Room Temperature Nematic Photoswitchable Liquid Crystals - Molecular Modularisation of Functional Elements

Article abstract of DOI:10.1002/ejoc.200300519

The successful design of photochromic self-assembly systems requires understanding (on both the molecular and sub-molecular scales) of diverse and often competing structure-property relationships. Such molecular systems are promising as components for pho

Full text of DOI:10.1002/ejoc.200300519

FULL PAPER
Room Temperature Nematic Photoswitchable Liquid Crystals — Molecular
Modularisation of Functional Elements
Michel Frigoli[a] and Georg H. Mehl*[a]
Keywords: Photochromism / Liquid crystals / Molecular switches / Siloxanes
The successful design of photochromic self-assembly systems
requires understanding (on both the molecular and sub-mo-
lecular scales) of diverse and often competing structure-
property relationships. Such molecular systems are promis-
an efficient approach to obtain such materials. The design,
synthesis and the photochromic and mesomorphic properties
of stable photochromic systems which exhibit nematic LC
phase behaviour at room temperature are reported.
ing as components for photonically switchable components ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
in opto-electronic devices. Molecular modularisation can be
Germany, 2004)
Introduction
The combination of liquid-crystallinity and photoch-
romic behaviour in molecular systems promises to be very
useful in optical technological devices.
This not only allows the study of photochromic behaviour,
but also the effects of photochromism on mesomorphic as-
sembly.
However to fulfil this potential, photochromic liquid
crystals have not only to be stable in both forms of the
photochromic systems, but they have to exhibit liquid crys-
talline phase behaviour (preferably the nematic phase) at
ambient or near-ambient temperatures. Furthermore, the
structure-property relationships of such a system should be
fully understood, or at least some clear rules of how to ob-
tain certain properties should be developed.[1] A further im-
portant feature is to be able to decouple the absorption be-
haviour of the photochromic systems from that of the mo-
lecular groups responsible for the formation of meso-
morphic behaviour. This is important to avoid competing
absorption of the mesogens in the UV region, and poten-
tially unwanted anisotropic FRET (Fluorescence Reson-
ance Energy Transfer) effects in the liquid crystal phase.[2]
At the current stage of knowledge in this field, these effects,
which influence the self-assembly properties of the molecu-
lar system, are both difficult to assess and control.
In order to achieve these goals, molecules I and II were
synthesised and investigated (Figure 1). They contain as a
photochromic core the 1,2-bis(2-methylbenzo[b]thiophen-3-
yl)hexafluorocyclopentene group (Figure 2), which has been
investigated in great detail and has been employed for me-
somorphic photochromes.[3] Compared to many other pho-
tochromic systems, substituted diarylethenes are thermally
very robust; both forms are stable even at high temperatures
and photonic switching at elevated temperatures is possible.
The photochromic core of II is symmetrically func-
tionalised in the 6- and 6Ј-positions with phenol units,
which are connected by a C4 spacer, a tetramethyldisiloxane
group and a further C4 spacer to a mesogenic group.[4,5]
This group is a rod-like (calamitic) mesogen, which is con-
nected by its long axis to the spacer and so to the photoch-
romic unit. This approach to some degree decouples the
soft assembly behaviour and the photochromic properties
of the system.
The use of laterally connected mesogens has been suc-
cessfully used in the investigation of organic/inorganic hy-
brids (silsesquioxanes), dendrimers (carbosilazanes) and LC
ferrocenes, where the liquid-crystalline phase behaviour and
the properties have been found to be broadly independent
of the ‘‘encapsulated core’’. Indeed, in such systems some
molecular modes, associated with the rotation around the
long axis of the molecules, are faster than in the corre-
sponding low molecular mass systems.[5] The selected ap-
proach is essentially modular. The mesogenic group is de-
signed in such manner that attachment to the photochromic
system does not alter the phase structure, so as to ensure a
nematic phase close to room temperature and (ideally), the
absence of crystallisation. Photochromic behaviour and
liquid-crystalline properties can then be addressed selec-
tively. Furthermore, the delocalised π-electron system,
which is responsible for the mesomorphic behaviour, has to
be interrupted in order to avoid an overlap with the absorp-
tion maximum of the photochromic group at the wave-
length of irradiation.[5bϪ5e] (In structurally related meso-
genic systems, LC phase behaviour is generally due to shape
anisotropy and van der Waals interactions of π-electrons).
[a]
Department of Chemistry, University of Hull,
Hull, HU6 7RX, UK
Fax: (internat.) ϩ 44-1482-466411
E-mail: g.h.mehl@hull.ac.uk
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DOI: 10.1002/ejoc.200300519
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Room Temperature Nematic Photoswitchable Liquid Crystals
FULL PAPER
Figure 1. Molecular structures of the photochromic liquid crystals I and II
Figure 2. Photochromic interconversion in I and II
In order to achieve this, an alicyclic group was incorpor- any liquid-crystalline behaviour, no further reactions were
ated, interrupting the electron conjugation in the mesogen. performed on it.
The synthesis of photochromic liquid crystals I and II
(Scheme 2) required the preparation of a photochromic
compound 11, which was obtained in 84 % yield from
Results and Discussion
10[1g,8] by treatment with 4-bromobut-1-ene in the presence
of potassium carbonate and butanone at reflux. Hydrosilyl-
ation of 11 with 9 at room temperature results in the forma-
tion of the glassy nematic compounds I and II. The use of
high dilution and one equivalent of the mesogenic group 9
gave the monosubstituted compound I in 49 % yield,
whereas use of two equivalents of 9 afforded the disubsti-
tuted compound II in 71 % yield.
Synthesis
The reduction of 4-(4-hydroxyphenyl)cyclohexanone (1)
with sodium borohydride in methanol at room temperature
gave selectively trans-4-(4-hydroxy-cyclohexyl)phenol (2)
after recrystallisation from ethanol.[6] The alcohol 2 was ob-
tained in 77 % yield, despite the poor solubility of the sub-
strate 1 in the solvent used. When 2 was treated with 4-
octyloxybenzoic acid in dichloromethane in the presence of
diisopropylcarbodiimide and DMAP, we surprisingly ob-
Photochromic Properties
The diarylethenes I and II underwent a reversible pho-
served the formation of 3 (8 %) and 5 (22 %) in addition to tochromic reaction in cyclohexane by alternate exposure to
the expected product 4, obtained in 44 % yield. Subsequent light at wavelengths of 313 nm (UV) and 546 nm (visible),
esterification, carried out in the same manner as for com- with retention of the isosbestic points. The colour of the
pounds 4 and 5, but with 4-octyloxy-2-(pent-4-en-1-yl- solutions of all the compounds was found to change from
oxy)benzoic acid (6), gave 7 and 8 in 35 % and 22 % yield colourless to red (520 nm). The photochromic behaviour of
respectively.[7] The ester 8 was converted to 9 by hydrosilyl- the new compounds was compared with that obtained for
ation with 1,1,3,3,5,5-hexamethyltrisiloxane and Karstedt’s the unsubstituted parent compound, 1,2-bis(2-methyl-1-
catalyst in 77 % yield. As the compound 7 did not exhibit benzothiophen-3-yl)hexafluorocyclopentene.[9]
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M. Frigoli, G. H. Mehl
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Scheme 1. Synthesis of the mesogenic unit 9
Scheme 2. Convergent synthesis of target compounds I and II
Figure 4. Absorption spectra of a cyclohexane solution (3.33 ϫ
10Ϫ5 mol LϪ1) of the open-ring isomer II, closed-ring isomer IIa
and in the photostastionary state under 313 nm radiation.
Figure 3. Absorption spectra of a cyclohexane solution (3.33 ϫ
10Ϫ5 mol LϪ1) of the open-ring isomer I, the closed-ring isomer Ia
and the photostastionary state under 313 nm radiation.
Figures 3 and 4 show the spectral changes of compounds from the absorption spectra obtained at the photostation-
I and II between the open- and the closed-ring isomers and ary state (PS) by using Equation (1),[10] which describes the
the system in photostationary state under 313 nm radiation. absorbance for a mixture of two compounds at a given
The absorption spectra of the closed forms were calculated wavelength.
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Room Temperature Nematic Photoswitchable Liquid Crystals
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[A]PS ϭ [A]OF (1 Ϫ Cv) ϩ [A]CF (Cv)
(1)
Quantum Yield and Conversion Measurements
where [A]PS: absorption obtained at the PS state, [A]OF
:
The quantum yields for the cyclisation and photocyclor-
eversion reactions of I and II were measured in cyclohex-
ane. Ref was used as reference. ΦOFǞCF was calculated from
the slope of the graph of absorbance (followed at the maxi-
mum wavelength of the closed-ring isomer) versus time at
the beginning of the photochromic process, by comparing
the values of the rate constant for the photocyclisation reac-
tion of the reference compound with that for the molecule
studied.[11] The conversions of compounds I and II under
313 nm radiation were determined by HPLC. The two
forms can be easily separated by HPLC (Merck-Hitachi) on
a silica column, eluting with a 96:4 cyclohexane/ethyl acet-
ate mixture. The results are summarised in Table 2 (the val-
ues for Ref are taken from the literature[9]).
absorption of the open form, [A]CF: absorption of the
closed form, and Cv: the degree of conversion from open-
to closed-ring isomers. [A]PS and [A]OF can be determined
by UV spectroscopy and Cv can be calculated using HPLC
data, by integration of the peaks for the open and the
closed forms.
As can be seen in Figures 3 and 4, the UV/Visible absorp-
tion spectra of compounds I and II are similar for both the
open and closed forms. Because the mesogenic groups are
connected to the photochromic core by an alkyl spacer and
absorb only below 305 nm (Figure 5), no differences are ob-
served between the absorption spectrum for the monomer
and that for the dimer above 305 nm.
Table 2. Quantum yield and conversion of cyclisation under 313 nm
radiation and quantum yield of cycloreversion under 546 nm radi-
ation
Compound Cyclisation[a] (313 nm)
Cycloreversion[b] (546 nm)
ΦCFǞOF
ΦΟFǞCF
Conversion
I
0.18
0.19
0.35
0.24
0.24
0.43
0.44
0.46
0.35
II
Ref
[a] All photochromic reactions were performed in cyclohexane solu-
[a]
tion.
For all compounds the conversion of the cycloreversion
reaction was 100 %.
The quantum yields of photocyclisation and photorever-
sion and the values of the conversions obtained at the
photostationary state (PS) are very similar for both I and
II (Table 2).
The lower quantum yield of photocyclisation for com-
pounds I and II, as compared to the unsubstituted pho-
tochromic compound Ref, is clearly not influenced by the
number of mesogenic groups attached to the central pho-
tochromic core. Similarly, a reduction in the quantum yield
of photocyclisation has been previously reported for di-
benzothienylethene derivatives having methoxy groups or
cholesterol derivatives at the 6,6Ј-positions.[8]
Figure 5. Absorption spectrum of 8 in cyclohexane
The absorption maximum of compounds Ia and IIa is
identical to that obtained for the parent molecule (520 nm).
However the ε value at this wavelength for compounds Ia
and IIa (between 23500 and 24000 molϪ1 L cmϪ1) is at least
2.5 times higher than for the parent molecule Ref (9100
molϪ1 L cmϪ1). The absorption maxima and ε values of
the open- and closed-ring isomers are summarised in
Table 1 (the values for Ref and Refa are taken from the lit-
erature[9]).
The quantum yield of photoreversion for I and II is
slightly larger than for the reference compound Ref, and the
conversions obtained at the photostationary state for I and
II were lower than measured for Ref (Table 2).
Table 1. Absorption maxima and ε values of the open forms I, II
and Ref and the closed forms Ia, IIa and Refa
Compound
λmax/nm (ε/molϪ1 L cmϪ1
)
Liquid-Crystalline Properties
I
258 (57600)
258 (79000)
II
The results of investigations into the mesomorphic
properties of the liquid crystalline intermediates using
differential scanning calorimetry (DSC) and optical polaris-
ing microscopy are summarised in Table 3 (the values for
Ref 1, Ref 2 and Ref 3 are taken from the literature[5b]).
Ref
Ia
IIa
Refa
258 (21000), 290 (6900), 299 (7400)
258 (46500), 310 (23000), 360 (20200), 520 (24000)
258 (67000), 310 (22500), 360 (20000), 520 (23600)
517 (9100)
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M. Frigoli, G. H. Mehl
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Table 3. Transition temperatures as determined by DSC
The clearing point of compound 3 is 255.3 °C; at lower
temperatures it shows a rich polymorphism of lamellar LC
phases (SmC: molecules in layers, tilted with respect to the
layer normal; SmA: layered structure, mesogens on average
orthogonal to the layer normal). The introduction of a lat-
eral alkyl group (8) lowers the transition temperature to
138.7 °C and the higher ordered LC phases are lost. Coup-
ling to the tetramethyldisiloxane group (9) results in a
further decrease in the transition temperature to 83.6 °C,
which is approximately 55 °C lower than in 8. The overall
reduction in phase stability from 3 to 9 is about 170 °C and
the reduction of the melting point (melting to glass tran-
sition temperature) is 40 °C. The liquid crystal phase range
is lower than in the fully aromatic system. Overall, the lower
transition temperatures are due to the lower transition tem-
peratures of the materials containing the alicyclic group, re-
flecting the smaller conjugated π-system. The observed
smaller reductions in the crystallisation temperatures are in
line with the known tendency of alicyclic compounds to
form crystals, in contrast to aromatic groups.
Compound
Thermal transition temperatures/°C[a]
3
Cr 87.4 SmC 106.6 SmA 156.2 N 255.3 Iso
Cr 64.6 Iso
Cr 84.3 N 138.7 Iso
7
8
9
Cr 45.9 N 83.6 Iso
Ref 1
Ref 2
Ref 3
Cr 140.4 SmA 213.7 N 300 Iso
Cr 99.4 N 173.2 Iso
Cr 67.1 (SmA, 51.3) N 123.8 Iso
[a]
Cr: crystalline, SmC: smectic C, SmA: smectic A, N: nematic,
Iso: Isotropic liquid; ( ) denotes a monotropic transition
The liquid crystalline behaviour of I and II and their
photostationary states IPS and IIPS are given in Table 4.
Table 4. Transition temperatures as determined by DSC
Compound
Thermal transition temperatures/°C[a]
The importance of detailed molecular engineering in the
design of these systems is highlighted when compounds 7
and 8 (which only differ in the position of the central cyclo-
hexyl groups) are compared. 7 does not show any LC be-
haviour and melts at 64.6 °C to an isotropic liquid, because
the conjugated π-electron system in 7 is close to the lateral
connecting group. In 8, the cyclohexyl ring separates the π-
electron system from the connecting group, leading to a bet-
I
Tg Ϫ10.4 N 14.2 Iso
Tg 2.6 N 54.3 Iso
Tg Ϫ1.9 N 14.3 Iso
Tg 7.4 N 51.3 Iso
II
IPS
IIPS
[a]
Tg: glass transition, N: nematic, Iso: Isotropic liquid
For a discussion of the mesomorphic behaviour, compari- ter electronic interaction between adjacent molecules and
son with the fully aromatic mesogens is instructive (Fig- mesomorphic behaviour.
ure 6).[5b,5c] The linear molecule Ref 1 has a clearing point
The fusion of one or two molecules of 9 to the photoch-
of 300 °C, but the introduction of a lateral alkyl group (Ref
2) reduces the phase stability to 173 °C and results in the
loss of all of the more highly ordered LC phases. The intro-
duction of a siloxane group (Ref 3) reduces the clearing
point to 123.8 °C, at which point a more highly ordered LC
phase (SmC) reappears as a thermodynamically non-stable
phase. Overall, the temperature at which the phases are
stable is reduced by about 175 °C, and the melting tempera-
ture is reduced by about 70 °C.
romic core leads to the target materials I and II. In the case
of I, which contains one mesogenic and one photochromic
group, a nematic phase was observed, which clears at 14.2
°C and vitrifies at Ϫ10.4 °C. This results in a liquid crystal
phase range of 24 °C at a temperature about 70 °C lower
than is required for 9. However, the overall impact on the
mesomorphic behaviour is smaller than for 9 and 7, because
both the transition and vitrification temperatures have been
modulated. Indeed, under photostationary conditions, the
systems clear at the same temperature (14.3 °C), but the
glass transition temperature is higher, which can be attri-
buted to a decrease in flexibility of the photochromic core
caused by the electrocyclisation. This is an indication of a
good decoupling of LC properties and mesomorphic behav-
iour, enabling them to be addressed independently.
For the disubstituted system II, the adverse effect of the
photochromic core on the mesomorphic behaviour is
smaller; it clears from the nematic state at 54.3 °C, which is
about 30 °C lower than in 9. However, the system has a
glass transition temperature of 2.6 °C, leading to a nematic
phase range of more than 50 °C, spanning the ambient tem-
perature range. The disubstituted system II at PS clears at
a similar temperature (51.3 °C), whereas the glass transition
temperature is slightly (4.8 °C) higher at 7.4 °C, reflecting
the increased stiffness of the photochromic core, in line with
the results for I and structurally related systems.[1g,8]
Figure 6. Molecular structures of reference compounds
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Room Temperature Nematic Photoswitchable Liquid Crystals
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dried over magnesium sulfate, and then concentrated under re-
duced pressure. The residue was recrystallised from cold ethanol to
give pure white crystals of 2 (8.7 g, 87 %). M.p.: 213Ϫ215 °C (ref.[6]
213Ϫ215 °C). 1H NMR ([D6]DMSO, 400 MHz): δ ϭ 1.20Ϫ1.45
(m, 4 H, CH2), 1.69 (d, J ϭ 12.4 Hz, 2 H, CH2), 1.86 (d, J ϭ
12.4 Hz, 2 H, CH2), 2.29 (m, 1 H, Ha), 3.41 (m, 1 H, Hb), 4.54 (s,
1 H, OH), 6.64 (d, J ϭ 8.6 Hz, 2 H, ArH), 6.96 (d, J ϭ 8.6 Hz, 2
H, ArH), 9.12 (s, 1 H, ArOH) ppm.
Conclusions
The use of a stable photochromic and functionalised di-
arylethene unit in conjunction with a mesomorphic group
which does not absorb at the same wavelengths as the pho-
tochrome is the basis of this system. Crucial to suppressing
the crystalline state and decoupling the electronic and me-
somorphic properties is the nature of the linkage; in this
work we have reported spacers containing both methylene
and siloxane units.
In conclusion, the identification of the structural ele-
ments that are responsible for the LC properties and a con-
vergent synthesis based on the modularisation of functional
elements is an efficient method by which to investigate the
structure-property relationships of photoswitchable room-
temperature nematic liquid crystals. This approach can now
be extended to related materials.
Compounds 3, 4 and 5: Dry CH2Cl2 (400 mL) was added to a mix-
ture of 2 (10 g, 52 mmol) and 4-octyloxybenzoic acid (13.02 g,
52 mmol), followed by DMAP (0.66 g, 5.2 mmol) and diisopro-
pylcarbodiimide (6.35 g, 52 mmol). After stirring overnight, the
solvent was distilled off and the residue purified by column chro-
matography on silica gel (eluent: CH2Cl2) to give pure 3, 4, and 5
as white solids.
Compound 3: Yield: 8 %. Transition temperatures (°C): Cr 87.4
1
SmC 106.6 SmA 156.2 N 255.3 Iso. H NMR (CDCl3, 400 MHz):
δ ϭ 0.82 (m, 6 H, CH3), 1.10Ϫ2.17 (m, 32 H, CH2), 2.55 (m, 1 H,
Ha), 3.96 (m, 4 H, OCH2), 4.94 (m, 1 H, Hb), 6.84 (m, 2 H, ArH),
6.90 (m, 2 H, ArH), 7.07 (m, 2 H, ArH), 7.20 (m, 2 H, ArH), 7.94
(m, 2 H, ArH), 8.07 (m, 2 H, ArH) ppm. C42H56O6 (656.89): calcd.
C 76.79, H 8.59; found C 76.70, H 8.64.
Experimental Section
General Remarks: Melting points (°C) were measured in capillary
tubes on a Gallenkamp apparatus and are uncorrected. Transition
temperatures were measured using a Mettler FP52 heating stage
and FP5 control unit in conjunction with an Olympus BH2 polariz-
ing microscope and were confirmed using differential scanning
calorimetry (PerkinϪElmer DSC 7 with an indium standard). 1H
and 13C NMR spectra were recorded on a JEOL Lambda 400 spec-
1
Compound 4: Yield: 44 %. M.p.: 148Ϫ149 °C. H NMR (CDCl3,
400 MHz): δ ϭ 0.88 (m, 3 H, CH3), 1.10Ϫ1.82 (m, 16 H, CH2),
1.93 (2 H, d, J ϭ 12.8 Hz, CH2), 2.11 (2 H, d, J ϭ 11.7 Hz, CH2),
2.51 (m, 1 H, Ha), 3.68 (m, 1 H, Hb), 4.02 (t, J ϭ 6.6 Hz, 2 H,
OCH2), 6.95 (m, 2 H, ArH), 7.11 (m, 2 H, ArH), 7.25 (m, 2 H,
ArH), 8.13 (m, 2 H, ArH) ppm. C27H36O4 (424.57): calcd. C 76.38,
H 8.55; found C 76.25, H 8.62.
1
trometer (at 400 MHz for H NMR and 100 MHz for 13C NMR),
using tetramethylsilane as an internal standard. UV/Visible spectra
were recorded on a Cary50Ϫdiode array spectrophotometer. El-
emental analyses of products were carried out using a Fisons EA
1108 CHN analyzer. All reactions were monitored by thin-layer
chromatography carried out on 0.2 nm Merck silica gel plates (60F-
254). Column chromatography was performed on silica gel
(Fluorochem 35Ϫ70u 60A). The purity of all final compounds was
checked (and found to be Ͼ 99.5 %) by HPLC analysis using a
Merck-Hitachi system fitted with a Rainin Dynamax Microsorb
5µm silica column (25 cm long, 4.6 mm internal diameter). The
identification of previously reported compounds was made by com-
parision of their 1H NMR spectra and melting points with the
literature data.
Compound 5: Yield: 22 %. M.p.: 105.9 °C. 1H NMR (CDCl3,
400 MHz): δ ϭ 0.88 (m, 3 H, CH3), 1.10Ϫ1.90 (m, 21 H, CH2),
2.53 (m, 1 H, Ha), 4.02 (t, J ϭ 6.4 Hz, 2 H, OCH2), 4.11 (m, 1 H,
Hb), 6.93 (m, 2 H, ArH), 7.10 (m, 2 H, ArH), 7.25 (m, 2 H, ArH),
8.12 (m, 2 H, ArH) ppm. C27H36O4 (424.57): calcd. C 76.38, H
8.55; found C 76.20, H 8.64.
Compound 7: Dry CH2Cl2 (60 mL) was added to a mixture of 5
(3.50 g, 8.24 mmol) and 4-octyloxy-2-(pent-4-enyloxy)benzoic acid
6 (2.75 g, 8.24 mmol), followed by DMAP (0.1 g, 0.83 mmol) and
diisopropylcarbodiimide (1.0 g, 8.24 mmol). After stirring over-
night, the solvent was distilled off and then the resulting products
were purified by column chromatography on silica gel (eluent:
CH2Cl2) to give pure 7 as white solid (3.28 g, 35 %). M.p.: 64.6 °C.
1H NMR (CDCl3, 400 MHz): δ ϭ 0.88 (m, 6 H, CH3), 1.10Ϫ2.25
(m, 34 H, CH2), 2.62 (m, 1 H, Ha), 4.02 (m, 6 H, OCH2), 4.97 (m,
2 H, CH2ϭCH), 5.30 (m, 1 H, Hb), 5.80 (m, 1 H, CHϭCH2), 6.47
(m, 2 H, ArH), 6.96 (d, J ϭ 8.6 Hz, 2 H, ArH), 7.12 (d, J ϭ 8.6 Hz,
2 H, ArH), 7.28 (m, 2 H, ArH), 7.87 (d, J ϭ 8.6 Hz, 1 H, ArH),
8.12 (d, J ϭ 8.6 Hz, 2 H, ArH) ppm. C46H62O7 (726.98): calcd. C
76.00, H 8.60; found C 75.89, H 8.68.
Chemicals: Solvents (purchased from Fisher Chemicals) were dried
over molecular sieves and used without further purification. Oc-
tyloxybenzoic acid and 4-octyloxy-2-(pent-4-en-1-yloxy)benzoic
acid were synthesised according to the literature procedure.[7]
Photochemical Measurements: Absorption spectra were performed
in spectrometric grade cyclohexane at 25 °C (Ϯ0.2 °C). The cell
(optical pathlength 10 mm) was placed in a thermostatted copper
block inside the sample chamber of a Cary 50-diode array spectro-
photometer. An Oriel 250 W high pressure Hg/Xe lamp was used
for irradiation. Mercury lines of 313 and 546 nm were isolated by
passing through a monochromator (Oriel).
Compound 8: Dry CH2Cl2 (100 mL) was added to a mixture of 4
(5.23 g, 12 mmol) and 6 (2.75 g, 12 mmol), followed by DMAP
(0.17 g, 1.4 mmol) and diisopropylcarbodiimide (1.46 g, 12 mmol).
After stirring overnight, the solvent was distilled off and the residue
purified by column chromatography on silica gel (eluent: CH2Cl2)
to give pure 8 as a white solid (1.91 g, 22 %). Transition tempera-
trans-4-(4-Hydroxycyclohexyl)phenol
(2):
4-(4-Hydroxyphe-
nyl)cyclohexanone (1) (10 g, 52 mmol) was dissolved in methanol
(300 mL). NaBH4 (2.13 g, 58 mmol) was added at room tempera-
1
tures (°C): Cr 84.3 N 138.7 Iso. H NMR (CDCl3, 400 MHz): δ ϭ
ture in small portions over 2 hours. After stirring overnight, the 0.88 (m, 6 H, CH3), 1.10Ϫ2.33 (m, 34 H, CH2), 2.57 (m, 1 H, Ha),
mixture was cooled in an ice-water bath, and then quenched with 4.01 (m, 6 H, OCH2), 5.01 (m, 3 H, CH2ϭCH ϩ Hb); 5.87 (m, 1
aqueous HCl (10 %). The solution was extracted several times with H, CHϭCH2), 6.45 (m, 2 H, ArH), 6.96 (d, J ϭ 8.6 Hz, 2 H, ArH),
ethyl acetate. The resulting organic phase was washed with water, 7.12 (d, J ϭ 8.6 Hz, 2 H, ArH), 7.25 (m, 2 H, ArH); 7.82 (d, J ϭ
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M. Frigoli, G. H. Mehl
8.6 Hz, 1 H, ArH), 8.12 (d, J ϭ 8.6 Hz, 2 H, ArH) ppm. C46H62O7 (57600). C81H102F6O9S2Si2 (1469.96): calcd. C 66.18, H 6.99; found
FULL PAPER
(726.98): calcd. C 76.00, H 8.60; found C 76.08, H 8.54.
C 66.25, H 6.92.
Dimer (II): Yield: 71 %, as a mixture of parallel and antiparallel
conformers (p:ap, 33:67). Transition temperatures (°C): Tg 2.6 N
54.3 Iso. H NMR (CDCl3, 400 MHz): δ ϭ 0.01 (s, 24 H, SiCH3),
Compound 9: 1,1,3,3,5,5-Hexamethyltrisiloxane (3.26 g, 24.8 mmol)
was added to a solution of 8 (1.8 g, 2.48 mmol) in anhydrous tolu-
ene (200 mL). Dry air was bubbled through the solution for 2 min
and a solution of Karstedt’s catalyst (5 % Pt solution in xylene, 32
µL) was added. After stirring the solution at room temperature
overnight with the exclusion of moisture, the solvent and the excess
of 1,1,3,3,5,5-hexamethyltrisiloxane were distilled off. The crude
product was purified by column chromatography on silica gel (elu-
ant: pentane/dichloromethane, 80:20) to give pure 9 as a white solid
(1.64 g, 77 %). Transition temperature (°C): Cryst 45.9 N 83.6 Iso.
1H NMR (CDCl3, 400 MHz): δ ϭ 0.01 (s, 6 H, SiCH3), 0.08 (d,
J ϭ 2.8 Hz, 6 H, SiCH3), 0.53 (m, 2 H, SiCH2), 0.81 (m, 6 H,
CH3), 1.10Ϫ1.80 (m, 32 H, CH2), 1.93 (m, 2 H, CH2), 2.18 (m, 2
H, CH2), 2.51 (m, 1 H, Ha), 3.96 (m, 6 H, OCH2), 4.60 (m, 1 H,
SiH), 4.94 (m, 1 H, Hb); 6.40 (m, 2 H, ArH), 6.89 (d, J ϭ 8.6 Hz,
2 H, ArH),), 7.06 (d, J ϭ 8.6 Hz, 2 H, ArH), 7.19 (m, 2 H, ArH);
7.76 (d, J ϭ 8.6 Hz, 1 H, ArH), 8.07 (d, J ϭ 8.6 Hz, 2 H, ArH)
ppm. C50H76O8Si2 (861.30): calcd. C 69.72, H 8.89; found C69.80,
H 8.86.
1
0.51 (m, 8 H, SiCH2), 0.82 (m, 12 H, CH3), 1.10Ϫ1.85 (m, 64 H,
CH2), 1.91 (m, 4 H, CH2), 2.05 (s, 3.98 H, ap CH3), 2.18 (m, 4 H,
CH2), 2.34 (s, 2.02 H, p CH3), 2.51 (m, 1 H, Ha), 3.91 (m, 16 H,
OCH2), 4.96 (m, 1 H, Hb); 6.40 (m, 4 H, ArH), 6.73 (dd, J ϭ 8.9
and 2.2 Hz, 0.67 H, p H-5), 6.89 (m, 5.33 H, 4 ϫ ArH ϩ 1.33 ap
H-5), 6.97 (0.67 H, d, J ϭ 2.2 Hz, p H-7), 7.06 (5.33 H, m, 4 ArH
ϩ 1.33 ap H-7), 7.19 (m, 4 H, ArH), 7.31 (0.67 H, d, J ϭ 8.9 Hz,
p H-4), 7.44 (1.33 H, d, J ϭ 8.9 Hz, ap H-4), 7.77 (d, J ϭ 8.6 Hz,
2 H, ArH), 8.07 (d, J ϭ 8.6 Hz, 4 H, ArH) ppm. UV/Vis (cyclohex-
ane): λmax (ε) ϭ 258 (79000). C131H176F6O18S2Si4 (2331.27): calcd.
C 67.49, H 7.70; found C 67.40, H 7.72.
Acknowledgments
We thank the EPSRC for funding.
Compound 11: K2CO3 (0.20 g, 1.41 mmol) and 4-bromobut-1-ene
(0.42 g, 3.10 mmol) were added to a solution of 10[1g,8] (0.72 g,
1.41 mmol) in dry butanone (14 mL). The resulting mixture was
refluxed for 24 h and then allowed to cool to room temperature.
The reaction mixture was filtered and the butanone was distilled off
under reduced pressure. The crude product was purified by column
chromatography on silica gel (eluent: CH2Cl2) to give pure 11 as a
glassy material (0.74 g, 84 %) as a mixture of parallel and antiparal-
lel conformers (p:ap, 36:64). 1H NMR (CDCl3, 400 MHz): δ ϭ 2.05
(s, 3.84 H, ap CH3), 2.33 (s, 2.16 H, p CH3), 2.47 (m, 4 H, CH2),
3.88 (s, 2.16 H, p OCH3), 3.95 (s, 3.84 H, ap OCH3), 5.07 (m, 4 H,
CH2ϭCH), 5.81 (m, 1 H, CHϭCH2), 6.70 (dd, J ϭ 8.9 and 2.2 Hz,
0.72 H, p H-5), 6.90 (dd, J ϭ 8.8 and 2.2 Hz, 1.28 H, ap H-5), 6.97
(d, J ϭ 2.2 Hz, 0.72 H, p H-7), 7.06 (d, J ϭ 2.2 Hz, 1.28 H, ap H-
7), 7.31 (d, J ϭ 8.9 Hz, 0.72 H, p H-4), 7.44 (d, J ϭ 8.9 Hz, 1.28
H, ap H-4) ppm. C31H26F6O2S2 (608.66): calcd. C 61.17, H 4.31;
found C 61.35, H 4.25.
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Monomer (I): Yield: 7 %, as a mixture of parallel and antiparallel
conformers (p:ap, 33:67). Transition temperatures (°C): Tg Ϫ10.4
N 14.2 Iso. 1H NMR (CDCl3, 400 MHz): δ ϭ 0.01 (s, 12 H,
SiCH3), 0.51 (m, 4 H, SiCH2), 0.83 (m, 6 H, CH3),1.10Ϫ1.85 (m,
32 H, CH2), 1.92 (m, 2 H, CH2), 2.05 (s, 4 H, ap CH3), 2.18 (m, 2
H, CH2), 2.34 (s, 2 H, p CH3), 2.51 (m, 1 H, Ha), 3.96 (m, 10 H,
OCH2), 4.96 (m, 3 H, CH2ϭCH ϩ Hb); 5.85 (m, 1 H, CHϭCH2),
6.40 (m, 2 H,, ArH); 6.70 (dd, J ϭ 8.9 and 2.2 Hz, 0.67 H, p H-
5), 6.89 (m, 3.33 H, 2 ϫ ArH ϩ 1.33 ap H-5), 6.97 (d, J ϭ 2.2,
0.67 H, p H-7), 7.06 (m, 3.33 H, 2 ArH ϩ 1.33 ap H-7), 7.19 (m,
2 H, ArH), 7.31 (d, J ϭ 8.9 Hz, 0.67 H, p H-4), 7.44 (d, J ϭ 8.9 Hz,
1.33 H, ap H-4), 7.77 (d, J ϭ 8.6 Hz, 1 H, ArH), 8.07 (d, J ϭ
8.6 Hz, 2 H, ArH) ppm. UV/Vis (cyclohexane): λmax (ε) ϭ 258
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Received August 14, 2003
642
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Eur. J. Org. Chem. 2004, 636Ϫ642