Dicyanamide Salts that Adopt Smectic, Columnar, or Bicontinuous Cubic Liquid-Crystalline Mesophases

Article abstract of DOI:10.1002/chem.201705794

Although dicyanamide (i.e., [N(CN)₂]^?) has been commonly used to obtain low-viscosity, halogen-free, room-temperature ionic liquids, liquid-crystalline salts containing such anions have remained virtually unexplored. Here we report a

Full text of DOI:10.1002/chem.201705794

A Journal of
Accepted Article
Title: Dicyanamide salts that adopt smectic, columnar or bicontinuous
cubic liquid-crystalline mesophases
Authors: Geonhui Park, Karel Goossens, Tae Joo Shin, and
Christopher W. Bielawski
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Dicyanamide salts that adopt smectic, columnar or bicontinuous
cubic liquid-crystalline mesophases
Geonhui Park,[a, b] Dr. Karel Goossens,*[a] Prof. Dr. Tae Joo Shin,[c] and Prof. Dr. Christopher
W. Bielawski*[a, b, d]
[a]
Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS),
Ulsan 44919, Republic of Korea
[b]
Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST),
Ulsan 44919, Republic of Korea
[c]
UNIST Central Research Facilities (UCRF) and School of Natural Science, UNIST, Ulsan
44919, Republic of Korea
[d]
Department of Energy Engineering, UNIST, Ulsan 44919, Republic of Korea
* E-mail addresses of the corresponding authors:
karel.cw.goossens@gmail.com
bielawski@unist.ac.kr
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Abstract
While dicyanamide (i.e., [N(CN)2]-) has been commonly used to obtain low-viscosity,
halogen-free, room-temperature ionic liquids, liquid-crystalline salts containing such anions
have remained virtually unexplored. Here we report a series of amphiphilic dicyanamide salts
that, depending on their structures and compositions, adopt smectic, columnar or bicontinuous
cubic thermotropic liquid-crystalline mesophases, even at room temperature in some cases.
Thermal properties were explored using polarized light optical microscopy, differential
scanning calorimetry, thermogravimetric analysis (including evolved gas analysis), and
variable-temperature synchrotron X-ray diffraction. A comparison of the thermal phase
characteristics of the new liquid-crystalline salts featuring “V-shaped” [N(CN)2]- anions with
those of structural analogues containing [SCN]-, [BF4]-, [PF6]- or [CF3SO3]- anions indicated
that not only the size of the counterion but also its shape should be considered in the
development of mesomorphic salts. Collectively, these discoveries should facilitate the design
of thermotropic ionic liquid crystals that form inverted-type bicontinuous cubic and other
sophisticated liquid-crystalline phases.
Keywords: liquid crystals, ionic liquids, self-assembly, mesophases, dicyanamide
Introduction
Liquid-crystalline salts or “ionic liquid crystals” (ILCs) have become the subject of
intensive research, partially due to the widespread interest in ionic liquids (ILs).[1-4]
The versatility of these materials lies in the fact that they effectively combine the
characteristics of liquid crystals (LCs) (e.g., anisotropic physical properties, dynamic
molecular order, etc.) with those of ionic liquids (e.g., ionic conductivity, tunable
physical properties, negligible vapor pressures under normal conditions, and so
forth).[1] Anion exchange is a relatively straightforward way to influence the
viscosities, phase transition temperatures, and other physical properties displayed by
ILs and ILCs. As such, a multitude of anions have been employed, including [BF4]-,
[PF6]-, [OTf]- (Tf = CF3SO2) and [NTf2]-. While low-viscosity, molten organic salts
containing dicyanamide (i.e., [N(CN)2]-) anions were reported about 15 years ago by
MacFarlane and co-workers,[5, 6] only one liquid-crystalline (LC) derivative, 1,3-bis(n-
dodecyl)-1,2,3-triazolium dicyanamide, has been disclosed thus far to the best of our
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knowledge.[7] The LC salt was reported to exhibit a smectic phase between 82 °C and
102 °C. Unlike commonly used (pseudo)spherical anions, such as Cl-, Br-, I-, [BF4]-,
[PF6]- and [ClO4]-, or linear species, such as [SCN]-, the dicyanamide anion is shaped
like the letter “V”.[8] Although the mesomeric forms of [N(CN)2]- indicate that the
negative charge is delocalized over the nitrogen atoms (Figure 1),[9] the anion retains
some Lewis basicity, in contrast to [BF4]-, [PF6]-, [OTf]- and [NTf2]-.[6, 10, 11] To better
understand how the thermal properties displayed by amphiphilic salts are influenced by
the [N(CN)2]- anion, we describe the synthesis and study of a series of halogen-free
dicyanamide salts that were found to adopt smectic, columnar or bicontinuous cubic
thermotropic liquid-crystalline mesophases, depending on their structures and
compositions. We also compare the thermal phase characteristics of the liquid-
crystalline [N(CN)2]- salts to an analogue outfitted with a linear [SCN]- anion and to
structurally related ILCs with [BF4]-, [PF6]- or [OTf]- anions that have been previously
reported.
Figure 1. The two mesomeric forms of the dicyanamide anion.
Results and Discussion
Molecular Design and Synthesis
To access a variety of different types of mesophases, we prepared six different [N(CN)2]-
salts (1–6) based on a N,N,N-trialkyl-N-benzylammonium motif (Scheme 1). Three
parameters were varied: (i) the number of n-alkyloxy chains attached to the benzyl group; (ii)
the length of the n-alkyloxy chains; and (iii) the size of the alkyl groups in the ammonium
headgroup. Analogous liquid-crystalline ammonium salts were previously reported by Kato
and co-workers.[12-16] As part of a series of controls, we also investigated the thermal phase
behavior of alkyl-substituted ammonium and imidazolium [N(CN)2]- analogues (7 and 8,
respectively)[17] and a related [SCN]- salt (9) (Scheme 2).
To synthesize compounds 1–6, either ethyl 3,4-dihydroxybenzoate or ethyl 3,4,5-
trihydroxybenzoate was first alkylated with 1-bromododecane or 1-bromooctadecane, and
then converted into the corresponding benzyl alcohol using LiAlH4. The latter was treated
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with PBr3 to prepare the corresponding benzyl bromide, which was then used to quaternize
either trimethylamine or triethylamine under Menschutkin-type[18] conditions. The resulting
quaternary ammonium bromide salts were subjected to an anion exchange with either (i)
Ag[N(CN)2] (which was prepared from AgNO3 and Na[N(CN)2]) or (ii) Na[N(CN)2] to form
the corresponding dicyanamide salts. Sodium dicyanamide was used instead of silver
dicyanamide to prepare 5 and 6 because the respective bromide salt precursors, 5-Br and 6-
Br, required elevated temperatures to be dissolved. Subsequent combustion elemental
analysis suggested to us that the bromide anions were successfully exchanged for [N(CN)2]-.
13
Quantitative C{1H} NMR analyses, conducted under appropriate measurement conditions
and in the presence of a paramagnetic relaxation agent (see the Supporting Information),
supported the composition assignments. Likewise, signals were not detected in the Na1s or
Br3d electron binding energy regions of the X-ray photoelectron survey spectra (XPS) that
were recorded for 5 and 6 (see the Supporting Information). Dicyanamide salt 7 was prepared
from the corresponding bromide salt using a slight excess of Ag[N(CN)2], whereas compound
8 was obtained from Ionic Liquids Technologies (IoLiTec GmbH). Thiocyanate salt 9 was
synthesized by reacting bromide salt 4-Br with KSCN in acetone. Additional experimental
details and analytical data can be found in the Supporting Information.
Scheme 1. Synthesis of dicyanamide salts 1–6. (i) LiAlH4, dry THF, r.t., N2; (ii) PBr3 (1 M in CH2Cl2), dry
CH2Cl2, 0‒5 °C  r.t., N2; (iii) N(CH3)3 (2 M in THF) or N(C2H5)3, dry toluene, 90 °C, N2; (iv) Ag[N(CN)2]
(1.1 eq.), EtOH/CH3CN (1:1), r.t.  40 °C (for 1‒4), or Na[N(CN)2] (40 eq.), EtOH, 75 °C (for 5‒6) (1: 96%, 2:
99%, 3: 99%, 4: 92%, 5: 73%, 6: 64%).
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Scheme 2. Dicyanamide salts 7 and 8, and thiocyanate salt 9.
Thermal and Synchrotron X-ray Diffraction Data
The thermal properties of the bromide, dicyanamide and thiocyanate salts were examined
by thermogravimetric analysis (TGA), polarized light optical microscopy (POM), and
differential scanning calorimetry (DSC). The liquid-crystalline mesophases adopted by
dicyanamide salts 1–8 and thiocyanate salt 9 were further investigated by synchrotron X-ray
diffraction (XRD) at various temperatures to facilitate full phase identification and
determination of the underlying lattice parameters, and to obtain more information about the
molecular packing in the respective phases. The synchrotron measurements were performed at
the PLS-II 6D UNIST-PAL Beamline of the Pohang Accelerator Laboratory (Pohang,
Republic of Korea).
Thermal stability. The TGA measurements revealed that dicyanamide salts 1–6 exhibited
higher thermal stabilities under an atmosphere of nitrogen when compared to their bromide
salt precursors 1-Br–6-Br (Figure 2), in accordance with previous results reported for ionic
liquids.[19, 20] Likewise, the N-benzyl-N,N,N-trimethylammonium salts were found to be more
stable than their N-benzyl-N,N,N-triethylammonium analogues. The residual weight
percentages recorded at 600 °C were measured to be similar across the series 1–6 as well as 1-
Br–6-Br.
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Figure 2. TGA data recorded for (a) bromide salts 1-Br–6-Br and (b) dicyanamide salts 1–6, 7, 8 and
10[21] (heating rate of 5 °C min-1; N2 atmosphere). T1% represents the temperature at which 1% weight loss
was measured (neglecting the initial small weight loss due to the release of H2O (< 1% for the [N(CN)2]-
salts)).
Evolved gas analysis using a TGA instrument coupled to a mass spectrometer (TGA-MS)
showed that the thermal decomposition of the bromide salts at temperatures below 270 °C
was accompanied by the evolution of CH3Br and N(CH3)3 or C2H5Br and N(C2H5)3 for the
benzyltrimethylammonium and benzyltriethylammonium salts, respectively (Figures S14–
S15). As such, we concluded that decomposition occurred through a reverse Menschutkin-
type reaction where nucleophilic attack of the bromide anions on the benzyltrialkylammonium
cations resulted in formation of the corresponding alkyl and benzyl bromide species.[22]
Hofmann elimination products (i.e., ethene and HBr) were not detected during the analysis of
the benzyltriethylammonium salts.
The benzylammonium dicyanamide salts exhibited a different decomposition pattern. The
TGA-MS data indicated that weight losses observed below 300 °C originated mainly from the
expulsion of N(CH3)3 (in case of 1, 3, and 5) or N(C2H5)3 (in case of 2, 4, and 6) (Figures 3
and S16). Although Kroon et al. predicted that CH3-N(CN)2 formed during the thermal
decomposition of [C4mim][N(CN)2] ([Cnmim]+ = 1-alkyl-3-methylimidazolium)[23] and we
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estimated the boiling points of CH3-N(CN)2 and C2H5-N(CN)2 to be approximately 210 °C at
atmospheric pressure from data reported by Benders and Hackmann,[24] signals attributed to
the formation of alkylated dicyanamides were not observed.[21] Likewise, signals consistent
with the formation of ethene or H-N(CN)2 were not observed during TGA-MS analysis of the
benzyltriethylammonium salts, which effectively excluded decomposition via Hofmann
elimination, Sommelet-Hauser rearrangement or Stevens rearrangement.
Figure 3. TGA-MS data recorded for the dicyanamide salt 4 (heating rate of 5 °C min-1; Ar atmosphere;
‘fr.’ = fragment, ‘alkyl’ refers to alkyl fragmentation).
Alkali metal salts containing the [N(CN)2]- anion (e.g., M[N(CN)2]; M+ = Na+, K+, Rb+)
have been reported to be stable to temperatures exceeding 300 °C.[25-27] At higher
temperatures, the dicyanamide undergoes trimerization and forms cyclic tricyanomelaminate
ions ([C6N9]3-) that contain the s-triazine ring system.[26-30] FT-IR measurements showed that
independently heating salts 4 or 10[21] to 225–260 °C (i.e., temperatures that corresponded to
the plateau regions in the respective TGA thermograms after the first weight loss event,
Figure 2(b); see also Table S2) resulted in the attenuation of the signals recorded between
2300 and 2000 cm-1, which are characteristic of the dicyanamide anion (i.e., s(C≡N), as(C–
N), s(C–N) and as(C≡N)) (Figures S21 and S22). This result suggested to us that the
dicyanamide-containing species may undergo cyclotrimerization after the expulsion of
N(C2H5)3. Our conclusion was supported in part by the observation of a weak but clearly
discernible exothermic signal in the DSC data obtained from simultaneous TGA-DSC
1
13
measurements (Figures S18–S20). Moreover, the H and C NMR spectra recorded for the
heat-treated products featured broader peaks as compared to the starting materials (Figures
S23–S26). The heat treatment apparently caused the 13C NMR signal from the [N(CN)2]-
anions (centered at ~120 ppm in the starting materials) to disappear. Combustion elemental
analyses yielded nitrogen contents that were largely consistent with the retention of the
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dicyanamide-containing species and/or their cyclotrimerization products in the heat-treated
materials (Table S3).
The benzyl group should play an important role in the thermal decomposition process of
dicyanamide salts 1–6 as [N(C16H33)2(CH3)2][N(CN)2] (7) was found to display a significantly
higher thermal stability, with the main weight loss occurring between 230 and 320 °C (Figure
2(b)). Similar decomposition temperatures were reported for [N(CH3)4][N(CN)2].[31] The
thermal stability and weight loss evolution of imidazolium salt [C16mim][N(CN)2] (8)
resembled that of compound 7 (Figure 2(b)). Finally, TGA analysis revealed that the
thiocyanate salt 9 exhibited a significantly lower thermal stability than the analogous
dicyanamide salt (i.e., 4) (Figure S22).
Liquid-crystalline properties. POM and DSC experiments showed that all of the
dicyanamide salts described above, except for compound 10[21], are thermotropic liquid
crystals, and that clearing to their isotropic liquid state occurs below the onset of their thermal
decomposition. Thiocyanate salt 9 was also found to be mesomorphic. The different
mesophases that were observed will be discussed in more detail below in concert with the
results that were obtained from synchrotron XRD measurements. Table 1 summarizes the
recorded phase transition temperatures and thermal data. Key XRD data are described in
Table 2.
Smectic phases. The XRD patterns of the LC mesophases adopted by compounds 1 and 2
are characterized by three to four sharp reflections in the small-angle region, in addition to a
diffuse wide-angle scattering signal centered at 4.5–4.6 Å (= h1) (Figure 5(a)). The latter
corresponds to the lateral short-range order of the molecules and is indicative of their liquid-
crystalline nature. The signal could be fitted with a single Lorentzian function; shoulders or
additional peaks that would distinguish different distances between the distinct molecular
segments were not observed. The reciprocal d-spacings of the sharp small-angle reflections
relate to each other as 1:2:3(:4) and originate from a layer-like ordering of the molecules
(first- and higher-order reflections). This smectic phase type assignment is consistent with the
focal conic texture that was observed by POM (Figure 4(a)). For both 1 and 2, good
homeotropic alignment was achieved by cooling samples sandwiched between two untreated
glass slides from their isotropic liquid states. These observations, in combination with the
absence of any sharp XRD signals in the medium- to wide-angle region, are indicative of a
smectic A phase (SmA).[32] There is no in-plane order in a SmA phase, but the relatively
intense (002) reflections that were observed for the SmA phases of 1 and 2 suggested to us
that adjacent smectic layers in those phases feature well-defined interfaces.[33] These
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observations are in agreement with other smectic ILCs that display high translational order
but low orientational order.[1, 34] Dicyanamide salts 7 and 8 also form SmA phases and
produce “oily streaks” when viewed by POM (Figures S4 and S5), but with a lower clearing
point as compared to 1 and 2 (Table 1). The differences were attributed to a beneficial effect
of the benzylammonium architecture on the mesophase stability.
For amphiphilic molecules of this kind, it can be assumed that, due to nanosegregation[35,
36]
and strong electrostatic interactions,[37] the cationic moieties and the [N(CN)2]- anions
segregate into ionic sublayers with a head-to-head arrangement of the ionic headgroups
(Figure 7(a)).[1, 38] The alkyl chains are homogeneously distributed on either side of the ion-
rich layers, resulting in an alternation of ionic and aliphatic sublayers. Indeed, when
calculating the cross-sectional area (or projection area) AM(T) that is occupied by an elemental
molecular assembly within the smectic layers via the relation N∙[Vmol(T)/d(T)] (with Vmol
being the molecular volume, d being the layer periodicity measured by XRD, and N being the
number of molecules), the values were found to be in agreement with a bilayer structure; for
N = 2, at 92 °C: AM ≈ 55.6 Å2 for 1 and AM ≈ 59.8 Å2 for 2. At this temperature, the cross-
sectional area of one fully stretched aliphatic chain, ch, equals about 22.4 Å2,[39] indicating
that the two molten alkyl chains that are connected to each cation are folded (and possibly
partially interdigitated) in the aliphatic sublayers.[40-43] The slightly larger AM value that was
calculated for 2 is consistent with its more voluminous cationic headgroup as compared to 1
(i.e., –N+(C2H5)3 versus –N+(CH3)3). Larger ionic headgroups typically result in smaller
smectic layer spacings due to increased chain folding and interdigitation in order to achieve a
balance between the area of the cationic headgroup/anion assembly that is projected onto the
smectic layer planes and the cross-sectional area(s) of the respective alkyl chain(s) attached to
the cation.[42, 44-48] In this case, however, d was found to be similar for the SmA phases
adopted by 1 and 2, respectively (Table 2 and Figure 6(a)). The result suggested to us that the
bulkier –N+(C2H5)3 headgroups may cause the ionic sublayers to expand in lateral as well as
longitudinal directions due to “rippling” or “undulation” (Figure 7(a)). Such an arrangement
also explains the less than expected difference in AM between 1 and 2 when considering the
estimated projection areas of the [N(CH3)4]+ and [N(C2H5)4]+ cations (Table 3). Moreover, the
difference in thickness of the ionic sublayers in the SmA phases of 1 and 2, respectively, was
estimated to be about 1.8 Å (Figure S16), which is more than would be expected based on the
relative sizes of the –N+(CH3)3 and –N+(C2H5)3 headgroups (Table 3). The increase of AM as a
function of temperature (Figure 6(a)) was attributed to the known[49] ‘spreading’ of the ionic
sublayers with increasing temperature, a process that is accompanied by an increased folding
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of the alkyl chains as reflected in the temperature evolutions of the aliphatic and ionic
sublayer thicknesses (Figure S16). The AM values that were determined for 1 and 2 at
temperatures close to their respective isotropization temperatures (based on linear fits of their
respective AM(T) experimental data) are similar: AM,limit is about 63.6–64.0 Å2 at 157 °C and
126 °C, respectively (Figure 6(a)). Thus, the breakdown of the ionic sublayers in the SmA
phases of 1 and 2 (which feature the same alkyl chain volume per molecule) results in
clearing to the isotropic liquid state and is governed by the temperature-dependent, molecular
cross-sectional area.[1, 42, 48, 50] The calculated AM,limit values are larger than those reported for
1-[3,4-bis(hexyloxy)benzyl]-3-methylimidazolium halides (~52.5 Å2)[48] and for 1-(4-
dodecyloxybenzyl)-3-methylimidazolium salts (~42.9–44.1 Å2),[42] presumably because 1 and
2 each contain two long n-dodecyl chains per molecule. It proved challenging to obtain a
more detailed structure of the ionic sublayers as the projection area of a N-benzyl-N,N,N-
tri(m)ethylammonium dicyanamide headgroup is presently not known and, particularly due to
the non-spherical shape of the [N(CN)2]- anion, may depend on its orientation.[51]
Similar conclusions were obtained by gleaning the XRD data recorded for the SmA phases
of compounds 7 and 8 (Table 2, Figures S8 and S9). The significantly different AM values
originate from the different molecular structures (c.f., two alkyl chains connected to the
cationic headgroups versus one). Higher-order reflections were not observed for these salts in
the small-angle region.
Columnar phases. In the small-angle region, the XRD patterns that were recorded for
compounds 3, 4 (at 56 °C), 5 and 6 are characterized by four or five sharp reflections with
reciprocal d-spacing ratios of 1:√3:2:√7(:3) (Figures 5(b)–(c)), which are indicative of a
columnar phase with a 2D hexagonal lattice (Colhex). Fan-like textures were seen via POM for
these Colhex phases (Figures 4(b)–(d)) and, in some cases, large homeotropic areas
spontaneously formed upon cooling from the isotropic liquid phase. The diffuse wide-angle
X-ray scattering signal centered at 4.5–4.6 Å (= h1) was fitted with a single Lorentzian
function in each case, and sharp wide-angle signals that would indicate a regular stacking of
molecules within the columns were not observed. These Colhex phases can therefore be
viewed as ‘disordered’ columnar phases (Colhex,d) in which the columns are formed through
nanosegregation and self-assembly of the amphiphilic molecules, with the column centers
being composed of the ionic moieties and the periphery of each column being composed of
the molten aliphatic chains (Figure 7(b)).
Although no discrete columnar slices exist in the aforementioned structure, it is useful to
estimate Nhex,slice, which approximates the average number of molecules that self-aggregate to
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form a hypothetical slice. Values for Nhex,slice were calculated using the relation
[h1(T)∙Shex(T)]/Vmol(T) (with Shex being the cross-sectional area of one column, which can be
estimated as (√3/2)ahex2; ahex is the hexagonal lattice parameter (Table 2)). At a particular
temperature, ahex is consistently larger for compounds 5 and 6 when compared to 3 and 4,
presumably due to the longer n-octadecyl chains in the former. The difference in alkyl chain
volume fraction between the pairs 3 : 4 and 5 : 6 is also reflected in the different relative
intensities of the small-angle XRD signals (c.f., Figures 5(b) and 5(c)). The temperature
evolution of ahex and Nhex,slice is displayed in Figure 6(b). The parameters ahex and Shex
decrease with increasing temperature as a result of increasing chain mobility and disorder,
which also leads to a decrease in the value of Nhex,slice.
[52] The ahex(T) and Nhex,slice(T) data that
were recorded for 5 and 6 have steeper slopes than that of 3. At 120 °C, Nhex,slice equals about
four for compounds 3, 5 and 6 (Figure 6(b), Figure 7(b)). The calculated value is about 2/3 of
those previously reported[48] at the same temperature for two 1-[3,4-bis(alkyloxy)benzyl]-3-
methylimidazolium bromide salts (Nhex,slice ~ 6 and Nhex,slice ~ 6.5 for dodecyloxy and
octadecyloxy substituents, respectively) and in agreement with the larger number of alkyl
chains per molecule in the dicyanamide salts (i.e., three instead of two).
We also found a relationship between Nhex,slice(T) and the clearing points, similar to that
described above for the AM,limit values calculated for the SmA phases. The Nhex,slice values
determined for 3, 5 and 6 are similar at temperatures close to their respective isotropization
temperatures. The data suggested to us that at least 3.6–3.7 molecules need to aggregate to
form the supramolecular columns in the Colhex phase (Figure 6(b)).
Based on its XRD pattern, the liquid-crystalline phase adopted by thiocyanate salt 9 was
also identified as a Colhex phase (POM images can be found in Figure S6). The (21) and (30)
reflections or the h2 scattering signal mentioned above were not observed (Table 2). The
Nhex,slice value showed a similar temperature evolution as the dicyanamide salts discussed
above (Figure S19).
Bicontinuous cubic phases. In addition to its Colhex phase, another LC mesophase was
detected for salt 4. When a pristine POM sample of 4 was heated and annealed, it lost all
birefringence (Figure S2) but remained viscous. Further heating caused transition to a higher-
temperature Colhex phase and the appearance of a texture (Figures 4(e)–(f) and S3). These
observations suggested to us that a cubic, optically isotropic LC mesophase had formed.
Judging from the position of the cubic phase in the phase sequence Cub  Colhex and from
the disappearance of the cubic phase for longer chain lengths (no cubic phase was observed
for 6), the possibility of a micellar cubic phase can be excluded and an “inverted-type”,
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multicontinuous structure can be assumed.[53-55] The XRD pattern of the cubic phase is
displayed in Figures 5(d) and S11. Up to 11 small-angle reflections were observed, with
reciprocal d-spacing ratios of √6:√8:√14:√16:√20:√22:√24:√26:√38:√48:√50. These signals
could be indexed as reflections (211), (220), (321), (400), (420), (332), (422), (431), (532) /
(611), (444), and (543) from a body-centered cubic lattice with Ia d symmetry (‘double
gyroid phase’).[56] The peak intensity distribution of the XRD pattern, which included an
intense reflection at small angles (i.e., (211)) and a progressively decreasing intensity with
increasing diffraction angle (up to reflection (400)), matches that of previously reported
thermotropic CubV(Ia d) phases adopted by polyphilic LCs.[57, 58]
The structural model for such cubic phases is based on two interpenetrating, branched 3D
networks of opposite chirality, separated by a type G infinite periodic minimal surface
(IPMS). Both networks are comprised of short channel segments. Each cubic unit cell
contains 24 of those segments, which are linked in a coplanar manner at junctions where three
segments join. The differential 3D electron density map that was constructed by Kato, Ungar,
Ohno and co-workers for two taper-shaped ILCs with [BF4]- or [PF6]- anions, respectively,
elegantly showed that the anions are located primarily in the center of the channels, close to
the cationic headgroups, with the channels being enclosed by the aliphatic continuum.[14]
A
similar structure can be envisioned for the CubV(Ia d) phase adopted by compound 4. It
should be noted that the volume of [N(CN)2]- is intermediate that of [BF4]- and [PF6]- (Table
3), yet with a different shape. Theoretically, the distance between two ternary junctions equals
(1/√8)aCub, with aCub being the cubic lattice parameter. The actual arrangement may be more
complex because, at the junctions, the channel segments may not perfectly fuse together and
are most probably separated by short gaps[48, 59-61] which complicates an estimation of the
number of molecules that are organized in a cylindrical fashion and bridge two junction
points.[62] Nevertheless, it can be assumed that the local structure of the Colhex phase of 4 is
retained during the Colhex-to-CubV phase transition upon cooling. Douce et al. pointed out that
such a transformation may take place through regular column undulations and
interconnections, which significantly increase with decreasing temperature.[48, 63] Similar to
that observed during analyses of the columnar phases (see above), a broad scattering signal
that was centered at ca. 9.5 Å (i.e., h2, see Figure S20) was detected apart from the wide-angle
scattering signal h1. Finally, by comparing the volume of one cubic unit cell, VCub, and the
molecular volume, Vmol, we estimated the total number of molecules per cell at a given
temperature as NCub ≈ VCub(T)/Vmol(T) (Table 2). As can be seen from Figure 6(c), this number
decreases with increasing temperature. The calculated NCub values were found to be
12
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approximately 2/3 of those reported[48] for 1-[3,4-bis(dodecyloxy)benzyl]-3-
methylimidazolium bromide, which contains two alkyl chains per molecule.
13
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Table 1. Phase transition temperatures and thermal data
recorded for dicyanamide salts 1–8 and for thiocyanate
salt 9.
H (kJ
mol-1)[c]
Cpd. Transition[a]
T (°C)[b]
78[d]
157
30.5
0.6
1
2
3
4
Cr  SmA
SmA  Iso
66[e]
126[f]
27.9
1.3
Cr  SmA
SmA  Iso
22
155[f]
38.0
0.9
Cr  Colhex
Colhex  Iso
24
41
84[g]
36.1
0.9
1.0[g]
Cr  CubV(Ia d)
CubV(Ia d)  Colhex
Colhex  Iso
68
145[f]
82.8
0.5
5
6
7
8
Cr  Colhex
Colhex  Iso
69
133[f]
76.3
0.4
Cr  Colhex
Colhex  Iso
47[h]
83
20.2
1.5
Cr  SmA
SmA  Iso
65
68.7
0.7
Cr  SmA
SmA  Iso
111
25[f][i]
91[f]
33.0
0.4
9
Cr  Colhex
Colhex  Iso
[a]
Abbreviations: Cr = crystalline phase; SmA = smectic A phase; CubV(Ia d)
= bicontinuous cubic phase (space group Ia d); Colhex = hexagonal columnar
[b]
phase (plane group p6mm); Iso = isotropic liquid phase. Onset temperatures
obtained by DSC during the second heating run (unless indicated otherwise) at
a heating rate of 10 °C min-1 and under an atmosphere of N2. The first heating
run was performed from 25 °C to 165 °C (1–6), 110 °C (7) or 130 °C (8–9); the
first cooling was run to -20 °C; subsequent heating/cooling cycles were run
[c]
between -20 °C and 165 °C (1–6), 110 °C (7) or 130 °C (8–9).
Enthalpy
change. [d] During the 2nd heating run, melting was preceded by an endothermic
solid-to-solid transition with a peak temperature of 48 °C (H = 5.8 kJ mol-1)
and was immediately followed by an exothermic recrystallization event with a
peak temperature of 57 °C (H = -0.6 kJ mol-1). During the 2nd heating run,
[e]
melting was preceded by an exothermic recrystallization event with a peak
temperature of 34 °C (H = -12.1 kJ mol-1). [f] Peak temperature. [g] Value from
the 1st heating run; no signal was observed during the 2nd heating run. [h] During
the 2nd heating run, the transition to the SmA phase was preceded by a melting
peak (Tonset = 36 °C, H = 33.8 kJ mol-1) which was immediately followed by
an exothermic signal that may stem from rapid recrystallization (Tonset = 41 °C,
H = -4.5 kJ mol-1). [i] During the 2nd heating run, melting was preceded by an
endothermic solid-to-solid transition with a peak temperature of 10 °C (H =
2.5 kJ mol-1).
14
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Table 2. Summary of synchrotron XRD data recorded for the LC mesophases adopted by dicyanamide salts 1–8
and thiocyanate salt 9, including calculated structural parameters.
Structural
dcalcd.
(Å)[a]
Type of LC
mesophase
Cpd.
1
T (°C)
dobs. (Å)[a]
I[b]
hkl[c]
parameters of the
LC mesophases[d]
SmA
92
36.65
18.33
12.22
4.5
VS (sh)
S (sh)
M (sh)
br
001
002
003
h1
36.66
18.33
12.22
d = 36.66 Å
Vmol ≈ 1020 Å3
AM ≈ 55.6 Å2
ch ≈ 22.4 Å2
SmA
92
83
36.59
18.30
12.19
9.14
VS (sh)
S (sh)
M (sh)
VW (sh)
br
001
002
003
004
h1
36.58
18.29
12.19
9.15
d = 36.58 Å
2
3
Vmol ≈ 1093 Å3
AM ≈ 59.8 Å2
ch ≈ 22.4 Å2
4.6
Colhex
32.92
18.99
16.45
12.43
10.96
9.1
VS (sh)
S (sh)
M (sh)
VW (sh)
VW (sh)
br
10
11
20
21
30
h2
32.92
19.00
16.46
12.44
10.97
ahex = 38.01 Å
Vmol ≈ 1333 Å3
Shex = 1251 Å2
Nhex,slice ≈ 4.2
4.5
br
h1
45[e]
35.14
30.43
23.03
21.54
19.26
18.36
17.57
16.89
13.95
12.43
12.17
18
VS (sh)
S (sh)
211
220
35.15
30.44
23.01
21.52
19.25
18.36
17.58
16.89
13.97
12.43
12.18
aCub = 86.10 Å
Vmol ≈ 1367 Å3
VCub = 638267 Å3
NCub ≈ 467
4
CubV(Ia d)
M (sh)
M (sh)
M (sh)
M (sh)
M (sh)
M (sh)
VW (sh)
VW (sh)
VW (sh)
br ( =
10.4 nm[f])
br
321
400
420
332
422
431
532, 611
444
543
D[g]
9.5
4.5
h2
h1
br
Colhex
56 (upon
cooling)
33.86
19.54
16.93
12.80
9.4
VS (sh)
S (sh)
M (sh)
VW (sh)
br
10
11
20
21
h2
33.86
19.55
16.93
12.80
ahex = 39.10 Å
Vmol ≈ 1388 Å3
Shex = 1324 Å2
Nhex,slice ≈ 4.3
4.5
br
h1
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Colhex
92
37.74
21.80
18.87
14.26
12.58
11.1
VS (sh)
M (sh)
M (sh)
M (sh)
W (sh)
br
10
11
20
21
30
h2
37.74
21.79
18.87
14.26
12.58
ahex = 43.58 Å
5
Vmol ≈ 1782 Å3
Shex = 1645 Å2
Nhex,slice ≈ 4.2
4.6
br
h1
Colhex
92
38.44
22.18
19.22
14.53
12.81
11.1
VS (sh)
M (sh)
M (sh)
M (sh)
W (sh)
br
10
11
20
21
30
h2
38.44
22.19
19.22
14.53
12.81
ahex = 44.39 Å
Vmol ≈ 1855 Å3
Shex = 1706 Å2
Nhex,slice ≈ 4.2
6
4.6
br
h1
SmA
SmA
Colhex
80
80
60
29.34
4.5
VS (sh)
br
001
29.34
34.14
d = 29.34 Å
Vmol ≈ 970 Å3
AM ≈ 66.1 Å2
ch ≈ 22.2 Å2
7
8
9
h1
34.14
4.5
VS (sh)
br
001
d = 34.14 Å
Vmol ≈ 646 Å3
AM ≈ 37.8 Å2
ch ≈ 22.2 Å2
h1
33.45
19.28
16.71
4.5
VS (sh)
S (sh)
M (sh)
br
10
11
20
h1
33.45
19.31
16.73
ahex = 38.63 Å
Vmol ≈ 1369 Å3
Shex = 1292 Å2
Nhex,slice ≈ 4.2
[a] The dobs. and dcalcd. values refer to the measured and calculated diffraction spacings, respectively. For the SmA
phases: dcalcd. = <d001> = [ld00ll]/N00l, in which N00l = the number of (00l) reflections. For the Colhex phases:
½
dhk,calcd. = [(√3/2)ahex]/(h2 + k2 + hk) , in which the hexagonal lattice parameter ahex was calculated from the peak
position of the most intense reflection at the smallest Bragg angle (i.e., (10)). For the CubV phase: dhkl,calcd.
=
aCub/(h2 + k2 + l2)½, in which the cubic lattice parameter aCub was calculated from the peak positions of all
[b]
observed reflections. I is the intensity of each reflection: VS, very strong; S, strong; M, medium; W, weak;
[c]
VW, very weak; sh, sharp reflection; br, broad reflection.
hkl are the Miller indices of the reflections. h1
[d]
indicates the center position of the diffuse wide-angle signal. h2: see text. D: see text. Vmol is the molecular
volume. Due to the lack of dilatometry data, Vmol(T) was estimated as (M/0.6022)f, in which M is the molecular
mass (in g mol-1) and f is a temperature-correcting factor (f = 0.9813 + 7.47410-4T with T in °C) (see Section
4.1 in the Supporting Information). AM is the cross-sectional molecular area as calculated from AM(T) =
2Vmol(T)/d(T) (see text). ch is the cross-sectional area of one fully stretched aliphatic chain.[39] ahex is the
hexagonal lattice parameter. Shex is the 2D hexagonal lattice area, i.e. the cross-sectional area of one column in
the Colhex phase (= (√3/2)ahex(T)2). Nhex,slice is the number of molecules within one hypothetical slice of a column
in the Colhex phase, with an assumed height h1, and was estimated as [h1(T)·Shex(T)]/Vmol(T) (see text). aCub is the
cubic lattice parameter. VCub is the volume of the cubic cell (= aCub(T)3). NCub is the number of molecules within
the cubic cell and was estimated as VCub(T)/Vmol(T). For comparison, Nhex,slice and NCub were also estimated using
[e]
crystal volume increments and similar values were obtained, see Section 4.1 in the Supporting Information.
The XRD pattern was recorded during the first heating run of a pristine sample. Table 1 lists the onset
temperature of the CubV-to-Colhex phase transition in the second heating run as 41 °C. Using POM and DSC, we
found that, for a pristine sample, the cubic phase can exist until higher temperatures (by up to 20 °C) than for
samples that had been previously heated to > 100 °C. We attribute the difference to the presence of small
amounts of water that effectively stabilize the microphase-segregated mesophase.[64, 65] Although all samples
[f]
were dried before analysis, the uptake of atmospheric moisture could not be avoided. : correlation length as
16
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[g]
estimated from the Scherrer equation with the shape factor K taken as 0.9.
The origin of this broad signal
centered at about 18 Å (Figure 5(d)) is unclear.
17
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Table 3. Sizes of selected ions that are commonly present in ILCs.
Ionic volume, Vion (Å3)
Projection area,
Ionic radius,
rion (Å)[b]
Ion[a]
[e]
[c]
[d]

ion (Å2)[f]
Vion,3
Vion,1
Vion,2
2.83 [66]
95 [66]
113 [67]
–
25
3.43 [66]
170 [66]
199 [67]
129 [68]
–
37
–
–
–
–
–
1.81 [69]
1.96 [69]
2.20 [69]
25
32
45
47 [67]
56 [67]
72 [67]
10
12
15
42 [70]
–
–
–
89 [68]
79 [70]
–
2.29 [66]
51 [66]
73 [67]
69 [70]
16
2.54 [66]
69 [66]
109 [67]
96 [70]
20
–
–
82 [66]
131 [68]
232 [68]
112 [70]
208 [70]
–
–
–
144 [66]
–
–
–
–
71 [67]
–
–
110 [71]
–
[a]
[b]
Only one canonical resonance structure is shown for [N(CN)2]- and [SCN]-. Listed for (pseudo)spherical
ions. Cl-, Br-, I-: rion as found in ionic crystals.[69] [N(CH3)4]+, [N(C2H5)4]+, [BF4]-, [PF6]-: rion as estimated by
Ue.[66] [c] Vion,1: calculated as (4/3)··rion3. Cl-, Br-, I-: based on the value for rion in the second column of the table.
[N(CH3)4]+, [N(C2H5)4]+, [BF4]-, [PF6]-, [OTf]-, [NTf2]-: values reported by Ue.[66]
Vion,2: ‘effective close-
[d]
packing ion volume’ in ionic crystals as reported by Jenkins, Glasser, Passmore and Marcus.[67, 68, 71, 72] The
values that were selected for [N(CN)2]-, [OTf]- and [NTf2]- in ref. 68 came from ref. 73. Vion,3: ion volume as
[e]
determined by AIM partitioning of electron density distributions that were reconstructed from the single-crystal
XRD data recorded for [C14mim][X] salts, as reported by Nelyubina et al.[70] [f] Estimated for ions whose shape
2
can be approximated as a sphere using the equation: ion = ·rion (where the value of rion is that listed in the
second column of the table).
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(a)
(c)
(b)
20 µm
75 µm
30 µm
(d)
(f)
(e)
Col
Cub
150 µm
150 µm
150 µm
substrate
substrate
Figure 4. Polarized light optical microscopy images as viewed through crossed polarizers and their
respective descriptions: (a) focal conic texture of the SmA phase of 1 at 97 °C; (b) fan-like texture of the
Colhex phase of 6 at 105 °C after cooling from its isotropic liquid state; (c) pseudo-focal conic fan-shaped
texture of the Colhex phase of 3 at 140 °C; (d) fan-shaped texture of the Colhex phase of 3 at 140 °C; (e)
Colhex phase and (f) cubic phase of 4 at 70 °C and at 40 °C, respectively, after cooling (the texture in (f) at
the edge of the sample originates from a small region of remaining Colhex phase). Additional POM images
can be found in the Supporting Information.
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Figure 5. (a) X-ray diffraction profile of 2 recorded at 92 °C (SmA phase). (b) X-ray diffraction profile of
3 recorded at 83 °C (Colhex phase). (c) X-ray diffraction profile of 6 recorded at 92 °C (Colhex phase). (d)
Small-angle X-ray diffraction profile of 4 recorded at 45 °C (CubV(Ia d) phase) (see also Figure S20). In
all cases, the X-ray wavelength used was 1.00 Å.
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Figure 6. (a) Evolution of the layer thickness, d, and of the “molecular area”, AM, as a function of
temperature for the SmA phases adopted by 1 and 2. The clearing points Tc of 1 and 2 are indicated by
vertical dashed lines. (b) Evolution of the hexagonal lattice parameter, ahex, and of the number of
molecules within one hypothetical slice of a column, Nhex,slice, as a function of temperature for the Colhex
phases adopted by 3, 5 and 6. The clearing points Tc of 3, 5 and 6 are indicated by vertical dashed lines.
(c) Evolution of the cubic lattice parameter, aCub, and of the number of molecules within the cubic cell,
NCub, as a function of temperature for the CubV(Ia d) phase adopted by 4 during the first heating run of a
pristine sample (see text).
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(a)
(b)
Figure 7. (a) Structural model of the SmA phases adopted by dicyanamide salts 1 and 2. Undulation of
the central ionic sublayers (represented here as a wavy ribbon) may occur to a greater extent for
compound 2, which features a relatively bulky benzyltriethylammonium cationic headgroup (see text).
(b) Structural model for the Colhex phase adopted by dicyanamide salt 4. The outer, solid circle delineates
the cross-section of one supramolecular column in the columnar phase. The inner, dashed circle may
reflect a non-aliphatic column center that is composed of the cationic benzyltrialkylammonium
headgroups and the [N(CN)2]- anions. Efficient space-filling is ensured by interdigitation of alkyl chains
from neighboring columns. The column cross-sections may be elliptical as the 2D p6mm symmetry was
inferred from space- and time-averaged XRD data. The model is an approximation as discrete columnar
slices are unlikely to exist in highly dynamic Colhex phases formed by flexible amphiphilic salts. It can be
assumed that the minicylinders in between ternary junctions in the CubV phase of 4 show a similar local
molecular arrangement. Comparable conclusions may be drawn for the structures of the Colhex phases
adopted by salts 3, 5 and 6.
Discussion
The evolution from the lamellar phases adopted by 1 and 2 through the bicontinuous cubic
and columnar phases of 3 and 4 to the exclusively columnar mesomorphism exhibited by 5
and 6 results from a progressively increasing curvature of the interface between the
incompatible molecular segments. Such mesophase behavior can be explained by the
Israelachvili theory of self-assembling hydrocarbon amphiphiles.[74] Similar phase sequences
have been observed for other thermotropic LC amphiphiles, and analogies to lyotropic
systems have been drawn.[35, 53] It should be noted that we did not find a micellar cubic phase,
CubI, for salts 5 and 6 upon heating; rather, in both cases, the Colhex phase cleared directly to
the isotropic liquid state. Cheng et al. observed CubI(Pm n) phases for some taper-shaped
imidazolium-based mesogens with three long alkyl chains and a short N-terminal chain (ethyl
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or n-propyl) on the imidazolium cations.[75, 76] The taper angles of compounds 5 and 6 are
presumably not appropriate to form a stable micellar cubic phase. We surmise that the
trialkylammonium headgroups also hinder close packing in micelles with polar cores.
Dendromesogens with trialkylammonium chloride apices, reported by Percec and co-workers,
also did not show micellar cubic phases, although analogues with imidazolium or pyridinium
headgroups did exhibit CubI(Pm n) phases.[77]
The appearance of a cubic phase for salt 4 and its apparent absence for the structurally
similar compound 3 merit further comment. The importance of the triethylammonium motif in
bicontinuous cubic LC materials was discovered by Kato and co-workers, who semi-
quantitatively explained the key contributors to mesophase formation in terms of molecular
geometry.[14] Theoretical dV/dr profiles showing the average radial distribution of volume for
an optimal molecular packing were constructed by Ungar and co-workers for a variety of
mesophases, including Colhex, CubV(Ia d(G)), CubV(Im m(P)), CubV(Im m(I)) and
CubV(Pn m(D)) phases.[57, 78, 79] Close to the central axis of the cylindrical columns or
network segments that are present in such phases, i.e. for small values of r, the profiles are
similar but not identical. Benzyltriethylammonium salts with three chains of appropriate size
and with suitable anions display cross-sectional area profiles, A(r), that match those of
bicontinuous cubic phases (i.c., the double gyroid phase) due to the three additional
methylene groups near their apices in comparison to the corresponding trimethylammonium
salts. Similarly, it had previously been demonstrated that alkali metal dihexadecylphosphates,
which all show a Colhex phase, exhibit an additional CubV(Ia d) phase when matched with
larger alkali metal cations (i.e., K+, Rb+ and Cs+).[80] The enantiotropic CubV(Ia d) phase at
low temperatures that was previously reported for benzyltriethylammonium salts with either
[BF4]- or [PF6]- anions[12-14] is also found for dicyanamide salt 4.
As mentioned above, the volume of [N(CN)2]- is intermediate that of [BF4]- and [PF6]-
(Table 3), but its shape is different. The difference may be important in thermotropic LCs as
efficient space-filling needs to be achieved in the LC state. The latter is less important in
conventional lyotropic LC systems, where water molecules can fill gaps of any shape. To the
best of our knowledge, the only other examples of ILCs containing relatively small, non-
mesogenic, V-shaped anions are the [Cnmim][F(HF)2] and [Cnmpyrr][F(HF)2] ([Cnmpyrr]+ =
N-alkyl-N-methylpyrrolidinium) salts that were reported by Xu et al. and which show only
SmA phases.[81-83] Our results demonstrate that the thermotropic CubV(Ia d) structure adopted
by taper-shaped ionic amphiphiles, such as 4, is apparently compatible with different anion
shapes, at least for moderately sized anions. Bulky [NTf2]- anions (which are
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conformationally flexible but can also adopt a bent shape) have already been incorporated in
bicontinuous cubic mesophases formed by low molar mass ionic and zwitterionic LCs but, in
those cases, the anions were located primarily on an infinite periodic minimal surface (IPMS),
where the steric restrictions are less severe than inside the polar channels of an inverted-type
CubV phase.[84-90]
We also note that thiocyanate salt 9 exhibits only a Colhex phase. Samples of compound 9
were heated and cooled at different rates (down to 1 °C min-1) but, in all cases, evidence for a
cubic phase was not observed by POM or XRD. Since the [SCN]- ion is similar in volume to
[BF4]- (Table 3), the result may be due to a shape effect as linear counterions may not be
compatible with thermotropic, inverted-type CubV(Ia d) phases. Presumably linear anions
cannot efficiently fill the space in the junction regions where three channel segments in the
cubic phase come together.
Inspection of the comparative LC data presented in Table S4 revealed the following
structure-property
relationships
for
ILCs
with
N,N,N-triethyl-N-(3,4,5-tris(n-
dodecyloxy)benzyl)ammonium cations and [SCN]-, [BF4]-, [N(CN)2]-, [PF6]- or [OTf]- anions:
(i) a bicontinuous cubic phase is only found for the salts featuring moderately sized and
pseudospherical or V-shaped [BF4]-, [N(CN)2]- or [PF6]- anions, and not for derivatives with
linear [SCN]- or [OTf]- anions; (ii) a Colhex phase is observed for the [SCN]- salt but the
mesomorphic properties vanish for the [OTf]- derivative; (iii) the melting as well as the
clearing point of the [N(CN)2]- salt are lower than those of the [BF4]- salt, which appears to be
in line with the generally observed evolution of phase transition temperatures with increasing
anion size,[1] although the [SCN]-, [PF6]- and [OTf]- salts deviate from this behavior. Given
the fact that the benzyltriethylammonium [OTf]- salts that are listed in Table S4 are not LC, it
may be expected that the corresponding salts with an even bulkier [NTf2]- anion may also not
exhibit mesomorphic characteristics. Structurally related ILCs with a 1-methylimidazolium
headgroup and [OTf]- or [NTf2]- anions only adopt enantiotropic Colhex phases when three
very long alkyl chains are attached to the cation (i.e., n-hexadecyl or n-octadecyl).[91]
When comparing the phase transition temperatures of salts 1 and 2 with those of 7 and 8,
the benzylammonium-based molecular architecture with two alkyl chains facilitated the
formation of SmA phases with higher clearing points (i.e., they exhibited higher
thermodynamic stabilities) than those adopted by other alkyl-substituted salts. To the best of
our knowledge, N,N,N-trialkyl-N-[3,4-bis(alkyloxy)benzyl]ammonium cations have not yet
been used to prepare ionic mesogens.
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The analogue of compound 8 with relatively bulky [NTf2]- anions, i.e. [C16mim][NTf2], is
not LC.[44] Although 1,3-bis(n-dodecyl)-1,2,3-triazolium dicyanamide shows a smectic phase
between 82 °C and 102 °C,[7] and the 1,3-bis(n-dodecyl)imidazolium salts with I-, [BF4]- or
[ClO4]- counterions exhibit SmA phases,[92, 93] 1,3-bis(n-dodecyl)imidazolium dicyanamide
does not exhibit LC properties.[93] [C14mim][N(CN)2] is also not LC,[70] while the
corresponding [NO3]-, [BF4]-, [PF6]- and [ClO4]- salts are mesomorphic.[1] We report here that
salt 8, with a slightly longer alkyl chain than [C14mim][N(CN)2], shows a SmA phase. The
fact that mesomorphism was observed for a chain length of 14 carbon atoms in the series
[Cnmim][PF6] and that a longer alkyl chain is required in the series [Cnmim][N(CN)2], despite
the smaller volume of the [N(CN)2]- anion (Table 3), indicates that not only the size of the
counterion[94] but also its shape should be considered in the design of an ILC.
Finally, it is important to note that salt 7 shows a SmA phase, which lacks long-range
intralayer ordering, whereas its bromide salt precursor is known to form a crystal smectic T
phase with 2D square ordering of the polar headgroups within the ionic sublayers.[95] Much in
the same way as a linear [SCN]- anion was found to be incompatible with such a tight packing
arrangement (in contrast to Br-, [BF4]- and [PF6]-),[46] the [N(CN)2]- anions may disrupt
intralayer ordering and cause the appearance of a SmA phase instead.
Conclusions
We describe the first series of ionic mesogens outfitted with halogen-free dicyanamide
anions that adopt smectic, columnar or bicontinuous cubic thermotropic LC mesophases
depending on their structures and compositions. These compounds cover the most common,
microphase-segregated mesophase morphologies displayed by thermotropic LC amphiphiles,
with the exception of micellar cubic phases. Synchrotron XRD experiments facilitated
elucidation of the molecular packing arrangements in the different types of mesophases.
The dicyanamide-based ILCs represent rare examples of LC salts with V-shaped anions.
The bent shape was found to influence the presence, stability range, and intralayer structural
order of lamellar phases. For example, salt 7 shows a SmA phase instead of the ordered
crystal smectic T phase adopted by its bromide analogue. On the other hand, the V-shaped
anions appeared to be compatible with the inverted-type columnar and bicontinuous cubic
mesophases adopted by the taper-shaped benzylammonium dicyanamide salts 3–6. For
comparison, cubic phases were not observed for salt 9, which features a linear thiocyanate
25
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10.1002/chem.201705794
Chemistry - A European Journal
anion. Our results reveal the importance of the shapes of anions in ionic mesogens, in addition
to their size (volume).
To the best of our knowledge, this is also the first study addressing the thermal stability of
N,N,N-trialkyl-N-benzylammonium dicyanamide salts.[96] Data from TGA and TGA-MS
measurements indicated that the initial stage of thermal degradation (below 300 °C) of
compounds 1–6 is specific to the benzylammonium motif and involves the expulsion of
tri(m)ethylamine as well as further reaction of the cyano groups present in the dicyanamide
moieties.
We are currently exploring ways to induce polymerization of the [N(CN)2]- anions within
the temperature ranges of the different mesophases, in order to obtain templated, nitrogen-rich
carbon materials. The Lewis basicity of the [N(CN)2]- anions could also be exploited when
these ILCs are used as organized media to facilitate selective, base-catalyzed chemistry.
Acknowledgments
G.P., K.G. and C.W.B. were supported in part by the Institute for Basic Science (IBS-R019-
D1). C.W.B. acknowledges the BK21 Plus Program as funded by the Ministry of Education
and the National Research Foundation of Korea for support. We thank Stanfield Lee for
recording the XPS survey spectra and Jihyun Park for recording the simultaneous TGA-DSC
data. We are also grateful to Dr. Sangho Jeon, Seong-hun Lee and Juhyun Yang for their
assistance with the synchrotron XRD measurements. The experiments at the PLS-II 6D
UNIST-PAL Beamline were supported in part by the Korean Ministry of Education, Science
and Technology (MEST), Pohang University of Science and Technology (POSTECH) and the
UNIST Central Research Facilities (UCRF).
TOC Graphic and Text
Ionic liquid crystals with “V-shaped” anions: A series of halogen-
free, amphiphilic dicyanamide (i.e., [N(CN)2]-) salts were found to
adopt smectic, columnar as well as bicontinuous cubic thermotropic
liquid-crystalline mesophases, even at room temperature in some
cases. The results show that not only the size of the anions, but also
their shape play important roles in determining the thermal phase
characteristics displayed by ILCs, a design parameter that is often
neglected. The thermal decomposition pathways of the dicyanamide
salts were also studied.
26
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10.1002/chem.201705794
Chemistry - A European Journal
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Chemistry - A European Journal
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29
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