Sterically crowding the bridge of dithienylcyclopentenes for enhanced photoswitching performance

Article abstract of DOI:10.1002/chem.201203111

Better switching: The introduction of bulky substituents into the bridge moiety of dithienylethenes led to derivatives exhibiting high photocyclization quantum yields. This novel and versatile form of substitution facilitated tuning of the switching performance without compromising on the optical and redox properties of the ring-open and ring-closed forms (see scheme). Copyright

Full text of DOI:10.1002/chem.201203111

DOI: 10.1002/chem.201203111
Sterically Crowding the Bridge of Dithienylcyclopentenes for Enhanced
Photoswitching Performance
Robert Gçstl, Bjçrn Kobin, Lutz Grubert, Michael Pꢀtzel, and Stefan Hecht*[a]
Diarylethenes (DAEs) are photochromic molecules with
outstanding physicochemical properties. DAEs undergo a
reversible 6p electrocyclic reaction upon irradiation with
light.[1] They are ideal for incorporation into materials for
optoelectronics because of their superior switching proper-
ties, in particular their thermal stability and high fatigue re-
sistance, together with the large optical and electronic
changes that occur upon their switching between ring-open
and ring-closed forms.[2] However, further improvement of
their switching efficiency[3] and reversibility[4] is of funda-
mental concern in view of their application in real devices.
The efficiency of the photocyclization reaction is governed
by its quantum yield (Foc), which is dependent on the equili-
brium between the parallel (p) and the antiparallel (ap) con-
formation (Scheme 1, top).[5,6] The quantum yield, Foc, is in-
trinsically limited to 0.5 for conventional DAEs because
photocyclization can take place in a conrotatory manner
from the ap conformation only. To achieve higher photocyc-
lization quantum yields it is necessary to bias the conforma-
Scheme 1. Schematic representation of the photocyclization of DAEs, as
governed by the equilibrium between parallel (p) and antiparallel (ap)
conformations (top). Steric repulsion can be used to bias the conforma-
tional equilibrium, through the introduction of bulky groups at either the
tional equilibrium and several approaches towards this goal
have been reported.
In one approach, mechanical locking of DAEs in the ap
conformation was achieved either by incorporating the pho-
tochromic unit into a polymer backbone[7] or by covalently
connecting bridging and aryl moieties.[8] However, these
methods are not generally applicable for small molecules
and do not allow versatile functionalization of the aryl moi-
eties. Alternatively, extraordinarily high photocyclization
quantum yields could be realized by incorporating attractive
hydrogen-bonding and sulfur–nitrogen interactions between
the bridge moiety and aryl termini of the switch.[3,9] Yet, a
fundamental drawback of using these directed noncovalent
interactions is their intrinsic solvent dependency, which is re-
flected in the varying Foc values.[3,9] In contrast, steric repul-
sion, which is largely independent of the environment, can
also be used to influence the equilibrium between p and ap
thiophene termini (bottom left) or in the bridge (bottom right).
conformation. Sterically demanding substituents can either
be introduced at the terminal aryl moieties or in the bridge.
The research group of Irie used the former approach to
obtain higher Foc values for one distinct type of derivative
(Scheme 1, bottom left).[5] However, the latter approach has
thus far not been systematically explored. Introducing steric
bias in the bridge should enable flexible functionalization of
the termini, an ability that is essential to realize and opti-
mize optoelectronic property changes (Scheme 1, bottom
right).
Herein, we report a synthetically straightforward method
for introducing bulky substituents into the bridge moiety of
dithienylcyclopentenes, thus leading to derivatives exhibiting
higher photocyclization quantum yield. We focused on
DAEs consisting of 3-thienyl termini linked by a cyclopen-
tene bridge because these systems are by far the most
widely employed.[1] The five-membered cyclopentene bridge
has been established to have the best balance of switching
performance and spectral differences between the two inter-
converting isomers.[10] Instead of the common perfluorocy-
clopentene bridging motif,[1] dithienylperhydrocyclopen-
[a] R. Gçstl, B. Kobin, Dr. L. Grubert, Dr. M. Pꢀtzel, Prof. S. Hecht
Department of Chemistry
Humboldt-Universitꢀt zu Berlin
Brook-Taylor-Str. 2, 12489 Berlin (Germany)
Fax : (+49) 30-2093-6940
E-mail: sh@chemie.hu-berlin.de
Homepage: http://www.hechtlab.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203111.
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Chem. Eur. J. 2012, 18, 14282 – 14285

COMMUNICATION
Scheme 2. Synthesis of dithienylethenes 20–27.
tenes[11] were chosen because they are readily accessible
through McMurry cyclization of the corresponding 1,5-dike-
tone precursors and their a-chlorothienyl termini allow for
modular postfunctionalization.[11]
To investigate the effect of substitution on the switching
efficiency, quantum yields of cyclization (Foc) and ring-
opening (Fco) were determined by absorption measurements
upon irradiation with either UV or visible light, respectively
(Table 1). From the data it is clear that compounds contain-
ing a,a’-disubstituted cyclopentene moieties have higher
quantum yields (Foc ꢀ0.6) than the parent unsubstituted
compound 20. Interestingly, neither the steric demand of
these a substituents, that is, whether the substituent is a
methyl group (21 and 22), an ethyl group (23), an isopropyl
group (24), or a phenyl group (25 and 26), nor the identity
of the diastereomer (21 versus 22 and 25 versus 26) has a
measurable effect on Foc. However, a,a,a’,a’-tetrasubstitut-
ed product 27 has the remarkably high Foc value of 0.83.
Our synthesis of the target photochromes 20–27 is based
on the easily accessible key a,a’-di- and a,a,a’,a’-tetrasubsti-
tuted 1,5-diketone intermediates 6–11 (Scheme 2). Starting
from 2-chloro-5-methyl-thiophene (1), alkanoylated com-
pounds 2–5 were easily obtained by employing Friedel–
Crafts acylation. Aldol condensation of 2–5 with formalde-
hyde afforded the corresponding a,b-unsaturated ketones,
which served as Michael acceptors in a conjugate addition
reaction with 2–5 to give a,a’-disubstituted diketones 6–9 in
moderate to excellent yields.[12] The transformation of ke-
tones bearing bulky substituents at the a position necessitat-
ed the use of a stronger base (KOtBu instead of KOH) to
achieve sufficiently efficient enolization. Notably, derivatives
bearing tertiary alkyl groups at the a position, such as tBu,
could not be prepared because the corresponding a,b-unsa-
turated ketone was not formed; only the aldol addition
product was isolated in this case. The a,a,a’,a’-tetramethyl
derivative 11 was obtained by methylation of precursor 10.
McMurry condensation of 1,5-diketones 6–11 gave dichloro-
substituted dithienylcyclopentenes 12–19, which were subse-
quently transformed into the target compounds 20–27
through in situ borylation followed by Suzuki cross-coupling
with p-tolyl bromide. In principle, any aryl halide could be
employed in this versatile late-stage functionalization reac-
tion. Notably, for the a,a’-disubstituted compounds, our syn-
thesis provides access to the target photochromes, which
were obtained as a mixture of cis and trans diastereomers of
varying composition; these diastereomers could be separat-
ed successfully and characterized by NMR analysis.
Table 1. Photocyclization (Foc) and ring-opening (Fco) quantum yields of
diarylcyclopentenes 20–29 in different solvents.
[a]
[b]
Compound
Foc
Fco
pss [%][c]
20[d]
21[d]
22[d]
23[d]
26[d]
27[d]
24[e]
25[e]
28[d]
29[d]
0.50
0.57
0.57
0.63
0.64
0.83
0.58
0.61
0.32
0.25[f]
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.04
0.54[g]
92
94
84
85
92
93
86
90
64
62
[a] lirr =313 nm. The uncertainty of measurements is estimated as being
Æ0.05 for ring closure. [b] lirr =546 nm. The uncertainty of measurements
is estimated as being Æ0.005 for ring opening. [c] Fraction of the ring-
closed isomer in the photostationary state (pss) as determined by ultra-
high-performance liquid chromatography (UPLC). [d] In MeCN. [e] In
MeCN, CH2Cl2, MeOH, toluene, and cyclohexane. [f] lirr =280 nm.
[g] lirr =436 nm.
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S. Hecht et al.
Most importantly, the large quantum yield of cyclization was
found to be independent of the polarity of the solvent, the
solvents tested being nonpolar, such as cyclohexane, to
polar, such as acetonitrile and the protic solvent, methanol.
The observed increase in the quantum yields of cycliza-
tion upon substitution can be rationalized by considering
the increased steric repulsion between the a substituents of
the cyclopentene bridge and both the a-methyl and b-hydro-
gen substituents of the adjacent terminal thienyl moieties, a
repulsion that would shift the conformational equilibria in
favor of the reactive ap conformer.
by the absorption spectra of the different photoswitches 20
and 27–29 (Figure 1). With increasing steric interactions be-
tween the substituents in the bridge and the thiophene ter-
To verify this assumption, ring-open forms of unsubstitut-
ed dithienylethene 20 and a,a,a’,a’-tetrasubstituted deriva-
tive 27 were investigated using variable-temperature
1H NMR spectroscopy. For compound 20, we were unable to
directly determine the ratio of the two conformers by inte-
gration of the respective signals since the signals derived
from the p and ap conformation were overlapping, even at
temperatures as low as À808C. For the best-performing
compound, dithienylethene 27, however, a nonlinear shifting
of various signals was observed upon temperature variation.
When the approach of Coudret and co-workers[13] was used,
the fraction of the ap conformer in the ring-open form at
258C was estimated as being 81Æ10% (see the Supporting
Information). This value is in good agreement with the ex-
perimentally determined quantum yield of cyclization, Foc =
0.83, and supports the underlying hypothesis that biasing the
conformational equilibrium towards the ap form is the key
mechanism that determines the improvement of photocycli-
zation quantum yields of dithienylethenes.
Figure 1. Maxima in the absorption spectra of ring-open (left) and ring-
closed (right) isomers of DAEs 20 and 27–29 (258C, ca. 10À5 m in MeCN)
with steric strain increasing in the direction of the arrow.
mini, the absorption maxima in the spectra of both the ring-
open and ring-closed isomers show considerable hypsochro-
mic shifts (see also the Supporting Information, Table S-1).
These shifts suggest that in the ring-open and the ring-
closed forms of diarylcyclopentenes 28 and 29, the p conju-
gation is heavily disturbed. Because 28 and 29 have photo-
stationary states that are composed of lower amounts of the
ring-closed form relative to that of 27, and the quantum
yields for ring opening are much higher (as high as 0.5), it is
reasonable to assume that the lower photocyclization quan-
tum yields result from lower thermodynamic stability of the
ring-closed isomers.
To gain further insight into the steric repulsion between
substituents, dithienylcyclopentene 28, which contains
methyl groups only at the 3- and 5-positions of the thio-
phene termini,[14] as well as dibenzothienylcyclopentene 29,
which contains substituents on both the bridge moiety and
the thiophene termini, were prepared and investigated
(Scheme 3). For 28, the photocyclization quantum yield was
In contrast, the introduction of substituents at the a and
a’ positions of the cyclopentene bridge does not affect the
absorption spectra, the spectra of derivatives 20–27 being
almost identical and showing absorption maxima at around
530 nm (see also the Supporting Information, Figure S-1 and
S-2). In addition, the quantum yields of ring opening, Fco, of
these dithienylcyclopentenes can be considered constant and
independent of the substitution pattern of the bridge
moiety.
Scheme 3. DAEs 28 and 29 with blocked a positions and increased steric
strain.
Considering the above data, tetramethylsubstituted deriv-
ative 27 exhibits the optimum substitution pattern for maxi-
mizing photocyclization quantum yield without compromis-
ing on other switching attributes.
Most importantly, the improved switching performance
does not lead to changes in optical and redox properties of
the two switching states, properties that are crucial for
gating optoelectronic functionality.[16] This key point is illus-
trated by a comparison of the absorption spectra and the
first reversible oxidation potentials of the ring-open and
ring-closed forms of the investigated photochromes
only 0.32 (see Table 1), which is significantly lower than that
of 27 and indicates either unfavorable steric or electronic in-
teractions; these data are in agreement with known b-
methyl substituted perfluorocyclopentenes, which show simi-
lar behavior.[15] Surprisingly, for 29, the photocyclization
quantum yield, Foc, was similarly low, thus suggesting that
the advantageous effect of the a substituents is negated by
the substituents on the thiophene termini, presumably be-
cause of excessive steric strain. This assumption is supported
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Enhanced Photoswitching of Dithienylcyclopentenes
COMMUNICATION
(Table 2). The large spectral separation between the ring-
open and ring-closed isomers of about 250 nm remains con-
stant throughout the series, that is, the absorption spectra do
Keywords: aldol reaction · diarylethenes · photochromism ·
quantum yield · steric hindrance
Table 2. Oxidation potentials and absorption maxima of ring-open and
-closed dithienylcyclopentenes 20–27 and their difference.
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Compound
Ring-open form
Ring-closed form
Difference Doc
a
a1
a1
Ep
lmax
[nm][b]
Ep
lmax
DEp
Dlmax
[V][a]
[V][a]
[nm]
[V][a]
[nm]
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21
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0.73
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0.80
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282
282
282
283
284
285
284
283
À0.02
À0.03
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À0.06
0.04
527
524
526
529
537
532
529
529
0.75
0.77
0.74
0.79
0.82
0.76
0.73
0.79
245
242
244
246
253
247
245
246
0.05
À0.01
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In conclusion, we have successfully developed a strategy
to improve the switching performance of DAE photo-
chromes. By introducing bulky substituents in the bridge
moiety of dithienylcyclopentenes, derivatives exhibiting en-
hanced photocyclization quantum yield were obtained, the
quantum yield being independent of the environment (sol-
vent). The improved switching efficiency was achieved with-
out compromising on the differences in the optical and
redox properties of the ring-open and ring-closed states,
thus rendering the new photoswitches ideal candidates for
applications in light-gated optoelectronic devices. Investiga-
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conformational equilibrium, and on the diastereoselectivity
of the ring-closing reaction are currently underway in our
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Acknowledgements
The authors are indebted to Jutta Schwarz for helpful synthesis, Martin
Herder for valuable discussions, and Prof. Dr. Clemens Mꢂgge and Kath-
arina Pfaff for VT-NMR measurements. Generous support by the
German Research Foundation (DFG via SFB 658) is gratefully acknowl-
edged. BASF AG, Bayer Industry Services, and Sasol Germany are
thanked for generous donations of chemicals.
Received: September 3, 2012
Published online: October 2, 2012
Chem. Eur. J. 2012, 18, 14282 – 14285
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14285