Controlling Mirror Symmetry Breaking and Network Formation in Liquid Crystalline Cubic, Isotropic Liquid and Crystalline Phases of Benzil-Based Polycatenars

Article abstract of DOI:10.1002/chem.202002869

Spontaneous development of chirality in systems composed of achiral molecules is important for new routes to asymmetric synthesis, chiral superstructures and materials, as well as for the understanding of the mechanisms of emergence of prebiotic chirality. Herein, it is shown that the 4,4′-diphenylbenzil unit is a universal transiently chiral bent building block for the design of multi-chained (polycatenar) rod-like molecules capable of forming a wide variety of helically twisted network structures in the liquid, the liquid crystalline (LC) and the crystalline state. Single polar substituents at the apex of tricatenar molecules support the formation of the achiral (racemic) cubic double network phase with Ia (Formula presented.) d symmetry and relatively small twist along the networks. The combination of an alkyl chain with fluorine substitution leads to the homogeneously chiral triple network phase with I23 space group, and in addition, provides a mirror symmetry broken liquid. Replacing F by Cl or Br further increases the twist, leading to a short pitch double gyroid Ia (Formula presented.) d phase, which is achiral again. The effects of the structural variations on the network structures, either leading to achiral phases or chiral conglomerates are analyzed.

Full text of DOI:10.1002/chem.202002869

Accepted Article
Accepted Article
Title: Controlling mirror symmetry breaking and network formation in
liquid crystalline cubic, isotropic liquid and crystalline phases of
benzil-based polycatenars
Authors: Tino Reppe, Silvio Poppe, and Carsten Tschierske
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To be cited as: Chem. Eur. J. 10.1002/chem.202002869
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01/2020

10.1002/chem.202002869
Chemistry - A European Journal
Controlling mirror symmetry breaking and network formation in liquid
crystalline cubic, isotropic liquid and crystalline phases of benzil-based
polycatenars
Tino Reppe, Silvio Poppe and Carsten Tschierske*
T. Reppe, Dr. S. Poppe, Prof. Dr. C. Tschierske
Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt-Mothes-Straße 2,
06120 Halle, Germany.
E-mail: carsten.tschierske@chemie.uni-halle.de
Abstract
Spontaneous development of chirality in systems composed of achiral molecules is important
for new routes to asymmetric synthesis, chiral superstructures and materials, as well as for the
understanding of the mechanisms of emergence of prebiotic chirality. Herein it is shown that
the 4,4’-diphenylbenzil unit is a universal transiently chiral bent building block for the design
of multi-chained (polycatenar) rod-like molecules capable of forming a wide variety of
helically twisted network structures in the liquid, the liquid crystalline (LC) and the
crystalline state. Single polar substituents at the apex of tricatenar molecules support the
formation of the achiral (racemic) cubic double network phase with Ia3¯d symmetry and
relatively small twist along the networks. The combination of an alkyl chain with fluorine
substitution leads to the homogeneously chiral triple network phase with I23 space group, and
in addition, provides a mirror symmetry broken liquid and also a crystalline conglomerate.
Replacing F by Cl or Br further increases the twist, leading to a short pitch double gyroid Ia3¯d
phase which is achiral again. The effects of the structural variations on the network structures,
either leading to achiral phases or chiral conglomerates are analyzed.
Keywords: Liquid crystals, mirror symmetry breaking, chirality, cubic phases, benzil, soft
matter
1
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1. Introduction
Dynamic self-assembled networks of any kind represent the fundamental basis of open-end
development in complex systems. For example, neural networks are responsible for the
development of the brain and consciousness, chemical networks are the basis of metabolism,
1 4]
data networks for information technology and so on.[ - Bicontinuous cubic (Cubbi) liquid
crystalline phases can be considered as soft self-assembled networks, which have attracted a
lot of interest for conducting and photonic materials, as matrices for protein crystallization
]
5,6
and as templates for sol-gel syntheses.[ Recently, these fluid networks were recognized to
be responsible for transmission and amplification of chirality in fluid systems, and thus could
7,8]
provide a new potential route to prebiotic chirality.[ Cubbi phases are known for different
9 11]
12
]
types of small molecular[ - or polymeric[ amphiphilic compounds, either in aqueous
6,13]
9,10,14]
systems (lyotropic phases)[
or as pure compounds (thermotropic phases).[
Amphiphilicity can be based on any intramolecular difference of intermolecular interactions
15
11]
]
leading to nano-segregation of incompatible units,[ like polar and apolar[ or rod-like
16-27]
polyaromatic cores and flexible chains.[
Figure 1. Schematics showing the networks of a) the Cubbi/Ia3¯d phase and b) the triple
network I23[*] phase; c) shows the transient chirality of the benzil unit and d) the
development of the helical twist by clashing of bulky end groups attached to the cores. The
polyaromatic cores are located in the networks and aligned perpendicular to the network
2
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directions, the continuum between them is filled by the terminal alkyl chains. b) was
reproduced from ref. 31 by permission of The Royal Society of Chemistry.
In the Cubbi phases of rod-like compounds the double gyroid phase with space group
6,9]
Ia3¯d involving two networks with three-way junctions (see Fig. 1a)[ and a more complex
triple network structure with larger lattice parameters and I23 space group, also involving
9,22,23]
exclusively three-way junctions, were observed.[
The networks are separated by the
alkyl chains organized around the infinite minimal surfaces located in the middle between
28
them.[ ] The I23[*]-type Cubbi phase is exceptional, not only as it is the only presently known
29
-
Cubbi phase with triple network structure, it was recently also found to be inherently chiral[
31]
and the discovery of this supramolecular chirality provided an improved general
understanding of the cubic phases formed by rod-like compounds.
Intermolecular
H-bonding
O
H2n+1CnO
H
N
N
H
OCnH2n+1
BABHs
O
O2N
H2n+1CnO
H
O
O
O
O
OCnH2n+1
NO2
H
Intramolecular
H-bonding
ANBCs
O
O
H2n+1CnO
H2n+1CnO
H2n+1CnO
OCmH2m+1
O
O
S
S
Polycatenars
Scheme 1. Structures of the BABH and ANBC based rod-like hydrogen bonded molecules
and supramolecules, and an example of a polycatenar mesogens forming Cubbi phases.
The most prominent are the 4’-n-alkyloxy-3’-nitrobiphenyl-4-carboxylic acids
32
(ANBCs)[ and the 1,2-bis(4’-n-alkyloxybenzoyl hydrazines (BABHs),[33,34] both involving
]
hydrogen bonding and having only one alkyl chain at each end, and the so-called polycatenar
35 37]
-
compounds, having more than only two terminal chains (Scheme 1).[
In the Cubbi phase
of these compounds the rod-like cores are organized in the networks with the long axes
perpendicular to the local net direction, and the clashing of the bulky molecular peripheries
leads to a helical twist along the networks (Fig. 1d) which is transmitted by the junctions to
38,29]
the macroscopic scale.[
The Ia3¯d phase with only two enantiomorphous networks with
39
]
opposite handedness is macroscopically achiral (Fig. 1a),[ whereas in the triple network
phase this degeneracy is broken by the third network (Fig. 1b) and a synchronization of the
helix sense between the interwoven networks leads to uniform chirality (I23[*] phase, the
40-43]
spontaneous chirality is indicated by [*]).[
3
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Notably, there are two distinct type of Ia¯3d phases, occurring on either side of the
I23[*] phase, a long pitch (low twist) Ia3¯d(L) and a short pitch (high twist) Ia3¯d(S) phase,
leading to the phase sequence Ia3¯d(L) – I23[*] - Ia3¯d(S) with growing twist between the
44 46]
-
molecules along the networks.[
In previous work we have shown that these rod-like
molecules do not only form mirror symmetry broken cubic phases, mirror symmetry breaking
[
is in some cases even observed in the adjacent isotropic liquid phases (Iso1 *]), forming liquid
[
conglomerates.[45,47] These Iso1 *] phases usually occur besides one of the Cubbi phases and
38,44]
therefore they are considered as liquid network phases.[
X2
X3
C10H21
C10H21
C10H21
C10H21
C10H21
C10H21
O
O
O
O
O
O
X4
O
O
X2
X3
2/X
O
X = H, F, CN, NO2, OCF3
O
O
O
O
O
OCnH2n+1
O
O
O
Y2
Y3
3/Yn
O
Y = H, F, Cl, Br; n = 6, 10
C10H21
C10H21
C10H21
O
O
O
O
O
OCnH2n+1
O
O
O
1/n
O
C10H21
C10H21
C10H21
O
O
O
O
O
R
O
O
R = -OOCR1
4/n R = -OCnH2n+1
5
:
:
Scheme 2. Structures of the compounds under investigation, for details of the structures, see
Tables 1-4.
The 4,4’-diphenylbenzil derived polycatenars 1/n (Scheme 2) with a torsion of the
48
38]
]
O=C-C=O bond,[ represent transiently chiral molecules[ which were recently shown to
even form a sequence of three mirror symmetry broken mesophases, thus transferring chirality
[
from the isotropic liquid (Iso1 *]) via the liquid crystalline (Cubbi/I23[*]) to the crystalline state
(CrIso *]).[44] Herein we expand this class of compounds by investigating the importance of the
[
core length (compounds 4/n and 5 in Scheme 2) and the effect of polar substituents in lateral
position (compounds 3/Yn) and at one end (at the apex, compounds 2/X in Scheme 2) on the
self assembly, network formation, cubic phase formation and spontaneous mirror symmetry
breaking in the liquid state.
2. Methods
2.1 Synthesis
4
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The target compounds were synthesized according to Scheme 3. Compounds 2/X, and 5 were
44]
obtained by acylation of the phenol 9[ using the experimental procedures given in the
Supporting Information (SI). The alkoxy compounds 4/n were synthesized from the aryl
bromide 8 by Suzuki cross-coupling with 4-alkoxyphenylboronic acid pinacol esters 7/n.
Scheme 3. Synthesis of the benzil based compounds 2-5. Reagents and conditions: (i) THF,
sat. NaHCO3 solution, [Pd(PPh3)4], reflux; (ii) SOCl2, abs. pyridine, DCM, DMAP, 25 °C.
2.2 Investigations
Investigations of the compounds were performed by polarizing optical microscopy between
crossed and slightly uncrossed polarizers (POM), differential scanning calorimetry (DSC) and
X-ray scattering (XRD) with the instrumentation described in the SI. The transition from the
isotropic liquid state to the Cubbi phases on cooling was identified by an increase of viscosity
associated with a small DSC peak (Fig. S1) while the phase remains optically isotropic (Fig.
2b,e). In the X-ray diffraction (XRD) patterns of the Cubbi phases there are several small angle
scatterings besides the diffuse wide angle scattering. The diffuse character of the wide angle
scattering with a maximum around d = 0.45 nm (Figs. S3b and S4b) confirms the LC state
5
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having no fixed positions of the individual molecules. The small angle reflections can be
indexed either to (211) and (220) of a Ia3¯d lattice or to (321), (400) and (420) of the I23[*]
lattice (Tables S2, S3). Figures S3 and S4 show representative XRD patterns of these two
cubic phase types. Additional confirmation of the cubic phase type was gained from optical
investigation under a polarizing microscope between polarizers slightly rotated by a few
degrees out of the exactly 90° twisted orientation where only the I23[*] phase shows a
conglomerate of chiral (dark and bright) domains which invert their brightness by changing
the twist direction of the analyzer (see Fig. 2a-c) whereas the Ia¯3d phase does not, and
therefore is achiral (Fig. 2c-f).[29] In a similar way the achiral isotropic melted state (Iso, Iso1)
[
is distinguished from the conglomerates formed by the mirror symmetry broken Iso1 *] phase.
The non-cubic LC phases were indicated by their typical birefringent textures as observed by
POM between crossed polarizers and confirmed by XRD investigation (see Section 3.3 and
Table S4).
Figure 2. POM images as observed between crossed (middle) and slightly uncrossed
polarizers (left, right) for a-c) the I23[*] phase of 1/10 at 105 °C as obtained after cooling from
[
the Iso1 *] phase and d-f) the achiral Ia3¯d phase of 3/2Br6 at 70 °C as observed on cooling
from Iso1.
3. Results and Discussion
3.1 The series of tetracatenar compounds 4/n
In the series of compounds 4/n with one benzoate unit shorter core than compounds 1/n only
the homologues with a chain length between n = 14 and 18 form a monotropic Cubbi phase on
cooling, whereas all other compounds have only crystalline phases (Table 1). Even for these
three compounds, due to rapid crystallization, the phase transitions can only be recorded on
6
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cooling and the investigations are therefore only based on DSC and optical investigations,
whereas crystallization takes place during the exposure time of attempted XRD investigations.
The assignment to the Ia3¯d space group is therefore only based on optical investigations,
indicating the absence of chiral domains in the optical isotropic mesophases with high
viscosity.
Table 1. Melting and crystallization temperatures of the homologous series of compounds 4/n
measured on heating and cooling (10 K min-1).[a]
Compd.
4/4
4/6
n
4
6
Heating: T/°C [ΔH/kJ mol-1]
Cr 82 [48.0] Iso
Cr 91 [64.2] Iso
Cr 96 [51.4] Iso
Cr 102 [54.7] Iso
Cr 80 [66.5] Iso
Cr 82 [78.3] Iso
Cr 83 [75.3] Iso
Cr 84 [92.3] Iso
Cr 86 [110.0] Iso
Cooling: T/°C [ΔH/kJ mol-1]
Iso 45 [19.5] Cr
Iso 42 [21.7] Cr
Iso 63 [45.2] Cr
Iso 81 [53.3] Cr
Iso 57 [49.4] Cr
4/8
8
4/10[44]
4/12
4/14
4/16
4/18
4/20
10
12
14
16
18
20
Iso 69 [6.5] Iso1 49 [1.3] Cub/Ia3¯d 31 [6.0] Cr[c]
Iso 70 [6.1] Iso1 55 [1.9] Cub/Ia3¯d 31 [2.5] Cr[c]
Iso 66 [19.9][b] Iso1 56 [1.3] Cub/Ia¯3d 37 [7.7] Cr[c]
Iso 66 [83.8] Cr
[a] DSC peak temperatures on heating at 10 K min-1, Abbreviations: Cr = crystalline solid, Iso = achiral isotropic
liquid; Cub/Ia3¯d = achiral bicontinuous cubic phase; though the direct assignment of the Ia¯3d type was not
possible, based on the large chain length a Ia¯3d(S) type appears more likely; for DSCs of compounds 4/14-4/18,
see Figs. 3 and S1a-c. [b] Transition is accompanied by partial crystallization. [c] Partial crystallization.
Figure 3. DSC of compound 4/14 showing the Iso-Iso1-Cubbi/Ia3¯d transition ranges; full
heating and cooling scans are shown in Fig. S1a, the DSCs of 4/16 and 4/18 are shown in Fig.
S1b-c.
7
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Only for 4/14 and 4/16 the transition from Iso to Cub/Ia3¯d takes place without partial
crystallization on cooling. The most interesting feature of these compound is an additional
transition in the liquid before the transition to the cubic phase (Table 1, Figs. 3 and 1a-c). On
cooling compound 4/14, as example, a broad feature occurs in the DSC traces in the
temperature range of the isotropic liquid phase, indicating a liquid state polyamorphism
38,44,46,49]
(liquid-liquid transitions, see Fig. 3).[
An additional sharp peak indicates the
transition to the cubic phase, associated with a sudden change from fluidity to viscoelasticity.
The majority of the enthalpy of this Iso-Iso1-Cubbi transition is involved in this broad feature
(6-7 kJ mol-1) which is interpreted as a growth of aggregates which fuse to a network structure
49]
and continuously increases network connectivity (Iso1 phase).[ At a certain degree of
network connectivity, the long range cubic lattice composed of two interwoven gyroid
networks with three way junctions is formed. This is indicated by a sharp peak with small
enthalpy (1.3-1.9 kJ mol-1). There is no indication of any mirror symmetry breaking in the
Iso1 phase range and therefore we assume a similar Ia¯3d-like network with three way
junctions, but without long range order for the structure of the achiral Iso1 phase. As the Iso1
phase flows under gravity like any other ordinary liquid, the network must be highly dynamic
and we consider it as a kind of percolated network liquid.[44] The very broad Iso-Iso1 transition
is interpreted as a continuous growth process fusing the cybotactic clusters to dynamic
38a,44]
networks.[
Though this broad DSC feature was also observed for compound 4/18, the
Iso-Iso1 transition is in this case accompanied by a partial crystallization, as indicated by the
changed shape and larger enthalpy of this broad transition (Fig. S1c).
3.2 Tetracatenar compound 5/n with an acyl chain and tricatenar compounds 5 with
cyclic units at the apex
Attempts to widen the cubic range of compounds 4/n by using acyloxy chains (5/12)
or alicyclic and polycyclic units (compounds 5/c4, 5/c12 and 5/Ad) instead of alkyloxy chains
failed (see Table 2), though in the series of polycatenar dicarboxylates of 5,5’-diphenyl-2,2’-
dithiophene (Scheme 1) a significant mesophases stabilizing effect of these cycloaliphatic
45]
units was found.[ As shown in Table 2 all these compound are crystalline solids, some of
them can be supercooled even down to room temperature without mesophases formation
before the onset of crystallization. It appears that the capability of the non-symmetric 5-ring
tetracatenars 4 for mesophases formation is limited to only few examples with long aliphatic
chains. Therefore, the focus of the following work is on compounds 2 and 3 with a
polyaromatic core involving six benzene rings.
8
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Table 2. Phase transitions of compounds 5 on heating and cooling (DSC at 10 K min-1).
C10H21O
C10H21O
O
O
O
R
O
C10H21O
T/°C [H kJ mol-1]
H: Cr 80 [66.5] Iso
C: Iso 57 [49.4] Cr
H: Cr 87 [53.4] Iso
C: Iso 65 [45.4] Cr
H: Cr 104 [67.0] Iso
C: Iso 45 [19.8] Cr
H: Cr 106 [66.8] Iso
C: Iso < 20 Cr
Compd.
R
4/12[b]
OC12H25
5/12
OOCC12H25
OOC
5/c4
OOC
12
5/c12
5/Ad
H: Cr 79 [35.5] Iso
C: Iso < 20 Cr
OOC
3.3 Tricatenar compounds with polar substituents at the apex
We expanded the structure of the tetracatenar six-ring compounds 1/n either by introduction
of halogens in lateral positions of the apex or by replacing the alkyl chain at the apex by polar
electron withdrawing substituents (tricatenar compounds 2/X). As shown in Table 3 electron
withdrawing groups, especially CN and NO2 lead to compounds with wide ranges of LC
phases, most probably due to increasing polar core-core interactions between the electron
deficit aromatics as well as by donor-acceptor interactions with the electron donor substituted
45,50]
trialkoxylated ends in an antiparallel packing.[
2/CN and 2/NO2 show an enantiotropic
phase sequence Iso-Iso1-SmA-Cubbi on cooling. The SmA phases were confirmed by their
typical fan-like textures with extinctions parallel to the directions of analyzer and polarizer
(Fig. 4a,b), and the optically isotropic appearance of their homeotropic aligned samples (see
insets in Fig. 4a,b), thus confirming optically uniaxial non-tilted SmA phases. The XRD
patterns show only one sharp small angle scattering with d = 5.5 (2/CN) or d = 5.3 nm
(2/NO2, see Table S4), being a bit larger than the molecular lengths Lmol = 4.5 and 4.6 nm,
respectively, measured for the most extended conformation of space filling models (Fig. 5a).
9
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Thus, it can be concluded that an almost fully intercalated antiparallel organization of the
molecules in the layers, providing a layer distance of 5.5 nm, is present (Fig. 5b).
Table 3. Phase transitions, lattice parameters (acub) and twist angles between the molecules in
neighboring rafts (Φ/°) in the Cubbi phases of compounds 2/X.[a]
acub
/
T/°C [H kJ mol-1]
Comp.
X4
X3 X2
nraft
nm
(Φ/°)
-
1/0[44]
1/1[44]
1/2[44]
H
H
H
H
H
H
H
H: Cr 126 [52.2] Iso; C: Iso 110 [27.3] Cr
H: Cr 127 [36.3] Iso; C: Iso 115 [36.4] Cr
H: Cr 120 [37.9] Ia3¯d (L) 127 [1.8] Iso1 135 [1.9] Iso
-
-
OMe
OEt
-
4.9
12.9
(6.9)
[
C: Iso 133 [1.3] Iso1 121 [0.2] Iso1 *] 110 [0.3] Ia3¯d L 105 [31.3] Cr
1/14[44]
OC14H29
H
H
3.2
11.3
(7.9)
H: Cr 108 [44.1] Ia3¯d (S) 135 [2.9] Iso1 138 [7.0] Iso
C: Iso 136 [3.1] Iso1 129 [1.2] Colhex 125 [0.4] Ia3¯d (S) 42 [16.9] CrIso
<20 Cr
F
H
H
H
H
H: Cr 126 [26.5] Iso; C: Iso 110 [27.3] Cr
H: Cr 145 [25.0] Ia3¯d (L) 165 [1.2] SmA[b] 176 [1.0] Iso1 183 Iso
-
-
2/F
CN
4.6
12.4
(7.2)
2/CN
C: Iso 178 [2.9] Iso1 173 [0.8] SmA 145 [0.9] Ia3¯d (L) 124 [34.4] Cr
H: Cr 149 [38.9] Ia3¯d (L) 181 [2.0] SmA[c] 187 [1.7] Iso1 193 [2.5] Iso
C: Iso 190 [3.0] Iso1 184 [1.5] SmA 167 [1.8] Ia3¯d (L) 130 [41.0] Cr
H: Cr 123 [24.9] Ia3¯d (L) 135 [2.2] Iso1 143 [4.0] Iso
C: Iso 141 [2.4] Iso1 126 [1.6] Ia3¯d (L) 115 [25.3] Cr
NO2
OCF3
F
H
H
F
F
H
H
H
F
4.5
3.8
3.6
3.5
12.1
(7.4)
2/NO2
2/OCF3
2/F3
11.3
(7.9)
10.9
(8.2)
H: Cr 123 [28.1] Ia3¯d (L) 130 [2.9] Iso1 138 [3.4] Iso
C: Iso 134 [3.4] Iso1 121 [2.2] Ia3¯d (L) 103 [26.4] Cr
F
10.8
(8.3)
2/F5
H: Cr 103 [39.7] Ia3¯d (L) 112 [2.0] Iso1 123 [5.2] Iso
C: Iso 118 [7.7] Iso1 100 [1.6] Ia3¯d (L) 84 [43.1] Cr
[a] DSC peak temperatures on heating/cooling (H/C) at 10 K min-1; nraft = number of molecules
in the cross section of the networks, calculated for rafts with a height of 0.45 nm; for details
of the calculations see Table S5; acub = cubic lattice parameter; Φ(Ia3¯d) = twist between
adjacent molecules (rafts), calculated according to Φ(Ia3¯d) = 70.5°/[0.354acub/0.45nm];[29a]
for DSCs, see Figs. 7 and S1 and for XRD data Tables S2-S5 and Figs. S3-S4; for the Iso-Iso1
transitions, representing broad features the maxima of the features are given, the complete
temperature ranges are listed in Table S1; abbreviations: SmA = non-tilted lamellar (smectic
A) phase; Ia3¯d(L) = long pitch (small Φ) Ia3¯d phase; Ia3¯d(L) = short pitch (large Φ) Ia3¯d
phase for the other abbreviations, see Table 1; [b] d = 5.5 nm; [c] d = 5.3 nm.
10
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In the cubic phases two main reflections are observed which can be indexed to (211)
and (220) of a Ia3¯d lattice; the calculated lattice parameters acub are 12.4 nm for 2/CN (155
°C) and 12.1 nm for 2/NO2 (160 °C, see Tables 3, S2 and S4). According to drod = √3 x acub/4
the distance between the two networks is drod = 5.4 and 5.2 nm, respectively (see Table S5).
This distance corresponds to the separation of the minimal surface and is similar to the layer
distances measured in the adjacent SmA phases. This is in line with a non-tilted fully
intercalated organization of the polyaromatic rods in the networks (Fig. 5b). The cubic phase
stability (and mesophases stability in general) of the 4-substituted compounds 2/X grows in
the order X = F < OCF3 < CN < NO2 with rising Hammett constants of the substituents
51
]
(m+p for F = 0.4; OCF3 = 0.7; CN = 1.2 and NO2 = 1.5).[
Figure 4. Textures of a) 2/CN at T = 147 °C and b) 2/NO2 at T = 167 °C in planar aligned
samples showing the growth of the Cubbi/Ia3¯d phase (dark area) into the fan textures of the
birefringent SmA phases on cooling; the insets show the homeotropic textures of the SmA
phases at 150 and 170 °C, respectively; the arrows indicate the orientation of polarizer and
analyzer. The width of the images is 0.8 mm.
Increasing degree of fluorination at the apex has a similar effect as CN/NO2
substitution. Though the monofluorinated compound 2/F does not show any LC phase,
probably due to its rapid crystallization on cooling, taking place already at 110 °C, the further
increase of the number of fluorines induces cubic phases. This works for aromatic
11
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52
]
fluorination[ (2/F3 and 2/F5) as well as for the fluorination of the OCH3 group (compound
2/OCF3, see Table 3), though there is a competition with the simultaneously growing steric
effect of fluorination. All compounds 2/X with polar substituents form exclusively the achiral
[
Ia3¯d phase and no I23[*] or Iso1 *] phase can be observed, whereas compounds with alkyloxy
chains, acting as donor-substituents (m+p = -0.22 for OC4H9)[51] form a sequence Ia3¯d(L)
-
I23[*] - Ia3¯d(S) upon chain elongation.[9b,44,45,53] The number of molecules in the cross-section
of the networks (nraft, see Tables 3 and S5) is 3.5-3.8 for the fluorinated compounds and 4.5-
4.6 for the NO2 and CN substituted compounds.
Figure 5. a) Space filling models of compound 2/CN and b) its antiparallel pair with fully
intercalated aromatic cores.
The question arises to which kind the Ia3¯d phases of compounds 2/X actually belong.
The twist angles of 7.9-8.3° for the fluorinated and 7.2-7.4° for the CN/NO2 substituted
compounds are intermediate between the values expected for the two cases (<7° for Ia3¯d(S)
31,44,45]
and >9° for Ia3¯d(L), respectively).[
As previously shown, the chiral I23[*] phase can be
induced between different Ia3¯d phase types, whereas no such phase is induced between Ia3¯d
phases belonging to the same type.[44,45,54-56] As shown in Fig. 6 a stripe of an I23[*] phase
with chiral conglomerate texture is observed in the contact region between the Cubbi phase of
1/14,[44] known to form the Ia3¯d(S) type, and the Ia3¯d phases of the compounds 2/CN, 2/OCF3
and 2/F3. This means that the developing concentration gradient leads to the sequence Ia3¯d(L)
- I23[*] - Ia3¯d(S) with increasing concentration of compounds 2. Therefore, the Ia3¯d phases of
all these polar substituted compounds 2/X must be considered as long pitch (low twist) Ia3¯d(L)
type phases.
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Figure 6. Development of chiral conglomerates of the induced I23[*] phase in the contact
regions between the achiral Ia3¯d phases of a-c) 1/14 (Ia3¯d(S))44 and 2/CN at 149 °C, d-f) 1/14
and 2/OCF3 at 121 °C and g-i) 1/14 with 2/F3 at 126 °C, at the left and right between slightly
rotated polarizers and in the middle between crossed polarizers. The induction of a I23[*]
ribbon indicates that both Ia3¯d phases should have different structure, i.e. that they represent
Ia3¯d(L) phases for compounds 2/X. The absence of any birefringence between crossed
polarizers indicates the absence of any birefringent non-cubic induced phases and the
inversion of the brightness by inverting the direction of the analyzer indicates the presence of
a chiral conglomerate in the I23[*] ribbons; the phase boundary in a, c) corresponds to the
Iso1-Cubbi transition of the 1/14+2/CN mixture.
[
No mirror symmetry broken Iso1 *] phase can be found besides any of the Ia3¯d(L) phases of
the polar substituted compounds 2/X (Table 3), though, as shown in Fig. 7 for compounds
2/NO2 and 2/OCF3, a broad feature occurrs in the DSC traces in the temperature range of the
isotropic liquid state. This indicates a transition to a percolated Iso1 phase before the cubic
phase is formed (see also Fig S1), independent if the Ia3¯d(L) phase is separated from the
isotropic liquid phase by an additional SmA range (2/NO2, 2/CN, Figs. 7a and S1e) or not
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(2/OCF3, 2/Fn, Figs. 7b and S1h-j). This shape of the diffuse DSC feature is similar to that
observed for compound 4/14 (Fig. 3a). In contrast, other polycatenar compounds, like the
more linear 5,5’-diphenyl-2,2’-dithiophene bisbenzoates (Scheme 1), allowing a denser core
29,45]
packing, do not show this kind of Iso-Iso1 transition.[
Related Iso-Iso1 transitions were
previously reported in the series of the hydrogen bonded dimers of the ANBCs with hydrogen
bonding within the cores, but not for the BABHs with intermolecular hydrogen bonding
9b,49]
between the cores (see Scheme 1),[
thus providing stronger core-core interactions. The
absence of an Iso-Iso1 transition in some cases could therefore be interpreted by a network
structure of the liquid phase, being present in the whole range of the isotropic liquid state.
This would mean that the isotropic liquids of the strongly aggregating molecules can be
considered as Iso1 phases, being present in the whole investigated temperature range of the
liquid state even at high temperatures, whereas for the weakly aggregating bent benzil
derivatives they develop only shortly before the transition to the Cubbi phase. Overall, the
broad Iso-Iso1 transition appears adjacent to LC network phases if the network develops in a
limited pre-transitional temperature range of the liquid phase.[37,49]
Figure 7. Representative DSC heating and cooling traces of compounds 2/X showing a) the
Iso-Iso1-SmA-Cubbi/Ia3¯dL transition of 2/NO2 and b) the Iso-Iso1-Cubbi/Ia3¯d(L) transition of
2/OCF3; for full scans and the DSC traces of the other compounds, see Fig. S1d,g.
14
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[
Chirality synchronization by transition to the mirror symmetry broken Iso1 *] phase,
takes place after reaching a certain degree of network connectivity, which is independent if
45,47]
there is an observable Iso-Iso1 transition (Fig. 8a,b,d) or not,[
meaning that chirality
synchronization in the liquid state takes place after the dynamic network with a sufficient
degree of connectivity has developed. However, not in all cases of liquid network phases
[
(Iso1) mirror symmetry breaking with formation of the Iso1 *] phase takes place (Figs. 7 and
8c,d). The reason could either be an insufficient connectivity of the network achieved in the
liquid state or a tendency to develop a racemic Ia3¯d-like network structure (see further
below).
3.4 Non-symmetric tetracatenars with additional fluorine substitution at the less
substituted terminal
Table 4. Comparison of the phase transitions of compounds 3/Yn having additional lateral
halogen substituents.[a]
C10H21O
O
O
O
O
O
OCnH2n+1
C10H21O
O
Y2
Y3
C10H21O
n
acub/
nm
18.1
T/°C [H kJ mol-1] [a]
H: Cr 118 [64.6] (Criso[*] 82 [17.3]) I23[*] 126 [2.2] Iso1 134 [5.7] Iso
Comd.
Y3
Y2
H
1/6[44
6
H
]
C: Iso 130 [7.1] Iso1 123 [0.5] Iso1 *] 118 [1.1] I23[*] 61 [15.3] CrIso
*
[
[ ]
3/3F6
3/2F6
6
6
F
H
F
F
F
H: Cr 93 [54.6] I23[*] 116 [0.4] Iso1 126 [9.5] Iso
17.1
C: Iso 123 [7.4] Iso1 108 [0.2] Iso1 *] 103 [0.8] I23[*] 78 [17.0] Cr
[
H: Cr 121[b] (CrIso[*] 96 [24.5] Iso1 *] 106 [0.2] Iso1 114 [3.0]) Iso
[
H
H
F
C: Iso 111 [3.1] Iso1 104 [0.2] Iso1 *] 89 [21.5] CrIso
*
[
[ ]
3/2F10
3/F26
10
6
H: Cr 103 [45.5] I23[*] 113 [1.5] Iso1 121 [4.3] Iso
18.1
C: Iso 120 [4.1] Iso1 111 [0.3] Iso1 *] 101 [0.5] I23[*] 82 [31] Cr
H: Cr 93 [16.4] I23[*] 126 [2.1] Iso1 133 [2.4] Iso
[
17.2
C: Iso 131 [2.1] Iso1 122 [0.3] Iso1 *] 117 [0.9] I23[*] 80 [17] Cr
[
3/2Cl6
3/2Br6
6
H
H
Cl H: Cr 109 [67.2] (Criso 84 [29.0] Iso1 93 [3.0]) Iso
C: Iso 86 [2.1] Iso1 78 [0.2] Ia3¯d(S) 73 [22.3] CrIso
Br H: Cr 77 [21.0] (Ia3¯d(S)[c] 80[d]) CrIso 84 [4.2] Iso1 90 [3.4] Iso
C: Iso 85 [4.5] Iso1 73 [2.4] CrIso
-
-
6
[a] DSC peak temperatures on heating/cooling (H/C) at 10 K min-1; values in brackets give monotropic
phase transitions observed in the second heating scan; abbreviations: Cr = birefringent crystalline
phase; CrIso = optically isotropic crystalline solid, CrIso[*] = mirror symmetry broken CrIso phase; for
15
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the other abbreviations, see Table 1 and 3; phases shown in bold and with [*] indicate mirror symmetry
broken phases; for DSCs, see Figs. 8, 9e and S1; the maxima of the Iso-Iso1 transitions are given, the
corresponding temperature ranges are shown in Table S1. [b] several crystalline modifications and
crystallization processes take place in first heating, only the crystalline phase with highest melting
point is given. [c] Due to crystallization no XRD could be performed. [d] The cubic phase can only be
observed in the first heating as long as CrIso is not formed.
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Figure 8. a-c) Representative DSC heating and cooling traces of a) compound 1/6 and b,c) the
44]
[
laterally core halogenated compounds 3/Y6;[ a,b) Iso-Iso1-Iso1 *]-Cubbi/I23[*] transitions of
compounds a) 1/6 and b) 3/F26 and c) Iso-Iso1-CrIso transition of compound 3/2Br6; for full
scans and the DSC traces of 3/F26 and 3/2Br6, see Fig. S1o,q. d) Schematic sketch showing
the transition from Iso via a cybotactic and a percolated liquid to Cubbi by increasing transient
network connectivity, the dots represent locally ordered clusters and the lines indicate the
connections between them, the vertical dotted lines indicate phase transitions.
44]
Compound 1/6[ was shown in previous work to form a unique sequence of three mirror
symmetry broken phases with conglomerate texture (Fig. 8a). At the transition from the
ordinary isotropic liquid (Iso) on cooling a cybotactic Iso1 phase[49] is formed around 130 °C.
This is achiral and becomes chiral at the next phase transition at 123 °C which is attributed to
[
a transition to an Iso1 *] network phase. This turns into the chiral Cubbi/I23[*] phase at 118 °C
which then on further cooling crystallizes at 61 °C with formation of an optically isotropic
[
44]
crystalline mesophase Criso *] composed of a chiral conglomerate (Fig. 8a and Table 4).[
Fluorination in the peripheral 3-position at the less substituted end (compound 3/3F6) retains
[
the fundamental phase sequence, though the Iso1 *] phases is only observable on cooling and
[
the isotropic Criso *] phase is replaced by a birefringent crystalline phase (Cr), see Table 4 and
Fig. S1k,l.
Fluorination at the inside directed 2-position (3/2F6) retains the mirror symmetry broken
[
[
Iso1 *] and Criso *] phases, but removes the cubic I23[*] phase of 1/6 completely, i.e. there is a
[
[
direct transition Iso1 *] to Criso *] (Table 4, Figs. 9 and S1m). There is no observable change of
[
[
the conglomerate texture at the transition from Iso1 *] to Criso *] (Fig. 9a-d), though it is
-1 57
]
associated with a significant transition enthalpy of 22-24 kJ mol .[ As also observed for
[
1/6,44 the XRD pattern of the Criso *] phase of 3/2F6 is characterized by a relatively broad
small angle scattering, with a maximum at d = 5.35 nm, being very similar to the length of
intercalated molecular pairs (Fig. 9f). The broadening of the small angle scattering indicates a
distorted structure with short correlation length of ~30 nm (Scherrer equation) corresponding
to about 5-6 network distances. The multiple wide angle scatterings indicate a crystalline
phase (Fig. 9g) and can be attributed to the mean alkyl chain distance (0.43 nm) and the
lateral edge-to-edge and face-to-face packings of the aromatics (0.56/0.37 nm), very similar to
[
the Criso *] phase reported for 1/6.[44] The unusually low intensity of the wide angle scatterings
58
]
(see Fig. S6)[ and the optical isotropy are in line with a strongly distorted structure with
crystallized polyaromatic cores and alkyl chain segments, with the disordered segments of the
59,60]
chains filling the remaining space.[
Additional birefringent crystalline phases with
melting points at 117 and 121 °C (magenta curve in Fig. 9e) slowly crystallize from the Iso1
or Iso phases, meaning that all mesophases of 3/2F6 are metastable.
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Figure 9. Investigation of compound 3/2F6. a, b) optically active domains in the Iso1 *] phase
[
[
at 100 °C and c, d) in the Criso *] phase at 60 °C, as observed between slightly uncrossed
polarizers after rotation in a), c) anticlockwise and b), d) clockwise direction (contrast
enhanced). The different degrees of intermediate brightness result from overlapping with
surface films having opposite chirality; the width of the pictures is 200 m; e) shows the
complete DSC heating and cooling traces (the section indicated by the dashed rectangle is
[
shown in Fig. S1m); f) small angle and g) wide angle XRD pattern in the Criso *] phase at 25
°C (see also Fig. S5 for the complete scattering pattern).
Either chain elongation (3/2F10) or 2,3-difluorination (3/F26) recovers the I23[*] phase and
[
[
retains the Iso1 *] phase, whereas the CrIso *] phase is removed and replaced by a birefringent
crystalline phase (Table 4). Note that the Cubbi-Iso1 transition temperature of the difluorinated
compound 3/F26 (Figs. 8c, S1o) is almost the same as that observed for the non-fluorinated
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compound 1/6, meaning that the steric mesophase destabilizing effect of 2-substitution is
compensated by the additional electron attracting substituent in 3-position, strengthening the
intermolecular attraction.
Figure 10. Temperature dependence of the d-value of the small angle scattering and the
correlation length (calculated with the Scherrer equation) of the clusters in the Iso phases of
compound 3/F26, measured on cooling; for compound 3/3F6, see Fig. 8b for the DSC traces,
the lift-off temperature of the broad DSC feature on cooling is around 147 °C, see Fig. S.5 for
the related curve of 3/3F6.
Investigation of the development of the correlation length  of the clusters in the isotropic
phase range by XRD on cooling was conducted for compound 3/F26 as example. The  = f(T)
curve is characterized by an increase of the slope at around 145-150 °C (Fig. 10). This
coincides with the onset of the broad DSC feature of the Iso-Iso1 transition (Fig. 8a). The
[
correlation length increases continuously from 13 nm to 36 nm in the Iso1 *] phase without
any distinct jump at the phase transitions. This is in line with the proposed model. The
increase of the d-value of the small angle scattering is also continuous, which is mainly
attributed to the transition from a diffuse scattering, not following the Bragg law, to a more
Bragg-like character of this scattering upon approaching the d-value in the LC phase. Similar
observations were made for 3/3F6, see Fig. S5.
3.5 Non-symmetric tetracatenars with other halogens at the less substituted terminal
The effect of replacing fluorine by the larger halogens Cl or Br was investigated for the
series 3/2Y6 with the halogen in the 2-position (compounds 3/2Cl6 and 3/2Br6, see Table 4).
The reason for choosing this position is that changes in this position provide a stronger effect
on the self-assembly. Whereas for 3/3F6 the fundamental phase sequence of 3/6 is retained,
the chiral I23[*] phase is removed for 3/2F6. Interestingly, replacing the fluorine of 3/2F6 by
chlorine restores the Cubbi phase, but the space group is changed to the achiral Ia3¯d phase,
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[
which occurs adjacent to the achiral Iso1 phase without additional Iso1 *] range. Moreover, the
Ia3¯d phases of 3/2Cl6 and 3/2Br6 are only monotropic and appear in small temperature
61
]
ranges, observable only on heating (Tab. 4).[ Though, due to partial crystallization, these
Cubbi phases are not accessible to XRD experiments, they can be investigated by POM. As
shown in Fig. S2d-f, in the contact region between the (previously confirmed) Cubbi/Ia3¯d(S)
44]
phase of 1/14[ and the Ia3¯d phase of compound 3/2Br6 no I23[*] phase with chiral
conglomerate texture is observed, meaning that this Cubbi phase is of the same short pitch
Ia3¯d(S) type. The increasing size of the lateral 2-substituent in the order H < F < Cl < Br
obviously changes the cubic space group from I23[*] to Ia3¯d(S), similar to the effect of alkyl
chain elongation in the series 1/n. The crystalline mesophase formed upon further cooling is
also optically isotropic as that formed by the fluorinated compound 3/2F6, but in this case it is
achiral (CrIso). It is likely that the network structure in this crystalline mesophase is Ia3¯d-like,
as in the adjacent cubic phase, which is intrinsically achiral (racemic). Also the chirality
synchronization in the liquid state is affected by the size of the halogen. Whereas for the non-
fluorinated compound 1/6 and all three investigated fluorinated compounds 3/2F6, 3/2F6 and
3/F26 chirality synchronization takes place in the temperature range of the percolated liquid
[
phase (Iso1 *], see Fig. 8b,c) it is not observed for 3/2Cl6 and 3/2Br6. A possible reason could
be that the local network structure in the percolated Iso1 phase also becomes Ia3¯d-like by the
steric effect of these larger halogens and therefore it is intrinsically achiral.
3.6 Local vs. total mirror symmetry breaking
It is noted that the observed conglomerate formation represents only a local mode of mirror
symmetry breaking. Total mirror symmetry breaking with stochastic outcome on a
macroscopic length scale can be achieved in the I23[*] phase if seed formation is slow and
growth of the cubic phase is fast and it becomes deterministic in the presence of traces of
external sources of chirality.[8,29,38,47a] Related effects were also observed in the mirror
symmetry broken soft crystalline phases formed by mesogenic trimers.[62] Tiny chirality
[
sources are also assumed to be responsible for “spontaneous” formation of I23[*] or Iso1 *]
phases with uniform chirality, as observed in some cases.[8] However, for the benzil based
compounds the tendency to form large homogeneous chiral domains is less developed if
compared with related 2,2’-bithiophene based polycatenars[29a,46] and formation of relatively
small chiral domains is dominating in this class of compounds (see Fig. 2a-c and 9a-d). A
20
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possible reason could be a higher viscosity or the bent molecular shape, slowing down the
growth processes.
3.7 Discussion of the importance of network formation for chirality synchronization
Chirality synchronization of the local helical structure by network formation, suppressing
helix reversal defects,[63] appears to be the key feature required for mirror symmetry breaking
of polycatenar mesogens in the liquid, LC and soft crystalline states. The helical networks,
[
developing in the Iso1/Iso1 *] and Cubbi Ia3¯d/I23[*] phases, can obviously be retained after
crystallization of the aromatic cores in the optically isotropic crystalline mesophases of
compounds 3/2Yn, where the polyaromatic cores and crystallized parts of the alkyl chains in
the networks assume a crystalline packing and the disordered segments of the alkyl chains fill
the remaining space. These crystalline mesophases are in some respect related to gels
(“solvent free gels”),[43,59,60,62,64-66] though details of their structures require further
investigations. A certain degree of net connectivity is in some cases also retained in the
67,68]
isotropic liquid phases, considered as percolated liquids[
and representing dynamic
networks, which become chiral after crossing the critical network density required for
47,38]
emergence of chirality synchronization.[
These percolated liquids can transform into
chirality synchronized cubic LC phases after assuming long range periodicity and to
disordered crystalline isotropic mesophases by freezing the molecular mobility.
For numerous compounds local clusters and network formation occurs in the liquid
state prior to the transition to LC and crystalline phases. Often, these transitions take place
already at high temperatures outside the investigated (or accessible) temperature range, or do
not show up in the DSC traces, because they take place over extremely broad temperature
ranges. However the DSC features of the Iso-Iso1 transition of the benzil derived polycatenars
appear in relatively narrow temperature ranges close to the phase transition to the Cubbi phase
and therefore become easily detectable. The degree of attractive intermolecular interactions
and the strength of segregation of the incompatible units appear to be responsible for the
distinct types of behaviour. Polyamorphism in the liquid state appears to be associated with
network formation and therefore is observed in the vicinity of Cubbi phases. It can also be
found if a lamellar or columnar phase separates the Iso1 and Cubbi phase (compounds 2/CN
and 2/NO2, Fig. 7a). It is however not observed at the transition to non-cubic LC phases
which are (with respect to molecular structure or temperature) far away from any Cubbi phase
range. Depending on the kind of network structure, the resulting liquid, LC or crystalline
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mesophases can be optically inactive if a racemic double network (Ia3¯d) is formed, or
optically active if a homochiral triple network develops (I23[*]). As the local structure can
change at the phase transitions also phase sequences combining chiral and achiral phases can
[
occasionally be observed (e.g. Ia3¯d-Iso1 *]).
An important point concerns the formation of the I23[*] structure instead of the usually
dominating double gyroid (Ia3¯d) structure, which is only observed if rod-like units are
involved in the molecules, leading to a helical twist along the networks. Therefore, it can be
assumed that this transition is chirality (helicity) induced. One way of thinking considers the
29a]
matching of the twist at the nodes in the individual nets (intra-network resonance).[
A
second important issue concerns the through-space interaction between the nets, i.e. the
interaction between adjacent close packed helix segments. Here a slight change of the twist
angle of the molecules along the helices could modify the sign and strength of inter-helix
69
]
interactions at a given angle and distance between the close packed adjacent helices.[
Preferred heterochiral interactions are obviously compatible with the Ia3¯d phase structure
composed of two enantiomeric networks, leading to chiral extinction (lowest energy racemic
state). However, an increasing preference of homochiral helix-helix interaction (lowest energy
homochiral state) could destabilize the double gyroid structure and requires a change of the
phase structure by retaining the network topology involving exclusively three way junctions.
The I23[*] structure appears to be the best solution for this case, though also non-cubic
40]
arrangements of helical networks, as for example in the tetragonal “SmQ phase”,[ could
have very similar energy and can compete under certain conditions.
The softening of the helix-helix interactions in the liquid state could change the
preferred mode of helix correlation and this could explain the transition between an achiral
[
double gyroid cubic phase and a conglomerate type Iso1 *] liquid (with local I23 structure)
observed in some cases. Though there is a substantial amount of work concerning the
70
]
investigation of helix-helix interactions in lipid membranes,[ the general understanding of
the helix packing depending on the sign and strength of the interaction parameters, the helical
twist, and the angle between the interacting helices needs to be studied and simulated in more
69]
detail to support this hypothesis.[
It is noted that there is a strong relation between the Cubbi-Iso[*] transitions reported
here for achiral molecules with transiently chiral conformations and the BPII-BPIII (BP =
71,72]
blue phase) transition observed for permanently chiral rod-like molecules.[
Whereas the
cubic BPII has a Ia3¯d double gyroid structure, the BPIII (blue fog phase) is an isotropic liquid
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with dynamic network structure. Even the typical DSC features of this transition resemble
those of the Cubbi-Iso[*] transitions, characterized by a broad diffuse feature in the liquid
49]
state.[ The main difference is that in the case of the Blue Phases the networks are chirality
induced, whereas for the achiral polycatenar compounds reported here the network structure
induces the chirality by destabilizing the achiral/racemic state which then bifurcates into a
conglomerate of chiral liquids due to the emergent cooperativity of chirality transfer provided
8]
by the network structure.[
Conclusions
It is shown that the 4,4’-diphenylbenzil unit is a universal transiently chiral bent building
block for the design of mesogens capable of forming network structures in the liquid, the LC
and the crystalline state. Cubbi network structures with Ia3¯d and I23[*] space groups were
obtained for non-symmetric tetracatenars (compoounds 1/n, 4/n and 3/Yn) and tricatenars
(compounds 2/X) if the total number of benzene rings incorporated in the core unit is at least
five. Polar substituents at the apex (X) were shown to support cubic phase formation, leading
to compounds with high transition temperatures and wide cubic ranges. These polar
substituents represent strong electron acceptors which reduce the electron density of the -
system of the cores and this strengthens the attractive core-core interactions, thus stabilizing
the cubic phases significantly, even against unfavorable steric effects of these substituents
(Table 3). The mesophase stabilizing effect grows in the order X = F < OCF3 < CN < NO2,
51]
roughly corresponding to the increasing Hammett constants of these substituents (Fig. 11).[
The cubic phases of all these compounds with polar apex represent long pitch Ia3¯d(L) phases,
thus indicating a relative weak helical twist between the molecules along the networks. Larger
twist angles were achieved with the longer alkoxy chains, supporting the formation of the
mirror symmetry broken I23[*] phase. Halogenation of the monoalkylated benzene ring
further supports the helical twist, leading to a transition from I23[*] to the achiral short pitch
Ia3¯d(S) phase for the larger halogens Cl and Br (Fig. 11). Especially strong is this effect in the
inside directed 2-position. Besides the effect on the twist, there is also a steric mesophase
destabilizing effect of the polar substituents X and Y, becoming more important in the
sequence of their positions 4 < 3 < 2. Thus, the steric effects of substituents become more
important and override the stabilizing electronic effects, the more they are shifted towards the
center of the polyaromatic core.[73,74] Figure 11 summarizes the relations between molecular
structure and formation of the distinct cubic and accompanying mirror symmetry broken
[
Iso1 *] phases gained herein.
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Figure 11. Summary of the relations between molecular structure, cubic phase type, cubic
phase stability and mirror symmetry breaking in the LC and isotropic liquid state.
Overall, this work contributes to the better understanding of the development of
networks and spontaneous mirror symmetry breaking in self-assembled systems, including
43,64-65,75]
crystalline helical aggregates,[
60,66]
38,59,60,62,76-79]
gels,[
LC phases[
and the most
intriguing case of mirror-symmetry broken isotropic liquids.[47] In general, the nano-structures
of the liquid phases and their polyamorphism (liquid-liquid phase transitions and liquid-liquid
phase separations) are still less investigated and represent almost unsolved scientific problems
80
]
with great importance.[ It appears that transitions in the liquid state with formation of
cybotactic clusters or dynamic networks are important features with relevance for the
understanding of the special properties of fluids, especially in the context of prebiotic and
8]
biological systems.[ It appears that Iso-Iso transitions (polyamorphism) can be found more
frequently than commonly thought. Network formation appears to be responsible for many of
these transitions, as well as it is a basic prerequisite for development of complexity in any
form of matter. Dynamic network formation in liquids obviously provides the basis of the
emergence of new features. Among them the bifurcation of achiral and racemic systems into
stable homochiral states, thus leading to the transition from achiral to chiral chemical systems,
8]
being the first step of the transition from chemical systems to biogenesis.[
Aknowledgements
This Work was supported by the Deutsche Forschungsgemeinschaft (Ts 39/24-2).
24
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