À1
Table 1 Phase transition temperatures (/1C, peak temperature) and enthalpies (/kJ mol , in parentheses) of novel disc–rod hybrids. Cr = crystal,
SmA = smectic A, I = isotropic, X = glassy state, Cub = cubic
Compound
Heating scan
Cooling scan
a
6
6
6
6
a
b
c
Cr 188.2 (105.6) I
I 173.4 (6.9) SmA 169.5 (12.4) Cub 156.4 (76.0) Cr
I 159.8 (17.7) SmA 76.1 (20.2) Cr
I 153.2 (22.5) SmA 63.9 (14.5) X
I 144.8 (21.6) SmA
Cr 131.0 (51.0) SmA 161.7 (17.3) I
Cr 109.4 (32.0) SmA 155.6 (22.6) I
Cr 93.5 (29.9) SmA 147.4 (23.7) I
d
a
Based on POM and DSC, see the ESI.w
three compounds, 6b–d, show similar diffraction patterns in
the mesophase. They consist of two sharp peaks in the
small-angle region, whose spacings are in the ratio 2 : 1, and
units attached radially to a central tricycloquinazoline discotic
core. These shape-amphiphilic oligomesogens exhibit
a
nanophase segregated morphology in a SmA phase, wherein
the calamitic and discotic moieties segregate into different
sub-layers. This is the first time that this kind of hierarchical
organization has been observed in oligomesogens having a
radial molecular topology and containing six rods. A cubic
phase has also been observed for the first time in this kind of
system, albeit monotropic in nature; hence, the detailed
molecular organization and symmetry of the phase could not
be established.
˚
a very diffuse peak at around 4.6 A in the wide-angle region
(
Fig. 1(c)). These features suggest that the mesophase is
smectic, consistent with their microscopy textures. The smectic
˚
periodicity, d, is found to be 42, 39 and 35 A for 6d, 6c and 6b,
respectively. In all cases, the ratio of d to the length of the fully
stretched molecule, l, is very close to 0.66. Interestingly, these
patterns also contain a diffuse peak in the small-angle region,
whose spacing varies slightly with spacer length, being ca. 20.1,
˚
7.5 and 16 A for 6d, 6c and 6b, respectively. Similar diffuse
1
peaks have been observed in the diffraction patterns of certain
9
mesogens containing both rod-like and disc-like moieties.
,11
Notes and references
On the basis of the X-ray and microscopy data, we propose a
structure for the mesophase of these compounds, which is
schematically illustrated in Fig. 1(d). In this SmA structure,
the rods and discs microphase separates into alternating
layers. This is made possible by the fact that the volume
fraction of the disc is roughly half that of the six rods in each
1 Handbook of Liquid Crystals, ed. D. Demus, J. W. Goodby,
G. W. Gray, H. W. Spiess and V. Vill, Wiley VCH, Weinheim,
1
998, vol. 1–3.
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3
K. Kawata, Chem. Rec., 2002, 2, 59; H. Mori, J. Disp. Technol.,
2005, 1, 179.
G. R. Luckhurst, Nature, 2004, 430, 413; D. W. Bruce, Chem. Rec.,
2
M. J. Freiser, Phys. Rev. Lett., 1970, 24, 1041.
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004, 4, 10.
4
5
9
molecule. The low d/l value of 0.66 is most probably a
consequence of the conformational flexibility of the spacers,
orientational disorder and the partial interdigitation of the
terminal cyano groups of the rod-like segments known to
6
7
S. R. Sharma, P. Palffy-Muhoray, B. Bergersen and
D. A. Dunmur, Phys. Rev. A: At., Mol., Opt. Phys., 1985, 32,
3752; A. G. Vanakaras, S. J. McGrother, G. Jackson and
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R. Pratibha and N. V. Madhusudana, Mol. Cryst. Liq. Cryst. Lett.,
9
,14
occur in cyanobiphenyls.
We cannot, though, rule out
the possibility of a tilt in either/both layers. However, the
observation of the microscopy texture characteristics of the
SmA in all of these compounds indicates that even if there is a
tilt, it is not correlated across the layers. The diffuse peak in
the wide-angle region corresponds to the average separation
of the rods and the face-to-face separation of the discs,
whereas the peak in the small-angle region arises from the
side-to-side distance between the discs. As is expected from the
molecular structure, the latter increases slightly with the length
1985, 1, 111; R. Hashim, G. R. Luckhurst, F. Prata and
S. Romano, Liq. Cryst., 1993, 15, 283.
8 I. D. Fletcher and G. R. Luckhurst, Liq. Cryst., 1995, 18, 175;
R. W. Date and D. W. Bruce, J. Am. Chem. Soc., 2003, 125, 9012.
9
P. H. J. Kouwer and G. H. Mehl, J. Am. Chem. Soc., 2003, 125,
1172; P. H. J. Kouwer and G. H. Mehl, Angew. Chem., Int. Ed.,
1
2003, 42, 6015; P. H. J. Kouwer, J. Pourzand and G. H. Mehl,
Chem. Commun., 2004, 66.
1
0 S. K. Pal, S. Kumar and J. Seth, Liq. Cryst., 2008, 35, 521;
C. T. Imrie, Z. Lu, S. J. Picken and Z. Yildirim, Chem. Commun.,
˚
of the spacer. The absence of a peak at around 3.8 A indicates
2
007, 1245; K. U. Jeong, A. J. Jing, B. Monsdorf, M. J. Graham,
that the discs do not stack up to form long columns in the
layers, but have only short-range positional correlations, as in
isotropic and nematic phases. From our diffraction data, it is
not possible to ascertain if the layers containing the discs form
a two-dimensional nematic phase or if their orientational
order is also short-range. Even if the discs have long-range
orientational order within each layer, the microscopy textures
again suggest a lack of long-range correlation across the
layers.
F. W. Harris and S. Z. D. Cheng, J. Phys. Chem. B, 2007, 111, 767;
M. L. Rahman, C. Tschierske, M. Yusoff and S. Silong,
Tetrahedron Lett., 2005, 46, 2303; Y. Shimizu, A. Kurobe,
H. Monobe, N. Terasawa, K. Klyohara and K. Uchida, Chem.
Commun., 2003, 1676.
11 P. H. J. Kouwer and G. H. Mehl, J. Mater. Chem., 2009, 19, 1564
and references therein.
1
2 S. Kumar, E. J. Wachtel and E. Keinan, J. Org. Chem., 1993, 58,
821; S. Kumar, D. S. S. Rao and S. K. Prasad, J. Mater. Chem.,
1999, 9, 2751.
3
13 V. Percec, M. Lee and H. Jonsson, J. Polym. Sci., Part A: Polym.
Chem., 1991, 29, 327.
In conclusion, a novel series of disc–rod oligomers have
been designed, synthesized and their mesomorphism studied.
These oligomers contain six calamitic alkoxy-cyanobiphenyl
1
4 R. Deschenaux, B. Donnio and D. Guillon, New J. Chem., 2007,
1, 1064; I. M. Saez and J. W. Goodby, J. Mater. Chem., 2005, 15,
26; C. Tschierske, J. Mater. Chem., 2001, 11, 2647.
3
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