DOI: 10.1002/cphc.201501166
Articles
d-Methyl Branching in the Side Chain Makes the
Difference: Access to Room-Temperature Discotics
Jochen Kirres,[a] Friederike Knecht,[b] Philipp Seubert,[a] Angelika Baro,[a] and
Sabine Laschat*[a]
Dedicated to Prof. Henning Hopf on the occasion of his 75th birthday
Although discotic liquid crystals are attractive functional mate-
rials, their use in electronic devices is often restricted by high
melting and clearing points. Among the promising candidates
for applications are [15]crown-5 ether-based liquid crystals
with peripheral n-alkoxy side chains, which, however, still have
melting points above room temperature. To overcome this
problem, a series of o-terphenyl and triphenylene [15]crown-5
ether derivatives was prepared in which d-methyl-branched
alkoxy side chains of varying lengths substitute the peripheral
linear alkoxy chains. The mesomorphic properties of the novel
crown ethers were studied by differential scanning calorimetry,
polarizing optical microscopy, and X-ray diffraction.
d-Methyl branching indeed lowers melting points resulting in
room-temperature hexagonal columnar mesophases. The
mesophase widths, which ranged from 87 to 30 K for o-ter-
phenyls, significantly increased to 106–147 K for the tripheny-
lenes depending on the chain lengths, revealing the beneficial
effect of a flat mesogen, due to improved p–p interactions.
1. Introduction
Thermotropic discotic liquid crystals, which self-assemble into
columnar mesophases upon a change in temperature have re-
ceived increasing interest since their discovery by Chandrase-
khar in 1977.[1,2] Their high charge-carrier mobility, ability to
self-heal defects, efficient synthetic access and good solubility,
as well as easy processing through spin-coating or drop-cast-
ing procedures, make them highly attractive as functional ma-
terials for various device applications, such as sensors,[3] organ-
ic solar cells,[4] organic light-emitting diodes (OLEDs),[5] and or-
ganic field-effect transistors (OFETs).[6] However, in contrast to
dendritic, amphiphilic, H-bonded, or star-shaped mesogens,[7]
most discotic liquid crystals, carrying a central rigid aromatic or
heteroaromatic core functionalized with flexible peripheral
alkyl side chains, enter their mesophases at elevated tempera-
tures.[8] Thus, melting points that are often far above room
temperature and clearing points close to the decomposition
temperature, limit further application in devices, for example,
ferroelectric switches.[9] Several strategies have therefore been
developed to circumvent this problem and to make room-tem-
perature mesophases possible, for example, the replacement
of n-alkoxy groups by thioethers lowered the melting transi-
tion temperature.[10] Another strategy based on seminal work
by Collard and Lillya[11] used the introduction of branched pe-
ripheral side chains rather than linear alkyl (or alkoxy) tails.[12]
In particular, core units bearing dove-tailed,[13] swallow-
tailed,[14] dihydrocitronellyl, phytyl,[15] or other bitailed sys-
tems[16,17] in the periphery were studied.
To address transition-temperature lowering in crown-ether-
based mesogens, previously we systematically varied the
crown size, symmetry, and salt complexation of o-terphenyl-
and triphenylene-substituted derivatives, such as 1 and 3, re-
spectively, with linear alkoxy side chains in the periphery
(Scheme 1).[18] The investigation of mesomorphic properties
showed that the complexation of salts with soft anions not
only increasing the flatness of the core from o-terphenyl to tri-
phenylene, but also improved the mesophase stability, howev-
er, melting transitions remained far above room tempera-
ture.[18c,19] The reduction of size, molecular symmetry and flexi-
bility of the central crown resulted in broader mesophase
widths and lower melting points, however, ambient tempera-
ture mesophases could not be accessed.[18b,c] Thus, an alterna-
tive method for melting-point depression was necessary with-
out compromising the mesophase stability. Taking the reports
on branched side chains[11,12] into account, we anticipated that
the attachment of d-methyl-branched tails to the most promis-
ing candidates among the previous series, 1 and 3, might lead
to room-temperature columnar mesophases. Moreover, we
might be able to check whether the improved p–p interaction
in the flat triphenylene[20] would improve the mesophase sta-
bility, as compared to o-terphenyl, while maintaining low melt-
ing transition temperatures, due to hydrophobic interactions
of the d-methyl-branched side chains.
[a] J. Kirres, P. Seubert, Dr. A. Baro, Prof. S. Laschat
Institut für Organische Chemie, Universität Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
[b] F. Knecht
Institut für Physikalische Chemie, Universität Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
Thus, our design concept featured o-terphenyls 2b–f and tri-
phenylenes 4b–f carrying d-methyl-branched side chains as
key target compounds (Scheme 1). These were compared with
Supporting Information for this article can be found under http://
ChemPhysChem 2016, 17, 1159 – 1165
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