1. Introduction
Chiral mesophases such as the chiral nematic (N*) or the blue phases (BPs) are regarded
as promising materials for the development of photonic materials, due to their periodic
nanostructures.[1] For example, the exoskeleton of the jewelled beetle, Chrysina gloriosa, is made of helicoidally stacked chitin nanofibrils.[2] These structures selectively reflect circularly polarised light and cause the characteristic
metallic green appearance of the beetle.
Chiral-nematic structures represent one-dimensional photonic crystals, because of
their periodically changing refractive indices along the director axis.[3] The reflected wavelength depends on the pitch (P) of the helical structure, the refractive index contrast (n
avg) of the compound and the angle of the incident light (sin(θ)), and can be described
by a deviation of the Bragg law.[4] Thereby, the helical pitch corresponds to the repeating distance of a full 360°
turn of the liquid crystalline molecules. Materials appear structurally coloured when
half of the pitch (P/2) of the chiral structure is in the region of the wavelength of visible light (see
[Figure 1]). Due to the helical photonic structure, the reflected light is circularly polarised
with a handedness determined by the chirality, and a maximum of 50% of the incident
light will be reflected. The remaining 50% light with opposite handedness transmits
through the film without loss of intensity.
Figure 1 Schematic of a usual phase sequence of a blue-phase liquid crystal. Phase sequence:
crystalline phase (Cr), chiral nematic phase (N*), blue phase (BP), and isotropic
liquid (Iso). Helix of a chiral nematic liquid crystal with reflection of circularly
polarised light and equation of Braggʼs law. A body-centered cubic unit cell of BP
I (A) and a simple cubic unit cell of BP II (B).
In 1888, Reinitzer observed the unusual melting behaviour of cholesteryl benzoate,
the first example of a chiral nematic phase.[5] However, it was Lehmann who comprehensively investigated this mesophase and coined
the term “liquid crystal” (LC).[6] They also observed the first BP, which was initially considered as a transition
phenomenon in the transition from the isotropic to the chiral nematic phase. But only
in 1973, these mesophases were recognised as distinct liquid crystalline phases by
Coates and Gray and named as “blue phase,” which relates back to the first observations
of Reinitzer observing a blue-coloured reflection of the mesophase.[7] Three kinds of BPs are known: BP I, BP II and BP III. While BP III represents a
disordered, amorphous structure,[8] BP I has a body-centered cubic symmetry ([Figure 1]A) and BP II a simple cubic symmetry ([Figure 1]B).[9] Both, BP I and BP II have a periodic nanostructure built by double-twisted cylinders
(DTCs). Since it is impossible to fill the three-dimensional space continuously by
the DTCs, line defects occur, the so-called disclination lines.
Due to this unique structures, BP possess interesting properties like sub-millisecond
response times to electrical fields, Bragg reflection of circularly polarised light
and, due to their cubic unit cell, optical isotropy.[10] Therefore, these materials are candidates for the usage in applications such as
displays[11], photonic sensors or switchable diffraction gratings ([Figure 2]).[12] With respect to photonic applications, BP I and BP II represent three-dimensional
photonic crystals, whereby the maximum of the reflected wavelength λ is described by:[9]
where n represents the average refractive index contrast, d is the lattice constant of the BP, θ is the angle between the incident light and the crystallographic direction, and h, k, and l are the Miller indices of the crystal orientation planes.
Figure 2 Milestones in BP research.
BPs are accessible by doping nematic host materials with chiral dopants with high
helical twisting powers (HTPs).[13] However, a limitation with respect to applications is the narrow temperature range
of only a few K, in which this specific mesophase usually appears. Therefore, in the
last few decades, a lot of efforts have been made into the stabilisation of BP for
making them appealing for application in optoelectronics or photonic sensing. To stabilise
the structural features of BPs, and thereby widening the temperature range, different
approaches have been followed like the introduction of polymers[11c] and nanoparticles (NPs),[14] or by doping with low-molecular weight additives.[15] Polymer stabilisation was first reported by Kikuchi et al. who could achieve a BP
temperature range of more than 60 °C.[16] This most prominent method to stabilise BPs is based on the accumulation of polymeric
material in the disclination areas of the three-dimensional structure ([Figure 3]A). Since the existence of defects requires sufficient energy, which is usually provided
as thermal energy, BP vanishes at lower temperatures due to a lack of energy. As soon
as the disclination lines are filled by a polymer, the energy requirement for the
maintenance of the defects is lower and the BP can exist over a broader temperature
range.
Figure 3 Disclination lines of BP II. Accumulation of polymer coils (left) or nanoparticles
(right) in the disclination volume.
Another common method is to dope BP liquid crystals (BPLCs) with NPs, which also accumulate
in the disclination lines ([Figure 3]B), and stabilise the BP structure by similar effects mentioned above.[17] Additionally, some NPs can positively influence the switching voltage of BPs.[18] For example, the LaF3-NPs utilised by Zhu et al. as dopants lowered the on-state voltage of polymer-stabilised
BPLCs to about 59% by increasing the anchoring energy of the mesogen molecules due
to their large dipole moments.
A comprehensive overview on polymer and NP stabilisation of BPs was given in a recent
review by Rahman et al.[17b]
In addition, a series of unconventional approaches have been reported. For example,
Zhao et al. doped their BPLC composites with aminoalkyl-substituted graphene oxide
flakes and successfully broadened the temperature range of the BP to 6.4 °C.[19] Furthermore, Lin et al. confined their BPLC in micrometer-scale two-dimensional
honeycomb microwells and showed that this leads to an increase in BP stability.[20] In addition to the broadening of the BP temperature range to ΔT = 7.1 °C, the reflectivity and the thermal stability of the reflection peak wavelength
have also been improved.
These examples show that a variety of methods have been employed to stabilise BPs
and broaden the temperature range of the chiral mesophase. In all cases, a balance
of attractive and repulsive intermolecular forces between the liquid crystalline system
and the stabilising additives is crucial for efficient stabilisation. Therefore, the
transfer of supramolecular principles and the specific targeting of non-covalent bonds
are a promising approach for superior BPLC materials. Here, especially the employment
of hydrogen bonding (H-bonding) and halogen bonding (X-bonding) appears promising,
since these non-covalent interactions are highly directional and easy to tune in their
strength. Furthermore, they provide a facile synthetic access to a variety of new
supramolecular mesogens, which can be tuned by simple replacement of the donor or
acceptor moiety. However, while a series of BPLCs are reported which were constructed
or stabilised by hydrogen bonding, to the best of our knowledge, efficient stabilisation
of BPs via halogen bonding has not been reported so far. Therefore, the aim of the
present review is to summarise the recent efforts made to stabilise BPs by employing
hydrogen bonds and the principles of supramolecular chemistry.
2. Stabilisation of Blue Phases by Supramolecular Methods
As pointed out in the Introduction section, the main disadvantage of BPLCs is the
narrow temperature range in which the BP occurs. However, since the formation of the
liquid crystalline state is the result of a balanced interplay of attractive and repulsive
intermolecular forces, supramolecular chemistry, as the chemistry of the non-covalent
bond, appears to be the perfect tool to address this challenge. Within the past three
decades, supramolecular chemistry has evolved to an effective approach towards functional
materials.[21] With respect to liquid crystals, especially hydrogen[22] and halogen bonds[23] have proven to be ideal supramolecular forces to induce and stabilise mesophases.
A number of studies employed hydrogen bonds for the stabilisation of BPLCs. However,
at the moment, the studies report mostly individual examples, and the empirical results
raise a lot of questions with respect to a comprehensive understanding of structure–property
relationships. In general, there are two different approaches to utilise hydrogen
bonding in the stabilisation of BPs found in the literature. The more common method
adds hydrogen-bonded assemblies (HBAs) as a dopant to a BPLC mixture (doping approach),
while the second approach uses hydrogen bonding for the formation of self-assembled
mesogens (design approach) with BPs. More specifically, in the doping approach, the
materials are based on conventional, mostly commercially available nematic host LCs,
which are doped with H-bonded additives to broaden the BP temperature range. In this
case, the hydrogen-bonded assemblies may be liquid crystalline, but they do not have
to be liquid crystalline. In contrast to this, the design approach employs hydrogen
bonding to construct new supramolecular mesogens forming BPs. Here, no separate host
material is needed to facilitate liquid crystallinity.
2.1. Doping Approach
The doping approach usually starts from a commercially available nematic LC host system.
A chiral dopant is added to induce chiral mesophases. The hydrogen-bonded assemblies
are added to stabilise the BPs by, e.g., introducing a bent-shaped structure, adding
structural flexibility to adopt the chiral structure and improve the compatibility
of the host and dopant system. In some examples, the HBAs bear chiral entities to
increase the HTP and thereby broaden the temperature and concentration ranges of the
BPs.
The first example using a HBA as an additive for BPLCs was reported by Guo et al.[24] The LC mixtures consisted of an achiral mesogenic host (8CB) and a chiral dopant (ISO(C6OBA)2
), and for stabilisation via polymerisation mono- and di-functional monomers together
with a photo-initiator. In their studies, they screened the influence of two different
HBAs ([Figure 4]), one terminated with a chiral branched alkyl chain (CB15 acid⋯ISIN) and another with an alkoxy chain (DOBA⋯ISIN), as additives in LCs on the stability of BPs. By varying the content of HBAs, they
could successfully broaden the BP temperature range up to ΔT = 13 °C in the cooling cycle for a mixture containing 8 wt% HBA CB15 acid⋯ISIN. They explained their observations by an increase in HTP and viscosity. The HBAs,
as isosorbide derivatives, can act as additional chiral dopants to increase the HTP
and thereby the chiral force in the whole system. This helps to stabilise the BP over
a wider temperature range. Additionally, due to their size and shape, the HBAs have
a high steric demand, which increases the viscosity of the material. This in turn
leads to a stronger supercooling effect and a longer persistence of the BP.
Figure 4 Hydrogen-bonded assemblies reported by Guo et al.[24] Colour code: host liquid crystal (blue), covalent chiral dopant (red) and hydrogen-bonded
assembly (green). Phase sequence on cooling for the mixture with the composition:
8CB/MF-LCM/C6 M/ISO(C6OBA)2
/CB15 acid⋯ISIN/Irgacure 651 64.72/18.30/3.48/5.5/8.0/0.44 wt%: not specified phase (X), blue phase
(BP), isotropic liquid (Iso). Transition temperatures were determined from the graphics
in the publication, unless stated in the text.
Later, the approach was extended to bent- and T-shaped HBAs as dopants for cholesteric
LCs. These molecular shapes are discussed to be beneficial for the stabilisation of
chiral mesophases. The reason for the stabilisation of BPs is not yet fully understood.
However, the addition of bent- or T-shaped HBAs is known to increase the HTP of a
chiral dopant present in the same mixture and dopants with high HTP values are known
to be beneficial for the formation of BPs.[25] Another factor which is discussed is the biaxiality of these additives, which seems
to contribute to the stabilisation of the DTCs of BPs.[26] Additionally, the bent-shaped structure can decrease the free energy for the disclinations
by introduction of elasticity to the surfaces of the DTCs. To understand this effect,
different possible factors have to be considered.[27] One is the reduction of interfacial tension due to higher elastic constants. The
other aspect is the reduction of the disclination line radius caused by a reorientation
of the molecules in the defect volume induced by the bent core additive. In 2003,
Shi et al. employed SLC1717 as a host system (83 wt%) doped with 10 wt% chiral dopant R811 and 7 wt% chiral dopant ISO(C6OBA)2
.[28] The BP temperature range could be successfully increased to ΔT = 15.1 °C for the mixture containing TBOA⋯MBIN ([Figure 5]). Additionally, the BP–N* transition temperature was brought closer to room temperature
(39 °C). The major difference with the previously reported systems was the non-linear
structure of the HBAs. According to the authors, the combination of chirality and
biaxiality was the main reason for the stabilisation of BP. In addition, tertiary
butyl and other branched end groups at the central core and the edges are beneficial
for the BP temperature range since they force a competition between attractive π–π
interactions and steric repulsion and thereby promote the double-twisted arrangement.
As compared to conventional covalently bound bent- and T-shaped molecules, the HBAs
are more flexible and can therefore adapt more efficiently to its environment resulting
in an increased miscibility. In addition, the flexibility of the assemblies leads
to weaker intermolecular interactions leading to lower BP–N* transition temperatures.
Figure 5 Bent-shaped hydrogen-bonded assemblies.[28] Colour code: covalent chiral dopant (red) and hydrogen-bonded assembly (green).
Phase sequence on cooling for the mixture with the composition: (SLC1717[29]
/R811/ISO(C6OBA)2
)/TBOA⋯MBIN (83.0/10.0/7.0)/16.67 wt%: chiral nematic phase (N*), blue phase (BP), isotropic
liquid (Iso). Transition temperatures were determined from the graphics in the publication,
unless stated in the text.
In an attempt to generate optically tunable BPLCs, Jin et al. doped H-bonded chiral
azobenzene switches in their LC system: SLC1717, S811 and TBOA⋯MBIN from the previous study of Guoʼs group.[28],[30] The dopants ([Figure 6]) are based on a binaphthyl core BNAzo connected to two peripheral pyridyl units via an azo-benzene-containing linker group.
Together with either octanoic OCA or 4-methyl hexanoic carboxylic acid MHA, the corresponding HBAs were obtained as dopants. With this further development of
the earlier system, the BP temperature ranges were also increased by the doping of
the LCs up to ΔT = 21 °C for the HBA BNAzo⋯MHA with the branched terminal chain. This effect was explained by an increase in HTP
leading to a wider BP temperature range. Additionally, they could show that irradiation
of BNAzo⋯OCA with UV light (365 nm, 10 mW · cm−1) induced photo-isomerisation of the azo moiety and results in an increase of the
lattice constant of the DTC, yielding a bathochromic shift in the reflection maximum
from 473 to 642 nm ([Figure 7]). The irradiation of the sample with visible light (450 nm, 15 mW · cm−2) caused the reversion of the trans–cis isomerisation and therefore the reflection wavelength shifts back to 473 nm. It should
be noted that all these processes occurred in a stable BP I and no phase transitions
could be observed during the experiment. Like mentioned above, the increased stability
of the BP was explained by an increased HTP in the system by substituting the isosorbide-based
dopant, ISO(C6OBA)2
, from the system of Shi et al. with the binaphthol-based dopant (BNAzo). This indicates that the HTP value of the employed chiral dopant is crucial for
the stability of the BP. The exact role of the H-bond in this system was not identified
by the authors but an increase in the size of the chiral dopant leading to an improved
HTP by the attachment of the carboxylic acids appears reasonable.
Figure 6 Hydrogen-bonded assembly building blocks reported by Jin et al.[30] Colour code: covalent chiral dopant (red) and hydrogen-bonded assembly (green).
Phase sequence on cooling for the mixture with the composition: (SLC1717[29]
/S811/BNAzo⋯MHA)TBOA⋯MBIN (71.5/25.0/3.5)20.0 wt%: chiral nematic phase (N*), blue phase (BP), isotropic liquid
(Iso). Transition temperatures were determined from the graphics in the publication,
unless stated in the text.
Figure 7 Shifting of absorption maxima during treatment with UV- or Vis-irradiation. Reprinted
with permission from Ref. [30]. Copyright 2014 Royal Society of Chemistry.[30]
Another series of supramolecular azobenzene switches ([Figure 8]) was reported by Wang et al.[31] In contrast to the previously reported system (see BNAzo, [Figure 6]),[30] the photo-switch is now introduced via DAIC in a peripheral position. The HBAs are composed of two pyridine-containing azo compounds
H-bonded to an isophthalic acid (IPA) or a 4-bromoisophthalic acid (BIPA) core. This gives them a bent-core shape resembling the non-switchable bent-core
dopants ([Figure 5]) from the same group.[28] Like in the previous study, a mixture of SLC1717 and S811 was used as a host system. Although the BPs generated by doping of the bent-core
HBAs are not as stable as the ones reported before, the optical switching properties
were different. In contrast to the earlier reports from Guoʼs group,[30] the new systems exhibited a hypsochromic shift of the reflected wavelength induced
by trans–cis isomerisation via irradiation with UV light (365 nm, 10 mW · cm−1). This effect is due to the back-folding of the peripheral H-bond acceptors towards
the core moiety of the HBAs and thereby reducing the lattice parameter due to the
reduced length of the assembly in the direction of the long axis. This explanation
is in concert with the one for the BNAzo system, meaning an expansion of the assemblies leads to a bathochromic shift, while
a contraction leads to a hypsochromic shift due to the changes in lattice constant.
Regarding the stability of the BP, the widest temperature range of ΔT = 16.7 °C was achieved by doping of the host LC with 3% of brominated HBA BIPA⋯DAIC. In contrast to the non-brominated derivative, there was no constant trend towards
lower BP transition temperatures with higher contents of additives but an increase
with higher concentrations. According to the authors, the presence of bromine in the
HBAs decreases their compatibility with the host LC system and therefore the BP transition
temperature increases with higher amounts of the dopant. This in turn increases the
total chirality of the LC system. Also, the reduction of free energy around the defect
lines due to the biaxiality of the HBAs and the better compatibility induced by the
H-bonds were mentioned as reasons for the increased BP temperature range. The reports
summarised so far indicate that the introduction of bent-shaped HBAs is beneficial
for the stabilisation of BPs and that the assemblies increase the HTP yielding a broader
temperature and concentration range of the BPs.[24],[28],[30],[31] In this context, the structural flexibility and reversibility of hydrogen bonds
are discussed to be beneficial since they more effectively adopt the double-twisted
structure of the BPs as compared to rigid covalent-bonded analogues.
Figure 8 Bent-shaped azo-benzene switches.[31] Colour code: covalent chiral dopant (red) and hydrogen-bonded assembly (green).
Phase sequence on cooling for the mixture with the composition: (SLC1717[29]
/S811)BIPA⋯DAIC (67/33)3.0 wt%: chiral nematic phase (N*), blue phase (BP), isotropic liquid (Iso).
Transition temperatures were determined from the graphics in the publication, unless
stated in the text.
In a study of He et al., chiral H-bonded molecules and their covalent bonded analogues
were synthesised ([Figure 9]) and employed as additives in chiral nematic LC systems, composed of 75 wt% of nematic
LC host SLC4 and 25 wt% of chiral dopant S811.[32] The additives were synthesised in different sizes, with 3, 4 or 5 hydrocarbon cycles
in their mesogenic core. Also, the terminal alkyl chain as well as the fluorination
pattern was varied. In general, longer alkyl chains and a higher number of rings led
to broader temperature range of the mesophases, chiral nematic and chiral smectic
A, with enantiotropic phase behaviour and higher clearing temperatures in the pure
compounds, which was attributed to the increased length-to-width ratio. When used
as additives in the host LC, it was shown that the HBAs, even though possessing lower
HTPs compared to the covalent molecules, can induce wider BP temperature ranges. The
broadest BP temperature range was about ΔT = 20 °C for the system doped with the HBA S8HBA⋯PyS8PBA ([Figure 9]) bearing chiral terminal groups on both ends, the H-bonding acceptor and the donor
unit. As explanation, the authors stated that the reversible and flexible H-bond can
more easily respond to structural changes in surroundings to assume lower energy positions
during self-assembly than the covalent analogues and therefore lowering the disclination
line volume and the corresponding free energy, which is in line with previous studies.
Interestingly, the authors found that the HBA, having a lower HTP value than the covalent
analogue stabilises the BP more efficiently. This contradicts previous statements
claiming higher twisting power to be beneficial for BP stability.
Figure 9 First generation of hydrogen-bonded assemblies reported by He et al.[32] Colour code: covalent chiral dopant (red) and hydrogen-bonded assembly (green).
Phase sequence on cooling for the mixture with the composition: (SLC4[29]
/S811)/S8HBA⋯PyS8PBA (75/25)/35 wt%: chiral nematic phase (N*), blue phase (BP), isotropic liquid (Iso).
Transition temperatures were determined from the graphics in the publication, unless
stated in the text.
This contradiction seems to support the findings by Li et al., where a chiral mesogen
M was doped with an achiral supramolecular assembly (A⋯PAB, [Figure 10]) to increase the BP temperature range.[33] The HBAs are assembled from a pyridine-containing molecule PAB as a hydrogen-bond acceptor (H-acceptor) and a carboxylic acid (8A, 10A, 12A or 14A) as a hydrogen-bond donor (H-donor), which were varied with respect to their alkyl
chain length. While doping the individual HB donors or acceptors to the LC mixture
had only a minor effect on the mesogenic behavior of the system, doping with the HBA
led to a significant increase in the BP temperature range as shown in [Figure 8]. Although the HBAs possess neither chirality nor any LC phases, the effect on the
BP of the LC after doping was evident. Interestingly, the LC system showed only a
chiral smectic and a cholesteric phase before doping with the achiral HBAs. However,
after the addition of the HBAs, BPs were induced, whereby the temperature range of
the BP depended on the chain length of the used carboxylic acids as well as their
concentrations ([Figure 11]). The best results were obtained for 10A⋯PAB with a dopant amount of 18 wt%, giving rise to a BP temperature range of ΔT = 45.6 °C. According to the authors, the presence of the H-bond promotes an intramolecular
twist in the neighbouring mesogen molecules. At the position of the H-bond, the diameter
of the mesogen is reduced, which leaves something like a notch. The turning of the
neighbouring molecules allows them to come closer to the HBA and slide into this notch.
This means the neighbours of the HBA turn in opposite senses around the H-bond to
practically squeeze it between them ([Figure 12]). Taking this into account, the proposed stabilisation mechanism is in fact in line
with the examples given above, reasoning the increase in twisting power in the system
to be the crucial factor for stabilising BPs over a wider temperature range.
Figure 10 Chiral mesogens and hydrogen-bonded assembly dopants.[33] Colour code: host liquid crystal (blue) and hydrogen-bonded assembly (green). Phase
sequence on cooling for the mixture with the composition: M/10A⋯PAB 82/18 wt%: crystalline phase (Cr), chiral smectic phase (Sm*), chiral nematic phase
(N*), blue phase (BP), isotropic liquid (Iso). Transition temperatures were determined
from the graphics in the publication, unless stated in the text.
Figure 11 Summary of BP temperature ranges compared to different additives at different concentrations.
Reprinted with permission from Ref. [33]. Copyright 2014 Royal Society of Chemistry.[33]
Figure 12 Schematic depiction of the tilt induced by the hydrogen-bonded assembly. Reprinted
with permission from Ref. [33]. Copyright 2014 Royal Society of Chemistry.[33]
In an approach of Kishikawa et al., the chiral succinimide derivative 1 bearing a hydroxyl group at the edge of its terminal alkyl chain was synthesised
([Figure 13]).[34] This compound was used as an additive in the commercially available LC N-(4-ethoxybenzylidene)-4-n-butylaniline (EBBA), to increase the BP temperature range to ΔT = 9.5 °C upon heating and ΔT = 22.7 °C upon cooling. Furthermore, the addition of alkane diols to the already
doped mixture led to an extension of the BP temperature range up to ΔT = 35 °C in the cooling cycle. The BP was remarkably stable at room temperature since
the lower transition temperature was below 20 °C. Also the impact of the length of
the diols was investigated during this study. The best results were achieved for octane
diol while the results for the two longer additives (decane diol slightly and dodecane
diol considerably) were inferior. Furthermore, the increase of amount of diol additive
was not favourable; the increase from 0.5 mol% to 1.0 mol% reduced the temperature
range of the BP to ΔT = 6.5 °C on heating. For comparison, a succinimide derivative without hydroxyl group
at the terminal alkyl chain was synthesised and tested as an additive together with
octane diol. In this case, the BP temperature range was much narrower, ΔT = 3.9 °C on heating and ΔT = 13.3 °C on cooling, which confirms the relevance of the lateral hydroxyl group.
The proposed mechanism for the stabilisation of the BP was the arrangement of the
bent-shaped succinimide molecules around the DTCs with their terminal alkyl chains
filling the disclination lines. The diols in the mixture form reversible hydrogen-bonded
networks throughout the disclination areas between the hydroxyl groups of the succinimide
molecules and thereby stabilise the BP structure. However, an explanation why some
of the diols have a positive effect and others do not was not provided by the authors.
Figure 13 Succinimide derivative reported by Kishikawa et al.[34] Colour code: host liquid crystal (blue) and covalent chiral dopant (red). Phase
sequence on cooling for the mixture with the composition: EBBA/1/1,8-octanediol 94.5/5.0/0.5 mol%: blue phase (BP), metastable chiral nematic phase
(N*), isotropic liquid (Iso). Transition temperatures were determined from the graphics
in the publication, unless stated in the text.
2.2. Design Approach
The second approach is to design the BP-forming mesogens with H-bonds. Here the host
and the dopant are obtained via self-assembly of hydrogen-bonded building blocks.
He et al. obtained their LC diad ([Figure 14]) by H-bonding of an achiral pyridine-bearing molecule PPI and a chiral carboxylic acid SHBA/SFBA.[35] Additionally, excess acid formed homo-dimers in the mixture. Careful variation of
the molar ratios of the two components, H-donor and H-acceptor, led to a BP with a
temperature range of ΔT = 23.0 °C at a molar ratio of about 2 : 1 (H-donor : H-acceptor) for a fluorinated
derivative (SFBA) of the acid. Since the excess carboxylic acid forms homo-dimers, it can be seen
as a chiral H-bonded dopant for the LC and therefore can contribute to the extension
of the BP temperature range. However, the acid does not show mesogenic behaviour and
too much additional acid supresses the formation of mesophases. Part of their explanation
for the widened BP was the flexibility of the H-bonds which facilitates the formation
of the double-twisted structure. Also, the presence of the fluorine substituents was
expected to have a positive effect on the BP stability since it can form weak H-bonds
with aliphatic protons to further stabilise the BP structure. In addition, the electron-withdrawing
character of the fluorine might assist in the packing of the aromatic rings by reducing
the electron density of the π-system.
Figure 14 Second generation of hydrogen-bonded assemblies reported by He et al.[35] Colour code: hydrogen-bonded assembly (green). Phase sequence on cooling for the
mixture with the composition: SFBA/PPI 2/1 molar ratio: crystalline phase (Cr), chiral smectic phase (Sm*), chiral nematic
phase (N*), blue phase (BP), isotropic liquid (Iso). Transition temperatures were
determined from the graphics in the publication, unless stated in the text.
Later, another derivative (SOCA⋯PPI, [Figure 15]) based on cinnamic acid was reported by He et al. to be H-bonded with the previously
mentioned achiral H-acceptor bearing a pyridine.[36] Although the BP temperature ranges were not as wide as in the previously reported
systems, only ΔT = 10 °C for the 2 : 1 complex, BPs could also be observed in the heating cycle, which
was not the case before. In this report, the stability of the BP was attributed again
to the chirality induction of the homo-dimer dopant and the flexibility of H-bonds.
Additionally, according to DFT calculations, the cinnamic acid-derived compounds possessed
a greater H-bond distance and inter-plane bending. Therefore, the enhanced flexibility,
induced by the longer H-bond and the additional double bond, of the newer molecules
as well as the stronger bent-shape character can explain the enantiotropic behaviour
of the BP. These two examples demonstrate that the flexibility provided by H-bonds
does not only positively influence the compatibility but also interactions in the
host system.
Figure 15 Third generation of hydrogen-bonded assemblies reported by He et al.[36] Colour code: hydrogen-bonded assembly (green). Phase sequence on cooling for the
mixture with the composition: SOCA/PPI 2/1 molar ratio: crystalline phase (Cr), chiral smectic phase (Sm*), chiral nematic
phase (N*), blue phase (BP), isotropic liquid (Iso). Transition temperatures were
determined from the graphics in the publication, unless stated in the text.
Huang et al. reported Schiff base derivatives ([Figure 16]) exhibiting mesogenic behaviour including BP.[37] This example stands out, since the mesogens are not built by hydrogen bonds rather
than covalent ones. However, in one series of molecules they possess hydroxyl groups
forming intramolecular H-bonds leading to the formation of the BP. These compounds
do not possess large BP temperature ranges; however, they nicely demonstrate the influence
of intramolecular H-bonding inside of the mesogens on the existence of BP. Two series
of molecules were synthesised, one bearing the branched chiral alkyl chain near the
salicylaldimine core (OH II) and one with the chiral centre far from the core (OH I). In the first series, a compound possessing a hydroxyl group exhibits a BP temperature
range of ΔT = 3.6 °C, while the structurally related derivative H I, missing the hydroxyl group, exhibits no BP. Also, the increase in terminal chain
length caused the BP to disappear, so only compounds of the first series with n = 6 and 7 exhibited BPs. For the ones with longer alkyl chains, the BP did not appear.
In contrast, all molecules of the second series did possess BPs, so no conclusions
could be drawn regarding the influences on the BP formation. The observation in the
first series, the disappearance of BP by removing the H-bonding moiety, was explained
by the enhanced rigidity and polarity induced by the H-bonds. This statement is in
clear contrast to the reasons claimed by other groups for the stabilisation of BPs,
which usually claim the flexibility of the HBA structure as a reason for the stabilisation.
Possibly, this is due to the fact that in this example discrete molecules are employed,
whereas in the other cases, HABs are utilised. But still, rigidity contradicts flexibility.
Figure 16 Schiffʼs base mesogens reported by Huang et al.[37] Colour code: covalent liquid crystal (blue). Phase sequence on cooling for the mixture
with the compound: OH I (n = 7): crystalline phase (Cr), chiral smectic phase (Sm*), chiral nematic phase (N*),
blue phase (BP), isotropic liquid (Iso). Transition temperatures were determined from
the graphics in the publication, unless stated in the text.
A comparative study was published by Wei et al. in which they investigated the different
effects of H-bonded mesogens and compared their properties with the covalent-bonded
counterparts ([Figure 17]).[38] A library of different molecules was synthesised including chiral and achiral as
well as fluorinated and non-fluorinated carboxylic acids as H-donors together with
chiral and achiral pyridine-bearing molecules as H-acceptors. Interestingly, only
in HBAs containing chiral H-donors BPs could be observed, while the system bearing
only a chiral centre at the H-acceptor did not form BPs. In addition to the different
combinations of donors and acceptors, also variations in the molar ratio were investigated
by increasing the amount of H-donor in the mixture. The results showed that the BP
temperature range increases with higher contents of acid up to a molar ratio of 3 : 1.
After that, the BP temperature range decreases rapidly. Herein, the chiral acid homo-dimers
acted again as a chiral dopant for the LC mixture to increase the HTP. Also lateral
fluoro-substitution is believed to have a positive effect on the BP temperature range.
Therefore, the highest BP temperature range of ΔT = 13.2 °C was observed for AF*⋯P*, a complex between a fluorinated chiral H-donor and a chiral H-acceptor in a molar
ratio of 3 : 1, respectively. An additional explanation for these results was the
favourable bent angles in the complexes showing BP, which should be between 132.1°
and 152.9°. Accordingly, the absence of BP in some of the compounds was explained
by unfavourable bent angles. A conclusive explanation on the role of the H-bond was
not provided.
Figure 17 Hydrogen-bonded assemblies and covalent analogues reported by Wei et al.[38] Colour code: covalent liquid crystal (blue) hydrogen-bonded assembly (green). Phase
sequence on cooling for the mixture with the composition: AF*/P* 3/1 molar ratio: crystalline phase (Cr), chiral nematic phase (N*), blue phase (BP),
isotropic liquid (Iso). Transition temperatures were determined from the graphics
in the publication, unless stated in the text.
To investigate the influence of the position of the H-bond in their bent-core supramolecular
systems, Han et al. expanded the library of compounds started by Wei et al.[38],[39] They observed a loss of LC behaviour in the HBAs ([Figure 18]) having their H-bond far from the bent core. The covalently bound analogues did
not show liquid crystalline behaviour. Consequently, the authors focused on derivatives
with H-bonds close to the core of the molecular structure. The variation of the terminal
alkyl chain at the H-acceptor showed that the elongation of the chain led to an increase
in the BP temperature range. Again the molar ratio played an important role in inducing
the BP. While none of the HBAs exhibited BP behaviour with a molar ratio of 1 : 1,
a slight excess of H-donor induced the BP, which is attributed to the already mentioned
effect of the formation of the homo-dimers of the acid molecules. Another interesting
observation was the effect of alkyl chain length on the range of molar ratios in which
BP is present. The lowest range was found for the HBAs with alkyl chains of the same
length on H-acceptor and H-donor (55 – 70 mol% AIIF* in PIIIC7
), while alkyl chains of different lengths were more beneficial for the molar ratio
range. This means the shorter or longer chain derivatives could take up more of the
acid homo-dimers (55 – 75 mol% AIIF* in PIIIC5
and 55 – 80 mol% AIIF* in PIIIC9
) before the BP disappeared. In contrast, the BP temperature range increased with
increasing length of the H-acceptor alkyl chain. The best results were obtained for
the supramolecular complex PIIIC9⋯AIIF* with a molar ratio of 7 : 3 (H-donor : H-acceptor) using the H-acceptor with the
longest (C9) chain. This system exhibited a temperature range for the BP of ΔT = 12 °C. In this study also, the fluorination effect was identified to be beneficial
for the BP stability, which supports previous examples of fluorinated compounds. The
importance of the position of the H-bond in the HBA is nicely demonstrated by the
loss of liquid crystallinity in the system possessing the inappropriately positioned
H-bond. However, a comprehensive understanding on the structure–property relationship
is still pending.
Figure 18 Hydrogen-bonded assemblies and covalent analogues reported by Han et al.[39] Colour code: covalent liquid crystal (blue) and hydrogen-bonded assembly (green).
Phase sequence on cooling for the mixture with the composition: PIIIC9/AIIF* 3/7 molar ratio: crystalline phase (Cr), chiral nematic phase (N*), blue phase (BP),
isotropic liquid (Iso). Transition temperatures were determined from the graphics
in the publication, unless stated in the text.
Han et al. reported deeper insights into the effects of substitution in HBAs by adding
more aromatic rings to the molecular structure of the mesogens previously reported
by Lin and co-workers.[38]–[40] These molecules were structurally related to the previously reported ones but the
terminal alkyl chain of the H-acceptor was extended by a cyano bisphenyl unit (see
[Figure 18]). Additionally, a H-donor extended by one benzoic acid unit was tested. Exclusively
the HBA PIII*⋯AII* ([Figure 19]) exhibited BP for a 1 : 1 ratio of H-donor and H-acceptor. In addition, additional
H-donor to the supramolecular mesogens yielded a decrease in the BP temperature range,
which was attributed to the poor miscibility of the supramolecular LC and the acid
homo-dimers. The BP temperature range of ΔT = 13.7 °C was explained by the appropriate HTP value and a large biaxial parameter,
as this parameter promotes the twisted arrangement of the mesogens in the system.[27] However, fluorination was not beneficial in this case, which is in contrast to the
previous reported findings where the introduction stabilised the BP.
Figure 19 Hydrogen-bonded assemblies reported by Han et al.[40] Bottom: colour code: hydrogen-bonded assembly (green). Phase sequence on cooling
for the mixture with the composition: PIII*/AII* 1/1 molar ratio: crystalline phase (Cr), chiral smectic phase (Sm*), chiral nematic
phase (N*), blue phase (BP), isotropic liquid (Iso). Transition temperatures were
determined from the graphics in the publication, unless stated in the text.
A modular approach for the investigation of BPs in supramolecular LCs was reported
by Giese and co-workers.[41] In this study, the HBAs were constructed using a H-donating core and a H-accepting
side-unit ([Figure 20]). As a H-bond-donating core, phloroglucinol (PHG) or fluoro-phloroglucinol (F-PHG) was used and combined with pyridyl-bearing stilbenes (St) and azo derivatives (Ap) as H-bond acceptors. Different combinations of acceptors were tested in this study;
however, the total ratio of core to side-unit was 1 : 3 in order to preserve the mesogenic
behaviour. To induce chirality in the system, the H-bond-accepting moieties were partially
substituted by their respective chiral equivalents. For example, the stilbene bearing
a terminal octyl chain was exchanged with a stilbene equipped with a citronellyl rest.
During a screening it became evident that a substitution of 50% of achiral side chains
with chiral ones produced the best results. For the complex PHG⋯(St1.5/St1.5*), only a broad chiral nematic phase was found, whereas for the PHG⋯(Ap1.5/Ap1.5*) assembly, a BP I from 75 to 67 °C (ΔT = 8 °C) could be observed. The introduction of fluorine into the core unit led to
a stabilisation of the mesophases in previous studies, so the same was done here.
However, the effects were of opposite nature. By changing PHG⋯(St1.5/St1.5*) to F-PHG⋯(St1.5/St1.5*), the temperature range of the cholesteric phase dropped from ΔT = 87 °C to ΔT = 81 °C. In the case of PHG⋯(Ap1.5/Ap1.5*), the BP temperature range was lowered by only 6 °C. Still the trends remained the
same. As the stilbazole-based assemblies exhibited wider mesophases, combinatorial
studies have been conducted by combining St with Ap* as well as Ap with St*. The assembly PHG⋯(St1.5/Ap1.5*) showed three mesophases, namely, a chiral nematic, a twist grain boundary, and a
chiral smectic A; however, no BP was observed. In contrast, the PHG⋯(Ap1.5/St1.5*) with a chiral moiety on the stilbazole instead of the azopyridine showed a BP temperature
range of ΔT = 10 °C. Also for this composition, the core units were exchanged for their fluorinated
counterparts. The resulting assembly F-PHG⋯(St1.5/Ap1.5*) showed only a cholesteric phase from 108 °C to 44 °C, while F-PHG⋯(Ap1.5/St1.5*) was most interesting. The latter exhibited a very stable BP I in a temperature range
of ΔT = 25 °C between 75 °C and 50 °C. These findings clearly indicate that the presence
of the achiral Ap is mandatory to facilitate good chirality transfer. Also the presence of fluorine
is beneficial due to non-classical H-bonding (C–F⋯H–C) between the core and the side
chain, as well as a more efficient packing due to reduced electron density in the
core. This is another example supporting the positive influence of fluorine on the
BP stability standing against the negative examples. To shed light on the structural
features of the BP I and chiral nematic phase, a solid-state 19F-NMR study was conducted.[41],[42] In principle, these experiments could confirm the three-dimensional structure of
the BP I by comparing the recorded with calculated spectra. The order parameter pseudo
exponent β was found to be 0.333 for the BP I and 0.168 for the cholesteric phase,
which correlate well with their phase symmetries.
Figure 20 Hydrogen-bonded assembly building blocks reported by Saccone et al.[41] Colour code: hydrogen-bonded assembly (green). Phase sequence on cooling for the
mixture with the composition: F-PHG/Ap/St* 1.0/1.5/1.5 molar ratio: crystalline phase (Cr), chiral nematic phase (N*), blue
phase (BP), isotropic liquid (Iso). Transition temperatures were determined from the
graphics in the publication, unless stated in the text.
The same system was reinvestigated recently to get more insight into the structure–property
relationship.[43] As the assemblies employing achiral Ap in the host system exhibit BP while the ones based on St do not, HTP values have been determined in the respective host LCs. It was shown
that both chiral acceptors possess higher HTPs in the Ap-based system. Furthermore, computer-assisted modelling was performed to investigate
the intermolecular interactions in the host systems. Here, the attractive forces between
the St molecules were found to be stronger leading to higher ordered arrangements. From
there, it can be rationalised that the presence of achiral Ap weakens the interactions in the host material facilitating an improved chiral transfer
from the chiral dopants. These findings nicely demonstrate the importance of not only
the nature of the chiral dopant, but also the ability of the host system to receive
the provided chiral information in the formation of BP.
