Impact of substitution pattern and chain length on the thermotropic properties of alkoxy-substituted triphenyl-tristriazolotriazines

Article abstract of DOI:10.3390/molecules25235761

Tristriazolotriazines (TTTs) with a threefold alkoxyphenyl substitution were prepared and studied by DSC, polarized optical microscopy (POM) and X-ray scattering. Six pentyloxy chains are sufficient to induce liquid-crystalline behavior in these star-shap

Full text of DOI:10.3390/molecules25235761

molecules
Article
Impact of Substitution Pattern and Chain Length on
the Thermotropic Properties of Alkoxy-Substituted
Triphenyl-Tristriazolotriazines †
Thorsten Rieth 1, Natalie Tober 1, Daniel Limbach 1, Tobias Haspel 1, Marcel Sperner 1
,
Niklas Schupp 1, Philipp Wicker 1, Stefan Glang 1, Matthias Lehmann 2,* and Heiner Detert 1,
*
1
Department for Chemistry, Johannes Gutenberg-University, 55099 Mainz, Germany;
thorstenrieth@gmx.de (T.R.); ntober@students.uni-mainz.de (N.T.); Limbach@ghc.de (D.L.);
t.haspel@gmx.de (T.H.); marcel.sperner@outlook.de (M.S.); nschupp@students.uni-mainz.de (N.S.);
pwicker@students.uni-mainz.de (P.W.); stglang@gmx.de (S.G.)
2
Institute for Organic Chemistry, Julius-Maximilians-University, 97074 Würzburg, Germany
*
Correspondence: matthias.lehmann@uni-wuerzburg.de (M.L.); detert@uni-mainz.de (H.D.);
Tel.: +49-931-318-3708 (M.L.); +49-6131-3924-189 (H.D.)
†
Dedicated to Horst Kunz, Mainz, on the occasion of his 80th birthday.
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Viorel Circu, Doina Manaila-Maximean and Valery A. Loiko
Received: 28 October 2020; Accepted: 2 December 2020; Published: 7 December 2020
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Tristriazolotriazines (TTTs) with a threefold alkoxyphenyl substitution were prepared and
studied by DSC, polarized optical microscopy (POM) and X-ray scattering. Six pentyloxy chains
are sufficient to induce liquid-crystalline behavior in these star-shaped compounds. Thermotropic
properties of TTTs with varying substitution patterns and a periphery of linear chains of different
lengths, branching in the chain and swallow-tails, are compared. Generally, these disks display
broad and stable thermotropic mesophases, with the tangential TTT being superior to the radial
isomer. The structure–property relationships of the number of alkyl chains, their position, length and
structure were studied.
Keywords: star-shaped compounds; discotic liquid crystals; X-ray diffraction; differential scanning
calorimetry; polarizing optical microscopy; swallow-tail; heterocycles; structure–property relation
1. Introduction
Tristriazolotriazines (TTTs), with a threefold alkoxyphenyl substitution [1–4] (Figure 1), not only
belong to the youngest classes of discotic liquid crystals (DLCs) [5–16] but also to the small subgroup
of DLCs with a heterocyclic [17 26] and electron-deficient core [27
–
–36]. Two C3-symmetrical isomeric
forms of these star-shaped molecules are known: the paddle-wheel with phenyl substituents in
tangential positions (t-TTT) , and, if the aromatic rings are attached to the radial positions, the propeller
(r-TTT) (Figure 1). This ostensible small change from tangential to radial substitution severely alters
the mesomorphic properties. The t-TTT is a reliable core for discotic liquid crystals with broad
mesophases [ 37 40], and phenyl substituents on this isomer are twisted out of the mean plane
1
2
1–4, –
by 21–80◦ [41]. Isomerization to the r-TTT provokes a flattening of the aromatic part and facilitates
crystallization. Reduced or even complete loss of mesomorphism is the typical consequence for
3,4- and 3,5-dialkoxy derivatives [42]. On the contrary, derivatives with a 3,4,5-trialkoxy periphery
have lower melting points and broader phases. Although calamitic LCs have contributed to a broad
range of technical innovations, discotic LCs have only found selected applications, such as in optical
compensating films or sensors [
9]. Understanding the structure and molecular dynamics is not a
trivial task. The comprehension of structure–property relationships is the key knowledge to tailor
Molecules 2020, 25, 5761; doi:10.3390/molecules25235761
www.mdpi.com/journal/molecules

Molecules 2020, 25, 5761
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mesomorphic, optical and electrical properties for the application of DLCs as anisotropic organic
semiconductors in organic field-effect transistors, light-emitting diodes or photovoltaic devices [13].
Columnar phases of DLCs comprise stacks of disk-like molecules arranged on a regular 2D-lattice.
Redox potentials and charge carrier mobilities are central properties for electronic applications;
the latter is highly dependent on intermolecular order, allowing orbital overlap, e.g., in staggered
cofacial conformations [14] or helical arrangements [15]. According to a theoretical study on columnar
mesophases of TTTs with an electron-rich periphery, a high number of
an ambipolar charge transport in TTTs on two main charge transport channels. Cyclic voltammetry
of alkoxyphenyl TTTs showed only nonreversible oxidation [ ], whereas the TTT core was shown
π-stacking contacts [16] render
2
to have electron-transporting characteristics [43]. The hole-transporting HOMO orbitals are located
in the peripheral region; the LUMO orbitals, located in the central TTT, take the electron transport.
From impedance spectroscopy, it is concluded that the conductivity in the columnar phase of
dialkoxy-TTTs is mostly ionic (low-frequency regime), probably due to ionic impurities [2].
Figure 1. Tangential tristriazolotriazines (t-TTTs) (a) and propeller tristriazolotriazines (r-TTTs) (b) and
the substitution pattern.
The established option for the mesophase engineering of DLCs is the variation of the periphery.
The number, length, position, composition and structure of flexible side chains form a highly advantageous
toolbox for custom-made mesomorphism and have been extensively studied, e.g., on triphenylenes [44
star systems [46 49] and macrocyclic discs [50 55]. Though some general trends have been found and
successfully used for the prediction of liquid crystal (LC) behavior, they are valid only for a certain
core [ ]. Furthermore, the scientist is often rewarded with nonlinearities and surprising exceptions.
,45],
–
–
7
The objective of this report is to elucidate the correlation between the mesomorphism of TTTs and the
composition of their periphery. This includes the number of side chains, the variation of their position,
their length and branching.
2. Results
2.1. Synthesis
The only effective synthesis of t-TTTs is the threefold acylation/ring transformation of tetrazoles
with cyanuric chloride (Scheme 1), according to Huisgen [56]. A few compounds require minor
modifications, but the main problem is the chromatographic separation of diaryl triazoles
by-products. Alkoxyaryl-tetrazoles are accessible via the addition of azide to alkoxybenzonitriles
or, if the respective hydroxybenzonitrile is not available, via the dehydration/azidination of
3 formed as
6
4

Molecules 2020, 25, 5761
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alkoxybenzamides
to high temperatures. A thermal isomerization converts
5
with triazidochlorosilane [57]. Despite earlier accounts [
to its isomer
the triazole rings, shifting the tangentially oriented aryl rings to radial positions. The cyclocondensation
of chlorotriazoles is the direct route to the isomeric r-TTTs 58 59]. Besides limited accessibility of
1
,
2
], t-TTTs
1
are sensitive
1
2 with different connectivity of
7
2 [ ,
the particular chlorotriazole, poor yields in the cyclocondensation step reduce the synthetic value of
this method. Therefore, nearly all r-TTTs known today were obtained by thermal isomerization of the
tangential isomer [42].
Scheme 1. Synthesis of t-TTTs and thermal rearrangement to r-TTTs.
2.2. Thermal Properties of 1-n-alkyloxy-t-TTTs
Some homologous series of t-TTTs with one to three linear alkyloxy were studied. Table 1 collects
the TTTs together with their thermal characteristics. The order follows the substitution pattern
(see Figure 1) and increasing chain length. Some compounds are reported in the literature, and the
preparation and analytical data of new TTTs are provided in the Supporting Information (SI). Thermal
properties were studied by differential scanning calorimetry (DSC) and polarized optical microscopy
(POM), and structure determination of the mesophases was performed on representative compounds
via X-ray diffraction (XRD). Details on equipment are given in the SI.
Table 1. Mono-, di- and tri-1-n-alkoxy t-TTTs and their thermal characteristics.
Entry
Substitution Pattern
Transition/◦C (Enthalpy/kJ/mol)
Ref
t-1
t-2
t-3
t-4
t-5
t-6
t-7
t-8
t-9
t-10
t-11
Ri = H
Mp. 360 ◦C
Mp. > 275
Cr 129.9 I; Tg = 55.4
Cr 93.8 I; Tg = 46.7
Cr 76 I
Cr 86.8 I; Tg = 36.7
Cr 78 I
Cr 134 I
[
[
[
56
42
37
]
]
]
R4 = OC3H7
R4 = OC6H13
R4 = OC8H17
R4 = OC10H21
R4 = OC12H25
R4 = OC13H27
R3 = OC10H21
R2 = OC10H21
R2 = R3 = OC10H21
R2 = R4 = OC10H21
SI
SI
SI
SI
SI
SI
SI
SI
Tg = −64 I
Cr 69 I
Cr 50 I

Molecules 2020, 25, 5761
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Table 1. Cont.
Transition/◦C (Enthalpy/kJ/mol)
Entry
Substitution Pattern
Ref
t-12
t-13
t-14
t-15
t-16
t-17
t-18
t-19
t-20
t-21
t-22
t-23
t-24
t-25
t-26
t-27
t-28
t-29
t-30
t-31
t-32
t-33
t-34
t-35
t-36
t-37
R2 = R5 = OC10H21
R3 = R4 = OC4H9
Cr 89 I
Cr 131 I
Cr 102 M 201 I
SI
SI
SI
SI
SI
R3 = R4 = OC5H11
R3 = R4 = OC6H13
R3 = R4 = OC7H15
R3 = R4 = OC8H17
R3 = R4 = OC9H19
R3 = R4 = OC10H21
R3 = R4 = OC11H23
R3 = R4 = OC12H25
R3 = R4 = OC13H27
R3 = R4 = OC14H29
R3 = R4 = OC16H33
R3 = R4 = OC18H37
R3 = R5 = OC6H13
R3 = R5 = OC7H15
R3 = R5 = OC8H17
R3 = R5 = OC10H21
R3 = R5 = OC12H25
R3 = R5 = OC12H25, R4 = Br
R3 = R5 = OC16H33
R3 = R4 = R5 = OC6H13
R3 = R4 = R5 = OC8H17
R3 = R4 = R5 = OC10H21
R3 = R4 = R5 = OC12H25
R3 = R4 = R5 = OC16H33
Cr 98 (21.0) M 232 (3.6) I
Cr 100 (22.7) M225 (4.7) I
Cr 100 (19.9) M 226 (4.5) I
Cr 94 (19.8) M 215 (5.7) I
Cr 89 (14.3) Colhd 207 (6.7) I
Cr 81 (5.7) M 200 (5.2) I
Cr 92.2 (19.3) Colh 207.6 (6.8) I
Cr 74 (12.4) M 175 (6.1) I
Cr-11(46.3) M2 68 (9.4) M1 181 (5.4) I
Cr 19 (58.7) M2 68 (9.4) M1 171 (5.7) I
Cr 40 (96.9) M2 74 (9.1) M1 164 (7.5) I
Cr 136 M 187 I
Cr 122 Colh 179 I
Cr 92 M 152 I
Cr 73 M 147 I
Cr 74 (6.8) Colh 143 (5.8) I
Cr 58 (9.5) M 120 I
[38
]
]
]
SI
[42
SI
[42
SI
[
[
42
42
]
]
SI
[
[
[
[
[
60
]
]
]
]
]
60
60
60
60
SI
SI
Cr 70 (7.4) M 115 (6.0) I
Cr 116 (19.9) Colhd 184 (4.6) I
Cr 126 (22.8) Colhd 184 (5.6) I
Cr 126 (18.6) Colhd 165 (3.7) I
Cr 128 (22.7) Colhd 153 (6.9) I
Cr 124 (24.6) M 144 (9.4) I
[
[
[
37
42
42
]
]
]
SI
SI
M, M1 and M2 refer to liquid-crystalline mesophases of unknown structure, Tg: glass transition temperature.
Triphenyl-t-TTT t-1 is a sparingly soluble and high-melting compound (360 ◦C) [56]. Its r-isomer
◦
r
-
1
melts even higher: 390–400 C [58]. The a◦ttachment of p-alkoxy chains◦significantly reduces
◦
the melting point (t-2: O-C3H7: >275 C to 76 C for O-C10H21
(
t-5), but 78 C for O-C13H27, t-7).
Although these compounds are not mesomorphous, they can form stable glasses with Tg in the
range of 35–60 ◦C [
1–
4
]. A comparison of the position of the alkyl chain, e.g., for decyloxy-t-TT◦Ts,
◦
reveals a strong impact: whereas 4-substituted TTT t-5 melts at 76 C, its 3-isomer t-8 at 134 C
and the 2-isomer t-9 is a viscous oil SI. The steric effect of the ortho-alkoxy group, pointing into the
center of the disks, is probably the reason for the lack of mesomorphism of TTTs with a 2,3-, 2,4- and
2,5-didecyloxy substitution (t-10
π-system, e.g., t-45 (Table 2).
–t-12) SI. This appears to be valid even for TTTs with an extended
The thermal behavior of 3,4-dialkoxyphenyl-substituted t-TTTs is completely different. This pattern
is almost a warranty for mesomorphism (Table 1, Figure 2). The width of the mesophase is typically
higher than
∆T = 100 K at a maximum of 192 K (t-23). Although the 3,4-dibutoxy derivative melts at
132 ◦C without any sign of mesomorphism, the higher homologs from 3,4-dipentyloxy (t-14: Cr 102 M
201 I), at least up to 3,4-dioctadecyloxy t-25, are discotic liquid crystals ([38,42], SI). With increasing chain
length, the clearing point successively drops from 232 ◦C (dihexyloxy, t-16) to 171 ◦C (dioctadecyloxy,
t-28), and the melting points from 102 ◦C (dipentyloxy, t-14) to 74 ◦C (ditridecyloxy, t-22). Further chain
elongation gives a surprising result. With the formation of an additional mesophase M2 (POM and DSC
below the typical LC range, e.g., with two tetradecyloxy chains (t-23), the melting transition occurs at
11 ◦C with an enthalpy of 46.3 kJ/mol and an M2–M1 transition at 68 ◦C (9.4 kJ/mol). This phase is
)
−
stable up to 181 ◦C (5.4 kJ/mol). Elongation of the side chains by two or four methylene groups (t-24
,
t-25) gives a similar phase behavior. Whereas the melting points increase significantly (Tm = 19 ◦C;
40 ◦C), the M2-M1 transition (Tt = 68 ◦C; 74 ◦C) and the clearing point are less affected (Tc = 181 ◦C;
171 ◦C). The phenomenon of the emergence of a second mesophase with increasing chain length has

Molecules 2020, 25, 5761
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been observed earlier, e.g., with triphenylene hexaesters and hexabenzoates [44
,
45]. Both the TTT and
triphenylene series have a columnar hexagonal structure of the upper LC phase in common. However,
although the transitions from the first to second mesophase in the triphenylene series occur within the
typical LC range, the additional mesomorphic phase of t-TTTs appears below this temperature regime.
Table 2. t-TTTs with extended p-systems and data from thermal analysis.
Entry
Substitution Pattern
Transition/◦C (Enthalpy/kJ/mol)
Ref
t-38
t-39
t-40
t-41
t-42
t-43
t-44
t-45
t-46
t-47
t-48
t-49
t-50
t-51
t-52
6-OC10H21-Nap [a]
Cr 121 I
SI
[38]
SI
SI
SI
[38]
SI
SI
[37]
[37]
[37]
[4]
[4]
SI
5,6-OC8H17-Nap [a]
5,6-OC10H21-Nap [a]
5,6-OC12H25-Nap [a]
3´,4´-OC8H17-Bip [a]
3´,4´-OC10H21-Bip [a]
3´,4´-OC12H25-Bip [a]
2’,5’-(OC12H25)2-Bip [a]
4’-(H13C6O)2-St [a]
3’,5’-(H13C6O)2-St [a]
3’,4’,5’-(H13C6O)2-St [a]
4’-Hexo-PEP [a]
3,4,5-(H17C8O)3-PEP [a]
3,4,5-(H25C12O)3-PEP [a]
3,4,5-(H17C8O)3-PEPEP [a]
Cr 112 (1.3) Colh 197(2.6) I
Cr 76 (7.6) M 210(2.3) I
Cr 59 (1.5) Colh 189(2.9) I
Cr 148 I
Cr 172 I
Cr 89 (31.1) M 199 (2.6) I
Cr 87 I
mp > 240
m.p. 124, Tg 13
Cr 175 M 200 I; Tg 178
Cr 245 I
Cr 82 I
Cr 80 I
Cr 159 M 204 I
[4]
[a] phenyl replaced by other aromatic unit; Nap = naphth-2-yl; Bip = biphenyl-4-yl; St = E-stilben-4-yl;
PEPEP = 4-(4-phenylethynyl)-phenylethynyl)phenyl; Tg: glass transition temperature.
Figure 2. Phase behavior of 3,4-di- (t-13–t-25), 3,5-di- (t-26–t-32) and 3,4,5-trialkoxyphenyl
(t-33–t-37)-substituted t-TTTs depending on chain length.
Miscibility of homologous t-TTTs allows the tuning of the phase transitions. Even a mixture of
25 mol % of nonmesomorphous butoxy-TTT t-13 and 75 mol % decyloxy-substituted DLC t-19 displays
a LC phase Cr 75 M 176, only ∆T = 13 K lower and ∆T = 31 K narrower compared to pure t-19.

Molecules 2020, 25, 5761
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The series with the isomeric 3,5-dialkoxy pattern (t-26
-
t-32) [60] shows a more pronounced descent
of phase transitions with increasing chain length ([60] SI). With decyl chains t-29, the melting point
reaches a plateau at about 72 ◦C. The width of the typically columnar hexagonal LC phase is about half
as broad as found for the 3,4-isomers (Table 1, Figure 2). It increases from hexyl t-26
decyl t-29 T = 74 K) and decreases to T = 45 K for hexadecyl t-32. An additional bromine atom
between dodecyloxy chains t-31 reduces the melting point and slightly destabilizes the mesophase
∆T = 62 K vs. T = 68 K, t-30) SI. Adding a third alkoxy chain in the four-position of t-30 enhances the
melting point and leads to the also mesomorphous tris(3,4,5-trialkoxyphenyl) t-TTTs, generally with
a columnar hexagonal-disordered structure ([37 42], SI). Tris-(3,4,5-trihexyloxyphenyl) substitution
(∆T = 50 K) to
(
∆
∆
(
∆
,
(
t-33) gives rise to a broad mesophase (Cr 116 Colhd 184 I) [37,42]. According to XRD, the hexagonal
columnar phase has cell parameters of a = 25.5 Å and d = 3.4 Å at 20 ◦C and in the mesophase at 145 ◦C.
◦
These values are a = 24.2 Å and d = 3.9 Å at 20 C. An ensemble of 16 molecules in a 2a × 2a × 4c
hexagonal supercell of t-33 consisting of 16 unit cells was modeled with the program Accelrys Materials
Studio 2017 R2 using the Compass2 force field [61]. In order to highlight the stabilization relative to
16 single mesogens, the energy differences were calculated for this model. As a result of the packing
the valence energy increased by 500 kcal/mol, but was opposed to an energy gain of
−
1615 kcal/mol for
the van der Waals and 448 kcal/mol for the electrostatic energy. The stabilization by nonbonding
−
intermolecular interactions was achieved by an offset of the cores from the center of the column.
Figure 3 indicates that there is a tendency to locate the peripheral trialkoxy phenyl ring above the
triazine center.
a = 25.5 Å
(b)
b)
(a)
Figure 3. Model of a supercell (2a × 2a × 4c) of t-33. (a,b) Top and side view, where hexyl chains are
(c)
(d)
a = 25.5 Å
omitted; (c,d) side and top view, including hexyl chains.
c)
d)
Figure 3. Model of a supercell (2a
omitted; (c,d) side and top view, including hexyl chains.
×
2a
×
4c) of t-33. (
a
,
b
) Top and side view, where hexyl chains are
◦
Chain elongation raises the melting point to a plateau at ca. 126 C, but the width of the
mesophase successively shrinks to ∆T = 20 K for hexadecyl chains t-37 due to decreasing clearing
points. A comparison of the number of methylene groups in the periphery of TTTs reveals that 3,5-and
3,4,5-substituted TTTs with a similar molecular weight show a similar phase width, e.g., 3,5-dihexadecyl
t-32
t-34
:
:
∆
∆
T = 39 K and 3,4,5-tri-decyl t-35
T = 58 K. Cammidge reported that the exchange of a single hexyloxy chain on triphenylene
: ∆T = 45 K and 3,5-didodecyl t-30: ∆T = 69 K and 3,4,5-tri-octyl
DLC HAT6 versus bromine dilated the width of the LC phase by almost a factor of three [62].
A comparison of t-36 (Cr 128 Col 153 I) and the didodecyloxy-bromo compound t-31 (Cr 58 M
120 I) gives a proportionate augmentation. Although the 3,5-pattern guarantees lower melting and
clearing points, the “less symmetrical” 3,4-series gives the broadest mesophases. Hexagonal structures

Molecules 2020, 25, 5761
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and π-π-distances of about 3.48–3.8 Å are common features of the mesophases of the 3,4-, 3,5- and
3,4,5-substituted TTTs.
2.3. Thermal Properties of t-TTTs with Extended π-Systems and a 1-n-alkyloxy Periphery
TTTs with extended π-systems have been prepared as new materials for organic electronics,
fluorophores and nonlinear optical materials. Like in the phenyl series, linear alkoxysubstitution can
induce mesomorphism of TTTs with additional phenyl-, styryl- or phenylethynyl groups.
The distension of the rigid part by the elongation of the phenyl arms to naphthyl, biphenylyl,
tolane or stilbene improves the
(4-decyloxyphenyl–6-decyloxynaphthyl,
T = +115 K) (SI). However, vicinal dialkoxy substitution in the lateral position is beneficial for the
π-π
interaction and heightens the melting points, e.g., t-5
–t-38
∆
T = +45 K) and t-3 t-49 (4-hexyloxyphenyl–4’-hexyloxytolanyl,
–
∆
mesomorphism of 3,4-dialkoxyphenyl-t-TTTs ([38], SI); even after extension of the rigid part, naphthyl
replacing phenyl, these disks form broad LC phases. From 5,6-di(octyloxy)naphth-2-yl t-39 to the dodecyl
homolog t-41, the melting point drops from 112 ◦C to 59 ◦C, and the LC phase widens from 85 ◦C to
130 ◦C. t-41 forms a hexagonal mesophase with a = 30.8 Å and a
π
-
π
-distance of 3.6 Å [38]. A further
augmentation, naphth-2-yl to biphenyl-4-yl, destabilizes the LC phase, mainly due to enhanced melting
points [38]. Although the 3’,4’-dioctyl and -decyl derivatives t-42 t-43 are not mesomorphic, dodecyl
chains t-44 are sufficient for a broad LC phase from T = 89 ◦C to 198 ◦C. Further extension of the rigid
part, stilbene (t-47 t-48), tolane (t-50, t-51) and phenylethynyltolane (t-52) replacing phenyl can give
DLCs ([ 37] SI). Whereas 3,5-dihexyloxy is insufficient for mesomorphism of stilbene derivative t-47,
,
,
4,
the 3,4,5-trihexyloxy periphery t-48 results in a narrow mesophase at high temperatures (Cr 175 M
200 I). This pattern with slightly longer octyl chains allows mesomorphism even of the extended
phenylethynyltolane TTT t-52 (Cr 159 M 204 I). POM suggests a discotic nematic structure for the 45 K
broad LC phase. The shorter homologs with tolane units and three octyl or dodecyl chains, t-50 t-51
,
have nearly identical melting points (82 ◦C, 80 ◦C), about
∆
T
≈
45 K lower than their phenyl homologs
t-34, t-36. Unfortunately, the tolane derivatives are not mesomorphous.
2.4. Thermal Properties of t-TTTs with Branched Side Chains, Swallow-Tails or Chains of Different Lengths
Chain branching is another useful concept to modify the thermal properties of calamitic and discotic
liquid crystals. This approach is particularly useful to reduce melting and clearing points, notably for
large-core mesogens [63
–71]. Two different strategies are applied: short-chain branching [35,36,72–79]
like 3,7-dimethyloctyl or 2-ethylhexyl and Weissflogs swallow-tails [80–85].
Short-chain branching has a strong impact on thermal properties, even on nonmesomorphous
4-alkoxyphenyl-TTTs (Table 3). Although p-decyloxy TTT t-5 melts at 76 ◦C, the diastereotopic mixture
of the p-(3,7-dimethyloctyloxy) derivative t-53 is a viscous oil. (SI) On the other hand, t-54, with a
voluminous but rigid neomenthyloxy periphery, is crystalline up to 185 ◦C; if the voids in the crystal are
filled with suitable solvent molecules, the crystal phase is stable up to 211 ◦C [41]. Short-chain branching
has a similar effect in the 3,4-dialkoxy series: change from n-octyloxy (t-17, Tc = 223 ◦C) to 3-octyloxy
(
t-55, Tc = 158 ◦C) or 2-ethylhexyloxy (t-56, Tc = 157 ◦C) provokes depression of the clearing point of
nearly
∆
T = 70 K and even more of melting points. Concerning longer chains, the impact of branching
◦
is leveled. Relative to the linear 3,4-didecyloxy TTT (t-29,Tc = 207 C), clearing of 4-ethyloctyl-,
as well as the 3,7-dimethyloctyl analog t-57
(R)-citronellyl chains Tc = 167 C [
,
t-58 is reduced to Tc = 182 ◦C (SI) for t-59 with enantiopure
◦
1]. Laschat reported a beneficial effect on the phase width of
short-chain branching in the
generalized since the 3,4,5-trialkoxy derivatives t-34
t-61 t-62 are not (SI). The LC phase of t-60 with the nonchiral and more rigid 3,3-dimethyloctyl chains is
δ-position on triphenylene systems [76]. Unfortunately, this cannot be
,
t-35 are mesomorphous, the branched isomers
,
stable only until 154 ◦C and crystallization occurs on the timescale of months (first heating: m = 56 ◦C
T ;
Tg = 8 ◦C). Depending on the cooling rate, POM textures either show a homeotropic growth or fan
structure (Figure 4).

Molecules 2020, 25, 5761
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Table 3. t-TTTs with branched chains, swallow-tails and mixed-chain lengths, and data from
thermal analysis.
Entry
Substitution Pattern
Transition/◦C (Enthalpy/kJ/mol)
Ref
R4 = O(CH2)2CH(CH3)(CH2)3CH(CH3)2
R4 = neomenthyloxy
t-53
t-54
t-55
t-56
t-57
t-58
t-59
t-60
t-61
t-62
t-63
t-64
t-65
t-66
t-67
t-68
t-69
t-70
t-71
t-72
t-73
t-74
t-75
t-76
t-77
t-78
t-79
I
SI
Cr 185 I
Colh 158 I
[41
]
R3 = R4 = O-CH(C2H5)C5H11
[1]
SI
SI
SI
[1]
SI
R3 = R4 = O-CH2CH(C2H5)C4H9
Cr −9 M 157 I
R3 = R4 = O-(CH2)3CH(C2H5)C4H9
R3 = R4 = O(CH2)2CH(CH3) (CH2)3CH(CH3)2
R3 = R4 = (R)-O(CH2)2CH(CH3)- (CH2)2CH=C(CH3)2
R3 = R4 = O(CH2)2C(CH3)2 C5H11
R3 = R4 = R5 = O-CH2-CH(C2H5)C4H9
R3 = R4 = R5 = O-(CH2)3CH(C2H5)C4H9
Cr72 (15.8) M 182 (5.7)
Cr (51) M 182 (5.8) I
Cr 41 Colh 167 I
Cr 56 (17.4) Colh 154 (4.6) I (Tg 8)
Cr 160 I [a]
Cr 124 I
SI
[42
]
R3 = OCH2CH(C8H17 2
R4 = OCH2CH(C6H13 2
R4 = OCH2CH(C8H17 2
R4 = OCH2CH(C10H21 2
R4 = OCH2CH(C12H25 2
R4 = OCH(C6H13
)
)
)
−64 (Tg) I
SI
SI
SI
SI
SI
SI
SI
SI
SI
SI
SI
SI
SI
SI
SI
SI
SI
14 (Tg) Colh 65 I
Cr −15 (2.1) M 101 (2.1) I
Cr −70 Tg 9 Tg Colh 85 (2,0) I
Cr −54 (19.5) Colh 82 (2.4) I
Cr 8 (1.6) I
)
)
)
R4 = O(CH2)2CH(C6H13
)
Cr −71 I
R4 = OCH2CH((CH2)2CH(CH3)2)2
Tg = 42 Tm = 47
Cr 95 (7.1) M 140 I [b] POM: 80–120
Cr 86 I
R4 = O-CH2CH(CH2CH(C2H5)C4H9)2
R3 = R4 = OCH2CH(C6H13 2
)
R3 = OC4H9, R5 = OC16H33
47 (Tg) M 91 (2.1) I
Cr 59 M 132 (4.5) I
Cr 98 I
Cr 103 (23.0) M 183 (4.0) I
Cr1 61 (6.0) Cr2 89 (5.3) I
Cr 127 (21.4) M 158 (4.9) I
Cr 80 (23.8) Colh 136 (1.5) I
R3 = OC6H13, R5 = OC14H29
R3 = OC16H33, R4 = OCH3
R3 = OC14H29, R4 = OC6H13
R3 = COOC10H21, R4 = OC10H21
R3 = R4 = OC6H13, R5 = OC12H25
R3 = R4 = OC6H13, R5 = O-(CH2)3CH(C2H5)C4H9
[a] determined by POM; Tg: glass transition temperature; [b] values from 1, heating scan.
Figure 4.
(cooling rate: 1 ◦C/min).
(a b
): Homeotropic growth of t-60 at 135 ◦C (cooling rate: 1 ◦C/min); ( ): fan textures at 137 ◦C
The XRD (SI) results reveal a hexagonal columnar mesophase of t-60. The structure is retained in
the solid phase, but the lattice constants increase (130 ◦C: ahex = 27.0 Å; 25 ◦C: ahex = 27.8 Å). It should be
noted that this lattice constant is larger than of the compound with the linear chain (3,4-O-decyloxy t-19
hex = 26.2 Å (20 ◦C) 26.8 Å (145 ◦C)), which can be attributed to the Thorpe–Ingold effect, preventing
:
a

Molecules 2020, 25, 5761
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the alkyl chains from interdigitation. The experimental (1.02 g/mL) and X-ray density (0.99 g/mL) are
nearly identical, and the
-distance in t-60 is assumed to correspond to d = 3.4 Å (25 ◦C). This accounts
π-π
for a single molecule per unit cell. The correlation length at 25 ◦C is 15–18 molecules. A simulation
using Materials Studio [61] gives stacks of molecules twisted about 60◦. These columns are arranged
in a hexagonal superstructure with a lattice constant a = 27.8 Å. (Figure 5). The self-assembly of
16 molecules in a supercell analogous to t-33 was geometry optimized (Figure 5). As a result of the
packing, the valence energy increases by 370 kcal/mol while the intermolecular energy gain amounts to
−
1607 kcal/mol for the van der Waals and 392 kcal/mol for the electrostatic interaction. This stabilization
of the derivatives with the branched chains is also achieved by an offset of the cores from the center of
the column and a tendency to locate the peripheral dialkoxy phenyl ring above the triazine centers.
−
a = 27.8 Å
a = 27.8 Å
(c)
(a)
(b)
(d)
Figure 5. Modeled liquid crystal (LC) phase of t-60: (a,b): top and side view on columns in the
c-direction, where alkyl chains are omitted; (c,d) top and side view.
If the rim of the TTT carries three branched 4-ethyloctyloxy chains per phenyl unit (t-62) [42],
the melting point of Tm = 124 ◦C matches that of the analog with linear decyl chains (t-35: Cr 126
Colhd 165 I), but mesomorphism is erased. Diasteromerism appears to be of minor importance for LC
properties. The phase width of the compound with chiral citronellyloxy chains [
2] (t-59: Cr 41 Colh
167 I) is comparable to the TTT prepared from a racemic mixture of 3,7-dimethyloctyl chains (t-58
:
Cr 51 M 182 I) (SI), but with the difference that the latter refrains from crystallization within the timescale
of a DSC experiment. It appears that branching, e.g., the exchange of decyl to 3,7-dimethyloctyl has a
comparable but more pronounced impact on TTTs as on triphenylenes [44,86].
Swallow-tail side chains are readily accessible via twofold malonester alkylation followed by
decarboxylation and reduction. They offer a successful strategy to greatly enhance the solubility
of large-core DLCs and generate mesomorphism in these materials [63
properties is expected to correlate with that of twofold alkylation with linear chains of similar
length. Several p-alkoxyphenyl-t-TTTs (t-64 t-71) with swallow-tails are investigated. The typical
–71]; their effect on thermal
–
“2,2-dialkylethyl” with alkyl equals hexyl to dodecyl are complimented by swallow-tails with additional
branching in the alkyl unit and by tails with the systematic variation of the branching position (SI).
para-Substitution with 2,2-dihexylethyl (2-hexyloct-1-yl) results in a mesomorphous material t-64 with
Tc = 65 ◦C. DSC reveals only a glass transition at 14 ◦C. The higher homolog t-65 (2,2-dioctyleth◦yl)
◦
displays a room-temperature LC phase with Tm
(
=
−
15 C (
∆
H = 2.1 kJ/mol) and Tc = 101 C
∆H = 2.1 kJ/mol). The low-melting enthalpy of a pristine sample indicates incomplete crystallization;

Molecules 2020, 25, 5761
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in subsequent experiments, no crystallization or melting was observed. The width of the LC phase
(116 K) is nearly as large as 3,4-dinonyloxy- and 3,4-didecyloxy-TTTs t-18 t-19 with a similar number
of atoms in the chains ( T ca. 120 K), but the transitions appear at significantly lower temperatures
T ca. 100 K). Further extension of the chains (t-66: p-2,2-didecylethyl, t-67: p-2,2-didodecylethyl)
also gives room-temperature DLCs with lower Tc (85 ◦C, 82 ◦C) and even lower Tm 70 ◦C, 54 ◦C)
,
∆
(
∆
−
(−
−
(SI). The structure of these phases is columnar hexagonal (XRD), and POM textures strongly depend
on the cooling rate and film thickness (Figure 6).
Figure 6. POM of t-66: (a): at 84 ◦C; (b): fan and homeotropic orientation at 65 ◦C.
The mesophase of t-66 has a hexagonal columnar structure with ahex = 30.78 Å. Slightly
decreasing with increasing temperature, the
π-π distance is 3.58 Å. This and the aberration of
X-ray density (0.79 g/mL) from the measured density (0.97 g/mL) enforced a new model for the
structure. For comparison, the lattice constants of the similar 3,4-didecyloxy t-19 (ahex = 26.2 Å) and
didodecyloxy t-21 (ahex = 30.4 Å) are significantly lower. The X-ray diffractogram of t-66 is also in
accordance with a hexagonal lattice with a = 30.78 Å and c = 5.85 Å. These parameters result in a
density of 0.97 Å. A simulation [24] leads to two contorted molecules per unit cell, allowing a π-π
distance of 3.58 Å and ahex = 30.8. Å. This model and the X-ray reflections give a correlation length of
<20 Å (Figure 7).
(a)
(b)
Figure 7. (a) Simulated structure of t-66: aromatic part of dimer; (b) view on 3 × 3 × 6 unit cells.
The series of related p-swallowtail t-TTTs with 1-hexylheptyloxy (t-68: Cr 8 I), 2-hexyloctyloxy (t-64
:
Tg 14 M 65 I) and 3-hexylnonyloxy (t-61: Cr 71 I) shows the strong impact of the branching position on
−
the thermal properties. Whereas the LC behavior of these low-melting compounds is severely restricted,
3,4-disubstituted t-TTTs t-16 and t-17 of similar mass are LCs with broad mesophases above 100 ◦C.
Additional branching in the swallow-tail destabilizes the LC phase; the p-(diisoamyl)ethyloxy t-70 TTT

Molecules 2020, 25, 5761
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is nonmesomorphous (SI). Although the diastereomeric p-2,2-di(2-ethylhexyl)ethyloxy derivative t-71 is
a LC with Cr 95 M 140 I, its isomer t-65 without additional branching, 2-octyldecyloxy, is characterized
by Cr
15 M 101 I (SI). Likewise, t-72 with two branched chains (3,4-di(2-hexyloctyl)) melts at 86 ◦C
−
without any sign of mesomorphism, whereas the isomer t-23 with two linear tetradecyl chains is
a LC with a phase width of 192 K. Excessive branching reduces the conformational freedom in the
aliphatic chains.
Discotic liquid crystals with alkoxy chains of different lengths appear only scarcely [87–93].
All compounds above are TTTs with three to nine peripheral chains of identical length in each
case, linear or branched. Three series of TTTs with different chains on the phenyl substituent
were prepared: compounds with a 3,4-3,5- and 3,4,5-di/tri-alkoxy pattern (SI). For example,
3,5-dialkoxy-substituted DLCs with either 3-butoxy-5-hexadecyloxy (t-73: 47(Tg) M 91(2.1) I) periphery
or 3-hexyloxy-5-tetradecyloxy substitution (t-74: Cr 59 Col(hex?) 123 (4.5) I) are isomers of t-29
(3,5-didecyloxy, Cr 73 M 147 I). The reduced symmetry has a strong impact on the thermal properties;
the more unbalanced the periphery, the lower appear the transitions (Tm, Tc). Since the melting point
is not as decreased as the clearing point, the width of the mesophase shrinks with the imbalanced
chain length.
Accordingly, a 3-hexadexyloxy-4-methoxy periphery (t-75
mesomorphism, but 3-tetradecyloxy-4-hexyloxy gives a broad mesophase (t-76: Cr103 M 183 I).
It is interesting that the melting point corresponds to the m.p. of the dihexyloxy derivative (t-15
Tm = 98 ◦C), whereas the clearing point is close to the ditetradecyloxy homolog (t-23: Tc = 181 ◦C).
A comparison of the LC phases of these “unsymmetrical” TTTs ( 73 t-74 t-76) with the thermal
: Cr 98 I) is not sufficient for
:
t
-
,
,
characteristics of the 3,4-didecyloxy isomer (t-19: Cr 89 Colhd 207 I) reveals that reduction of parity
reduces liquid crystallinity. This is further substantiated by the following example: when changing
the notoriously mesomorphic TTTs with a 3,4-disubstituted periphery from 3,4-didecyloxy t-19 to
3-decyloxy-4-decyloxycarbonyl t-77, the LC character vanishes (SI). Thermal transitions of t-78 with
a 3,4-dihexyloxy-5-dodecyloxy periphery (Cr 127 M 158 I) appear nearly at the same temperatures
as of t-36 with three dodecyloxy chains (Cr 128 Colhd 153 I); here, like with t-76, the longest
chain accounts for the clearing point. With the combination of lower symmetry and branching,
t-79 (3,4-dihexyloxy-5-(4-ethyl)octyloxy) is rewarded by a largely reduced melting point, whereas the
width of the LC phase is maintained (Cr 80 Colh 136 I, ∆T = 56 K; t-34: ∆T = 58 K).
The synthesis of “nonsymmetrical” 3,5-dialkoxy-TTTs is straight forward (Scheme 2): alkylation
of methyl 3,5-dihydroxybenzoate
8
with a bromoalkane, the separation of di- and monoalkylated ester
9
, followed by the reaction of the latter with a second bromoalkane and conversion of the ester 10 via
amide to the tetrazole 3 (SI). Similarly, partial alkylation of ethyl gallate with 1-bromohexane gives ethyl
3,4-dihexyloxy-5-hydroxybenzoate. Alkylation with a suitable reagent followed by the transformation
of the ester to tetrazole and reaction with cyanuric chloride yields TTTs with a “nonsymmetrical”
3,4,5-trialkoxy periphery. The 3-alkoxy-4-alkoxy’ disubstitution requires more steps. The alkylation of
5-bromosalicylic aldehyde 11 with the first bromoalkane, Dakin reaction to yield bromoalkoxyphenol
13 and alkylation with the second bromoalkane, followed by the Rosenmund–von Braun reaction with
CuCN to yield nitrile 15 and the addition of azide lead to tetrazole 3 SI.

Molecules 2020, 25, 5761
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Scheme 2. Synthesis of TTTs with chains of different lengths. i: R1Br, K2CO3; ii: R2Br, K2CO3; iii:
NaOH, iv: HCl; v: SOCl2; vi: NH3 aq; vii: ClSi(N3)3; viii: mCPBA; ix: CuCN; x: Et3NHN3.
2.5. Thermal Properties of Alkoxyphenyl-Substituted TTTs with Radial Structure
Prolonged heating at temperatures above 150 ◦C provokes rearrangement of the tristriazolotriazine.
In three steps, the annulation geometry changes from the original [3,4]-annulation of triazoles to the
central triazine ring to [1,5]-annulation. This brings “tangentially” oriented alkoxyaryl substituents
into “radial” positions. The flattened structure of r-TTTs facilitates crystallization and concomitantly
enhances the melting point [38,58]. The ‘radial’ isomers of t-TTTs and their thermotropic characteristics
are collected in Table 4.
In addition, 3,4- and even 3,4,5-dialkoxy substitution can give mesogens [42]. Unlike the t-TTT
series, 3,4-didecyloxy (r-19: Cr 136 I) and didodecyloxy (r-21: Cr1 90 Cr2 131 I)-substituted r-TTT are
crystalline. Mesomorphism starts with 13 carbons per chain (r-22: Cr 94 (16.8) M 119 (9.9) I) and those
with 14 (r-23: Cr 128 Col 136 I) or 16 carbons per chain (r-24: Cr 36 M1 97 M2 125 Colh 132 I) are also
DLCs. Similar to the 3,4–didodecyloxy-TTT r-21, its 3,5-isomer r-30 is not mesomorphous, but nothing
more than hexyl is required for mesomorphism in the 3,4,5-trisubstituted series, (r-33: Cr 13 Colh
180), and the higher octyl (r-34: Cr 35 Col 205 I) and decyl (r-35: Cr 121Colh 181 I) members display
very broad mesophases without recrystallization in DSC experiments. The hexagonal mesophases
(XRD) are built from columns with a helical arrangement of the disks. A pitch of 51 Å, corresponding
to 14 molecules, was found for r-34. The propensity of 3,4,5-trialkoxyphenyl-substituted cores to
form broad mesophases is well-known [
9] but distinctively different from the tangential isomers.
Branching in the periphery (r-62: 3 4-ethyloctyloxy) gives a DLC with a lower melting point and
×
much broader mesophase (Cr 75 Colh 207 I) than that found for the isomer r-35 with linear decyl chains
(Cr 126 Colh 165 I). Though swallow-tails in the para-position are a successful way to t-TTTs with
broad mesophases at low temperature, this concept fails for the r-isomers. 2-Decyldodec-1-yl results
in crystalline r-66 with Tm = 6 ◦C. The low enthalpy (
∆H = 0.7 kJ/mol) is attributed to incomplete
crystallization. The swallow-tail drastically reduces the melting point, and the related r-19 with two
vicinal decyloxy chains melt about 130 ◦C higher. Swallow-tail 2-octyldecyloxy in the m-position on
t-TTT t-63 effectively inhibited crystallization (Tm
=
64 ◦C), but the r-isomer r-63 melts only at 75 ◦C
−
without any sign of mesomorphism. Additional branching (2-(2-ethylhexyl)-4-ethyloctyloxy) stiffens
the flexible parts, thus increasing the melting point of r-71 (Cr 136 I), identical to the didecyloxy analog
r-19. Reducing the symmetry, 3-butoxy-5-hexadecyloxy t-73, does not provoke mesomorphism (Cr 99
I), but the slightly more balanced 3-hexyloxy-5-tetradecyloxy isomer t-74 is liquid crystalline even at
room temperature (Cr 13 M 70 I).
The structure of TTT r-33 with a 3,4,5-trihexyloxyphenyl substitution was studied via X-ray
diffraction and in silico [61]. r-33 is a DLC with a broad mesophase (
∆T = 167 K) and hexagonal
columnar structure. The experimental cell parameters are ahex = 28.1 Å and c = 3.49 Å, and the density

Molecules 2020, 25, 5761
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is 0.92 g/cm3. The model of an ensemble of 16 TTTs r-33 in a supercell is depicted in Figure 8 [61] As a
result, the self-assembly is stabilized by large van der Waals ( 1467 kcal/mol) and electrostatic energy
547 kcal/mol), whereas the packing results in a slight increase in valence energy (197 kcal/mol) owing
−
(−
to the geometric adaption of the molecular scaffold. Again, the mesogen cores are offset from the center
of the columns and trialkoxy phenyl rings are preferentially located over the triazine cores.
A comparison of the 3,4,5-trihexyloxyphenyl-substituted DLCs r-33 and t-33 reveals a cell
parameter a for the r-isomer being 2.5 Å higher than for t-33. This can be attributed not only to the more
compact packing of t-33 versus r-33 but also to the radial topology, which generates space between the
arms of the star-shaped mesogen, which is not as easily filled compared with the t-33 derivative. As a
result, the disorder in r-33 is higher, which accounts for the much lower melting point of r-33 (13 ◦C)
compared to t-33 (116 ◦C).
Table 4. Substitution pattern of r-TTTs and thermal properties.
Entry
Substitution Pattern
Transition/◦C (Enthalpy/kJ/mol)
Ref
r-1
r-2
r-3
r-5
Ri = H
Cr > 390
Cr > 350
Cr 178 I
Cr 142 I
Cr 146 I
Cr 136 I
[58]
[42]
SI
SI
SI
[42]
[42]
SI
[42]
[42]
SI
SI
SI
SI
SI
[42]
SI
SI
SI
SI
R4 = OC3H7
R4 = OC6H13
R4 = OC10H21
r-15
r-19
r-21
r-22
r-23
r-24
r-30
r-33
r-34
r-35
r-61
r-62
r-63
r-66
r-71
r-73
r-74
r-80
R3 = R4 = OC6H13
R3 = R4 = OC10H21
R3 = R4 = OC12H25
R3 = R4 = OC13H27
R3 = R4 = OC14H29
R3 = R4 = OC16H33
R3 = R5 = OC12H25
R3 = R4 = R5 = OC6H13
R3 = R4 = R5 = OC8H17
R3 = R4 = R5 = OC10H21
R3 = R4 = R5 = O-CH2CH(C2H5)C4H9
R3 = R4 = R5 = O-(CH2)3CH(C2H5)C4H9
Cr1 90 Cr2 131(25.2) I
Cr 94 (16.8) M 119 (9.9) I
Cr 128 (25.4) Col 136 (1.6) I
Cr 36 M1 97 M2 125 (23.4) Colh 132 (1.4) I
Cr 127 I
Cr 13 (8.9) Colh180 (3.0) I
Cr 35 (16.9) [a] Col 205 (2.7) I
Cr 121 Colh 181 (3.0) I [a]
Cr 241 I [b]
Cr 75 (2.8) Col 207(4.8) I
Cr 75 I
Cr 6 (0.7) I
Cr 136 I
Cr 99 (6.5) I
R3 = OCH2CH(C8H17 2
R4 = OCH2CH(C10H21 2
)
)
R4 = O-CH2CH(CH2CH(C2H5)C4H9)2
R3 = OC4H9, R5 = OC16H33
R3 = OC6H13, R5 = OC14H29
R3 = Cl, R4 = OC6H13
Cr 13 (4.4) M 70 (3.4) I
Cr 178 I
SI
SI
[a] values from 1, heating scan; [b] determined by POM; Tg: glass transition temperature.

Molecules 2020, 25, 5761
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a = 28.1 Å
(c)
(a)
(d)
(e)
(b)
Figure 8. Model of a supercell (2a
(c,d) top and side view, including hexyl chains; (e) side view, where hexyl chains are omitted.
×
2a
×
4c) of r-33. (a,b) Top view, where hexyl chains are omitted;
3. Conclusions
An extensive study of mesomorphous TTTs with di- or trialkoxyphenyl substitution reveals that
the 3,4-dialkoxy pattern is nearly a guarantee for broad mesophases, whereas no 2-alkoxy chain is
tolerated. 3,5-Di- and 3,4,5-trialkoxy derivatives with comparable numbers of methylene groups show
similar phase widths, and the 3,5-pattern gives lower melting points. The main effect of increasing
chain lengths is the reduction of clearing points. A similar effect provokes short-chain branching on
the 3,4-pattern, but this is disastrous on the 3,4,5-derivatives. One swallow-tail chain per phenyl ring
of t-TTTs largely reduces the melting point. If this group is in the para-position, broad mesophases
are obtained. On the other hand, no r-TTT with a swallow-tail chain is a DLC. TTTs with chains of
identical lengths are LCs with broader mesophases at higher temperatures compared to their isomers
with chains of different lengths. Moderate irregularity such as short-chain branching and varying chain
lengths is an effective tool to tune thermal properties. The longest chains appear to control the clearing
point. This study shows that the thermal properties of TTTs are highly dependent on the substitution
pattern, with individual consequences on t- and r-TTTs; however, within some homologous series,
property changes seem to be predictable.
Supplementary Materials: The following are available online, Experimental procedures, spectroscopical and
analytical data, NMR spectra, DSC and POM.
Author Contributions: Conceptualization: H.D.; XRD: M.L., T.R., N.T. and D.L.; synthesis, spectroscopy and
thermal analysis: T.R., N.T., P.W., N.S., M.S., S.G., H.D. and T.H.; writing—original draft preparation, H.D.;
writing—review and editing, H.D. and M.L.; project administration, H.D.; funding acquisition, H.D. All authors
have read and agreed to the published version of the manuscript.
Funding: This research was funded by Deutsche Forschungsgemeinschaft, grant number DE 515/9-1 and DE
515/12-1.
Acknowledgments: The authors thank M. Müller (DSC), J. Liermann, and D. Schollmeyer (XRD) for
many measurements.

Molecules 2020, 25, 5761
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Conflicts of Interest: The authors declare no conflict of interest.
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