Synthesis, characterization, and computational studies of cycloparaphenylene dimers

Article abstract of DOI:10.1021/ja307373r

Two novel arene-bridged cycloparaphenylene dimers (1 and 2) were prepared using a functionalized precursor, bromo-substituted macrocycle 7. The preferred conformations of these dimeric structures were evaluated computationally in the solid state, as well as in the gas and solution phases. In the solid state, the trans configuration of 1 is preferred by 34 kcal/mol due to the denser crystal packing structure that is achieved. In contrast, in the gas phase and in solution, the cis conformation is favored by 7 kcal/mol (dimer 1) and 10 kcal/mol (dimer 2), with a cis to trans activation barrier of 20 kcal/mol. The stabilization seen in the cis conformations is attributed to the increased van der Waals interactions between the two cycloparaphenylene rings. These calculations indicate that the cis conformation is accessible in solution, which is promising for future efforts toward the synthesis of short carbon nanotubes (CNTs) via cycloparaphenylene monomers. In addition, the optoelectronic properties of these dimeric cycloparaphenylenes were characterized both experimentally and computationally for the first time.

Full text of DOI:10.1021/ja307373r

Article
pubs.acs.org/JACS
Synthesis, Characterization, and Computational Studies of
Cycloparaphenylene Dimers
Jianlong Xia,† Matthew R. Golder,† Michael E. Foster,‡ Bryan M. Wong,‡ and Ramesh Jasti*,†
†Department of Chemistry, Boston University, 24 Cummington St., Boston, Massachusetts 02215, United States (USA)
‡Materials Chemistry Department, Sandia National Laboratories, Livermore, California 94551, United States (USA)
S
* Supporting Information
ABSTRACT: Two novel arene-bridged cycloparaphenylene
dimers (1 and 2) were prepared using a functionalized
precursor, bromo-substituted macrocycle 7. The preferred
conformations of these dimeric structures were evaluated
computationally in the solid state, as well as in the gas and
solution phases. In the solid state, the trans configuration of 1
is preferred by 34 kcal/mol due to the denser crystal packing
structure that is achieved. In contrast, in the gas phase and in
solution, the cis conformation is favored by 7 kcal/mol (dimer
1) and 10 kcal/mol (dimer 2), with a cis to trans activation
barrier of 20 kcal/mol. The stabilization seen in the cis conformations is attributed to the increased van der Waals interactions
between the two cycloparaphenylene rings. These calculations indicate that the cis conformation is accessible in solution, which is
promising for future efforts toward the synthesis of short carbon nanotubes (CNTs) via cycloparaphenylene monomers. In
addition, the optoelectronic properties of these dimeric cycloparaphenylenes were characterized both experimentally and
computationally for the first time.
the [n]CPPs were first conceptualized and the initial attempts
INTRODUCTION
■
were made at their preparation.28 These highly strained
Since their discovery in 1991,1 carbon nanotubes (CNTs) have
captured the interest of physicists, engineers, and chemists due
to their unique architecture, desirable optoelectronic properties,
as well as their high tensile strength.2 Over the last two decades,
chemical vapor deposition3 and laser ablation methods4 have
been developed further, allowing for the scalable production of
carbon nanotubes. These synthetic methods, however, provide
very little control over CNT diameter or chirality  the two
structural features that determine the band gap of CNTs.2,5 To
take advantage of the unique properties of CNTs for
applications in nanotechnology, new synthetic approaches to
circumvent these shortcomings must be pursued.6−9
aromatic macrocycles were later revisited by Vogtle and co-
̈
workers in 1993, but they too were unsuccessful in synthesizing
the desired [n]CPPs.29 It was not until 2008 that the CPPs
were finally prepared by Jasti and Bertozzi utilizing novel
reductive aromatization methodology.22 Since then, our
group,22−25,30 as well as the groups of Itami16,31−35 and
Yamago,21,36,37 have made the synthetic availability of [n]CPPs
(n = 6−16, 18) possible. Most recently, our group has
developed the gram-scale synthesis of two different sized CPPs,
heightening the interest in using these fascinating structures in
materials science applications and as possible precursors to
uniform CNTs.38
The [n]cycloparaphenylenes ([n]CPPs) represent the short-
est possible fragment of an [n,n]armchair carbon nanotube
(CNT). Because of this structural relationship, the CPPs have
recently attracted significant attention due to their potential as
monomeric precursors or seeds for the bottom-up synthesis of
uniform [n,n]armchair CNTs.5−8,10−19 Additionally, CPPs
exhibit size-dependent, tunable optoelectronic proper-
ties,16,20−26 which position them as novel carbon quantum
dots for optoelectronic applications.23 Furthermore, the
cycloparaphenylenes possess unique nanosized cavities which
can be exploited for supramolecular assemblies25,27 or as
components of novel graphitic materials and porous organic
frameworks. With these promising features in mind, the CPPs
have become highly touted non-natural synthetic targets.
Only recently have the cycloparaphenylenes succumbed to
synthesis. As early as 1934, well before the discovery of CNTs,
Despite recent contributions by Itami,16,31−35 Yamago,21,36,37
and our group,22−25,30 a synthetic procedure toward the
functionalization of cycloparaphenylenes has not been reported
to date. In particular, synthetic methods that can connect CPP
units may allow access to wider polyaromatic hydrocarbon belts
and ultimately CNTs. Herein, we describe the first synthesis of
arene-bridged cycloparaphenylene dimers (Figure 1) as well as
their optoelectronic characterization (Figure 2). We also report
computational studies indicating that in the gas phase and in
solution these arene-bridged cycloparaphenylene dimers prefer
cis conformations in which the two cycloparaphenylene units
are stacked on top of each other in a nanotube-like geometry.
Received: July 26, 2012
© XXXX American Chemical Society
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cyclohexadiene units as masked benzene rings to prepare
compound 6 on multigram scale. This building block was
successfully used in the synthesis of [6]cycloparaphenylene,24
the smallest CPP to date, as well as the first gram-scale
synthesis of [8]CPP and [10]CPP.25 We expected that bromo-
substituted macrocycle 7 could be prepared efficiently by taking
advantage of readily available building block 6.
Diiodide 5 (Scheme 1), which bears a vinyl bromide
functionality within the cyclohexadiene unit, is easily prepared
in two steps. The synthesis started from 2-bromo-4,4-
dimethoxycyclohexa-2,5-dienone 3,39 which underwent an
addition of (4-iodophenyl)lithium, and subsequent deprotec-
tion with 10% AcOH to generate bromodieneone 4 in 90%
yield. Deprotonation of alcohol 4 with sodium hydride followed
by addition of (4-iodophenyl)lithium, and subsequent methyl-
ation in a one-pot sequence25 produced brominated monomer
5 in 78% yield and in high diastereoselectivity. Diiodide 5 and
diboronate 624,25 underwent Suzuki coupling/macrocyclization
in the presence of Pd(OAc)2 (0.1 equiv), Cs2CO3 (4 equiv) in
DMF/iPrOH (10:1) at 100 °C for 24 h to deliver macrocycle 7
in 30% isolated yield. Notably, the less reactive vinyl bromide
remained intact under these reaction conditions.
Figure 1. Cycloparaphenylene dimers are short segments of armchair
carbon nanotubes.
With bromo-substituted macrocycle 7 in hand, we next
investigated the reactivity of the vinyl bromide for metal-
catalyzed cross-coupling reactions to prepare macrocyclic
precursors to arene-bridged dimers 1 and 2 (Scheme 2).
Figure 2. Normalized UV−vis absorption (solid line) and fluorescence
spectra (dashed line) of 1, 2 and [8]CPP in dichloromethane.
Scheme 2. Dimerization and Reductive Aromatization to
Synthesize 1 and 2
This result along with synthetic access to linked CPP structures
opens up the possibility of using these dimeric structures to
synthesize short CNT fragments via cycloparaphenylene
monomers. More broadly, the synthetic methodology devel-
oped in this work will be amenable to preparing a wide variety
of monofunctionalized CPPs for use in materials science
applications.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. To couple two cyclo-
paraphenylene molecules, installation of a functional handle for
dimerization was required. We hypothesized that a late-stage
functionalization reaction of CPP would likely lead to mixtures
so we turned our attention to preparing a monofunctionalized
macrocyclic precursor. We envisioned bromo-substituted
macrocycle 7 as a key intermediate  a structure that could
be further derivatized through transition metal catalyzed cross-
coupling reactions (Scheme 1). Recently, we reported a
sequential oxidative dearomatization/addition procedure using
After careful exploration, we found that macrocycle 7 and
commercially available 1,4-benzenediboronic acid bis(pinacol)
ester 8 underwent an efficient Pd-catalyzed cross-coupling
reaction in the presence of Pd(PPh3)4/Cs2CO3 in toluene/H2O
at 80 °C for 24 h to deliver the dimeric macrocycle 10 in 66%
isolated yield. Under the same conditions, the 1,5-naphthalene
bridged dimeric macrocycle 11 was also assembled by coupling
Scheme 1. Synthesis of Bromo-Substituted Macrocycle 7
of 7 with 1,5-naphthalenediboronic acid bis(pinacol) ester40
9
in 49% yield.
Subjecting 10 and 11 to sodium naphthalenide25 at −78 °C
for 2 h and then quenching with I2 led to the arene-bridged
CPPs 1 and 2 in 75% and 48% isolated yield, respectively. Both
compounds exhibit clusters of overlapping peaks around 127.4
and 137.8 ppm in the 13C NMR spectra  consistent with the
chemical shifts observed for [8]CPP.21 The 1H NMR spectra of
both compounds exhibit multiple overlapping peaks ranging
from 7−8 ppm as expected. The MALDI-TOF mass spectra of
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1 and 2 show single peaks at 1291.5924 and 1341.1713,
respectively.
Optoelectronic Characterization. With the CPPs pos-
sessing unique optoelectronic properties, we were curious
about the behavior of these novel dimeric structures. Both 1
and 2 exhibit an absorption maximum at 340 nm, similar to that
of [8]CPP and other CPPs (Figure 2). The extinction
coefficient (ε) of dimer 1 is 0.87 × 10−5 M−1 cm−1, which is
slightly smaller than that of [8]CPP (ε = 1.0 × 10−5 M−1
cm−1),21 while dimer 2 exhibits an increased ε value of 1.7 ×
10−5 M−1 cm−1 (Figures S1 and S2 of the Supporting
Information). The corresponding emission spectra have the
same maximum as [8]CPP at about 540 nm (Figure 2), with
improved fluorescence quantum yields of 0.18 and 0.15
(Figures S3 and S4 of the Supporting Information) respectively
in contrast to that of [8]CPP (0.10).30 The enhanced quantum
yields may indicate a more rigid structure. Cyclic voltammetry
analysis showed that the half-wave oxidation potentials of 1 and
2 are 0.64 and 0.68 V (vs Fc/Fc+), which are larger than that of
[8]CPP (0.59 V).21
Figure 4. Potential energy curve (B3LYP-D/6-31G(d,p)) of the trans
to cis transition for 1.
COMPUTATIONAL STUDIES
■
Conformational Analyses. With the first cycloparapheny-
lene dimers in hand, we were interested in the conformational
dynamics of these molecules in solution. One can envision two
extreme orientations  a trans conformer in which the two
CPP rings are positioned as far apart as possible from each
other (Figure 4, leftmost structure) or a cis conformer in which
diffracting crystals could be obtained. The samples suffered
rapid decomposition during handling due to solvent loss, and
further decomposition during X-ray data collection due to
radiation damage. The resulting data sets were highly
incomplete and although a preliminary solution for dimer 1
clearly showed two linked macrocycles in the trans geometry,
the structure could not be refined.48 Because of the uncertainty
in the crystallographic data, we sought to verify this result
computationally by performing periodic boundary condition
calculations using the PBE-D functional with projector
augmented wave pseudopotential on the experimental crystal
structure (trans) (Figure 3) and a representative cis crystal
structure (Figure S11 of the Supporting Information). Upon
optimization of the unit cell and atomic positions, the trans
crystal structure is predicted to be lower in energy by 34 kcal/
mol per molecule. In addition, the unit cell volume of the
optimized trans structure is smaller by 16% resulting in a denser
crystal packing. This computational result is consistent with the
preliminary X-ray data that was acquired.
Next, we examined the dynamics of the CPP dimer systems
in both the gas phase and in solution. An analysis of the
conformational barrier from trans to cis in the gas phase was
conducted for 1 using DFT-based ab initio calculations.49,50 It
is important to note that to properly model these nanohoop
systems, a computational method capable of modeling van der
Waals interactions must be used, such as the B3LYP-D
functional. The B3LYP functional alone is incapable of
modeling van der Waals interactions and is therefore
quantitatively incapable of modeling these types of systems.
We also investigated these systems using the M06−2X
functional, which, like B3LYP-D, is designed for modeling
van der Waals interactions. For both types of calculations, the
same quantitative results were achieved.
The potential energy curve (PEC) (Figure 4) corresponding
to the trans to cis conformational change in compound 1 was
calculated by starting with the optimized trans geometry and
creating a series of structures mapping the conformational
change (provided movie in the Supporting Information). These
structures were optimized, minimizing the total energy, while
constraining two dihedral angles (Figure S7 of the Supporting
Information). At the B3LYP-D/6-31G(d,p) level of theory, the
Figure 3. Optimized (PBE-D functional) solid state packing of trans 1.
the CPP rings are oriented on top of each other in a nanotube-
like arrangement (Figure 4, rightmost structure). If the cis
conformer is accessible in solution, further oxidative carbon−
carbon bond forming reactions (e.g., Scholl reaction) may be
feasible to lock the molecule into a nanotube-like geometry. In
addition to this enticing possibility, the dimeric CPPs can also
be envisioned as supramolecular host molecules which can
readily switch orientation.41−47 The energetics and pathways
for this conformational isomerization are relevant for both
types of future studies. In an effort to unravel the conforma-
tional dynamics of these molecules, we first considered NMR
analysis. Unfortunately, the high levels of symmetry in these
molecules impede this type of study. Therefore, we decided to
investigate these issues in silico (vide infra).
As an initial starting point for our investigations, we decided
to examine the solid state structure of these dimeric CPPs.
Accordingly, several attempts were made to acquire the crystal
structures of dimers 1 and 2. Unfortunately, due to the
insolubility of these compounds, only very small, weakly
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energy barrier from trans to cis (i.e., left to right) is 13 kcal/
mol. The energy barrier from cis to trans (i.e., right to left) is 20
kcal/mol, in which the lowest energy pathway is that in which
one CPP ring flips on top of the other as opposed to a rotating
motion around the aryl linker axis (see provided movie in the
Supporting Information). In contrast to the solid state
structure, the cis conformation is found to be 7 kcal/mol
more stable in energy in the gas phase because of the increased
van der Waals interactions between the two rings. The cis
conformation of dimer 2 is predicted to be 10 kcal/mol lower
in energy than the trans conformation (Figure S8 of the
Supporting Information). Furthermore, SCRF calculations
using a polarizable continuum model (PCM) and the B3LYP-
D functional showed no changes in the preferred conformation
or relative stability of 1 and 2 via the inclusion of
dichloromethane solvent effects. These results together are
very promising for future efforts toward the synthesis of short
carbon nanotubes via cycloparaphenylene monomers.
Table 1. Significant Optical Contributions Obtained via TD-
DFT Calculations
a
Inspired by arene-bridged cycloparaphenylene dimers 1 and
2, we were also interested in the conformational dynamics of a
direct CPP dimer (i.e., no arene-linker) (Figure 5). This
Figure 5. Side-view (left) and top-view (right) of lowest-energy
conformation of directly linked [8]CPP dimer.
a
Calculations were performed at the B3LYP-D/6-31G(d,p) level of
molecule is especially fascinating because, if it can be prepared,
successful cyclodehydrogenation would deliver an ultrashort
CNT in one step. To probe this type of structure further, we
again analyzed the potential energy surface of the correspond-
ing cis to trans conformational change by DFT calculations
(Figure S10 of the Supporting Information). Surprisingly, the
trans conformation is predicted to be a relatively unstable local
minimum in comparison to the cis conformation, which is
predicted to be 30 kcal/mol lower in energy. We therefore
propose directly linked CPP dimers as exciting new synthetic
targets that our laboratory is actively pursuing.
TD-DFT Calculations. To gain a more in depth under-
standing of the optical data for the CPP dimers, we carried out
TD-DFT calculations at the B3LYP-D/6-31G(d,p) level of
theory for both the cis and trans conformations of 1 and 2. The
calculations revealed that although the HOMO→LUMO
transition is forbidden for [8]CPP (f = 0), the HOMO→
LUMO transition of dimers 1 and 2 have nonzero oscillator
strengths (Table 1).51 Consistent with this result, we observe
weak shoulder peaks centered around 400 nm for both dimers
1 and 2 that is not present in the absorption spectrum of
[8]CPP (vide supra, Figure 2). Furthermore, TD-DFT results
indicate that the maximum absorption of the cis conformation
of 1 can be assigned to a combination of HOMO-3→LUMO,
HOMO→LUMO+2, and HOMO→LUMO+3 transitions,
whereas the maximum absorption for cis 2 can be assigned to
HOMO-4→LUMO, HOMO-3→LUMO, and HOMO→
LUMO+3. The maximum absorption of the trans conformation
at approximately 375 nm can be attributed to a blending of
b
theory. H = HOMO, L = LUMO.
HOMO-2→LUMO and HOMO→LUMO+2 transitions for
both 1 and 2. TD-DFT calculations predict very similar optical
absorption spectra for both the cis and trans conformers of
dimers 1 and 2. Hence, the preferred conformations of 1 and 2
in solution could not be addressed by analysis of the UV−vis
data and TD-DFT calculations.
In addition to the TD-DFT calculations, we also analyzed the
frontier molecular orbitals of dimer 1 (Figure 6). In contrast to
the predicted absorption spectra, the frontier molecular orbitals
of the cis and trans configurations of 1 are localized quite
differently. Most notably, in the cis configuration (C1
symmetry), the HOMO and LUMO lie almost exclusively on
one cycloparaphenylene ring on either side of the arene bridge.
However, the HOMO and LUMO are more evenly distributed
across the arene bridge in the higher symmetry (Ci) trans
configuration. An FMO analysis on dimer 2 led to similar
results (Figure S10 of the Supporting Information). The higher
level of symmetry in the trans conformers is also responsible for
the larger oscillator strengths compared to the cis conformers
(Table 1).
CONCLUSIONS
■
In summary, the syntheses of arene-bridged CPP dimers 1 and
2 were accomplished using macrocycle precursor 7. More
broadly, this general synthetic route will be useful for the
preparation of a variety of monofunctionalized CPPs via cross-
D
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Figure 6. Major electronic transitions (TD-DFT) and representative FMOs for 1 cis (left) and trans (right), calculated at the B3LYP/6-31G(d,p)
level of theory.
coupling reactions. From computational analyses, we find that 1
and 2 can adopt favored cis conformations in the gas phase and
in solution, although the trans is the preferred conformer in the
solid state. As a result, further carbon−carbon bond forming
reactions (e.g., cyclodehydrogenations) that lock the cis
conformation and generate a nanotube-like structure are
possible. Additionally, there is potential to exploit the cis
conformation in interesting supramolecular chemistry applica-
tions. Furthermore, we report that a directly linked CPP dimer
can adopt a stable cis conformation. We are currently
investigating the synthesis of directly linked CPP dimers, as
well as cyclodehydrogenation reactions of arene-linked dimers
1 and 2. We will report the results of these investigations in due
course.
133.72, 133.48, 133.46, 133.30, 132.98, 132.80, 132.76, 132.61,
131.32, 128.10, 127.30, 127.18, 127.15, 126.87, 126.79, 126.38,
126.31, 126.24, 104.96, 78.83, 78.78, 74.59, 74.51, 74.02, 73.96,
52.43 (OMe), 52.12, 52.09, 51.80. MALDI-TOF m/z calcd for
C54H49BrO6 (M)+: 873.87, found (isotopic pattern): 872.1226,
874.1280, 875.1285. IR (neat): 736, 772, 820, 949, 1005, 1016,
1080, 1174, 1265, 1397, 1449, 1491, 2823, 2932 cm−1.
Phenyl-bridged Macrocycle 10. A mixture of brominated
[8]macrocycle 7 (288 mg, 0.330 mmol), 1,4-benzenediboronic
acid bis(pinacol) ester 8 (50.0 mg, 0.150 mmol), Pd(PPh3)4
(38.0 mg, 0.0330 mmol) and Cs2CO3 (430 mg, 1.32 mmol)
was dissolved in Toluene/H2O (7 mL, 6:1) and stirred at 80 °C
for 24 h under nitrogen. After cooling down to room
temperature, 10 mL water was added. The aqueous phase
was extracted with dichloromethane (3 × 10 mL) and the
combined organics were washed with water (3 × 10 mL) and
dried over sodium sulfate. After removing the solvent under
vacuum, the crude mixture was purified by silica column
chromatography (ethyl acetate/hexane = 1:1) to give 10 as a
EXPERIMENTAL SECTION
■
Bromo-Substituted Macrocycle 7. Diiodide 5 (1.25 g,
2.00 mmol), diboronate 6 (1.52 g, 2.00 mmol), Pd(OAc)2 (135
mg, 0.200 mmol, 0.1 equiv) and Cs2CO3 (2.61 g, 8.00 mmol, 4
equiv) were charged in a 500 mL Schlenk flask under nitrogen,
then degassed DMF (350 mL) and 2-isopropanol (35 mL) was
added. The resulting mixture was heated to 100 °C and stirred
for 24 h. After cooling down to room temperature, the mixture
was filtered through a short plug of Celite, and 250 mL water
was added to the filtrate. After extraction with dichloromethane
(3 × 100 mL), the combined organic phase was washed with
water (8 × 100 mL) and dried over sodium sulfate. After
removing the solvent under vacuum, the crude mixture was
purified by silica column chromatography (ethyl acetate/hexane
= 2:3) to give the brominated [8]macrocycle 7 as a white solid
1
white solid (165 mg, 66%, d.p. > 300 °C.). H NMR (400
MHz, CDCl3): δ(ppm) 7.47−7.52 (m, 20H, Ar), 7.40 (d, J =
8.4 Hz, 4H, Ar), 7.27 (d, J = 6 Hz, 8H, Ar), 7.20 (d, J = 6 Hz,
8H, Ar), 7.04 (d, J = 8.8 Hz, 4H, Ar), 6.68 (d, J = 2 Hz, 2H,
Vinyl-H), 6.02−6.21 (overlap, 20H, Vinyl-H), 3.40−3.48
(overlap, 30H, OMe), 3.12 (s, 6H, OMe). 13C NMR (100
MHz, CDCl3): δ(ppm) 143.30, 142.80, 142.72, 140.86, 140.25,
140.21, 139.67, 139.49, 139.45, 139.34, 138.18, 136.69, 135.57,
134.00, 133.51, 133.40, 133.19, 133.10, 132.81, 132.32, 128.82,
127.58, 127.22, 126.97, 126.83, 126.73, 126.59, 126.49, 126.23,
126.18, 78.78, 76.30, 74.58, 74.54, 74.05, 74.03, 52.10 (OMe),
51.97, 51.84, 51.69. MALDI-TOF m/z calcd for C114H102O12
(M)+: 1664.02. Found: 1664.3682. IR (neat): 819, 950, 1016,
1079, 1174, 1491, 2822, 2931 cm−1.
1
(530 mg, 30%, mp 155 °C.) H NMR (400 MHz, CDCl3):
δ(ppm) 7.45−7.53 (m, 14H, Ar), 7.35 (d, J = 8.4 Hz, 2H, Ar),
7.23 (t, J = 8.8 Hz, 2H, Ar), 7.08 (d, J = 8 Hz, 2H, Ar), 6.79 (d,
J = 1.6 Hz, 1H, Vinyl-H), 6.23−6.26 (m, 2H, Vinyl-H), 6.12−
6.15 (overlap, 4H, Vinyl-H), 6.04−6.08 (overlap, 4H, Vinyl-H),
3.47−3.49 (overlap, 12H, OMe), 3.40 (s, 6H, OMe). 13C NMR
(100 MHz, CDCl3): δ(ppm) 143.35, 143.31, 143.02, 142.85,
140.53, 140.11, 139.46, 139.43, 139.40, 138.70, 138.30, 134.55,
Naphthyl-brided Dimer 11. A mixture of brominated
[8]macrocycle 7 (73.2 mg, 0.0840 mmol), 1,5-naphthalenedi-
boronic acid bis(pinacol) ester 9 (16.0 mg, 0.0420 mmol),
Pd(PPh3)4 (9.70 mg, 0.00840 mmol) and Cs2CO3 (110 mg,
0.340 mmol) was dissolved in Toluene/H2O (2 mL, 6:1) and
E
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stirred at 90 °C for 24 h under nitrogen. After cooling down to
room temperature, 5 mL water was added. The aqueous phase
was extracted with dichloromethane (3 × 5 mL) and the
combined organics were washed with water (3 × 5 mL) and
brine (5 mL) and dried over sodium sulfate. After removing the
solvent under vacuum, the crude mixture was purified by silica
column chromatography (ethyl acetate/hexane =30% to 50%)
to give compound 11 as an off-white solid (35 mg, 49%, d.p. >
300 °C). 1H NMR (400 MHz, CDCl3): δ(ppm) 8.28 (br s, 1H,
Ar), 7.53−7.47 (overlap, 28H, Ar), 7.29−7.27 (overlap, 8H,
Ar), 7.12−7.00 (overlap, 9H, Ar), 6.61 (br d, 2H, vinyl-H), 6.46
(d, J = 10.5 Hz, 2H, vinyl-H), 6.36 (d, J = 10.5 Hz, 2H, vinyl-
H), 6.17−6.05 (overlap, 16H, vinyl-H), 3.59 (overlap, 6H,
OMe), 3.49 (overlap, 12H, OMe), 3.48 (overlap, 12H, OMe),
3.26 (overlap, 6H, OMe); 13C NMR (125 MHz, CDCl3):
δ(ppm) 143.32, 142.96, 142.87, 140.77, 140.40, 140.36, 139.78,
139.52, 139.41, 137.04, 136.14, 135.47, 133.94, 133.55, 133.50,
133.25, 133.12, 133.05, 132.87, 132.75, 132.37, 128.05, 127.62,
127.44, 126.94, 126.76, 126.36, 126.32, 126.28, 126.24, 126.22,
125.42, 124.26, 79.81, 76.41, 74.63, 74.52, 74.13, 74.10, 52.76,
52.51, 52.11, 51.86. MALDI-TOF m/z calcd for C118H104O12
(M)+: 1714.08, Found: 1713.4715. IR(neat): 729, 819, 950,
1016, 1079, 1174, 1490, 2822, 2934 cm−1.
sssure, the crude yellow solid was purified by chromatography
(hexanes/dichloromethane = 2:3) to give compound 2 as a
yellow solid (13 mg, 48%, d.p. > 350 °C). 1H NMR (400 MHz,
CDCl3): δ(ppm) 8.00 (m, 1H), 7.82 (overlap, 2H), 7.68 (br s,
1H) 7.49 (overlap, 54H), 7.14 (overlap, 8H), 7.01 (overlap,
2H). 13C NMR (100 MHz, CDCl3): δ(ppm) 137.38−138.23
(multiple overlapping peaks), 127.38 (multiple overlapping
peaks). MALDI-TOF m/z calcd for C106H68 (M)+: 1341.67,
Found: 1341.1713. IR(neat): 729, 815, 1074, 1262, 1482, 1586,
2364, 2980, 3021.
ASSOCIATED CONTENT
■
S
* Supporting Information
General experimental considerations, syntheses and character-
ization of all new compounds, UV−vis, fluorescence, and cyclic
voltammetry spectra, details of calculations, and movie of
conformational change. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jasti@bu.edu.
Preparation of Sodium Naphthalenide (1.0 M in THF).
To a 25 mL dry round-bottom flask charged with a solution of
naphthalene (768 mg, 6.00 mmol) in 6 mL dry THF was added
sodium metal (207 mg, 9.00 mmol) under nitrogen. The
reaction mixture was stirred for 18 h at room temperature. After
this time, a green solution containing sodium naphthalenide
(1.0 M in THF) was formed.
Notes
The authors declare no competing financial interest.
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G
dx.doi.org/10.1021/ja307373r | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX