Light-controlled supramolecular helicity of a liquid crystalline phase using a helical polymer functionalized with a single chiroptical molecular switch

Article abstract of DOI:10.1021/ja711283c

Control over the preferred helical sense of a poly(n-hexyl isocyanate) (PHIC) by using a single light-driven molecular motor, covalently attached at the polymer's terminus, has been accomplished in solution via a combination of photochemical and thermal isomerizations. Here, we report that after redesigning the photochromic unit to a chiroptical molecular switch, of which the two states are thermally stable but photochemically bistable, the chiral induction to the polymer's backbone is significantly improved and the handedness of the helical polymer is addressable by irradiation with two different wavelengths of light. Moreover, we show that the chiral information is transmitted, via the macromolecular level of the poly isocyanate, to the supramolecular level of a lyotropic cholesteric liquid crystalline phase consisting of these stiff, rodlike polymers. This allows the magnitude and sign of the supramolecular helical pitch of the liquid crystal film to be fully controlled by light.

Full text of DOI:10.1021/ja711283c

Published on Web 03/12/2008
Light-Controlled Supramolecular Helicity of a Liquid
Crystalline Phase Using a Helical Polymer Functionalized with
a Single Chiroptical Molecular Switch
Dirk Pijper, Mahthild G. M. Jongejan, Auke Meetsma, and Ben L. Feringa*
Department of Organic and Molecular Inorganic Chemistry, Stratingh Institute, UniVersity of
Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
Received December 20, 2007; Revised Manuscript Received January 30, 2008 ; E-mail: b.l.feringa@rug.nl
Abstract: Control over the preferred helical sense of a poly(n-hexyl isocyanate) (PHIC) by using a single
light-driven molecular motor, covalently attached at the polymer’s terminus, has been accomplished in
solution via a combination of photochemical and thermal isomerizations. Here, we report that after
redesigning the photochromic unit to a chiroptical molecular switch, of which the two states are thermally
stable but photochemically bistable, the chiral induction to the polymer’s backbone is significantly improved
and the handedness of the helical polymer is addressable by irradiation with two different wavelengths of
light. Moreover, we show that the chiral information is transmitted, via the macromolecular level of the
polyisocyanate, to the supramolecular level of a lyotropic cholesteric liquid crystalline phase consisting of
these stiff, rodlike polymers. This allows the magnitude and sign of the supramolecular helical pitch of the
liquid crystal film to be fully controlled by light.
Introduction
Whereas only a slight orientational order is present in an
ordinary nematic LC matrix, the cholesteric (or chiral nematic)
LC phase is characterized by an additional change of the
direction of the individual mesogens in a helical fashion
perpendicular to the original director. The resulting large
supramolecular chiral organization is indicated by the sign and
magnitude of the cholesteric pitch (p), which is the distance
along the helix axis across which the director rotates a full 360°.
The nematic-to-cholesteric phase transition can be induced by
the addition of certain chiral dopant molecules to an LC formed
by achiral mesogens.8 The development of switchable dopants,
which can change shape under the influence of an external
stimulus (for instance light or heat) and consequently alter their
chiral induction toward the LC medium, holds great promise
toward future application.9 A series of chiroptical molecular
switches10 and light-driven molecular motors based on sterically
overcrowded alkenes11 developed in our laboratories proved to
be excellent photoswitchable dopants, due to the inversion of
the intrinsic helicity of the molecules involved with the
The transmission of chiral information from the molecular
to the macro- and supramolecular level1 is a powerful mecha-
nism used by nature in the controlled self-assembly of complex
macromolecular superstructures such as proteins and DNA.2 In
artificial systems, it offers the opportunity to control the
properties of materials in a dynamic and reversible manner. In
particular, it has been demonstrated that smart materials can be
triggered to alter their properties upon switching of chiral
structures at the molecular level.3 The amplification of chirality
has been realized with several dynamically helical systems, e.g.,
chiral aggregates and gels,4 synthetic helical foldamers,5 or
polymers.6 Liquid crystals (LCs) are especially suitable as a
host material for this purpose, due to their sensitivity to small
molecular chiral perturbations.7
(1) (a) Feringa, B. L.; Van Delden, R. A. Angew. Chem., Int. Ed. 1999, 38,
3418-3438. (b) Crego-Calama, M., Reinhoudt, D. N., Eds. Supramolecular
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Okamoto, Y. J. Am. Chem. Soc. 1995, 117, 11596-11597. (c) Maeda, K.;
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(3) (a) De Jong, J. J. D.; Hania, P. R.; Pugzˇlys, A.; Lucas, L. N.; De Loos, M.;
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Int. Ed. 2005, 44, 2373-2376. (b) Eelkema, R.; Pollard, M. M.; Vicario,
J.; Katsonis, N.; Ramon, B. S.; Bastiaansen, C. W. M.; Broer, D. J.; Feringa,
B. L. Nature 2006, 440, 163. (c) Eelkema, R.; Pollard, M. M.; Katsonis,
N.; Vicario, J.; Broer, D. J.; Feringa, B. L. J. Am. Chem. Soc. 2006, 128,
14397-14407. (d) Uchida, K.; Sukata, S.; Matsuzawa Y.; De Jong, J. J.
D.; Katsonis, N.; Kojima, Y.; Nakamura, S.; Areephong, J.; Meetsma, A.;
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J. AM. CHEM. SOC. 2008, 130, 4541-4552
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A R T I C L E S
Pijper et al.
isomerization steps of these systems. This concept was extended
to chiral amplification by irradiation with circularly polarized
light (CPL) of a nematic LC host doped with a racemic mixture
of a chiroptical switch, resulting in deracemization of the switch,
consequently generating a cholesteric phase.7a The use of light-
driven molecular motors as the chiral guests led to the
development of light addressable dopant-LC mixtures with
pitches short enough to reflect visible light. The thermal and
photochemical isomerizations of these motors led to reversible
color changes of the LC film, yielding a system which possesses
the basic requirements to generate addressable color pixels
covering the full visible spectrum, potentially useful in future
generation LC displays.12 Irradiation of a molecular motor doped
cholesteric LC even led to a unidirectional rotational reorganiza-
tion of the surface texture of an LC film.3b,c
Besides these solvent-free thermotropic LC systems, there
are many solvent-solute systems, where aggregation of the
solutes at sufficiently high concentration leads to liquid crystal-
linity. Rigid rodlike polymers are well-known to form such a
lyotropic LC phase.7b The photoinduced trans-to-cis isomeriza-
tion of photochromic azobenzene groups introduced in the side
chains of, for instance, copolyacrylates generally causes a
disruption of the nematic LC phase, leading to a large change
in birefringence.13 Furthermore, CPL irradiation has been shown
to lead to a long-range helical arrangement of the azobenzenes,14
an effect that could be further amplified by a supramolecular
chiral organization in polymeric LCs.15 A series of polymeric
LCs with photochromic switch units bearing a chiral pendant
group, either covalently attached to the polymer side chains or
seeded as a dopant in the LC material, were used to achieve
photochemically induced changes in the cholesteric pitch.16
An intriguing class of rigid, LC-forming polymers are the
polyisocyanates.6a,17 The backbone of these polymers adopts a
helical conformation, of which the equal amounts of left- and
right-handed helical conformations dynamically interconvert. A
small chiral induction can lead to a strong preference for one
helical twist sense, due to the strong cooperativity of the
monomeric units in the helical conformation and the resulting
infrequency of helix reversals along the polymer chain. Am-
plification of chirality has been demonstrated with these
polymers taking advantage of the “sergeants-and-soldiers”18 and
“majority-rules”19 effects, and even by the substitution in each
side chain of one hydrogen atom by a deuterium atom.20 The
transformation of a nematic lyotropic LC formed by poly(n-
hexyl isocyanate) to the cholesteric state was also demonstrated
by the addition of both various chiral small molecules and
optically active polyisocyanates as dopants.21 In an elegant
approach toward switching of the helicity, Green and co-workers
employed a polyisocyanate bearing structurally different chiral
side chains, which compete with each other favoring a left- and
right-handed helical sense of the polymer backbone.22 Since the
chiral bias of the competing units depends on temperature in
different ways, the polymer was shown to switch its helical
conformation between left- and right-handed as a function of
temperature.23 With the use of these temperature-dependent
helicity-switchable polyisocyanates, also the helical pitch and
twist sense of a cholesteric lyotropic LC formed by these
polymers could be controlled thermally.22b The introduction of
azobenzene photochromic groups containing two stereogenic
centers in the side chains of a copolyisocyanate allowed the
photochemically induced transition of excess M to P helicity
of the polymer backbone in dilute solution.24 As the chiral
induction of the switch units toward the polymer backbone
depends on a subtle interplay between the different components
(the stereogenic centers do not change upon photoisomerization),
the ability of switching of macromolecular helicity with these
systems has proven to be rather sensitive to various parameters.25
Furthermore, switching of the helical pitch of a lyotropic
cholesteric LC formed by these azobenzene-functionalized
polyisocyanates has not been demonstrated, although one
example has been published in which the helical pitch could
only be slightly altered in a reversible way.16d Also by irradiating
an axially chiral bicyclic ketone moiety with CPL, introduced
in the side chains of a copolyisocyanate, the preferred handed-
ness of the polymer’s backbone was switched in dilute solu-
tion.26
(10) (a) Jager, W. F.; De Jong, J. C.; De Lange, B.; Huck, N. M. P.; Meetsma,
A.; Feringa, B. L. Angew. Chem., Int. Ed. 1995, 34, 348-350. (b) Feringa,
B. L.; Huck, N. M. P.; Schoevaars, A. M. AdV. Mater. 1996, 8, 681-683.
(c) Feringa, B. L.; Van Delden, R. A.; Koumura, N.; Geertsema, E. M.
Chem. ReV. 2000, 100, 1789-1816.
In all these photoswitchable polymeric systems, the chiral
information of multiple photochromic switches randomly dis-
(11) (a) Koumura, N.; Zijlstra, R. W. J.; Van Delden, R. A.; Harada, N.; Feringa,
B. L. Nature 1999, 401, 152-155. (b) Koumura, N.; Geertsema, E. M.;
Meetsma, A.; Feringa, B. L. J. Am. Chem. Soc. 2000, 122, 12005-12006.
(c) Koumura, N.; Geertsema, E. M.; Van Gelder, M. B.; Meetsma, A.;
Feringa, B. L. J. Am. Chem. Soc. 2002, 124, 5037-5051.
(12) (a) Van Delden, R. A.; Koumura, N.; Harada, N.; Feringa, B. L. Proc.
Natl. Acad. Sci. U.S.A. 2002, 99, 4945-4949. (b) Van Delden, R. A.; Van
Gelder, M. B.; Huck, N. P. M.; Feringa, B. L. AdV. Funct. Mater. 2003,
13, 319-324. (c) Eelkema, R.; Feringa, B. L. Chem. Asian J. 2006, 1,
367-369.
(13) (a) Natansohn, A.; Rochon, P. Chem. ReV. 2002, 102, 4139-4175. (b) Ikeda,
T. J. Mater. Chem. 2003, 13, 2037-2057.
(14) Some amount of linear polarization in the CPL, essentially leading to
elliptically polarized light, appeared to be required, see: (a) Kim, M.-J.;
Shin, B.-G.; Kim, J.-J.; Kim, D.-Y. J. Am. Chem. Soc. 2002, 124, 3504-
3505. (b) Kim, M.-J.; Yoo, S.-J.; Kim, D.-Y. AdV. Funct. Mater. 2006, 16,
2089-2094.
(15) (a) Nikolova, L.; Nedelchev, L.; Todorov, T.; Petrova, T.; Tomova, N.;
Dragostinova, V.; Ramanujam, P. S.; Hvilsted, S. Appl. Phys. Lett. 2000,
77, 657-659. (b) Iftime, G.; Labarthet, F. L.; Natansohn, A.; Rochon, P.
J. Am. Chem. Soc. 2000, 122, 12646-12650. (c) Wu, Y.; Natansohn, A.;
Rochon, P. Macromolecules 2004, 37, 6801-6805.
(18) Green, M. M.; Reidy, M. P.; Johnson, R. D.; Darling, G.; O’Leary, D. J.;
Willson, G. J. Am. Chem. Soc. 1989, 111, 6452-6454.
(19) Green, M. M.; Garetz, B. A.; Munoz, B.; Chang, H.; Hoke, S.; Cooks, R.
G. J. Am. Chem. Soc. 1995, 117, 4181-4182.
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Chem. Soc. 1988, 110, 4063-4065.
(21) Green, M. M.; Zanella, S.; Gu, H.; Sato, T.; Gottarelli, G.; Jha, S. K.;
Spada, G. P.; Schoevaars, A. M.; Feringa, B. L.; Teramoto, A. J. Am. Chem.
Soc. 1998, 120, 9810-9817.
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J. V.; Garetz, B. A. J. Am. Chem. Soc. 2003, 125, 7313-7323.
(23) For representative examples of other temperature-based helical-switching
polymers, see: (a) Watanabe, J.; Okamoto, S.; Satoh, K.; Sakajiri, K.;
Furuya, H.; Abe, A. Macromolecules 1996, 29, 7084-7088. (b) Maeda,
K.; Okamoto, Y. Macromolecules 1998, 31, 5164-5166. (c) Fujiki, M. J.
Am. Chem. Soc. 2000, 122, 3336-3343. (d) Zhao, H.; Sanda, F.; Masuda,
T. Macromolecules 2004, 37, 8888-8892. (e) Maeda, K.; Mochizuki, H.;
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Hasegawa, T.; Morino, K.; Tanaka, Y.; Katagiri, H.; Furusho, Y.; Yashima,
E. Macromolecules 2006, 39, 482-488.
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(24) Maxein, G.; Zentel, R. Macromolecules 1995, 28, 8438-8440.
(25) The system is very sensitive to small structural changes, as well as variations
in the amount of switch units on the polymer chain and its concentration,
see: (a) Mu¨ller, M.; Zentel, R. Macromolecules 1996, 29, 1609-1617.
(b) Mayer, S.; Maxein, G.; Zentel, R. Macromolecules 1998, 31, 8522-
8525.
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L. B.; Selinger, J. V. Angew. Chem., Int. Ed. 1999, 38, 3138-3154.
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Chem. Soc. 2000, 122, 2603-2612.
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Light-Controlled Supramolecular Helicity
A R T I C L E S
Scheme 1. Photochemical and Thermal Isomerization Steps for
Chain-End Molecular Motor Functionalized 1-PHIC (ref 6d)
Figure 1. Schematic representation of the hierarchical transmission of chiral
information from the molecular level of a single chiroptical switch molecule,
via the macromolecular level of a polyisocyanate, to the supramolecular
level of a cholesteric LC phase.
tributed along the polymer chain was transferred to the backbone
of the polymer. Besides having a relatively small chiral influence
toward the helical backbone, this also fundamentally changes
the structure, and physical properties (e.g., solubility and
aggregation behavior in the LC phase), of the polymer.
Alternatively, incorporation of the switch unit at the terminus
of the polymer leads to little change in the properties of the
polymer, except for the chirality. Recently, we showed that
effective chiral induction by a single chiral photochromic unit
(a light-driven molecular motor), covalently attached at the
terminus of a poly(n-hexyl isocyanate) (PHIC) (1 in Scheme
1), allows fully reversible control over the preferred helical sense
of the polymer backbone.6d A drawback of our original system
is the fact that both light and heat are used as control elements
to achieve the chiral induction toward the polymer. As tem-
perature variation by itself, regardless of the switching aspects
in this system, can have a large effect on the cholesteric state
of a lyotropic LC, we anticipated that a system that is fully
controllable with light at one set temperature would be
particularly suitable to achieve precise control over the helicity
of a cholesteric LC phase.
Figure 2. (a) Benzamide-functionalized chiroptical molecular switch 2,
chain-end chiroptical switch functionalized poly(n-hexyl isocyanate) 3, and
N-acylated chiroptical molecular switch 4 and (b) schematic representation
of the reversible inversion of the preferred helical twist sense of a polymer
backbone induced by a chiroptical molecular switch at its terminus. (2′S)-
(P)-3-PHIC induces a preferred M helical twist of the polymer backbone.
UV irradiation (λ ) 365 nm) of the photochromic switch, yielding (2′S)-
(M)-3-PHIC, leads to an induced preferred P helicity of the polymer.
Subsequent irradiation with visible light (λ > 480 nm) reverts the system
to (2′S)-(P)-3-PHIC with a preferred M helicity of the polymer.
reported system 1-PHIC is achieved in a four-step switching
cycle by a combination of two photochemically induced cis-
trans isomerizations, each followed by an essentially irreversible
and fast thermal isomerization (Scheme 1).27 This makes control
of the exact composition of the mixture of isomers and the
associated chiral induction toward the polymer chain upon
irradiation of 1-PHIC at room temperature a highly complex
process.
In order to obtain better control over the chiral induction
toward the helical polymer, the photochromic unit attached at
the polymer chain’s terminus first was redesigned into a
photochemically bistable chiroptical switch which has two
thermally stable states (3-PHIC in Figure 2). By attaching the
polymer chain to the upper-half of the molecule in 3-PHIC
(Figure 2a) instead of to the lower-half as was the case in
1-PHIC, a symmetrical fluorenyl-based lower-half is obtained
while positioning the polymer in the fjord region of the switch
unit. As the two thermally stable isomers in the rotary cycle
(Scheme 1), as well as the two unstable isomers, are now
degenerate, this effectively reduces the number of possible
isomers from four to only two. The molecular motor now
operates as a chiroptical switch with two distinct states: (2′S)-
(P)-3-PHIC is converted, after a photochemically induced cis-
trans isomerization around the central olefinic bond, to (2′S)-
(M)-3-PHIC, resulting in an inversion of the intrinsic helical
Herein, we show that the molecular chirality of the chiroptical
switch can be transmitted, via the macromolecular level of the
polyisocyanate, further to the supramolecular level of a cho-
lesteric LC phase generated by these functionalized polymers
(Figure 1). The current system is actually the first in which full
control of the cholesteric handedness of the lyotropic LC phase
is allowed with light. Furthermore, this is accomplished by
simply modifying the end-group, inducing minimal change in
the physical properties of the polymer, whereas in all previously
developed systems in which a small cholesteric pitch alteration
can be induced photochemically, multiple switches are ap-
pended.16 Our system is also unique as the chirality of the switch
unit gets completely inverted. To the best of our knowledge,
our system is the first where the sign and magnitude of the
chirality of the LC material can be completely switched by light
using a single molecular switch in each lyotropic LC component.
Molecular Design
Unidirectional rotation with the molecular motor system
attached at the polymer chain’s terminus in the previously
(27) The half-lives at 20 °C for the thermal isomerizations involved with second-
generation light-driven molecular motor 1 are approximately 5 min.
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A R T I C L E S
Pijper et al.
Scheme 3. Synthesis of Upper-Half Ketone Precursor 13
Scheme 2. Retrosynthesis for 2
shape of the molecule from P to M (Figure 2b). In 3-PHIC,
one state induces a preferred M helicity, and the other induces
a preferred P helicity of the polymer backbone. Moreover, using
two different wavelengths, the two states of the chiroptical
switch are addressable with light in a fully reversible manner.
The thermal helix inversion that follows the photochemical
isomerization (step 2 and step 4 in Scheme 1), which involves
the passage of both the naphthyl moiety and the methyl
substituent in the upper-half of the molecule past the lower-
half, reverts the molecule to the stable isomer with an intrinsic
P helical shape. In order to obtain two thermally stable states
and create a system that is fully photoaddressable, this thermal
helix inversion step needs to be blocked. Therefore, the five-
membered ring in the upper-half of the motor molecule
connected to the central double bond in 1-PHIC was changed
to a six-membered ring in 3-PHIC. It was reasoned that in
3-PHIC, the naphthyl moiety in the upper-half of the molecule
is pushed toward the lower-half, hampering the passage of this
group along the planar and rigid fluorenyl-based lower part
during the thermal helix inversion step, resulting in a strongly
increased energy barrier for this thermal isomerization. The
presence of the amide substituent at a position on the naphtha-
lene in the upper-half of the molecule where it points toward
the lower-half probably raises this energy barrier even further.
The photochemically induced isomer can, however, be converted
back to the original isomer (2′S)-(P)-3-PHIC by irradiation with
visible (λ > 480 nm) light, as at these wavelengths only (2′S)-
(M)-3-PHIC absorbs (vide infra).
Scheme 4. Synthesis of Benzamide-Functionalized Chiroptical
Switch 2 (xantphos ) 4,5-Bis(diphenylphosphino)-9,9-
dimethylxanthene)
the harsh conditions involved with the synthesis of the upper-
half thioketone precursor, the benzamide functionality was
introduced in the last step of the synthesis via well-precedented
palladium-catalyzed amidation conditions developed by Yin and
Buchwald.30
Synthesis of ketone precursor 13 was feasible in four steps
(Scheme 3). First, 2,7-naphthalenediol 8 was transformed into
7-bromo-naphthalen-2-ol 9 by treatment with PPh3‚Br2 at
elevated temperature following a literature procedure.31 Sub-
sequently, alkylation using tosylate 10 in refluxing DMF yielded
compound 11 which, by the oxidation of the terminal primary
alcohol, was converted to acid 12. Regioselective ring-closure
of the acid was achieved in polyphosphoric acid at elevated
temperatures, yielding ketone 13.
After the conversion of ketone 13 to the corresponding
thioketone 6 by treatment with Lawesson’s reagent, the diazo-
thioketone coupling using diazofluorenone 7 provided episulfide
14 in 33% yield over the two steps (Scheme 4). Thioketone 6
proved to be rather unstable and was used immediately after a
quick purification by flash column chromatography. Desulfu-
rization of 14 by triphenylphosphine yielded alkene 5, which
was then used in the palladium-catalyzed amidation reaction30
to introduce the benzamide functionality. Enantioresolution of
2 was achieved by CSP-HPLC (Chiralpak OD, heptane/2-
propanol ) 95:5).
As a result of these changes in design, 3-PHIC can be
switched between two states that are fully photoaddressable and
thermally stable (Figure 2b). (2′S)-(P)-3-PHIC is anticipated to
induce a preferred M helical twist sense of the polymer
backbone, as the fluorenyl moiety of the lower-half of the
molecule resides at one specific side of the polymer chain.28 In
(2′S)-(M)-3-PHIC, the fluorenyl-based lower-half is present at
the other side of the polymer chain, which therefore should
induce a preferred P helical twist sense of the polymer backbone.
Synthesis
The most important step in the synthetic route toward the
envisioned chiroptical switch 2, bearing a benzamide functional-
ity to be used as an initiator of the n-hexyl isocyanate
polymerization, is the formation of the sterically overcrowded
central olefinic bond by coupling the two halves of the molecule
via a Staudinger-type diazo-thioketone reaction29 (Scheme 2).
To avoid problems with the amide functionality arising from
(28) In (2′S)-(P)-cis-1-PHIC, which also induces a preferred M helical twist
sense of the polymer backbone, the upper-half of the motor molecule resides
on the same side of the polymer chain as the fluorenyl-based lower-half of
the chiroptical switch in the present system.
Photochemistry of Benzamide-Functionalized
Chiroptical Switch 2
(29) (a) Barton, D. H. R.; Willis, B. J. J. Chem. Soc., Chem. Commun. 1970,
1225-1226. (b) Buter, J.; Wassenaar, S.; Kellogg, R. M. J. Org. Chem.
1972, 37, 4045-4060.
The photoisomerizations of 2 (Scheme 5) were studied using
1
UV-vis,32 circular dichroism (CD) (Figure 3), and H NMR
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4544 J. AM. CHEM. SOC. VOL. 130, NO. 13, 2008

Light-Controlled Supramolecular Helicity
A R T I C L E S
Scheme 5. Photoisomerizations of Benzamide-Functionalized
Chiroptical Switch 2
the case of photochemically generated isomer (2′S*)-(M*)-2
(Figure 5b), the twisted boat conformation of this six-membered
ring in the upper-half is not inverted with respect to the
conformation in (2′S*)-(P*)-2. The methyl substituent retains
a pseudoaxial orientation, yet it ends up at the other face of the
fluorenyl-based lower-half with respect to the substituted
naphthyl moiety, resulting in the twisted structure shown.
In order to observe the reversal of the spectral changes,
corresponding to the conversion of (2′S)-(M)-2 to (2′S)-(P)-2
via the thermal helix inversion process, heating to at least 80
°C was required. The kinetics and thermodynamic parameters
of this thermal helix inversion were determined by monitoring
the UV signal at 470 nm over time in the dark at five different
temperatures ranging from 80 to 100 °C. Using the various rate
constants, the Gibbs free energy of activation was calculated
using the Eyring equation: ∆qG° ) 112.9 kJ‚mol-1 (∆qH° )
112.0 kJ‚mol-1, ∆qS° ) -3.0 J‚mol-1‚K-1).32 By extrapolation
of the kinetic data, a half-life of (2′S)-(P)-2 at 20 °C of 459
days was determined (k ) 1.75 × 10-8 s-1), indicating that for
the irradiation experiments using the switchable LC film at room
temperature (vide infra) this thermal step is effectively blocked.
(Figure 4) spectroscopy. Irradiation of a sample of (2′S)-(P)-
233 in Et2O at 20 °C (λ ) 365 nm) resulted in the appearance
of a distinct absorption band around 450 nm in the UV-vis
spectrum and a change in sign of the CD absorptions, indicative
for the helix inversion of the molecule to provide diastereoi-
somer (2′S)-(M)-2. Upon irradiation (λ ) 365 nm) of a sample
of a racemic mixture of (2′S*)-(P*)-234 in CD2Cl2 for 3 h at
20 °C, new signals corresponding to the newly formed isomer
1
(2′S*)-(M*)-2 appeared in the H NMR spectrum (Figure 4).
The signal of the methyl substituent, a doublet at 1.42 ppm for
(2′S*)-(P*)-2, shifts downfield to 1.58 ppm for (2′S*)-(M*)-2.
Using HPLC analysis, a ratio of (2′S)-(P)-2 to (2′S)-(M)-2 of
9:91 at the photostationary state (PSS) of this photoequilibrium
was determined.
Polymerization
The polymerization of n-hexyl isocyanate (HIC) was ac-
complished via a slightly altered version of the procedure
developed by Ahn et al.35 After the addition of NaH to a solution
of enantiomerically pure (2′S)-(P)-2 in THF at room tempera-
ture, the mixture was cooled to -90 °C and 100 equiv of the
monomer (HIC) was added (Scheme 6). After 45 min, acetyl
chloride and pyridine were added to end-cap the polymers.
Precipitation of the polymer from MeOH afforded the polymer
in 71% yield. Analysis by GPC (Mw ) 32 650)36 and 1H NMR37
spectroscopy indicated an average degree of polymerization of
approximately 200. Acetylation of (2′S)-(P)-2 afforded switch
molecule (2′S)-(P)-4 (Scheme 6), which was used as a CD
reference compound: the CD spectra of the different isomers
of 4 allowed us to discriminate between the CD absorptions of
the helical polymer and that of the chiroptical switch molecule
connected to it.
Subsequent irradiation of the samples containing the PSS
mixture with visible light (λ > 480 nm) lead to a near complete
1
reversal of the changes in the UV-vis, CD, and H NMR
spectra, as at these wavelengths the extinction coefficient of
(2′S)-(M)-2 is much higher than that of (2′S)-(P)-2 (Figure 3a),
leading to the efficient conversion back to (2′S)-(P)-2. The PSS
ratio of (2′S)-(P)-2 to (2′S)-(M)-2 of this photoequilibrium, as
determined by HPLC analysis, was 89:11, indicating that
efficient and selective switching in two directions is indeed
possible with this system.
The geometry of the two isomers was unequivocally deter-
mined by X-ray analysis of the racemates (Figure 5). A solution
of racemic (2′S*)-(P*)-2 in CH2Cl2 was irradiated (λ ) 365
nm) for 5 h. Thereafter, via slow evaporation under n-pentane
atmosphere, crystals suitable for X-ray analysis, containing both
(2′S*)-(P*)-2 and (2′S*)-(M*)-2, were obtained. In the (2′S*)-
(P*)-2 isomer, the upper heterocyclic ring adopts the twisted
boat conformation (Figure 5a) which was observed before with
structurally analogous chiroptical switches and molecular
motors.10a,11c The methyl substituent adopts a pseudoaxial
orientation, which allows it to point away from the lower-half
of the molecule, in the same direction as the naphthyl group
does. With the previously described systems,10a,11c the twisted
boat conformation of the six-membered ring in the upper-half
is inverted in the isomer obtained after the photochemically
induced cis-trans isomerization. With the molecular motors this
forces the methyl substituent in a pseudoequatorial orientation,
in close proximity to the lower-half of the molecule and residing
on the same side of this lower-half as the naphthyl moiety. In
Photochemistry of Chiroptical Switch Functionalized
Polyisocyanate 3
The photoisomerizations of 3-PHIC were followed by CD
spectroscopy, using the CD spectra of the different isomers of
4 to distinguish between the CD contributions of the helical
polymer and those of the chiroptical molecular switch connected
to it (Figure 6). The spectra of the polymer and the acetylated
switch completely overlap between 300 and 500 nm. In this
wavelength region, polyisocyanate does not absorb and the CD
signals of the polymer-switch hybrid originate solely from the
attached switch molecule. Between 210 and 280 nm a strong
deviation of the CD curve of 3-PHIC from the CD curve of
acetylated switch 4 was observed (Figure 6a), and subtraction
(35) Ahn, J.-H.; Shin, Y.-D.; Nath, G. Y.; Park, S.-Y.; Rahman, M. S.; Samal,
S.; Lee, J.-S. J. Am. Chem. Soc. 2005, 127, 4132-4133.
(36) As the GPC columns were calibrated using standard solutions of polystyrene,
a polymer with a hydrodynamic volume differing from that of polyisocy-
anate, the value obtained with this technique is not very accurate and can
only be used as an indication.
(37) By comparing the integrals of the signals of the C6H13 side chains in every
monomeric unit and the signals of the aromatic protons of the photochromic
unit at the polymer’s terminus in the 1H NMR spectrum, an estimate of
the chain length can be made.
(30) Yin, J. J.; Buchwald, S. L. Org. Lett. 2000, 2, 1101-1104.
(31) Lustenberger, P.; Diederich, F. HelV. Chim. Acta 2000, 83, 2865-2883.
(32) See the Supporting Information for details.
(33) Assignment of the absolute configuration was done by comparison with
CD data of related compounds (see refs 11b and 11c).
(34) The asterisks indicate the fact that a racemic mixture of (2′S)-(P)-2 and
(2′R)-(M)-2 was used; the absolute configuration and helicity of one of the
enantiomers is given here for clarity.
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A R T I C L E S
Pijper et al.
Figure 3. UV-vis (a) and CD (b) spectra (Et2O, 20 °C) of (2′S)-(P)-2 (solid line), the PSS mixture with (2′S)-(P)-2 and (2′S)-(M)-2 (dotted line) after UV
irradiation (λ ) 365 nm), and the PSS mixture of (2′S)-(P)-2 and (2′S)-(M)-2 (dashed line) after irradiation with visible light (λ > 480 nm).
Figure 5. Pluto drawings of (a) (2′S*)-(P*)-2 and (b) (2′S*)-(M*)-2. One
enantiomer is shown for both isomers; this structure does not express the
absolute stereochemistry of the molecule.
Scheme 6. Anionic Polymerization of n-Hexyl Isocyanate Using
the Sodium Salt of Benzamide-Functionalized Chiroptical Switch 2
as the Initiator (Top), and Acetylation of the Amide Yielding
Chiroptical Switch 4 (Bottom)
to that obtained with the previously described system 1,6d was
found. Apparently, the chiral induction from the chiroptical
switch to the polymer chain is significantly increased in 3-PHIC
compared to 1, which is attributed to a more pronounced
influence of the molecular helix structure of the switching unit
at this new position of attachment (in the fjord region) to the
photochromic molecule. We reason that this effect is also caused
Figure 4. 1H NMR (20 °C, CD2Cl2) of (a) (2′S*)-(P*)-2 before irradiation,
by the fact that the five-membered ring in the upper-half of the
(b) the PSS mixture of (2′S*)-(P*)-2 and (2′S*)-(M*)-2 after UV irradiation
motor molecule connected to the central double bond in 1 has
(λ ) 365 nm), and (c) the PSS mixture of (2′S*)-(P*)-2 and (2′S*)-(M*)-2
expanded to a six-membered ring in 3 by introduction of the
oxygen atom. Together with the naphthyl moiety in the upper-
half, the polymer chain is thereby “pushed” closer to the
fluorenyl-based lower-half of the molecule, leading to the
enhanced chiral induction.
after irradiation with visible light (λ > 480 nm).
of the CD spectrum of (2′S)-(P)-4 from the CD spectrum of
(2′S)-(P)-3-PHIC yielded a CD difference spectrum typical of
PHIC with an excess of the M (left-handed) helical sense of
the backbone (solid line in Figure 6d). Interestingly, a 3-fold
increase in intensity of the molar elipticity (Figure 6d), compared
Upon irradiation of a sample of (2′S)-(P)-3-PHIC in Et2O at
20 °C with UV light (λ ) 365 nm), providing (2′S)-(M)-3-
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Figure 6. CD spectra (Et2O, 20 °C) of (a) (2′S)-(P)-3-PHIC (38.3 mg/L) (solid) and (2′S)-(P)-4 (3.2 µM) (dashed), (b) the PSS mixture with (2′S)-(P)-3-
PHIC and (2′S)-(M)-3-PHIC (solid) and the PSS mixture with (2′S)-(P)-4 and (2′S)-(M)-4 (dashed) after UV irradiation (λ ) 365 nm), (c) the PSS mixture
of (2′S)-(M)-3-PHIC and (2′S)-(P)-3-PHIC (solid) and the PSS mixture of (2′S)-(M)-4 and (2′S)-(P)-4 (dashed) after irradiation with visible light (λ > 480
nm), and (d) molar elipticity of the polymer backbone (CD difference spectra polymer 3 - acetylated switch 4) for the stable (2′S)-(P)-isomer of the
chiroptical switch (solid), after UV irradiation (λ ) 365 nm) (dotted), and after irradiation with visible light (λ > 480 nm) (dashed).
PHIC,38 the prominent absorption maxima around 220 and 260
nm in the CD spectrum inverted (Figure 6b). Irradiation of a
sample of (2′S)-(P)-4 under the same conditions resulted in an
inversion of the CD absorptions in accordance with the inversion
of the intrinsic chirality of the molecule to provide (2′S)-(M)-
4. Most obvious is the inversion of the CD difference spectrum
(solid vs dotted line in Figure 6d), indicating that the polymer
backbone has switched from a predominant M to P helical sense.
Subsequent irradiation of the sample containing (2′S)-(M)-
3-PHIC with visible light (λ > 480 nm), providing (2′S)-(P)-
3-PHIC,39 resulted again in an inversion of the prominent CD
signals of the polymer (Figure 6c). Irradiation under the same
conditions of a sample containing (2′S)-(M)-4 resulted also in
an inversion of the CD absorptions, again in accordance with
the inversion of the intrinsic helicity of the acetylated switch
to provide (2′S)-(P)-4. The inverted CD signals of the polymer,
again clearest observed from the CD difference spectrum
(dashed line in Figure 6d), indicate that the polymer backbone
has switched back to a preferred M helical twist sense. The CD
spectrum does not return completely to the spectrum obtained
before the irradiation experiments, as a PSS mixture still
containing a small amount of (2′S)-(M)-3-PHIC is obtained.
an optical microscope equipped with crossed polarizers. Initially,
a polygonal fingerprint texture was observed (Figure 7a), which
is typical for a cholesteric LC phase with an alignment of the
cholesteric helix axis parallel to the surface.41 The periodicity
of the polygonal texture corresponds to half a pitch length, which
was used to determine a pitch length of 6.0 µm.42 The sample
was then followed in time at a fixed location throughout the
irradiation experiments. Upon UV irradiation (λ ) 365 nm),
leading to the conversion of (2′S)-(P)-3-PHIC, a polymer with
a preferred M helical twist sense, to (2′S)-(M)-3-PHIC with
preferred P helicity, the distance between the lines of the
polygonal texture clearly increased, indicating an elongation of
the cholesteric helical pitch of the LC. Initially, only a slight
increase in the line distance in the fingerprint texture was
observed, as after 15 min of irradiation a pitch length of 6.2
µm was determined (Figure 7b). After 45 min, the distance
between the lines had increased much further, and the lines
slowly started to fade out until almost no fingerprint texture
was observed anymore (Figure 7c). At this point the pitch has
elongated to such an extent that the cholesteric lines are no
longer detectable by eye. Upon further UV irradiation the
fingerprint texture redeveloped, indicating a shortening of the
pitch length. Interestingly, during this process, initially circular
patterns were observed (Figure 7d, after 90 min of UV
irradiation).43 During continual irradiation, the circular patterns
slowly merged (Figure 7e), generating more homogeneous areas
of the polygonal fingerprint texture. The UV irradiation was
stopped after 150 min, as no changes in the polygonal fingerprint
texture seemed to occur anymore. However, a fully homoge-
neous fingerprint texture, similar to that observed before the
irradiation experiment, was observed after the sample had been
left in the dark overnight. At this point, a pitch length of 4.5
Photochemistry of the LC Matrix
The effect of the photoinduced inversion of the predominant
helicity of the polymer, on a lyotropic cholesteric liquid
crystalline phase generated by these polymers, was subsequently
investigated.40 A 30 wt % solution of (2′S)-(P)-3-PHIC in
toluene was placed between two flat KBr plates in a liquid IR
cell, in which the layer thickness of 200 µm was controlled
using a Teflon spacer. The LC was allowed to organize and
orient for a period of 16 h, after which it was analyzed using
(41) A 520 nm cutoff filter was placed between the light source of the microscope
and the sample, in order to prevent any photoconversion to be caused by
this light source. For this reason the pictures of the UV irradiation series
(Figure 7) are slightly darker compared to the pictures of the visible light
irradiation series (Figure 8).
(42) The cholesteric pitch length of the sample was determined from the
periodicity of the fingerprint texture. The values presented are an average
of three measurements per optical micrograph. The average deviation is
always within 5%.
(43) The circular patterns that emerge initially during the re-formation of the
fingerprint texture (Figure 7, parts d and e) have been observed previously
with other cholesteric LC materials: Bouligand, Y. J. Phys. 1973, 34, 603-
614.
(38) A PSS mixture of (2′S)-(P)-3-PHIC and (2′S)-(M)-3-PHIC is obtained. For
2 a PSS ratio of (2′S-(P)-2/(2′S)-(M)-2 ) 9:91 was determined; for 3 a
similar PSS is anticipated.
(39) A PSS mixture of (2′S)-(P)-3-PHIC and (2′S)-(M)-3-PHIC is obtained. For
2 a PSS ratio of (2′S-(P)-2/(2′S)-(M)-2 ) 89:11 was determined; for 3 a
similar PSS is anticipated.
(40) After the addition of (2′S)-(P)-2 as a chiral dopant to an LC film of
unfunctionalized PHIC (formed by initiation of the polymerization with
benzanilide, see ref 35) in toluene, the LC remained nematic. Apparently,
the intermolecular interactions of the switch with the polymers alone are
not sufficient to induce a nematic-to-cholesteric phase transition of the LC
matrix.
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Figure 7. Optical micrographs of a thin film (thickness, 200 µm) of (2′S)-(P)-3-PHIC in toluene (30 wt %) (a) before irradiation (p ) 6.0 µm), (b) after
15 min (p ) 6.2 µm), (c) 45 min, (d) 90 min, and (e) 150 min of UV irradiation (λ ) 365 nm), after which a PSS mixture is obtained that consists of a large
excess of (2′S)-(M)-3-PHIC over (2′S)-(P)-3-PHIC, and (f) after leaving the irradiated sample in the dark overnight (p ) 4.5 µm) (the inset shows an
enlarged section under 5× higher magnification). Scale bar, 50 µm. The arrows indicate the directions of the crossed polarizers.
µm was determined, which is shorter than the pitch of 6.0 µm
observed before the irradiation experiment (Figure 7, part f vs
part a).
Initially the sample contains solely (2′S)-(P)-3-PHIC, a
polymer with a preferred M helical twist sense, leading to a P
helical cholesteric LC phase46 with a tight pitch (Figures 7a
and 9a). Upon UV irradiation (λ ) 365 nm), (2′S)-(P)-3-PHIC
is converted to (2′S)-(M)-3-PHIC with a preferred P helical twist
sense, which induces a cholesteric LC phase with the opposite
helicity (M). This results in the observed elongation of the
cholesteric pitch length upon UV irradiation (Figures 7b and
9b). At a certain point, equal amounts of M and P helical
polymer are present in the LC film, effectively canceling each
other’s helical induction toward the cholesteric LC. This results
in a near-infinite pitch length, and a sample that appears to be
nematic, explaining the observed disappearance of the choles-
teric texture (Figures 7c and 9c). Further irradiation leads to an
excess (2′S)-(M)-3-PHIC over (2′S)-(P)-3-PHIC, resulting in a
shortening of the pitch length of the cholesteric phase, which
has now the M helicity, observed as the reappearance of the
fingerprint texture (Figure 7, parts d and e, and Figure 9d).
Continuous UV irradiation gradually increases the (2′S)-(M)-
3-PHIC to (2′S)-(P)-3-PHIC ratio, explaining the observed
shortening of the periodicity of the cholesteric corrugation, until
the PSS is reached (Figures 7f and 9e). Subsequent irradiation
with visible light (λ > 480 nm) leads to the conversion of (2′S)-
(M)-3-PHIC back to (2′S)-(P)-3-PHIC. This results in the
Subsequent irradiation of the sample with visible light (λ >
480 nm), leading to the conversion of (2′S)-(M)-3-PHIC, a
polymer with a preferred P helical twist sense, to (2′S)-(P)-3-
PHIC with preferred M helicity of the polymer, resulted again
in an increase in the distance between the lines in the polygonal
texture, indicating an increase in the cholesteric pitch length.
After 5 h of irradiation, an increase to 7.8 µm was determined
(Figure 8a).44 Continuing irradiation of the phase led to a further
elongation of the pitch (13 µm after 6 h, see Figure 8b) and
eventually a disappearance of the cholesteric lines (Figure 8c).
The fingerprint texture reappeared upon further visible light
irradiation, and the distance between the lines slowly decreased
with time, indicating a shortening of the pitch length from
approximately 22 µm45 after 8 h (Figure 8d) to 9.0 µm after 9
h (Figure 8e). After 24 h of visible light irradiation, the
fingerprint texture of the LC phase ceased to change, at which
point a pitch length of 3.7 µm was reached (Figure 8f), again
remarkably shorter than the 6.0 µm pitch observed before the
irradiation experiments. The irradiation cycles of the liquid
crystalline system are reversible and can be repeated several
times.
(44) Owing to the lower intensity of the light source used for the visible light
irradiation compared to the UV irradiation, longer irradiation times are
needed here in order to obtain the same changes of the LC phase.
(45) The value given for the pitch here is only approximate, due to the
inhomogeneity of the periodicity in this micrograph.
(46) A rationalization of the relationship between the twist sense of the
polyisocyanate and the handedness of the cholesteric LC formed by these
polymers can be found in ref 21.
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A R T I C L E S
Figure 8. Optical micrographs of a thin film (thickness, 200 µm) of the PSS mixture obtained upon UV irradiation that consists of a large excess of
(2′S)-(M)-3-PHIC over (2′S)-(P)-3-PHIC in toluene (30 wt %), after (a) 5 h (p ) 7.8 µm), (b) 6 h (p ) 13 µm), (c) 7 h, (d) 8 h (p ) 22 µm), (e) 10 h (p
) 9.0 µm), and (f) 24 h of irradiation with visible light (λ > 480 nm), after which a PSS mixture is obtained that consists of a large excess of (2′S)-(P)-
3-PHIC over (2′S)-(M)-3-PHIC (p ) 3.7 µm) (the inset shows an enlarged section under 5× higher magnification). Scale bar, 50 µm. The arrows indicate
the directions of the crossed polarizers.
reversal of the changes of the cholesteric LC phase and a
repetition of the observations that were described before (Figures
8 and 9).
A puzzling observation is the fact that the pitch length of the
cholesteric phase at both PSSs (4.5 µm at PSS365, Figure 7f,
and 3.7 µm at PSS>480, Figure 8f) are substantially shorter than
the pitch length of the nonirradiated sample (6.0 µm, Figure
7a). A slight reduction of the pitch has also been observed after
irradiation of an LC based on an azobenzene-functionalized
helical polymer and was attributed to solvent evaporation due
to heating of the sample under the UV lamp, leading to a change
in concentration.16d Upon increasing the concentration of (2′S)-
(P)-3-PHIC in the lyotropic LC phase from 30 to 40 wt %
indeed a decrease in pitch length from 6.0 to 2.9 µm was
observed.32 To verify if this could provide an explanation for
the observed effect, a sample of (2′S)-(P)-3-PHIC in toluene
was submitted to a number of irradiation cycles, in which first
1 h of UV irradiation was applied resulting in an increase in
pitch length of the LC sample to almost nematic, after which it
was irradiated 20 h with visible light. The initial pitch length
of the sample, before irradiation was applied, was 6.9 µm.50
After the first irradiation cycle, the pitch had decreased to a
value of 4.4 µm,32 a similar decrease in pitch length over one
Discussion and Conclusions
We have demonstrated that irradiation with light of different
wavelengths allows fully reversible control over sign and
magnitude of the cholesteric helical pitch of the LC phase of
(2′S)-(P)-3-PHIC. The thermal reorganization observed for the
UV irradiated sample after it had been stored in the dark
overnight (Figure 7, part e vs part f) implies that the reorganiza-
tion of the LC is slow47 compared to the photoisomerization of
the chiroptical switch-polymer hybrid.48 As the photoisomer-
ization upon visible light irradiation (the reverse process) takes
place on a much longer time scale compared to the UV
irradiation experiment, caused by the fact that a light source of
much lower intensity was used, this thermal reorganization
process was not observed after terminating the visible light
irradiation. In this case, the photoisomerization and the thermal
reorganization of the LC phase appear to take place on the same
time scale.49
(49) A slower reorganization would render a lyotropic LC system inferior to
switchable thermotropic LCs, where the reorganization of the LC phase
generally follows the switching of the dopant directly. However, the long
irradiation times needed in the current experiments can be attributed to the
experimental setup and relatively weak light sources used. Furthermore,
lyotropic LCs have an important advantage over thermotropic LCs, in that
there is no temperature-dependent phase boundary in these systems, as stated
in ref 22b.
(47) An apparent deficiency of this switchable system, in view of its potential
applicability, appears to be the slow reorganization of the LC, a feature
that seems to be common for any switching system based on a lyotropic
LC, see, e.g., refs 9a, 16, and 22b. However, it has been observed that
switching of the temperature responsive lyotropic LC described in ref 22b,
when the temperature-based helicity-switching polymer was applied as the
mesogen, followed the temperature change immediately.
(50) This is somewhat longer than the 6.0 µm observed with the previous
unirradiated sample. We assume this is caused by a slightly lower
concentration of this particular sample.
(48) Switching of the photochromic unit, when an appropriate light source is
used, takes place on the time scale of picoseconds.
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A R T I C L E S
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lecular level of a cholesteric liquid crystalline phase formed by
these helically switchable polymers, allows for the full control
of the magnitude and sign of the supramolecular helical pitch
of this LC phase using two different wavelengths of light.
Experimental Section
General Remarks. Chemicals were used as received from Aldrich
or Acros; solvents were reagent grade and distilled and dried before
use according to standard procedures. Column chromatography was
performed on silica gel (Aldrich 60, 230-400 mesh). Uniplate thin-
layer chromatography plates were purchased at Analtech (Newark, DE);
dimensions, 20 × 20 cm; layer thickness, 2000 µm. 1H and 13C NMR
spectra were recorded on a Varian Gemini-200 (50.32 MHz) or on a
Varian AMX400 (100.59 MHz) spectrometer in CDCl3. Chemical shifts
are denoted in δ-unit (ppm) relative to CDCl3 (1H δ ) 7.23, 13C δ )
1
77). For H NMR, the splitting parameters are designated as follows:
s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), and b
(broad). For 13C NMR, the carbon atoms are assigned as follows: q
(primary carbon), t (secondary carbon), d (tertiary carbon), s (quarter-
nary carbon). MS (EI) and HRMS (EI) spectra were obtained with a
JEOL JMS-600 spectrometer. Melting points are taken on a Bu¨chi
B-545 melting point apparatus. HPLC analyses were performed on a
Shimadzu 10AD-VP system using a Chiralcel AD (Daicel) column.
Preparative HPLC was performed on a Gilson HPLC system consisting
of a 231XL sample injector, a 306 (10SC) pump, an 811C dynamic
mixer, a 805 manometric module, with a 119 UV-vis detector, and a
202 fraction collector, using the Chiralcel AD (Daicel) column. Elution
speed was 1 mL/min. UV measurements were performed on a Hewlett-
Packard HP 8453 FT spectrophotometer, and CD spectra were recorded
on a JASCO J-715 spectropolarimeter using Uvasol-grade solvents
(Merck). Irradiation experiments were performed with a Spectroline
ENB-280C/FE UV lamp at 365 nm and a 200 W Oriel Xe lamp adapted
with a 480 nm cutoff filter. Materials needed for the polymerization:
n-hexyl isocyanate (Aldrich, 97%) was dried over CaH2 and vacuum
distilled, THF was distilled under N2 after refluxing with sodium for 5
h. The polymerizations were carried out in a flame-dried Schlenk tube
under N2 atmosphere. GPC measurements were performed on Waters
GPC equipment, with THF as solvent and polystyrene as reference,
using refractive index and viscosity detection in series and universal
calibration via the intrinsic viscosities of the polystyrene reference
samples and the polyisocyanates to provide the correct molecular weight
distributions.
Figure 9. Schematic representation of the full photocontrol of the
magnitude and sign of the supramolecular helical pitch of a cholesteric LC
phase generated by a polyisocyanate with a single chiroptical molecular
switch covalently linked to the polymer’s terminus. (a) Initially, (2′S)-(P)-
3-PHIC adopts a P helical cholesteric LC phase with a tight pitch. (b) UV
irradiation (λ ) 365 nm) converts the switchable polymer to (2′S)-(M)-3-
PHIC, leading to an elongation of the cholesteric pitch of the LC. (c) Equal
amounts of (2′S)-(P)-3-PHIC and (2′S)-(M)-3-PHIC are present in the
mixture, resulting in an infinite pitch of the cholesteric LC, which appears
as an optically nematic phase. (d) After prolonged UV irradiation, (2′S)-
(M)-3-PHIC is present in excess, resulting in the LC matrix returning to
cholesteric with inverted helicity (M). (e) A PSS is reached, predominantly
consisting of (2′S)-(M)-3-PHIC, resulting in an M helical cholesteric LC
phase with a tight pitch. Irradiation with visible light (λ > 480 nm) leads
to a reversal of the process (e f a).
switch cycle as observed before (6.0-3.7 µm). Three subsequent
switch cycles, however, did not result in a further decrease in
pitch length: similar pitch values, varying between 4.0 and 4.4,
were obtained. It is therefore highly unlikely that the observed
decrease in pitch length after irradiation is caused by solvent
evaporation due to heating of the sample under the irradiation
conditions applied, as a gradual further decrease in pitch length
over subsequent irradiation cycles would be expected. We
propose that the observed tightening of the pitch is actually the
result of the photoinduced switching of the polymer’s helicity
causing an overall improvement in the packing of the polymers
in the helical organization of the LC phase. Such an improve-
ment in the packing might well be accompanied by a reduction
in the amount of helix reversal points along the polymer chains,
leading to the observed tightening of the cholesteric pitch.51
In the work described here, we have shown that the preferred
helical twist sense of a polyisocyanate can be controlled by a
single photochemically bistable chiroptical molecular switch,
of which the two thermally stable states are fully addressable
with light, which is covalently attached to the polymer’s
terminus. The hierarchical transmission of chiral information
from the molecular level of the chiroptical switch, via the
macromolecular level of the helical polymer to the supramo-
Preparation and Study of the Liquid Crystalline Phase. The liquid
crystalline phase was prepared by dissolving 30 wt % of (2′S)-(P)-3-
PHIC in freshly distilled toluene. The liquid crystalline phase was
generated between two KBr plates of a liquid IR cell (demountable
liquid IR cell as sold by Sigma-Aldrich no. Z112003-1KT). The Teflon
spacer placed between the two plates was chosen at a thickness of 200
µm. The liquid crystalline phase images were recorded in transmission
using an Olympus BX 60 microscope, equipped with crossed polarizers
and a Sony 3CCD DXC 950P digital camera, attached to a personal
computer with Matrox Inspector 8.0 imaging software.
7-Bromonaphthalen-2-ol 9. A procedure slightly modified from the
one described in ref 31 was followed. To a vigorously stirred mixture
of triphenylphosphine (31.5 g, 120 mmol) in MeCN (50 mL), Br2 (19
g, 6.2 mL, 120 mmol) was added dropwise at 0 °C. The reaction mixture
was allowed to reach room temperature, and 2,7-dihydroxynaphthalene
8 (16 g, 100 mmol) was added in one portion. The mixture was heated
to 70 °C for 30 min, after which the solvent was removed by rotary
evaporation. The flask was equipped with a gas trap, and the black
residue was heated to 250 °C for 45 min. After cooling to room
temperature, the mixture was dissolved in 200 mL of CH2Cl2 and
filtered over a plug of silica. The pure product was obtained after
column chromatography (SiO2, pentane/CH2Cl2 ) 1:1, Rf ) 0.22, to
pure CH2Cl2) as a beige powder (18.6 g, 83.4 mmol, 84%). Mp 127-
(51) This improvement in the packing might also be achieved by applying
heating/cooling cycles; in preliminary experiments, however, no significant
change in pitch and texture was observed after one heating-cooling cycle
(up to 40 °C).
9
4550 J. AM. CHEM. SOC. VOL. 130, NO. 13, 2008

Light-Controlled Supramolecular Helicity
A R T I C L E S
129 °C. 1H NMR (400 MHz, CDCl3) δ 5.15 (b, 1H), 7.01 (d, J ) 2.6
Hz, 1H), 7.08 (dd, J ) 8.8, 2.6 Hz, 1H), 7.37 (dd, J ) 8.8, 1.8 Hz,
1H), 7.59 (d, J ) 8.4 Hz, 1H), 7.68 (d, J ) 8.8 Hz, 1H), 7.79 (d, J )
2.2 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ 108.7 (d), 118.1 (d), 120.8
(s), 127.0 (d), 127.3 (s), 128.3 (d), 129.4 (d), 129.9 (d), 135.7 (s), 154.0
(s). m/z (EI+, %) ) 224, 222 (M+, 100), 115 (38). HRMS (EI+): calcd
for C10H779BrO, 221.9680; found, 221.9676. Anal. Calcd for C10H7-
BrO: C, 53.84; H, 3.16. Found: C, 53.85; H, 3.12.
the mixture was vigorously stirred for 3 h. After cooling to 70 °C, the
reaction mixture was cautiously poured on ice (250 g), and the resulting
aqueous solution was stirred at room temperature overnight. The
aqueous mixture was extracted with ethyl acetate (4 × 150 mL), the
organic extracts were washed with brine and dried (Na2SO4), and the
solvent was removed under reduced pressure, yielding a brown oil.
Further purification could be performed by column chromatography
(SiO2, pentane/EtOAc ) 4:1, Rf ) 0.8), providing a light-brown solid
1
(2.3 g, 8.0 mmol, 62%). Mp 87-89 °C. H NMR (400 MHz, CDCl3)
3-Hydroxy-2-methylpropyl-4-methylbenzenesulfonate 10. To a
solution of 2-methyl-1,3-propanediol (9.0 g, 8.9 mL, 0.10 mmol) in
CHCl3 (100 mL), pyridine (15.8 g, 16.0 mL, 200 mmol) and
p-toluenesulfonyl chloride (21 g, 0.11 mol) were added at 0 °C. The
stirred solution was allowed to reach room temperature overnight. After
addition of water (200 mL), the mixture was extracted with ethyl acetate
(3 × 200 mL). The organic extracts were washed with aqueous solutions
of HCl (2 M, 100 mL), saturated NaHCO3 (100 mL), and brine, and
dried (Na2SO4). Evaporation of the solvent yielded a slightly yellow
oil. The pure product was obtained after column chromatography (SiO2,
gradient pentane/EtOAc ) 2:1, Rf ) 0.17, to pure EtOAc), yielding a
δ 1.21 (d, J ) 7.0 Hz, 3H), 2.87 (m, 1H), 4.19 (dd, J ) 9.2, 8.8 Hz,
1H), 4.54 (dd, J ) 11.3, 5.1 Hz, 1H), 7.01 (d, J ) 9.1 Hz, 1H), 7.42
(d, J ) 8.4 Hz, 1H), 7.49 (d, J ) 8.4 Hz, 1H), 7.75 (d, J ) 8.8 Hz,
1H), 9.64 (s, 1H). 13C NMR (50 MHz, CDCl3) δ 11.0 (q), 41.0 (d),
72.0 (t), 110.9 (s), 119.0 (d), 124.5 (s), 127.5 (s), 128.0 (d), 128.1 (d),
129.5 (d), 132.6 (s), 136.7 (d), 163.9 (s), 195.4 (s). m/z (EI+, %) )
290, 292 (M+, 59), 248, 250 (100). HRMS (EI+): calcd for C14H11
-
79
BrO2, 289.9942; found, 289.9926. Anal. Calcd for C14H11BrO2: C,
57.76; H, 3.81. Found: C, 57.80; H, 3.77.
Dispiro[9-bromo-2-methyl-2,3-dihydro-1H-naphto[2,1-b]pyran-
1,2′-thiirane-3′,9′′-(9′′H)-fluorene] 14. A solution of 13 (500 mg, 1.70
mmol) and P2S5 (1.2 g, 5.2 mmol) in benzene (20 mL) was refluxed
for 1 h. After cooling to room temperature, the mixture was directly
filtered over a plug of silica, eluted with pentane/CH2Cl2 ) 1:1, to
quickly purify the rather unstable thioketone 6. The first deep-green
colored fraction (thioketone 6) was collected and directly added to a
solution of diazofluorenone 7 (660 mg, 3.4 mmol) in toluene (75 mL).
The solution was heated at reflux overnight, and after cooling to room
temperature, the solvent was removed under reduced pressure. The pure
product was obtained after column chromatography (SiO2, pentane/
CH2Cl2 ) 8:1, Rf ) 0.3) as a yellow solid (270 mg, 0.57 mmol, 33%).
Mp 177-179 °C. 1H NMR (400 MHz, CDCl3) δ 1.18 (d, J ) 6.6 Hz,
3H), 3.13 (m, 1H), 3.61-3.67 (m, 2H), 6.13 (d, J ) 7.7 Hz, 1H), 6.54
(t, J ) 8.1 Hz, 1H), 6.77 (d, J ) 8.8 Hz, 1H), 7.08 (t, J ) 8.1 Hz, 1H),
7.32 (t, J ) 8.3 Hz, 1H), 7.44 (t, J ) 8.2 Hz, 1H), 7.50-7.66 (m, 5H),
7.76 (d, J ) 7.3 Hz, 1H), 9.48 (s, 1H). 13C NMR (100 MHz, CDCl3)
δ 19.1 (q), 39.6 (d), 53.4 (s), 56.3 (s), 72.4 (t), 116.4 (s), 118.7 (d),
119.4 (d), 120.5 (d), 120.8 (s), 123.5 (d), 124.2 (d), 124.8 (d), 126.0
(d), 126.4 (d), 126.6 (d), 127.6 (d), 127.7 (s), 128.4 (d), 129.5 (d),
130.4 (d), 135.5 (s), 140.0 (s), 141.9 (s), 142.8 (s), 142.9 (s), 155.9
(s). m/z (EI+, %) ) 470, 472 (M+, 45), 438, 440 (100). HRMS (EI+):
calcd for C27H1979BrOS, 470.0340; found, 470.0358.
1
colorless oil (7.42 g, 31 mmol, 31%). H NMR (400 MHz, CDCl3) δ
0.86 (d, J ) 7.0 Hz, 3H), 1.95 (m, 1H), 2.39 (s, 3H), 2.68 (b, 1H),
3.43-3.53 (m, 2H), 3.96 (d, J ) 5.8 Hz, 2H), 7.30 (d, J ) 8.4 Hz,
2H), 7.73 (d, J ) 6.6 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ 13.0
(q), 21.5 (q), 35.4 (d), 63.5 (t), 71.9 (t), 127.8 (d), 129.8 (d), 132.7 (s),
144.8 (s). m/z (CI+, %) ) 262 (M + NH4+, 100), 108 (2.6).
3-(7-Bromonaphthalen-2-yloxy)-2-methylpropan-1-ol 11. To a
stirred solution of 9 (12.7 g, 57.2 mmol) in DMF (500 mL), Cs2CO3
(37 g, 114 mmol) and 10 (18 g, 74 mmol) were added, and the mixture
was refluxed for 48 h. After cooling to room temperature, the mixture
was diluted in ethyl acetate and washed with an aqueous solution of
HCl (10%, 3 × 200 mL), saturated NaHCO3 (200 mL), and brine, and
dried (Na2SO4). After removal of the solvent under reduced pressure,
the pure product was obtained after column chromatography (SiO2,
pentane/Et2O ) 2:1, Rf ) 0.54) as a white solid (12.4 g, 42 mmol,
74%). Mp 77-79 °C. 1H NMR (400 MHz, CDCl3) δ 1.06 (d, J ) 7.0
Hz, 3H), 2.10-2.26 (m, 2H), 3.71 (d, J ) 6.9 Hz, 2H), 4.00 (d, J )
6.2 Hz, 2H), 6.99 (d, J ) 2.6 Hz, 1H), 7.10 (dd, J ) 8.8, 2.6 Hz, 1H),
7.36 (dd, J ) 8.4, 1.8 Hz, 1H), 7.57 (d, J ) 8.8 Hz, 1H), 7.65 (d, J )
8.8 Hz, 1H), 7.82 (d, J ) 2.2 Hz, 1H). 13C NMR (100 MHz, CDCl3)
δ 13.6 (q), 35.6 (d), 65.7 (t), 70.8 (t), 105.8 (d), 119.2 (d), 120.5 (s),
126.9 (d), 127.3 (s), 128.6 (d), 129.2 (d), 129.3 (d), 135.7 (s), 157.5
(s). m/z (EI+, %) ) 294, 296 (M+, 30), 222, 224 (100). HRMS (EI+):
calcd for C14H1579BrO2, 294.0255; found, 294.0243. Anal. Calcd for
C14H15BrO2: C, 56.97; H, 5.12. Found: C, 57.15; H, 5.06.
9-Bromo-1-(9H-fluoren-9-ylidene)-2-methyl-2,3-dihydro-1H-ben-
zo[f]chromene 5. A solution of episulfide 10 (0.30 mg, 0.64 mmol)
and triphenylphosphine (0.83 g, 3.2 mmol) in toluene (20 mL) was
heated at reflux overnight. After removal of the solvent under reduced
pressure the alkene was purified by column chromatography (SiO2,
pentane/CH2Cl2 ) 8:1, Rf ) 0.41), yielding a yellow solid (0.25 g,
3-(7-Bromonaphthalen-2-yloxy)-2-methylpropanoic Acid 12. To
a stirred solution of 11 (8.9 g, 30 mmol) in acetone (250 mL), KMnO4
was added (24 g, 0.15 mol), after which stirring was continued
overnight. An aqueous saturated solution of NaHSO3 was added to
destroy the excess KMnO4, and the aqueous mixture was acidified by
addition of aqueous HCl (10%) and extracted with CH2Cl2 (3 × 200
mL). The organic extracts were washed with brine, dried (Na2SO4),
and the solvent was removed under reduced pressure. The pure product
was obtained after column chromatography (SiO2, EtOAc, Rf ) 0.66)
1
0.57 mmol, 89%). Mp 176-177 °C. H NMR (400 MHz, CDCl3) δ
1.41 (d, J ) 7.0 Hz, 3H), 4.20 (m, 1H), 4.37-4.44 (m, 2H), 6.47 (d,
J ) 8.1 Hz, 1H), 6.74 (t, J ) 7.7 Hz, 1H), 7.12-7.18 (m, 2H), 7.32-
7.41 (m, 3H), 7.62 (d, J ) 8.8 Hz, 1H), 7.68 (d, J ) 7.3 Hz, 1H), 7.76
(d, J ) 8.8 Hz, 1H), 7.81 (d, J ) 7.0 Hz, 1H), 7.92-7.94 (m, 2H). 13
C
NMR (100 MHz, CDCl3) δ 15.8 (q), 32.8 (d), 72.5 (t), 113.1 (s), 118.6
(d), 119.1 (d), 120.0 (d), 121.9 (s), 124.5 (d), 125.0 (d), 126.6 (d),
126.8 (d), 126.9 (d), 127.3 (d), 127.4 (d), 127.5 (s), 127.6 (d), 129.8
(d), 131.5 (d), 133.0 (s), 133.1 (s), 136.9 (s), 137.4 (s), 138.3 (s), 139.7
(s), 140.8 (s), 154.2 (s). m/z (EI+, %) ) 438, 440 (M+, 100), 423, 425
(14). HRMS (EI+): calcd for C27H1979BrO, 438.0619; found, 438.0639.
Anal. Calcd for C27H19BrO: C, 73.81; H, 4.36. Found: C, 73.55; H,
4.28.
1
as a white solid (5.6 g, 18 mmol, 60%). Mp 156-159 °C. H NMR
(400 MHz, DMSO-d6) δ 1.22 (d, J ) 7.0 Hz, 3H), 2.90 (m, 1H), 4.12
(dd, J ) 9.4, 5.4 Hz, 1H), 4.22 (dd, J ) 8.2, 7.0 Hz, 1H), 7.18 (d, J
) 9.0 Hz, 1H), 7.36 (s, 1H), 7.45 (d, J ) 8.4 Hz, 1H), 7.80 (d, J ) 8.8
Hz, 1H), 7.84 (d, J ) 9.2 Hz, 1H), 8.07 (s, 1H). 13C NMR (100 MHz,
DMSO-d6) δ 13.8 (q), 38.9 (d), 69.5 (t), 106.2 (d), 119.1 (d), 119.8
(s), 126.5 (d), 127.0 (s), 128.4 (d), 129.4 (d), 129.7 (d), 135.6 (s), 157.1
(s), 175.1 (s). m/z (EI+, %) ) 308, 310 (M+, 48), 222, 224 (100). HRMS
(EI+): calcd for C14H1379BrO3, 308.0048; found, 308.0063. Anal. Calcd
for C14H13BrO3: C, 54.39; H, 4.24. Found: C, 54.40; H, 4.26.
9-Bromo-2-methyl-2,3-dihydro-1H-benzo[f]chromen-1-one 13. To
mechanically stirred polyphosphoric acid (100 mL) at 70 °C, 12 (4.0
g, 13 mmol) was added. The temperature was raised to 110 °C, and
N-(1-(9H-Fluoren-9-ylidene)-2-methyl-2,3-dihydro-1H-benzo[f]-
chromen-9-yl)benzamide 2. Following the procedure of Yin and
Buchwald,30 a mixture of 5 (0.14 mg, 0.32 mmol), benzamide (96 mg,
0.80 mmol), 4,5-bis(diphenylphosphino)-9,9-dimethyl-xanthene (45 mg,
77 µmol), Pd2(dba)3 (24 mg, 26 µmol), and Cs2CO3 (209 mg, 0.64
mmol) was stirred in dioxane (2 mL) at 100 °C overnight. After the
9
J. AM. CHEM. SOC. VOL. 130, NO. 13, 2008 4551

A R T I C L E S
Pijper et al.
mixture was cooled to room temperature, it was diluted in CH2Cl2 (20
mL) and filtered over a plug of silica. After removal of all volatiles
under reduced pressure, the product was further purified by column
chromatography (SiO2, pentane/EtOAc ) 4:1, Rf ) 0.5), yielding a
127.0 (d), 127.4 (d), 127.8 (d), 127.9 (d), 128.5 (2×d), 128.8 (2×d),
129.0 (s), 130.3 (d), 131.9 (d), 132.1 (d), 132.9 (s), 133.1 (s), 135.7
(s), 137.9 (2×s), 138.5 (s), 138.6 (s), 140.2 (s), 140.8 (s), 154.7 (s),
173.0 (s), 173.4 (s). m/z (EI+, %) ) 521 (M+, 100), 416 (15). HRMS
(EI+): calcd for C36H27NO3, 521.1991; found, 521.1992.
1
yellow solid (150 mg, 0.31 mmol, 98%). Mp 180-182 °C. H NMR
(400 MHz, CDCl3) δ 1.38 (d, J ) 7.3 Hz, 3H), 4.19 (m, 1H), 4.35-
4.43 (m, 2H), 6.56 (d, J ) 8.1 Hz, 1H), 6.75 (t, J ) 8.1 Hz, 1H), 7.06
(d, J ) 8.8 Hz, 1H), 7.13 (t, J ) 7.5 Hz, 1H), 7.27-7.41 (m, 5H),
7.45 (s, 1H), 7.58-7.64 (m, 4H), 7.73-7.82 (m, 3H), 7.89 (d, J ) 8.8
Hz, 1H), 8.20 (d, J ) 8.8 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ
15.9 (q), 33.1 (d), 72.3 (t), 113.1 (s), 113.8 (d), 117.3 (d), 118.0 (d),
119.0 (d), 119.9 (d), 124.5 (d), 125.1 (d), 126.4 (s), 126.7 (d), 126.9
(d), 127.1 (2×d), 127.2 (d), 127.3 (d), 128.3 (2×d), 129.5 (d), 131.5
(d), 131.7 (d), 132.3 (s), 132.5 (s), 134.8 (s), 137.6 (s), 137.8 (s), 138.1
(s), 138.4 (s), 139.4 (s), 140.7 (s), 154.1 (s), 165.8 (s). m/z (EI+, %) )
479 (M+, 100), 374 (10). HRMS (EI+): calcd for C34H25NO2, 479.1885;
found, 479.1908. Separation of the enantiomers was achieved by CSP-
HPLC (Chiralcel OD, heptane/2-propanol ) 95:5, flow rate ) 1.0 mL/
min, retention times: 35.8 min for (2′S)-(P)-2 and 40.6 min for (2′R)-
(M)-2.
N-(1-(9H-Fluoren-9-ylidene)-2-methyl-2,3-dihydro-1H-benzo[f]-
chromen-9-yl)-N-acetylbenzamide, (2′S)-(P)-4. Sodium bis(trimeth-
ylsilyl)amide (50 µL of a 1.0 M solution in THF, 50 µmol) was added
to a solution of (2′S)-(P)-2 (10 mg, 21 µmol) in THF (4 mL) and stirred
at -78 °C. After 5 min acetyl chloride (1.6 µL, 1.7 mg, 22 µmol) was
added to the mixture, which was stirred another 5 min and allowed to
warm to room temperature. The solution was quenched with a saturated
aqueous NaHCO3 solution (50 mL), which was then extracted with
ether (3 × 75 mL). The organic phase was washed with brine, dried
(Na2SO4), and the solvent was removed under reduced pressure. The
pure product was obtained after preparative TLC (SiO2, pentane/EtOAc
) 8:1, Rf ) 0.25), providing a yellow solid (5.3 mg, 11 µmol, 52%).
Mp 193-195 °C. 1H NMR (400 MHz, CD2Cl2) δ 1.39 (d, J ) 7.0 Hz,
3H), 1.62 (s, 3H), 4.18 (m, 1H), 4.40-4.46 (m, 2H), 6.51 (d, J ) 8.8
Hz, 1H), 6.64 (t, J ) 7.5 Hz, 1H), 7.09-7.15 (m, 2H), 7.20-7.25 (m,
3H), 7.33-7.41 (m, 5H), 7.63-7.67 (m, 2H), 7.82 (d, J ) 6.8 Hz,
1H), 7.85-7.89 (m, 2H), 7.94 (d, J ) 7.3 Hz, 1H). 13C NMR (100
MHz, CD2Cl2) δ 15.7 (q), 24.6 (q), 33.3 (d), 73.0 (t), 114.4 (s), 119.6
(d), 119.7 (d), 120.3 (d), 124.3 (d), 124.9 (d), 125.1 (d), 125.3 (d),
(2′S)-(P)-3-PHIC. NaH (1 mg, 50% in oil, 21 µmol) was washed
with pentane in a Schlenck tube under N2, after which (2′S)-(P)-2 (9
mg, 19 µmol) dissolved in THF (5 mL) was added. After stirring at
room temperature for 10 min, the mixture was cooled to -90 °C, and
n-hexyl isocyanate (0.56 µL, 0.48 g, 3.8 mmol) was added, upon which
the mixture slowly turned more viscous. After 45 min, acetyl chloride
(18 µL, 20 mg, 0.25 mmol) and pyridine (20 µL, 20 mg, 0.25 mmol)
were added to the mixture, which was then allowed to warm to room
temperature. The mixture was then poured in MeOH (100 mL), and
the precipitated poly(n-hexyl isocyanate) was filtered off, washed three
times with MeOH, and dried in vacuo, which yielded a yellow solid
(340 mg, 71%). 1H NMR (400 MHz, CD2Cl2) δ 0.85 (b, 3H), 1.30 (b,
6H), 1.60 (b, 2H), 3.70 (b, 2H), 6.50-8.00 (low-intensity signals:
aromatic protons). 13C NMR (100 MHz, CDCl3) δ 14.0 (q), 22.6 (t),
26.2 (t), 28.4 (t), 31.5 (t), 48.5 (t), 156.8 (s). GPC: Mn ) 25 850, Mw
) 32 650, D ) 1.26.
Acknowledgment. Financial support from The Netherlands
Foundation for Scientific Research (NWO-CW) and NanoNed
is gratefully acknowledged. We thank Ing. E. J. Vorenkamp
for performing the GPC analysis and Dr. M. M. Pollard and T.
C. Pijper for helpful discussions.
Supporting Information Available: 1H NMR and APT
spectra of all compounds, UV-vis spectra for the photocon-
version of (2′S*)-(P*)-2 to (2′S*)-(M*)-2 and vice versa, detailed
experimental and crystal data for the X-ray structure determi-
nation of (2′S*)-(P*)-2 and (2′S*)-(M*)-2, crystallographic
information files (CIF), Eyring plot of thermal helix inversion
of 2, optical micrographs of the LC film at PSS>480 during a
series of irradiation cycles. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA711283C
9
4552 J. AM. CHEM. SOC. VOL. 130, NO. 13, 2008