A Chiroptical Molecular Switch with Distinct Chiral and Photochromic Entities and Its Application in Optical Switching of a Cholesteric Liquid Crystal

Article abstract of DOI:10.1002/chem.200305276

Two new structurally related photoswitches are described, in which azobenzene photochromism is combined with the chirality of a 2,2′-dihydroxy-1,1′-binaphthyl unit. In system 1 the chiral binaphthyl moiety is bridged by a methylene tether, locking the biaryl chirality while in system 2 the biaryl core is unbridged and has considerable conformational flexibility. Both compound are capable of inducing cholesteric liquid crystalline phases and proved to be good photoswitches both in solution and in a liquid crystalline matrix. Compound 2 is capable of completely reversing the liquid crystalline chirality which is unique for a chiroptical molecular switch where the switching unit and the chiral moiety are separate entities.

Full text of DOI:10.1002/chem.200305276

FULL PAPER
A Chiroptical Molecular Switch with Distinct Chiral and Photochromic
Entities and Its Application in Optical Switching of a Cholesteric Liquid
Crystal
[a]
Richard A.van Delden, [a] Tommaso Mecca,[b, c] Carlo Rosini,*[b] and Ben L.Feringa*
Abstract: Two new structurally related
photoswitches are described, in which
azobenzene photochromism is com-
bined with the chirality of a 2,2’-dihy-
droxy-1,1’-binaphthyl unit. In system 1
the chiral binaphthyl moiety is bridged
by a methylene tether, locking the
biaryl chirality while in system 2 the
biaryl core is unbridged and has con-
siderable conformational flexibility.
Both compound are capable of induc-
ing cholesteric liquid crystalline phases
and proved to be good photoswitches
both in solution and in a liquid crystal-
line matrix. Compound 2 is capable of
completely reversing the liquid crystal-
line chirality which is unique for a chi-
roptical molecular switch where the
switching unit and the chiral moiety
are separate entities.
Keywords: biaryls
¥
chirality
¥
¥
isomerization liquid crystals
¥
molecular switches ¥ photochromism
Introduction
nanotechnological application. Typical materials include
photochromic (doped) polymers[4] or polypeptides,[5] crys-
tals,[6] and liquid crystals.[7] Usually photochromism is moni-
tored by UV/Vis spectroscopy at the absorption maxima of
the bistable molecular system but in an application as light-
controlled molecular switches this implies destructive read-
out. In order to meet the requirement of non-destructive
read-out chiroptical molecular switches based on the photo-
chromism of chiral sterically overcrowded alkenes have
been developed.[8] These overcrowded alkenes are unique
because switching results in a reversal of the intrinsic chirali-
ty, changing the handedness of the helicity of the system.
Two types of switches based on sterically overcrowded al-
kenes were developed (Figure 1): A) enantiomeric photobi-
stable molecules where two enantiomers are interconverted
at a single wavelength by changing the handedness of the
circularly polarized light, and B) diastereomeric photobista-
ble molecules where the photochromic compound consists
of two pseudoenantiomeric forms of opposite helicity which
Molecular switches are prominent members of the toolbox
of nanotechnology.[1] The broad current interest in bistable
molecular systems, which can function as molecular
switches, has resulted in a large variety of organic structures
which potentially can be applied for example in optical data
storage and processing.[2] The most widely studied class of
molecular switches are based on photochromic molecules
where light can be used as a stimulus for switching.[3] Re-
quirements for applications of these systems, such as high
switching selectivity and fatigue resistance, have been met
by fine-tuning of the different molecular structures.[2]
number of switching systems was also shown to function in
processable media, which is absolutely essential for any
A
[a] Dr. R. A. v. Delden, Prof. Dr. B. L. Feringa
Department of Organic Chemistry
Stratingh Institute, University of Groningen
Nijenborgh 4
9747 AG Groningen (The Netherlands)
E-mail: feringa@chem.rug.nl
[b] Dr. T. Mecca, Prof. Dr. C. Rosini
Dipartimento di Chimica
Universit‡ della Basilicata
Via N. Sauro 85
85100 Potenza (Italy)
E-mail: rosini@unibas.it
[c] Dr. T. Mecca
Current address:
C.N.R.–Istituto di Chimica Biomolecolare (Sez. di Catania)
Via del Santuario 110
Figure 1. Schematic representation of different types of chiroptical molec-
95028 Valverde, Catania (Italy)
ular switches (X*=chiral auxiliary group).
61
Chem. Eur. J. 2004, 10, 61 70
DOI: 10.1002/chem.200305276
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

FULL PAPER
can undergo photoisomerization
by irradiation at two different
wavelengths l1 and l2.
Both systems can be read-out
in a non-destructive manner by
optical
rotation
employing
wavelengths far outside the ab-
sorption bands of either of the
bistable molecular forms of the
switchable system. The photo-
modulation of intrinsic chirality
also offers the possibility of
using these systems as triggers
of liquid crystalline phase transi-
tions where the chirality of a
Scheme 1. Synthesis of enantiomerically pure chiroptical molecular switches (S)-1 and (S)-2 starting from (S)-
6,6’-dibromo-2,2’-dihydroxy-1,1’-binaphthalene 3: a) K2CO3, DMF, CH2I2, 808C, 78%;[17] b) K2CO3, DMF,
CH3CH2Br, 808C, 88%; c) nBuLi, ꢀ788C; d) BF(OCH3)2, ꢀ788C; e) NaOH/H2O2, 0 208C (yields (S)-6: 86%,
(S)-7: 72%); f) benzenediazonium tetrafluoroborate salt, pyridine and K2CO3 in anhydrous THF, ꢀ208C
chiral
nematic
(cholesteric) (yields (S)-8: 38%, (S)-9: 43%), g) MeI/K2CO3 in DMF (yields (S)-1: 65%), (S)-2: 66% yield).
liquid crystal is fully controlled
by using a chiroptical molecular
switch as a dopant.[9] Steric hin-
drance resulting in an intrinsic helical structure is an essen-
tial feature in all the switchable chiral alkenes developed so
far. There are a number of other examples of chiral optical
switches but in most of these systems the photochromic part
and the chiral part are separate entities. In these cases
switching does not change the chirality of the system al-
though it can have a substantial influence on the chiral
properties (Figure 1C).[2,10] Some of these systems, which are
based on different types of photochromism, have been em-
ployed in liquid crystals and could influence the chiral prop-
erties of the systems but not control the handedness of the
cholesteric phase, that is, the magnitude but not the sign of
the cholesteric pitch could be switched by light in a reversi-
ble manner.[11] Very recently, for example, an axially chiral
photoswitchable compound based on azobenzene photo-
chromism combined with binaphthalene chirality was re-
ported.[12] With this system doped in nematic liquid crystal-
line hosts, it was possible to induce cholesteric phases by the
influence of the chiral binaphthyl part of the dopant and to
influence the chiral properties by photoinduced trans cis
isomerization of the photochromic azobenzene part of the
dopant. The handedness of the liquid crystalline phase, how-
ever, remained the same. Here we report on two structurally
related systems, in which azobenzene photochromism is
combined with the chirality of 2,2’-dihydroxy-1,1’-binaphthyl
(BINOL), a source of chirality which is nowadays easily
available and has found a myriad of applications as chiral
ligand and catalyst in asymmetric synthesis.[13] The new pho-
tochromic compounds (S)-1 and (S)-2 are shown in
Scheme 1. These two compound were designed as phototrig-
gers for liquid crystalline phase transitions. Functionalized
1,1’-binaphthalenes are very efficient dopants to induce
chiral nematic LC phases and high helical twisting powers
are observed due to the intrinsic chirality of the binaphtha-
lene core (see below).[14] It was established that the chiral in-
duction is highly dependent on the dihedral angle (q) be-
tween the two naphthalene moieties.[15] In the most com-
monly observed cisoid conformation (q <908), an (S)-bi-
naphthalene compound induces a positive cholesteric phase
while in the transoid (q >908) conformation a negative cho-
lesteric phase is induced by an (S)-binaphthalene com-
pound.
For compounds 1 and 2, the intrinsic chirality of the bi-
naphthyl-based core should guarantee the induction (in a
nematic solvent) of cholesteric mesophases. The azobenzene
moieties are oriented in the same direction as the axis join-
ing the two naphthalene moieties. In LC hosts, binaphtha-
lenes are known to orient with this axis parallel to the nem-
atic director.[14a] In compound 1 the chiral binaphthalene
core is rigidified by a methylene bridge in order to minimize
the influence of azobenzene photoisomerization on rotation
around the biaryl bond as the compound is fixed in a cisoid
conformation (note that racemization cannot occur under
the present conditions). In compound 2 the chiral binaph-
thalene core has more rotational freedom and the influence
of photoisomerization of the azobenzene on the overall ge-
ometry of the molecule could be more pronounced. We an-
ticipated a major influence of azobenzene switching on the
overall conformation, and as a consequence the chirality, of
the compounds which could lead to chiroptical switching of
the handedness of these cholesteric phases.
The photochromic and chiral properties of these two sys-
tems were studied both in solution and in liquid crystalline
(LC) environment.
Results and Discussion
Synthesis: Compounds (S)-1 and (S)-2 were synthesized
starting from (S)-6,6’-dibromo-2,2’-dihydroxy-1,1’-binaphtha-
lene[16] in four steps: 1) protection of the hydroxy groups in
1,1’-position of the binaphthalene rings;[17] 2) substitution of
the two bromine atoms in 6,6’-position with two hydroxy
groups; 3) coupling reaction with the benzenediazonium tet-
rafluoborate salt and 4) methylation of the two hydroxy
groups in 6,6’-position of the binaphthalene moiety
(Scheme 1).
The presence of two azobenzene moieties in both com-
pound 1 and 2 allows the existence of three isomers, the en-
ergetically preferred all-trans C2-symmetric isomer (tt), the
62
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 61 70

Chiroptical Molecular Switches
61 70
mixed cis trans isomer (ct) and the all-cis C2-symmetric
isomer (cc). These forms can be interconverted by photoin-
duced isomerization of the photochromic azobenzene moiet-
ies (Scheme 2). A substantially steric influence of the bi-
naphthalene core on the overall geometry and chirality
Scheme 2. Possible photochemical interconversions between three com-
pounds differing in azobenzene geometry.
upon photoisomerization is expected. Whereas in the all-
trans configuration of 1 and 2 the linear orientation around
the azo double bond allows for the phenyl group to simply
bend away, upon trans to cis isomerization steric interactions
are severely increased and the chiral binaphthyl unit is ex-
pected to exert an effect on the orientation of the azo-
phenyl moiety. Pure–energetically favorable–tt-(S)-1 and
tt-(S)-2 were obtained by heating any mixture of tt, ct and cc
isomers to allow thermal equilibration. The UV/Vis and CD
spectra of both tt-(S)-1 and tt-(S)-2 are depicted in Figure 2.
Upon comparison of compound 1 and 2 distinct differen-
ces in the UV/Vis absorption are observed. First of all, it is
interesting to compare the absorption spectra of (S)-1 and
(S)-2 with the corresponding spectra of (S)-2,2’-methylene-
dioxy-1,1’-binaphthyl (S)-10 and (S)-2,2’-dimethoxy-1,1’-bi-
naphthyl (S)-11, that is, the compounds which are the model
compounds of the core of (S)-1 and (S)-2 (Figure 3).[18]
In the spectra of those model compounds, two main
region of absorption, can be easily singled out: a) 350
250 nm and b) 250 200 nm. As it is well known, region a is
due to the 1La transition (short axis polarization) while
Figure 2. UV/Vis and CD spectra of (S)-tt-1 (c) and (S)-tt-2 (a) in
n-heptane solution.
Figure 3. Model compounds (S)-10 and (S)-11.
1
region b) is due to the B transition (long axis polarization)
of the naphthalene chromophore.[18] Interestingly, the ab-
sorption of these model compounds is almost negligible at
250 nm and the extinction coefficient at about 300 nm is in
the range of 7000 8000, therefore the strong absorption
bands (e 20000 or even more), present in the spectra of
both (S)-1 and (S)-2 at wavelengths longer than 300 nm, are
related to the aromatic azo-moieties. As a matter of fact,
trans-phenyl-N=N-(1-naphthyl) shows absorptions at 454 nm
(emax 450, n!p* transition) and 371 nm (emax 22900, p!p*
transition),[19] therefore we can conclude that the bands ob-
served for (S)-1 and (S)-2 at about 460 and 400 nm are due
to the n!p* and p!p* transitions of the aromatic azo-
chromophore, respectively. The wavelength shift is due the
presence of the substituents in both (S)-1 and (S)-2. By con-
trast, the intense bands at l <250 nm are certainly due to
the binaphthyl chromophore, in particular the sequence of a
shoulder at about 230 nm followed by a maximum at about
220 nm is clearly indicative of the presence of a small dihe-
dral angle between the naphthyl rings (in fact (S)-1 is a
bridged binaphthyl compound) whilst the broad peak at
230 nm for (S)-2 clearly reveals a larger dihedral angle be-
tween the aromatic rings,[18] as it is certainly true for the
open derivative (S)-2. In the CD spectra, the excitations of
the azo-moieties (wavelengths longer than 300 nm) give rise
to only relatively weak Cotton effects, whilst a strong, posi-
tive couplet in the spectra of (S)-1 and (S)-2 shows clearly
the existence of cisoid binaphthyl chromophores, taking into
account that both these compounds have (S) absolute con-
figuration.[18] The absorption band at 390 nm of tt-(S)-1 is
bathochromically shifted to 405 nm for tt-(S)-2. Similarly,
the absorption band at 450 nm of cc-(S)-1 is red-shifted to
460 nm for cc-(S)-2. This effect can be ascribed to an in-
63
Chem. Eur. J. 2004, 10, 61 70
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

C. Rosini, B. L. Feringa et al.
FULL PAPER
Table 1. Ratios of the three geometrical isomers of (S)-1 at the photosta-
creased contribution of the lone pairs of the oxygens in 1,1’-
position of the binaphthyl group to the delocalized p elec-
tronic system.[20] This should have a small but distinct effect
on the photochemical properties.
tionary states in chloroform.[a]
l [nmꢀ1
]
tt-(S)-1 [%]
ct-(S)-1 [%]
cc-(S)-1 [%]
396
450
19
67
46
30
35
3
[a] As determined by HPLC analysis on silica gel using CHCl3/Et2O 98:2
at 0.25 mLminꢀ1 and 296 nm (which is an isosbestic point of the three
form this eluent) as a detection wavelength. The elution order is: tt-(S)-1
(16.5 min), ct-(S)-1 (23.0 min) and cc-(S)-1 (28.5 min).
Isomerization in solution: Solutions of pure tt-(S)-1 in
chloroform and n-heptane (10ꢀ5 m) were irradiated at differ-
ent wavelength (391, 396, 450 nm) and the change in UV/
Vis absorption and circular dichroism (CD) was monitored.
From the ratio of the different absorption curves, the most
ideal wavelength for trans to cis and the reverse cis to trans
isomerization were determined to be 396 and 450 nm, re-
spectively. The UV/Vis and CD characteristics of compound
(S)-1 in chloroform and n-heptane are similar. The UV/Vis
and CD spectra corresponding to the two photostationary
states at 396 and 450 nm in n-heptane solution are depicted
in Figure 4, together with the UV/Vis absorption of the ini-
tial tt-(S)-1 solution.
The photostationary state ratios of the three forms
(Table 1) were analyzed by HPLC and the three geometrical
isomers could be assigned by diode-array UV/Vis analysis.
Only starting from chloroform solution full baseline separa-
tion of the three isomers was achieved and the ratios in this
solvent are reported. Analysis of the not fully separated
HPLC traces and UV/Vis and CD spectra from n-heptane
solution revealed similar switching selectivities in this sol-
vent. The first (least polar) eluted fraction shows a UV/Vis
spectrum coincident with the spectrum of the initial com-
pound after the thermal isomerization, and it was assigned
the trans trans (tt) structure. The third (most polar) fraction
shows a UV/Vis spectrum characterized by the disappear-
ance of the absorption bands at l=390 nm and l=480 nm
and the appearance of a broad band at l=450 nm that is
typical of a cis-azobenzene moiety.[21] It was assigned the
cis cis (cc) structure. The second substance eluted (inter-
mediate polarity) shows an UV/Vis spectrum that is exactly
the average of the spectra of the two previous compounds,
so it is safe to assign to this compound the cis trans (ct)
structure.
After 396 nm irradition 58% of all azobenzene units are
switched to the cis configuration, subsequent irradiation at
450 nm results in a photostationary state in which 82% of
all azobenzene units are present in trans form. The presence
of an isosbestic point at 269 nm indicates a clean switching
process which was shown to be fully reversible. From the
photostationary state ratios and UV/Vis spectra, the absorp-
tion spectra of ct-1 and cc-1 could be calculated (Figure 5).
These spectra and also analysis of the UV/Vis spectra ob-
tained directly from the HPLC detector show that the ab-
sorption spectrum of ct-(S)-1 is the exact average of the
spectra of the all-trans and all-cis forms of (S)-1. In this C2-
symmetrical system, the two azobenzene units act as sepa-
rate entities. The ratios observed for both photostationary
Figure 4. UV/Vis and CD spectra of (S)-1 in n-heptane solution: c,
photostationary state at 450 nm (excess trans); a, photostationary
states at 396 nm (excess cis). The original spectra of pure tt-(S)-1 are
plotted for comparison (g).
Figure 5. Calculated UV/Vis spectra of the three isomers tt (c), ct
(a) and cc (S)-1 (g) in n-heptane.
64
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 61 70

Chiroptical Molecular Switches
61 70
Table 2. Photostationary state ratios of the three isomers of (S)-2 in
states are a near statistical distribution of the amount of
trans and cis azobenzene units. The most apparent feature
of these spectra is that the difference between the UV/Vis
spectra of the all-trans compound tt-(S)-1 and both cis-com-
pounds ct-(S)-1 and cc-(S)-1 is relatively small compared to
other azobenzene photochromic compounds reported.[22]
This small difference results in a relatively modest switching
efficiency but nevertheless it is shown here that compound
(S)-1 can readily function as a molecular switch in chloro-
form and n-heptane solution. From the CD spectra of both
photostationary states and the initial tt-(S)-1 isomer
(Figure 4) only a minor influence of azobenzene isomeriza-
tion on the overall geometry and chirality of this compound
is observed. This was expected since the intrinsic chirality of
the binaphthalene core is fixed by the methylene tether.
Compound (S)-2 was analyzed in the same manner using
a 10ꢀ5 m solution of pure tt-(S)-2 in chloroform and n-hep-
tane. As for compound 1, UV/Vis and CD spectra as well as
switching selectivities were comparable in both solvents.
The most efficient wavelengths for the switching process
were determined to be 405 nm for the trans to cis isomeriza-
tion and 466 nm for the reverse process. The photoisomeri-
zation showed a clear isosbestic point at 271 nm (Figure 6).
The photostationary state ratios are shown in Table 2.
chloroform.[a]
l [nmꢀ1
]
tt-(S)-2 [%]
ct-(S)-2 [%]
cc-(S)-2 [%]
405
466
33
56
46
40
21
4
[a] As determined by HPLC analysis on silica using CHCl3/Et2O 98:2
(0.25 mLminꢀ1) and HPLC analysis using the same conditions as for 1
using a detection wavelength of 245 nm (isosbestic point of the three
forms in this eluent). The elution order is the same as for compound 1:
tt-(S)-2 (13.8 min), ct-(S)-2 (18.3 min) and cc-(S)-2 (21.4 min).
easily assignable to the tt, ct and cc (S)-2. Also for this com-
pound the absorption spectrum of ct-(S)-2 is the exact aver-
age of the spectra of the all-trans and all-cis forms of (S)-2,
and the absorption difference between the cis- and trans-
forms are less pronounced than expected. The calculated
spectra for ct-2 and cc-2 are depicted in Figure 7.
Comparable to compound 1, the HPLC analysis of the
photostationary states show the presence of three isomers
Figure 7. Calculated UV/Vis spectra of the three isomers tt (c), ct
(a) and cc (S)-2 (g) in n-heptane.
When the wavelength of 405 nm is used to promote the
trans- to cis-isomerization process, only 44% of all trans-
azobenzene moieties present are switched to the cis-form.
Subsequent irradiation at 466 nm results in a photostation-
ary state in which 76% of the azobenzenes units are present
in the trans-form. Also, the observed ratios reflect a near
statistical distribution of trans- and cis-azobenzene units and
no combined effect of the two units present in each mole-
cule is observed.
Also for compound 2 the effect of cis trans isomerization
on the chirality of the compound was studied by circular di-
chroism (Figure 6). The most striking observation here is
that the effect of this isomerization is similar to the effect
observed for (S)-1. Although some changes in CD can be
observed upon irradiation the overall chirality of the system
is more or less retained even though the rotational freedom
of the binaphthalene core is far larger than for compound
(S)-1. The bisignate CD band around 240 nm reflecting the
chirality of the binaphthalene core is hardly changed upon
photoirradiation. The relative changes in the bathochromic
side of the spectrum are more pronounced, comparable to
Figure 6. UV/Vis and CD spectra of (S)-2 in n-heptane solution: c,
photostationary state at 466 nm (excess trans), a, photostationary
states at 405 nm (excess cis). The original spectra of pure tt-(S)-2 are
plotted for comparison (g).
65
Chem. Eur. J. 2004, 10, 61 70
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

C. Rosini, B. L. Feringa et al.
FULL PAPER
the changes observed for compound (S)-1. Compound (S)-2
was shown to be a–though moderate–molecular switch in
these solution experiments. Both compounds were subse-
quently examined in liquid crystalline hosts.
already anticipated because the binaphthalene core, expect-
ed to be largely responsible for chiral induction in a liquid
crystal, is fixed by a methylene tether and azobenzene iso-
merization hardly has any effect on the chirality of the core
as already observed in CD experiments in chloroform solu-
tion.
Photomodulation of cholesteric liquid-crystalline mesophas-
es: The behavior of (S)-1 and (S)-2 was studied in E7 (a
commercially available nematic mixture of different biphe-
nylcarbonitrile-based mesogens, liquid crystalline up to
588C). All samples were prepared with a concentration of
active compound (tt-(S)-1 or tt-(S)-2) of three weight%. The
pitches of the induced cholesteric mesophases were deter-
mined by the Grandjean Cano technique.[23] The sign of the
cholesteric phases was determined using a contact method
where mixing of the samples with a doped cholesteric
liquid-crystal of known negative screw sense was tested.[24]
The wavelengths used for photoswitching were the ideal
wavelength determined for CHCl3 solution.
Compound (S)-2, although structurally similar to (S)-1,
shows completely different behavior in a liquid crystalline
matrix. Again, three weight% of pure tt-(S)-2 in nematic E7
resulted in a clear cholesteric phase with a pitch of +
3.05 mm, indicating a relatively low helical twisting power of
+10.9 mmꢀ1. Irradiation with 402 nm light inverted the cho-
lesteric pitch to ꢀ5.52 mm while subsequent irradiation at
466 nm reverted the pitch to +21.87 mm.[26] During these
photoinduced cholesteric helix inversions the Grandjean -
Cano lines observed through a polarization microscope
moved outwards, then disappeared and upon continued irra-
diation appeared again. This is a unique case of direct obser-
vation of gradual cholesteric helix inversion via a compen-
sated nematic phase using a chiroptical molecular switch.
The photostationary state ratios of the three isomers tt-(S)-
2, ct-(S)-2 and cc-(S)-2 in the liquid-crystalline phase deter-
mined by HPLC, using the same method as for compound 1,
are summarized in Table 4.
For the LC sample doped with three weight% tt-(S)-1 a
clear cholesteric phase was observed with a pitch of +
2.12 mm, indicating a helical twisting power (b) of
+
15.2 mmꢀ1.[25] The positive screw sense of the cholesteric
packing induced by an (S)-binaphthalene based dopant is in
accordance with comparable compounds.[14] Irradiation with
396 nm increased the cholesteric pitch to +2.24 mm while
subsequent irradiation with 450 nm decreased the pitch to
+2.18 mm. The ratio of the three isomers tt-(S)-1, ct-(S)-1
and cc-(S)-1 in the photostationary states in the liquid-crys-
talline phase were determined by HPLC (Table 3).
Table 4. Photostationary state ratios of the three isomers of (S)-2 doped
in mesogenic host E7 (3 wt%).[a]
l [nmꢀ1
]
tt-(S)-2 [%]
ct-(S)-2 [%]
cc-(S)-2 [%]
pitch [mmꢀ1
]
100
35
58
+ 3.05
ꢀ5.52
402
466
51
37
14
5
Table 3. Photostationary state ratios of the three isomers of (S)-1 doped
+ 21.87
in mesogenic host E7 (3 wt%).[a]
[a] As determined by HPLC analysis on silica gel using a gradient from
pure chloroform to chloroform/ether 98:2 (0.5 mLminꢀ1) and 245 nm
(which is an isosbestic point of the three form this eluent) as a detection
wavelength. The elution order is: tt-(S)-1 20.7 min, ct-(S)-1 22.8 min, cc-
(S)-1 24.2 min. Cholesteric pitches were determined by the Grandjean
Cano method.
l [nmꢀ1
]
tt-(S)-1 [%]
ct-(S)-1 [%]
cc-(S)-1 [%]
pitch [mmꢀ1
]
100
51
82
+ 2.12
+ 2.24
+ 2.18
396
450
39
16
10
2
[a] As determined by HPLC analysis on silica gel using a gradient from
pure chloroform to chloroform/Et2O 98:2 (0.5 mLminꢀ1) and 296 nm
(which is an isosbestic point of the three form this eluent) as a detection
wavelength. The elution order is: tt-(S)-1 20.9 min, ct-(S)-1 22.2 min, cc-
(S)-1 23.3 min. Cholesteric pitches were determined by the Grandjean
Cano method.
Also thermal switching back to tt-(S)-2 starting from the
402 nm photostationary state was investigated and after
heating the sample to 508C (where the sample is still liquid
crystalline) the mixture fully reverts to >99% tt-(S)-2 as
confirmed by HPLC analysis. The helical twisting powers of
ct-(S)-2 and cc-(S)-2 were calculated to be ꢀ8.4 mmꢀ1 and
ꢀ39.3 mmꢀ1, respectively. Compound 2 can function as an ef-
ficient molecular switch in a liquid crystalline matrix and
photoswitching between three different forms can be in-
duced by light. Due to the differences in helical twisting
powers of the three forms, positive for tt-(S)-2 and negative
for ct-(S)-2 and cc-(S)-2, the helicity of a cholesteric liquid
crystal can efficiently be controlled. Essential here are the
opposite helical twisting powers for the all-trans and the iso-
merized forms of 2.
Comparing these results in an LC phase with those of a
CHCl3 solution (Table 2), shows a less selective trans to cis
isomerization. The cis to trans isomerization on the other
hand is more selective. These minor effects can be attributed
to changes in the relative absorption of the various isomers
in a different environment. From the experimental photosta-
tionary ratios and the cholesteric pitches the helical twisting
powers of ct-(S)-1 and cc-(S)-1 could be calculated to be +
17.2 mmꢀ1 and +13.0 mmꢀ1. All three forms have comparable
helical twisting powers and azobenzene isomerization was
proven to occur in a liquid crystalline environment. The
switching selectivity in a LC host is similar to the switching
selectivity in chloroform solution but irradiation times are
prolonged. Because of the similar helical twisting powers
found for tt-1, ct-1 and cc-1, however, no efficient switching
of cholesteric phase chirality is possible for (S)-1. This was
At this point it is difficult to provide a detailed interpreta-
tion of the very different behavior of (S)-1 and (S)-2, how-
ever by comparison with model compounds (S)-10 and (S)-
11 (Figure 3), respectively, we can make the following com-
ments. First of all, (S)-10, the model compound for the core
66
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 61 70

Chiroptical Molecular Switches
61 70
of (S)-1, shows a very strong HTP (b = +85 mmꢀ1) in PCB
(a biphenyl-like nematic liquid crystal),[14c] whilst the HTPs
of the three isomers of (S)-1 range between b=+13 and +
17 mmꢀ1. This indicates that the main source of the cholester-
ic induction of (S)-1 is still the binaphthyl core which is
fixed in a cisoid conformation by the methylene bridge. The
significant reduction of HTP going from (S)-10 to (S)-1
could be ascribed to a more difficult alignment of the mole-
cules of the nematic liquid crystal around the (S)-1 solute,
as a result of the steric hindrance due to the azo-substitu-
ents. It is also interesting to note that the above variation of
HTP could also be related to a different alignment of the tt,
ct, cc isomers of (S)-1 in the nematic solvent, owing to the
different shape of these compounds. Ferrarini and Spada
et al. have in fact recently demonstrated that biphenyl com-
pounds with the same absolute configuration ((P), for in-
stance) can induce a (P)- or (M)-cholesteric phase in E7, de-
pending on their disk-like or rod-like behaviour, which is, in
turn, related to the molecular shape of these derivatives.[27]
On the contrary, the HTP of (S)-11, the model compound
for the core of (S)-2, in K15 (a nematic solvent structurally
similar to E7) is only +1.5 mmꢀ1.[14b] Therefore the calculat-
ed HTP values of b+10.9, ꢀ8.4 and ꢀ39.3 mmꢀ1 of tt-, ct-
and cc-(S)-2, respectively, could largely be attributed to the
geometry around the azobenzene moieties which changes
upon photoisomerization. It is remarkable that the changes
in HTP on passing from the tt-(S)-2 to ct-(S)-2 to cc-(S)-2
are almost additive, indicating that the effects on the overall
HTP value due to the two azobenzene isomerizations are
almost independent. Although the intervention of a cisoid
transoid isomerization of the binaphthyl moiety cannot be
ruled out, the CD spectrum of (S)-2 shows only very little
changes after the photochemical irradiation in solution. A
positive couplet is always observable: this means that the
critical value of the dihedral angle for sign inversion of the
CD couplet (q = 1108) is not attained.[18] Therefore, both
before and after irradiation the dihedral angle of the binap-
thalene moiety of (S)-2 ranges between 90 and 1108. These
values can be easily accepted, considering that other 1,1’-bi-
naphthyl compounds with these characteristics are
known.[18b,c] Only in the case of 2,2’-bis(bromomethyl)-1,1’-
binaphthyl a dihedral angle larger than 1108 was repor-
ted.[14b]
azobenzene isomerization changes the alignment of the
nematic solvent around the solute resulting in a different
cholesteric phase.
Conclusion
We have shown that optically active binaphthyl derivatives
containing two azobenzene chromophores can be prepared
by straightforward chemistry in satisfactory yields starting
from enantiomerically pure BINOL. Compounds (S)-1 and
(S)-2 show interesting photochromic properties and they can
act as molecular switches in solution. While compound (S)-1
in the liquid crystal host E7 shows only very small variation
of the cholesteric pitch, compound (S)-2 in the same liquid
crystalline host is capable of reversibly inverting the choles-
teric packing upon photoisomerization. The all-trans form of
(S)-2 has a positive helical twisting power whereas both
forms with cis-azobenzene moieties induce a negative helici-
ty. The helical twisting power of the all-cis form of (S)-2 is
remarkably large. These features were thus far only ob-
served for switches where photoisomerization resulted in a
full reversal of the chirality of the molecular structure (in-
herently dissymmetric chiroptical switches).
The present investigation clearly shows that the chemical
coupling of the photochemical properties of the azo-group
with the chirality of the BINOL skeleton constitutes a prac-
ticable approach to a new class of useful chiral molecular
switches. Compound (S)-2 is the first example of a chiropti-
cal molecular switch where the switching unit and the chiral
moiety are separate entities that is capable to completely re-
verse liquid crystalline chirality.
Experimental Section
General procedures: 1H NMRspectra were recorded in CDCl on a
3
Bruker Aspect 300 (300 MHz), Varian VXR-300 (300 MHz) or Varian
Gemini-200 (200 MHz). 13C NMRspectra were recorded in CDCl on a
3
Bruker Aspect 300 (75 MHz), Varian VXR-300 (75 MHz) or Varian
Gemini-200 (50 MHz). Chemical shifts are denoted in d unit (ppm) rela-
tive to CDCl3. The splitting patterns are designated as follows: s (singlet),
d (doublet), dd (double doublet), t (triplet), q (quartet), and m (multip-
let) for 1H NMR. For 13C NMRthe carbon atoms are assigned as q (pri-
mary carbon), t (secondary carbon), d (tertiary carbon) and s (quaternary
carbon). Chemical ionization MS spectra were obtained with a AEI MS-
902 spectrometer. The EI-MS spectra were obtained with a Jeol JMS-600
spectrometer. HPLC analyses were performed on a Waters HPLC system
equipped of a 600E solvent delivery system and a 996 photodiode array
detector. CD and UV/Vis analyses were performed on a JASCO J-715
spectropolarimeter and a Hewlett Packard HP 8453 FT Spectrophotome-
ter. Optical rotations were measured with a JASCO DIP-370 digital po-
larimeter and a Perkin Elmer 241 polarimeter. THF was freshly distilled
prior its use on sodium/benzophenone and stored under nitrogen atmos-
phere. CH2Cl2 was distilled from P2O5 and stored over activated 4 ä mo-
lecular sieves. Pyridine was distilled over CaH2 and stored under nitrogen
on KOH. n-Butyllithium (Aldrich) was a 1.6m solution in n-hexane. Ad-
dition of organometallics were performed using syringe/septum cap tech-
niques under nitrogen atmosphere. Commercial reagents were used with-
out purification. Column chromatography was carried out with silica gel
Merck 60 (80 230 mesh).
However, it is interesting to note, as we did above, that
the variation of the HTP value increases with the extent of
trans cis isomerization of the azobenzene moieties. This sup-
ports the major role of the azobenzene isomerization on the
overall geometry of the molecule. Next to a possible cisoid
transoid passage of the core itself due to the different steric
hindrance around the binaphthyl moiety (see above), photo-
induced cis trans isomerization of the azobenzene moieties
in (S)-2 can induce a change the overall geometry and helic-
ity of (S)-2. The observed variations of the HTP can be ex-
plained as follows; either one or two photoinduced trans to
cis isomerizations of the azobenzene moieties in (S)-2 give
rise to a compound with an overall helicity opposite to the
helicity of the binaphthyl core resulting in a reversal of the
handedness of the cholesteric phase. An alternative explana-
tion (following again Ferrarini, Spada et al.)[27] could be that
Photochemical isomerizations: Photoirradiation in solution were per-
formed by irradiation of a sample in a 1 cm quartz cuvette with an 200 W
67
Chem. Eur. J. 2004, 10, 61 70
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

C. Rosini, B. L. Feringa et al.
FULL PAPER
Oriel 60100 Xe-lamp attached to an Oriel Cornerstone 260 monochroma-
tor. Photostationary states are ensured by monitoring composition
changes in time by taking UV spectra at distinct intervals until no
changes were observed. Ratios of the different isomers of the molecular
switches were determined by HPLC by monitoring at the isosbestic
point. Photoisomerizations in liquid crystalline films were performed
with the same light source monitoring the phase transitions through a
Olympus BX 60 polarization microscope. Ratios of the different isomers
were again determined by HPLC.
was added and the temperature was slowly increased until ꢀ108C. A sol-
ution of 30% aq. H2O2 (2.875 mL, 25 mmol) and 0.5m aq. NaOH (3 mL)
was added and the overall reaction mixture was stirred while reaching
RT. The reaction mixture was diluted with Et2O (200 mL), washed with
brine and the organic layer was dried over anhydrous Na2SO4 and con-
centrated in vacuo. The residue was purified by column chromatography
(silica gel, Et2O/n-hexane 8:2) affording pure (S)-6,6’-dihydroxy-2,2’-
methylenedioxy-1,1’-binaphthalene (6) as a white solid (710 mg, 86%).
1H NMR(300 MHz, CDCl 3): d=7.74 (d, J=8.8 Hz, 2H), 7.37 (d, J=
8.8 Hz, 2H), 7.36 (d, J=9.2 Hz, 2H), 7.18 (d, J=2.5 Hz, 2H), 6.86 (dd,
J=9.2, 2.5 Hz, 2H), 5.60 (s, 2H), 4.9 (s, 2H); 13C NMR(300 MHz,
CDCl3): d = 100.5 (t), 107.5 (d), 115.4 (d), 119.0 (d), 123.7 (s), 124.8 (s),
126.1 (d), 126.2 (d), 130.5 (s), 146.9 (s), 150.2 (s); CI-MS: m/z: calcd for
C12H14O4: 330.09; found: 348 [M+NH4+].
Preparation and analysis of liquid crystalline samples: The pitches of the
liquid crystalline phases were determined by the Grandjean Cano techni-
que using an Olympus BX 60 polarization microscope.[28] Grandjean Ca-
no textures were obtained by alignment of a cholesteric liquid crystalline
material on a polyimide-covered glass surface. For this purpose a glass
surface (typically 6.25 cm2) was carefully cleaned with aqueous detergent
and with organic solvent (2-propanol). This clean and dry surface was
spin-coated (at approximately 3000 rpm for 1 min) with commercially
available polyimide AL1051 (purchased from JSR, Belgium). These
coated glass surfaces were allowed to harden at 1708C in vacuum over-
night. The surface was then linearly rubbed with a velvet cloth and the
LC material doped with the appropriate amount of dopant dissolved in
toluene (ꢁ5 mgmLꢀ1) was slowly poured onto this alignment layer. After
evaporation of the solvent at room temperature a suitable aligned LC
film was obtained. The Grandjean Cano texture was obtained by apply-
ing a plan-convex converging lens of known radius (R=25.119, 30.287,
40.388 or 50.481 mm; Linos Components; Radiometer) onto this liquid
crystalline covered surface. The sign of the cholesteric phases was deter-
mined using a contact method where mixing of the samples with a doped
cholesteric liquid crystal of known negative screw sense, consisting of
dopant ZLI-811, obtained from Merck in the appropriate liquid crystal-
line host, was tested.
(S)-6,6’-Dihydroxy-2,2’-diethoxy-1,1’-binaphthalene (7):
A solution of
nBuLi 1.6m in n-hexane (3.9 mL, 6.25 mmol) was added under nitrogen
at ꢀ708C to a solution of (S)-6,6’-dibromo-2,2’-diethoxy-1,1’-binaphtha-
lene (5; 1.250 g, 2.5 mmol) in dry THF (35 mL). The solution was stirred
at ꢀ708C for 15 min, BF(OCH3)2 (0.750 mL, 6.25 mmol) was added and
the temperature was slowly increased to ꢀ108C. A solution of 30% aq.
H2O2 (2.875 mL, 25 mmol) and 0.5m aq. NaOH (3 mL) was added and
the overall mixture was stirred while reaching RT. The reaction mixture
was diluted with Et2O (200 mL), washed with brine and the organic layer
was dried over anhydrous Na2SO4 and concentrated in vacuo. The resi-
due was purified by column chromatography (silica gel, Et2O/n-hexane,
8:2) affording pure (S)-6,6’-dihydroxy-2,2’-diethoxy-1,1’-binaphthalene
(7) as a white solid (670 mg, 72%). 1H NMR(300 MHz, CDCl 3): d =
7.74 (d, J=8.9 Hz, 2H), 7.38 (d, J=8.9 Hz, 2H), 7.12 (d, J=2.5 Hz, 2H),
7.03 (d, J=9.1 Hz, 2H), 6.81 (dd, J=9.1, 2.5 Hz, 2H), 4.91 (s, 2H), 4.0
(q, J=7.0 Hz, 4H), 1.05 (t, J=7.0 Hz, 6H); 13C NMR(300 MHz, CDCl ):
3
d = 12.6 (q), 63.2 (t), 106.9 (d), 114.6 (d), 115.6 (d), 118.7 (s), 124.9 (2d),
126.9 (s), 127.8 (s), 149.2 (s), 150.1 (s); CI-MS: m/z: calcd for C24H22O4:
374.15; found: 392 [M+NH4+].
Synthesis
(S)-6,6’-Dibromo-2,2’-methylenedioxy-1,1’-binaphthalene (4): K2CO3
(3.5 g, 25 mmol) was added to a solution of (S)-6,6’-dibromo-2,2’-dihy-
droxy-1,1’-binaphthalene (2.66 g, 6.0 mmol) in dry DMF (30 mL) under
nitrogen. The reaction mixture was stirred at 808C for 30 min, CH2I2
(0.72 mL, 9 mmol) was added and stirring continued overnight at 808C.
After cooling to RT, the reaction mixture was diluted with Et2O
(300 mL), washed with H2O and brine, the organic layer was dried over
anhydrous Na2SO4 and concentrated in vacuo. The product was purified
by column chromatography (silica gel, pet. ether/Et2O 85:15) and a suc-
cessive crystallization from Et2O/n-hexane affording pure (S)-6,6’-dibro-
(S)-6,6’-Dihydroxy-5,5’-bisbenzeneazo-2,2’-methylenedioxy-1,1’-binaph-
thalene (8): Dry pyridine (0.4 mL, 5 mmol) and K2CO3 (690 mg, 5 mmol)
was added at ꢀ108C to a solution of (S)-6,6’-dihydroxy-2,2’-methylene-
dioxy-1,1’-binaphthalene (6; 363 mg, 1.1 mmol) in dry THF (20 mL)
under a nitrogen atmosphere. The solution was stirred at ꢀ108C for
5 min, dry benzenediazonium tetrafluoborate (634 mg, 3.3 mmol) was
added and the reaction mixture was stirred for 40 h at ꢀ108C. The red
solution was diluted with Et2O (200 mL), washed with NH4Cl, H2O and
brine. The organic layer was dried over anhydrous Na2SO4 and concen-
trated in vacuo. The residue was purified by two successive column chro-
matography (silica gel; toluene/Et2O 96:4, then silica gel, CH2Cl2) afford-
ing pure (S)-6,6’-dihydroxy-5,5’-bisbenzeneazo-2,2’-methylenedioxy-1,1’-
binaphthalene (8) as a red solid (225 mg, 38%). [a]2D5 = ꢀ150 (c=1.0 in
chloroform). 1H NMR(200 MHz, CDCl 3): d=16.23 (s, 2H), 8.79 (d, J=
9.5 Hz, 2H), 7.81 (d, J=7.3 Hz, 4H), 7.45 7.6 (m, 8H), 7.36 (t, J=
7.3 Hz, 2H), 6.82 (dd, J=9.8, 2.2 Hz, 2H), 5.68 (s, 2H); 13C NMR
(300 MHz, CDCl3): d =100.5 (t), 116.5 (d), 116.6 (d), 120.1 (d), 121.4 (d),
122.6 (d), 124.4 (s), 124.8 (s), 125.5 (d), 127.1 (d), 127.5 (s), 129.3 (s),
134.4 (d), 142.8 (s), 148.7 (s), 166.5 (s); CI-MS: m/z: calcd for
C33H22N4O4: 538.16; found: 539 [M+H+].
mo-2,2’-methylenedioxy-1,1’-binaphthalene as white crystals (2.13 g,
1
78%). H NMR(250 MHz, CDCl ): d = 8.10 (d, 2H), 7.88 (d, J=9.0 Hz,
3
2H), 7.50 (d, 2H), 7.38 (dd, J=9.0, 1.9 Hz, 2H), 7.30 (d, 2H), 5.69 (s,
2H); EI-MS: m/z: calcd for C21H12Br2O2: 456.92; found: 456 [M +].
[16]
(S)-6,6’-Dibromo-2,2’-diethoxy-1,1’’-binaphthalene (5):
K2CO3 (3.0 g,
22 mmol) was added under nitrogen to a solution of (S)-6,6’-dibromo-
2,2’-dihydroxy-1,1’-binaphthalene (2.22 g, 5.0 mmol) in dry DMF
(18 mL). The reaction mixture was stirred at 808C for 30 min, bromo-
ethane (1.1 mL, 15 mmol) was added and stirring continued overnight at
808C. After cooling to RT, the reaction mixture was diluted with Et2O
(300 mL), washed with H2O and brine, the organic layer was dried over
anhydrous Na2SO4 and concentrated in vacuo. The product was purified
by column chromatography (silica gel, CH2Cl2/n-hexane 4:6) and a suc-
cessive crystallization from Et2O/n-hexane affording pure (S)-6,6’-dibro-
mo-2,2’-diethoxy-1,1’-binaphthalene as white crystals (2.20 g, 88%).
1H NMR(400 MHz, CDCl 3): d=8.01 (d, 2H), 7.85 (d, J=9.0 Hz, 2H),
7.42 (d, 2H), 7.27 (dd, J=9.0, 2.0 Hz, 2H), 6.96 (d, 2H), 4.1 4.2 (m, 4H),
1.06 (t, 6H); EI-MS: m/z: calcd for C24H20Br2O2: 497.98; found: 500 [M +
].
(S)-6,6’-Dihydroxy-5,5’-bisbenzeneazo-2,2’-diethoxy-1,1’-binaphthalene
(9): Dry pyridine (about 0.2 mL, 3 mmol) and K2CO3 (414 mg, 3 mmol)
was added at ꢀ108C to a solution of (S)-6,6’-dihydroxy-2,2’-diethoxy-1,1’-
binaphthalene (7; 224 mg, 0.6 mmol) in dry THF (10 mL) under a nitro-
gen atmosphere. The solution was stirred at ꢀ108C for 5 min, dry benze-
nediazonium tetrafluoborate (345 mg, 1.8 mmol) was added and the reac-
tion mixture was stirred for 40 h at ꢀ108C. The red solution was diluted
with Et2O (150 mL), washed with NH4Cl, H2O and brine. The organic
layer was dried over anhydrous Na2SO4 and concentrated in vacuo. The
residue was purified by two successive column chromatography separa-
tions (silica gel, toluene/CH2Cl2/Et2O 70:27:3 then silica gel, n-hexane/
Et2O 6:4) affording pure (S)-6,6’-dihydroxy-5,5’-bisbenzeneazo-2,2’-dieth-
Preparation of BF(OCH3)2: B(OCH3)3 (6 mL, 74 mmol) and BF3¥Et2O
(2.86 mL, 23.3 mmol) were mixed and stirred at RT for 30 min. The pure
product was obtained by distillation collecting fractions between 50
528C. The product was immediately used for the following reactions.
oxy-1,1’-binaphthalene (9) as a dark red solid (149 mg, 43%). [a]D25
=
ꢀ28 (c=1.0 in chloroform). 1H NMR(300 MHz, CDCl 3): d=16.15 (s,
2H), 8.64 (d, J=9.0 Hz, 2H), 7.48 (t, J=7.3 Hz, 4H), 7.35 (d, J=9.0 Hz,
4H), 7.28 (t, J=7.3 Hz, 2H), 7.20 (d, J=9.8 Hz, 2H), 6.66 (d, J=9.8 Hz,
2H), 4.06 (q, J=6.9 Hz, 4H), 1.14 (t, J=6.9 Hz, 6H); 13C NMR
(200 MHz, CDCl3): d=13.3 (q), 63.3 (t), 114.1 (d), 116.6 (d), 120.8 (s),
(S)-6,6’-Dihydroxy-2,2’-methylenedioxy-1,1’-binaphthalene (6): A solution
of nBuLi 1.6m in n-hexane (3.9 mL, 6.25 mmol) under nitrogen was
added at ꢀ708C to a solution of (S)-6,6’-dibromo-2,2’-methylenedioxy-
1,1’-binaphthalene (4; 1.140 g, 2.5 mmol) in dry THF (35 mL). The solu-
tion was stirred at ꢀ708C for 15 min, BF(OCH3)2 (0.750 mL, 6.25 mmol)
68
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 61 70

Chiroptical Molecular Switches
61 70
121.5 (d), 124.1 (d), 125.3 (d), 126.2 (s), 127.1 (s), 128.1 (d), 128.9 (s),
136.8 (d), 143.1 (s), 153.4 (s), 171.2 (s); CI-MS: m/z: calcd for
C36H30N4O4: 582.23; found: 583 [M+H+].
[4] See for example: a) O. Pieroni, A. Fissi, G. Popova, Prog. Polym.
Sci. 1998, 23, 81 123; b) F. Ciardelli, O. Pieroni, A. Fissi, C. Carlini,
A. Altomare, Br. Polym. J. 1989, 21, 97 106; c) M. Irie, Adv. Polym.
Sci. 1990, 94, 27 67; d) Applied Photochromic Polymer Systems
(Ed.: C. B. McArdle), Blackie, Glasgow (UK), 1992; e) F. Ciardelli,
O. Pieroni, A. Fissi, J. L. Houben, Biopolymers 1984, 23, 1423 1437;
f) O. Pieroni, F. Ciardelli, Trends Polym. Sci. 1995, 3, 282 287; g) T.
Kinoshita, Prog. Polym. Sci. 1995, 20, 527 583; h) I. Willner, Acc.
Chem. Res. 1997, 30, 347 356.
[5] F. Ciardelli, O. Pieroni in Chiroptical Molecular Switches (Ed.: B. L.
Feringa), Wiley-VCH, Weinheim, 2001, Chapter 13, pp. 399 441.
[6] M. Irie, S. Kobatake, M. Horichi, Science 2001, 291, 1769 1772.
[7] T. Ikeda, A. Kanazawa in Chiroptical Molecular Switches (Ed.: B. L.
Feringa), Wiley-VCH, Weinheim, 2001, Chapter 12, pp. 363 397.
[8] a) B. L. Feringa, R. A. van Delden, N. Koumura, E. M. Geertsema,
Chem. Rev. 2000, 100, 1789 1816; b) B. L. Feringa, R. A. van Del-
den, M.K. J. ter Wiel in Molecular Switches (Ed.: B. L. Feringa),
Wiley-VCH, Weinheim, 2001, Chapter 5, pp. 123 163.
(S)-6,6’-Dimethoxy-5,5’-bisbenzeneazo-2,2’-methylenedioxy-1,1’-binaph-
thalene (1): K2CO3 (138 mg, 1.0 mmol) was added under nitrogen at RT
to
a solution of (S)-6,6’-dihydroxy-5,5’-bisbenzeneazo-2,2’-methylene-
dioxy-1,1’-binaphthalene (8; 108 mg, 0.20 mmol) in dry DMF (2 mL). The
solution was stirred at 608C for 30 min, MeI (38 mL, 0.60 mmol) was
added and the reaction mixture was stirred overnight at 608C. After cool-
ing to RT, the reaction mixture was diluted with Et2O (50 mL), washed
with H2O and brine, the organic layer was dried over anhydrous Na2SO4
and concentrated in vacuo. All the successive operations were performed
keeping the product away from light. The residue was dissolved in tolu-
ene, stirred at 808C for 1 h and concentrated in vacuo. The product was
purified by two successive column chromatography separations (silica
gel, CH2Cl2/pet. ether 85:15 then silica gel, n-hexane/Et2O 1:1) affording
pure (S)-6,6’-dimethoxy-5,5’-bisbenzeneazo-2,2’-methylenedioxy-1,1’-bi-
naphthalene (1) as a dark orange solid (73 mg, 65%). [a]2D5 =ꢀ78 (c=
0.05 in chloroform); 1H NMR(300 MHz, CDCl 3): d=8.46 (d, J=9.0 Hz,
2H), 8.09 (m, 4H), 7.55 7.65 (m, 6H), 7.52 (d, J=9.0 Hz, 2H), 7.24 (d,
J=9.0 Hz, 2H), 5.71 (s, 2H), 3.98 (s, 6H); 13C NMR(200 MHz, CDCl 3):
d = 55.9 (q), 101.6 (t), 113.6 (d), 121.0 (d), 121.3 (d), 124.0 (d), 124.3 (s),
125.7 (s), 126.4 (s), 127.7 (d), 127.9 (d), 129.7 (d), 135.4 (s), 145.6 (s),
148.8 (s), 151.9 (s); CI-MS: m/z: calcd for C35H26N4O4: 566.20; found:
567 [M+H+].
[9] B. L. Feringa, N. P. M. Huck, H. A. van Doren, J. Am. Chem. Soc.
1995, 117, 9929 9930.
[10] See for example: a) T. Yamaguchi, K. Uchida, M. Irie, J. Am. Chem.
Soc. 1997, 119, 6066 6071; b) T. Yamaguchi, Y. Tanaka, H. Nakazu-
mi, K. Uchida, T. Yamada, M. Irie, Enantiomer 2001, 6, 309 311;
c) Y. Yokoyama, S. Uchida, Y. Shimizu, Y. Yokoyama, Mol. Cryst.
Liq. Cryst. 1997, 297, 85 91; d) Y. Yokoyama, S. Uchida, Y. Yo-
koyama, Y. Sugawara, Y. Kurita, J. Am. Chem. Soc. 1996, 118,
3100 3107; e) L. Eggers, V. Bu˚, Angew. Chem. 1997, 109, 885 887;
Angew. Chem. Int. Ed. Engl. 1997, 36, 881 883; f) A. Miyashita, A.
Iwamoto, T. Kuwayama, H. Shitara, Y. Aoki, M. Hirano, H. Nohira,
Chem. Lett. 1997, 965 966; g) T. Ikeda, T. Sasaki, K. Ichimura,
Nature 1993, 361, 428; h) T. Kusumoto, K. Sato, K. Ogino, T.
Hiyama, S. Takehara, M. Osawa, K. Nakamura, Liq. Cryst. 1993, 14,
727 732; i) M. Negishi, O. Tutsumi, T. Ikeda, T. Hiyama, J. Kawa-
mura, M. Aizawa, S. Takehara, Chem. Lett. 1996, 319 320; j) M.
Negishi, K. Kanie, T. Ikeda, T. Hiyama, Chem. Lett. 1996, 583 584;
k) T. Ikeda, O. Tsutsum, Science 1995, 268, 1873 1875.
[11] See for example: a) S. Z. Janicki, G. B. Schuster, J. Am. Chem. Soc.
1995, 117, 8524 8527; b) Y. Yokoyama, T. Sagisaka, Chem. Lett.
1997, 687 688; c) C. Denekamp, B. L. Feringa, Adv. Mater. 1998, 10,
1080 1082; d) C. Ruslim, K. Ichimura, Adv. Mater. 2001, 13, 37 40;
e) G. Heppke, H. Marshall, P. Nurnberg, F. Oestreicher, G. Sherow-
sky, Chem. Ber. 1981, 114, 2501 2518.
[12] S. Pieraccini, S. Masiero, G. P. Spada, G. Gottarelli, Chem. Commun.
2003, 598 599.
[13] a) C. Rosini, L. Franzini, A. Raffaelli, P. Salvadori, Synthesis 1992,
503 517; b) L. Pu, H.-B. Yu, Chem. Rev. 2001, 101, 757 824;
c) B. L. Feringa, Acc. Chem. Res. 2000, 33, 346 353.
(S)-6,6’-Dimethoxy-5,5’-bisbenzeneazo-2,2’-diethoxy-1,1’-binaphthalene
(2): K2CO3 (104 mg, 0.75 mmol) was added at RT under nitrogen to a sol-
ution of (S)-6,6’-dihydroxy-5,5’-bisbenzeneazo-2,2’-diethoxy-1,1’-binaph-
thalene (9; 87 mg, 0.15 mmol) in dry DMF (1.5 mL). The solution was
stirred at 608C for 30 min, MeI (28 mL, 0.45 mmol) was added and the re-
action mixture was stirred overnight at 608C. After cooling to RT, the re-
action mixture was diluted with Et2O (40 mL), washed with H2O and
brine, the organic layer was dried over anhydrous Na2SO4 and concen-
trated in vacuo. All the successive operations were performed keeping
the product away from light. The residue was dissolved in toluene, stirred
at 808C for 1 h and concentrated in vacuo. The product was purified by
two successive column chromatography separations (silica gel, CH2Cl2/
pet. ether 85:15 then silica gel, n-hexane/Et2O 1:1) affording pure (S)-
6,6’-dimethoxy-5,5’-bisbenzeneazo-2,2’-diethoxy-1,1’-binaphthalene (2) as
a dark orange solid (60 mg, 66%). [a]2D5 =ꢀ445 (c=0.05 in chloroform);
1H NMR(300 MHz, CDCl 3): d=8.47 (d, J=9.4 Hz, 2H), 8.05 (m, 4H),
7.5 7.65 (m, 6H), 7.45 (d, J=9.4 Hz, 2H), 7.15 7.25 (m, 4H), 4.02 (q, J=
6.9 Hz, 4H), 1.08 (t, J=6.9 Hz, 6H); 13C NMR(200 MHz, CDCl 3): d=
13.6 (q), 56.0 (q), 63.8 (t), 113.9 (d), 116.1 (d), 119.0 (s), 121.2 (d), 122.6
(s), 123.0 (d), 127.2 (d), 127.7 (d), 128.6 (s), 129.4 (d), 135.1 (s), 145.4 (s),
151.9 (s), 152.1 (s); CI-MS: m/z: calcd for C38H34N4O4: 610.26; found:
611 [M+H+].
[14] a) G. Gottarelli, M. Hibert, B. Samori, G. Solladiÿ, G. P. Spada, R.
Zimmermann, J. Am. Chem. Soc. 1983, 105, 7318 7321; b) G. Got-
tarelli, G. P. Spada, R. Bartsch, G. Solladiÿ, R. Zimmermann, J. Org.
Chem. 1986, 51, 589 592; c) G. Solladiÿ, G. Gottarelli, Tetrahedron
1987, 43, 1425 1437; d) P. V. Shibaev, K. Schaumberg, T. Bjornholm,
Mol. Cryst. Liq. Cryst. 2000, 338, 181 195; e) G. Gottarelli, G. P.
Spada, Mol. Cryst. Liq. Cryst. 1985, 123, 377; f) P. V. Shibaev, R. Vi-
nokur, H.-J. Deussen, T. Bj˘rnholm, K. Schaumberg, SPIE 1997,
3319, 185 .
Acknowledgments
C.R. and T.M. thank the Universit‡ della Basilicata, and MIUR-COFIN
2001 (New theoretical and experimental methods for the assignment of
the molecular absolute configuration in solution™) for financial support.
Financial support from the National Research School Catalysis (NRSC-
C) to B.L.F. and R.A.v.D. and The Netherlands Foundation for Scientific
Research (NWO-CW) are gratefully acknowledged.
[15] a) A. Ferrarini, G. J. Moro, P. L. Nordio, Liq. Cryst. 1995, 189, 397
399; b) A. Ferrarini, G. J. Moro, P. L. Nordio, Phys. Rev. E 1996, 53,
681 688.
[16] G. D. Y. Sogah, D. J. Cram, J. Am. Chem. Soc. 1979, 101, 3035 3042.
[17] H. J. Deussen, E. Hendrickx, C. Boutton, D. Krog, K. Clays, K.
Bechgaard, A. Persoons, T. Bjornholm, J. Am. Chem. Soc. 1996, 118,
6841 6852.
[18] a) S. F. Mason, R. H. Seal, D. R. Roberts, Tetrahedron 1974, 30,
1671 1682; b) C. Rosini, I. Rosati, G. P. Spada, Chirality 1995, 7,
353 358; c) C. Rosini, S. Superchi, H. W. I. Peerlings, E. W. Meijer,
Eur. J. Org. Chem. 2000, 61 71.
[1] a) Sci. Amer. Special Issue: Nanotech: The Science of Small Gets
Down to Business, September 2001; b) R. P. Feynman in Miniaturi-
zation (Ed.: H. D. Gilbert), New York, 1971; c) K. E. Drexler, Nano-
systems: Molecular Machinery, Manufacturing and Computation,
Wiley, New York, 1992; d) R. D. Astumian, Sci. Amer. 2001, 285,
56 64.
[19] H. Suzuki, Electronic Absorption Spectra and Geometry of Organic
Molecules, Academic Press, New York, 1967, p. 503.
[2] Molecular Switches (Ed.: B. L. Feringa), Wiley-VCH, Weinheim,
2001.
[20] J. J. G. S. van Es, H. A. M. Biemans, E. W. Meijer, Tetrahedron:
Asymmetry 1997, 8, 1825 1831.
[3] Special Issue: Photochromism: Memories and Switches, Chem. Rev.
2000 (Guest Ed.: M. Irie), 100, 1683 1890.
69
Chem. Eur. J. 2004, 10, 61 70
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

C. Rosini, B. L. Feringa et al.
FULL PAPER
[21] H. Rau in Photochromism, molecules and systems (Eds.: H. D¸rr, H.
Bouas-Laurent), Elsevier, Amsterdam, 1990, Chapter 4, pp. 165 192.
[22] a) Z. Sekkat, W. Knoll J. Opt. Soc Am. B 1995, 12, 1855 1867; b) O.
Watanabe, M. Tsuchimori, A. Okada, J. Mater. Chem. 1996, 6,
1487 1492; c) C. Egami, Y. Suzuki, O. Sugihara, N. Okamoto, H.
Fujimura, K. Nakagawa, H. Fujiwara, Appl. Phys. B 1997, 64, 471
478; d) C. Wang, H. Fei. , Y. Yang, Z. Wei, Y. Qui, Y. Chen, Opt.
Commun. 1999, 159, 58 62.
[23] a) G. Heppke, F. Oesterreicher, Z. Naturforsch. A 1977, 32, 899
901; b) G. Gottarelli, B. Samori, C. Stremmenos, G. Torre, Tetrahe-
dron 1981, 37, 395 399.
[24] G. Solladiÿ, R. G. Zimmermann, Angew. Chem. 1984, 96, 335 349;
Angew. Chem. Int. Ed. Engl. 1984, 23, 348 362.
[25] Helical twisting power: b=(pcr)-1, where p is the pitch (mm), c is
the concentration (g of dopant per gram of LC solution) and r is the
enantiomeric purity of the dopant.
[26] Light of 402 nm proved to be a slightly more selective wavelength
for switching in an LC phase, in solution the selectivity observed
upon irradiation with this wavelength is slightly (about 2%) less
than the ideal 405 nm.
[27] A. di Matteo, S. M. Todd, G. Gottarelli, G. Solladiÿ, V. E. Williams,
R. P. Lemieux, A. Ferrarini and G. P. Spada, J. Am. Chem. Soc.
2001, 123, 7842 7851.
[28] a) F. Grandjean, C. R. Acad. Sci. 1921, 172, 71; b) R. Cano, Bull.
Soc. Fr. Mineral 1968, 91, 20.
Received: June 26, 2003 [F5276]
70
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 61 70