Fluorescent supramolecular liquid crystalline polymers from nucleobase-terminated monomers

Article abstract of DOI:10.1039/b307877a

While thymine or N⁶-(4-methoxybenzoyl)adenine terminated bis(4-alkoxy)-substituted bis(phenylethynyl)benzene monomers show no liquid crystalline behaviour by themselves, mixing the complementary nucleobase monomers together results in the forma

Full text of DOI:10.1039/b307877a

Fluorescent supramolecular liquid crystalline polymers from
nucleobase-terminated monomers†
Sona Sivakova and Stuart J. Rowan*
Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert
Road, Cleveland, Ohio, USA. E-mail: sjr4@cwru.edu; Fax: +1 216 368 4202; Tel: +1 216 368 4242
Received (in Cambridge, UK) 10th July 2003, Accepted 20th August 2003
First published as an Advance Article on the web 4th September 2003
While thymine or N6-(4-methoxybenzoyl)adenine termi-
nated bis(4-alkoxy)-substituted bis(phenylethynyl)benzene
monomers show no liquid crystalline behaviour by them-
selves, mixing the complementary nucleobase monomers
together results in the formation of thermotropic liquid
crystalline phases.
alkoxy-substituted bis(phenylethynyl)benzene. This mesogen
with pendant octyloxy chains and no nucleobases attached has
been shown to display both nematic (N) and smectic (S) phases
with the following thermal transition temperatures (°C): K–
128.3–S1–167–S2–182–N–218–I.9
The synthesis of the BP-3-BP monomers (Scheme 1) was
achieved by reacting, under basic conditions, either thymine (T)
or N6-anisoyl adenine (AAn) with either 1a or 1b, which contain
a C6 or C9 alkyl chain, respectively. The resulting product 2 (a
or b) was then reacted with 1,4-diethynylbenzene under Heck
coupling conditions to yield the desired bis(phenylethy-
nyl)benzene-derived supramolecular monomers. Using this
synthetic route we prepared four different monomers which not
only varied the nucleobase they contained but also the length of
the alkyl chain between the nucleobase and the bis(phenylethy-
nyl)benzene core.
Over the last decade the area of supramolecular polymerisation,
i.e. the self-assembly of small molecules into polymer-like
materials through the use of the non-covalent bond, has received
a growing amount of attention.1 As a result it has been found
that material properties of small molecules can be drastically
altered by attachment of appropriate binding motifs that induce
formation of “supramolecular polymers” by a self-assembly
process. In addition, if the binding motif is asymmetric then the
desired change in properties should only be expressed when the
two complementary binding motifs are present. There are a
number of material properties, e.g. mechanical, optical, phase
organisation and viscosity, that can be altered using such a
methodology. One possibility is the use of the non-covalent
bond to induce non-mesogenic small molecules to form liquid
crystalline phases.2 In fact, the formation of an ordered liquid
crystalline phase will also aid the degree of assembly (or
polymerisation) of the small molecules.2,3 The individual
nucleobases are one of natures more interesting binding motifs
and the possibility of using them in the material realm has
started to gain attention in recent years.4 In this communication,
we present our initial studies on the use of the nucleobase
derivatives, based on thymine (T) and N6-(4-methoxybenzoyl)-
adenine (AAn), to control the assembly of the fluorescent low
molecular weight core, bis(4-alkoxy)-substituted bis(phenyle-
thynyl)benzene, and result in the formation of supramolecular
polymeric liquid crystalline phases.
Individually, the four monomers show no propensity for LC
behavior and have very high melting points. For example, Figs
1a,b show the DSC thermograms for AAn–3a–AAn and T–3a–
T. However, upon mixing and melting these two com-
plementary nucleobase-terminated monomers, a dramatic de-
crease in the melting temperature is observed (Fig. 1c) along
with concurrent formation of a viscous bifringent phase
between 125–154 °C. Fig. 2a shows the polarized optical
While DNA sequences are one of the most predictable
supramolecular binding motifs,5 the individual nucleobases are
less well behaved. For example, the purines (adenine and
guanine) are able to bind though two different binding sites
(Watson–Crick and Hoogsteen),6 and as a result can form multi-
component complexes. Therefore, we decided to investigate the
binding capability of N6-anisoyl protected adenine (AAn) which
results in one of the binding sites of the adenine moiety being
blocked. While such a synthetic modification will reduce the
degree of binding of adenine,7 it is known that the amide group
will tend to block the Watson–Crick face of the nucleobase on
account of unfavorable interactions between the lone pair
electrons of the amide carbonyl and the N7 of the adenine. We
have measured the binding constant in CDCl3 between T and
AAn to be ca. 22 M21. While this confirms that the AAn and T
still interact with each other it does correspond to a reduction in
the binding constant compared to the literature value of the A–T
interaction (CDCl3 KA–T = 100 M21).8 The core units to which
the nucleobase derivatives were attached is the known mesogen
Scheme 1 Synthesis of the nucleobase terminated monomers.
1
† Electronic supplementary information (ESI) available: This includes H
NMR and DSC data for all monomeric compounds, as well as DSC data of
the mixtures, the NMR titration data for the binding study and photo-
luminescence spectra of the AAn–3a–AAn : T–3a–T mixture and its
individual components. See http://www.rsc.org/suppdata/cc/b3/b307877a/
Fig. 1 DSC thermograms of the second heating of (a) T–3a–T; (b) AAn–3a–
AAn; and (c) AAn–3a–AAn: T–3a–T (1 : 1 molar ratio) with an expansion of
the bifringent region (inset).
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This journal is © The Royal Society of Chemistry 2003

Table 1 Thermal transitions observed upon mixing complementary monomers of BP–3a–BP and BP–3b–BP
,
,
Monomer mixture (1 : 1)
Transitions heating/°Ca b
Transitions cooling/°Ca,c
Bifringent range/°Ca b
AAn–3a–AAn : T–3a–T
AAn–3a–AAn : T–3b–T
AAn–3b–AAn : T–3a–T
AAn–3b–AAn : T–3b–T
94, 105, 116, 125, 154, 172
77, 96, 119, 137, 146, 166, 179
88, 110, 174
154, 115, 110, 101
176, 161, 142, 134, 117, 95
162, 86
125–154
120–178
110–175
117–185
75, 84, 117, 163, 184, 192
182, 163, 117, 80
a DSC and POM measurements. b Second heating. c Second cooling.
Fig. 3 (a) Optical micrograph (310) of a fibre obtained from the LC phase
of AAn–3a–AAn and T–3a–T under non-polarized light, (b) fibre diffraction
pattern of the fibre and (c) optical micrograph (3100) of the fibre when
excited with UV light (365 nm).
AAn–3a–AAn and T–3a–T, and the 1 : 1 mixture, which were
solution cast from chloroform onto glass slides, show only
slight differences in the shape of the emission peaks.†
In conclusion, we have shown that while addition of the
nucleobase thymine and the nucleobase derivative N6-(4-me-
thoxybenzoyl)adenine to a mesogenic alkoxy-substituted bi-
s(phenylethynyl)benzene results in a loss of liquid crystal phase
formation, simple mixing and annealing of the two com-
plementary nucleobase-derived monomers results in the forma-
tion of relatively stable LC phases. Concurrent with the
formation of the viscous bifringent phases the material also
demonstrates the ability to form oriented fluorescent fibres.
Thus we have utilized the functionality of the core unit to aid LC
formation and impart fluorescent behaviour, in conjunction
with the self-assembly capability of the nucleobases, which not
only aids LC formation but also imparts polymer-like properties
to the material, i.e. fibre formation.
Fig. 2 Polarized optical micrographs of bifringent textures observed for (a)
AAn3aAn : T–3a–T at 130 °C; (b) AAn3aAn : T–3b–T at 160 °C; (c) AAn
–
3c–AAn : T–3a–T at 155 °C (d) AAn–3b–AAn : T–3b–T at 120 °C
(magnification 5003 ). See ESI for optical micrographs (3100).†
micrograph of this material at 130 °C. Above 175 °C, the
material behaves as a free-flowing liquid, which becomes more
viscous again upon cooling below 175 °C. At 153 °C, a viscous
bifringent phase was observed which became solid below 120
°C. It should be noted that these phenomena are not observed on
the first heating of the 1 : 1 samples obtained either from solid
or solution-state mixing, but are consistently present upon
subsequent heating and cooling cycles. Similar observations
were made for the nine carbon monomers AAn–3b–AAn and T–
3b–T. Individually, these monomers exhibit high melting
temperatures which decrease upon mixing, and a viscous
bifringent phase (Fig. 2b) can be observed between 118 °C and
180 °C. In both cases, it is postulated that the formation of the
viscous bifringent phase occurs as a result of the monomers
aggregating into a supramolecular polymer.
Intermixing of the C6 and C9 monomers with complementary
moieties in 1 : 1 ratios also produced similar results with viscous
bifringent phases forming at lower temperatures compared to
the high melting, non-bifringent individual monomers. For
example, the AAn–3b–AAn : T–3a–T material shows a texture
indicative of a nematic phase at 155 °C (Fig. 2c). All mixtures
showed numerous transitions as measured by DSC (Table 1) of
which most could be attributed to transitions between crystal-
line, viscous bifringent, and isotropic states. Most transitions
were observed as extremely broad peaks consistent with the
presence of a polydispersed polymeric mixture.
We thank the Case School of Engineering and the Dow
Chemical Company for helping to fund this work. We also
thank Professor C. Weder for useful discussions. SJR also
thanks the NSF for a CAREER Award (CHE-0133164).
Notes and references
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In addition to forming viscous bifringent phases the mixed
materials also displayed polymer-like properties. For example,
fibres could be obtained from the LC phase of these systems.
Fig. 3a shows an optical micrograph of a fibre obtained from the
bifringent phase of the AAn–3a–AAn and T–3a–T mixture at ca.
145 °C. Fig. 3b shows the X-ray fibre diffraction data which
confirm that these fibres are oriented.
Furthermore, these fibres are also fluorescent. Fig. 3c shows
the fibre when it is excited with UV light (365 nm).
Fluorescence spectra of films of the individual components,
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