3D and 2D supramolecular assemblies and thermotropic behaviour of a: Carbo -benzenic mesogen

Article abstract of DOI:10.1039/c7cc02430d

Hexaalkoxy-di(phenylethynyl)-tetraphenyl-carbo-benzenes bearing six C_nH_2n+1 aliphatic chains (n = 12, 18) have been synthesized and characterized, including by single crystal XRD analysis. For n = 12, a liquid crystal behaviour was observed in the range of 109-130 °C. DSC, POM and PXRD analyses evidenced a rectangular columnar mesophase at 115 °C. The determined 3D lattice constants are consistent with 2D STM images of the so-called carbo-mesogen deposited on an HOPG surface.

Full text of DOI:10.1039/c7cc02430d

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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DOI: 10.1039/C7CC02430D
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3D and 2D Supramolecular Assemblies and Thermotropic
Behavior of a Carbo-benzenic Mesogen
Chongwei Zhu,a Tsai-Hui Wang,b Chien-Jhang Su, Shern-Long Lee,c Arnaud Rives,a Carine
Duhayon,a Brice Kauffmann,d Valérie Maraval,*a Chun-hsien Chen,*c Hsiu-Fu Hsu*b and Remi
Chauvin*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Hexaalkoxy-di(phenylethynyl)-tetraphenyl-carbo-benzenes
moderate size (C12 ring) and discoid character (so = 3). This
bearing six CnH2n+1 aliphatic chains (n = 12, 18) have been challenge actually meets former designs of discotic LCs based
synthesized and characterized, including by single crystal XRD on larger shape-persistent primitive macrocyclic cores of the
analysis. For n = 12, a liquid crystal behavior was observed in the oligo-phenylacetylene type.7 A "tubular" discotic LC devised
range 109-130 °C. DSC, POM and PXRD analyses evidenced a from a hollow phenylacetylene macrocycle (PAM C30 ring)
rectangular columnar mesophase at 115 °C. The determined 3D decorated with six dispersion groups providing the mesogen
lattice constants are consistent with 2D STM images of the so-called with a regular hexagonal discoid character (so = 6) has thus
"carbo-mesogen" deposited on a HOPG surface.
been evidenced in the range 144-110°C.7a A larger mesogenic
core (C58 ring) filled with intra-annular polar groups was shown
to give stable columnar mesophases which were fully
characterized at 120°C or 110°C.7b With the view to gaining
insight into the mesogenic properties of truly monocyclic (non-
Since the early report by S. Chandrasekhar et al. describing
mesomorphic properties of
a
hexasubstituted benzene
-conjugated rigid cores
derivative,1 a large variety of disc-like

has been envisaged in the design of discotic-type liquid crystal
(LC) mesogens. Such cores have been diversely substituted by
two, three, four, six or more dispersion groups, conferring them
the self-assembling properties governing the mesophase
formation. In the case of extended cores such as discoid
polycyclic aromatic hydrocarbons (PAHs), or heterocyclic
counterparts, nanosegregation between the aromatic cores and
outer aliphatic chains has been shown to act as a driving force
for the formation of columnar mesophases.2 The discoid
symmetry order of these cores varies from so = 3 for triangular
triphenylene3 and its expanded dehydrobenzoannulene (DBA)
counterparts,4 to so = 4 for tetragonal porphin,5 and up to so =
6 for hexagonal hexabenzocoronene (HBC).6 Most of the latter
are thus condensed polycycles, where the peripheral
macrocycle encompasses smaller rings filling the discoid interior
(e.g. 5 and 13 primitive rings in porphin and HBC, respectively).
In contrast, due to the decrease of aromaticity vs ring size, the
mesogenic properties of macro-monocycles have been sparsely
investigated beyond the case of DBA,4 where rigidity is restored
by both double insaturations and benzannelation, albeit for a
benzannelated) macro-aromatics, the carbo-benzene ring of
1
appears as a relevant candidate (Scheme 1), both in terms of
size (C18, diameter ≈ 0.8 nm) and discoid character (so = 6). As a
three-fold expanded carbo-meric version of benzene,8 the 18-

-electron carbo-benzene ring is aromatic just as the 6--
electron parent is.9 This C18 core also possesses other
remarkable features, such as an all-carbon composition, an
extended flexible flatness, a strong magnetic anisotropy, and a
structural/chromophoric analogy with the C16N2 porphyrin core,
thus providing carbo-benzene derivatives with unique
properties.10 The same features make the same C18 core also
attractive for LC design upon anchoring of selected dispersion
groups. Lipidic carbo-benzenes p-(CnH2n+1)2C18Ph4 bearing two
n-alkyl chains of various lengths (n = 2, 8, 14, 20) at the para-
positions were recently described,11 but none of them were
found to display any LC behaviour. An increase of the number
of dispersion groups is thus here envisaged by anchoring two
pyrogallol ether groups at the same para positions, and more
precisely two long-chain trialkoxyphenylethynyl groups.
The synthesis of the targets 1a and 1b was intended from the
[6]pericyclynedione 2 12
, following a method widely experienced
for the preparation of other para-disubstituted tetraphenyl-
carbo-benzenes.8b,8c,11 The precursors of the selected dispersion
groups classically used in LC design, the ethynylpyrogallol ethers
3a and 3b with alkyl chain lengths n = 12 and 18, respectively,
were prepared from commercially available 5-bromo-1,2,3-
trimethoxybenzene.13 Addition of two equivalents of the
a.CNRS, LCC (Laboratoire de Chimie de Coordination)
205 route de Narbonne, BP44099, 31077 Toulouse Cedex 4 (France)
Université de Toulouse, UPS, ICT-FR 2599, 31062 Toulouse Cedex 9 (France).E-mail:
valerie.maraval@lcc-toulouse.fr, chauvin@lcc-toulouse.fr
b.Department of Chemistry, Tamkang University, 251 Taipei (Taiwan).E-mail:
hhsu@mail.tku.edu.tw
c. Department of Chemistry and Center for Emerging Material and Advanced
Devices, National Taiwan University, 106 Taipei (Taiwan).E-mail:
chhchen@ntu.edu.tw
d.Institut Européen de Chimie et Biologie, 2, Rue Robert Escarpit, 33607 Pessac (
France).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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information available should be included here]. See DOI: 10.1039/x0xx00000x
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bromomagnesium salt of 3a or 3b to the diketone
the expected [6]pericyclynediols 4a and 4b as mixtures of
diastereoisomers with 64 and 25 % yields, respectively. The low
isolated yield of 4b was due to the tricky procedure required for
the purification of such a highly paraffinic molecule. Reaction of
Journal Name
2
afforded
OR
View Article Online
OMe Ph
OMe Ph
DOI: 10.1039/C7CC0O2R430D
Ph
OMe
Ph
Ph
OMe
1) 3a or 3b
+ base
OR
HO
O
O
2) H+
OH
RO
RO
MeO
MeO
Ph
Ph MeO
2
Ph MeO
OR
2
with the lithium salt of 3a (generated with n-BuLi) was also
R = C12H24, base = EtMgBr 4a, 64 %
R = C18H37, base = EtMgBr,4b, 25 %
R = C12H24, base = n-BuLi, 4a, 75 %
attempted, allowing the diadduct 4a to be obtained in up to 75
% yield (Scheme 1). After treatment of 4a and 4b with SnCl2 and
HCl, the carbo-benzenes 1a and 1b were finally obtained in 58
% and 18 % yield, respectively (Scheme 1). The two products
were isolated as dark blue-violet solids, giving red-violet
dichloromethane (DCM) or chloroform solutions. The dodecyl
derivative 1a was however found to be much more soluble than
the octadecyl homologue 1b, the latter dissolving in chloroform
only and at very low concentration. Both 1H and 13C NMR
spectra of 1b could however be recorded in CDCl3 solution at
320 K. A signature of the presence of the aromatic C18
macrocycle of 1a and 1b is the diatropic ring current-induced
deshielding of the ortho-1H nuclei of the phenyl substituents at
ca 9.5 ppm, as observed for all aryl-substituted carbo-benzenes.
Both 1a and 1b also exhibit similar UV-vis absorption spectra in
chloroform solution, with the same maximum absorption
wavelength at 490 nm. In most cases, whatever the solvents
and conditions employed, 1a and 1b were found to give thin
hair-like crystals, or incidentally bipyramidal crystals (see
photographs in ESI), of extremely low diffracting power.
Nevertheless, after many attempts, single crystals suitable for
OR
OR
OR
1) SnCl2/HCl, DCM
2) NaOH
H
Ph
Ph
R = C12H25, 3a
R = C18H37, 3b
RO
RO
RO
OR
OR
OR
Ph
Ph
R = C12H25, 1a, 58 %
R = C18H37, 1b, 18 %
R'
RO
RO
OR
OR
RO
RO
RO
OR
OR
N
HN
N
RO
OR
OR
NH
R'
5a
6
Scheme 1. Synthesis of the hexaalkyloxy-di(ethynylphenyl)-tetraphenyl-carbo-benzenes
1a and 1b, and previously reported reference congeners 5a and 6a.
XRD analysis were obtained from a chloroform solution of 1a
,
thus allowing confirmation of the structure (Figure 1).14 Due to
weak diffraction intensities and low resolution, non-hydrogen
atoms could only be refined with isotropic displacement
parameters, but in the absence of any solvate molecule, no
disorder is found in the structure, in particular in the dodecyl
chains aligned in
(approximately along the a axis and the ab plane). .
a parallel manner in parallel triads
Figure 2. Polarizing micrograph of 1a at 109 ºC on heating as a viscous liquid showing
birefringent textures indicating the phase to be liquid crystalline (the identity of LC phase
cannot be determined from the POM texture only).
The phase behavior of 1a was investigated by the combination
of polarizing optical microscopy (POM), differential scanning
calorimetry (DSC), and powder X-ray diffraction (PXRD). Under
POM, a viscous birefringent liquid was observed upon heating
between 100 and 130 ºC (Figure 2). Further heating was found
to give a dark image, indicating either decomposition of 1a or
formation of its isotropic phase, which remained the same upon
cooling even to room temperature. The birefringent texture of
the viscous liquid on heating (usually not as indicative as natural
textures obtained by cooling a perfect liquid phase, since solid
state attributes can remain and interfere) does not show
characteristic features for an explicit identification of phases. In
the DSC thermogram between 0 and 120 °C, signals can be
detected on heating only, and a phase transition at 109 °C is in
accordance with POM observations. Further heating leads to an
exothermic signal starting at ca. 130 °C, interpretable as the
signature of either a recrystallization or decomposition process.
As no signal is observed in the second or third heating/cooling
runs, the initial exothermic signal at 136 ºC is ascribed to
Figure 1. Molecular view of the X-ray crystal structure of 1a, with thermal ellipsoids
drawn at 30 % probability level. For clarity, hydrogen atoms are omitted.
Mesophases of 1a, more soluble and available in larger amount
than 1b, could also be analysed in detail. As a literature
reference, the oligophenyleneethynylene (OPE) 5a, carbo-meric
parent of 1a, was recently reported for its self-assembling
properties in solution, but the possible thermotropic LC
behaviour of the pure substance was not mentioned.15
Porphyrinic analogues of type
6 with two or four
trialkoxyarylethynyl dispersion groups, were also reported to
exhibit LC behaviour at low temperature.16
2 | J. Name., 2012, 00, 1-3
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thermal decomposition. The lowest reported melting- as a macrocycle decorated by six phenyl-like grouVpiesw(AFritgiculerOenl4in)e.
DOI: 10.1039/C7CC02430D
decomposition temperature for a tetraaryl-carbo-benzene is Although the 2D packing is not perfectly regular and compact, it
just 12 °C higher (148 °C for p-dieicosyl-tetraphenyl-carbo- clearly shows a 48 x 43 Å rectangular pattern analogous to the
benzene),11 and although most hexaaryl- and dialkynyltetraaryl- one determined by PXRD analysis. The distances between
carbo-benzenes reported to date have been shown to melt or neighbouring molecules of 1a are found to be ca. 16, 21, 24, and
decompose above 200 °C, the "π-frustration" brought by the 25 Å. As observed in the single crystal along the a axis,18 the
trialkoxyphenylethynyl groups onto the π-electron-rich C18 core alkyl chains are aligned in a parallel manner with the [0110]
of 1a can account for a specific thermal instability.17 When the direction of the underlying graphite substrate, and are
DSC thermogram of 1a is recorded between 0 and 115 °C to interdigitated between neighbouring molecules, thus locking
avoid thermal decomposition, the non-reproducibility of the rotation of the mesogen about the column axes.
signal at 109 °C is ascribed to the thermal history of the virgin A schematic molecular packing deduced from XRD and STM
crystals (powders). However, after one night annealing at room investigations is shown in Figure 5. For the 2D lattice, STM
temperature, a new signal at 55.1 ºC appears on heating and a resolves 3 chains per molecule of 1a while the other chains are
signal at 27.8 ºC can be observed on cooling. These DSC data for presumably fluxional in the solution. This is a common packing
two cycles between 0 and 115 °C are consistent with POM pattern for alkane-decorated aromatics because of the limited
observations (see ESI).
space and insufficient attractions from the substrate.19 For the
The LC behaviour of 1a was confirmed by PXRD, allowing 3D columnar phase, the fluxional chains inferred from STM
investigation of molecular packing patterns in different states. images are likely coplanar with the molecular plane due to van
Upon heating from the crystalline phase at temperatures above der Waals interactions between alkyl chains of the nearest
109 ºC and below 130 ºC, a wide angle broad halo at 4.7 Å of upper and lower layers (vide infra, Figure 5c).18
molten chains evolved, indicating the fluid phase formation.18 The macrocyclic cores thus stack face-to-face into columns by
In the small angle regime, signals are simplified by comparison
to the complicated patterns of the crystalline phase. The XRD interdigited molten side-chains and

-

interactions, assisted by nanosegregation of domains of
interaction of the
-
pattern at 115 °C (Figure 3) can be indexed to a columnar peripheral phenyl/phenylethynyl groups. The alkyl chains are
rectangular (Colrec) phase. Detailed analysis of XRD data allowed arranged in a trans manner with respect to para-dialkynyl axis,
the determination of the two-dimensional lattice constants of thus driving an anisotropic in-plane packing by chain
the Colrec phase at 115°C (Table 1): a = 48.5 Å, b = 45.0 Å, and Z interdigitation.
= 3 (Z: number of molecules in a unit cell). No signal could be
a
b
c
attributed to the intracolumnar correlation along the c-axis.
10 nm
5 nm
Figure 4. STM images of 1a. (a) STM image showing the in-plane molecular packing into
ordered arrays; (b) high resolution STM image exhibiting the macrocycle organization by
intermolecular chain interdigitation framing a rectangular pattern of 48 x 43 A2 tiles (see
TOC Scheme); (c) proposed packing for Panel b. Conditions: substrate, HOPG (highly
oriented pyrolytic graphite); solvent, 1,2,4-trichlorobenzene; concentration, ca. 0.1 mM;
Ebias, –800 mV; itunneling, 0.18 nA; room temperature.
Figure 3. Powder X-ray diffractogram of 1a at 115 °C in the LC phase range indicating a
columnar rectangular geometry. Left: 2D-XRD; right: 1D-XRD.
Table 1. PXRD data of 1a at 115°C (see Figure 3).
Phase
Colrec
d-spacing (Å)
Miller indices
Lattice constants (Å)
obsd
calcd
33.01
24.26
16.51
15.22
12.13
11.71
33.01
24.26
16.53
15.19
12.10
11.37
4.68
11
20
22
31
40
a = 48.52
b = 45.04
n = 3
41
(alkyl)
Figure 5. Molecular packing patterns based on PXRD and STM investigations of 1a.
(a)Spaced-filled model and to-scale cartoon representation. (b) Top view cartoon
representation of the columnar rectangular lattice. (c) Side-view cartoon representation
of the columnar molecular packing.
The 2D lattice constants of the LC phase are consistent with a
specific domain in STM images of 1a, which is primarily revealed
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The extended aromatic core of the carbo-benzene 1a (58 sp2- or
sp-C atoms) is embedded in a 34 % more extended oxyparaffinic
surrounding (78 sp3-CH2 or O centers). This balance allows a LC
behaviour of the pure substance in a rectangular columnar
mesophase resulting from unprecedented differential supra-
molecular associations around C18 macro-monocycles. The
results open natural prospects for a new generation of
functional tubular discotic LCs based on such a type of rigid
cores.
Maldonado and R. Chauvin, Chem. Eur. J. 2015, 21, 14186-
14195. For single molecule conductanDceOIp: 1r0o.p10e3rV9tii/eeCws7ACortCfic0lce2aO4r3nbl0ionDe-
benzenes see: b) Z. Li, M. Smeu, A. Rives, V. Maraval, R.
Chauvin, M. A. Ratner and E. Borguet, Nat. Commun. 2015, 6,
6321.
11 C. Zhu, A. Rives, C. Duhayon, V. Maraval and R. Chauvin, J. Org.
Chem. 2017, 82, 925-935.
12 a) L. Maurette, C. Tedeschi, E. Sermot, M. Soleilhavoup, F.
Hussain, B. Donnadieu and R. Chauvin, Tetrahedron 2004, 60
,
10077-10098; b) L. Leroyer, C. Zou, V. Maraval and R. Chauvin,
C. R. Chimie 2009, 12, 412-429. For references on pericyclynes
see: c) L. T. Scott, G. J. DeCicco, J. L. Hyun and G. Reinhardt, J.
Am. Chem. Soc. 1983, 105, 7760-7761; d) L. T. Scott, G. J.
DeCicco, J. L. Hyun and G. Reinhardt, J. Am. Chem. Soc. 1985,
107, 6546-6555.
Acknowledgements
C.Z. thanks the China Scholarship Council for his Ph.D.
scholarship. The ANR program (ANR-11-BS07-016-01) is
acknowledged for the postdoctoral fellowship of A.R. and for
funding of working facilities. R.C. thanks the Centre National de
la Recherche Scientifique (CNRS) for half a teaching sabbatical
in 2015-2016. The authors also thank Dr. Nathalie Saffon-
Merceron for her efforts in trying to solve the structure of 1a by
X-ray diffraction analysis of single crystals thereof. In the
framework of the CNRS ЯÉCIPROCS network, this work has
benefited from the X-ray facility of the Biophysical and
Structural Chemistry platform at IECB, CNRS UMS3033, INSERM
US001, Bordeaux University. C.-h. C. and H.-F. H. would like to
thank Ministry of Science and Technology for financial support.
13 J. Wu, M. D. Warson, L. Zhang, Z. Wang and K. Müllen, J. Am.
Chem. Soc. 2004, 126, 177-186.
14 C130H174O6, pink red plate, M = 1832.81 g.mol-1, monoclinic, a
= 49.45(2), b = 15.293(11), c = 7.3526(14) Å,
 =  = 90°,  =
91.49(3), V = 5558(2) Å3, T = 100 K, space group P21/c, Z = 2,
(Cu Kα) = 0.490 mm-1; 13385 reflections measured and 4780
unique (2max = 102.6°, Rint = 0.086), 2434 reflections [I > 2.5
(I)] used in all calculations; R = 0.2493, Rw = 0.2680 (more


details in ESI). CCDC 1546671 contains the supplementary
crystallographic data for this paper. The data can be obtained
free of charge from the Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/getstructures.
15 M. J. Mayoral, C. Rest, J. Schellheimer, V. Stepanenko and G.
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18 Similar distances occur in the single crystal, where the
shortest distance between C atoms of parallel sections of
nearest dodecyl chains varies in the range 4.5-4.8 Å in
intramolecular triads (approximately along the b axis), and in
2
3
4
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the range 4.1-4.3
Å between intercalated molecules
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10 For two-photon absorption properties of carbo-benzenes see:
a) I. Baglai, M. de Anda-Villa, R. M. Barba-Barba, C. Poidevin,
G. Ramos-Ortíz, V. Maraval, N. Saffon-Merceron, J.-L.
4 | J. Name., 2012, 00, 1-3
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