Symmetric six-fold arrays of photo- and electrochromic dithienylethene switches

Article abstract of DOI:10.1021/ol1006295

Figure presented The direct synthesis of six-fold symmetric hexaphenylbenzenes with multiple photochromic dithienylethene units via a cobalt-catalyzed cyclotrimerization is reported. This approach allows for six photochromic units to be held in proximity with a well-defined spatial separation without affecting the photochromic properties of each unit.

Full text of DOI:10.1021/ol1006295

ORGANIC
LETTERS
2010
Vol. 12, No. 9
2132-2135
Symmetric Six-Fold Arrays of Photo-
and Electrochromic Dithienylethene
Switches
Jetsuda Areephong, Hella Logtenberg, Wesley R. Browne,* and Ben L. Feringa*
Center for System Chemistry, Stratingh Institute for Chemistry, Faculty of Mathematics and
Natural Sciences, UniVersity of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands
w.r.browne@rug.nl; b.l.feringa@rug.nl
Received March 17, 2010
ABSTRACT
The direct synthesis of six-fold symmetric hexaphenylbenzenes with multiple photochromic dithienylethene units via a cobalt-catalyzed
cyclotrimerization is reported. This approach allows for six photochromic units to be held in proximity with a well-defined spatial separation
without affecting the photochromic properties of each unit.
The design and synthesis of highly symmetric (e.g., star-
shaped) macromolecules such as hexaarylbenzenes are of
current interest in material science owing to their potential
for application in host-guest systems,1 self-assembly,2
energy storage in chromophore aggregates,3 and liquid crystal
technologies,4 through the control they provide over the
orientation of functional units.5 Hexaarylbenzenes, due to
their symmetry and relative rigidity, are key structural
components on which to assemble,6 in a highly spatially
controlled manner, redox and photoresponsive molecular
units. Assemblies containing multiple redox- and photoactive
(4) Geng, Y.; Fechtenko¨tter, A.; Mu¨llen, K. J. Mater. Chem. 2001, 11,
1634–1641.
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Chem. Soc. 2003, 125, 3035–3045. (b) Maly, K. E.; Gagnon, E.; Maris, T.;
West, J. D. J. Am. Chem. Soc. 2007, 129, 4306–4322.
(2) Sajiri, K.; Sugisaki, T.; Moriya, K. Chem. Commun. 2008, 3447–
3449.
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5849–5860.
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A. P. H. J. J. Am. Chem. Soc. 2007, 129, 16190–16196.
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Bottari, G.; Torres, T. Chem. Commun. 2004, 2668–2669.
10.1021/ol1006295  2010 American Chemical Society
Published on Web 04/01/2010

units require, in addition to a rigid scaffold, robust and
synthetically accessible functional units.
Scheme 1. Synthesis of 1 and 2
Dithienylethenes show remarkable photo- and electro-
chromic behavior that can be tuned readily through synthetic
modification. Importantly, they show often excellent pho-
tostationary states, reversibility of switching, and high fidelity
between open (colorless) and closed (colored) forms upon
irradiation with UV and visible light, respectively.7 In
addition, the dithienylethene unit shows bidirectional redox-
driven switching both intrinsically8 and through appended
redox-active units.9 The versatility of dithienylethene subunits
as components for functional materials has been demon-
strated already7 and allows for reversible photochemical
control of properties as diverse as fluorescence,10 self-
assembly,11 and molecular conductivity.12 In designing a
multichromophoric array, an important consideration is
interchromophore interactions and their effect on the ef-
ficiency of photo- and electrochemical switching.
Multicomponent molecular systems for multimode switch-
ing based on the dithienylethene photochromic unit have been
reported recently where two or more switches are connected
covalently via a single methylene,13 ethynylene,14 diyne,15
phenylene,16 or silyl17 bridge. The photochemical properties
of these multicomponent systems have proven to be sensitive
to the nature of the bridging unit ranging from complete
inhibition of double ring closing13-15 to a complete absence
of interaction and hence ring closure of all units.16,17 A key
challenge is therefore to achieve a high density of switching
units in a controlled spatial arrangement without affecting
the individual switching properties of the constituent units.
Here we report the synthesis, photo- and redox chemistry
of star-shaped dithienylethene-substituted hexaphenylben-
zenes 1 and 2 (Scheme 1), prepared via cobalt-catalyzed
cyclotrimerization of alkynes, in which six (1, 2) or one (3)
photochromic units are connected around a central benzene
core. Importantly the photo- and electrochemical behaviors
of the dithienylethene photochromic units are unaffected
when held in the six-fold symmetric molecular arrangement
despite the individual switching units being in proximity.
Compound 3, in which only one dithienylethene unit is
attached to a hexaphenylbenzene core, was prepared by
treatment18 of dithienylethene 4 with t-BuLi at ambient
temperature. This was followed by reaction with tri-n-
butylborate to generate a boronic acid intermediate, which
was reacted immediately with iodohexaphenylbenzene19
5
in the presence of a palladium catalyst to yield 3 (51%). A
similar approach using a six-fold Suzuki coupling reaction
with hexakis-bromo-hexaphenylbenzene and the boronic ester
prepared from dithienylethene 4 (even with 12 equiv of the
boronic acid) resulted in recovery of starting materials and
a mixture of regioisomers. Cobalt-catalyzed cyclotrimeriza-
tion,20 a well-known method for the synthesis of hexaaryl-
benzenes with diverse substituents, typically proceeds in good
yields, even where the substituents are dendrons with large
molecular volumes.21 In the present study, this route proved
equally effective and provided direct access to hexaphenyl-
benzenes 1 and 2 in good yield (Scheme 1). Treatment18 of
dithienylethene 4 with t-BuLi and subsequent reaction with
tri-n-butylborate generated a boronic acid intermediate, which
was reacted with 4-bromo-4′-iodobiphenyl22 in the presence
of a palladium catalyst to yield compound 6 in 74% yield.
The introduction of a branched alkyl chain was achieved by
demethylation of compound 6 with boron tribromide, fol-
lowed by ether formation using (S)-1-bromo-3,7-dimethy-
loctane, to yield dithienylethene monomer 7 in 57% overall
isolated yield.
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L. N.; Uchida, K.; van Esch, J. H.; Feringa, B. L. Chem.sEur. J. 2005, 11,
6430–6441.
A Stille coupling reaction23 was used instead in the
synthesis of bis-dithienylethene acetylenes 8 and 9. Bis-
(tributylstannyl)acetylene served as the linker for 2 equiv of
either 6 or 7. The bis-dithienylethene acetylenes 8 and 9 were
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(18) See Supporting Information.
(19) Shen, X.; Ho, D. M.; Pascal, R. A., Jr. J. Am. Chem. Soc. 2004,
126, 5798–5805.
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B. J. Phys. ReV. Lett. 2003, 91, 207402. (b) Kronemeijer, A. J.; Akkerman,
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Sonogashira coupling procedure of Grieco and co-workers was employed
initially. However, in our hands, this method provided lower yields; 18%
yield of 8. Mio, M. J.; Kopel, L. C.; Braun, J. B.; Gadzikwa, T. L.; Hull,
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Org. Lett., Vol. 12, No. 9, 2010
2133

Table 1. Electronic Absorption and Redox Data for Open and PSS365nm (Closed) States
open form
closed form
a
b
a
a
b
abs λmax/nm (ε/103 M-1 cm-1
)
Ep,a
abs λmax/nm (ε/103 M-1 cm-1
)
E1/2
Ep,c
1
2
3
6
7
8
9
316 (50)
1.34(irr)
1.29(irr)
1.42(irr)
1.34(irr)
1.34(irr)
1.40(irr)
1.32(irr)
359 (31.5), 602 (20.1)
357 (201.3), 602 (120.9)
359 (222), 602 (131)
345 (31.0), 596 (17.3)
348 (35.3), 602 (21.4)
396 (89.5), 605 (48.3)
366 (91.0), 607(50.9)
0.79, 0.86
0.78, 0.87
0.78, 0.92
0.80, 0.93
0.80, 0.93
0.79, 0.90
0.79, 0.92
-1.24 (irr), -1.61(irr)
-1.33(irr), -1.85 (irr)
-1.20(irr), -1.58(irr)
n.d.
n.d.
n.d.
n.d.
324 (290)
324 (304)
290 (65.6)
312 (51.9)
347 (106.1)
344 (102)
a In toluene at 298 K. b With 0.3-1.1 mM in CH2Cl2 with 0.1 M TBAPF6 at 298 K; irr ) irreversible.
prepared in 66 and 67% yield, respectively, by a double
Stille-type24 reaction. A cobalt-catalyzed25 cyclotrimerization
of bis-dithienylethene acetylenes 8 and 9 was carried out
with catalytic Co2(CO)8 heated at reflux in 1,4-dioxane under
an argon atmosphere, affording compounds 1 and 2 in 44
and 66% yield, respectively. 1 and 2 were soluble in common
organic solvents, such as dichloromethane, chloroform, and
toluene, and were characterized by 1H/13C NMR spectroscopy
and MALDI-TOF mass spectrometry.18 Formation of the
hexaphenylbenzene core in 1 and 2 was confirmed by
comparison with the 13C NMR spectra of 8 and 9.18 In
particular, the characteristic resonances corresponding to the
sp-hybridized C atom at δ ) 90.2 ppm for compounds 8
and 9 are not present for 1 and 2, whereas an additional
resonance around δ ) 136.8 ppm is observed in the spectra
of 1 and 2. The signal is attributed to the six equivalent sp2-
hybridized C atoms of the central phenyl cores of 1 and 2,
and the data are similar to those described for hexaarylben-
zenes.26 MALDI-TOF mass spectrometry showed molecular
ions at m/z ) 3824 and 4581, as expected for 1 and 2,
respectively.
UV-vis absorption data of 1-3 and 6-9 are summarized
in Table 1. Irradiation of the open form with UV light (365
nm) results, in all cases, in formation of the closed form
with a photostationary state (PSS) of >90% (vide infra). The
switching behavior of compounds 1-3 is excellent, and
photochromic switching can be performed over several cycles
without degradation. Figure 1a shows the UV-vis absorption
spectra of the open and closed form of the star-shaped
dithienylethene-substituted hexaphenylbenzenes 1 and 2 in
toluene. Upon irradiation of 1 and 2 at 365 nm, the colorless
solution of the open forms turned blue, with the appearance
of an absorption maximum at 602 nm. Importantly, the rates
of ring closure for the hexa- (1/2) and monosubstituted (3)
hexaphenylbenzenes are essentially identical, indicating that
each switching unit behaves independently.18 The absorption
maxima of the closed forms of 1 and 2 are essentially the
same as those of 3 and 6-9, respectively.
Figure 1. Left: UV-vis absorption spectra of open and PSS365nm
of 1 and 3 (note that the molar absorptivity (ε) of 3 has been
multiplied by 6 to allow for comparison of spectral shape) in
toluene. Right: Cyclic voltammetry of 1o at a glassy carbon working
electrode in 0.1 M TBAPF6/CH2Cl2 vs SCE at 0.1 V s-1
.
the diphenylacetylene or hexaphenylbenzene cores. The
absorption spectra of 1o and 3o are shown in Figure 1. Upon
irradiation at 365 nm, both 3o and 1o undergo photocycliza-
tion, and two absorption bands appear in the visible region,
corresponding to 1c and 3c, respectively, with the same
maximum at 602 nm. The molar absorptivity of 1c is 6 times
that of 3c, confirming that the PSS365nm (in terms of the total
number of photochromic units) for 1 is essentially identical
to 3.27
FTIR spectroscopy has been shown previously to be a
versatile method for nondestructive readout of the state of
photochromic switches, not least dithienylethenes.28 Since
absorbance is linearly dependent on concentration, FTIR
spectroscopy can be used to estimate the photostationary state
achieved upon irradiation at λ365nm. In Figure 2, FTIR ATR
spectra are shown of a thin film of 2c (i.e., at the PSS365nm).
Upon irradiation at >400 nm, full conversion to the ring-
opened state is observed. Subsequent irradiation of the
ring-opened film at 365 nm leads to >90% conversion to
the ring-closed state.
The absence of a significant difference indicates that little
or no interaction between chromophores is mediated by either
(27) Kobatake, S.; Kuma, S.; Irie, M. Bull. Chem. Soc. Jpn. 2004, 77,
945–951.
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Barrett, L. J.; McGarvey, J. J.; van Esch, J. H.; Feringa, B. L. Org. Biomol.
Chem. 2006, 4, 2387–2392. (b) Takata, A.; Yokojima, S.; Nakagawa, H.;
Matsuzawa, Y.; Murakami, A.; Nakamura, S,; Irie, M.; Uchida, K. J. Phys.
Org. Chem. 2007, 20, 998–1006. (c) Bianco, A.; Bertarelli, C.; Rabolt, J. F.;
Zerbi, G. Chem. Mater. 2005, 17, 869–874.
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M.; Klute, W.; Tam, W. Chem. ReV. 1996, 96, 49–92.
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2134
Org. Lett., Vol. 12, No. 9, 2010

were confirmed by resonance Raman spectroscopy.18 Cyclic
voltammetry of 2o, which bears alkyl chains to increase
solubility, shows electrochemical behavior similar to that of
3o.
Overall, it is important to note that irrespective of the
number of the redox-active dithienylethene groups (i.e., six
dithienylethene units in 1 and 2, two in compounds 8 and 9,
and one in the compound 3) these dithienylethenes show
essentially the same characteristic cyclic voltammetry,
indicating that the dithienylethene units are not coupled
electronically and are sufficiently well-separated to minimize
electrostatic interactions.30 Thus, the redox waves observed
for 1c and 2c can be assigned to simultaneous multiple
electron transfer31 to produce highly charged stable species.
Here we have described the synthesis and spectroscopic and
electrochemical characterization of C6-symmetric star-shaped
hexa(dithienylethene)-substituted hexaphenylbenzenes 1 and
2 prepared using a simple cobalt-catalyzed cyclotrimerization.
The synthetic approach facilitates the synthesis of hexaphe-
nylbenzene-centered multidithienylethene systems in which
the dithienylethene units behave independently; that is, each
unit can be switched photo- or electrochemically. Impor-
tantly, there is no indication of intramolecular interaction
between the dithienylethene moieties, which is likely to be
due to the twisted conformation of the hexaphenylbenzene
core. The orientation of the phenyl rings in a paddle-wheel-
type conformation is anticipated to be the origin of the
absence of intercomponent communication and offers to be
a versatile future scaffold for high-density arrangement of
functional units. This approach to arranging chromophores
is expected to be general and allow for straightforward
synthetic access to systems in which multiple photochemical
and redox-active units are arranged with a high degree of
order and retention of molecular properties. Furthermore, the
solubility of these systems allows for further modification
at their periphery with additional functional units. This
provides an additional advantage when constructing highly
ordered functional arrays.
Figure 2. FTIR spectra of 2o (thick line) and at the PSS365nm (thin
line); samples in CH2Cl2 were deposited by evaporation onto a
ZnSe/diamond ATR crystal. The band at 897 cm-1 is present only
in the open state and at 914 cm-1 in the closed state.
The electrochemical properties of all compounds (Table
1) are in accordance with those of related dithienylethene
switches reported previously.8 In the open state, all com-
pounds undergo an irreversible oxidation at 1.42 V (vs SCE),
and on the return cycle, two new reversible redox processes
at 0.78 and 0.92 V (vs SCE) are observed at potentials
coincident with those of the closed forms. This indicates that
oxidative ring closure to 3c2+, for example, is occurring.
Intermediate 3c2+ can then be reduced, first to 3c+ and finally
to 3c. In addition, two irreversible reduction steps are
observed at -1.24 and -1.61 V (vs SCE) (not shown). The
characteristic cyclic voltammetry together with the absence
of a redox process between -2 and 1.5 V for the iodo-
hexaphenylbenzene 10 supports the assignment that the
oxidative processes observed are due to the dithienylethene
units.
The hexadithienylethene-substituted 1 shows the charac-
teristic irreversible oxidation at Ep,a ) 1.34 V (vs SCE),
leading to oxidative ring closing to 1c2+ (Figure 1), which
is subsequently reduced first to 1c+ and finally to 1c. The
cyclic voltammetry shows distinct differences with that of
3o because the highly charged polycationic species formed
upon oxidation is insoluble in CH2Cl2 and deposits on the
electrode surface. As a consequence, on the subsequent
cathodic sweep, a desorption spike29 at 0.83 V is observed,
where the precipitated polycationic molecules are reduced
to the more soluble 1c6+ and subsequently neutral 1c state,
allowing it to desorb from the surface of the electrode and
return to solution. The adsorption and desorption of 1c12+
Acknowledgment. We thank Ubbo Emmius Scholarship
(J.A.), NWO-Vidi (W.R.B., H.L.), Nanoned (W.R.B., B.L.F.),
and the ERC for Advanced Investigator Grant No. 227897
(B.L.F.) for financial support.
Supporting Information Available: Synthesis and char-
acterization of all compounds. NMR, FTIR, Raman and
UV-vis absorption spectra, and electrochemical data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL1006295
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