Arylene ethynylene macrocycles with intramolecular π-π Stacking

Article abstract of DOI:10.1021/ol101914f

Arylene ethynylene macrocycles containing 9,10-anthrylene or 1,4-naphthylene units were synthesized. In chloroform, significant resonance upfield shifting was observed with β-protons of anthrylene and naphthylene in NMR spectra. This was considered to result from partial stacking of these aromatic units intramolecularly, driven by attractive π-π interactions. DFT calculations supported the proposed intramolecular stacking motif. Moreover, a liquid-crystal phase was exhibited by the anthrylene-containing macrocycle, by virtue of the unique discotic shape.

Full text of DOI:10.1021/ol101914f

ORGANIC
LETTERS
2010
Vol. 12, No. 21
4784-4787
Arylene Ethynylene Macrocycles with
Intramolecular π-π Stacking
Shusen Chen, Qifan Yan, Tian Li, and Dahui Zhao*
Beijing National Laboratory for Molecular Sciences, Department of Applied Chemistry
and Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education,
College of Chemistry, Peking UniVersity, Beijing 100871, China
dhzhao@pku.edu.cn
Received August 13, 2010
ABSTRACT
Arylene ethynylene macrocycles containing 9,10-anthrylene or 1,4-naphthylene units were synthesized. In chloroform, significant resonance
upfield shifting was observed with ꢀ-protons of anthrylene and naphthylene in NMR spectra. This was considered to result from partial
stacking of these aromatic units intramolecularly, driven by attractive π-π interactions. DFT calculations supported the proposed intramolecular
stacking motif. Moreover, a liquid-crystal phase was exhibited by the anthrylene-containing macrocycle, by virtue of the unique discotic
shape.
Despite its relatively weak strength, as an important type of
noncovalent force π-π stacking interactions ubiquitously
influence crystal packing, molecular recognition, biomacro-
molecule conformations, and so on. Hence, considerable
research has been conducted to explore the nature as well
as applications of aromatic interactions.1 Particularly, inter-
molecular π-π stacking plays an important role in supramo-
lecular self-assembly of polycyclic aromatic systems.2,3
Electronic properties of the molecules are affected by π-π
interactions, resulting in exciton coupling, charge or energy
transfer, excimer or exciplex formation, etc.4,5 However,
intramolecular π-π stacking is employed for designing and
creating synthetic architectures with higher order structures.6
Owing to the shape-persistent, planar geometry, arylene
ethynylene macrocycles (AEMs) effectively harness π-π
interactions as the major driving force for intermolecular
association and self-assembly.7,8 Diverse supramolecular
architectures have been realized by AEMs via self-assembly,
including three-dimensional nanostructures,9 extended tubular
channels,10 discotic liquid crystals,11 host-guest com-
plexes,7b and so forth. Recently, self-assembled AEMs were
(6) (a) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S.
Chem. ReV. 2001, 101, 3893. (b) Saraogi, I.; Hamilton, A. D. Chem. Soc.
ReV. 2009, 38, 1726. (c) Ni, B.-B.; Yan, Q.; Ma, Y.; Zhao, D. Coord. Chem.
ReV. 2010, 254, 954
.
(7) (a) Zhao, D.; Moore, J. S. Chem. Commun. 2003, 807. (b) Ho¨ger,
S. Chem.sEur. J. 2004, 10, 1320. (c) Zhang, W.; Moore, J. S. Angew.
Chem., Int. Ed. 2006, 45, 4416
.
(1) (a) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112,
5525. (b) Grimme, S. Angew. Chem., Int. Ed. 2008, 47, 3430. (c) Wheeler,
S. E.; Houk, K. N. J. Am. Chem. Soc. 2008, 130, 10854. (d) Ringer, A. L.;
Sherrill, C. D. J. Am. Chem. Soc. 2009, 131, 4574.
(8) For recent examples of AEMs, see: (a) Mo¨ssinger, D.; Chaudhuri,
D.; Kudernac, T.; Lei, S.; De Feyter, S.; Lupton, J. M.; Ho¨ger, S. J. Am.
Chem. Soc. 2010, 132, 1410. (b) Chan, J. M. W.; Tischler, J. R.; Kooi,
S. E.; Bulovic´, V.; Swager, T. M. J. Am. Chem. Soc. 2009, 131, 5659. (c)
Kissel, P.; Schlu¨ter, A. D.; Sakamoto, J. Chem.sEur. J. 2009, 15, 8955.
(d) Tahara, K.; Fujita, T.; Sonoda, M.; Shiro, M.; Tobe, Y. J. Am. Chem.
Soc. 2008, 130, 14339. (e) Ishikawa, T.; Shimasaki, T.; Akashi, H.; Toyota,
S. Org. Lett. 2008, 10, 417. (f) Hartley, S. C.; Moore, J. S. J. Am. Chem.
Soc. 2007, 129, 11682. (g) Becker, K.; Lagoudakis, P. G.; Gaefke, G.; Ho¨ger,
S.; Lupton, J. M. Angew. Chem., Int. Ed. 2007, 46, 3450. (h) Toyota, S.;
(2) Klosterman, J. K.; Yamauchi, Y.; Fujita, M. Chem. Soc. ReV. 2009,
38, 1714
.
(3) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J.
Chem. ReV. 2005, 105, 1491
(4) Chen, Z.; Lohr, A.; Saha-Mo¨ller, C. R.; Wu¨rthner, F. Chem. Soc.
ReV. 2009, 38, 564
(5) Wu, J.; Pisula, W.; Mu¨llen, K. Chem. ReV. 2007, 107, 718
.
.
.
Goichi, M.; Kotani, M. Angew. Chem., Int. Ed. 2004, 43, 2248.
10.1021/ol101914f  2010 American Chemical Society
Published on Web 09/28/2010

shown to perform functions by virtue of their semiconductive
properties, which has further enhanced their research val-
ues.12,13 Here, we report an AEM that exhibited intramo-
lecular π-π stacking in solution and manifested a columnar
liquid crystal (LC) phase.
Previously, we studied an AEM composed of alternative
o-PE (phenylene ethynylene) and p-PE units which exhibited
notable photoconductive properties.13,14 Aimed to further
exploit related functions, an analogous macrocycle AEM-A
was designed (Figure 1). In addition to o-PE, this macrocycle
acetylene terminated molecule.14 The latter was obtained
upon removing the isopropanol protective groups from 2-A
(see the Supporting Information for details). A similar
reaction initially performed with a dibromo-substituted
analogue of 1-A was not successful, due to the lower
reactivity of aryl bromide in the Sonogashira reaction. The
dodecyloxy side chains on o-phenylenes conferred the target
molecule and precursors sufficient solubility in common
organic solvents, and allowed the use of flash column
chromatography for purification. The structure and purity of
1
AEM-A were fully characterized by H and 13C NMR,
MALDI-TOF mass spectroscopy, and elemental analysis.
In examining the 1H NMR spectrum of AEM-A, interest-
ing observations emerged (Figure 2). With a highly sym-
1
Figure 2
.
The aromatic region of H NMR (400 MHz, CDCl3)
spectra of 1-A, 2-A, and AEM-A with resonance assignments
(signals labeled a, b, c, etc. were assigned to protons with
corresponding tags in Figure 1; signals with an asterisk were from
residual CHCl3 in CDCl3).
metric structure, AEM-A exhibited only three resonances
in the aromatic region. Two multiplets at about 8.6 and 6.7
ppm were assigned to protons b and c of the anthrylene unit,
respectively. On the basis of previously published reports,
chemical shifts of ꢀ protons (2-, 3-, 6-, and 7-positions) of
anthracene, with 9- and 10-positions substituted with alkoxy-
phenylethynyl groups, should be in the range of 7.6 to 7.7
ppm.15 Apparently, in AEM-A the resonance of protons c
was significantly shifted upfield. Additionally, we noticed
that in the spectrum of 1-A protons c′ also exhibited an
unusual chemical shift of less than 7.0 ppm, although 2-A
gave a relatively normal value of ca. 7.7 ppm for its protons
c. Moreover, the resonance of protons a on dialkoxy
o-phenylene in both AEM-A and 1-A appeared abnormally
downfield at >7.3 ppm, although their usual chemical shift
should be around 7.0 ppm, as observed in a previously
synthesized macrocycle analogue.14 Namely, the aromatic
protons of AEM-A and 1-A displayed atypical chemical
shifts, while 2-A showed relatively normal values. Evidently,
the observed shifting cannot be fully justified by electronic
effects imparted by chemical structures. Furthermore, since
the chemical shifts and UV-vis absorption (Figure S2,
Supporting Information) of AEM-A both exhibited concen-
tration-independent properties within the investigated con-
Figure 1. Chemical structures of studied AEMs and their synthetic
precursors.
accommodated 9,10-anthrylene units. The structure was
accomplished via a multistep synthetic route. The final
reaction was a one-pot intermolecular Sonogashira cross-
coupling followed by intramolecular cyclization, carried out
between diiodo-functionalized precursor 1-A and a bis-
(9) Kim, J. K.; Lee, E.; Kim, M. C.; Sim, E.; Lee, M. J. Am. Chem.
Soc. 2009, 131, 17768
(10) Ono, K.; Tsukamoto, K.; Hosokawa, R.; Kato, M.; Suganuma, M.;
Tomura, M.; Sako, K.; Taga, K.; Saito, K. Nano Lett. 2009, 9, 122
.
.
(11) Previous examples of AEM macrocycles exhibiting a liquid crystal
state: (a) Zhang, J.; Moore, J. S. J. Am. Chem. Soc. 1994, 116, 2655. (b)
Ho¨ger, S.; Enkelmann, V.; Bonrad, K.; Tschierske, C. Angew. Chem., Int.
Ed. 2000, 39, 2268. (c) Ho¨ger, S.; Cheng, X. H.; Ramminger, A.-D.;
Enkelmann, V.; Rapp, A.; Mondeshki, M.; Schnell, I. Angew. Chem., Int.
Ed. 2005, 44, 2801. (d) Seo, S. H.; Jones, T. V.; Seyler, H.; Peters, J. O.;
Kim, T. H.; Chang, J. Y.; Tew, G. N. J. Am. Chem. Soc. 2006, 128, 9264.
(e) Shimura, H.; Yoshio, M.; Kato, T. Org. Biomol. Chem. 2009, 7, 3205
(12) (a) Zang, L.; Che, Y.; Moore, J. S. Acc. Chem. Res. 2008, 41, 1596.
(b) Che, Y.; Yang, X.; Zhang, Z.; Zuo, J.; Moore, J. S.; Zang, L. Chem.
.
Commun. 2010, 46, 4127
(13) Luo, J.; Yan, Q.; Zhou, Y.; Li, T.; Zhu, N.; Bai, C.; Cao, Y.; Wang,
J.; Pei, J.; Zhao, D. Chem. Commun. 2010, 46, 5725
.
(15) (a) Nakatsuji, S.; Matsuda, K.; Uesugi, Y.; Nakashima, K.;
Akiyama, S.; Fabian, W. Perkin Trans. 1 1992, 755. (b) Schmidt, R.;
Go¨ttling, S.; Leusser, D.; Stalke, D.; Krause, A.-M.; Wu¨rthner, F. J. Mater.
Chem. 2006, 16, 3708. (c) Valentini, L.; Bagnis, D.; Marrocchi, A.; Seri,
M.; Taticchi, A.; Kenny, J. M. Chem. Mater. 2008, 20, 32.
.
(14) Zhu, N.; Hu, W.; Han, S.; Wang, Q.; Zhao, D. Org. Lett. 2008, 10,
4283
.
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centration range, the possibility was excluded that the shifting
observed in 1H NMR spectra was caused by intermolecular
aggregation.
The above observations with chemical shifts led us to
proposing that in CDCl3 the anthrylene units of AEM-A
partially stacked with one another intramolecularly. Such a
hypothesis was put forward mainly for two reasons. First,
the relative spatial distance and orientation of the anthrylene
moieties in this molecule are confined by the o-PE linker,
so they have to partially overlap with one another intramo-
lecularly, unless adopting a significant dihedral angle with
the o-phenylene units. However, the noncoplanar conforma-
tion disrupts π-conjugation and may thus be disfavored. More
importantly, the observed chemical shifts served as sup-
portive evidence for the overlapped conformation. When the
anthracenes are intramolecularly stacked, certain ꢀ-protons
on one anthrylene may fall in the shielded region imposed
by the ring current of another anthrylene, thus exhibiting an
upfield shift in the NMR spectrum.16 In such an overlapping
conformation, protons on the stacked, intraannular benzo-
moieties should experience a different environment from
those on the nonstacked, extraannular benzo-groups. How-
ever, at even -50 °C the NMR spectrometer did not
distinguish these protons (Figure S5, Supporting Informa-
tion). It implied that the anthrylenes were still undergoing
rapid rotation around ethynylenes at such a low temperature
and the NMR thus detected averaged chemical shifts.
Precursor 1-A contained a pair of anthrylene units positioned
in a similar fashion with those in AEM-A. Due to the
asymmetric substitution, the ꢀ-protons of anthrylene in 1-A
were differentiated into two sets (c′ and c′′) and both
experienced upfield shifting, although to a different extent.
The overlapping conformation also explained the down-
field shifting of protons a in AEM-A and 1-A. When the
anthrylenes were intramolecularly stacked, their aromatic
rings were approximately in plane with the o-phenylene units.
Consequently, their ring current exerted a deshielding effect
on protons a, whereas in 2-A, since no such π-π interaction
stabilized anthrylene to be in plane with o-phenylenes, the
ring current entailed deshielding effect was less pronounced.
This was consistent with the experimental observation that
the resonance of a′′ appeared at ca. 7.2 ppm.
Figure 3. B3LYP/3-21G* optimized geometry of compounds 1-A
and AEM-A. Dodecyl chains were replaced by methyl groups in
the calculations.
exist among three equivalent conformers as depicted in
Figure 3, resulting in averaged chemical shifts in NMR
measurements.
Regarding the driving force for such an intramolecularly
stacked conformation, two possibilities were speculated. First,
stacking of the anthrylenes was driven by attractive π-π
interactions. Alternatively, the interactions among anthrylenes
are repulsive by nature but the steric repulsion was over-
whelmed by the conjugation energy favoring the coplanar
conformation of anthrylenes with respect to o-phenylenes,
forcing the anthrylenes to stack. To delineate the origin of
the stacking conformation, a new macrocycle, AEM-N, was
synthesized. It differed from AEM-A only by three naph-
thylene units in place of anthrylenes. Since 1,4-naphthylene
has only one benzene ring fused to the 1,4-phenylene, if the
aromatic rings were repulsive to each other, the benzo-portion
of naphthylenes may be flanked outside (extraannular) of
the macrocycle, but still remain in plane with o-phenylene
and nondisruptive to π-conjugation. On the other hand, if
the aromatic units were attracted to each other, AEM-N
would also display the stacked conformation. Hence, AEM-N
would provide an unambiguous clue for the nature of
interactions among these aromatic units.
As shown in Figure 4, protons c of macrocycle AEM-N
and c′ of 1-N exhibited similarly upfield-shifted chemical
Theoretical calculations were then conducted to help
analyze and illustrate the intramolecular stacking motif of
the anthrylenes. DFT-optimized conformations of AEM-A
and 1-A confirmed that the anthrylenes tended to stack with
another anthrylene. Under such a conformation, protons c
and c′ were located on top of one of the benzene rings from
the adjacent anthracene unit, where a strong shielding effect
should operate (Figure 3). The model also predicted that in
AEM-A two of the anthrylenes overlapped, while repelling
the third anthrylene to be out of plane with o-phenylenes.
Imaginably, at room temperature a rapid equilibration should
1
Figure 4
.
The aromatic region of H NMR (400 MHz, CDCl3)
spectra of 1-N, 2-N, and AEM-N with resonance assignments
(signals labeled a, b, c, d, etc. were assigned to protons with
corresponding tags in Figure 1; signals with an asterisk were from
residual CHCl3 in CDCl3).
(16) For other examples of molecules exhibiting resonance upfield
shifting in 1H NMR due to intramolecular π-π stacking and/or ring current
shielding effect in constrained structures, see: (a) Ting, Y.; Lai, Y. H. J. Am.
Chem. Soc. 2004, 126, 909. (b) Mei, X.; Wolf, C. J. Org. Chem. 2005, 70,
2299. (c) Nandy, R.; Subramoni, M.; Varghese, B.; Sankararaman, S. J.
Org. Chem. 2007, 72, 938.
shifts of about 7.0 ppm. In comparison, the resonance of
protons c appeared at nearly 7.7 ppm in 2-N. This result
suggested that in CDCl3 naphthylenes in AEM-N and 1-N
also stacked intramolecularly. Reasonably, the downfield
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Org. Lett., Vol. 12, No. 21, 2010

shifting of protons a in AEM-N and 1-N was less pro-
nounced compared to that in AEM-A and 1-A, appearing at
ca. 7.2 ppm. Despite the intramolecular stacking conforma-
tion, in the absence of the extraannular benzo-units, the
deshielding effect on protons a was evidently tempered in
AEM-N and 1-N. On the basis of the NMR spectrum of
AEM-N, it was concluded that the intramolecular π-π
interactions were attractive forces in these systems, rendering
both anthrylene and naphthylene units to stack intramolecu-
larly in chloroform solution.
Figure 5
A.
.
DSC trace and a POM image (taken at 153 °C) of AEM-
Interesting thermotropic behaviors were discovered for
AEM-A, which were characterized by differential scanning
calorimetry (DSC), polarized optical microscopy (POM), and
X-ray diffraction (XRD). The DSC traces were obtained at
a scan rate of 5 deg min-1 and multiple transitions were
detected for AEM-A (Figure 5).17 With POM, an evident
liquid-crystal (LC) texture with a strong birefringence was
observed at elevated temperature, indicative of an LC phase
of AEM-A (Figure 5). The existence of a mesophase phase
was further confirmed by XRD analysis (Figure S1, Sup-
porting Information). Further studies on this LC phase are
being carried out and will be reported in due course.
In conclusion, we have synthesized two shape-persistent
space ring-current effect to bring about the experimentally
observed proton resonance shifting. An LC phase was
detected for AEM-A, which is believed to be pertinent to
the unique intramolecularly stacked structure.
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (Projects
50603001 and 20774005), the Ministry of Science and
Technology of China (No. 2006CB921600), and the Fok
Ying-Tung Educational Foundation (No. 114008).
1
AEMs, AEM-A and AEM-N. Evidence from the H NMR
spectra suggested that the anthrylene and naphthylene units
in the macrocycles partially stacked intramolecularly, favored
by attractive π-π stacking interactions. DFT calculations
confirmed that the stacked conformation may confer through-
Supporting Information Available: Syntheses, optical
spectroscopy, wide-angle X-ray diffraction of the LC phase,
varied-temperature NMR spectra of AEM-A, and calculation
details. This material is available free of charge via the
Internet at http://pubs.acs.org.
(17) The DSC trace shown in Figure 5 is the first cooling and second
heating scans; the first heating scan was neglected to exclude the influence
of thermal history of the sample.
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