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CPK models showing the organization of the molecules in the cross
‡ In the first heating scan here are crystalꢀcrystal transitions at 51 °C and
section of two adjacent triangular cylinders (one 2D unit cell); d) shows a 60 76 °C before the melting point at 109 °C, the given enthalpy is the total of
model of the triangular honeycomb phase and e) (36)ꢀnet.
all three transitions.
§ The dark red color of the material is also dominating in the optical
isotropic areas.
¶ The orientation of the optical axis can in this case not be determined
65 with certainty by observing the birefringence color shift using a lambda
retarder plate, due to the color of the material covering the birefringence.
An alternative honeycomb formed by hexagonal cylinders can
be excluded as this would provide insufficient space filling of the
larger hexagonal cells and would require a lattice parameter of
ahex= 31/2 L which is much larger than the experimental
determined value. If the rodꢀlike units would be arranged parallel
to the column long axis and surrounded by the fluid alkyl chains
5
1
(a) I. Mcculloch, M. Heeney, C. Bailey, K. Genevicius, I. Macdonald,
M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W. M. Zhang, M.
L. Chabinyc, R. J. Kline, M. D. McGehee and M. F. Toney, Nat.
Mater., 2006, 5, 328ꢀ333; (b) M. M. Ling and Z. N. Bao, Chem.
Mater., 2004, 16, 4824ꢀ4840.
10 as recently proposed for the rodꢀbundle phases, 24 then ahex should
be significantly less than 3.6 nm. 27 This value is much smaller
than the experimentally observed lattice parameter and was
estimated from twice the length of the lateral chain (1.35 nm in
all-trans conformation) plus twice the diameter of the aromatic
15 core (0.45 nm).
70
75
2
3
M. O’Neill and S. M. Kelly, Adv. Mater., 2011, 23, 566–584.
X. Zeng, R. Kieffer, B. Glettner, C. Nürnberger, F. Liu, K. Pelz, M.
Prehm, U. Baumeister, H. Hahn, H. Lang, G. A. Gehring, C. H. M.
Weber, J. K. Hobbs, C. Tschierske and G. Ungar, Science, 2011, 331,
1302ꢀ1306.
The shrinkage of the trigonal cylinders with rising temperature
can be explained by the increasing mobility of the molecules and
especially the alkyl chains with increasing temperature and this is
typically observed for columnar phases and especially for
20 polygonal honeycomb phases.12,13 The thermal expansion occurs
predominately along the cylinder long axis which reduces the
coreꢀcore packing density along this direction. This is in line with
the shift of the position of the diffuse wide angle scattering from
d = 0.44 nm at T = 40 °C to d = 0.46 nm at T = 140 °C. Also in
25 line with the enhanced mobility is the significant broadening of
the small angle scatterings with rising temperature (Fig. 2a). This
indicates a continuously decreasing correlation length of the
hexagonal lattice with rising temperature which might be due to
the incorporation of a growing number of rhombic cells.15 These
30 rhombic cells can be formed by fusion of two adjacent triangular
cells and hence provide more space for the lateral chains. These
cells also allow an enhanced flexibility, as in contrast to triangles
the rhombs can more easily change the angles. This leads to a
reduced correlation length of the hexagonal lattice. At T = 148 °C
35 the 2Dꢀlattice becomes short range at the phase transition to the
isotropic liquid state which is assumed to be composed of small
disordered segments of the previous honeycomb.
In Summary, the first example of a sexithiophene based
bolaamphiphile decorated with four lateral decyl chains and two
40 terminal glycerol groups was synthesized. It represents the
bolaamphiphilic mesogens with the longest πꢀconjugated
oligothiophene core so far, which selfꢀassembles into a triangular
honeycombꢀlike LC phase over a broad temperature region. This
mode of selfꢀassembly is different to all previously reported
45 sexithiophenes and opens new possibilities for the directed
organization of πꢀconjugated organic materials into complex
superstructures. Further work will be focused on modification of
the πꢀconjugated core to achieve closer πꢀstacking along the
honeycomb walls. Such optimized compounds could be of
50 interest for selfꢀassembled nanoꢀscale organic electronic devices.
4
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b Institute of Chemistry Organic Chemistry, Martin-Luther University
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† Electronic Supplementary Information (ESI) available: [Synthesis,
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