9, 22.7, 14.3, 0.19 ppm; IR (CH2Cl2 cast): n˜ = 2924, 2853, 2151, 1578,
1590 cmꢁ1; MALDI-TOF MS (dithranol as the matrix): m/z: calcd for
Compound 25: See the general procedure for the Pd/Cu coupling reac-
tion. The materials used were 23 (0.27 g, 0.27 mmol), 1,4-diiodobenzene
(24) (0.040 g, 0.12 mmol), [PdCl2(PPh3)2] (0.01 g, 0.013 mmol), CuI
(0.003 g, 0.016 mmol), and TEA (20 mL) at room temperature overnight.
The residue was purified by flash column chromatography with 25–33%
CH2Cl2 in hexanes to give product 25 (0.203 g, 81%) as a yellow-green
solid. 1H NMR (400 MHz, CDCl3): d=7.52 (s, 4H), 7.015 (s, 2H), 7.008
(s, 2H), 6.998 (s, 2H), 6.96 (s, 2H), 4.05–3.99 (m, 16H), 1.87–1.84 (m,
16H), 1.51–1.26 (m, 112H), 1.15 (s, 42H), 0.92–0.87 ppm (m, 24H);
13C NMR (100 MHz, CDCl3): d=153.93, 153.85, 149.4, 149.3, 131.6, 123.5,
119.4, 118.7, 117.1 (2ꢂ), 117.0, 116.7, 115.1, 113.6, 106.0, 94.7, 94.4, 93.0,
88.3, 88.1, 69.83, 69.79, 69.4, 69.3, 32.1, 29.9, 29.87, 29.84, 29.83, 29.80,
29.79, 29.65, 29.63, 29.57, 29.37, 29.33, 26.29, 26.25, 26.21, 26.20, 22.9,
C
273H426O18Si3: 4077; found: 4078 [M]+.
1,2-Bis(decyloxy)-4-iodo-5-trimethylsilylethynylbenzene (20): See the
general procedure for the Pd/Cu coupling reaction. The materials used
were TMSA (2.20 mL, 15.5 mmol), 19[122] (8.83 g, 13.7 mmol), [PdCl2-
(PPh3)2] (0.25 g, 0.36 mmol), CuI (0.10 g, 0.53 mmol), TEA (20 mL), and
THF (100 mL) at room temperature overnight. The residue was purified
by flash column chromatography with 10% CH2Cl2 in hexanes to give
product 20 (3.55 g, 42%) as a yellow oil. 1H NMR (400 MHz, CDCl3):
d=7.21 (s, 1H), 6.96 (s, 1H), 3.95 (m, 4H), 1.80 (m, 4H), 1.45 (m, 4H),
1.33 (m, 24H), 0.89 (t, 3J(H,H)=6.7 Hz, 6H), 0.28 ppm (s, 9H);
13C NMR (100 MHz, CDCl3): d=150.3, 149.0, 123.0, 121.9, 117.2, 107.2,
96.6, 90.8, 69.54, 69.47, 32.13, 29.81, 29.78, 29.6, 29.3, 29.25, 29.16, 26.13,
19.0, 14.3, 11.6 ppm; FTIR (CH2Cl2 cast): n=2923, 2853, 2203, 2145 cmꢁ1
;
MALDI-TOF MS (sulphur as the matrix): m/z: calcd for C140H222O8Si2:
2089; found: 2089 [M]+.
22.9, 14.3, 0.14 ppm; FTIR (CH2Cl2 cast): n˜
= 2954, 2923, 2855,
2153 cmꢁ1; HRMS (EI): m/z: calcd for C31H53IO2Si: 612.2860; found:
612.2849 [M]+.
Compound 27: See the general procedure for the Pd/Cu coupling reac-
tion. The materials used were 23 (0.67 g, 0.66 mmol), 1,3,5-tribromoben-
zene (26) (0.022 g, 0.07 mmol), [PdCl2(PPh3)2] (0.006 g, 0.008 mmol), CuI
(0.002 g, 0.009 mmol), and TEA (13 mL) at 758C overnight. The residue
was purified by flash column chromatography with 25–35% CH2Cl2 in
hexanes to give product 27 (0.165 g, 76%) as a yellow waxy solid.
1H NMR (500 MHz, CDCl3): d=7.65 (s, 3H), 6.999 (s, 3H), 6.998 (s,
3H), 6.992 (s, 3H), 6.95 (s, 3H), 4.04–3.98 (m, 24H), 1.8 (m, 24H), 1.5(m,
24H), 1.33–1.23 (m, 144H), 1.14 (s, 63H), 0.91–0.84 ppm (m, 36H);
13C NMR (125 MHz, CDCl3): d=153.91, 153.90, 149.3, 149.2, 134.1, 124.4,
119.4, 118.7, 117.3, 117.2, 116.9, 116.6, 115.2, 113.4, 105.9, 94.5, 93.3, 93.0,
88.1, 87.4, 69.9 (2ꢂ), 69.4, 69.3, 32.13, 32.12, 32.11, 29.92, 29.90, 29.88,
29.85, 29.83, 29.81, 29.791, 29.788, 29.67, 29.64, 29.62, 29.58, 29.57, 29.56,
29.36, 29.33, 26.27, 26.25, 26.21, 26.20, 22.91, 22.90, 22.89, 18.98, 14.34,
1,2-Bis(decyloxy)-4-triisopropylsilylethynyl-5-trimethylsilylethynylben-
zene (21): See the general procedure for the Pd/Cu coupling reaction.
The materials used were TIPSA (2.00 mL, 8.92 mmol), 20 (3.50 g,
5.71 mmol), [PdCl2(PPh3)2] (0.12 g, 0.17 mmol), CuI (0.054 g, 0.28 mmol),
and TEA (50 mL) at 508C overnight. The residue was purified by flash
column chromatography with 10–20% CH2Cl2 in hexanes to give product
21 (3.77 g, 99%) as a pale yellow oil. 1H NMR (400 MHz, CDCl3): d=
6.91 (s, 1H), 6.88 (s, 1H), 3.98 (m, 4H), 1.81 (m, 4H), 1.46 (m, 4H), 1.31
3
(m, 24H), 1.16 (s, 21H), 0.89 (t, J(H,H)=6.7 Hz, 6H), 0.25 ppm (s, 9H);
13C NMR (100 MHz, CDCl3): d=149.5, 149.4, 119.3, 119.0, 117.3, 117.27,
105.9, 104.2, 96.5, 93.1, 69.6 (2ꢂ), 32.3, 30.0, 29.97, 29.8, 29.76, 29.5,
29.48, 26.3, 23.1, 19.2, 14.5, 11.8, 0.46 ppm; FTIR (CH2Cl2 cast): n˜
=
2924, 2923, 2859, 2151 cmꢁ1; HRMS (EI): m/z: calcd for C42H74O2Si2:
14.33, 11.6 ppm; FTIR (CH2Cl2 cast): n˜ = 2923, 2852, 2207, 2149 cmꢁ1
;
MALDI-TOF MS (sulfur as the matrix): m/z: calcd for C207H330O12Si3:
3095; found: 3095 [M]+.
666.5227; found: 666.5235 [M]+.
1,2-Bis(decyloxy)-4-ethynyl-5-triisopropylethynylbenzene (22): Com-
pound 21 (3.77 g, 5.65 mmol), K2CO3 (1.66 g, 12.0 mmol), and THF/
MeOH (1:1, 80 mL) were added to a round bottom flask equipped with a
magnetic stirrer. The reaction mixture was stirred for 3 h at room temper-
ature and then quenched with satd NH4Cl and diluted with hexanes. The
organic layer was washed with water (1ꢂ), dried over MgSO4, and the
solvent was removed in vacuo. The residue was purified by flash column
chromatography with 10–15% CH2Cl2 in hexanes to give product 22
(3.19 g, 95%) as a pale yellow oil. 1H NMR (400 MHz, CDCl3): d=6.94
(s, 1H), 6.92 (s, 1H), 3.99 (m, 4H), 3.17 (s, 1H), 1.81 (m, 4H), 1.46 (m,
4H), 1.31 (m, 24H), 1.15 (s, 21H), 0.89 ppm (t, 3J(H,H)=6.7 Hz, 6H);
13C NMR (100 MHz, CDCl3): d=149.5, 149.2, 119.8, 118.1, 116.8, 116.6,
Compound 33: TBAF (0.15 mL, 0.15 mmol) was added to a solution of
16 (173 mg, 0.0424 mmol) dissolved in THF (5 mL). The reaction mixture
was stirred for 1 min at room temperature and then quenched with satd
NH4Cl and diluted with diethyl ether. The organic layer was washed with
water (1ꢂ), dried over MgSO4, the slurry was filtered and the solvent
was removed from the filtrate in vacuo. All organic residues were subject
to the next reaction without further purification (see the general proce-
dure for the addition reaction of C60 using LHMDS). The materials used
were C60 (229 mg, 0.318 mmol), LHMDS (0.64 mL, 0.64 mmol), and THF
(220 mL). The reaction was quenched at 1 h with TFA (0.15 mL,
2.0 mmol). Crude products were dissolved in CS2, mixed with silica gel,
3654
ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 3643 – 3658

Fullerene–Phenylene Hybrids
FULL PAPER
and loaded onto
a
column with hexanes/CS2/CH2Cl2 (100:1:1). The
onto a column with hexanes/CS2/CH2Cl2 (100:1:1). The column was
eluted with hexanes/CS2/CH2Cl2 (100:1:1) to remove unreacted C60, and
then with hexanes/CS2/CH2Cl2 (50:50:1) for complete removal of trace
C60, and finally with hexanes/CS2/CH2Cl2 (75:1:25 to 65:1:35) to afford
product 35 (0.13 g, 55%) as a brown solid. 1H NMR (500 MHz, CDCl3):
d=7.45 (s, 3H), 7.28 (s, 3H), 7.174 (s, 3H), 7.171 (s, 3H), 7.09 (s, 3H),
6.88 (s, 3H), 4.13 (t, 3J(H,H)=6.6 Hz, 6H), 4.09 (t, 3J(H,H)=6.7 Hz,
6H), 4.01 (t, 3J(H,H)=6.7 Hz, 6H), 3.87 (t, 3J(H,H)=6.7 Hz, 6H), 1.92–
1.87 (m, 18H), 1.65 (m, 6H), 1.53 (m, 18H), 1.45–1.12 (m, 150H), 0.90–
0.87 (m, 27H), 0.80 ppm (t, 3J(H,H)=7.0 Hz, 9H); 13C NMR (125 MHz,
CDCl3): d=153.9, 153.8, 149.6, 149.3 (4 signals from sp2-C in the aromat-
ic ring), 151.7, 151.6, 147.6, 147.4, 146.6, 146.41, 146.40, 146.24, 146.23,
145.8, 145.7, 145.6, 145.444, 145.436, 145.37, 144.7, 144.5, 143.2, 142.60,
142.58, 142.09, 142.06, 142.0, 141.9, 141.7, 141.6, 140.290, 140.285, 136.0,
135.2 (30 signals from sp2-C in the C60 core), 133.9, 124.0, 120.2, 117.9,
117.3, 117.1, 115.8, 115.7, 114.6, 113.9, 95.2, 94.1, 93.3, 88.8, 87.1, 83.0,
70.0, 69.7, 69.3, 69.2, 62.2 (CH in the C60 core), 55.5 (quaternary sp3-C in
the C60 core), 31.97, 31.95, 31.94, 29.67, 29.67, 29.66, 29.63, 29.62, 29.47,
29.46, 29.45, 29.41, 29.394, 29.387, 26.3, 26.15, 26.069, 26.056, 22.763,
22.755, 22.724, 22.716, 14.24, 14.20, 14.15 ppm; FTIR (CH2Cl2 cast): n˜ =
2921, 2851, 1509 cmꢁ1; MALDI-TOF MS (dithranol as the matrix): m/z:
calcd for C360H270O12: 4787; found: 4788 [M]+.
column was eluted with hexanes/CS2/CH2Cl2 (30:70:1) to remove un-
reacted C60, and then with hexanes/CS2/CH2Cl2 (50:1:50) to afford prod-
uct 33 (82 mg, 32%) as a brown solid. 1H NMR (500 MHz, CDCl3): d=
7.66 (s, 3H), 7.29 (s, 3H), 7.18 (s, 3H), 7.14 (s, 3H), 7.07 (s, 3H), 7.06 (s,
3H), 7.05 (s, 3H), 7.03 (s, 3H), 4.15 (t, 3J(H,H)=6.4 Hz, 12H), 4.07 (m,
24H), 1.98–1.83 (m, 36H), 1.70–1.50 (m, 36H), 1.45–1.14 (m, 216H),
0.91–0.80 ppm (m, 54H); 13C NMR (125 MHz, CDCl3): d=154.6, 153.8,
153.57, 153.56, 153.55, 153.48 (6 signals from the C-O in the aromatic
ring), 151.7, 151.5, 147.7, 147.4, 146.7, 146.47, 146.46, 146.3, 145.9, 145.8,
145.7, 145.52, 145.51, 145.4, 144.8, 144.6, 143.3, 142.7, 142.6, 142.2, 142.11,
142.05, 141.97, 141.74, 141.67, 140.45, 140.37, 136.2, 135.3 (30 signals from
sp2-C in the C60 core), 133.9 (CH on the central ring), 124.2 (substituted
C on the central ring), 117.3, 117.2, 116.9, 115.2, 114.7, 114.4, 114.2, 113.5,
112.9, 97.6, 93.3, 92.0, 91.7, 91.6, 91.5, 87.2, 80.3, 69.9, 69.8, 69.7, 69.6,
69.5, 62.0 (CH in the C60 core), 55.6 (quaternary sp3-C in the C60 core),
32.0, 31.95, 31.943, 31.940, 31.91, 30.0, 29.845, 29.837, 29.78, 29.77, 29.74,
29.72, 29.71, 29.68, 29.66, 29.60, 29.57, 29.53, 29.52, 29.49, 29.48, 29.45,
29.43, 29.41, 29.39, 29.37, 29.35, 26.6, 26.14, 26.09, 26.06, 26.02, 22.74,
22.72, 22.70, 14.18, 14.16, 14.152, 14.146, 14.140 ppm; FTIR (CH2Cl2
cast): n˜ =2923, 2852, 1505, 1464 cmꢁ1; MALDI-TOF MS (dithranol as the
matrix): m/z: calcd for C444H402O18: 6021; found: 6021 [M]+. (Additional
NMR information is in the Supporting Information.)
Blank test reaction of [60]fullerene with LHMDS: A dark brownish
slurry of pristine C60 (45 mg, 0.063 mmol) in THF (30 mL) was ultrasoni-
cated under N2 for 4 h. LHMDS (0.31 mL, 0.31 mmol) was then added
dropwise over 15 min and the mixture was kept stirring at room tempera-
ture for 2 h until a green color was observed. The reaction was quenched
with TFA (0.1 mL) and the solvents were removed in vacuo. The resulted
brownish solid was applied to a short silica plug and eluted with CS2 af-
fording the unreacted fullerene (41 mg, 0.057 mmol, 90% recovery). Af-
terwards, the silica plug was eluted with CS2/CH2Cl2 (1:1) giving a brown
Compound 34: Compound 25 (0.16 g, 0.077 mmol), TBAF (0.2 mL,
0.2 mmol), and THF (15 mL) were added to a round bottom flask equip-
ped with a magnetic stirrer. The reaction mixture was stirred for 1 h at
room temperature and then quenched with satd NH4Cl and diluted with
hexanes. The organic layer was washed with water (1ꢂ), dried over
MgSO4, and the solvent was removed in vacuo. All organic residues were
subject to the next reaction without further purification (see the general
procedure for the addition reaction of C60 by using LHMDS). The mate-
rials used were C60 (0.17 g, 0.23 mmol), LHMDS (0.6 mL, 0.6 mmol), and
THF (100 mL). The reaction was quenched at 2 h with TFA (0.3 mL,
3.9 mmol). Crude products were dissolved in CS2 and directly loaded
onto a column with hexanes/CS2/CH2Cl2 (100:1:1). The column was
eluted with hexanes/CS2/CH2Cl2 (100:1:1) to remove unreacted C60, and
then with hexanes/CS2/CH2Cl2 (55:45:1) for complete removal of trace
C60, and finally with hexanes/CS2/CH2Cl2 (75:1:25 to 40:1:60) to afford
product 34 (0.12 g, 48%) as a brown solid. 1H NMR (500 MHz, CDCl3):
d=7.39 (s, 4H), 7.27 (s, 2H), 7.18 (s, 2H), 7.16 (s, 2H), 7.09 (s, 2H), 6.91
(s, 2H), 4.13 (t, 3J(H,H)=6.7 Hz, 4H), 4.09 (t, 3J(H,H)=6.5 Hz, 4H),
4.01 (t, 3J(H,H)=6.4 Hz, 4H), 3.88 (t, 3J(H,H)=6.7 Hz, 4H), 1.91–1.87
(m, 12H), 1.69 (m, 4H), 1.50 (m, 12H), 1.45–1.15 (m, 100H), 0.90–
0.83 ppm (m, 24H); 13C NMR (125 MHz, CDCl3): d=153.84, 153.81,
149.6, 149.3 (4 signals from C-O in the aromatic ring), 151.7, 151.6, 147.7,
147.4, 146.6, 146.41, 146.40, 146.254, 146.245, 145.8, 145.7, 145.6, 145.5,
145.43, 145.37, 144.7, 144.5, 143.2, 142.62, 142.57, 142.09, 142.06, 142.0,
141.9, 141.7, 141.6, 140.3 (2ꢂ), 136.0, 135.2 (29 resolved signals from sp2-
C in the C60 core), 131.4 (CH on the central ring), 123.2 (substituted C
on the central ring), 120.2, 117.8, 117.2, 117.0, 115.9, 115.7, 114.4, 114.1,
95.2, 94.7, 94.1, 88.8, 88.0, 83.0, 69.9, 69.6, 69.3, 69.2, 62.2 (CH in the C60
core), 55.5 (quaternary sp3-C in the C60 core), 31.95, 31.94, 31.93, 29.74,
29.71, 29.69, 29.67, 29.66, 29.63, 29.62, 29.61, 29.58, 29.53, 29.47, 29.45,
29.43, 29.41, 29.39, 29.38, 29.32, 29.18, 29.16, 26.20, 26.15, 26.07, 26.06,
22.75, 22.74, 22.72, 22.71, 14.22, 14.18, 14.15 ppm; FTIR (CH2Cl2 cast): n˜
= 2920, 2850, 2205 cmꢁ1; MALDI-TOF MS (sulfur as the matrix): m/z:
calcd for C242H182O8: 3218; found: 3217 [M]+. (Additional NMR informa-
tion is in the Supporting Information.)
1
solid (2 mg), which was characterized by H NMR and MALDI-TOF MS
as described in the text.
Electrochemistry: Cyclic voltammetry experiments were performed on a
BAS-CV 50W instrument. A non-aqueous Ag/AgNO3 electrode was used
to serve as the reference electrode. A glass carbon electrode was used as
the working electrode and a platinum wire as the counter-electrode. All
experiments were conducted under a N2 atmosphere using 3–5 mL so-
lution containing 0.1m Bu4NBF4 as the supporting electrolyte. The formal
potential of 2 mm ferrocene in 0.1m Bu4NBF4 and acetonitrile is 70 mV
versus the Ag/AgNO3 reference electrode.
Third-order NLO and TPA studies: The molecular third-order hyperpo-
larizabilities were measured by a differential optical Kerr effect (DOKE)
setup as reported elsewhere.[73] A multi-pass Ti/sapphire laser amplifier
output 800 nm, 90 fs, 700 mJ pulses at a repetition rate of ~1 kHz. The
beam was split into pump and probe beams, with the beamsplitter provid-
ing a 20:1 pump/probe energy ratio. The probe pulse was time-delayed
with respect to the pump pulse by a computer controlled retro-reflector
delay stage along the probe arm. The pump pulse is chopped by a 50%
1
duty cycle chopper (CH) at = of the repetition rate (~270 Hz). Using a
4
half-wave plate, the pump pulse was polarized 458 to the horizontal and
was focused onto a 1 mm path-length quartz cuvette that was filled with
a sample solution. The pump beam was directed to the sample and was
nearly collinear (ca. 38) with the circular-polarized probe beam. Attenua-
tion control was present for both beams. At the sample, 0.05 mJ and 1–
4 mJ pulses were typically used for the probe and pump beams respective-
ly. The pump beam was blocked after the sample, while the probe beam
was allowed to travel to a Wollaston polarizer acting as the analyzer.
Here, the two transmitted beams were separated and directed to bal-
anced photodiodes. Photodiode A received the horizontally polarized
beam, and photodiode B received the vertically polarized beam. The sum
and difference (A+B and AꢁB, respectively) of these signals were sent
to separate lock-in amplifiers. The AꢁB signal was detected at the chop-
ped frequency, and A+B was detected at the laser repetition rate fre-
quency. Finally, these signals were sent to a data acquisition board en-
route to a personal computer that is used for analysis and delay-stage
control.
Compound 35: Compound 27 (0.16 g, 0.051 mmol), TBAF (0.2 mL,
0.2 mmol), and THF (17 mL) were added to a round bottom flask equip-
ped with a magnetic stirrer. The reaction mixture was stirred for 1 h at
room temperature and then quenched with satd NH4Cl and diluted with
hexanes. The organic layer was washed with water (1ꢂ), dried over
MgSO4, and the solvent was removed in vacuo. All organic residues were
subject to the next reaction without further purification (see the general
procedure for the addition reaction of C60 by using LHMDS). The mate-
rials used were C60 (0.16 g, 0.22 mmol), LHMDS (0.5 mL, 0.5 mmol), and
THF (100 mL). The reaction was quenched at 1 h with TFA (0.2 mL,
2.6 mmol). Crude products were dissolved in CS2 and directly loaded
Chem. Eur. J. 2005, 11, 3643 – 3658
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ꢁ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3655

F. A. Hegmann, J. M. Tour et al.
When the input pump polarization is aligned with one of the analyzerꢀs
axes (i.e., vertical or horizontal, in our case) the detected signal is pro-
portional to the imaginary component of the nonlinearity, Im(g), and
thus gives the nonlinear absorption response of the samples. On the
other hand, when the input pump polarization is 458 to the analyzing di-
rections, the detected signal is proportional to both the real and imagina-
ry components of g. Thus, by iterative scans with both polarizations, the
real and imaginary components of the third-order optical nonlinearities
are separated.[73]
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Acknowledgements
This work was supported by the Welch Foundation of Texas, the Defense
Advanced Research Projects Agency (DARPA), the Office of Naval Re-
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Received: November 23, 2004
Published online: April 7, 2005
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