Heterolithic azobenzene-containing supermolecular tripedal liquid crystals self-organizing into highly segregated bilayered smectic phases

Article abstract of DOI:10.1039/c2jm33751g

Synthesis, self-organization, and optical properties of supermolecular tripedal liquid crystals incorporating various prototypical mesogenic units such as alkoxy-azobenzene (AZB), alkoxy-biphenylene (BPH) or alkoxy-cyanobiphenyl (OCB) derivatives are repo

Full text of DOI:10.1039/c2jm33751g

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PAPER
Heterolithic azobenzene-containing supermolecular tripedal liquid crystals
self-organizing into highly segregated bilayered smectic phases†
a
a
a
bc
Zsuzsanna T. Nagy, Benoıt Heinrich, Daniel Guillon, Jaroslaw Tomczyk, Joachim Stumpebc
^
a
*
and Bertrand Donnio
Received 11th June 2012, Accepted 16th July 2012
DOI: 10.1039/c2jm33751g
Synthesis, self-organization, and optical properties of supermolecular tripedal liquid crystals
incorporating various prototypical mesogenic units such as alkoxy-azobenzene (AZB), alkoxy-
biphenylene (BPH) or alkoxy-cyanobiphenyl (OCB) derivatives are reported. Different molecular
systems were designed in order to sequentially incorporate the smectogenic-like alkoxy-azobenzene-
based chromophore within the molecular structure, whose relative proportion is selectively varied by
exchanging with the other mesogens. A divergent synthetic mode was elaborated for their synthesis,
starting from the regioselective functionalization of the phloroglucinol-based (PG) inner core. This
methodology allowed the preparation of several sets of unconventional tripedal oligomers with
conjugated heterolithic structures (made of different blocks, e.g. PG6AZBxBPH3ꢀx and
PG6AZBxOCB3ꢀx, x ¼ 1 or 2) along the homolithic parents (all identical blocks, e.g. PGzAZB3, z ¼ 6
or 11, z is the number of methylene in the spacer between PG and the protomesogen, PG6BPH3, and
PG6OCB3), respectively. Essentially all the synthesized systems behave as thermotropic liquid crystals
and show various types of highly segregated multilayered smectic phases, or, in one case, a nematic
phase, depending on the nature of the constitutive anisotropic blocks and on the molecular topology
(homolithic versus heterolithic, mesogenic ratio x : 3 ꢀ x). The effects of these structural modifications
on the mesomorphism (mesophase structures, temperature ranges, and thermodynamic stability) have
been investigated by differential scanning calorimetry and small-angle X-ray diffraction experiments
combined with dilatometric measurements. Models describing the various supramolecular
organizations of these tripedes into such multilayered structures are proposed and discussed.
Preliminary results of the investigations of their optical properties will also be presented.
(LCs) materials continuously emerge as prime candidates due to
1. Introduction
their dynamic nature, function-integration and stimuli-respon-
siveness.2 This class of soft materials which combines order with
fluidity within various types of mobile and low-dimensional
periodic structures seems indeed suited in the future development
of organic-based organization-dependent technological (e.g.
optic- and electronic-based) and biological (molecular machines,
guest–host systems, sensors, catalysts, switches, signal amplifi-
cation devices, therapeutic and transfecting agents, etc.) appli-
cations.3 Moreover, the properties of the single molecules can be
transferred and considerably enhanced within ordered supra-
molecular soft assemblies and occasionally even new properties
and functions may emerge.4 The judicious choice and careful
molecular design of the primary constitutive building blocks
(chemical shape, interaction strength, specificity and direction-
ality) permit us to partially predict the nature of such dynamic
supramolecular assemblies, although the strict control of self-
assembling and self-organizing processes and the function
transfer from nano- to micrometric scale by molecular engi-
neering still remain challenging.
Supramolecular chemistry is a powerful tool to elaborate pre-
programmed self-assemblies of single molecules, interacting by
non-covalent intermolecular forces and specific molecular
recognition criterion, into highly complex and exquisite func-
tional networks mimicking living systems of interest in life and
material sciences.1 Among the diversity of materials able to self-
assemble into ordered supramolecular edifices, liquid-crystalline
aInstitut de Physique et Chimie des Mateꢀriaux de Strasbourg (IPCMS),
CNRS-Universiteꢀ de Strasbourg (UMR7504), 23 rue du Loess, BP43,
67034 Strasbourg Cedex 2, France. E-mail: bertrand.donnio@ipcms.
unistra.fr; Tel: +33 3 8810 7156
bFraunhofer Institute for Applied Polymer Research, Geiselbergstraße 69,
14476 Golm-Potsdam, Germany. Tel: +49 33 1568 1259
cUniversity of Potsdam, Department of Chemistry, Karl-Liebknecht-
Strasse 24-25, 14476 Golm-Potsdam, Germany
† Electronic supplementary information (ESI) available: TGA and DSC
curves, POM images, table of volume fractions, UV-Vis spectra, synthetic
details, and analytical and spectroscopic data. See DOI:
10.1039/c2jm33751g
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The majority of low-molecular weight thermotropic mesogens
possesses a prototypical molecular structure made of at least two
segments with contrasting chemical or structural character
(amphiphilicity),5 most often a rigid anisometric core, e.g. with a
rod-like or a disk-like shape, equipped with flexible alkyl or
generated in the heterolithic systems. The effects of the nature
of the constitutive anisotropic blocks (AZB, BPH, OCB) and
the molecular topology (homolithic versus heterolithic, meso-
genic ratio x : 3 ꢀ x) on the mesomorphism (mesophase struc-
tures and stability) are discussed and models describing the
various supramolecular organizations into such multilayer
structures are presented. Preliminary investigations of their
optical properties are also given.
perfluorinated end-chains.2 Such
a straightforward design
usually lends to nematic and/or smectic phases for the former
and to columnar mesophases for the latter. The discovery that
other types of amphipathic molecules based on different struc-
tural paradigms, such as those made of a bent core6 or with
poliphilic T- and X-shaped7,8 architectures, was also compatible
with mesomorphism (generation of new families of LC phases
e.g. polar phases6 and polygonal honeycombs,7,8 respectively),
revived considerable interest in the design of molecules for pre-
programmed self-assemblies with increasing complexity.9 Alter-
natively, driven by the need to expand further the range of
mesomorphic organizations and to develop original and multi-
functional materials, oligomeric molecular systems, with
perfectly controlled geometry (symmetrical or not) and identifi-
able molecular sub-units, have also been considered as poten-
tially interesting candidates.10–13 In addition to their intrinsic
advantages, such intricate molecular systems, which combine
traits of discrete low-molecular-weight materials (monodisperse)
with those of polymers (various degrees of conformational flex-
ibility, size-modulated viscosity, glass transition, etc.), represent
indeed an attractive way of adding functionality (e.g., multi-
functional and high control of functionality specificity and
hierarchy) and in designing ‘‘multitask materials’’ with tunable
properties (combination of properties of the inserted molecular
species, induction of cooperative and synergistic behavior by self-
organization).14 The basic structure of these functional super-
molecules usually consists of protomesogenic units linked
together through flexible spacers (alkyl, siloxane chains, etc.) to
generate linear oligomeric systems,10,11,14 or alternatively via a
central focal multivalent node to yield molecular structures with
branched architectures such as polypedes12 and dendrimers.13
The structural diversity offered by such supermolecular LCs
(connectivity, topology, restricted flexibility) is translated into
the nature of the supramolecular organizations formed which
often present unusual morphologies induced by the segregation
at the molecular level between the various building species.10–14
For this study, unconventional liquid-crystalline tripedal
molecular systems with homogeneous and heterogeneous
structures were synthesized. A specific divergent synthesis was
elaborated in order to selectively incorporate the proto-
mesogenic alkoxy-azobenzene-based chromophore within the
supermolecular structure. As such, the relative proportion
between alkoxy-azobenzene (AZB) and alkoxy-biphenylene
(BPH) or alkoxy-cyanobiphenyl (OCB) mesogens, respectively,
could be varied stepwise in a controlled manner. Thus, this
synthetic methodology afforded the preparation of several sets
of phloroglucinol (PG)-based tripedal oligomers with homo-
lithic (all identical blocks, e.g. PG11AZB3, PG6AZB3,
PG6BPH3, and PG6OCB3) and conjugated heterolithic
2. Results and discussion
2.1. Design and synthesis
The various homolithic and heterolithic phloroglucinol-based
(PG)
trimeric
materials,
hereafter
abbreviated
as
PGzAZBxBPH3ꢀx or PGzAZBxOCB3ꢀx, where z ¼ 6 or 11 is the
number of methylene units within the alkyl spacer between PG
and the peripheral mesogenic units, were prepared by a divergent
protocol involving the repetition of etherification reactions from
phloroglucinol (homogeneous PG11AZB3 trimer) or hydroxyl
protected phloroglucinol (PG6-based trimers). The mesogenic
units e.g. 4-alkoxy-azobenzene (AZB), 4-alkoxy-biphenyl (BPH)
and 4-alkoxy-cyanobiphenyl (OCB) based-groups were chosen
for their specific molecular structures, easy synthetic access and
tendency to form liquid-crystalline mesophases.2 In order to tune
and eventually compare both mesomorphic and optical proper-
ties of these materials, the AZB/BPH or AZB/OCB ratios
(x : (3 ꢀ x)) were varied within the supermolecular structure
according to the sequence 3 : 0 / 2 : 1 / 1 : 2 / 0 : 3.
The synthesis of the two homolithic trimers incorporating
azobenzene functional groups, PG6AZB3 and PG11AZB3, was
carried out according to two different protocols. The diver-
gent synthesis of PG11AZB3 was achieved in two steps by the
complete etherification of phloroglucinol from the commer-
cially available 11-bromo-1-undecanol via Mitsunobu reac-
tion, and the subsequent Williamson etherification of the
resulting tribromide derivative (1) with 4-hydroxy-40-(unde-
cyloxy)azobenzene (2), prepared beforehand (Scheme 1). The
synthesis of PG6AZB3 was also performed by a divergent
procedure but involved the preparation of the difunctionalized
intermediate ‘‘PG6AZB2OH’’ (5), as a useful precursor for the
synthesis
of
the
corresponding
mixed
materials
(PG6AZB2BPH, PG6AZB2OCB, vide infra, Scheme 2). This
procedure included firstly the double etherification of 3,5-
dihydroxyphenyl benzoate (3) by 6-bromo-1-hexanol via
Mitsunobu reaction, the etherification of the end-bromide
derivative (4) with 4-hydroxy-40-(undecyloxy)azobenzene (2),
followed by the hydrolysis of the protecting benzoate group to
lead to the hydroxyl PG derivative (5). Then, in the second
stage, the ether coupling of PG6AZB2OH with excess of 1,6-
dibromohexane resulted in the bromide derivative (6), and the
subsequent etherification with (2) provided the homogeneous
trimer PG6AZB3 (Scheme 1).
The heterogeneous trimers PG6AZB2BPH and PG6AZB2OCB
were directly obtained in a one step procedure by etherification of
the intermediate phenolic derivative PG6AZB2OH (5) and the
corresponding mesogenic brominated monomers 4-(6-bromohex-
yloxy)-40-(hexyloxy)biphenyl (8) and 40-(6-bromohexyloxy)
biphenyl-4-carbonitrile (10), previously synthesized according to
(different blocks, e.g. PG6AZBxBPH3ꢀx and PG6AZBxOCB3ꢀx
)
structures, respectively.15 Essentially all compounds are meso-
morphic (except PG6BPH3), and the mesophase structures have
been thoroughly investigated by small-angle X-ray diffraction.
In particular, bilayered SmC- or SmA-like mesophases were
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Scheme 2 Synthesis of the heterolithic (PG6AZB2BPH, PG6AZB2OCB
PG6BPH2AZB and PG6OCB2AZB) and homolithic (PG6BPH3 and
PG6OCB3) compounds. Reagents and conditions: (i) K2CO3, acetone, 50
ꢁC; (ii) K2CO3, KI, 18 crown-6, solvent (DMF: 80 ꢁC, or acetone: 50 ꢁC,
or THF: 65 ꢁC, or DMF–THF: 65 ꢁC); (iii) KOH, THF–H2O, reflux.
Scheme 1 Synthesis of homolithic PGzAZB3. Reagents and conditions:
(i): DIAD, PPh3, THF, 0 ꢁC to reflux; (ii): K2CO3, DMF, 90 ꢁC; (iii):
KOH, THF–H2O, reflux.
are stable above 300 ꢁC and exhibit similar decomposition
pathways, except PG11AZB3, which starts degrading just above
ꢁ
200 C (ESI†).
standard procedures starting from 40-(hexyloxy)biphenyl-4-ol (7)
and 40-hydroxybiphenyl-4-carbonitrile(9), respectively (Scheme 2).
The mixed conjugates i.e. PG6BPH2AZB and PG6OCB2AZB,
were prepared similarly by ether coupling of 4-(6-bromohex-
yloxy)-40-(hexyloxy)azobenzene (11) with the phenolic deriva-
tives, PG6BPH2OH (12) and PG6OCB2OH (13), respectively
(Scheme 2). These two phenolic derivatives were obtained from
the ether coupling of monoprotected PG (3) with the bromide
derivatives (8) and (10), respectively, followed by hydrolysis.
During these syntheses, two by-products were formed in situ,
likely due to the simultaneous deprotection of the 3,5-dihy-
droxyphenyl benzoate, which were isolated as the two homoge-
neous PG6BPH3 and PG6OCB3 compounds, respectively.
All the synthetic details are given in the Experimental section
The two homolithic azobenzene derivatives (PG6AZB3 and
PG11AZB3) are mesomorphic, although the phase domain is
strongly reduced when the spacer chain-length is shortened:
PG11AZB3 shows a mesophase between ca. 100 and 120 ꢁC,
whereas the phase becomes monotropic for the shortest homo-
logue (PG6AZB3). This is the result of the decrease of the melting
temperature for the longer spacer, the mesophase-to-isotropic
phase transition temperatures remaining unchanged for both
compounds (Table 1). The texture of the enantiotropic phase,
shown in Fig. 1, is characteristic of a tilted disordered SmC phase
as identified by the presence of Schlieren features and striated
domains.16 The monotropic phase was harder to detect due to its
short temperature range (about 3 ꢁC only on cooling), and could
not be unequivocally assigned. At the isotropic-to-mesophase
temperature transition, the furtive formation of small focal
conics was sufficient to assign the formation of a SmC phase too.
Diffraction patterns of PG11AZB3 recorded between 105 and
120 ꢁC confirmed the LC nature of the mesophase. The dif-
fractogram registered at 110 ꢁC, described as a representative
example (Fig. 2), shows a sharp and intense small-angle reflection
1
of the ESI.† The structure of all materials was confirmed by H
and 13C NMR. The purity and the structure of the eight final
products (PG11AZB3, PG6AZB3, PG6BPH3, PG6OCB3,
PG6BPH2AZB, PG6OCB2AZB, PG6AZB2BPH and PG6AZ-
B2OCB) were further confirmed by additional Mass Spectros-
copy (MALDI-TOF) and C, H, N elemental analysis.
ꢁ
whose position gives a periodicity of 37.25 A (d001), along the
ꢁ
presence of a large and diffuse signal at ca. 4.5 A associated with
2.2. Thermal behavior and liquid-crystalline properties
the liquid-like lateral ordering of the molten chains and meso-
genic cores (hch + hc). This, along with POM observation,
permits the unequivocal assignment of the mesophase as SmC.
Despite its narrow domain of existence, a diffraction pattern of
The thermal behaviour of the various phloroglucinol-based
derivatives was investigated by TGA, DSC, POM and SA-XRD,
and is summarized in the table below (Table 1). All compounds
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Table 1 Phase transition temperatures (ꢁC) and corresponding enthalpy DH (J gꢀ1) or specific heat variation DCp (J gꢀ1 ꢁCꢀ1 a,b
)
PG11AZB3
LamCr 100.3 (ꢀ) LamCr0 107.2 (83.9)c SmC 120.5 (21.2) I
I 121.4 (ꢀ25.4) SmC 90.8 (ꢀ) LamCr0 78.5 (ꢀ48.4)c LamCr
LamCr 100.4 (50.9) SmC 120.7 (22.1) I
PG6AZB3
LamCr 66.0 (8.5) LamCr0 87.9 (1.01) LamCr00 123.7 (48.0) I
I 121.0 (ꢀ) SmC 118.0 (ꢀ58.7)c LamCr00 79.5 (ꢀ0.4) LamCr0 63.3 (ꢀ2.5) LamCr
LamCr 70.6 (10.2) LamCr0 123.9 (55.6) I
PG6AZB2BPH
LamCr 76.6 (14.6) LamCr0 108.3 (37.0)c SmC 116.0 (ꢀ) I
I 115.8 (ꢀ13.9) SmC 81.8 (ꢀ8.0) LamCr0
LamCr 75.3 (3.2) Cr 84.6 (ꢀ9.8) Cr0 108.6 (37.4)c SmC 118 (ꢀ) I
PG6BPH2AZB
LamCr 119.3 (64.7) I
I 118 (ꢀ31.3) SmC 101.9 (ꢀ31.1) LamCr
PG6BPH3
LamCr 147.3 (76.7) I
I 130.3 (ꢀ76.0) LamCr
PG6AZB2OCB
Cr 105.0 (6.4) Cr0 116.0 (57.8) SmA 122.8 (12.8) I
I 124.8 (ꢀ12.9) SmA 9.8 (0.25) G
G 17.4 (0.28) SmA 42.5 (ꢀ4.4) Cr 66.8 (4.6) Cr0 70.0 (ꢀ24.0) Cr00 103.7 (19.3) Cr000 108.0 (ꢀ24.9) Cr0000 114.2 (26.6) SmA 123.5 (11.42) I
PG6OCB2AZB
Cr 55.9 (24.8) Cr0 62.0 (ꢀ28.3) Cr00 91.2 (37.4) SmA 116.0 (9.2) I
I 117.1 (ꢀ9.2) SmA 3.6 (0.43) G
G 7.4 (0.45) SmA 116.2 (9.8) I
PG6OCB3
Cr 133.6 (67.0) I
I 108.8 (ꢀ3.9) N 18.2 (0.4) G
G 21.8 (0.4) N 108.4 (2.7) I
a
Onset temperature (DSC), Cr–Cr0000: crystalline phases, LamCr–LamCr00: lamellar crystalline phases, G: glassy state, SmA/C: smectic A/C phases, N:
nematic phase, and I: isotropic liquid. b First heating, first cooling, and second heating. c Cumulated enthalpy.
Fig. 1 Polarized optical microphotographs of the LC trimers, observed
on cooling from the isotropic liquid state. Clockwise from top left:
PG11AZB3 (SmC, T ¼ 116 ꢁC), PG6AZB2BPH (SmC, T ¼ 112 ꢁC),
PG6AZB2OCB (SmA, T ¼ 117 ꢁC), and PG6OCB3 (N, T ¼ 70 ꢁC).
the monotropic mesophase (PG6AZB3) could luckily be obtained
at 120 ꢁC, in the vicinity of the isotropic liquid: it revealed a
Fig. 2 Representative small-angle X-ray patterns of PG11AZB3 (SmC,
100 ꢁC) and PG6AZB2BPH (SmC, 100 ꢁC).
single small-angle reflection corresponding to the lamellar peri-
ꢁ
odicity (d001 z 60.9 A). For both compounds, the SmC meso-
phase transforms on further cooling into crystalline phases with a
highly pronounced layered character and sharp interfaces as
testified by XRD, thereafter labelled as LamCr phases. Patterns
measured down to room temperature systematically revealed up
to four sharp small-angle reflections in the ratio 1 : 2 : 3 : 4 and
the occurrence of several sharp large-angle peaks (three to five)
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Table 2 Structural parameters of the mesophasesa
jAZB/ꢁ,
(jBPH/jOCB)
3
Vmol/A , Amol/A , nAmesi
2
2
aAZB/A , (aBPH/aOCB
ꢁ
d/A, NL, nLmesi
ꢁ
ꢁ
ꢁ
ꢁ
dAZB/A, (dBPH/dOCB
Compounds
)
)
PG11AZB3, SmC (T ¼ 110 ꢁC)
PG6AZB3, SmC (T ¼ 120 ꢁC)
37.2, 1, 1
60.9, 2, 2
55.1, 2, 1 : 1
51.2, 2, 1 : 1
30.8, 1, 1 : 1
28.8, 1, 1 : 1
2820, 75.7, 3
25.2, (ꢀ)
10.95, (ꢀ)
10.6, (ꢀ)
13.8, 6.1
6.95, (12.3)
[12.2]
z25, (ꢀ)
z30, (ꢀ)
0, (z55)
z55, (0)
0, (0)
2405, 39.5, 1.5
2196, 39.9, 2 : 1
2043, 39.9, 1 : 2
2062, 67.1, [2 + 1]
1715, 59.7, [1 + 2]
26.3, (ꢀ)
PG6AZB2BPH, SmC (T ¼ 100 ꢁC)
PG6BPH2AZB, SmC (T ¼ 115 ꢁC)
PG6AZB2OCB, SmA (T ¼ 114 ꢁC)
PG6OCB2AZB, SmA (T ¼ 102 ꢁC)
20.0, (40.0)
40.0, (20.0)
22.5, (22.5)
22.5, (18.5)*
[13.3]
0, (0)*
a
d is the smectic periodicity, NL is the number of mesogenic layers per smectic period (NL ¼ 1 for a monolayer and NL ¼ 2 for a bilayer), nLmesi is the
number of sub-layers containing the mesogenic groups of type i (i ¼ AZB, BPH or OCB) in a smectic layer, Vmol is the molecular volume, Amol is the
molecular area (Amol ¼ Vmol/d), nAmesi is the number of mesogenic groups of type i covering the area Amol in sub-layers [for OCB-containing trimers,
mesogen sublayers are not differentiated and thus appear constituted by both mesogens, see text], ai is the mesogenic area, ai ¼ Amol/nAmesi (*calculated
for a monolayer OCB packing, but the small value reveals a change of the packing type, see text), di is the sub-layer thickness di ¼ fi ꢂ d/nLmesi [average
thickness deduced from the sum of both mesogens’ volume fractions, fi (ESI†)], j is the tilt angle, j ¼ arccos(s/ai) ¼ arccos(di/li) with s z VCH /1.27 and
2
ꢁ
lAZB z 12.2, lPBH z 10.8, lOCB z 11.5 A.
rising indicative of the chain crystallization and 3D crystalline
packing.
intensities of first and second order reflections then depend on a
subtle manner from the respective thickness ratios of these sub-
layers, from the electronic density increase within aliphatic sub-
layers due to PG nodes and from the sharpness of their
interfaces, i.e. the quality of the segregation between mesogens
and aliphatic parts. A different intensity ratio may thus be
expected for PG6BPH2AZB, having an opposite number of
undecyl and hexyl terminal chains, and therefore a different sub-
The sequential exchange of azobenzene by biphenylene groups
along the series PG6AZB3 / PG6AZB2BPH / PG6BPH2AZB
/ PG6BPH3 does not modify significantly the liquid crystalline
properties, and in particular the SmC-to-I transition temperature
is almost constant for all AZB-containing compounds (Table 1).
However, for both heterolithic systems the melting temperatures
ꢁ
are depressed by 13 and 20 C with respect to ideal mixtures of
layer thickness ratio. Unfortunately, such a quantitative
similar composition for PG6AZB2BPH and PG6BPH2AZB,
respectively. Such a thermal behaviour (of the heterolithic
compounds) resembles that associated with eutectic solid phases’
macrosegregation in mixtures,17 although here the two compo-
nents can only microsegregate at the molecular level in alter-
nating sub-layers since they are chemically connected. As for
technically used liquid crystal mixtures, this melting point
depression, conjugated to a quite ideal clearing temperature
variation (here between 116 and 121 ꢁC, Table 1), logically leads
to mesophase induction at intermediate compositions. Thus,
both heterolithic tripedes show the smectic C phase on a broader
range than PG6AZB3, whereas mesomorphism is not observed in
the homolithic biphenylene-containing compound (PG6BPH3):
comparison (first-to-second order intensity-ratio) could not be
performed since only one reflection of low intensity was detected
ꢁ
(d001 ¼ 51.2 A) in this case, during the substantially shortened
acquisition, due to the mesophase metastability (monotropic
SmC). As for the pure azobenzene derivatives, both heterolithic
compounds form low-temperature crystalline phases with a
pronounced layering, as evidenced by XRD (LamCr).
The same melting temperature decrease is also observed when
the azobenzene units are sequentially exchanged by the stronger
cyanobiphenyl mesomorphic promoter along the series
PG6AZB3 / PG6AZB2OCB / PG6OCB2AZB / PG6OCB3.
However, discrepancy from ideality is much larger here, expected
due to the different chemical structures of both constitutive
mesogens (AZB and OCB). Consequently, the induction of
mesomorphism is also larger in this series. For instance, the
depression is around 11 ꢁC for the low OCB-content trimer
PG6AZB2OCB and reaches even 40 ꢁC for PG6OCB2AZB,
ꢁ
PG6AZB2BPH exhibits an enantiotropic domain (ca. 10 C on
heating) which can be supercooled (e.g. dowꢁn to ca. 80 ꢁC),
whilst the mesophase is monotropic by only 1 C for the conju-
gate homolog.
ꢁ
The SmC phase was confirmed by the observation of typical
Schlieren textures (Fig. 1) and by XRD (Fig. 2). Diffraction
patterns recorded for PG6AZB2BPH between 115 and down to
85 ꢁC are all identical: they display two intense, sharp, small-
angle reflections in the ratio 1 : 2, indexed as d001 ¼ 55.1 for the
giving rise to enantiotropic mesophase domains of 7 and 25 C,
respectively. In contrast, mesophases of both homolithic
compounds are monotropic. Nevertheless, for all the OCB-con-
taining compounds, the mesophases can be supercooled and
frozen into a glassy state close to room-temperature (DSC,
ESI†). It should be noted that the thermal behaviour of the low
OCB-content trimer is, however, not fully reversible on subse-
quent heating and exhibits a complicated thermal behaviour with
a succession of crystallization and melting events. In contrast to
the previous system, PGzAZBxBPH3ꢀx, the mesophase exhibited
by both heterolithic compounds is different from that of both
homolithic compounds: thus, at both extremes, SmC (for
PG6AZB3) and N (for PG6OCB3, Fig. 1) phases are present,
whilst a SmA mesophase was induced in both intermediate
compounds, readily identified by POM (Fig. 1, ESI†). It is
ꢁ
most intense, and d002 ¼ 27.55 A for the second order, charac-
teristic of the layering of the molecular arrangement, and the
diffuse scattering corresponding to the molten chains and
mesogenic cores (Table 2). The abnormal intensity profile in the
reflection series, with a relatively intense (002) reflection with
respect to (001), indicates segregation at the molecular level, with
alternation of several high-electronic density sub-layers, respec-
tively, associated with two different mesogens, with low-elec-
tronic density sublayers, associated with the aliphatic tails and
with the alkyl spacers (vide infra, Fig. 4). The respective
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noteworthy that the behaviour exhibited in this series reproduces
the typical behaviour of classical calamitic mesogens within
homologous series, with the N phase observed at the short chain-
length, the SmC phase at the longer chain-length, and the SmA
phase at the intermediate chain-length.18 X-ray patterns of both
samples confirm the smectic nature of the mesophase, and show
only one sharp small-angle reflection along the broad and diffuse
scattering in the wide-angle range (hch + hc, Fig. 3): layer peri-
and nodes (case a), or with differentiated aliphatic sub-layers
(case b),
(ii) micro-segregation of the mesogens in alternated layers
(AB-type bilayer) either with a large tilt within the sub-layers of
the mesogens in minority (case a), or with a strong curvature of
the sub-layer interfaces, and a possible evolution toward an
alternative type of packings (case b),
(iii) and finally a segregation of the mesogens into a three-layer
arrangement of the ABB-type.
ꢁ
odicities are of the order of 30 A (Table 2) and no appreciable
variation of the d-spacing with temperature signifies the absence
The latter ABB arrangement (iii) is excluded by X-ray data: the
layer periodicities and the intensity modulation of the lamellar
reflection series are compatible with either single or bilayer
structures only (Table 2).15
of significant tilt angle variations.
2.3. Self-organization properties
Since the mesogens are all identical, the mixture of mesogens in
one layer trivially occurs for PG11AZB3 only, likely because the
length of the terminal chains and spacer are identical, allowing
inter-lamellar exchange without affecting aliphatic sub-layer
thickness (Fig. 4(ia)), and with the threefold nodes randomly
distributed in those sub-layers: the supramolecular arrangement
results of the simple alternation of mesogenic (AZB moieties are
slightly tilted) and aliphatic (tails and spacers undifferentiated)
layers (Fig. 4(ia)). This was clearly proven by the absence of a
double periodicity in long-time acquisition X-ray patterns and
the resulting molecular areas. However, this arrangement is no
more valid for the smaller homolithic azobenzene-based
compound, as the d-spacing corresponds to a periodicity of two
mesogenic layers, very likely since tails and spacers, which have
different lengths, are forced into identifiable layers of different
thicknesses, with an average antiparallel orientation of the nodes
(Fig. 4(ib)). In contrast, the embedding mesogenic sub-layers are
undifferentiated.
The mesogens involved in these compound series are typical
calamitic smectogens, therefore smectic mesomorphism was
expected. However, due to the specific tripedal architecture and
nodes, the conformational freedom is reduced, and thus the
layering is substantially constrained. Indeed, the homolothic
tripedes are not liquid-crystalline or show only monotropic
behaviour, but quite remarkably mesomorphism was promoted
in the heterolithic tripedes. As shown above, this promotion is
actually due to the depression of the melting points associated
with the co-crystallization of segregating mesogens in separated
layers, and not to the clearing point variation, the relative devi-
ation from ideality being rather modest. Despite the complexity
introduced by the additional segregation between mesogens due
to 2 : 1 stoichiometry in heterolithic systems (which does not
occur in homolothic compounds), the accommodation of the
different sub-layer areas can take place according to different
processes as listed below (Fig. 4):
(i) intimate mixing of incompatible mesogens within one single
sub-layer alternating with either sub-layers of mixed tails, spacers
Fig. 4 Possible processes of trimers packing in smectic phases (d001
:
smectic phase periodicity and the blue line is the corresponding schematic
electronic density variation). Top row (from left to right): cases (ia, ib and
iii) (not observed here). Bottom row (from left to right): cases (iia and iib).
For the homolithic systems PG11AZB3 and PG6AZB3, one-colour brick
for ia and ib (see text).
Fig. 3 Representative small-angle X-ray patterns of PG6AZB2OCB
(SmA, 114 ꢁC) and PG6OCB2AZB (SmA, 102 ꢁC).
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For the heterolithic BPH-containing systems, segregation
between the two types of mesogens occurs, leading to a more
intricate organization as for the latter, with an additional
differentiation between two mesogenic sub-layers (Fig. 4(iia)).
Both conjugates give rise to bi-layered organizations with a very
high tilt-angle of the low-content component, segregating in a
SmC-like sub-layer from non-tilted SmA-like sub-layers of the
high-content component. Indeed, the molecular area, identical
for both systems, is imposed by the cross-section sum of the high-
content mesogen (Table 2). Nevertheless, the slight loss in the
molecular area values (10% smaller compared to the estimated
2
ꢁ
cross-section, zsCH z 22.5 A , Table 2) suggests that a
2
restricted amount of inter-layer diffusion occurs, simultaneously
Fig. 5 UV-Vis spectra of PG6BPH2AZB (top) and PG6OCB2AZB
(bottom) in CHCl3. Arrows indicate the absorption spectra change after
UV (366 nm) light exposure. (A) core band; (B1) mesogenic BPH band;
(B2) mesogenic OCB band; (C and D) azobenzene bands.
reducing the tilt angle to 55ꢁ (instead of 60ꢁ in the ideal case of
perfectly segregated mesogens of identical cross-sections).
Unexpectedly, layer-un-differentiation is total for the OCB-con-
taining materials, as evidenced by the monolayer SmA phase (see d
and Amol values, Table 2, and also POM, Fig. 1), composed of
apparently untilted mesogens mixed in the same layer instead of the
bilayer SmC phase formed by the BPH-containing systems just
described. Yet, good segregation between the two different meso-
gens in separated individual layers is a priori expected because of
their strong structural difference. In reality, this result is not in
contradiction with the occurrence of segregation at the local scale
between the mesogens, but the substitution of the alkyl chain by a
cyano group reduces the segregation with the aliphatic medium, this
hypothesis being supported by the fact that PG6OCB3 shows the N
phase and not a smectic phase. Therefore, the compensation of the
various areas is promoted by a layer undulation process (Fig. 4(iib))
which facilitates permutation between the mesogen sub-layers. At
the larger scale-length, this permutation leads to an average
apparent sub-layer made of untilted mixed mesogens similar to
classical SmA–SmC phases found in calamitic compounds.19 In the
particular cases of the OCB-containing systems, the suppression of
the terminal chains gives the possibility of a monolayer packing
(SmA1) similar to the mesogens with terminal chains, but also to a
bi-layer packing (SmA2) through cyano pairing, or to a mixture of
both (SmAd).20 The molecular area of the low-OCB content tripede
is fully compatible with a SmA1 type, whilst the smaller molecular
area of the high-OCB content compound agrees with a SmAd type,
with 30 to 40% of pairing.
(C) p / p* transition of the E-form of the azobenzene with a
maximum at 360 nm;
(D) n / p* transition of the azobenzene at 450 nm.
It is clearly seen that the absorption bands of the azobenzene
(C and D) are separated from the mesogen absorption bands (B1
or B2). Moreover, it can be noticed that there is a red shift in the
alkoxy-cyanobiphenyl absorption bands (B2) which is caused by
the electron acceptor cyano group. Existence of the electron
acceptor and electron donor alkoxy groups on biphenyl creates a
push–pull effect.21
The photoisomerisation process of the azobenzene moiety of
PG6AZB2BPH in chloroform under UV exposure is shown in
Fig. 6. Irradiation at 366 nm results in the photoisomerisation
establishing a steady state between the thermodynamically stable
E isomeric form of azobenzene and its meta-stable Z-form. As a
result, a decrease in the p / p* absorption between two
isosbestic points (322 and 431 nm) and simultaneously an
increase in the absorption in the range of the n / p* transition
(431–490 nm) are observed (Fig. 6). Subsequent Vis exposure
(532 nm) results in the establishment of a new steady state rich
with E isomers.
The UV-Vis spectra of the other trimeric compounds in solu-
tion were similar and variable peak intensities were obtained due
to a different proportion of the azobenzene to the mesogenic
3. Photochemical behaviour
The photochemical behaviour of the three-armed star-like liquid
crystalline molecules containing 1 to 3 photochromic azobenzene
groups was investigated. The UV-Vis absorption spectra of such
systems in solutions were measured and the photoisomerisation
process of azobenzene moieties was studied. Based on these results
thelightinducedanisotropywasinvestigatedin filmsin thenextstep.
The UV-Vis spectra of initial and irradiated solutions were
recorded, and are characterised by four spectral bands as follows
(Fig. 5):
(A) absorption of the aromatic core located below 260 nm;
(B1) absorption of the mesogenic alkoxy-biphenyl (BPH)
groups at 265 nm (PG6BPH2AZB and PG6AZB2BPH);
(B2) absorption of the mesogenic alkoxy-cyanobiphenyl
(OCB) groups at 298–302 nm (PG6AZB2OCB and
PG6OCB2AZB);
Fig. 6 UV-Vis spectra of PG6AZB2BPH in CHCl3 (C ¼ 2.1 ꢂ 10ꢀ5 mol
dmꢀ3). Arrows indicate the absorption spectra change after UV (366 nm)
light exposure. C and D: azobenzene bands.
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Table 3 Absorption maxima (lmax), molar absorption coefficients (3) of
three-arm star-like LC molecules with a different proportion of azo-
benzenes to mesogens obtained in chloroform at room temperature
lmax [nm] (3 [Mꢀ1 cmꢀ1])
Compound
p / p*
n / p*
PG11AZB3
PG6AZB3
PG6BPH2AZB
PG6AZB2BPH
PG6AZB2OCB
PG6OCB2AZB
360 (73 000)
360 (76 000)
360 (30 000)
360 (46 000)
360 (61 000)
360 (29 000)
452 (9000)
448 (8700)
450 (3500)
450 (6000)
451 (6800)
453 (3300)
Fig. 8 Polarized UV-Vis absorption spectra of the PG6AZB2BPH film
measured with a beam polarized parallel (red-dashed line) and perpen-
dicular (blue-dashed line) to the polarization direction of the incident
beam of an Ar+ laser (488 nm, 160 mW cmꢀ2, 43 min) compared with the
initial spectrum (black-solid line).
groups (ESI†). The absorption maxima (lmax) together with the
molar absorption coefficients (3) for all investigated materials are
collected in Table 3.
However, as shown in Fig. 7, the absorption maximum of the
p / p* transition of spin-coated PG6AZB2BPH films is
significantly blue shifted up to 336 nm when compared to the
spectrum in chloroform. Moreover, the bands have an asym-
metric shape. Both observations indicate that a high amount of
azobenzene groups exist as H-aggregates.22 This phenomenon is
caused by head-to-head stacking of the azobenzene moieties
probably promoted by further intra-/intermolecular interactions
especially by the van der Waals forces of the terminal tails. In
contrast PG6AZB2OCB and PG6OCB2AZB show a much
smaller blue shift of only 5 nm. Based on the results of photo-
isomerisation in solution the suitable wavelength of the light was
selected for induction of optical anisotropy in films.
The photoisomerisation of azobenzene occurs also in films
(Fig. 8). In contrast to the solution, the steady state of the
aggregated films is characterized by much less formed Z isomers.
The reversibility of photoisomerisation can be used as a light
driven motor for the molecular motion in films. The irradiation
at the appropriate wavelengths which both isomers absorb
results in a number of E–Z and Z–E photoisomerisation cycles.
As shown in many studies, the linearly polarized exposure can
cause an orientation of the azobenzene groups. The azobenzene
chromophores oriented parallel to the electric field vector of the
incident light undergo preferably a multiple photoisomerisation
cycle up to a situation in which mobile azobenzene moieties
become oriented in the direction perpendicular to the light elec-
tric field vector, thus optical anisotropy is induced in the film.
Fig.
9 Absorption polar diagram plotted for 366 nm for the
PG6AZB2BPH film before (black-solid line) and after irradiation (red-
dashed line) with linear polarized laser light (488 nm, 160 mW cmꢀ2, 43
min).
The absorption spectra of the initial and irradiated
PG6AZB2BPH films are depicted in Fig. 9. Polarized measure-
ments indicate a weak anisotropy (dichroism of about D ¼ 0.13)
with the absorption maximum perpendicular to the electric field
vector of the incident light. The spectroscopy revealed that the
amount of chromophores oriented in the direction to field vector
has been decreased while no increase is observed in the direction
perpendicular to the electric field vector of incident light (Fig. 9).
Such changes suggest that the orientation process is more complex
and contains probably an out-of-plane orientation component of
the mobile non-aggregated molecules in the film. The p / p*
maximum at 366 nm is not shifted by the exposure indicating that
the film is still strongly aggregated. The anisotropy did not increase
due to subsequent annealing of films of the LC derivatives.
4. Conclusions
New supermolecular tripedal homo- and heterolithic liquid crys-
tals have been synthesized. Smectic mesomorphism is observed in
most compounds as expected since the building blocks are typical
calamitic smectogens. Depending on the molecular topology,
SmC- or SmA-like phases with monolayer (PG11AZB3) or bilayer
Fig. 7 Absorption spectra of PG6AZB2BPH in chloroform solution
(green-dashed line) and in the film (black-solid line) with the maximum of
p / p* transition.
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C. H. M. Weber, J. K. Hobbs, C. Tschierske and G. Ungar,
Science, 2011, 331, 1302–1306.
structures were formed. For the heterolithic systems, segregation
between the two types of mesogens occurs, leading to more
intricate organizations than for the homolithic ones, with a good
differentiation between the two mesogenic sub-layers. These
various smectic organizations were comprehensively described by
the help of classical concepts, such as partial molecular volumes
and transverse cross-sections of molecular segments applied at all
interfaces generated by the tripedal architecture. A comparison of
the UV-Vis spectra of tripedal star-like azobenzene functionalized
derivatives in the solution and in spin-coated films revealed a
strong ‘‘blue’’ shift of the maximum absorption band indicating
the formation of H-aggregates. The aggregated azobenzenes
however do not undergo photo-isomerisation nor photo-orien-
tation upon linearly polarized exposure. Consequently, only a
small dichroism was induced by light. More detailed optical
investigations of the heterolithic derivatives of this series will be
presented in a subsequent publication.
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