Switchable trans-cis interconversion of an amphiphilic anthracene trimer

Article abstract of DOI:10.1039/b912365b

Three anthracene rings connected by m-phenylene spacers with hydroxy groups generate isolable trans- and cis-atropisomers whose interconversion is solvent sensitive and can be activated at room temperature by an external stimulus (base).

Full text of DOI:10.1039/b912365b

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Switchable trans–cis interconversion of an amphiphilic anthracene
trimerw
Junji Iwasa,a Kosuke Ono,a Makoto Fujita,a Munetaka Akitab and Michito Yoshizawa*bc
Received (in Cambridge, UK) 23rd June 2009, Accepted 18th August 2009
First published as an Advance Article on the web 3rd September 2009
DOI: 10.1039/b912365b
Three anthracene rings connected by m-phenylene spacers with
hydroxy groups generate isolable trans- and cis-atropisomers
whose interconversion is solvent sensitive and can be activated at
room temperature by an external stimulus (base).
afforded 1. Compound 1 was fully characterized by MS,
NMR, and EA. The FAB-MS spectrum showed a single peak
1
at m/z = 746.8 ([M]+), but the H NMR spectrum revealed
the formation of a mixture of the trans and cis isomers of 1 in
ca. 2 : 1 ratio. The proton signals derived from hydroxy groups
and anthracene rings (HaBg) appeared around 4.7–4.9 and
7.5–8.6 ppm, respectively. The atropisomers were separated by
recycled gel permeation chromatography (GPC) (Fig. 2A,B).7
After isolation, no interconversion between the isomers was
observed in organic solvents at room temperature after several
months due to steric interactions between the bulky anthracene
units and the ortho-hydroxy groups.
Rotation around an aryl–aryl single bond is a fundamental
molecular motion.1 Unsubstituted biphenyl rotates freely at
room temperature but bulky ortho-substituents effectively
restrict and hinder bond rotation due to steric repulsion
between the substituents.2 Most aryl–aryl atropisomers
require elevated temperatures to interconvert, and switching
between freely rotating and non-rotating states at a moderate
temperature is rare. Controlling the rotation around an
aryl–aryl single bond by external stimuli is a fascinating
subject in terms of basic stereochemistry as well as potentially
useful for molecular-based switches and machines.3,4
Interconversion between the atropisomers of 1 could be
controlled with solvent and was activated at room temperature
in a basic solution. At room temperature in THF no
interconversion was observed but, as expected, upon heating
trans-1 (1.0 mM) at 80 1C torsional rotation occurred and
afforded a 3 : 2 trans : cis equilibrium mixture after 12 hours
(Table 1). Similar behavior was observed in a benzene
solution. In pyridine at 80 1C, the equilibrium favors the
trans-atropisomer to a greater extent (trans : cis = 4 : 1) but
after heating a methanol solution in a sealed tube at 120 1C,
the equilibrium slightly favored the cis form (trans : cis = 2 : 3).
In an aqueous 1 M NaOH solution, trans-1 almost fully
converted to cis-1 at room temperature in a 1 : 9 trans : cis
equilibrium mixture after one week.8
Here we report that anthracene trimer 1 shows solvent- and
pH-controllable trans–cis atropisomerism (Fig. 1).5
Compound 1 consists of three anthracene moieties connected
by m-phenylene spacers functionalized with hydroxy groups,
providing both hydrophobic and hydrophilic surfaces. The
aryl–aryl bonds of 1 are hindered by the three ortho-substituents,
which restrict the bond rotation and force the aryl subunits to
adopt orthogonal conformations. As a result, the tape-like
structure of 1 can take on a zig–zag (trans-1) or curved (cis-1)
form. In this communication, we report that the trans–cis
interconversion of 1 occurs at room temperature in a basic
aqueous solution. Not only is the free bond rotation activated
in the presence of base, but the cis isomer becomes thermo-
dynamically favored in an aqueous solution.
The remarkable effects of the basic aqueous solution
on the bond rotation9 and the equilibrium distribution are
rationalized as follows: typically, steric interactions with the
ortho-hydroxy group prevent anthracene rotation (Fig. 3A); after
deprotonation, there exists a quinoid resonance form with the
carbanion located on the anthracene part, stabilizing the planar
transition station state (Fig. 3B) and thus lowering the rotation
barrier.
Anthracene trimer 1 was synthesized by the following four-
step reaction sequence (Fig. 2). First, treatment of anthraquinone
with 2,4-dimethoxyphenyllithium and immediate reduction
gave 9,10-bis(2,4-dimethoxyphenyl)anthracene (2; R1 = H)
in 54% yield.6 After bromination, the Suzuki–Miyaura
coupling of brominated 2 (R1 = Br) with 9-anthrylboronic acid
yielded 3 in 59%. Demethylation of 3 by BBr3 quantitatively
The base-mediated trans–cis interconversion of 1 is general
and also occurred in a basic methanol solution at room
temperature but the trans–cis equilibrium ratio matched 1 in
non-basic methanol (trans : cis = 2 : 3). Gas-phase calculations
indicated that the two isomers have essentially identical
potential energies but the cis isomer of 1 is a curved,
a Department of Applied Chemistry, School of Engineering,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo,
113-8656, Japan
b Chemical Resources Laboratory, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
E-mail: yoshizawa.m.ac@m.titech.ac.jp; Fax: +81 45-924-5284;
Tel: +81 45-924-5230
c Precursory Research for Embryonic Science and Technology
(PRESTO), Japan Science and Technology Agency (JST),
4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
w Electronic supplementary information (ESI) available: Experimental
details for the preparation, characterization and properties of 1. See
DOI: 10.1039/b912365b
Fig. 1 Schematic representation of trans–cis conformational inter
conversion of anthracene trimer 1 through the aryl–aryl bond rotations.
ꢀc
This journal is The Royal Society of Chemistry 2009
5746 | Chem. Commun., 2009, 5746–5748

View Article Online
amphiphilic structure—the solvent exposed, hydrophilic
exterior is covered by the hydroxy groups and the concave
aromatic interior is hydrophobic (Fig. 3C). In an aqueous
solution, solvent–solute interactions and hydrophobic induced
self-aggregation thermodynamically favored the cis isomer.
1
Self-aggregation was evidenced by H NMR; the spectrum of
cis-1 was severely broadened in a basic aqueous solution,
indicative of aggregation, whereas that of trans-1 displayed
sharp signals.10
The UV-vis spectrum of 1 in EtOH shows a broadened
absorption band (lmax = 390 nm, e = 1.9 Â 104) which is
red-shifted (Dl = 30 nm) compared to anthracene (Fig. 4A).
The UV-vis spectrum of 9,10-bis(2,4-dihydroxyphenyl)anthracene,
the basic core unit of 1, was similar to that of 1 indicating that
each anthracene unit is electronically isolated due to the
orthogonal conformations and limited p-conjugation. In a
basic aqueous solution, a broad shoulder band at B450 nm
was present and stems from the phenoxide. In EtOH, 1 is
emissive (lmax = 436 nm, f = 0.33) but the emission was
completely quenched in a basic aqueous solution due to
photoinduced electron transfer (PET) from the phenoxide to
the anthracene fragment (Fig. 4B).11
Fig. 2 Synthesis of anthracene trimer 1: (a) 2,4-dimethoxyphenyl-
lithium, THF, À78 1C; (b) HI (aq), H3PO2, AcOH, 80 1C; (c) DBH,
THF, 0 1C; (d) 9-anthrylboronic acid, PdCl2(PhCN)2/P(t-Bu)3, DMF,
80 1C; (e) BBr3, CH2Cl2, 0 1C. 1H NMR spectra (500 MHz, CDCl3, r.t.)
of (A) cis-1 and (B) trans-1 (*: CHCl3).
Table 1 Trans–cis thermodynamic equilibria for 1 starting from
trans-1 in various solvents at different temperatures
Run
Solvent
Temperaturea/1C
trans : cis ratiob
1
2
3
4
5
THF
80
70
3 : 2
3 : 2
4 : 1
2 : 3
1 : 9
Benzene
Pyridinec
MeOH
80
Fig. 4 (A) UV-vis spectra of anthracene trimer 1 (red line) and
anthracene (blue line) in EtOH, and 1 in aqueous NaOH solution
(green line) at r.t. (B) Fluorescence spectra (lex = 385 nm) of
anthracene trimer 1 in EtOH (red line) and aqueous NaOH solution
(green line) at r.t.
120d
30
H2O + NaOH
a
b
trans-cis interconversion temperatures. trans-cis thermodynamic
equilibrium ratios determined by 1H NMR in CDCl3 at r.t. Starting
d
from cis-1. In a sealed tube.
c
In summary, we have prepared curved, amphiphilic
structure 1 with large hydrophobic anthracenes and hydrophilic
m-phenylene spacers. Rotation about the aryl–aryl bonds is
sterically restricted due to the large ortho-substituents, and the
trans and cis atropisomers were stable enough to be isolated.
The bond rotation and trans–cis interconversion at room
temperature were activated in basic solution in addition to
standard elevated temperatures. Although there is no significant
energy difference between the trans and cis atropisomers, the
concave cis isomer was thermodynamically favored in aqueous
solution and self-aggregated. We expect that the incorporation
of this dynamic trans–cis switch into static organic and
inorganic structures can provide novel molecular switches
and machines.
M. Y. is grateful to a Challenging Research Award from
Tokyo Institute of Technology and a Grant-in-Aid for Young
Scientists from the Ministry of Education, Culture, Sports,
Science and Technology of Japan for financial support of this
research. K. O. thanks the Japan Society for the Promotion of
Science (JSPS) for a Research Fellowship for Young Scientists.
The authors would like to thank Dr Jeremy K. Klosterman for
his helpful discussions.
Fig. 3 (A, B) Proposed mechanisms of the trans–cis interconversion
of 1 under neutral and basic conditions at room temperature.
(C) Optimized structures of trans-1 and cis-1, and their hydrophilic
and hydrophobic properties.
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5746–5748 | 5747

View Article Online
1H NMR in CDCl3. Due to the efficient shielding effect by the
surrounding three anthracene rings, the Hh signal of cis-1 was
observed higher up-field than that of trans-1 (Dd = À0.1 ppm). In
addition, the GPC retention time of trans-1 (a zig–zag form) is less
than that of cis-1 (a curved form).
Notes and references
1 I. Cepanec, Synthesis of biaryls, Elsevier, Dordrecht, The Netherlands,
2004.
2 (a) C. H. Christie and J. Kenner, J. Chem. Soc., Trans., 1922, 121,
614–620; (b) G. Bringmann, A. J. P. Mortimer, P. A. Keller,
M. J. Gresser, J. Garner and M. Breuning, Angew. Chem., Int.
Ed., 2005, 44, 5384–5427.
3 (a) Molecular Machines & Motors, ed. J.-P. Sauvage, Springer,
Berlin, 2001; (b) V. Balzani, A. Credi and M. Venturi, Molecular
Devices and Machines: Concepts and Perspectives for the
Nanoworld, Wiley-VCH, Weinheim, 2008.
4 (a) B. L. Feringa, R. A. van Delden and M. K. J. ter Wiel, Pure
Appl. Chem., 2003, 75, 563–575; (b) T. R. Kelly, Acc. Chem. Res.,
2001, 34, 514–522.
5 Unique cis–trans isomerization of ortho-substituted bisarylanthracenes
by 1O2 and heat (320 1C) has been recently reported, see: (a) D. Zehm,
W. Fudickar and T. Linker, Angew. Chem., Int. Ed., 2007, 46,
7689–7692; (b) D. Zehm, W. Fudickar, M. Hans, U. Schilde,
A. Kelling and T. Linker, Chem.–Eur. J., 2008, 14, 11429–11441;
(c) W. Fudickar and T. Linker, Chem. Commun., 2008, 1771–1773.
8 The thermodynamic equilibrium ratios were determined from not
only the various solutions of trans-1 but also those of cis-1.
9 (a) E. P. Kyba, G. W. Gokel, F. de Jong, K. Koga, L. R. Sousa,
M. G. Siegel, L. Kaplan, G. D. Y. Sogah and D. J. Cram, J. Org.
Chem., 1977, 42, 4173–4184; (b) A. K. Yudin, L. J. P. Marytn,
S. Pandiaraju, J. Zheng and A. Lough, Org. Lett., 2000, 2, 41–44.
10 The NMR spectrum of cis-1 in basic MeOH (and neutral MeOH)
display sharp signals (Fig. S13w). The progressive addition of basic
water (10, 30, 50%) into the MeOH solutions of 1 produced almost
no broadening effects for the cis-1 signals. Only in basic water (due
to the insolubility of cis-1 at neutral pH) does cis-1 form aggregates
and gives the broadened 1H NMR signals and UV-vis changes.
Concentration-dependent UV-vis spectra (2.5–150 mM) of cis-1 in
basic water showed a new broadened band around 420 nm only
under higher concentration (150 mM) (Fig. S15w).
11 (a) A. P. de Silva and S. A. de Silva, J. Chem. Soc., Chem.
Commun., 1986, 1709–1710; (b) R. A. Bissell, A. P. de Silva, H.
Q. N. Gunaratne, P. L. M. Lynch, G. E. M. Maguire and K. R. A.
S. Sandanayake, Chem. Soc. Rev., 1992, 21, 187–195;
(c) L. Fabbrizzi and A. Poggi, Chem. Soc. Rev., 1995, 24, 197–202.
6 U. Muller and M. Baumgarten, J. Am. Chem. Soc., 1995, 117,
¨
5840–5850.
7 Although we examined the trans–cis interconversion experiments
of 1 in various solutions, the assignment of the trans–cis isomers
and the thermodynamic equilibrium ratios were determined by
ꢀc
This journal is The Royal Society of Chemistry 2009
5748 | Chem. Commun., 2009, 5746–5748