Modulation of the absorption, fluorescence, and liquid-crystal properties of functionalised diarylethene derivatives

Article abstract of DOI:10.1002/chem.200305682

Systematic variation of the molecular symmetry in a photochromic system based on a 1,2-bis(2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopentene group, connected by decyl spacers to two cyanobiphenyl groups as mesogens, allows for a systematic investigatio

Full text of DOI:10.1002/chem.200305682

FULL PAPER
Modulation of the Absorption, Fluorescence, and Liquid-Crystal Properties
of Functionalised Diarylethene Derivatives
[a]
Michel Frigoli and Georg H.Mehl*
Abstract: Systematic variation of the molecular symmetry in a photochromic
system based on a 1,2-bis(2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopentene
group, connected by decyl spacers to two cyanobiphenyl groups as mesogens,
allows for a systematic investigation of the correlations between molecular shape
and symmetry, electronic effects, photochromic conversion and liquid-crystalline
properties.
Keywords: fluorescence
crystals molecular switches
photochromism
·
liquid
·
·
Introduction
The concept of switching molecular or
supramolecular systems reversibly from
one state to another by using light, in
which the states have significantly different
properties, is widespread in biological sys-
tems and has been in the focus of materials
research for many years.[1,2] Such synthetic
systems are promising as high-density data-
storage systems, sensors, photonic switches
and molecular logic gates. The concept of
optically addressable molecules promises
miniaturisation to the molecular scale and
potentially of optical computing on that
level. In the area of liquid crystal (LC) re-
search, concerned with information trans-
mission, the power of light modulation was
recognised early on, and systems based on
Figure 1. Photochromic liquid crystal systems.
azo groups, spyropyranes, spirooxazines or,
more recently, fulgides have been investi-
gated for data storage or for the alignment
of LCs on surfaces (command surfaces).[2g,3–5] However, re-
laxation phenomena, the colouring of both states and ther-
mal stability have been the main concerns in such systems.
Thus, progress in research on functionalised diarylethenes,
some of which are thermally very stable and which can be
mixed into LC systems or can be functionalised to be liquid-
crystalline, is very interesting. For such materials the corre-
lation between molecular structure, electronic properties
and mesomorphic behaviour remains to be investigated.
Our approach for this investigation is modular, making
use of a photochromic core, a 1,2-bis(2-methylbenzo[b]thio-
phen-3-yl)hexafluorocyclopentene system which is connect-
ed to two cyanobiphenyl groups as mesogens via spacers of
ten methylene groups, as shown in Figure 1. The position of
the ether linking group to the mesogens was varied from
6,6’-substitution in 1, 5,6’-functionalisation in 2 to a 5,5’-sub-
stitution of the methylbenzothiopene cores in 3. This allows
for a systematic investigation of the influence of molecular
shape and electronic properties on the absorption behaviour
[a] Dr. M. Frigoli, Dr. G. H. Mehl
Department of Chemistry
University of Hull
Hull HU67RX (UK)
Fax (+44)1482-466411
E-mail: g.h.mehl@hull.ac.uk
Chem. Eur. J. 2004, 10, 5243 – 5250
DOI: 10.1002/chem.200305682
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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FULL PAPER
and the photochromic properties, as well as the correlation
of these properties with the mesomorphic behaviour of the
systems.
thiopen-3-yl)hexafluorocyclopentene allows only the 6-posi-
tion of the photochromic core to be targeted.[6] Thus, in
order to obtain 4–6, the starting material must be substitut-
ed by a methoxyl group.
6-Methoxybenzothiophene (10) and 5-methoxybenzothio-
phene (14) were prepared according to procedures de-
scribed in the literature.[7] The symmetrical photochromes 4
and 6 were obtained in three steps from the corresponding
methoxybenzothiophenes 10 and 14, which were methylated
in the 2-position by using nBuLi at ꢀ408C followed by addi-
tion of methyliodide at ꢀ788C to afford 11 and 15 in yields
of 92 and 96%. Bromination at the 3-position of 11 and 15
with bromine in CHCl3 at room temperature gave 12 and 16
in 82 and 47% yield, respectively.
The formation of a not yet identified byproduct in the
bromination of 15 is responsible for the moderate yield.
Treatment of 12 and 16 in THF with nBuLi at ꢀ788C fol-
lowed by the addition at the same temperature of a semi-
equivalent amount of octafluorocyclopentene yielded photo-
chromes 4 and 6 (62 and 68%, respectively). In this reac-
tion, the product of monocondensation, which is useful for
the investigation of asymmetric photochromes, can be ob-
tained selectively by using an excess of octafluorocyclopen-
tene.
Results and Discussion
Synthesis: The preparation of photochromic liquid crystal
systems 1–3 requires the synthesis of photochromic com-
pounds which are suitably functionalised. The synthetic
pathway to these new compounds is outlined in Schemes 1
and 2.
The arylhexafluorocyclopentenes are usually obtained by
a reaction involving nucleophilic attack of an aryllithium on
octafluorocyclopentene, followed by elimination of fluoro
groups.[2b] Direct substitution of 1,2-bis(2-methylbenzo[b]-
Compound 13 is obtained in 82% yield from 12 by using
nBuLi at ꢀ788C followed by addition of three equivalents
of octafluorocyclopentene. Condensation of the lithiated de-
rivative of 16 with 13 at ꢀ788C afforded 5 in 56% yield.
The synthesis of the photochromic liquid crystal systems
is achieved in two steps from 4–6 (Scheme 2). Deprotection
of the methoxyl group with boron tribromide in CH2Cl2 at
room temperature resulted in 7–9 in yields higher than
80%. An etherification reaction between 7–9 and mesogenic
groups in butanone at reflux with K2CO3 afforded the tar-
geted molecules 1–3 in good yield.
Photochromic properties: All synthesised diarylethenes un-
derwent reversible photochromic reactions in cyclohexane
with alternating light at wavelengths of 313 (UV) and
546 nm (visible) with retention of isosbestic points. The
colour of the solutions changed from colourless for all com-
pounds to red (1a, 4a), red-purple (2a, 5a) or purple (3a,
6a). The photochromic behaviour of the new compounds
was compared with that of the unsubstituted parent com-
pound 1,2-bis(2-methyl-1-ben-
Scheme 1. Reagents and conditions: a) nBuLi, MeI, THF, 92%; b) Br2,
CHCl3, 45%; c) nBuLi, 0.5 equiv octafluorocyclopentene, THF, 62%;
d) nBuLi, MeI, THF, 97%; e) Br2, CHCl3, 82%; f) nBuLi, 0.5 equiv octa-
fluorocyclopentene, THF, 68%; g) nBuLi, 3 equiv octafluorocyclopen-
tene, THF, 68%; h) lithiated derivative of 16, THF, 65%.
zothiophen-3-yl)hexafluorocy-
clopentene, denoted Ref.[8]
The effect of the position of
methoxyl groups on the benzo-
thiophene part of the photoac-
tive core on the absorption
properties was examined. Fig-
ures 2 and 3 show absorption
spectra of the open-ring iso-
mers 4, 5, and 6 and the corre-
sponding closed-ring isomers
4a, 5a, and 6a. The absorption
spectra of the closed forms
Scheme 2. Synthesis of the photochromic liquid-crystalline systems 1–3.
were calculated from the ab-
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Photochromic Liquid Crystals
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The absorption maxima and absorption coefficients of the
closed forms depend on the positions of the methoxyl
groups. As can be seen from the spectra, 6a absorbs at the
highest wavelength (569 nm) with the lowest
(11300 Lmolꢀ1 cmꢀ1), while 4a absorbs at the lowest wave-
length (520 nm) with the highest value
e value
e
(20400 Lmolꢀ1 cmꢀ1). The absorption maximum and e value
of 5 (542 nm, 15000 Lmolꢀ1 cmꢀ1) are intermediate between
those of 4 and 6. In this series, the e values decrease with
shifting of the absorption maxima to longer wavelengths.
This suggests that, in line with classical concepts of the rela-
tive positions of maxima in the absorption spectra, the
energy difference between the ground state and the first ex-
cited state for 4a is higher than for the other systems. This
could be due to electronic or steric effects (the dipoles of
the perfluoro and ether groups point in different directions
in 4a). In the absence of reported systematic MO calcula-
tions on substituted benzothiophene-based diarylethene sys-
tems, any explanation must be very tentative. However, Par-
iser–Parr–Pople molecular orbital calculations for linear azo
dyes, in which the substitution pattern was varied systemati-
cally, allowed us to relate the values of the extinction coeffi-
Figure 2. Absorption spectra of the open-ring isomers 4–6 in cyclohexane.
cients to the inverse mean absorption wavelength 1/l.[11]
A
similar effect might occur for the closed forms of the substi-
tuted diarylethenes.
Differences are noticeable between the absorption spectra
of the open- and closed-ring isomers of compounds 1 and 4
(Figure 4). The presence of two cyanobiphenyl groups in 1
Figure 3. Absorption spectra of the closed-ring isomers 4a–6a in cyclo-
hexane.
sorption spectra obtained at the photostationary state (PS)
by using Equation (1) for the absorbance of a mixture of
two compounds at any wavelength.[9]
½AŠ ¼ ½AŠOFð1ꢀCvÞ þ ½AŠCFðCvÞ
ð1Þ
PS
In Equation (1) [A]PS is the absorption obtained at the PS
state, [A]OF the absorption of the open form, [A]CF the ab-
sorption of the closed form, and Cv the conversion from
open- to closed-ring isomer; [A]PS and [A]OF can be deter-
mined by UV spectroscopy, and Cv can be calculated from
HPLC data by integration of the peaks for the open and
closed forms.
Figure 4. Absorption spectra of open-ring isomers 1 and 4 and closed-
ring isomers 1a and 4a.
leads to a significant increase in the absorption coefficient e
of the open-ring isomer compared with parent molecule 4.
At the irradiation wavelength (313 nm), the e value of 4 is
approximately eight times higher than that of 1. Indeed, the
absorption spectrum of 1 corresponds roughly to the sum of
the spectra of the photochromic core and the cyanobiphenyl
groups; the alkyl spacer that connects these two entities is a
nonconjugating junction. Compounds 1a and 4a have the
same absorption properties in the wavelength interval be-
tween 332 and 650 nm at which cyanobiphenyl groups do
not absorb. The electronic effects induced by methoxyl or
decanoxyl groups on the photochromic core are broadly the
same. The absorption coefficient of the closed-ring isomer
Variation of the position of the methoxyl groups leads to
changes in the absorption coefficient e at the absorption
maximum of the open forms in the far and near UV without
modifying much the position of the absorption maximum in
the UV spectrum (Figure 2). At the irradiation wavelength
(313 nm), the e value of compound 6 (5,5’-OMe) is approxi-
mately three times higher than those of 4 (6,6’-OMe) and
Ref.[8] The e value of compound 5 (5,6’-OMe) lies between
those obtained for 4 and 6. Structurally related dithienyle-
thene derivatives with methoxyl groups at the reaction cen-
tres were reported to exhibit a decrease in the absorption
coefficient.[10]
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G. H. Mehl and M. Frigoli
FULL PAPER
Table 3. Quantum yield and conversion of cyclisation with 313 nm light
and quantum yield of cycloreversion with 546 nm light.
4a is approximately seven times higher than that of the
open-ring isomer 4. This explains in part the low conversion
obtained for 4 (see Table 3) at the photostationary state
with irradiation at 313 nm. At low concentration, the per-
Compounds
Cyclisation[a]
Conversion
Cycloreversion[b]
FCF!OF
FOF!CF
1
2
3
4
5
6
0.19
0.12
0.05
0.20
0.14
0.23
0.35
0.56
0.63
0.64
0.22
0.48
0.68
0.43
0.41
0.19
0.16
0.48
0.21
0.17
0.35
Table 1. Absorption maxima and coefficients of the open-ring isomers 1–
6.[a]
lmax [nm] (e [molꢀ1 Lcmꢀ1])
lmax [nm] (e [molꢀ1 Lcmꢀ1])
1
2
3
280 (57000)
292 (63000)
292 (58000)
4
5
6
267 (23800), 305 (4500)
265 (21300), 306 (6600), 316 (6600)
262 (15000), 307 (8400), 316 (9000)
Ref[c]
[a] All photochromic reactions were performed in cyclohexane solution.
[b] For all compounds the conversion of the cycloreversion reaction was
1. [c] Taken from ref. [8].
Ref 258 (21000), 290 (6900), 299 (7400)
[a] All absorption spectra were recorded in cyclohexane.
centage conversion from the open-ring to the closed-ring
isomers at the photostationary state is defined by Equa-
tion (2) in which eOF and eCF are the absorption coefficients
of the open- and closed-ring isomers at the irradiation wave-
lengths.
these materials are compared with the value obtained for
the unsubstituted photochromic compound Ref (0.35). The
quantum yields of photocyclisation for 4–6 were found to be
in the range of 0.14–0.23. For structurally related dithienyl-
ethene derivatives in which the methoxyl groups are at the
reaction centers a reduction in the quantum yield of photo-
cyclisation has been reported, too.[10]
Conversion ¼ eOFFOF!CF=ðeOFFOF!CF þ eCFFCF!OF
Þ
ð2Þ
The quantum yield of the photoreversion with irradiation
at 546 nm depends on the relative positions of the methoxyl
groups in the molecule. When the two methoxyl groups are
in 6,6’-positions (4), the quantum yield of photoreversion in-
creases slightly relative to Ref. By contrast, when the two
methoxyl groups are in the 5,6’- (5) or 5,5’-positions (6), the
quantum yield for photoreversion decreases to 40% for 5
and 52% for 6.
Compounds 1 (0.19) and 2 (0.12) have quantum yields for
cyclisation similar to that of parent compounds 4 (0.20) and
5 (0.14). The quantum yield of 3 (0.05) is 4.6 times lower
than that of parent molecule 6 (0.23). By contrast, no dis-
similarity of the values of the quantum yields of cyclorever-
sion is observed for each group of molecules.
The eOF and eCF can be determined by UV spectroscopy,
and FOF!CF calculated from the slope of absorbance (moni-
tored at the maximum wavelength of the closed-ring
isomer) versus time at the beginning of the photochromic
process when the photoreversion reaction can be neglected,
by comparing the values of rate constant of photocyclisation
reaction of the reference compound and of the molecule
studied.[12] When the ratio of the quantum yields eOFFOF!CF
CFFCF!OF is low, conversion is weak. Similar observations
are valid for the pairs 2 and 5 and 3 and 6.
/
e
Absorption maxima and coefficients of the open- and
closed-ring isomers are summarised in Tables 1 and 2.
The conversions obtained at the photostationary state
were also dependent on the position of the methoxyl groups
in the molecule. Compound 6 (5,5’-OMe) has the highest
conversion (0.68), while 4 (6,6’-OMe) has the lowest (0.22).
An intermediate value (0.48) was obtained for 5 (5,6’-OMe),
which is very close to that obtained for Ref (0.43). For 1, 2
and 3, conversions were in the range of 0.56–0.64. The con-
versions of 1 (0.56) and 2 (0.63) are higher than those of the
parent molecules 4 (0.22) and 5 (0.48). This increase is di-
rectly correlated with the absorption properties of the open
and closed forms at the wavelength of irradiation in accord-
ance with Equation (2). Indeed, at 313 nm, the ratio be-
tween the absorption coefficient of the closed-ring isomer
4a and that of the open-ring isomer 4 is 7:1, and this ratio is
2:1 for 5. In contrast, for 1 and 2, the absorption coefficients
of the two forms are roughly the same, and this leads to an
enhanced conversion compared to the parent compounds.
Indeed, in the systems 1–3, active participation of the cyano-
biphenyl groups (donors) in the photochromism takes place,
as can be deduced from the results of fluorescence studies
given in Table 4; in other words, the photochromic process
is assisted by a fluoresecence resonance energy transfer
(FRET) or Förster-type effect.
Table 2. Absorption maxima and coefficients calculated for the closed-
ring isomers 1a–6a.
lmax [nm] (e [molꢀ1 Lcmꢀ1])
1a
2a
3a
4a
294 (66000), 360 (15600), 520 (20400)
292 (72000), 365 (13200), 542 (15300)
292 (69000), 368 (11000), 569 (10500)
310 (21000), 360 (15000), 520 (21300)
268 (15300), 301 (16200), 366 (12900), 542 (15000)
278 (22500), 366 (11600), 569 (11300)
517 (9100)
5a
6a
Ref[a]
[a] Taken from ref. [8].
Quantum yield and conversion measurements: The quantum
yield of cyclisation and photocycloreversion of 1–6 were
measured in cyclohexane; Ref was used as reference. The
conversions of 1–6 on irradiation with 313 nm light were de-
termined by HPLC. The two forms can be easily separated
(see Experimental Section for conditions). The results are
summarised in Table 3.
A decrease in the quantum yield of photocyclisation for
all the compounds 1–6 was observed. This is not influenced
by the position of the methoxyl groups on the benzothio-
phene part of the photochromic core, when the results for
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Photochromic Liquid Crystals
5243 – 5250
Table 4. Relative fluorescence and emission quantum yield (Fem) for the
Table 5. Transition temperatures [8C] by DSC.
open forms 1–3 and for the systems at the PS state.
Thermal transition[a]
Thermal transition[a]
[b]
Compound
Fluorescence
wavelength [nm]
Fluorescence
intensity[a]
Fem
[b]
1
2
3
Cr 33.3 SmC 74.5 N 78.5 Iso
Cr 45.4 SmC 79.5 N 92.1 Iso
Cr 141.3 (N 88.8) Iso
1PS
2PS
3PS
Tg 26.5 SmX 54.6 N 75.9 Iso
Tg 31.8 SmC 92.6 Iso
Cr 121.9 (SmX 63.5) Iso
1
1PS
2
2PS
3
3PS
338
338
338
338
338
338
530
340
405
300
390
300
0.019
9.2”10ꢀ3
0.011
[a] Cr: crystalline, SmX: as yet unidentified smectic, SmC: smectic C, N:
nematic. [b] Taken from ref. [5f].
7.5”10ꢀ3
0.011
9.1”10ꢀ3
[a] At 313 nm; arbitrary units: 3.33”0ꢀ5 molLꢀ1 solutions of compounds
in cyclohexane. [b] At 313 nm; all compound are referenced to 4-cyano-
The liquid-crystalline properties of the materials in this
series are modulated both by the substitution pattern of the
photochromic core and the conversion from the colourless
open forms to the deeply coloured closed forms.
4’-butyloxybiphenyl (Fem =0.74).[13]
.
All compounds 1–3 exhibit fluorescence properties which
are attributed to the presence of cyanobiphenyl groups. This
is supported by the observation that parent compounds 4–6
display only very weak fluorescence properties with a fluo-
rescence maximum wavelength of 440 nm when exited at
313 nm. Furthermore, the fluorescence spectra of all com-
pounds 4–6 are similar to that obtained for the cyanobiphen-
yl group, with a fluorescence maximum at a wavelength of
338 nm.
Even though 1–3 have two cyanobiphenyl groups which
are strong fluorophores (Fem =0.74), the quantum yields of
fluorescence observed are generally low. This indicates that
an energy transfer from the cyanobiphenyl groups to the
open forms takes place when the molecules are exited at
313 nm. This phenomenon is also observed when the mole-
cules are exited in the region in which the cyanobiphenyl
groups absorb. At the photostationary state, the systems dis-
play a lower intensity of emission fluorescence than is ob-
tained for the corresponding open forms (Figure 5). This in-
The liquid-crystalline phase behaviour for all of these sys-
tems 1–3 and 1PS–3PS is broadly similar in terms of the type
of phase structures and the range of the isotropisation tran-
sition temperatures, if viewed in the context of the structure
of the mesogens. There are significant differences with
regard to the thermodynamic stability of phases and the oc-
currence of some LC phases and the range of the mesomor-
phic state.
For 1–3, the highest stable phase is a nematic phase. For 1
and 2 this phase is thermodynamically stable; for 3, it is
only observable on cooling (monotropic LC behaviour). The
material with the most linear molecular structure of this
series, that is, 2, has the highest isotropisation temperature
of 92.18C, followed by that of the “V-shaped” compound 1
of 72.18C. Compound 3, in which the central perfluorinated
cyclopentene ring points in the same direction as the con-
nections of the ether linkages to the mesogens, shows a
monotropic transition at 88.88C. In addition, compounds 1
and 2 exhibit an SmC phase (tilted lamellar structure) at
74.58C for 1 and at 79.58C for 2.
Irradiation with UV light (313 nm) modifies these proper-
ties significantly. Under PS conditions the stability of the
nematic phase for 1 is reduced by 2.68C for 1PS to 75.98C;
more importantly, at lower temperatures a smectic phase of
unidentified structure (SmX) at 54.68C is found, and the
material vitrifies at 26.58C. For 2PS the nematic phase is lost
altogether, and the mesomorphic state is enhanced by
0.58C; the material melts from an SmC phase to the isotrop-
ic liquid at 92.68C. Additionally the crystalline low-tempera-
ture state is replaced by a glassy state with a transition Tg at
31.88C. For 3PS the melting point is reduced (to 121.98C)
relative to 3, and the monotropic nematic phase of 3 is re-
placed by a highly ordered, but unidentified, smectic phase
(SmX) at 63.58C, which represents a reduction by 25.38C
compared to the open form.
Figure 5. Emission fluorescence spectra of open-ring isomer 1 and at PS
state
Detailed comparisons of these three systems are not pos-
sible. Due to different conversion rates at the PS state they
are at different positions in their binary phase diagrams of
open and closed forms. Nevertheless, we note that the
system with the most linear molecular shape of the series,
that is, 2, is the least affected by the modulation in molecu-
lar rigidity in the central core on going from the open to the
closed form; indeed, a small enhancement of the mesomor-
phic state occurs, and the presence of the more rigid closed
systems promotes the formation of a higher ordered LC
phase. For 1 and 3 the reduction in flexibility due to ring
dicates that the closed forms can also act as acceptors.
These conclusions are in line with earlier results for a dihe-
teroarylethene (acceptor) coupled via an alkyl spacer to Lu-
cifer Yellow (donor).[14]
Liquid-crystalline properties: The results of the investigation
of the mesomorphic properties of the open-ring isomers 1–3
and at the photostationary state (irradiation wavelength
313 nm) 1PS–3PS by DSC and optical polarizing microscopy
are summarised in Table 5.
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G. H. Mehl and M. Frigoli
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6-Methoxy-2-methylbenzo[b]thiophene (11): 6-Methoxy-benzo[b]thio-
phene (10; 20.20 g, 0.12 mol) diluted in anhydrous THF (450 mL) was in-
troduced into a three-necked round-bottomed flask under nitrogen. The
mixture was cooled to ꢀ608C, and a solution of nBuLi (2.5m, 64 mL,
0.14 mol) in hexane was added dropwise. The resulting mixture was stir-
red for 90 min at ꢀ408C and then cooled to ꢀ788C. Methyl iodide (52 g,
0.36 mol) was added. The mixture was allowed to return to room temper-
ature, then hydrolysed with water (250 mL). The water/THF mixture was
extracted three times with Et2O (300 mL). The resulting organic phase
was washed several times with saturated aqueous NaCl solution, dried
over magnesium sulfate and then concentrated under reduced pressure.
The solid collected was recrystallised from methanol to give a white solid
(21.25 g, 97%). M.p. 88–898C; 1H NMR (CDCl3, 400 MHz): d=2.50 (s,
3H; CH3), 3.80 (s, 3H; OCH3), 6.83 (s, 1H; H-3), 6.90 (dd, J=8.6,
2.4 Hz, 1H; H-5), 7.21 (d, J=2.5 Hz, 1H; H-7), 7.49 ppm (d, J=8.6 Hz,
1H; H-4); 13C NMR (CDCl3, 100 MHz): d=16.1 (q), 56.4 (q), 104.8 (d),
113.9 (d), 121.6 (d), 123.3 (d), 134.3 (s), 137.9 (s), 140.9 (s), 156.6 ppm
(s); elemental analysis calcd (%) for C10H10OS (178.25): C 65.82, H 4.91;
found: C 65.90, H 4.84.
closure reduces the mesomorphic stability, and for 3, in
which all functionalities of the central aromatic core unit
(perfluorinated group, connections to the mesogens) are
crowded on one side of the long axis of the central photo-
chromic core, this results in a dramatic reduction of the sta-
bility of the mesomorphic state and the self-assembly behav-
iour.
The combination of diarylethene-based photochromic
groups with cyanobiphenyl moieties as mesogens allows for
the investigation of molecular switches, in which the colour,
absorption behaviour, liquid-crystalline properties and fluo-
rescence behaviour can be systematically modulated. They
can be considered as molecular logic gates (depending on
the addressing scheme several logical functions can be con-
sidered (e.g., YES, NO functions), and more complex opera-
tions are conceivable. This is particularly interesting as the
miscibility of these systems with commercial LC mixtures
(e.g., E7) points towards an avenue for optical computing
using LC technology.[2b,c]
3-Bromo-6-methoxy-2-methylbenzo[b]thiophene (12): A mixture of 11
(20.63 g, 0.13 mol) and CHCl3 (250 mL) was cooled in an ice-water bath.
A solution consisting of Br2 (20.08 g, 0.14 mol) and CHCl3 (90 mL) was
added dropwise. After the addition was completed, the mixture was stir-
red for 30 min at room temperature and then hydrolysed with saturated
aqueous Na2S2O6 solution (300 mL). The aqueous phase was extracted
twice with CHCl3 (300 mL). The combined organic phases were washed
several times with water, dried over magnesium sulfate and then concen-
trated under reduced pressure. The resulting solid was recrystallised from
methanol to give a white solid (28.50 g, 82%). M.p. 80–818C; 1H NMR
(CDCl3, 400 MHz): d=2.50 (s, 3H; CH3), 3.86 (s, 3H; OCH3), 6.90 (dd,
J=8.8, 2.4 Hz, 1H; H-5), 7.21 (d, J=2.4 Hz, 1H; H-7), 7.57 ppm (d, J=
8.8 Hz, 1H; H-4); 13C NMR (CDCl3, 100 MHz): d=15.3 (q), 55.6 (q),
104.9 (d), 105.8 (s), 114.4 (s), 114.5 (d), 123.1 (d), 132.4 (s), 138.2 (s),
157.8 ppm (s); elemental analysis calcd (%) for C10H9BrOS (257.15): C
46.71, H 3.53; found: C 46.93, H 3.60.
Experimental Section
Materials: THF was dried by distillation from Na/benzophenone. The
other solvents (Fisher Chemicals) were used without further purification
other than drying over molecular sieves. Melting points, measured in ca-
pillary tubes on a Gallenkamp apparatus, are uncorrected. Transition
temperatures were measured by differential scanning calorimetry
(Perkin–Elmer 7 series DSC with an indium standard). The phase type
and structure was confirmed by optical polarising microscopy using a
Mettler FP52 heating stage and FP5 control unit in conjunction with an
Olympus BH2 polarizing microscope (the coloured molecules were an-
nealed in the dark state, and the light of the microscope was switched on
for only very short periods of time, as decolouration takes place with the
broadband-emitting tungsten bulb of the miscroscope). 1H and 13C NMR
spectra were recorded on a JEOL Lambda 400 spectrometer (400 and
100 MHz for 1H and 13C, respectively) with tetramethysilane as internal
standard. Chemical shifts are given in parts per million and coupling con-
stants in hertz. UV/Vis spectra were recorded on a Cary 50-diode array
spectrophotometer. Elemental analyses were carried out with a Fisons
EA 1108 CHN analyzer. All reactions were monitored by thin-layer chro-
matography on 0.2 nm Merck silica gel plates (60F-254). Column chroma-
tography was performed on silica gel (Fluorochem 35-70u 60A). The pu-
rities of all final compounds were checked by GLC analysis on a Chrom-
pack 9001 capillary gas chromatograph fitted with a WCOT fused silica
column (CP-Sil5 CB 0.12 m, 10 m long, 0.25 mm internal diameter) with
nitrogen as carrier gas and by HPLC analysis on a Merck-Hitachi system
fitted with a Rainin Dynamax Microsorb 5 mm silica column (25 cm long,
4.6 mm internal diameter) and were found to be >99.5%. Previously re-
ported compounds were identified by comparison of 1H NMR spectra
and melting points with literature data.
1-[6-Methoxy-2-methylbenzo[b]thiophen-3-yl]heptafluorocyclopentene
(13): A solution of nBuLi (2.5 m, 11.90 mL, 0.034 mol) in hexane was
added dropwise to a stirred solution of 12 (7.61 g, 0.03 mol) in anhydrous
THF (120 mL) at ꢀ788C under nitrogen. The resulting mixture was stir-
red for 30 min at ꢀ788C, then octafluorocyclopentene (20 g, 0.09 mol)
was added in a single portion. After the addition was completed, the mix-
ture was allowed to return to room temperature, then hydrolysed with
water (120 mL). The THF/water mixture was extracted three times with
Et2O (150 mL). The resulting organic phase was washed several times
with saturated aqueous NaCl solution, dried over magnesium sulfate and
then concentrated under reduced pressure. The crude product was puri-
fied by column chromatography on silica gel with pentane/CH2Cl2 (100:0
to 80:20) to give slightly yellow crystals (8.12 g, 68%). M.p. 41–428C;
1H NMR (CDCl3, 400 MHz): d=2.48 (s, 3H; CH3), 3.86 (s, 3H; OCH3),
7.03 (dd, J=8.8, 2.4 Hz, 1H; H-5), 7.27 (d, J=2.4 Hz, 1H; H-7),
7.41 ppm (d, J=8.8 Hz, 1H; H-4); 13C NMR (CDCl3, 100 MHz): d=14.4
(q), 55.6 (q), 104.9 (d), 113.9 (s), 114.8 (d), 122.2 (d), 132.0 (d), 139.7 (s),
141.3 (s), 157.6 ppm (s); elemental analysis calcd (%) for C15H15F7OS
(370.29): C 48.65, H 2.45; found: C 48.45, H 2.59.
5-Methoxy-2-methylbenzo[b]thiophene (15): This compound was pre-
pared from 14 according the same procedure as described for the synthe-
sis of the 11. White solid (92% yield). M.p. 97–988C; 1H NMR (CDCl3,
400 MHz): d=2.56 (s, 3H; CH3), 3.84 (s, 3H; OCH3), 6.89 (m, 2H; H-3,
H-5), 7.12 (d, J=2.5 Hz, 1H; H-4), 7.59 ppm (d, J=8.6, 1H; H-6);
13C NMR (CDCl3, 100 MHz): d=16.3 (q), 55.6 (q), 105.2 (d), 113.2 (d),
121.5 (d), 122.7 (d), 132.0 (s), 141.5 (s), 142.3 (s), 157.4 ppm (s); elemen-
tal analysis calcd (%) for C10H10OS (178.25): C 65.82, H 4.91; found: C
65.95, H 4.81.
Photochemical measurements: Absorption spectra were performed in cy-
clohexane solutions of spectrometric grade (Fisher Chemicals) at 25ꢁ
0.2. The analysis cell (optical path lengh 10 nm) was placed in a thermo-
stated copper block inside the sample chamber of a Cary 50-diode array
spectrophotometer. An Oriel 200 W high-pressure Hg/Xe lamp was used
for irradiation. Mercury lines of 313 and 546 nm were isolated by passage
through a monochromater (Oriel).
Fluorescence measurements: Emission spectra were measured in cyclo-
hexane solutions of spectrometric grade (Fisher Chemicals) at 25ꢁ0.28C
by using a Shimadzu RF-1501 spectrofluorophotometer.
3-Bromo-5-methoxy-2-methylbenzo[b]thiophene (16). This compound
was prepared from 15 according to the same procedure as described for
the synthesis of 12. Oily product (45% yield). 1H NMR (CDCl3,
400 MHz): d=2.50 (s, 3H; CH3), 3.86 (s, 3H; OCH3), 6.92 (dd, J=8.7,
2.5 Hz, 1H; H-6), 7.12 (d, J=2.5 Hz, 1H; H-4), 7.53 ppm (d, J=8.7 Hz,
1H; H-6); 13C NMR (CDCl3, 100 MHz): d=15.7 (q), 55.6 (q), 104.9 (d),
106.3 (s), 115.0 (d), 123.0 (d), 129.3 (d), 136.5 (s), 139.5(s), 158.2 ppm (s);
Conversion measurements: The open- and the closed-ring isomers were
easily separated by HPLC using a Merck-Hitachi system fitted with a
Rainin Dynamax Microsorb 5 mm silica column (25 cm long, 4.6 mm in-
ternal diameter) with hexane as eluent for 4–6 and hexane/ethyl acetate
(94:6) for 1–3.
5248
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 5243 – 5250

Photochromic Liquid Crystals
5243 – 5250
elemental analysis calcd (%) for C10H9BrOS (257.15): C 46.71, H 3.53;
found: C 46.78, H 3.41.
(100:0 to 90:10) to give a white solid (93% yield). M.p. 236–2378C;
1H NMR ([D6]acetone, 400 MHz): d=2.16 (s, 3.18H; ap CH3), 2.39 (s,
2.82H; p CH3), 6.73 (dd, J=8.6, 2.2, 0.94H; p H-5), 6.91 (dd, J=8.6,
2.2 Hz, 1.06H; ap H-5), 7.12 (d, J=2.2 Hz, 0.94H; p H-7), 7.20 (d, J=
2.2 Hz, 1.06H; ap H-7), 7.34 (d, J=8.6 Hz, 0.94H; p H-4), 7.38 (d, J=
8.6 Hz, 1.06H; ap H-4), 9.66 (s, 0.47H; p OH), 9.76 ppm (s, 0.53H; ap
OH); parallel conformer/antiparallel conformer=47:53; elemental analy-
sis calcd (%) for C23H14F6O2S2 (500.48): C 55.20, H 2.82; found: C 55.25,
H 2.72.
1,2-Bis(6-methoxy-2-methyl-benzo[b]thiophen-3-yl)hexafluorocyclopen-
tene (4): A solution of nBuLi in hexane (2.5 m, 17.9 mL, 0.044 mol) was
added dropwise to a stirred solution of 12 (10 g, 0.039 mol) in anhydrous
THF (150 mL) at ꢀ788C under nitrogen atmosphere. The resulting mix-
ture was stirred for 30 min at ꢀ788C, then octafluorocyclopentene
(4.12 g, 0.019 mol) was added in a single portion. After the addition was
completed, the mixture was allowed to return to room temperature, then
hydrolysed with water (150 mL). The THF/water mixture was extracted
three times with Et2O (150 mL). The resulting organic phase was washed
several times with saturated aqueous NaCl solution, dried over magnesi-
um sulfate and then concentrated under reduced pressure. The crude
product was purified by column chromatography on silica gel with pen-
tane/CH2Cl2 (100:0 to 70:30). The resulting solid was recrystallised from
methanol to give a white solid (6.22 g, 62%). M.p. 128–1298C; 1H NMR
(CDCl3, 400 MHz): d=2.13 (s, 3.84H; ap CH3), 2.42 (s, 2.16H; p CH3),
3.77 (s, 2.16H; p OCH3), 3.84 (s, 3.84H; ap OCH3), 6.79 (dd, J=8.9,
2.2 Hz, 0.72H; p H-5), 6.98 (dd, J=8.8, 2.2 Hz, 1.28H; ap H-5), 7.07 (d,
J=2.2 Hz, 0.72H; p H-7), 7.16 (d, J=2.2 Hz, 1.28H; ap H-7), 7.39 (d, J=
8.9 Hz, 0.72H; p H-4), 7.52 ppm (d, J=8.9 Hz, 1.28H; ap H-4); parallel
(p) conformer/antiparallel (ap) conformer=36:64; UV/Vis (cyclohex-
ane): lmax (e)=267 (23800), 305 nm (4500); elemental analysis calcd (%)
for C25H18F6O2S2 (528.53): C 56.81, H 3.43; found: C 56.88, H 3.53.
1-(5-Hydroxy-2-methylbenzo[b]thiophen-3-yl)-2-(6’-hydroxy-2’-methyl-
benzo-[b]-thiophen-3’-yl)hexafluorocyclopentene (8): This compound
was prepared from 5 according to the same procedure as described for
the synthesis of 4. White solid (91% yield). M.p. 227–2288C; 1H NMR
([D6]acetone, 400 MHz): d=2.20 (s, 1.67H; ap 2-CH3 or ap 2’-CH3), 2.21
(s, 1.67H; ap 2-CH3 or ap 2’-CH3), 2.42 (s, 1.33H; p 2-CH3 or p 2’-CH3),
2.52 (s, 1.33H; p 2-CH3 or p 2’-CH3), 6.73–6.75 (m, 0.87H; p H-6 and p
H-5’), 6.85 (dd, J=8.6, 2.2 Hz, 1.13H; ap H-6), 6.93 (dd, J=8.6, 2.2 Hz,
1.13H; ap H-5’), 6.98 (sl, 1H; p H-4 and ap H-4), 7.13 (d, J=2.2 Hz,
0.46H; p H-7’), 7.20 (d, J=2.2 Hz, 0.54H; ap H-7’), 7.35–7.41 (m, 1H; p
and ap H-4’), 7.58 (d, J=8.6 Hz, 0.46H; p H-7), 7.66 (d, J=8.8 Hz,
0.54H; ap H-7), 9.51 (s, 0.23H; p 5-OH), 9.65 (s, 0.50H; ap 5-OH and p
6’-OH), 9.65 ppm (s, 0.37H; ap 6’-OH); parallel conformer/antiparallel
conformer=44:56; elemental analysis calcd (%) for C23H14F6O2S2
(500.48): C 55.20, H 2.82; found: C 55.35, H 2.90.
1-(5-Methoxy-2-methyl-benzo[b]thiophen-3-yl)-2-(6’-methoxy-2’-methyl-
benzo[b]thiophen-3’-yl)hexafluorocyclopentene (5): A solution of nBuLi
(2.5m, 7.2 mL, 0.018 mol) in hexane was added dropwise to a stirred solu-
tion of 16 (4.78 g, 0.018 mol) in anhydrous THF (100 mL) at ꢀ788C
under nitrogen. The resulting mixture was stirred for 30 min at ꢀ788C,
then a solution of 13 (7 g, 0.018 mol) in of anhydrous THF (40 mL) was
added slowly. After the addition was completed, the mixture was allowed
to return to room temperature, then hydrolysed with water (140 mL).
The THF/water mixture was extracted three times with Et2O (150 mL).
The resulting organic phase was washed several times with saturated
aqueous NaCl solution, dried over magnesium sulfate and then concen-
trated under reduced pressure. The crude product was purified by
column chromatography on silica gel with pentane/CH2Cl2 of increasing
polarity (100:0 to 70:30). The resulting solid was recrystallised from
methanol to give a white solid (5.42 g, 57%). M.p. 118–1198C; 1H NMR
(CDCl3, 400 MHz): d=2.18 (s, 3.72H;ap 2-CH3, ap 2’-CH3), 2.40 (s,
1.14H; p 2-CH3 or p 2’-CH3), 2.56 (s, 1.14H;p 2-CH3 or p 2’-CH3), 3.72
(s, 1.14H;p 5-OCH3 or p 6’-OCH3), 3.78 (s, 1.14H;p 5-OCH3 or p 6’-
OCH3), 3.84 (s, 1.86H; ap 5-OCH3 or ap 6’-OCH3), 3.87 (s, 1.86H; ap 5-
OCH3 or ap 6’-OCH3), 6.79–7.20 (m, 4H), 7.41–7.60 ppm (m, 2H); paral-
lel conformer/antiparallel conformer=38:62; UV/Vis (cyclohexane): lmax
(e)=265 (21300), 306 (6600), 316 nm (6600); elemental analysis calcd
(%) for C25H18F6O2S2 (528.53): C 56.81, H 3.43; found: C 56.89, H 3.48.
1,2-Bis(5-hydroxy-2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopen-
tene (9): This compound was prepared from 6 according to the same pro-
cedure as described for the synthesis of the 4. White solid (87% yield).
1
M.p. 250–2518C; H NMR ([D6]acetone, 400 MHz): d=2.23 (s, 4.44H; ap
CH3), 2.48 (s, 1.56H; p CH3), 6.75 (dd, J=8.6, 2.2 Hz, 0.52H; p H-6),
6.85 (dd, J=8.8, 2.4 Hz, 1.48H; ap H-6), 6.93 (d, J=2.2 Hz, 0.52H; p H-
4), 6.97 (d, J=2.4 Hz, 1.48H; ap H-4), 7.57 (d, J=8.6 Hz, 0.52H; p H-7),
7.64 (d, J=8.8 Hz, 1.48H; ap H-7), 9.38 (s, 0.26H; p OH), 9.64 (s, 0.74H;
ap OH); parallel conformer/antiparallel conformer=26:74; elemental
analysis calcd (%) for C23H14F6O2S2 (500.48): C 55.20, H 2.82; found: C
55.26, H 2.86.
Compound 1: A mixture of 4 (0.15 g, 0.30 mmol), 4’-(6-bromohexyloxy)-
biphenyl-4-carbonitrile (0.26 g, 0.72 mmol), K2CO3 (0.10 g, 0.72 mmol)
and dry butanone (20 mL) was refluxed for 24 h, then allowed to return
to room temperature. The reaction mixture was filtered, and the buta-
none distilled off under reduced pressure. The crude product was purified
by column chromatography on silica gel with CH2Cl2 to give a white
solid (0.26 g, 87% yield). Transition T/8C: Cr 40.1 SmC 74.5 N 78.5 Iso
liq; 1H NMR (CDCl3, 400 MHz): d=1.25–1.48 (m, 24H), 1.80–1.86 (m,
8H), 2.13 (s, 3.92H; ap CH3), 2.43 (s, 2.08H; p CH3), 3.98–4.93 (m, 8H;
OCH2), 6.80 (dd, J=8.6, 2.2 Hz, 0.70H; p H-5), 6.96–7.06 (m, 5.30H; 4H-
arom+ap H-5), 7.05 (d, J=2.2 Hz, 0.70H; p H-7), 7.15 (d, J=2.2 Hz,
1.30H; ap H-7), 7.39 (d, J=8.6 Hz, p 0.70H; H-4), 7.52 (m, 5.30H; 4H-
arom+ap H-4), 7.60–7.70 ppm (m, 8H; 8H-arom); parallel conformer/
antiparallel conformer=35:65; UV/Vis (cyclohexane): lmax (e)=280 nm
(57000); elemental analysis calcd (%) for C69H68F6N2O4S2 (1167.41): C
70.99, H 5.87, N 2.40; found: C 70.94, H 5.89, N 2.45.
1,2-Bis(5-methoxy-2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopen-
tene (6): This compound was prepared from 16 according the same pro-
cedure as described for the synthesis of 4. White solid (69% yield); M.p.
182–1838C; 1H NMR (CDCl3, 400 MHz): d=2.10 (s, 3.32H; ap CH3),
2.38 (s, 2.82H; p CH3), 3.60 (s, 2.82H; p OCH3), 3.79 (s, 3.32H; ap
OCH3), 6.78 (dd, J=8.8, 2.4 Hz, 0.92H; p H-6), 6.78 (dd, J=8.8, 2.4 Hz,
1.08H; ap H-6), 6.92 (d, J=1.6 Hz, 0.92H; p H-4), 7.06 (d, J=1.6 Hz,
1.08H; ap H-4), 7.41 (d, J=8.8 Hz, 0.92H; p H-7), 7.49 ppm (d, J=
8.8 Hz, 1.08H; ap H-7); parallel conformer/antiparallel conformer=
45:55; UV/Vis (cyclohexane): lmax (e)=262 (21000), 307 (8400), 316 nm
(9000); elemental analysis calcd (%) for C25H18F6O2S2 (528.53): C 56.81,
H 3.43; found: C 56.78, H 3.51.
Compound 2: This compound was prepared from 8 according to the
same procedure as described for synthesis of 1. White solid (78% yield);
Transition T/8C: Cr 45.4 SmA 79.5 N 92.1 Iso liq; 1H NMR (CDCl3,
400 MHz): d=1.30–1.48 (m, 24H), 1.68–1.83 (m, 8H), 2.15 (s, 3.90H; ap
2-CH3, ap 2’-CH3), 2.37 (s, 1.05H; p 2-CH3 or p 2’-CH3), 2.49 (s, 1.05H; p
2-CH3 or p 2’-CH3), 3.81–4.00 (m, 8H; OCH2), 6.79–7.20 (m, 8H), 7.41–
7.68 ppm (m, 10H); parallel conformer/antiparallel conformer=35:65;
UV/Vis (cyclohexane): lmax (e)=292 nm (63000); elemental analysis
calcd (%) for C69H68F6N2O4S2 (1167.41): C 70.99, H 5.87, N 2.40; found:
C 71.10, H 5.78, N 2.44.
1,2-Bis(6-hydroxy-2-methylbenzo[b]thiophen-3-yl)-hexafluorocyclopen-
tene (7): Compound (4) (5.60 g, 0.01 mol) and dried CH2Cl2 (100 mL)
were introduced into a three-necked round-bottomed flask under nitro-
gen. The resulting mixture was cooled in an ice-water bath, then BBr3
(4.91 mL; 0.05 mol) was added dropwise. The solution was warmed to
room temperature and stirred for 6 h. Then, the mixture was cooled in an
ice-water bath, and 10% aqueous HCl solution (100 mL) was added
slowly. The chlorinated phase was separated from the aqueous phase, and
the latter was extracted several times with ethyl acetate. The combined
organic phases were washed with water, dried over magnesium sulfate
and then concentrated under reduced pressure. The crude product was
purified by column chromatography on silica gel with CH2Cl2/Et2O
Compound 3: This compound was prepared from 9 according to the
same procedure as described for synthesis of 1. White solid (84% yield);
Transition T/8C: Cr 141.3 (N 88.8) Iso liq; 1H NMR (CDCl3, 400 MHz):
d=1.29–1.53 (m, 24H), 1.68–1.81 (m, 8H), 2.15 (s, 3.35H; ap CH3), 2.41
(s, 2.65H; p CH3), 3.81–3.98 (m, 8H; OCH2), 6.80 (dd, J=8.6, 2.2 Hz,
0.88H; p H-6), 6.93–697 (m, 6H; 4H-arom+ap H-6+p H-4), 7.05 (d, J=
1.6 Hz, 1.12H; ap H-4), 7.43 (d, J=8.6 Hz, 0.88H; p H-7), 7.47–7.52 (m,
5.12H; 4H-arom+ap H-7), 7.59–7.67 ppm (m, 8H; 8H-arom); parallel
conformer/antiparallel conformer=44:56; UV/Vis (cyclohexane): lmax
Chem. Eur. J. 2004, 10, 5243 – 5250
www.chemeurj.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5249

G. H. Mehl and M. Frigoli
FULL PAPER
(e)=292 nm (58000); elemental analysis calcd (%) for C69H68F6N2O4S2
(1167.41): C 70.99, H 5.87, N 2.40; found: C 71.07, H 5.82, N 2.47.
N. L. Boiko, Polym. Sci. Part A 2001, 43, 1040; e) H. Hattori, T.
Uryu, Liq. Cryst. 1999, 26, 1085; f) M. Frigoli, G. H. Mehl, Chem-
PhysChem 2003, 4, 101; g) M. Frigoli, G. H. Mehl, Eur. J. Org.
Chem. 2004, 633.
[6] a) K. Matsuda, M. Irie, Chem. Eur. J. 2001, 7, 3466; b) S. Kobatake,
M. Yamada, T. Yamada, M. Irie, J. Am. Chem. Soc. 1999, 121, 8450;
c) M. Irie, O. Miyatake, K. Uchida, J. Am. Chem. Soc. 1994, 116,
9874.
Acknowledgement
We thank the EPSRC for funding.
[7] K. Takeuchi, T. J. Kohn, D. J. Sall, M. L. Denney, J. R. McCowan,
G. F. Smith, D. S. Gifford-Moore, Bioorg. Med. Chem. Lett. 1999, 9,
759.
[8] K. Uchida, E. Tsuchida, Y. Aoi, S. Nakamura, M. Irie, Chem. Lett.
1999, 63.
[1] a) Recent review: Electron Transfer in Chemistry, Vols. 1–5, (Ed.: V.
Balzani), Wiley-VCH, Weinheim, 2001, particularly Vols. 3 and 5 (all
chapters); b) V. Balzani, A. Credi, M. Venturi, Chem. Eur. J. 2002,
8, 5525; c) G. J. Brown, A. P. de Silva, S. Pagliari, Chem. Commun.
2002, 2461.
[9] A. W. Adamson, A Textbook of Physical Chemistry, Academic Press,
London, 1973.
[10] a) S. Takami, T. Kawai, M. Irie, Eur. J. Org. Chem. 2002, 3796; b) K.
Morimitsu, K. Shibata, S. Kobatake, M. Irie, J. Org. Chem. 2002, 67,
4574.
[11] a) R. M. Christie, Colour Chemistry, RSC, Cambridge, 2001. Note:
the extinction coefficient has been related to the oscillator strength
fosc, which can be calculated as fosc =4.703”1029 ”(M2/lM), where M
is the transition dipole moment and lM is the mean wavelength of
the absorption band.
[2] Recent reviews: a) Molecular Switches (Ed.: B. L. Feringa), Wiley-
VCH, Weinheim, 2001; b) M. Irie, Chem. Rev. 2000, 100, 1683–
1685; c) M. Irie Chem. Rev. 2000, 100, 1685–1715; d) G. Berkovic,
V. Krongauz, V. Weiss, Chem. Rev. 2000, 100, 1741–1754; e) B. L
Ferringa, R. A. van Delden, N. Konimura, E. M. Geertsema, Chem.
Rev. 2000, 100, 1789–1816; f) S. Kawata, Y Kawata, Chem. Rev.
2000, 100, 1777–1789; g) K. Ichimura, Chem. Rev. 2000, 100, 1847–
1874.
[12] T. Tsujioka, M. Kume, M. Irie, J. Photochem. Photobiol. A 1997,
104, 203.
[13] M. Van Damme, J. Hofkens, F. C. De Schryver, T. G. Ryan, W.
Rettig, A. Klock, Tetrahedron 1989, 45, 4693.
[14] a) L. Giordano, T. M. Jovin, M. Irie, E. A. Jares-Erijman, J. Am.
Chem. Soc. 2002, 124, 7481; b) M. Frigoli, G. H. Mehl, Chem.
Commun. 2004, 817.
[15] a) G. M. Tsivgoulis, J.-M. Lehn, Angew. Chem. 1995, 107, 1188;
Angew. Chem. Int. Ed. Engl. 1995, 34, 1119; b) E. Kim, Y.-K. Choi,
M.-H Lee, Macromolecules 1999, 32, 4855; c) M.-S. Kim, T. Kawai,
M. Irie, Opt. Mater. 2002, 21, 271.
[3] a) E. Sackmann, J. Am. Chem. Soc. 1971, 93, 7088; b) K. Ogiura, H.
Hirabayashi, A. Uejima, K. Nakamura, Jpn. J. Appl. Phys. 1982, 21,
969; c) M. Eich, J. H. Wendorff, Makromol. Chem. Rapid Commun.
1987, 8, 59; d) T. Ikeda, A. Kanzawa in Molecular Switches (Ed.:
B. L. Feringa), Wiley-VCH, Weinheim, 2001.
[4] a) K. Uchida, Y. Kawai, Y. Shimizu, V. Vill, M. Irie, Chem. Lett.
2000, 654; b) G. Subramanian, J. M. Lehn, Mol. Cryst. Liq. Cryst.
2001, 243, 364; c) K. E. Maly, M. D. Wand, R. P. Lemieux, J. Am.
Chem. Soc. 2002, 124, 7898.
[5] a) H. Dꢁrr in Organic Photochromic and Thermochromic Com-
pounds (Eds.: J. C. Crano, R. J. Guglielmetti), Plenum Press, New
York, 1999; b) V. Krongauz in Applied Photochromic Polymer Sys-
tems (Ed.: C. B. McArdle), Blackie, Glasgow, 1992; c) I. Cabrera, V.
Krongauz, Nature 1987, 326, 582; d) V. P. Shibaev, A. Y. Bobrovsky,
Received: October 24, 2003
Published online: September 13, 2004
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