Switching diarylethenes reliably in both directions with visible light

Article abstract of DOI:10.1002/anie.201509875

A diarylethene photoswitch was covalently connected to two small triplet sensitizer moieties in a conjugated and nonconjugated fashion and the photochromic performance of the resulting compounds was investigated. In comparison with the parent diarylethene (without sensitizers) and one featuring saturated linkages, the conjugated photoswitch offers superior fatigue resistance upon visible-light excitation due to effective triplet energy transfer from the biacetyl termini to the diarylethene core. Our design makes it possible to switch diarylethenes with visible light in both directions in a highly efficient and robust fashion based on extending π-conjugation and by-product-free ring-closure via the triplet manifold. Flipping the spin: When biacetyl triplet sensitizers were linked to a dithienylethene core in a conjugated fashion, the photoswitching performance dramatically improved. This design makes it possible to switch diarylethenes with visible light in both directions in a highly efficient and robust fashion.

Full text of DOI:10.1002/anie.201509875

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International Edition: DOI: 10.1002/anie.201509875
German Edition: DOI: 10.1002/ange.201509875
Photoswitches
Hot Paper
Switching Diarylethenes Reliably in Both Directions with Visible Light
Sebastian Fredrich, Robert Gçstl, Martin Herder, Lutz Grubert, and Stefan Hecht*
Abstract: A diarylethene photoswitch was covalently con-
nected to two small triplet sensitizer moieties in a conjugated
and nonconjugated fashion and the photochromic perfor-
mance of the resulting compounds was investigated. In
comparison with the parent diarylethene (without sensitizers)
and one featuring saturated linkages, the conjugated photo-
switch offers superior fatigue resistance upon visible-light
excitation due to effective triplet energy transfer from the
biacetyl termini to the diarylethene core. Our design makes it
possible to switch diarylethenes with visible light in both
directions in a highly efficient and robust fashion based on
extending p-conjugation and by-product-free ring-closure via
the triplet manifold.
ring-closed isomer. In addition to the issue of fatigue
resistance, several attempts have been made to red-shift the
absorption of the open form in order to utilize visible light
also for ring-closing. This issue has been addressed directly by
elongating the conjugated p-system[11] and indirectly by
attaching (rather extended) antenna chromophores.[12] Here
we show that both of these critical aspects can be addressed
simultaneously by attaching small triplet-sensitizing moieties
to a diarylethene core via a p-conjugated linkage. Thus, our
design takes advantage of both the deliberate manipulation of
the singlet–triplet transition and the bathochromically shifted
absorption due to the extended p-system to achieve ring-
closure via the triplet manifold upon visible-light excitation.
Our approach is based on recent findings showing that by-
product formation occurs exclusively from an excited singlet
state[9c,13] and hence ring-closure via the triplet manifold
represents an attractive strategy to circumvent this detrimen-
tal side reaction. In fact, triplet sensitization of diarylethenes
has previously been reported using transition-metal com-
plexes[14] as well as solely organic[9c,15] triplet sensitizers in
either intermolecular or intramolecular fashion. Indeed,
formation of the by-product during the conversion of
a parent diarylethene 1o to the closed isomer 1c could be
avoided by intermolecular triplet sensitization using an excess
of biacetyl (Scheme 1).[9c] Although biacetyl offers a high
intersystem crossing efficiency of practically 100%[16] and
a high-energy triplet state able to sensitize the open form of
the diarylethene, it suffers from an extremely low extinction
coefficient of 20mÀ1 cmÀ1 at 400 nm originating from the
corresponding symmetry-forbidden n!p* transition. As
a result, a large excess of biacetyl (250 equivalents) was
needed to ensure sufficient absorption at 400 nm and local
concentration in the bimolecular sensitization process. In
order to avoid this concentration dependence we have
covalently connected the biacetyl and diarylethene units,
using saturated and p-conjugated linkages to afford diaryl-
ethenes 2 and 3, respectively (Scheme 1).
To synthesize target compounds 2 and 3 the modular route
developed by Feringa and co-workers[17] was adapted by
attaching the appropriate biacetyl-substituted phenyl residues
to the preformed diarylethene core via a Suzuki cross-
coupling reaction. The corresponding biacetyl-substituted
phenylbromides were readily available from diethyl ketal
monoprotected biacetyl via aldol condensation with benzal-
dehydes. In the case of the saturated linker, the double bond
was reduced by transfer hydrogenation. Suzuki cross-coupling
of the phenyl bromides to 1,2-bis(2-chloro-5-methylthien-3-
yl)cyclopentene involving in situ borylation followed by final
deprotection provided photoswitches 2 and 3 as their open
isomers. For further details on the synthetic procedures and
compound characterization, consult the Supporting Informa-
tion.
O
ver the past years molecular switches[1] have received
much attention due to their promising role as active control
elements in functional materials and devices.[2] The unique
characteristic of a switch as compared to a fuse lies in its
reversibility. However, in order to exploit this distinguishing
feature truly robust systems associated with a pronounced
fatigue resistance—the capacity to undergo many switching
cycles without considerable degradation—are needed.
Another important practical demand relates to the necessity
to operate the switch (in both directions) by visible light,[3] for
example to reduce harmful effects of high-energy UV light
and to ensure deeper penetration into the (bio)material.
Most families of molecular switches rely on two distinct
mechanisms: Either E/Z isomerization of a double bond,
involving a significant geometrical change, or 6p electro-
cyclization of a triene system that enables strong electronic
modulation. Azobenzenes,[4] the most prominent family in the
former class, are known to perform reliably and can be
addressed directly by visible-light excitation.[5] In contrast,
diarylethenes,[6] representing arguably the most attractive
family in the latter class, require an elaborate design to
achieve high fatigue resistance. Whereas in the solid state the
molecular environment of a single crystal allows for thou-
sands of switching cycles without pronounced degradation
upon alternating irradiation with UV and visible light,[7]
highly repetitive switching remains a critical issue in solu-
tion.[8] To meet this challenge, the two main strategies involve
incorporation of electron-withdrawing groups[9] and steric
hindrance[10] to prevent formation of a common by-product
caused by a UV-induced irreversible photoreaction of the
[*] S. Fredrich, Dr. R. Gçstl, Dr. M. Herder, Dr. L. Grubert,
Prof. Dr. S. Hecht
Department of Chemistry, Humboldt-Universität zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
E-mail: sh@chemie.hu-berlin.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201509875.
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Irradiation of an ace-
tonitrile solution of 2o at
lirr = 313 nm results in
quantitative photocon-
version to the closed
isomer 2c, which displays
a
characteristic visible
absorption band at lmax
=
526 nm similar to 1o and
typical for ring-closed
diarylethenes.
Analo-
gously, irradiation of the
conjugated photoswitch
3o in acetonitrile at lirr
=
313 nm leads to forma-
tion of the respective
ring-closed isomer 3c.
During ring-closure, the
band at lmax = 390 nm
diminishes and two new
bands appear: One at
lmax = 330 nm and the
other one in the visible
range at lmax = 608 nm. In
Scheme 1. Evolution of the photoswitch design starting from intermolecular triplet sensitization of the parent
dithienylethene 1 with biacetyl (top) to intramolecular sensitization via a decoupled system 2 (center) to the best
performing p-conjugated switch 3 (bottom), which does not display irreversible by-product (bp) formation upon
visible-light irradiation (l=405 nm).
The absorption spectra of the saturated photoswitch 2
closely resemble those of the parent diarylethene 1 (Figure 1).
The presence of the (decoupled) biacetyl moiety is revealed
by a weak shoulder reaching into the visible range of the
spectrum. In contrast, the absorption band of the unsaturated
comparison with 2c, the latter is strongly red-shifted by 82 nm
(Figure 1), again illustrating the extended p-conjugation. The
photoconversion, that is, the amount of isomer 3c that can be
attained in the photostationary state (PSS) upon ring-closure
at lirr = 313 nm, is lower than in the case of 2o!2c due to an
increasing absorption of the ring-closed isomer upon irradi-
ation at 313 nm (Table 1).
derivative 3o is strongly bathochromically shifted to lmax
=
Because of the large bathochromic shift of 3o, it should
also be possible to induce ring-closure in the blue region of
the visible spectrum. Indeed, when irradiation at lirr = 405 nm
was employed, complete photoconversion of 3o to 3c is
observed, whereas 1o and 2o do not undergo any detectable
photocyclization under these conditions. This is most likely
due to the lack of absorbance of 1o and the negligible
extinction coefficient of 2o at lirr = 405 nm as well as the
stronger absorbance of the formed closed isomers at the same
wavelength, thus favoring cycloreversion instead. The signifi-
cant effect of the conjugated linkage on shifting a high
extinction band into the visible range has also been verified
independently for model compounds (see Figure S6 in the
Supporting Information). Furthermore, the presence of
oxygen in solution has a considerable effect on the time
required to reach the PSS in the irradiation of 3o, while
irradiations of 1o and 2o were not affected.
To quantify the efficiencies of the photochemical ring-
opening and ring-closure reactions of 2 and 3, quantum yields
were determined both in the presence and absence of oxygen
(Table 1). The quantum yields for ring-closure of 2o upon
lirr = 313 nm irradiation are comparable to those of the parent
dithienylethene 1[9c,18] and are not dependent on the presence
of oxygen, similar to most conventional diarylethenes. For the
Figure 1. Absorption spectra of the investigated dithienylethenes with-
out biacetyl (1, black), with a saturated linker (2, blue), and with
a conjugated linker (3, red) as their open (solid) as well as closed
isomers (dashed) in acetonitrile (2”10À5 m) at room temperature.
390 nm, as compared to lmax = 282 nm (for 2o), due to the
elongated p-system and its push–pull character originating
from the electron-rich thiophene and the electron-withdraw-
ing diketone units. Most importantly, the molar absorptivity at
400 nm increases by about 1000-fold, rendering the excitation
of photoswitch 3o in the visible range a lot more efficient than
that of 2o.
conjugated photoswitch 3o
a different behavior was
observed. Upon irradiation of a degassed solution of 3o at
lirr = 405 nm a quantum yield of 30% was determined,
whereas in the presence of oxygen it drastically decreased
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Table 1: Quantum yields (F) and composition of the photostationary
state (PSS) upon irradiation of the given switch isomers at the depicted
wavelengths (lirr) in aerated or degassed acetonitrile (2”10À5 m) at
258C.[a]
quantum yield for the ring-opening of 3c is lower than that of
the the ring-opening of the saturated compound 2c and the
parent diarylethene 1c may be explained by a competing
population of the unreactive triplet as well as the generally
less efficient cycloreversion in the excited singlet state as
a consequence of the elongated p-system.[23] This in fact
illustrates an intrinsic drawback of our approach as the
enhanced fatigue resistance in the triplet manifold is accom-
panied by a reduced ring-opening efficiency.
To gather further evidence for the involvement of a triplet
state in the cyclization of 3o oxygen was replaced by another
quencher, for which the concentration could easily be
adjusted. Anthracene was chosen because it features a low-
lying triplet state, which (in the absence of oxygen) thermally
relaxes without any by-product formation, and its absorption
spectra display no extinction at the irradiation wavelength
(lirr = 405 nm). We measured the quantum yield of the ring-
closure of 3o at varying concentrations of the anthracene
quencher and the resulting Stern–Volmer analysis showed
a linear relationship (see Figure S9 in the Supporting
Information), providing clear evidence that cyclization pro-
ceeds via a triplet state.
Photoreaction
lirr [nm]
F [%]
PSS composition
313
405
43[9c]
–
92% 1c[18]
0% 1c
1o!1c
1o+biacetyl!1c
405
405
–
0% 1c
(aerated)
1o+biacetyl!1c
36
100% 1c
(degassed)
2o!2c
313
405
43
–
100% 2c
0% 2c
(aerated)
2o!2c
313
405
42
–
100% 2c
0% 2c
(degassed)
3o!3c
313
405
0.5
0.9
66% 3c, 34% 3o
100% 3c
(aerated)
3o!3c
313
405
18
30
66% 3c, 34% 3o
100% 3c
(degassed)
One advantage of triplet sensitization is the possible use
of light with lower energy in contrast to conventional
diarylethenes. As UV light is known to induce side reactions
arising from the ring-closed isomer, the fatigue resistance of
photoswitch 3 should be improved. To investigate this aspect,
1c!1o[9c]
2c!2o
3c!3o
546
546
579
0.8
0.62
0.028
100% 1o
100% 2o
100% 3o
switching cycles consisting of alternating ring-closure at lirr
=
405 nm and ring-opening with lirr > 500 nm were carried out
(Figure 2). Repeatedly the long-wavelength band of the
closed isomer at lmax = 608 nm could be reestablished, indi-
cating no loss of material as the same amount of ring-closed
isomer is consistently regenerated over the course of many
switching cycles. This is in strong contrast to conventional
diarylethenes, such as parent dithienylethene 1, which upon
an increasing number of switching cycles exhibit a continuous
[a] Biacetyl was added in 250-fold excess.
to 0.9%. This finding is a clear indication for the population
of an excited triplet state of 3o, which is quenched by
molecular oxygen and thereby acts as a singlet-oxygen
sensitizer.[19] In the meantime not only the quantum yield
but also the fatigue resistance decreases in the presence of air
primarily due to formation of reactive singlet oxygen, which
yields various oxidation by-products.[20] A similar tendency is
observed for irradiation of 3o at lirr = 313 nm; however, the
generally lower quantum yields compared to those upon
irradiation at lirr = 405 nm indicate a less efficient population
of the reactive triplet excited state. A significantly reduced
intersystem-crossing quantum yield compared to that of the
parent biacetyl is not likely since comparable values have
been determined for benzil with FISC = 0.92.[21] Note that the
effective quantum yield for the intermolecular sensitization of
1o to 1c using excess biacetyl upon irradiation at lirr = 405 nm
was 36%, which is comparable to the value obtained for the
ring-closure of 3o. No trace of 1c was observed in the
experiments without biacetyl or in the presence of oxygen at
lirr = 405 nm.
The ring-closed isomer 3c can readily be converted back
to its open form 3o by irradiation of the long-wavelength
band at l > 500 nm. The associated quantum yield of the
cycloreversion reaction is rather low and is independent of the
presence of oxygen. As earlier work has shown that the
cycloreversion in the excited triplet state is inhibited due to
large thermal barriers,[22] it is likely that the ring-opening
reaction of 3c proceeds in the singlet state. The fact that the
Figure 2. Composition of the photostationary state (PSS, right) as
measured by the long-wavelength absorption maximum of 3c at
lmax =608 nm during alternating switching cycles consisting of ring-
closure (lirr =405 nm, t=20 min, light gray areas) and ring-opening
(lirr >500 nm, t=100 min, gray areas) of a 2”10À5 m degassed aceto-
nitrile solution irradiated with a 1000 W high-pressure Xe lamp. The
maximum for pure 3c is indicated by the dashed line.
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decay of the visible band indicating an increasing loss of
Current efforts are devoted to designing alternative sensi-
material due to by-product formation (see Figure S10 in the
Supporting Information). Note that the number of cycles that
can conveniently be obtained for diarylethene 3 is limited by
the rather poor cycloreversion efficiency necessitating
extended irradiation times for backswitching, which was not
pushed to completion for time reasons (see varying minima in
Figure 2).
tizer-switch constructs to optimize the ring-opening quantum
yields.
Acknowledgements
Generous support by the German Research Foundation
(DFG via SFB 658, project B8), the European Research
Council (ERC via ERC-2012-STG_308117 “Light4Func-
tion”), and the European Commission (via ITN “iSwitch”)
is gratefully acknowledged. We thank BASF AG, Bayer
Industry Services, and Sasol Germany for generous donations
of chemicals.
For a true performance test 3 was exposed to long-time
irradiation in order to force by-product formation (Figure 3).
Keywords: diarylethenes · photochromism · sensitizers ·
singlet oxygen
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Angew. Chem. 2016, 128, 1226–1230
[1] Representative recent overviews are given in: a) Molecular
Switches (Eds.: B. L. Feringa, W. R. Browne), 2nd ed., Wiley-
VCH, Weinheim, 2011; b) New Frontiers in Photochromism
(Eds.: M. Irie, Y. Yokoyama, T. Seki) Springer, Japan, 2013.
[2] For reviews, see: a) J. M. Abendroth, O. S. Bushuyev, P. S. Weiss,
C. J. Barrett, ACS Nano 2015, 9, 7746 – 7768; b) E. Orgiu, P.
Samorì, Adv. Mater. 2014, 26, 1827 – 1845; c) M.-M. Russew, S.
Hecht, Adv. Mater. 2010, 22, 3348 – 3360.
[3] D. BlØger, S. Hecht, Angew. Chem. Int. Ed. 2015, 54, 11338 –
11349; Angew. Chem. 2015, 127, 11494 – 11506.
[4] For a review, see: a) H. M. D. Bandara, S. C. Burdette, Chem.
Soc. Rev. 2012, 41, 1809 – 1825.
Figure 3. Evolution of the absorbance at the long-wavelength band
associated with the ring-closed isomer for the given switches upon
constant irradiation with a 1000 W high-pressure Xe lamp at the
depicted wavelengths in degassed acetonitrile solutions (2”10À5 m).
[5] a) R. Siewertsen, H. Neumann, B. Buchheim-Stehn, R. Herges,
C. Näther, F. Renth, F. Temps, J. Am. Chem. Soc. 2009, 131,
15594 – 15595; b) A. A. Beharry, O. Sadovski, G. A. Woolley, J.
Am. Chem. Soc. 2011, 133, 19684 – 19687; c) D. BlØger, J.
Schwarz, A. M. Brouwer, S. Hecht, J. Am. Chem. Soc. 2012,
134, 20597 – 20600.
[6] For a comprehensive recent review, see: M. Irie, T. Fukaminato,
K. Matsuda, S. Kobatake, Chem. Rev. 2014, 114, 12174 – 12277.
[7] a) M. Irie, K. Uchida, T. Eriguchi, H. Tsuzuki, Chem. Lett. 1995,
10, 899 – 900; b) S. Kobatake, T. Yamada, K. Uchida, N. Kato, M.
Irie, J. Am. Chem. Soc. 1999, 121, 2380 – 2386; c) S. Kobatake, M.
Yamada, T. Yamada, M. Irie, J. Am. Chem. Soc. 1999, 121, 8450 –
8456; d) H. Jean-Ruel, R. R. Cooney, M. Gao, C. Lu, M. A.
Kochman, C. A. Morrison, R. J. D. Miller, J. Phys. Chem. A 2011,
115, 13158 – 13168.
Even after extended continuous irradiation with lirr = 405 nm
light of high intensity (1000 W Xe lamp equipped with an
interference filter) no sign of degradation was observed as
judged by the intensity of the long-wavelength absorption of
the closed isomer. In contrast, diarylethene 1 shows a fast
decay upon irradiation at lirr = 313 nm as a result of the
formation of the well-known by-product from the closed
isomer.[9c] A similar behavior was observed in the case of the
saturated switch 2 (313 nm), which does not ring-close via the
triplet excited state.
Even upon prolonged UV irradiation at lirr = 313 nm 3
proves to be significantly more stable than diarylethenes
1 and 2 (Figure 3). Clearly, upon excitation at lirr = 313 nm the
triplet pathway plays an important role and helps to avoid by-
product formation. Apart from this, it is important to note
that diarylethene 3 can be used to generate singlet oxygen
selectively, that is, only by irradiation of the ring-opened
isomer 3o but not from 3c.
In summary, by attaching triplet-sensitizing units to
a diarylethene we have been able to significantly improve
its photoswitching performance. Specifically, our improved
photoswitch 3 can reliably be operated with visible light only
and shows negligible fatigue even after extended irradiation
times. Besides extending the conjugated p-system our design
is based on a deliberate use of the triplet manifold, which
should prove beneficial for the development of improved
photoswitches[3] as well as photoswitchable sensitizers.[19]
[8] M. Hanazawa, R. Sumiya, Y. Horikawa, M. Irie, J. Chem. Soc.
Chem. Commun. 1992, 206 – 207.
[9] a) J. J. D. de Jong, L. N. Lucas, R. Hania, A. Pugzlys, R. M.
Kellogg, B. L. Feringa, K. Duppen, J. H. van Esch, Eur. J. Org.
Chem. 2003, 1887 – 1893; b) Y.-C. Jeong, D. G. Park, E. Kim, K.-
H. Ahn, S. I. Yang, Chem. Commun. 2006, 1881 – 1883; c) M.
Herder, B. Schmidt, L. Grubert, M. Pätzel, J. Schwarz, S. Hecht,
J. Am. Chem. Soc. 2015, 137, 2738 – 2747.
[10] a) M. Irie, T. Lifka, K. Uchida, S. Kobatake, Y. Shindo, Chem.
Commun. 1999, 747 – 750; b) Y.-C. Jeong, S. I. Yang, E. Kim, K.-
H. Ahn, Tetrahedron 2006, 62, 5855 – 5861; c) Y.-C. Jeong, D. G.
Park, I. S. Lee, S. I. Yang, K.-H. Ahn, J. Mater. Chem. 2009, 19,
97 – 103; d) P. D. Patel, I. A. Mikhailov, K. D. Belfield, A. E.
Masunov, Int. J. Quantum Chem. 2009, 109, 3711 – 3722.
[11] a) G. M. Tsivgoulis, J.-M. Lehn, Chem. Eur. J. 1996, 2, 1399 –
1406; b) O. Tosic, K. Altenhoner, J. Mattay, Photochem. Photo-
biol. Sci. 2010, 9, 128 – 130.
Angew. Chem. Int. Ed. 2016, 55, 1208 –1212
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1211

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Communications
Chemie
[12] a) R. T. F. Jukes, V. Adamo, F. Hartl, P. Belser, L. De Cola,
Inorg. Chem. 2004, 43, 2779 – 2792; b) T. Fukaminato, T. Hirose,
T. Doi, M. Hazama, K. Matsuda, M. Irie, J. Am. Chem. Soc. 2014,
136, 17145 – 17154.
van Esch, R. M. Kellogg, B. L. Feringa, Eur. J. Org. Chem. 2003,
155 – 166.
[18] R. Gçstl, B. Kobin, L. Grubert, M. Pätzel, S. Hecht, Chem. Eur. J.
2012, 18, 14282 – 14285.
[19] A photoswitchable singlet-oxygen sensitizer based on a diaryl-
ethene linked to a palladium porphyrin has recently been
described in: L. Hou, X. Zhang, T. C. Pijper, W. R. Browne, B. L.
Feringa, J. Am. Chem. Soc. 2014, 136, 910 – 913.
[20] K. Higashiguchi, K. Matsuda, T. Yamada, T. Kawai, M. Irie,
Chem. Lett. 2000, 29, 1358 – 1359.
[13] D. Mendive-Tapia, A. Perrier, M. J. Bearpark, M. A. Robb, B.
Lasorne, D. Jacquemin, Phys. Chem. Chem. Phys. 2014, 16,
18463 – 18471.
[14] For reviews, see: a) E. C. Harvey, B. L. Feringa, J. G. Vos, W. R.
Browne, M. T. Pryce, Coord. Chem. Rev. 2015, 282 – 283, 77 – 86;
b) F. Jukes, V. Adamo, F. Hartl, P. Belser, L. De Cola, Coord.
Chem. Rev. 2005, 249, 1327 – 1335.
[15] a) G. Favaro, U. Mazzucato, F. Ortica, P. Smimmo, Inorg. Chim.
Acta 2007, 360, 995 – 999; b) T. Fukaminato, T. Doi, M. Tanaka,
M. Irie, J. Phys. Chem. C 2009, 113, 11623 – 11627; c) R. Murata,
T. Yago, M. Wakasa, Bull. Chem. Soc. Jpn. 2011, 84, 1336 – 1338;
d) R. Murata, T. Yago, M. Wakasa, J. Phys. Chem. A 2015, 119,
11138 – 11145.
[21] A. A. Lamola, G. S. Hammond, J. Chem. Phys. 1965, 43, 2129 –
2134.
[22] M. T. Indelli, S. Carli, M. Ghirotti, C. Chiorboli, M. Ravaglia, M.
Garavelli, F. Scandola, J. Am. Chem. Soc. 2008, 130, 7286 – 7299.
[23] a) M. Irie, T. Eriguchi, T. Takada, K. Uchida, Tetrahedron 1997,
53, 12263 – 12271; b) A. T. Bens, D. Frewert, K. Kodatis, C.
Kryschi, H.-D. Martin, H. P. Trommsdorff, Eur. J. Org. Chem.
1998, 2333 – 2338.
[16] T. N. Singh-Rachford, F. N. Castellano, J. Phys. Chem. A 2009,
113, 5912 – 5917.
[17] a) L. N. Lucas, J. van Esch, R. M. Kellogg, B. L. Feringa, Chem.
Commun. 1998, 2313 – 2314; b) L. N. Lucas, J. J. D. de Jong, J. H.
Received: October 22, 2015
Published online: December 10, 2015
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