The design and investigation of porphyrins with liquid crystal properties at room temperature

Article abstract of DOI:10.1039/c2tc00286h

The design, synthesis and investigation of a series of porphyrins displaying smectic liquid crystal phase behaviour at room temperature are described. The molecules are constructed in a modular approach, consisting of porphyrin units linked via hydrocarbon and siloxane spacers to calamitic cyanobiphenyl units. The investigation of the phase behaviour, based on optical polarizing microscopy, differential scanning calorimetry and XRD studies, is discussed in detail.

Full text of DOI:10.1039/c2tc00286h

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Materials Chemistry C
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The design and investigation of porphyrins with liquid
crystal properties at room temperature†
Cite this: J. Mater. Chem. C, 2013, 1,
144
*
*
Christopher J. Wilson, Daniela A. Wilson,‡ Ross W. Boyle and Georg H. Mehl
The design, synthesis and investigation of a series of porphyrins displaying smectic liquid crystal phase
behaviour at room temperature are described. The molecules are constructed in a modular approach,
consisting of porphyrin units linked via hydrocarbon and siloxane spacers to calamitic cyanobiphenyl
units. The investigation of the phase behaviour, based on optical polarizing microscopy, differential
scanning calorimetry and XRD studies, is discussed in detail.
Received 26th September 2012
Accepted 8th November 2012
DOI: 10.1039/c2tc00286h
www.rsc.org/MaterialsC
previously. It was demonstrated that by attachment of suitable
substituents on the two phenyl rings a calamitic type behaviour
Introduction
could be induced.10 Melting points of calamitic type LC
porphyrins were found to be high, however, incorporation of
secondary substituents on the 2-positions of the phenyl ring has
reduced transition temperatures considerably, pointing thus
towards a methodology for modifying such materials, resulting
in lower liquid crystal phase transition temperatures.11
Porphyrins play a vital role in photosynthesis, cellular respira-
tion, oxygen transport, and as components of metalloenzymes.1
The functions performed by porphyrins in natural systems have
driven research into the use of synthetic porphyrins in many
areas including articial photosynthetic materials,2 as oxidation
catalysts3 and in photodynamic therapy.4 Incorporation of
porphyrins into functional materials is an area which has
received considerable attention, due to the possibility of using
these materials to fabricate devices, which retain some of the
photochemical and redox properties associated with porphyrins
and their metal complexes. Porphyrins have previously been
incorporated into bulk materials either by electro-
polymerisation onto electrodes, where they have shown appli-
cations in the electrochemical activation of thiols,5 or by
incorporation into polymers for use as antibacterial materials.6
Octa-substituted porphyrins with liquid crystalline (LC) behav-
iour were rst reported more than thirty years ago with the
materials having only a very narrow phase range, however, over
the years the stability of the mesophase has been expanded
considerably.7 Tetraphenyl porphyrins with LC behaviour have
also been reported and for such materials LC behaviour close to
room temperature has been described.8 The predominant LC
behaviour found with both octa-substituted and tetraphenyl
porphyrins is columnar, due to the planar discotic structure of
the porphyrin macrocycle.9
To date there are no single molecule based examples of
calamitic porphyrins exhibiting liquid crystalline phases at or
close to room temperature. The aim of the work reported here
was to obtain porphyrins which show smectic phase behaviour
and to devise a systematic route towards this goal. To achieve
this objective the porphyrin macrocycle was extended in one
direction, destabilizing potential columnar phase formation
completely. An additional target was to control the transition
temperatures with application accessible phase transitions
being the focus. Moreover we demonstrate that the concept is
architecturally robust and that LC behaviour can be obtained
with or without metals in the central cavity.
Results and discussion
As a core structure 5,15-di(4-hydroxyphenyl)porphyrin was
selected. The porphyrin core is essentially planar and the two
phenyl rings positioned in the 5 and 15 positions whilst
essentially being orthogonal to the porphyrin have some rota-
tional exibility. In order to induce calamitic phase behaviour
when functionalizing an essentially discotic centre, the attach-
ment of calamitic mesogens using a exible spacer was
employed. This has been explored for a number of systems12
and has been reviewed recently.13
In order to promote low melting points, or the formation of
glass transitions a tetramethyl disiloxane group was introduced
into the spacer unit separating the porphyrin from the calamitic
mesogenic unit. The spacer units connecting porphyrin to
siloxane, and siloxane to mesogen groups were selected to be of
In order to obtain the liquid crystalline phase behaviour with
a layer like register and to break the dominating effect of the
macrocycle on mesophase formation, a number of zinc(II)
complexes of 5,15-diphenyl porphyrins have been explored
Department of Chemistry, University of Hull, Kingston-upon-Hull, East Yorkshire, HU6
7RX, UK. E-mail: g.h.mehl@hull.ac.uk; r.w.boyle@hull.ac.uk
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2tc00286h
‡ Current address: Institute for Molecules and Materials, Radboud University
Nijmegen, Heyendaalseweg 135, 6525 AJ, Nijmegen, The Netherlands.
144 | J. Mater. Chem. C, 2013, 1, 144–150
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different lengths to ensure a liquid like character of the spacer hand, the rst hydrosilylation was attempted. Alkene 3 was
over a very wide temperature range. One of the spacers was functionalised with tetramethyldisiloxane using platinum(0)-
selected to consist of a pentyl chain, the other of an undecyl 1,1,3,3-tetramethyl-1,3-divinyldisiloxane (Karstedt’s catalyst) to
chain to induce a lowering of melting points and glass transi- yield the silylated compound 4 in a yield of 90% according to
tion temperatures, by disfavouring antiparallel assembly of literature procedures.18 Siloxane 4 was further reacted with
sublayers of the nal materials. Such an approach has been porphyrin 2, again in the presence of Karstedt’s catalyst. Thin
found to be very versatile, particularly when using the cyano- layer chromatographic investigation of the reaction product
biphenyl unit to promote the calamitic phase behaviour.14
revealed two major products, which were separated by column
chromatography. Interestingly, it was found that the two reac-
tion products, which were obtained in approximately equal
amounts, were the desired di-hydrosilylated product 5 and a
mono-hydrosilylated product 6 with the remaining terminal
double bond internalised to a less reactive form observed in the
1H NMR as a multiplet at 5.4 ppm. Although the yield of the di-
hydrosilylated porphyrin (50%) was somewhat disappointing,
the overall yield of both di- and mono-hydrosilated products
was excellent (90%). The hydrosilylation reaction of porphyrin 2
thus generated two potentially interesting compounds for
study, both in acceptable yields.
Synthesis
In order to explore the role of mesogenic groups on the liquid
crystalline properties of 5,15-diphenyl porphyrins, we sought to
devise a versatile method by which these groups could be
attached easily to the common porphyrin core. Hydrosilylation
of both the porphyrin and mesogens could be easily achieved
via alkyl chains possessing terminal double bonds distal to the
porphyrin core; this encouraged us to explore the route
further.15 The 5,15-di(4-hydroxyphenyl)porphyrin core was syn-
thesised according to the procedure of Boyle et al.16 to yield
compound 1 and functionalised by Williamson etherication
with 11-bromoundec-10-ene to yield 5,15-di(11-endec-1-enyl-
DSC and optical polarising microscopy
phenyl)porphyrin, compound 2 (Scheme 1). The rst mesogenic Porphyrins 5 and 6 were investigated rst by differential scan-
unit bearing a terminal alkene was prepared according to ning calorimetry (DSC) (Fig. 1 and ESI Fig. 2†) and their thermal
literature procedures from 4-cyano-40-hydroxy biphenyl and 1- properties are summarized in Table 1. The mono hydrosilylated
ꢁ1
ꢀ
bromopent-4-ene using similar etherication conditions to porphyrin 6 showed a rst transition at 105 C (3.0 kJ mol
)
those used for the porphyrins to yield 40-(pent-4-enyloxy) and a second transition at 163 ꢀC (9.5 kJ molꢁ1), while the
biphenyl-4-carbonitrile 3.17 With the two coupling partners in dihydrosilylated porphyrin 5 showed a single transition at
Scheme 1 Reagents and conditions: (i) K2CO3, butanone, KI, C5H9Br, (ii) K2CO3, butanone, KI, C11H21Br and (iii) Karstedt’s cat., toluene, 1,1,3,3-tetramethyldisiloxane.
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Fig. 2 Optical polarising microscopy image of 5 at 20 ꢀC (magnification 100ꢂ).
behaviour at room temperature (Fig. 2), only transforming to an
isotropic liquid at 178 ꢀC, arguably a very wide liquid crystal
phase range.
The most striking feature of increasing the number of
calamitic mesogenic groups from one in 6 to two in 5 is not the
relatively small increase in the stability of the mesophase by
15 ꢀC, but the strong destabilisation of the crystalline phase
present in 6 but absent in 5 and the formation of a very wide
range SmA phase. Based on the positive results obtained for the
free base porphyrins this method was extended to explore the
effect of complexation of the porphyrin with metal ions on
the liquid crystal phases.
Fig. 1 Differential scanning calorimetry plots for (a) compound 6 and (b)
compound 5.
To achieve this, porphyrin 2 was metalated with zinc(II)
acetate and nickel(II) acetate respectively using previously pub-
lished conditions.19 Formation of the metal complex was evi-
denced by the collapse of the four Q-bands in the UV-spectrum
along with the loss of the aminal protons of the macrocycle at
Table 1 Differential scanning calorimetry results for liquid crystalline 5,15-
diphenyl porphyrins and metal complexes
1
ꢁ3 ppm in the H NMR.19 Hydrosilylation of the two resulting
Transition temperatures (ꢀC),
metallo-porphyrins, Zn(II)2 and Ni(II)2, proceeded smoothly and
in a similar manner to that observed for the metal free
porphyrin 2. Approximately equal amounts of the di-hydro-
silylated products, 5-Zn(II) and 5-Ni(II), and mono-hydrosilylated
products, 6-Zn(II) and 6-Ni(II), were obtained. DSC analysis of
the metal complexes indicated the formation of liquid crystal-
line behaviour for all of the hydrosilylated porphyrin products,
however the di-substitution products 5-Zn(II) and 5-Ni(II) both
showed liquid crystalline behaviour at lower temperatures,
when compared with their mono-substituted analogues
(Table 1). The zinc dimer 5-Zn(II) exhibits a transition at 145 ꢀC
turning from a SmA phase to an isotropic liquid, the corre-
sponding nickel dimer undergoes this transition at 157 ꢀC. The
nickel complex 5-Ni(II) shows a low temperature crystalline or
partially crystalline phase with a melting point of 56 ꢀC. It is
noted that the transition enthalpy with a value of 10 kJ molꢁ1 is
relatively low for a crystal to liquid crystal transition, suggesting
the possible formation of a higher ordered lamellar phase. The
lower transition temperatures for the metalated porphyrins 5-
Zn(II) and 5-Ni(II) when compared to the bare porphyrin 5
indicates that the electron density associated with the incor-
poration of metal ions does not enhance LC properties, indeed
Compound
6
enthalpies (DH/(kJ molꢁ1))
Cr 105 (3.0) SmA 163 (9.5) Iso
Iso 162 (ꢁ9.5) SmA glass
Cr 117 (9.2) SmA 150 (10.2) Iso
Iso 149 (ꢁ8.8) SmA 93 (ꢁ3.5) SmX
Cr 93 (1) SmA 163 (10.4) Iso
Iso 162 (ꢁ10.1) SmA glass
SmA 178 (18) Iso
6-Zn(II)
6-Ni(II)
5
Iso 178 (ꢁ19) SmA
5-Zn(II)
5-Ni(II)
SmA 145 (9.5) Iso
Iso 145 (ꢁ9.5) SmA
Cr 56 (10.0) SmA 157 (16.0) Iso
Iso 157 (ꢁ16) SmA glass
178 ꢀC (18 kJ molꢁ1). Further investigation by optical polarising
microscopy (OPM) (Fig. 2 and ESI Fig. 3–5†) indicated that, in
the case of 6, the two phase changes observed are the crystalline
to SmA transition at 105 ꢀC and the SmA to isotropic liquid
ꢀ
transition at 163 C.
Porphyrin 5, bearing two mesogenic cyanobiphenyl groups
linked by alkyl chains and siloxy groups, exhibited SmA
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the metal centre reduces the LC phase stability. For the metallo-
porphyrins containing only one calamitic mesogenic group the
difference in the isotropisation temperatures was around 13 ꢀC.
X-ray diffraction results
In order to elucidate the mechanism of self-assembly operating
in these porphyrin based LC systems using X-ray diffraction
patterns datasets were collected spanning both the small and
wide angle regions. The data gathered from the small angle
region allowed for the phase structure to be elucidated whereas
the wide angle region delivers crucial information on the nature
of the ordering in these systems. The symmetric and non-
symmetric structures, bearing two and one cyanobiphenyl
groups respectively, were analysed by X-ray diffraction (Fig. 4
and ESI Fig. 6 and 7†). A representative example is shown for
compound 5 and the corresponding d-spacings are presented in
Table 2. The temperature dependent layer spacing for the 01
reection is shown in Fig. 5. In all cases for the highest
temperature phase three sharp reections were observed,
ꢀ
ꢀ
The melting points vary from 93 C for 6-Ni(II) to 117 C for 6-
Zn(II); 6-Ni(II) can be supercooled to a room temperature glassy
state, whereas 6-Zn(II) at 93 ꢀC forms a monotropic higher
ordered LC phase, which will be discussed in more detail below.
Notable is that the metal containing dimers exhibit lower
isotropisation temperatures than the respective mono-func-
tionalized materials. At present there is no conclusive expla-
nation for this behaviour, though it can be surmised that in the
dimeric materials a layer register is induced at both ends of the
molecules, and, given the detrimental effect of the metal centres
on the LC phase behaviour, this induces a stability penalty,
when compared to the mono-substituted compounds where the
porphyrin ends have more freedom to pack efficiently.
Optical polarising microscopy
A striking observation for all of the investigated materials was
the ease of LC texture formation, a feature more typical for low
molar mass liquid crystals, with the rapid formation of typical
defect textures when cooled from the isotropic state, an
encouraging feature of the molecular design approach adopted
here. An example is shown in Fig. 2, where the focal texture of 5
in the SmA phase is depicted, the red colouring is due to the
absorption properties of the porphyrin core in the visible region
of the electromagnetic spectrum. The low temperature liquid
crystal phase of 6-Zn(II) is characterized by the rapid formation
of a schlieren texture, when cooled, evolving as a front in the
Fig. 4 (a) 2D powder diffraction of the non-symmetric compound 6 and (b) 2D
homeotropic areas of the preceding SmA phase (Fig. 3 and ESI powder diffraction of symmetric compound 5; inset: expansion of the small angle
region.
Fig. 5†).
Additionally a ngerprint like texture developed from the
focal conic and fan areas of 6-Zn(II). The formation of schlieren
and ngerprint texture suggests the formation of a higher
ordered tilted smectic phase. Though these ngerprint regions
were not investigated in great detail, it is noted that aer
formation no signicant change in the apparent spacings of the
ngerprints with changing the temperature was observed.
Table 2 d-Spacings for the high temperature SmA phase of compound 5
q nmꢁ1
d-Spacing A
˚
Peak
001
002
003
0.199
0.399
0.599
31.4
15.7
10.5
Fig. 3 Optical polarising microscopy image showing the transition from the SmA
to SmX phase in compound 6-Zn(II) on cooling from 130 ꢀC to 92 ꢀC (magnifi-
cation 100ꢂ).
Fig. 5 Temperature dependent layer spacings for compound 5.
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Table 3 d-Spacings for the high temperature SmA phases
q nmꢁ1
d-Spacing A
˚
Compound
5
6
0.199
0.196
0.191
0.191
0.200
0.200
31.4
32.1
32.9
32.9
31.4
31.4
Zn(II)5
Zn(II)6
Ni(II)5
Ni(II)6
corresponding to the 01, 02 and 03 reections of a layered
structure together with a characteristically broad halo in the
wide angle region. The q-values of the collected intensity
maxima and the associated d-spacings for the small angle
region of the symmetric compound 5 are listed in Table 2. The
layer spacings corresponding to the 01 reection for all
compounds are shown in Table 3.
Fig. 6 Example of the least squares fit to the broad scattering of compound 5.
Although the experiments were carried out in the presence of
a ꢃ0.5 T magnetic eld perpendicular to the capillary axis,
complete preferential axis alignment was not obtained for any
of the samples, though some preferential alignment could be
observed with the meridional small angle and the equatorial
wide angle intensities being stronger.
This would suggest a minimal preferential orientation of the
molecules parallel to the magnetic eld. Temperature depen-
dent XRD studies showed that a single liquid crystal phase
structure for almost all of the materials (exception high ordered
low temperature phase for Zn(II)6) was observed throughout the
mesophase range. All observations were in good agreement with
the DSC data and no discernible change in the layer spacing was
seen, an example for temperature dependent behaviour is
shown in Fig. 5. Radial integration of the wide angle region of
the 2D diffractograms for the high temperature phase showed a
crystals displaying smectic phase morphology has been repor-
ted previously and attributed to in plane correlations in the SmA
phase.20 It was possible to t the broad peak to three Lorentzian
type peaks by least squares tꢁti1ng. The peaks are centred
ꢁ1
ꢁ1
˚
˚
˚
˚
˚
˚
around q: 1.25 A ¼ 5.0 A, 1.44 A ¼ 4.4 A and 1.68 A ¼ 3.7 A
respectively (Fig. 6). In line with earlier work this asymmetric
peak appearance has been attributed as follows: the rst of the
˚
three peaks at q ¼ 1.25 ¼ 5.0 A is associated with the liquid
correlations of the linear calamitic aromatic groups in the SmA
ꢁ1
˚
˚
phase. The remaining peaks at q ¼ 1.44 A
¼ 4.4 A, the
dominant peak in the wide angle region to theꢁla1teral distance
˚
˚
of the molten aliphatic chains, and q ¼ 1.68 A ¼ 3.7 A were
attributed to intra-layer correlations of the large porphyrin
ꢁ1
˚
groups. It is noted that the value of the peak at q ¼ 1.68 A
¼
˚
3.7 A is at the value typically observed for the separation of
extended aromatic ring systems in columnar phases.13 It can
ꢁ1
˚
characteristic main broad peak centred around q ¼ 1.4 A
¼
˚
˚
4.4 A. Closer inspection revealed that the peak was not
symmetric in nature. The presence of multiple peaks in the
wide-angle region of the XRD diffractograms of polyphilic liquid
clearly be seen that the 4.4 A peak dominates over the much less
˚
intense peak at 3.7 A. For the low temperature smectic phase of
6-Zn(II) observed by both optical polarising microscopy and
Fig. 7 Composite models showing three potential molecular packing models and the corresponding cross-section profiles A(x).
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DSC, as well as X-ray diffraction experiments, it was not possible
to assign a LC phase structure unambiguously. The XRD results
indicate clearly the absence of a hexatic ordering and a partial
crystallisation of sublayers as has been observed earlier (see
ESI†).12a,c
Molecular organisation
A model for the molecular organisation has to take into account
the observation that the observed layer spacings are much
˚
˚
smaller than the estimated molecular lengths of 91 A and 68 A
respectively (see ESI†). Moreover the enhanced in-plane
ordering imparted by the large planar aromatic core of the
porphyrin macrocycle needs to be considered.20 Using the
collected XRD data and well established methods the recon-
structed electron density proles were calculated.21 The models
are described in detail in the ESI† (Fig. 13 and 14). A number of
likely arrangements are possible and they are outlined in Fig. 7.
The three principal assembly modes are: sub-layer forma-
tion with regions of concentration of cyanobiphenyl-, siloxane-
and porphyrin groups with some disorder in the hydrocarbon
groups as shown in Fig. 7a (only one half of a molecule is
shown), a sublayer formation as shown in Fig. 7a but with
strongly tilted and highly ordered aliphatic chains (Fig. 7b) and
the formation of antiparallel correlations as shown in Fig. 7c.
Some related schematic assembly models are shown in Fig. 8.
Though principally possible, antiparallel correlations shown in
Fig. 7c resulting in a self-assembly behaviour shown in Fig. 8b
seem to be the most unlikely model of the series. This is due to
the smallest modulation in the estimated electron densities
and mainly as there is very little shape amphiphilic micro-
segregation and a co-facial assembly of the disks which has
been observed for multimesogenic systems, consisting of LC
groups of different types before.12a,c A model of micro-
segregated rod shaped and disk shaped groups, as shown in
Fig. 8a, is in our view the most likely arrangement and the
building up of this assembly via some sub-layers with rather
disordered hydrocarbon chains as shown in Fig. 7a seems to be
the most likely case. This model also ts with the observation
that the materials containing only one calamitic mesogen
(materials 6, 6-Zn(II) and 6Ni-(II) have only slightly smaller layer
spacings than the materials 5, 5-Zn(II) and 5-Ni(II), they show
very similar OPM textures to the dimer mesogen materials and
full miscibility in the liquid crystal state. As previously dis-
cussed by Bruce et al.10,11 co-facial interactions of the porphyrin
macrocycles drive self-assembly in liquid crystalline 5,15-
diphenyl porphyrin systems. Where this mechanism predom-
inates, defects arising from the asymmetric substitution
pattern in the molecule are much better tolerated than in a
head to tail motif where inter-layer interactions should be
much less favoured. It could even be expected that in this
conguration a nematic phase would arise. Hence the model
which is based on a co-facial arrangement of porphyrins with
distinct cyanobiphenyl sublayering prohibiting columnar
phase formation is favoured, as it can explain quite easily
the observed properties both for the symmetric and non-
symmetric compounds.
Fig. 8 (a) Proposed co-facial packing model for a single sub-layer unit cell and
the associated proposed bulk packing model. (b) Proposed head to tail packing
model showing a single sub-layer unit cell and the associated proposed bulk
packing model.
Conclusions
In this report we present the rst examples of liquid crystalline
porphyrin based materials displaying lamellar phases at room
temperature and being stable over large temperature ranges.
Furthermore we provide a model for the mechanisms driving
assembly in porphyrin based liquid crystalline systems. The
porphyrin macrocycle dominates liquid crystal phase formation
and strong interactions govern the nature of the phase formed.
Variation of the macrocycle structure and the connection to
calamitic mesogens via hydrocarbon siloxane spacers allow for
a suppression of the crystalline phase behaviour and the
ꢀ
formation of a SmA phase up to 178 C. The selected architec-
ture is robust, and the inclusion of metals does not change the
transitions signicantly. This design, maximizing metalated
porphyrin interactions within layers whilst controlling the
organisation of these moieties in smectic layers, might provide
a useful tool to enhance the utilisation of porphyrin based
materials.
Notes and references
1 (a) The Porphyrin Handook, ed. K. M. Kadish, K. M. Smith and
R. Guillard, Academic Press, San Diego, 2000; (b)
L. R. Milgrom, Colours of Life, Oxford University Press,
Oxford, 1997.
2 (a) S. D. Straight, G. Kodis, Y. Terazono, M. Hambourger,
T. A. Moore, L. A. Moore and D. Gust, Nat. Nanotechnol.,
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C, 2013, 1, 144–150 | 149

View Article Online
Journal of Materials Chemistry C
Paper
2008, 3, 280–283; (b) D. Gust, T. A. Moore and A. L. Moore,
Acc. Chem. Res., 2001, 34, 40–48.
3 Metalloporphyrins in Catalytic Oxidations ed. R. A. Sheldon,
Marcel Dekker Inc., New York, 1994.
4 (a) L. B. Josefsen and R. W. Boyle, Met.-Based Drugs, 2008, 1,
276109; (b) I. J. MacDonald and T. J. J. Dougherty,
J. Porphyrins Phthalocyanines, 2001, 5, 105–129; (c)
E. D. Sternberg, D. Dolphin and C. Bruckner, Tetrahedron,
1998, 54, 4151–4202.
7003–7006; (e) S. K. Pal, S. Kumar and J. Seth, Liq. Cryst.,
2008, 35, 521–525; (f) K. U. Jeong, A. J. Jing, B. Mansdorf,
M. J. Graham, D. K. Yang, F. W. Harris and S. Z. D. Cheng,
Chem. Mater., 2005, 19, 2921; (g) M. L. Rahman,
C. Tschierske, M. Yusoff and S. Silong, Tetrahedron Lett.,
2005, 46, 2303–2306; (h) C. H. Yu, C. Schubert, B. J. Tang,
C. Welch, M. G. Tamba and G. H. Mehl, J. Am. Chem. Soc.,
2012, 134, 5076–5079; (i) C. Schubert, M. G. Tamba and
G. H. Mehl, Chem. Commun., 2012, 48, 6851–6853.
5 S. Griveau, V. Albin, T. Pauporte, J. H. Fagal and F. Bedioui, 13 (a) S. Laschat, A. Baro, N. Steinke, F. Giesselmann, C. Hagele,
J. Mater. Sci., 2002, 12, 225–232.
6 J. Mosinger, O. Jirsak, P. Kubat, K. Lang and B. Mosinger,
J. Mater. Chem., 2007, 17, 164–166.
7 (a) J. W. Goodby, P. S. Robinson, B. K. Teo and P. E. Cladis,
Mol. Cryst. Liq. Cryst., 1980, 56, 303–309; (b) M. Castella,
G. Scalia, R. Judele, E. Kapatsina, S. Sauer, A. Schreivogel and
M. Tosoni, Angew. Chem., Int. Ed., 2007, 46, 4832–4887; (b)
C. T. Imrie and P. A. Henderson, Chem. Soc. Rev., 2007, 36,
2096–2124; (c) Chemistry of Discotic Liquid Crystals, ed. S.
Kumar, CRC Press, Boca Raton, 2011.
F. Lopez-Calahora and D. Velasco, Liq. Cryst., 2002, 29, 14 (a) G. H. Mehl and J. W. Goodby, Chem. Commun., 1999, 13;
559–565; (c) M. Castella, F. Lopez-Calahora, D. Velasco and
H. Finkelmann, Chem. Commun., 2002, 2348; (d) A. Segade,
M. Castella, F. Lopez-Calahora and D. Velasco, Chem.
(b) K. J. Shepperson, P. H. J. Kouwer, W. F. Jager, W. J. Mijs,
S. J. Picken and G. H. Mehl, Mol. Cryst. Liq. Cryst., 2004, 411,
387.
Mater., 2005, 17, 5366–5374; (e) C. M. Drain, A. Varotto and 15 (a) B. D. Karstedt, US Pat., 3775452, Dow Corning 1973; (b)
I. Radivojevic, Chem. Rev., 2009, 109, 1630–1658.
8 (a) B. A. Gregg, M. A. Fox and A. J. Bard, J. Am. Chem. Soc.,
D. Apreutesei and G. H. Mehl, J. Mater. Chem., 2007, 44,
4711–4715.
1989, 111, 3024–3029; A. Nowak-Krol, D. Gryko and 16 R. W. Boyle, C. Bruckner, J. Posakony, B. R. James and
D. T. Gryko, Chem.–Asian J., 2010, 5, 904–909. D. Dolphin, Org. Synth., 1999, 287–293.
9 (a) Y. Shimizu, A. Ishikawa and S. Kusabayashi, Chem. Lett., 17 K. J. Shepperson, T. Meyer and G. M. Mehl, Mol. Cryst. Liq.
1986, 7, 1041–1044; (b) A. S. Ahsen, A. Segade, D. Velasco and Cryst., 2004, 411, 1227–1233.
Z. Z. Ozturk, J. Porphyrins Phthalocyanines, 2009, 13, 1188– 18 R. Alvarez and G. H. Mehl, Mol. Cryst. Liq. Cryst., 2005, 439,
1195; J. Z. Li, T. Tang, F. Li and M. Li, Dyes Pigm., 2007, 77, 2125–2133.
395–401; (c) I. N. Fedulova, N. A. Bragina, N. V. Novikov, 19 T. P. Wijesekera and D. Dolphin in Metalloporphyrins in
A. F. Mironov, V. V. Bykova, N. V. Usul’tseva and
G. A. Ananeva, Mendeleev Commun., 2008, 18, 324–326.
Catalytic Oxidations, ed. R. A. Sheldon, Marcel Dekker Inc.,
New York, 1994. p218.
10 (a) D. W. Bruce, D. A. Dunmur, L. S. Santa and M. A. Wali, 20 L. M. Blinov, T. A. Lobko, B. I. Ostrovskii, S. N. Sulianov and
J. Mater. Chem., 1992, 2, 363–364; (b) D. W. Bruce, F. G. Tournilhac, J. Phys. II, 1993, 3(7), 1121–1139.
M. A. Wali and Q. M. Wang, J. Chem. Soc., Chem. Commun., 21 (a) G. Ungar, K. Nobel and V. Percec, J. Mater. Sci., 2000, 35,
1994, 2089–2090.
5241–5246; (b) B. M. Rosen, C. J. Wilson, D. A. Wilson,
M. Peterca, M. R. Imam and V. Percec, Chem. Rev., 2009,
109(11), 6275–6540; (c) B. M. Rosen, D. A. Wilson,
C. J. Wilson, M. Peterca, B. C. Won, C. Huang, L. R. Lipski,
X. Zeng, G. Ungar, P. A. Heingy and V. Percec, J. Am. Chem.
Soc., 2009, 131(47), 17500–17521; (d) M. Peterca,
M. R. Immam, P. Leowanawat, B. M. Rosen, D. A. Wilson,
C. J. Wilson, X. Zeng, G. Ungar, P. A. Heiney and V. Percec,
J. Am. Chem. Soc., 2010, 132(32), 11288–11305.
11 (a) Q. M. Wang and D. W. Bruce, Chem. Commun., 1996,
2505–2506; (b) Q. M. Wang and D. W. Bruce, Tetrahedron
Lett., 1996, 45, 7641–7644.
12 (a) P. H. J. Kouwer and G. H. Mehl, J. Mater. Chem., 2009, 19,
1564–1575; (b) P. H. J. Kouwer, J. Pourzand and G. H. Mehl,
Chem. Commun., 2004, 66; (c) P. H. J. Kouwer and G. H. Mehl,
Angew. Chem., Int. Ed., 2003, 42, 6015; (d) H. K. Bisoyi,
V. A. Raghuathan and S. Kumar, Chem. Commun., 2009,
150 | J. Mater. Chem. C, 2013, 1, 144–150
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