Added Alkane Allows Thermal Thinning of Supramolecular Columns by Forming Superlattice?-An X-ray and Neutron Study

Article abstract of DOI:10.1021/jacs.6b01172

We report a columnar superlattice formed by blends of dendron-like Li 3,4,5-Tris(n-Alkoxy)benzoates with n-Alkanes. Without the alkane, the wedge-shaped molecules form liquid crystal columns with 3 dendrons in a supramolecular disk. The same structure exi

Full text of DOI:10.1021/jacs.6b01172

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Communication
Added alkane allows thermal thinning of supramolecular
columns by forming superlattice - an X-ray and neutron study
Ming-Huei Yen, Jitrin Chaiprapa, Xiangbing Zeng, Yongsong
Liu, Liliana Cseh, Georg H. Mehl, and Goran Ungar
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b01172 • Publication Date (Web): 22 Apr 2016
Downloaded from http://pubs.acs.org on April 23, 2016
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Added Alkane Allows Thermal Thinning of Supramolecular Columns
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by Forming Superlattice – an X-ray and Neutron Study
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†
†,‡
†
§
║,
◊
Ming-Huei Yen, Jitrin Chaiprapa, Xiangbing Zeng, Yongsong Liu, Liliana Cseh, Georg H.
║
§,†,*
Mehl, Goran Ungar
†Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, U.K.
§Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China
║Department of Chemistry, University of Hull, Hull HU6 7RX, UK
◊Institute of Chemistry Timisoara of Romanian Academy, Timisoara – 300223, Romania
Supporting Information Placeholder
ing with increasing temperature (T) due to lateral expansion of the
ABSTRACT: We report a columnar superlattice formed by
blends of dendron-like Li 3,4,5-tris(n-alkoxy)benzoates with n-
alkanes. Without the alkane, the wedge-shaped molecules form
liquid crystal columns with 3 dendrons in a supramolecular disk.
The same structure exists in the blend, but on heating one dendron
is expelled from the disks in every third column and is replaced
by the alkane. This superlattice of unequal columns is confirmed
by complementary X-ray and neutron diffraction studies. Lateral
thermal expansion of dendrons normally leads to the expulsion of
excess molecules from the column, reducing the column diameter.
However in the already narrow columns of pure Li salt, expulsion
of one of only three dendrons in a disk is not viable. The added
alkane facilitates the expulsion, as it replaces the missing dendron.
Replacing the alkane with a functional compound can potentially
lead to active nanoarrays with relatively large periodicity by using
only small molecules.
flexible alkyl chains caused by their increased conformational
disorder. For any specific dendron at a given T there is an optimal
number of molecules   2/ required to complete the full disk.
As a result of increasing  with increasing T, the number of den-
drons  decreases and the excess dendrons are ejected from the
columns; these then aggregate to form new columns. As a conse-
quence on heating the average diameter of all columns decreases
9
steadily. Similar behavior is also observed in the cubic LC phase,
where the number  of conical dendrons in a spherical micelle
decreases with increasing temperature, causing 3-d shrinkage of
9
the unit cell. Nevertheless, the bulk thermal expansivity of both
the columnar and the cubic phases remains positive.
Taper-shaped mesogens, notably dendrons, are some of the
most fundamental and widespread building blocks in supramolec-
1
ular chemistry. They have been attached to moieties such as or-
2
3
4
ganic semiconductors, fullerenes, ionic conductors, etc. This
way “dendronized” functional materials may be created that can
be used in a variety of applications. The most common supra-
5,6
Scheme
1
Chemical structure of the 3,4,5-tris (n-
alkoxy)benzoate salts used. Transition temperatures (in C) were
determined by DSC and XRD on first heating. Cr = crystal, Colh
hexagonal columnar, Cols = columnar superlattice, BCC =
body-centred cubic, LQC = dodecagonal quasicrystal. The ratios
mean weight ratios in the blend.
molecular objects that such dendrons or dendronized compounds
form are columns and spheres. The former, in turn, tend to ar-
range on a 2-d lattice, most often hexagonal (Figure 1a), while the
=
7
,8
latter form 3-d lattices or even quasicrystals. The most widely
used dendrons producing 2-d and 3-d ordered liquid crystal (LC)
or soft crystal structures consist of an aromatic core and flexible
terminal chains. While dendrons of this type up to fifth generation
Here we report on rather unusual behavior of the columnar
phase of some taper-shaped “minidendron” alkali metal salts of
,4,5-tris(dodecyloxy)benzoic acid. While the columns of most
1
have been synthesised and studied, it turned out that most LC
3
phases are observed already with the simplest tapered mesogens,
the so-called “minidendrons”, containing just one benzene ring, a
weekly interacting group at the apex (1-position) and three or two
flexible chains attached to positions 3, 4 and/or 5. The group at
salts become thinner when heated, those of Li salt change little, in
fact expand slightly. Surprisingly, however, when free n-alkane is
added, above a certain temperature the column diameter starts to
decrease abruptly concomitant with the transition to a new super-
lattice phase. Using deuterated alkane and a combination of X-ray
and neutron diffraction, we arrive at a detailed structure and a
rationale for the formation of this unusual superlattice. The find-
ings illuminate the intriguing phenomenon of “thinning” and
“non-thinning” columns.
9
,10
11
the apex could be carboxylate salt
group or other.
sulfonate salt, an onium
4
b,e
It is known that in the hexagonal columnar (Colh) phase the
columns consisting of tapered mesogens shrink in diameter on
9
,12
heating. This has been explained by the taper angle  increas-
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The “minidendrons” used in this work, referred to hereafter as
“dendrons” for short, are shown in Scheme 1. The synthesis of the
(
a)
(b)
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1
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,10
symmetrical dendrons was described previously,
2-12-18Li is new. The details are given in Supporting Infor-
mation (SI). All three compounds show a crystalline phase at low
temperatures, followed by the Col phase on heating, then by the
BCC cubic phase (see SI). Changes in the hexagonal lattice pa-
rameter a with temperature for 12-12-12Rb and 16-16-16Rb are
shown in Figure 1b. They are given as examples of the steep de-
crease in column diameter, or intercolumnar distance ( = a ) on
while that of
1
h
h
h
9
heating, typical for Na, K, Rb and Cs salts. In contrast, Li salts
show no such decrease, and in fact the columns undergo slight
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lateral expansion. The plots of a
2-18Li in the Col range are also shown in Figure 1c.
When an n-alkane is added to the dendron, a Col phase is ob-
h
vs T for pure 12-12-12Li and 12-
1
h
h
served again, but with an enlarged ah. Within 5% error, the expan-
sion of the unit cell can be fully accounted for by the added vol-
ume of the alkane. n-Alkanes C15
ed C19 40 have all been found to show the same behaviour. SAXS
traces recorded at increasing T for 12-12-18Li + C19 40 70/30 are
shown in Figure 1b. The lattice parameter is plotted vs T in Figure
1c, both for this blend and for 12-12-12Li + C19 40 80/20. At
lower temperatures a remains nearly constant in both cases. How-
ever, at around 110C for 12-12-12Li + C19 40 80/20 and at
around 90C for 12-12-18Li + C19 40 70/30 a starts to decrease
relatively steeply and quite suddenly. This is particularly clear for
2-12-18Li + C19 40 70/30, both from Figures 1b and 1c. We note
H32, C17H36 and the perdeuterat-
(c)
D
D
D
D
D
1
D
that when the net bulk thermal expansion is taken into account,
and considering that all expansion is confined to the xy plane (see
below and Figure S7), a significant increase in a would be ex-
pected on heating. E.g. in 12-12-12Li + C19D40 80/20 the expected
increase between 70C and 130C would be 2.5%. Based on this
a should increase from 34.3 Å to 35.0 Å (see Table S4); instead it
decreases to 33.8 Å (Figure 1b), a drop by 3.6% linear, or 7% in
area and volume, from the expected value.
The other significant observation is that, at the point where the
lattice starts to contract, additional relatively weak X-ray diffrac-
tion peaks appear, denoted (10)
additional reflections, together with the ones carried over from the
low-T Col phase, can all be indexed on a 2-d hexagonal superlat-
tice with a unit cell parameter a , (see Table S1), where
and a refer, respectively, to the low-T Col , and the high-T
superlattice Col . As the extra reflections of the superlattice are
weak, and the area of the Col unit cell is 3 times that of the Col
s s
and (21) in Figure 1b. These
h
s
=
3
a
h
a
h
s
h
Figure 1. (a) Self-assembly of dendrons in a columnar phase
schematic). (b) SAXS curves for 12-12-18Li + C19 40 70/30
recorded during first heating. SAXS experiments were done at
beamline I22 at Diamond, UK. (c) Temperature dependence of
intercolumnar distance in Colh and Cols phases of 12-12-12Rb,
2-12-12Li, 12-12-18Li, 12-12-12Li + C19
18Li + C19 40 70/30. The trendline for 16-16-16Rb from ref. 9 is
also added.
s
(
D
s
h
cell, it is likely that the superlattice cell contains three columns
that are similar but not identical. To test this conjecture we recon-
structed electron density (ED) maps of the Col and Col phases
h s
using the intensities of the Bragg peaks e.g. in Figure 2a,b – for
details see SI.
1
D40 80/20 and 12-12-
D
The preferred maps for 12-12-12Li + C19
in Figures 2c and e, respectively, for the Col
Col
D
h
40 80/20 are shown
phase at 70C and
s
phase at 130C. The high density in the centre of the columns
(purple) comes from the electron-rich aromatic cores, while the
electron-poor aliphatic chains form the surrounding low-density
continuum (red). As can be seen, in the superlattice (Figure 2e)
two out of three columns in the cell have a high density core,
while the density of the third is apparently lower. However, due to
the ambiguity of the phase choice in the construction of ED maps,
solely on the basis of XRD we could not exclude the alternative
model with one dendron-rich and two dendron-poor columns (see
SI). However, the neutron diffraction data favor the former model
in 12-12-12Li + C19
D40 80/20 quite conclusively (see below). At
the same time, however, the latter model is the most likely scenar-
io in the case of 12-12-18Li + C19
ature, as explained in SI.
D40 70/30 at the highest temper-
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30C. Rounded to the nearest integer, this gives 3 in the Col
h
h
(
c)
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4
5
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7
8
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phases and ’8 in the superlattice.
(a)
Thus a stratum, or molecular layer, in a column of the Col
phase contains ca. three dendrons, the same as the pure 12-12-
12Li salt. The SLD map of the Col phase in Figure 2d indicates
that C19 40 (high SLD) preferentially settles in the interstices
between each three neighbouring columns. This accounts for the
six purple SLD maxima surrounding each column in Figure 2d,
the maximum in the column centre being mainly due to the aro-
matic rings (see Table S3).
h
D
H
L
(d)
In order to determine the number of dendrons in the two col-
umn types in the superlattice, we integrate the excess electron
density of the column cores above the ED of the aliphatic back-
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ground in the map in Figure 2e. Thus we obtain for the Col
s
of 12-
(
e)
12-12Li + C19 40 80/20 at 130C, 1.57 as the ratio between the
H
number of molecules in the dendron-rich (purple) and dendron-
poor (green) columns. Thus there are ca. 3 dendrons in each of the
two dendron-rich columns and 2 dendrons in the remaining den-
dron-deficient column in the unit cell, totaling 3+3+2 = 8 den-
drons.
(
b)
The situation in one stratum of the two phases is depicted
schematically in Figures 3a and b. In view of the above model, the
neutron SLD map in Figure 2f is easily understood. As in the Colh
phase, there are still six SLD maxima surrounding each column
due to C19D40 clustering in the triangular channels. But notably
the dendron-deficient columns have now lost their central SLD
maximum as well as the low-density circle around it. The former
is explained by the lowering of the total scattering cross-section of
the column centre due to the missing dendron with its aromatic
ring. The latter, i.e. the filling of the low-density ring, is explained
by the high-SLD deuterated alkane replacing the low-SLD hy-
drogenous alkyl tails of the missing dendron.
(f)
Figure 2. (a,b) Small-angle X-ray (black) and neutron powder
diffractograms (red) of 12-12-12Li + C19 40 80/20 in the (a) Col
phase at 70 C and (b) the superlattice phase at 130 C. (c) ED
and (d) neutron SLD map of the Col phase. (e) ED and (f) SLD
. Neutron experiments at station D16 at ILL, Gre-
D
h
h
map of the Col
noble.
s
As shown in SI, the neutron SLD map of 12-12-12Li + C19
0/20 at 130C proves conclusively that the arrangement in the
Col unit cell is “2*3+1*2”, i.e. two 3-dendron columns and one
-dendron column. However, such configuration cannot be ascer-
tained for the Col phase at all compositions of the blend. For a
D
40
8
s
The reason for using perdeuterated n-C19D40 is that neutrons
can clearly distinguish it from the hydrogenous alkyl tails of the
dendrons (Section 3 and Table S3). Experimental small-angle
neutron and X-ray diffraction curves of 12-12-12Li + C19D40
2
s
smaller alkane percentage the dendron-deficient column may
contain a mixture of two-dendron and three-dendron strata. More
8
0/20 are compared in Figures 2a and 2b, respectively, for the
significantly, in the case of the 12-12-18Li + C19
at the highest temperature of 112C (see Figure 1c) the calculated
number of dendrons in a Col cell is only 7; there a “2*3+1*2”
arrangement is impossible, and it is likely that instead a
1*3+2*2” configuration is adopted. Thus dendron-deficient col-
D40 70/30 blend
Colh and Cols phase. As one can see, the X-ray and neutron dif-
fractograms are very different: X-rays “see” the aromatic do-
mains, while neutrons are most strongly scattered on the deuterat-
ed alkane and, to a lesser extent, on the aromatic regions (Table
S3). Particularly striking is the relatively high intensity of the
additional superlattice reflections in the neutron pattern (labelled
s
“
umns that are strictly uniform, i.e. with all strata having the same
integer number of dendrons, cannot be maintained at all tempera-
tures.
1
0s, 20s, 21s, Figure 2b), suggesting a major rearrangement of the
added alkane upon the Colh-Cols transition. The neutron scatter-
ing length density (SLD) maps are shown in Figures 2d and 2f for
the Colh and Cols phases. For details of the construction of the
maps see SI. Like the ED map, the SLD map of the Cols phase
also shows the presence of two different types of columns.
As already discussed, adding alkane to the Li salts makes it
possible for a unit stratum to eject a dendron by partially replacing
it with the alkane. However questions, such as why a superlattice
is formed and why 3-dendron and 2-dendron cells are not distrib-
uted randomly, are still to be addressed. We propose that by
grouping all smaller 2-dendron strata in a separate column the
overall lattice strain can be minimized. Thus we might think of a
network of line dislocations, with the deficient strata all grouped
in an array of lines.
Next we proceed to determine the number of molecules in the
unit cell and in each of the constituent columns. Both the Col
Col phases can be classed as “ordered” columnar phases and both
have a molecular stacking periodicity c = 4.2 Å along the column
axis, the same as that of pure 12-12-12Li. This is determined from
the meridional reflection in the fibre patterns (Figures S7 and S8).
From the calculated unit cell volume and material density the
number of molecules in the cell has been derived for pure 12-12-
h
and
s
1
2Li and the 12-12-12Li + C19
and S5. In pure 12-12-12Li there are =3.2 molecules per unit cell
or column stratum). In the blend there are =2.9 dendrons and
=1.6 alkane molecules in the average Col cell at 70C, and
’=7.9 dendrons and ’=4.4 alkanes in the superlattice cell at
D40 80/20 mixture – see Tables S4
(

h
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ACKNOWLEDGMENT
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For help with synchrotron SAXS and neutron experiments we
thank Prof. N. Terrill at I22, Diamond, and Drs. A. Perkins and B.
Deme at D16, ILL. Financial support is acknowledged from NSF-
EPSRC
(
(
project
EP/D058066), Leverhulme Trust (RPG-2012-804), NSFC China
21274132).
“RENEW”
(EP/K034308),
EPSRC
REFERENCES
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Figure 3. Schematic model of a layer of the 12-12-12Li + C19D40
0/20 blend in the (a) Colh and (b) superlattice phase (pink:
minidendrons; green: n-alkane).
8
1 (a) Sun, H.-J.; Zhang, S.; Percec, V. Chem. Soc. Rev. 2015, 44, 3900;
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The remaining question to answer is why the columns of the Li
salts do not show thermal contraction like the other alkali metal
9
2
salts, exemplified here by 12-12-Rb and 16-16-16Rb (Figure 1c).
(a) Würthner, F.; Thalacker, C.; Diele, S.; Tschierske, C. Chem. Eur.
We propose that the columns of the Li salts resist contraction
because with the number () of dendrons as small as three, the
loss of one dendron would create unoccupied space that the ex-
panding tails of the remaining two dendrons could not fill at any
temperature within the range of the Col phase. Thus, without
h
added alkane, lateral contraction could not occur for Li salts. By
J. 2001, 7, 2245-2253; (b) An, Z.; Yu, J.; Domercq, B.; Jones, S. C.; Bar-
low, S.; Kippelen, B.; Marder, S. R. J. Mater. Chem. 2009, 19, 6688-6698;
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J. Am. Chem. Soc. 2011, 133, 12197-12219.
3
(a) Li, W. S.; Yamamoto, Y.; Fukushima, T.; Saeki, A.; Seki, S.; Ta-
contrast, in the salts of other alkali metals  is larger than 3 (in
gawa, S.; Masunaga, H.; Sasaki, S.; Takata, M.; Aida, T. J. Am. Chem.
Soc. 2008, 130, 8886; (b) Nierengarten, J.-F.; Oswald, L.; Eckert, J.-F.;
1
2-12-12Rb and 16-16-16Rb  is close to 4), hence the loss of
one molecule leaves a proportionally smaller gap that the remain-
ing dendrons could fill. Moreover, the Col phase in the salts with
Nicoud, J.-F.; Armaroli, N. Tetraheron Lett. 1999, 40, 5681.
4
(a) Ungar, G.; Batty, S. V.; Percec, V.; Heck, J.; Johansson, G. Adv.
h
Mater. Opt. Electro. 1994, 4, 303–313; (b) Ichikawa, T.; Yoshio, M.;
Hamasaki, A.; Mukai, T.; Ohno, H.; Kato, T. J. Am. Chem. Soc. 2007,
larger cations is of the “disordered” type, i.e. a true liquid crystal
and not a “soft crystal” with a degree of 3-d order. As can be seen
in the fibre pattern of 12-12-12Rb in Figure S8, there is no sharp
Bragg diffraction on or near the meridian, hence no periodic -
stacking. Such disordered columnar structures are more likely to
tolerate a non-integer . We also add that the column thinning is
partially reversible (see Figure S9).
1
29, 10662–10663; (c) Peterca, M.; Percec, V.; Dulcey, A. E.; Nummelin,
S.; Korey, S.; Ilies, M.; Heiney, P. A. J. Am. Chem. Soc. 2006, 128, 6713–
720; (d) Pecinovsky, C. S.; Hatakeyama, E. S.; Gin, D. L. Adv. Mater.
6
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B.; Taguchi, S.; Ohno, H.; Liu, F.; Ungar, G.; Kato, T. J. Am. Chem. Soc.
2
015, 137, 13212-13215.
5
Feng, X.; Tousley, M. E.; Cowan, M. G.; Wiesenauer, B. R.; Nejati,
In conclusion, we have presented a new type of superlattice in
liquid crystals. At the thermal transition from the simple hexago-
nal to the superlattice columnar phase the added alkane diluent is
redistributed into an ordered array of dendron-deficient columns.
This mitigates the large relative drop from 3 to 2 in the number of
dendrons per stratum. The results shed new light on the intriguing
phenomena of heat-thinning columns and supramolecular super-
lattices. This new type of superlattice also shows the way to dis-
tribute a potentially active functional additive in an ordered 2-D
pattern of sizeable period using only small molecules.
S.; Choo, Y.; Noble, R. D.; Elimelech, M.; Gin, D. L. O.; Osuji, C. O.
ACS Nano, 2014, 8, 11977–11986.
6
Feng, X.; Nejati, S.; Cowan, M. G.; Tousley, M. E.; Wiesenauer, B.
R.; Noble, R. D.; Elimelech, M.; Gin, D. L. O.; Osuji, C. O. ACS Nano,
2015, 10, 150-158.
7
Ungar, G.; Zeng, X. B. Soft Matter, 2005, 1, 95-106.
8
Ungar, G.; Liu, F.; Zeng, X. B. Chap. 7 in Handbook of Liquid Crys-
nd
tals, 2 Ed., Vol. 5, ed. Goodby, J. W.; Collins, P. J.; Kato, T.;
Tschierske, C.; Gleeson, H. F.; Reynes, P., VCH_Wiley, Weinheim, 2014,
3
63-436.
9
Ungar, G.; Percec, V.; Holerca, M. N.; Johansson, G.; Heck, J. A.
Chem. Eur. J., 2000, 6, 1258-1266.
ASSOCIATED CONTENT
Supporting Information
DSC; X-ray and neutron experimental and additional data; density
maps; calculation; molecular modelling.
1 0
Percec, V.; Holerca, M. N.; Uchida, S.; Cho, W. D.; Ungar, G.; Lee, Y.
S.; Yeardley, D. J. P. Chem. Eur. J. 2002, 8, 1106-1117.
1
1
(a) Beginn, U.; Yan, L. ; Chvalun, S. N.; Shcherbina, M. A.; Baki-
rov, A.; Möller, M. Liq. Cryst. 2008, 35, 1073-1093; (b) Shcherbina, M.
A.; Bakirov, A.; Yakunin, A. N.; Beginn, U.; Yan, L. ; Möller, M.;
Chvalun, S. N. Soft Matter, 2014, 10, 1746-1757.
1 2
AUTHOR INFORMATION
Kwon, J.-K.; Chvalun, S. N.; Blackwell, J.; Percec, V.; Heck, J. A.
Macromolecules 1995, 28, 1552-1558.
Corresponding Author
*g.ungar@sheffield.ac.uk
Present Addresses
‡
Synchrotron Research Inst., Nakhon Ratchasima, Thailand.
Notes
The authors declare no competing financial interests.
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