From tectons to luminescent supramolecular ionic liquid crystals

Article abstract of DOI:10.1039/c0cc03970e

New phosphorescent and room-temperature liquid-crystalline materials were obtained by combining dicyanometallate anions with dicationic bisamidinium based tectons bearing four peripheral lipophilic pyrogallate moieties. The Royal Society of Chemistry 2011

Full text of DOI:10.1039/c0cc03970e

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From tectons to luminescent supramolecular ionic liquid crystalsw
a
Pierre Dechambenoit,z Sylvie Ferlay,*a Bertrand Donnio,b Daniel Guillonb and
Mir Wais Hosseini*a
Received 20th September 2010, Accepted 15th October 2010
DOI: 10.1039/c0cc03970e
New phosphorescent and room-temperature liquid-crystalline
materials were obtained by combining dicyanometallate
anions with dicationic bisamidinium based tectons bearing four
peripheral lipophilic pyrogallate moieties.
Liquid crystals (LCs) constitute a class of soft materials,
combining order with fluidity within various types of mobile
and low-dimensional periodic structures.1 Their dynamic
nature, function-integration and stimuli-responsiveness
abilities render them interesting for technological applications.2
Scheme 1
However, the control of self-assembling and self-organizing
processes from functional nano-segregated structures to
groups) were found to behave in a similar fashion, demon-
micrometric scale by molecular engineering still remains a
strating that the presence of alkyl substituents at these two
challenge. Over the last two decades, the supramolecular
positions do not alter the recognition pattern observed
approach, based on molecular recognition events,3 has been
considered4 for generating novel LC assemblies integrating
new functions. The majority of reported supramolecular LCs
is mainly based on the use of non-ionic5 or ionic6 H-bonding.
Functional mesophases displaying emissive properties are
mostly achieved using either purely organic luminophores7 or
transition metal8 and lanthanide9 complexes. Hereafter, we report
on new phosphorescent LC materials elaborated by molecular
tectonics, an approach until now sparingly used in this area.
Molecular tectonics,10 a strategy allowing the assembly of
molecular building blocks (tectons) into pre-designed 1–3D
periodic architectures in crystalline phases or on surfaces,11
seems particularly well-suited for the generation of LCs based
on the formation of molecular networks composed of at least
one mesogenic tecton. Indeed, using charge-assisted H-bonding,12
it was shown that dicationic bisamidinium-based tectons
combined with alkoxybenzoates lead to the formation of LC
mesophases.6c Moreover, association of such dicationic
tectons with cyanometallate anions induces the formation of
porous networks in the crystalline phase.13 In particular, thꢀe
combination of compound 12+ 14 (Scheme 1) with M(CN)2
(M = Ag or Au) leads to the formation of neutral 1D
H-bonded networks (Fig. 1a).15 The formation of the latter
results from both electrostatic charge–charge interactions and
directional H bonds between the acidic H atoms of 12+ and N
atoms of CN ligands of M(CN)2ꢀ. Furthermore, analogues
of 12+ bearing alkyl chains 22+ (propyl,16 hexyl or dodecyl
between the dication and M(CN)2 (Fig. 1a).17
ꢀ
In order to avoid crystallization and to induce the formation
of mesophases, while maintaining the formation of 1D
networks (Fig. 1b), the dicationic tecton 32+ bearing four
peripheral lipophilic pyrogallate units (Scheme 1) was considered
ꢀ
as an appropriate unit to be combined with M(CN)2
.
As stated above, the design of 3+ is based on the tetra
H-bond-donor-unit bisamidinium central core bearing four
mesogenic 3,4,5-trisalkoxybenzyl groups offering 12 flexible
chains disposed in a symmetrical fashion. The phenyl spacer
connecting the two cyclic amidinium moieties was chosen
because not only it increases considerably the stability of the
tecton towards hydrolysis but in addition it imposes the
required distances between the two acidic H atoms on each
face of the tecton and thus a distance of ca. 4.0–4.2 A between
the two metal centers (Ag or Au) and furthermore enhances
the axial anisotropy of the system. This rather short M–M
distance of ca. 4.0–4.2 A leads to metallophilic interactions18
between adjacent metallic centers (Ag or Au).
a Laboratoire de Chimie de Coordination Organique,
´
UMR CNRS 7140, Universite de Strasbourg, Institut Le Bel, 4,
rue Blaise Pascal, CS 90032 67081 STRASBOURG Cedex, France
b IPCMS, UMR 7504, CNRS-Universite´ de Strasbourg,
23 rue du Loess, BP 43, F-67034 STRASBOURG Cedex 2, France
w Electronic supplementary information (ESI) available: Complete
synthesis of [32+][Clꢀ]2 and [32+][Ag(CN)2ꢀ], DSC measurements
and SA-XRD for [32+][Clꢀ]2. See DOI: 10.1039/c0cc03970e
z Present address: CRPP, Centre de Recherche Paul Pascal, 115 Avenue
Schweitzer 33600 PESSAC, France.
Fig.
1 Schematic representation of 1D H-bonded networks
(ladder-like type) formed upon the combination of the dicationic
organic tecton 12+ (a) or lipophilic 32+ (b) with dicyanometallates.
c
734 Chem. Commun., 2011, 47, 734–736
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The interconnection of the mesogenic unit to the cyclic
amidinium moieties was achieved using positions 1 and 4 of
the phenyl group and the position 3 on the cyclic amidinium
moiety. The junction between the rigid and flexible units was
an ether group. As for the length of the alkyl chain, the
dodecyl fragment was chosen.
For [32+][Ag(CN)2ꢀ]2, at 40 1C, the X-ray pattern
was composed of two sharp signals, shown in Fig. 2, with
reciprocal d-spacing in the ratio 1 : O3. This is compatible
with the assignment of the two peaks as (10) and (11)
reflections of the 2D hexagonal lattice with p6mm planar
group (Colh phase) (Fig. 2).
Considering a mean density of ca. 1 g cmꢀ3 for both salts in
their respective phase, about one molecular equivalent
([32+][Cl2ꢀ]2) or one elementary repeat unit ([32+][Ag(CN)2ꢀ]2)
occupies the elementary columnar slice 4.5–4.6 A thick (Table 1).
The specific Ag–Ag distances (in the 201 o 2y o 251 range)
cannot be detected by X-ray diffraction, due to the presence of the
diffuse and intense signal corresponding to the molten chains.
The detailed synthetic strategy as well as preparative
procedures for [32+][Clꢀ]2 and [32+][Ag(CN)2ꢀ]2 are given in
ESI.w
The mesomorphic behaviour of [32+][Xꢀ]2 (X = Cl or
Ag(CN)2) was analyzed by polarized optical microscopy
(POM), differential scanning calorimetryꢀ (DSC, Fig. S1
for [32+][Clꢀ]2 and S2 for [32+][Ag(CN)2
] in ESIw) and
2
ꢀ
small-angle X-ray scattering (SAXS). As deduced by POM
and DSC, [32+][Clꢀ]2 and [32+][Ag(CN)2ꢀ]2 self-organize into
fluid liquid crystalline mesophases from RT to 80 1C and
110 1C, respectively (for the latter compound, the glass
transition temperature (Tg) was detected during the first
heating at 64 1C, but not on subsequent cycles). As expected,
the large number of divergent chains on the cations are
responsible for the low melting-temperature behaviour.19 In
both cases, the mesophase optical textures were birefringent
and homogeneous but not sufficiently characteristic, prohibiting
definitive mesophase assignments.
As previously observed for [12+][Ag(CN)2
]
2
in the
crystalline
phase,15
the
mesophase
generated
by
[32+][Ag(CN)2ꢀ]2 also displays luminescence (Fig. 3). Indeed,
ꢀ
excitation of [32+][Ag(CN)2
]
at 380 nm leads to blue
2
luminescence at 430 nm. In contrast, no emission is observed
for [32+][Clꢀ]2 implying probably that the luminescence is
related to Ag–Ag interactions.22 Moreover, the mesogenic
structural changes between [32+][Clꢀ]2 and [32+][Ag(CN)2
]
2
ꢀ
Temperature-dependent SAXS experiments confirmed and
allowed LC phases’ symmetry identification (Table 1). Both
appeared as disordered mesophases as shown by the rather
strong and diffuse signal in the wide-angle region (at around
4.5–4.6 A) reflecting the liquid-like state of the molten alkyl
chains. For [32+][Clꢀ]2, in the low angle region, several sharp
and intense reflections are observed indicating the presence of
2D columnar arrangements, as shown in Fig. S3 (see ESIw).
At 40 1C, up to 8 sharp reflections were observed. Their
indexation satisfied the conditions for the non-centered
rectangular p2gg planar group (Colr) (Fig. S3, ESIw).
Fig. 2 Small-angle XRD diffraction pattern and peaks indexation for
[32+][AgCN2ꢀ]2, recorded at 313 K, second heating (Gaussian fit of
the diffuse scattering halo).
Table 1 Mesomorphic behaviour and mesophase structural data for [32+][Xꢀ]2. Temperatures in 1C, DH in kJ molꢀ1, and DCp in J molꢀ1 Kꢀ1
(glass transition)
Mesomorphic behavioura
dexp/Ab
Ic
[hk]d
dtheo/Ab
Parameterse
h/Ae, N
[32+][Clꢀ]2
Cr-12 (42.5)
Colr-p2gg 80
(3.3) I
34.1
30.45
19.75
17.05
15.6
11.26
9.5
8.55
4.5
29.87
17.22
4.6
VS (sh)
M (sh)
M (sh)
M (sh)
W (sh)
W (sh)
M (sh)
M (sh)
VS (br)
VS (sh)
W (sh)
VS (br)
11
20
12
22
40
33
24
44
34.1
30.45
19.5
17.05
15.22
11.37
9.75
8.52
hch
T = 40 1C
Colr-p2gg
a = 60.9 A
b = 41.15 A
S = 2506 A2
Vmol = 4800 A3
N B 1
[32+][Ag(CN)2
g (2.7) 64 Colh
110 (—)f I
]
10
11
29.85
17.23
hch
T = 40 1C
Colh-p6mm
a = 34.46 A
S = 1029 A2
Vmol = 4950 A3
N B 1
ꢀ
2
a
Abbreviations: Colh = hexagonal columnar phase; Colr = rectangular columnar phase; Cr = crystalline phase; g = glass; I = isotropic
b
liquid. dexp and dtheo are the experimentally measured and theoretical diffraction spacings. The distances are given in A. Intensity of the
c
d
reflections: vs: very strong, m: medium, W: weak; br and sh stand for broad and sharp reflections, respectively. [hk] are the Miller indices of the
reflections.20 e Mesophase parameters a, b, S, are deduced from the following mathematical expressions. For Colh, the lattice parameter
a = 2[Shkdhk(h2 + k2 + hk)1/2]/O3Nhk where Nhk is the number of hk reflections, and the lattice area (i.e. columnar cross-section)
S = a231/2/2. For Colr, hdhki = 1/[(h2/a2 + k2/b2)1/2], the lattice area S = a ꢁ b (columnar cross-sections = S/2). N is the number of molecules
(or molecular equivalents): N = hchS/Vmol21 where S is the columnar cross-section, Vmol is the molecular volume (estimated with a density close to
1) and hch is the average thickness of the molecule for repeating stacking distance along the column (i.e. the diffuse scattering corresponding to the
f
molten aliphatic chains). Transition not detected by DSC, but by POM and XRD.
c
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columnar mesophase might resulꢀt from interconnections of disks
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electrostatic interactions. The imposed rather short Ag–Ag
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acknowledged for financial support and for a scholarship
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c
736 Chem. Commun., 2011, 47, 734–736
This journal is The Royal Society of Chemistry 2011