2,6-Dibromogallates as a new building block for controlling π-stacking, network formation and mirror symmetry breaking

Article abstract of DOI:10.1039/d1cc01922h

Achiral multi-chain (polycatenar) compounds based on the 2,7-diphenyl substituted [1]benzothieno[3,2-b]benzothiophene (BTBT) unit and a 2,6-dibromo-3,4,5-trialkoxybenzoate end group lead to materials forming bicontinuous cubic liquid crystalline phases with helical network structures over wide temperature ranges.

Full text of DOI:10.1039/d1cc01922h

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2,6-Dibromogallates as a new building block for
controlling p-stacking, network formation and
mirror symmetry breaking†
Cite this: Chem. Commun., 2021,
57, 6491
Received 11th April 2021,
Accepted 2nd June 2021
Ohjin Kwon,‡a Xiaoqian Cai,‡b Azhar Saeed,b Feng Liu, *b Silvio Poppea and
a
Carsten Tschierske
*
DOI: 10.1039/d1cc01922h
rsc.li/chemcomm
Achiral multi-chain (polycatenar) compounds based on the 2,7-diphenyl out, and macroscopic chirality develops by mirror symmetry
substituted [1]benzothieno[3,2-b]benzothiophene (BTBT) unit and a breaking. Polycatenar molecules involving
a p-conjugated
2,6-dibromo-3,4,5-trialkoxybenzoate end group lead to materials form- core-unit, such as [1]benzothieno[3,2-b]benzothiophene (BTBT)15
ing bicontinuous cubic liquid crystalline phases with helical network provide access to new charge carrier materials with conduction
structures over wide temperature ranges.
pathways in all three spatial direction (Fig. 1).16 However, often an
unfavourable herringbone packing is assumed, reducing the charge
Supramolecular self-assembly1 is an effective route to design nano- carrier mobility,3b,16 and in addition, the Cubbi phases are accom-
structured materials with tailored properties.2 Among them, liquid panied by parasitic birefringent mesophases with non-cubic 3D
crystalline (LC) and solution processable p-conjugated compounds lattices.14,16
are of growing interest for optical, electronic, light harvesting3a and
Herein, we introduce the 2,6-dibromo-3,4,5-trialkoxybenzoate
photonic applications.4 The 3,4,5-trialkoxybenzoate unit derived unit as a new building block for solution processable and soft self-
from gallic acid has developed to an important building block for assembled functional materials, more specifically, for the molecular
the design of such materials, allowing the control of the mode of design of compounds being capable of forming surprisingly broad
self-assembly from lamellar via columnar to spherical.5–7 A single ranges of mirror symmetry broken helical network phases, and we
3,4,5-trialkoxybenzoate unit attached to one end of an extended compare them with related non- and mono-brominated analogues.§
p-conjugated rod-like unit could even lead to bicontinuous cubic It is surprising that the bulky bromines at both sides of the molecule
phases (Cubbi, Fig. 1c and d),8–10 being composed of interwoven do not significantly reduce the LC phase stability, but widen the LC
networks and representing an intermediate structure at the transi-
tion from lamellar to columnar self-assembly.11 In these networks
the molecular rods are organized antiparallel to each other and
almost perpendicular to the local network direction, and the colli-
sion of the bulky trisubstituted ends induces a helical twist (Fig. 1b).8
The network junctions provide a long range synchronization of the
helix sense along the networks.10 If the Cubbi phase is formed by
%
only two networks, an achiral Ia3d phase composed of two enantio-
morphic networks is formed (double gyroid, Fig. 1c). If there are
three networks, as in the I23 structure12 (previously known as
‘‘Im3m’’,11,13,14a see Fig. 1d), the network chirality cannot cancel
a Institute of Chemistry, Martin Luther University Halle-Wittenberg,
Kurt-Mothes Str. 2, 06120 Halle (Saale), Germany.
E-mail: carsten.tschierske@chemie.uni-halle.de
b State Key Laboratory for Mechanical Behaviour of Materials,
Fig. 1 (a) Structures of the discussed compounds Hn, Bn and BBn and
(b) chirality developing due to the twist along the networks of the Cubbi
Shaanxi International Research Center for Soft Matter, Xi’an Jiaotong University,
Xi’an 710049, P. R. China. E-mail: feng.liu@xjtu.edu.cn
%
phases; (c) double network structure of the Ia3d phase and (d) the triple
† Electronic supplementary information (ESI) available: Methods, syntheses,
analytical data, additional XRD and structural data. See DOI: 10.1039/d1cc01922h
‡ O. K. and X. C. contributed equally.
network of the I23 phase; (c and d) were reproduced with permission,12
The Royal Society of Chemistry 2020.
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Fig. 2 (a) Phase transitions as observed by DSC on cooling (10 K minÀ1); the
data for Bn were taken from ref. 16. The related transitions on heating are
shown in Fig. S3, ESI;† the numerical data and enthalpy values are collated in
Tables S1–S3 (ESI†) (for DSCs, see Fig. S1 and S2, for XRD, see Fig. S6–S11 and
Tables S4–S11, ESI†); abbreviations: Cr = crystalline solid, Cubbi *]/I23 = Cubbi
[
%
%
phase with chiral I23 lattice; Cubbi/Ia3d = Cubbi phase with achiral Ia3d lattice;
Tet = tetragonal 3D phase; M = highly birefringent 3D phase with unknown
structure; Iso1[*] = isotropic liquid with chiral conglomerate structure; the
achiral isotropic liquid (Iso) is at the right; (b–d) representative WAXS patterns
of selected compounds from each series at the indicated temperatures.
Scheme 1 Synthesis of compounds Hn and BBn. Reagents and conditions:
(i) 1. SOCl2, 2. dry pyridine, RT; (ii) 1.1 eq. Br2, (iii) 2.2 eq. Br2, or (iv) 3.3 eq. Br2,
DCM, RT; (v) THF, sat. aqu. NaHCO3, [Pd(PPh3)4], reflux; (vi) CnH2n+1Br,
2-butanone, K2CO3; TBAI; reflux. See Schemes S2–S4 (ESI†) for details.
Compounds Bn have been reported previously.16
ranges, remove the birefringent phases and change the preferred
p-stacking mode from edge-to-face to face-to-face.
The synthesis of the new non-brominated and dibrominated
compounds Hn and BBn, respectively, is shown in Scheme 1
(see also, ESI†). Compounds BBn can be prepared by bromination
of the boronate 1 to give 1Br2, followed by Suzuki coupling reaction
with 1.5 equivalents of 3Brn (Scheme 1, blue). Alternatively, by using
a Pd[PPh3]4 catalyst, prepared according to ref. 17 the Suzuki
coupling18 of the boronate 1 with 2Br3 takes place preferentially
with the bromine at the BTBT unit, leading to the phenol BBOH
which was alkylated in the final step.
In all three series of compounds (Fig. 2a) there are two different
types of optically isotropic Cubbi phases which can be distin-
guished by the small angle X-ray scattering (SAXS) patterns and
chiroptical properties (see Fig. S4–S8 and Tables S4–S9, ESI†). The
dibrominated compound BB7 with the shortest apex chain shows
%
an Ia3d lattice with acub = 9.80 nm (Fig. 3a). The reconstructed
electron density (ED) map confirms a double gyroid structure,
where the aromatic cores with the highest ED (purple - green)
form the networks and the alkyl chains form the continuum
around them (Fig. 3b). The same network phase is found for
BB8, but the Cubbi phases of the longer homologues have three
main SAXS peaks which can be indexed to the (321), (400) and
(420) reflections of the I23 lattice (acub = 14.72 nm for BB9, Fig. 3c).
This cubic phase with a lattice parameter being about 50% larger
%
%
Fig. 3 SAXS patterns at the indicated temperatures. (a) Cubbi/Ia3d phase
of BB7 and (c) the Cubbi *]/I23 phase of compound BB9 with (b and d) the
[
corresponding ED maps reconstructed from the diffraction patterns, the
aromatic cores are located in the networks formed by the green iso-surfaces,
the space in between is filled by the alkyl chains, as shown in Fig. S12 and S13
(ESI†); for the network models, see Fig. 1c and d; for numerical values and
additional XRD date, see Fig. S4–S8 and Tables S4–S11 (ESI†).
than in the Ia3d phase has a triple network structure, again with combination for ED reconstruction is described in previous
the aromatic cores forming the networks and the alkyl chains in work.9,10,12
the continuum (Fig. 3d). The lattice parameters are collated in
The Cubbi phase of BB7 and BB8 is achiral (Fig. 4a and Fig. S4b,
%
Table S11 (ESI†) and the method of selecting the correct phase ESI†), in line with the Ia3d lattice (Fig. 1c). In contrast, the Cubbi
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phase of BB9–BB11 shows a conglomerate of dark and bright highly birefringent platelets, which rapidly transforms into Cubbi or
domains (Fig. S4d, ESI†), if observed by optical microscopy Tet with time or upon cooling (Fig. 4a and b). This mesophase is
between polarizers rotated by a small angle out of the precise observed in all three series of compounds in a very narrow
[ ]
%
901 orientation. The domain brightness is inverted by changing temperature range during the transition from Iso1 * to Cubbi/Ia3d
the sense of rotation, thus indicating that this optically isotropic or Tet on cooling. It is metastable, rapidly transforms into the Cubbi
Cubbi phase is chiral, confirming a triple network I23 structure or Tet phase, and therefore it cannot be studied in more detail.
(Fig. 1d).8–12 The transition from the achiral Ia3d to the chiral I23 It appears to be associated with the Ia3d - (Tet) - I23 transition
%
%
phase upon chain elongation (Fig. 2a) is due to the growing volume upon chain elongation and it is removed as soon as either the
%
of the periphery which increases the twist between the molecules Cubbi/I23 or the Ia3d phase becomes the dominating phase. Double
(F).9,21 At a certain critical twist angle (F 4 9.21, see Table S11, ESI†) bromination removes the M-phase for n 4 7 (Fig. 2).
%
the transition from Ia3d to I23 takes place in all three series of
In all mesophases (Cubbi, Tet, M, Colhex), the wide-angle scatter-
%
compound. However, the Ia3d - I23 transition is shifted by ing (WAXS) is diffuse with a maximum around 0.45–0.46 nm, as
bromination to shorter chain length (Hn: 9 - 12; Bn: 8 - 11; typical for LC phases (Fig. 2b and c). For H10 there is a very weak
BBn: 8 - 9, see Fig. 2a). This means that the bromines obviously shoulder at larger 2y values which for Bn and BBn becomes a second
contribute to the expansion of the molecular periphery and thus diffuse scattering with increased intensity around 0.35–0.37 nm.
%
favour mirror symmetry breaking by inducing the Ia3d - I23 (Fig. 2d and Fig. S10 ESI†). This is independent on the phase type,
transition. Thus, substitution of H by Br has a similar effect as but decreases in intensity with rising temperature (Fig. S11, ESI†).
an elongation of the apex chain by one additional CH2 unit, We attribute this additional scattering to a change of the self-
being in line with the similar volumes of Br (3.3 Â 10À2 nm3) assembly of the p-conjugated rods. For Hn the preferred average
and CH2 (2.5 Â 10À2 nm3).19 Though the melting temperatures organization is edge-to-face, leading to a dominating WAXS
are almost the same in all three series (see Tables S1–S3 and maximum at 0.45–0.46 nm, whereas bromination disfavour this
Fig. S1, S2, ESI†), there is a reduction of the crystallization arrangement and supports a parallel face-to-face stacking of the
tendency, leading to an improved supercooling of the crystal- p-systems of compounds Bn and BBn with a mean distance of
lization with growing number of bromines, allowing the 0.35–0.37 nm.20 It appears that the face-to-face packing is
formation of long time persistent Cubbi phases down to mainly caused by the increased aspect ratio of the brominated
ambient temperature (Fig. 2a and Fig. S2a–e, ESI†).
compounds (Hn { Bn o BBn). This disfavours the edge-to-
At the transition between the two different Cubbi phases two face and supports the face-to-face packing of the aromatic
types of birefringent mesophases occur (Tet, M in Fig. 4). Compar- cores, which then allows the overlapping of adjacent p-
ison of the three series shows that an increasing degree of bromina- systems.20 However, the coherence length of the face-to-face
tion reduces the Tet and M phase ranges and even completely stacking is still relatively short, as indicated by the
eliminates the Tet phase for compounds BBn (Fig. 2a). The Tet diffuse WAXS.
phases of Hn and Bn have very similar low birefringent optical
For all investigated compounds the viscoelastic 3D mesophases
textures (Fig. 4c) and comparable a-parameters of the tetragonal are accompanied by additional highly fluid mesophases. One is a
[ ]
lattices (a = 17.7 and 17.8 nm, respectively), and therefore, both mirror symmetry broken isotropic liquid Iso1 * phase (Fig. S4a,
should belong to the same kind. Only the parameter c is increased ESI†) which is observed on cooling from the achiral isotropic
by bromination from c = B16 nm for H10 to c = B20–22 nm for Bn liquid (Iso).21,22 Its temperature range is the largest for Hn and it
(Table S11, ESI†).16 The transition between the double and triple decreases with increasing bromination in the order Hn 4 Bn c
network Cubbi phase is also associated with the formation of an BBn. In the series BBn it is only observed in a small temperature
additional small range of a metastable mesophase M, growing as range on cooling the shortest compound BB7 before the transition
%
to the M and Ia3d phases, whereas for all compounds BBn with
longer chains it is replaced by a Colhex phase (Fig. 2 and Fig. S4c,
S9, S11, ESI†). This effect of bromination could be attributed to the
increase of interface curvature by the bulky bromines. Columnar
self-assembly is additionally supported by the change of the
preferred mode of packing from edge-to-face to face-to-face
(see above), thus providing a taper-like molecular shape. In this
Colhex phase the lattice parameter ahex = 4.33 nm (B11) is in good
agreement with the total molecular length Lmol = 4.8 nm (see
Fig. S14, ESI†) after considering the chain folding. The calculated
number of B4.5 molecules per 0.45 nm thick raft of the columns
(nraft, Table S11, ESI†) is close to the number of molecules organized
Fig. 4 Optical textures of (a) BB7 at 226 1C as observed on cooling
between two polarizers slightly rotated out of the 901 twisted configu-
ration (arrows) and showing the transition from the Iso1 *] phase (chiral
[
%
in the rafts of the networks of the Ia3d phases, ranging from 3
%
conglomerate) to Cubbi/Ia3d with simultaneous growth of the M-phase
(BB11) to 4 (H8) molecules.
and (b and c) of H10 between fully crossed polarizers; (b) high birefringent
[ ]
In both series Hn and Bn an Iso1 * phase is found at the high
M phase developing from the Iso1 *] phase and the co-developing low
[
%
temperature side of the I23, Ia3d and Tet phases. The proximity to
birefringent Tet phase (bottom, right) at 215 1C and (c) fully developed
texture of the Tet phase at 205 1C (longer exposure time).
[ ]
the chiral I23 phase suggests a local I23-like structure of these Iso1 *
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replaced by Br,5 whereas in this work the bromines represent additional
substituents.
phases retaining long range chirality synchronization, even after
loss of the long-range cubic lattice in the liquid state.10 Because the
%
I23 phase can develop from the Ia3d phase upon increasing twist
1 (a) J. M. Lehn, Science, 2002, 295, 2400; (b) K. Ariga, J. P. Hill,
M. V. Lee, A. Vinu, R. Charvet and S. Acharya, Sci. Technol. Adv.
Mater., 2008, 9, 014109.
2 T. Kato, M. Yoshio, T. Ichikawa, B. Soberats, H. Ohno and
M. Funahashi, Nat. Rev. Mater., 2017, 2, 17001.
3 (a) M. Hecht and F. Wu¨rthner, Acc. Chem. Res., 2021, 54, 642;
(b) R. Ramakrishnan, M. A. Niyas, M. P. Lijina and M. Hariharan,
Acc. Chem. Res., 2019, 52, 3075.
along the networks,8–12 it is likely that thermal chain expansion can
increase the helical twist and transforms the achiral Cubbi/Ia3d
%
lattice into a chiral I23-like local structure in the adjacent mirror
[ ]
symmetry broken Iso1 * phase.
Remarkably, the LC - Iso/Iso1 *] transition temperatures
[
are almost the same in all three series of compounds if the
same chain lengths are compared (Fig. S3, ESI†). This is
surprising, as it is known from previous work, that bulky
substituents in the 2,6-positions at opposite sides of an aro-
matic core have a dramatic destabilizing effect on LC phases
(see for example Scheme S1, ESI†).23 Hence, it appears that the
unfavourable steric effect of bromination is compensated by
mesophase stabilizing effects. Besides the different p-stacking
modes, the increased London dispersion interactions of the
highly polarizable bromines could stabilize the LC phases.
Moreover, the bromines can in an antiparallel arrangement
efficiently fill the free space around the adjacent ester linkage
(Fig. S14, ESI†), and this increases the contact area between the
molecules, thus strengthening the dispersion interactions.
Overall, peripheral double bromination of the tapered end of
the p-conjugated polycatenars allows the formation of broad
regions of mirror symmetry broken network phases, even covering
ambient temperature. It also leads to a removal of birefringent 3D
4 (a) R. K. Gupta and A. A. Sudhakar, Langmuir, 2019, 35, 2455–2479;
(b) Z.-G. Zheng, Y.-Q. Lu and Q. Li, Adv. Mater., 2020, 1905318.
5 H. T. Nguyen, C. Destrade and J. Malthete, Adv. Mater., 1997, 9, 375.
¨
6 (a) K. Borisch, S. Diele, P. Goring and C. Tschierske, Chem. Commun.,
1996, 237; (b) B. M. Rosen, C. J. Wilson, D. A. Wilson, M. Peterca,
M. R. Imam and V. Percec, Chem. Rev., 2009, 109, 6275;
(c) C. Tschierske, Angew. Chem., Int. Ed., 2013, 52, 8828.
7 (a) B. T. Diroll, D. Jishkariani, M. Cargnello, C. B. Murray and
B. Donnio, J. Am. Chem. Soc., 2016, 138, 10508; (b) H. Li, S. S. Babu,
S. T. Turner, D. Neher, M. J. Hollamby, T. Seki, S. Yagai, Y. Deguchi,
¨
H. Mohwald and T. Nakanishi, J. Mater. Chem. C, 2013, 1, 1943;
(c) L. Chen, C. Chen, Y. Sun, S. Lu, H. Huo, T. Tan, A. Li, X. Li,
G. Ungar, F. Liu and M. Zhang, Angew. Chem., Int. Ed., 2020,
59, 10143.
8 C. Dressel, F. Liu, M. Prehm, X. B. Zeng, G. Ungar and C. Tschierske,
Angew. Chem., Int. Ed., 2014, 53, 1115.
9 C. Dressel, T. Reppe, S. Poppe, M. Prehm, H. Lu, X. Zeng, G. Ungar
and C. Tschierske, Adv. Funct. Mater., 2020, 202004353.
10 (a) T. Reppe, C. Dressel, S. Poppe and C. Tschierske, Chem. Com-
mun., 2020, 56, 711; (b) T. Reppe, S. Poppe, X. Cai, Y. Cao, F. Liu and
C. Tschierske, Chem. Sci., 2020, 11, 5902; (c) M. Alaasar, S. Poppe,
Q. Dong, F. Liu and C. Tschierske, Angew. Chem., Int. Ed., 2017,
56, 10801.
11 S. Kutsumizu, Isr. J. Chem., 2010, 52, 844.
%
phases occurring at the Ia3d–I23 transition and hinders the
12 X. B. Zeng and G. Ungar, J. Mater. Chem. C, 2020, 8, 5389.
13 X. B. Zeng, G. Ungar and M. Imperor-Clerc, Nat. Mater., 2005, 4, 562.
14 (a) A.-M. Levelut and M. Clerc, Liq. Cryst., 1998, 24, 105; (b) H. Lu,
X. Zeng, G. Ungar, C. Dressel and C. Tschierske, Angew. Chem., Int.
Ed., 2018, 57, 2835; (c) J. Matraszek, D. Pociecha, N. Vaupotic,
M. Salamonczyk, M. Vogrine and E. Gorecka, Soft Matter, 2020,
16, 3882.
15 (a) H. Ebata, T. Izawa, E. Miyazaki, K. Takimiya, M. Ikeda,
H. Kuwabara and T. Yui, J. Am. Chem. Soc., 2007, 129, 15732;
(b) H. Iino and J. Hanna, Polym. J., 2017, 49, 23.
16 O. Kwon, X. Cai, W. Qu, F. Liu, J. Szydłowska, E. Gorecka, M. J. Han,
D. K. Yoon, S. Poppe and C. Tschierske, Adv. Funct. Mater., 2021,
2102271.
17 S. O. Mihigo, W. Mammo, M. Bezabih, K. Andrae-Marobela and
B. M. Abegaz, Bioorg. Med. Chem., 2010, 18, 2464.
crystallization. These surprising effects of double-bromination
are opposite to the effect observed for halogenation of related
molecules at other positions. Especially, halogenation in the centre
of the core unit of polycatenars is known to suppress Cubbi phase
formation.24 Even more remarkable is the formation of a parallel
alignment of the p-faces of the aromatic cores which is important
for the tailoring of photophysical properties and expected to
support charge transportation.15,16 Hence, the work provides
general guidelines for the controlled design of helical network
forming and mirror symmetry broken soft materials based on
p-conjugated rods. These are of current interest for numerous
applications, as mentioned in the introduction.
18 N. Miyaura and A. Suzuki, J. Chem. Soc., Chem. Commun., 1979,
19, 866.
19 A. Immirzi and B. Perini, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr.,
Theor. Gen. Crystallogr., 1977, 33, 216.
20 (a) C. A. Hunter and J. K. M. Sanders, J. Am. Chem. Soc., 1990,
112, 5525; (b) C. R. Martinez and B. L. Iverson, Chem. Sci., 2012,
3, 2191.
21 (a) C. Tschierske, Liq. Cryst., 2018, 45, 2221; (b) C. Tschierske and
G. Ungar, ChemPhysChem, 2016, 17, 9; (c) C. Tschierske and
C. Dressel, Symmetry, 2020, 12, 1098.
This work is supported by the European Union (EFRE) and the
National Natural Science Foundation of China (No. 21761132033,
21374086). The authors are grateful to Beamline BL16B1 at SSRF
(Shanghai Synchrotron Radiation Facility, China) for providing the
beamtime.
22 C. Dressel, T. Reppe, M. Prehm, M. Brautzsch and C. Tschierske,
Nat. Chem., 2014, 6, 971.
Conflicts of interest
23 (a) D. W. Bruce and S. A. Hudson, J. Mater. Chem., 1994, 4, 479;
(b) J. W. Goodby, I. M. Saez, S. J. Cowling, J. S. Gasowska,
R. A. MacDonald, S. Sia, P. Watson, K. J. Toyne, M. Hird,
R. A. Lewis, S.-E. Lee and V. Vaschenko, Liq. Cryst., 2009, 36, 567.
24 A. I. Smirnova, B. Heinrich, B. Donnio and D. W. Bruce, RSC Adv.,
2015, 5, 75149.
There are no conflicts to declare.
Notes and references
§ Previously, the 2,6-dichloro-3,4,5-trialkoxybenzoate unit was used for 25 J. Matraszek, J. Mieczkowski, D. Pociecha, E. Gorecka, B. Donnio
bent-core molecules.25 In polycatenars one or two alkyl chains were
and D. Guillon, Chem. – Eur. J., 2007, 13, 3377.
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