Liquid crystalline order from ortho-phenylene ethynylene macrocycles

Article abstract of DOI:10.1021/ja060354b

Triangular ortho-phenylene ethynylene (o-PE) cyclic trimers represent a novel member of shape-persistent macrocycles. Shape-persistent cyclic structures remain of great interest as molecular components in the fields of supramolecular materials, host-guest

Full text of DOI:10.1021/ja060354b

Published on Web 06/30/2006
Liquid Crystalline Order from ortho-Phenylene Ethynylene Macrocycles
Sang Hyuk Seo,† Ticora V. Jones, Helga Seyler, Jack O. Peters, Tae Hyung Kim,
Ji Young Chang,† and Gregory N. Tew*,
School of Materials Science and Engineering, Hyperstructured Organic Materials Research Center, Seoul National
UniVersity, Seoul 151-742, Korea, and Polymer Science and Engineering Department, UniVersity of Massachusetts,
Amherst, Massachusetts 01003
Received January 17, 2006; Revised Manuscript Received June 3, 2006; E-mail: tew@mail.pse.umass.edu
Triangular ortho-phenylene ethynylene (o-PE) cyclic trimers,
such as M1, represent a novel member of shape-persistent macro-
cycles. In general, shape-persistent cyclic structures remain of great
interest as molecular components in the fields of supramolecular
materials, host-guest chemistry, and materials science due to their
unique properties.1 Some of these shape-persistent macrocycles have
shown discotic liquid crystalline (LC) properties as a result of their
pseudo-planar, disk-like topology.1-3 These disk-like molecules can
Figure 1. Structure of ortho-phenylene ethynylene macrocycles.
arrange into one-dimensional columnar structures which are
governed by strong π-π stacking, van der Waals, dipole, and
hydrophobic interactions.2,3 o-PE macrocycles have attracted in-
creasing attention in the past few years because of their unique
properties, including a small interior pore, which is particularly well
suited for transition metal binding; they are structural units of
graphyne and exhibit unique electronic properties.1 It is one of the
most compact shape-persistent macrocycles known. Despite this
Figure 2. Polarized optical micrograph of (a) the Colr mesophase of M1,
(b) the Colh mesophase of M2, and (c) discotic nematic mesophase of M3
interest in o-PE macrocycles, liquid crystalline materials have
remained elusive.
obtained with a cooling rate of 10 °C/min at 25 °C on the second cooling.
In this paper, we report novel discotic LC properties from
triangular-shaped o-PE macrocycles containing branched alkoxy-
and/or triethylene glycol (TEG) side chains (see Figure 1). Using
polarized optical microscopy (POM), differential scanning calo-
rimetry (DSC), and X-ray diffraction (XRD), the macrocycles were
shown to form thermotropic rectangular columnar (Colr) (for M1),
hexagonal columnar (Colh) (for M2), and discotic nematic (for M3)
mesophases at room temperature.
temperature. The DSC trace of M1 showed one weak transition at
81 °C with an enthalpy value of 2.52 J/g, consistent with a LC to
isotropic transition. Upon cooling M1 from 100 °C, a weak broad
transition at 44 °C was observed with an enthalpy value of 3.86
J/g, corresponding to the isotropic to LC phase transition. A
birefringent phase with the same texture appeared at 44 °C on
cooling from the isotropic melt and remained until room temperature
without any change (Figure 2a). M2 and M3 showed thermal
behaviors similar to that of M1 with DSC. Phase transition
temperatures and corresponding enthalpy values for compounds M1,
M2, and M3 are given in Table 1.
Figure 3a,b shows the XRD scan collected for the birefringent
phase of M1 and M2 at room temperature, respectively. The
birefringent phases were confirmed as a Colr and Colh phase through
assignment of the reflections, respectively. The XRD profile of M1
(Figure 3a) shows seven sharp reflection peaks corresponding to d
spacings of 17.9, 15.3, 12.3, 8.9, 8.2, 7.4, and 6.8 Å, which were
indexed in sequence as (100), (010), (110), (200), (210), (020),
and (120) of a Colr lattice with the lattice parameter of a ) 17.9
and b ) 15.3 Å. In fact, more reflections were visible but began to
overlap with scattering from the Mylar sample holder. Regardless,
these seven reflections allow definite assignment of the structure.
These spacings agree well with those expected from the macrocycle
based on a diameter calculated to be 20 Å (Figure 3c,d). It is likely
that the branched alkyl side chains interdigitate along the a,b-axis
of the structure and that the rectangular organization is due to the
triangular shape, or irregular disk, of this macrocycle with short
branched alkyl side chains. Other o-PE macrocycles have been
reported with unbranched alkyl side chains that did not induce LC
order.5a,d
Synthesis of the macrocycles is outlined in the Supporting
Information but uses the traditional ultra-dilute ring closing reaction
of an A-B functionalized molecule4 as the final step. Much recent
attention has been given to metathesis cyclization, which can greatly
improve the macrocycle yield.5 However, metathesis does have
limitations, including the inability to synthesize M3. Metathesis
would also create mixtures of M1 and M2 by scrambling the
monosubstituted side chains, leading to a collection of isomers.
Therefore, although metathesis is a powerful new, and sometimes,
high yielding approach, traditional macrocyclization is still an
important synthetic approach to building designer materials.
Macrocycle M1 is freely soluble in a variety of organic solvents,
including chloroform, tetrahydrofuran, heptane, and methanol. The
solubility of macrocycles M2 and M3 is similar to that of M1 except
in hydrocarbons due to the hydrophilic TEG side chains. Macro-
cycles M1, M2, and M3 were isolated as a sticky (for M1) and
oily (for M2 and M3) material at room temperature that exhibited
birefringent mesophases when examined with POM. The textures,
shown in Figure 2, are nonspecific with short-rod-like (for M1),
thread-like (for M2), and wave-like (for M3) structures present.
As a result, the macrocycles were assigned to a LC phase at room
† Seoul National University.
University of Massachusetts.
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J. AM. CHEM. SOC. 2006, 128, 9264-9265
10.1021/ja060354b CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Phase Transition Temperatures and Enthalpies [In
meta series has an internal hydrogen to hydrogen distance of about
8 Å, while the spacing-filling molecular model of M1 in Figure 3c
shows the internal cavity is extremely small, approximately 2.4 Å
across the macrocycle. In addition to the central cavity size
distinction, the connectivity of the ortho versus meta series leads
to differences in the nature of conjugation for the two systems.
This work shows clearly that electron-rich PE macrocycles can form
LC materials.
Parentheses] for Macrocycles M1, M2, and M3 as Determined by
a
DSC (scan rate ) 10 °C min-1
)
1
a
T (oC) [ Ht, J g-
∆ ]
compound
heating (oC)
cooling (oC)
M1
M2
M3
Colr 81 [2.5] I
Colh 55 [12.4] I
ND 54 [2.2] I
I 44 [3.9] Colr
I 55 [13.0) Colh
I 52 [1.7] ND
In summary, novel LC materials are reported based on ortho-
PE macrocycles for the first time. The macrocycle M1 contains
short alkyl side chains. Alternatively, if the side chain is hydrophilic,
as is the case with M2, then the amphiphilicity provides a driving
force for organization. These structures demonstrate two side chain
chemistries for creating self-assembling materials from ortho-PE
macrocycles. The ability to create ordered self-assembling materials
from these novel electron-rich macrocycles is important in nano-
technology and may enable new materials for applications in
membranes and electronics.
a Colr, rectangular columnar; Colh, hexagonal columnar; ND, discotic
nematic; I, isotropic.
Acknowledgment. We thank the NSF for financial support
(NSF-CAREER CHE-0449663 and MRSEC-DMR 9400488), BK
21 Project, HOMRC to S.H.S., and Ford Foundation (T.V.J.).
Supporting Information Available: Experimental procedures of
macrocycles. including synthesis, UV-vis spectra, and XRD of M3 at
room temperatures. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Figure 3. X-ray diffractograms for the (a) rectangular columnar phase of
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