UV-visible absorption study of the selfassociation of non-ionic chromonic triphenylenes TP6EOnM (n = 2, 3, 4) in dilute aqueous solutions: Impact of chain length on aggregation

Article abstract of DOI:10.2533/chimia.2015.520

A series of triphenylenes with oligoethoxy chains of various length, TP6EOnM with n = 2, 3, 4, has been synthesised and purified by HPLC. The self-association of these disc-shaped molecules in dilute aqueous solutions (~10^-7 to ~4 × 10^-4 M) has been studied by UV-visible absorption spectroscopy. The free energy of association decreases as the length of the chains increases. As a result, for a given concentration, the average size of aggregate diminishes as the chain length increases. While the absorption properties of the monomer are identical for the three molecules, the extinction coefficients of solutions of the three triphenylenes at a given concentration are significantly different and are directly linked to the average size of the aggregates. The change of epsilon values upon aggregation could explain the trend generally observed with dyes for solar cells substituted with chains of increasing length showing increasing extinction coefficient values.

Full text of DOI:10.2533/chimia.2015.520

520 CHIMIA 2015, 69, No. 9
doi:10.2533/chimia.2015.520
Supramolecular chemiStry
Chimia 69 (2015) 520–523 © Schweizerische Chemische Gesellschaft
UV-visible Absorption Study of the Self-
association of Non-ionic Chromonic
Triphenylenes TP6EOnM (n = 2, 3, 4) in
Dilute Aqueous Solutions: Impact of Chain
Length on Aggregation
Antoine Herbaut and Etienne Baranoff*
Abstract: A series of triphenylenes with oligoethoxy chains of various length, TP6EOnM with n = 2, 3, 4, has
been synthesised and purified by HPLC. The self-association of these disc-shaped molecules in dilute aqueous
solutions (~10–7 to ~4 × 10–4 M) has been studied by UV-visible absorption spectroscopy. The free energy of
association decreases as the length of the chains increases. As a result, for a given concentration, the average
size of aggregate diminishes as the chain length increases. While the absorption properties of the monomer
are identical for the three molecules, the extinction coefficients of solutions of the three triphenylenes at a
given concentration are significantly different and are directly linked to the average size of the aggregates. The
change of epsilon values upon aggregation could explain the trend generally observed with dyes for solar cells
substituted with chains of increasing length showing increasing extinction coefficient values.
Keywords: Aggregation · Self-association · TP6EO2M · Triphenylene · UV-visible absorption
ical micelle concentration.[1] Chromonic
mesogens are formed of a rigid and solvo- oxy)-ethoxy)triphenylene (TP6EO2M,
phobic core, generally aromatic, surround- Fig. 1) is a triphenylene-based chromonic
2,3,6,7,10,11-hexa(2-(2-methoxyeth-
1. Introduction
The aggregation of molecules in solu-
ed by solubilising groups. An important mesogen with six peripheral 2-(2-meth-
tion is of great interest for a broad variety
of topics including biology, drugs, inks,
and organic electronics. Among all supra-
molecular architectures, there is a growing
interest in chromonic liquid crystals be-
cause of numerous potential applications
including the fabrication of polarizing
films.[1]
difference with thermotropic liquid crys- oxyethoxy)ethoxy chains acting as the
tals, in which LC behaviour is controlled solubilising groups.[2] When mixed with
solely by temperature, is that the molecular D2O at room temperature, TP6EO2M
organisation in chromonic liquid crystals forms a nematic columnar phase for con-
is also dependent on the concentration. centration above ~20wt% and a columnar
As the concentration increases, the mole- hexagonal phase at higher concentration.
cules spontaneously aggregate into colum- This behaviour has been studied by X-ray
1
nar stacks. As stacks grow, they become diffraction and H NMR[3] and results are
Chromonic liquid crystals are a par-
ticular class of lyotropic liquid crystals
that aggregate in solution without a crit-
ordered and form the liquid crystalline well supported by theoretical studies.[4]
phases.
However, only limited studies of the ag-
gregation at low concentration exist, as
only one NMR study is reported.[5] Based
on this study relating the average aggregate
length to the concentration, the photophys-
ical properties, absorption and emission, of
TP6EO2M have been investigated to show
that the exciton hops along the columnar
aggregate at a rate of at least 1011 s–1.[6]
TP6EO3M does not display liquid
crystalline properties in water because the
additional ethyleneoxy unit provides too
much hydrophilic character.[3a] It can there-
fore be safely assumed that TP6EO4M,
which has a further additional ethyleneoxy
unit, would also not show liquid crystal-
line properties with water. Consequently
the aggregation properties of these com-
pounds in dilute aqueous solution have not
been studied.
*Correspondence: Dr. E. Baranoff
School of Chemistry
The University of Birmingham
Edgbaston, B15 2TT Birmingham, UK
Tel.: +44 121 414 2527
The three molecules TP6EOnM, n = 2,
3, 4, have a common aromatic core with
Fig. 1. Chemical structures of TP6EOnM
(n = 1, 2, and 3).
E-mail: e.baranoff@bham.ac.uk

Supramolecular chemiStry
CHIMIA 2015, 69, No. 9 521
well studied photophysical properties and with a mesyl group results in the crude tion step is key for the aggregation study
therefore can be considered as a very good TP6EOnM
compound.[3a,7] Purification as the impurities have a clear impact on
model to study the impact of chains on ag- is crucial to perform reliable aggregation the self-association of the triphenylene at
gregation and on the absorption properties study. We have therefore evaluated the very low concentration. Indeed we have
of the aggregates. The relation between
purity of the triphenylene derivative by also isolated by HPLC a ‘technical grade’
aggregation size and absorption proper- HPLC. TP6EO2M is discussed as a rep- TP6EO2M containing only a small amount
ties would be important in areas related to resentative example. After extensive col- of the second impurity and compared its
sensing and dyes chemistry and applica- umn chromatography on silica gel (eluent: aggregation in water with the ‘HPLC
tions, e.g. solar cells.
CHCl to CHCl /MeOH 5%), two impuri- grade’ TP6EO2M (see below).
Herein we have prepared a series of ties re3main in th3e product (Fig. 2).Analysis
TP6EOnM (n = 2, 3, 4) and investigated
of these impurities by mass spectrometry 2.2 UV-visible Absorption.
by UV-visible absorption the self-associa- revealed that the impurity at ~35 min is
The absorption spectra as a function of
tion of these three triphenylene derivatives the triphenylene with only five 2-(2-meth- concentration of TP6EO2M, TP6EO3M
in dilute aqueous solution (concentra- oxyethoxy)ethoxy chains. This impurity
tion between ~10–7 and ~4 × 10–4 M). We originates from incomplete substitution Fig. 3. The measurements were taken at
have found that the purity of the materials of the hexahydroxytriphenylene. The sec- 20 ºC. The spectra at the lowest concen-
strongly influences the aggregation at very ond impurity very close to the TP6EO2M
tration (~10–7 M) are virtually identical
and TP6EO4M in H2O are shown in
low concentration but not at higher con- is a triphenylene with a methoxy instead for the three compounds. By analogy
centration. Considering the aggregation of one 2-(2-methoxyethoxy)ethoxy chain. with hexa-alkoxytriphenylene derivatives,
process of such molecules as isodesmic It originates from an incomplete cleavage the main absorption band at 276 nm has
and using an exciton-coupling model to of the methoxy groups of hexamethoxy- been attributed to the allowed S ← S0
link the absorption of the solution to the triphenylene. We could fully remove these transition and the broad band at 3046 nm to
state of aggregation, we show that the impurities only by HPLC, which gave us the allowed S ← S transition.[6,8] As the
‘bonding energy’ between molecules in ‘HPLC grade’ materials. This purifica-
concentration 3increa0ses, the shape of the
the aggregates decreases as the length of
the chains increases. The main conclusion
of this work is that the extinction coeffi-
cient of solution of TP6EOnM (n = 2, 3,
4) will be similar when the size distribu-
tion of aggregates is similar, not when the
concentrations are the same. In the broader
context of dyes for solar cells, this would
explain the trends of increasing extinction
coefficient as the length of pendent alkyl
chains increases.
2. Results and Discussion
2.1 Synthesis
The series TP6EOnM was prepared
following literature procedures with slight
modifications. Briefly, trimerisation of ver-
atroleusingFeCl3 gave 2,3,6,7,10,11-hexa-
methoxytriphenylene, which is reacted
with HBr to provide 2,3,6,7,10,11-hexa-
hydroxytriphenylene. Reaction with the
required oligoethoxy alcohol activated Fig. 2. HPLC trace of TP6EO2M after column chromatography.
Fig. 3. UV-visible absorption spectra of increasing concentration in H2O of a) TP6EO2M from 1.02 × 10–7 to 3.99 × 10–4 M; b) TP6EO3M from 1.01 ×
10–7 to 2.86 × 10–4 M; c) TP6EO4M from 1.00 × 10–7 to 3.86 × 10–4 M.

522 CHIMIA 2015, 69, No. 9
Supramolecular chemiStry
spectra gradually changes because of the
formation of columnar aggregates of larger
size. In the aggregate, the chromophores
are interacting with neighbouring mole-
cules, which impact the properties of the
excited chromophore.
2.3 Aggregation
The plots of extinction coefficient as
a function of concentration are shown in
Fig. 4. To analyse the change of extinction
coefficient as a function of the concentra-
tion we used a known model of isodesmic
aggregation.[9] In short, the aggregation of
molecules A can be described by succes-
sive equilibria of the form An–1 + A1 A (n
⇌
n
= 2 to infinity), with equilibrium constants
of the form Kn = [An]/(K2K3…
K [A1]n). In
the simplest aggregation modenl–,1 all equi-
librium constants are equal, which means
that the energy of interaction between two
molecules, α, is independent of the size of
Fig. 4. Epsilon at 276 nm as a function of concentration in H2O of TP6EO2M (), TP6EO3M (),
and TP6EO4M (). Black lines are from the fits (see text). Inset: comparison between TP6EO2M
‘HPLC grade’ () and TP6EO2M ‘technical grade’ (see text) () between 0.1 and 8 mM. The
the aggregate. To link the distribution of
aggregates with the absorption properties
of the solutions, we have considered exci-
ton coupling theory as previously done for
other small molecules, including chromon-
black line for the latter has no physical meaning and is simply a guide for the eye.
Table 1. Fitting parameters for the extinction coefficient of solution of TP6EOnM ‘HPLC grade’ in
ics.[9c,10] The fits are plotted in Fig. 4 and
H2O as a function of concentration.
fitting parameters are summarised in Table
1. In this preliminary study, we focus on
b
α [kT]a
12.6 ± 0.2
β
ε
mono [103 M–1 cm–1]c
110 ± 2
The ‘bond energy’ is themofnroee energy
the ‘bond energy’, α, and the absorption
TP6EO2M
TP6EO3M 11.4 ± 0.2 –24600 ± 800 114 ± 1
TP6EO4M 9.3 ± 0.2 –29800 ± 1000 116 ± 1
–28600 ± 1000
coefficient of the monomer, ε
.
of association between two molecules; in
the isodesmic self-association considered
here, α is independent of the size of the aBond energy between two molecules in an aggregate. bCoupling constant of the exciton.
cExtinction coefficient of the monomer.
aggregate. For TP6EO2M, α was found to
be 12.6 ± 0.2 kT, which is similar to the
value found by NMR (14 kT) and by at-
–1
–1
omistic simulations (14.7 kT
using OPLS
similar epsilon value (~80,000 M cm ) explain the results obtained with the ‘tech-
as a 15 × 10 M solution of TP6EO4M (av- nical grade’ TP6EO2M, which still con-
–5
force field).[4d,5] The NMR study was per-
formed at 17 ºC, which is a slightly lower
temperature than our study (20 ºC) and
can explain our lower values. On going to
erage size of aggregates ~2). The approxi- tained minute amounts of the 2-methoxy-
mation made is that the coupling constant 3,6,7,10,11-pentakis(2-(2-methoxy-
β would not vary with a change of chain ethoxy)ethoxy)triphenylene impurity (see
TP6EO3M and TP6EO4M, α decreases to
11.4 and 9.3 kT respectively. Compared to
of the present study; as a simple note, the Comparing the extinction coefficient of
length. This aspect is outside the scope
HPLC trace in the Supplementary Data).
TP6EO2M, thelongerchainsofTP6EO3M
exciton coupling value is expected to vary this ‘technical grade’ TP6EO2M with the
and TP6EO4M allow for additional water
with the interdisc distance, which is an- ‘HPLC grade’ TP6EO2M at concentra-
ticipated to increase slightly as the length tions below 2 × 10–6 M (inset in Fig. 4), it is
molecules to interact with the oxygen at-
oms of the oligoethoxy chains, hence in-
creasing the hydrophilic character of the
of the chains increases by analogy with clear that the ‘technical grade’ TP6EO2M
hexa-alkoxytriphenylenes.[11]
has an abnormal behaviour as the epsilon
triphenylene.[4c,d] This means that, for a
Furthermore, it should be emphasised values are decreasing as the concentration
given concentration, as the length of the
chains increases, the distribution of aggre-
gates moves towards smaller average size
of aggregates.
that the impact of aggregate size distribu- decreases. The two grades of TP6EO2M
tion on the extinction coefficient is strong- behave the same at concentrations above 4
er as the concentration decreases. For × 10–6 M. These measurements are repro-
example, the average size of aggregates ducible over time using the same solutions
Importantly, the distribution of aggre-
gate size has a significant effect on the
extinction coefficient of the solutions: as
the average size of aggregates increases,
the extinction coefficient decreases. As a
for TP6EO2M increases from just above
and also when preparing new solutions
1 to ~1.1 molecule per aggregate on go- from the same batch. As the present im-
ing from ~10–7 to ~10–6 M. In this range purity has one methoxy group instead of
of concentration, the extinction coefficient one 2-(2-methoxyethoxy)ethoxy chain, it
significantly drops from ~120,000 M–1 L–1 will be less hydrophilic than TP6EO2M.
–1 –1
result, solutions of different TP6EOnM
to ~85,000 M L . This result can simply
The lower hydrophilicity will favour ag-
with similar extinction coefficient will be
obtained at different concentration but will
have similar distribution of aggregates. For
explain the trend in extinction coefficients gregation hence larger size aggregates
of series of organic dyes with increasing compared to the pure compound, which
pendent chains.[12]
leads to lower extinction coefficient. As
the concentration increases, the impurities
example, a 10–5 M solution of TP6EO2M
The impact of aggregate size distribu-
(average size of aggregates ~2) has a
tion on the extinction coefficient can also will be buried inside larger size aggregates

Supramolecular chemiStry
CHIMIA 2015, 69, No. 9 523
and less accessible to water molecules[4d] changes of average size of aggregates lead
so that the effect of lower hydrophilicity to a significant decrease of extinction co-
4.4 UV-visible Spectroscopy
TP6EOnM (n = 2, 3, 4) solutions
on aggregates size will become marginal.
efficient. In short, dyes with longer alkyl
were prepared in distilled water at a con-
centration from ~1 × 10–7 M to ~10–4 M.
UV-Visible spectroscopy was carried out
on a Cary 50 Spectrophotometer at a tem-
perature of 20 ± 1 °C. 0.1 cm, 1 cm (from
Starna scientific) and 5 cm (from Agilent
Finally it should be noted that the ex- chains have higher extinction coefficients
perimental extinction coefficients of the because, at a given concentration, they
~10–7 M solutions are the same for the three form smaller size aggregates than dyes
triphenylenes pointing to the existence of with shorter alkyl chains.
only the monomeric species at this concen-
tration. The drop in epsilon values is faster
Technologies) length path quartz cuvettes
than expected from the isodesmic self-as- 4. Experimental
were used. Absorption spectra were re-
corded from 200 to 400 nm. Molar extinc-
tion coefficient ε was taken at λ = 276 nm.
sociation model for concentrations up to
~3 × 10–7 M. This means that, at these con-
Detailed experimental methods and
centrations, there are larger size aggregates additional figures can be found in the
than anticipated by the model. We attribute Supplementary Data.
this observation to a larger ‘bond energy’
Acknowledgments
We thank the School of Chemistry,
University of Birmingham, for a studentship
and the EPSRC for underpinning support.
for the formation of the dimer as pointed 4.1 Materials
out by theoretical calculations[4d] but nev-
Dichloromethane, methanol, N,N'-
dimethylformamide, acetic acid and
er observed experimentally for TP6EOnM
Received: June 25, 2015
systems. Further studies at very low con- sulphuric acid were purchased from
centration are required to confirm this Fisher. Diethyl ether was purchased
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4.2 HPLC
We report the synthesis and careful pu-
Analytical chromatograms were re-
rification of a series of triphenylenes with corded on a Shimatdzu device using a
oligoethoxy peripheral chains of increas- Phenomenex Kinetex 5 µm, C18, 100 Å
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[6] a) R. Hughes, A. Smith, R. Bushby, B.
ing length, TP6EOnM with n = 2, 3, 4. We
column at a constant temperature of 35 °C
have studied by UV-visible absorption the and a flow rate of 1 mL/min. All samples
self-association of these triphenylenes in were filtered through a 0.45 µm Acrodisc®
dilute aqueous solution with concentra- syringe filter (Supplier: Pall Corporation)
tions between ~10–7 to ~4 × 10–4 M, a much before injection. Injection volume was
larger range of concentration than previ- 20 µL. Detection was made by UV-visible
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ously reported for TP6EO2M only. In this
absorption at a wavelength of 254 nm.
preliminary study, we analysed the aggre- Compound purities were calculated from
gation behaviour using a simple isodesmic the HPLC relative areas assuming that the
self-association model.
‘energy bond’ between two haviour as the compound.
TP6EO2M molecules is found to be ~12.6 Preparative HPLCs were done using a
kT, close to reported values for this system. Kinetex 5 µm C18 100A, AXIA Packed,
equipped with a Security Guard PREP
impurities have the same absorption be-
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The
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which leads to smaller size aggregate dis- were carried out at room temperature and
tribution for a given concentration as the at a flow rate of 10 mL/min. All samples
length of the chains increases. At concen- were filtered through a 0.45 µm Acrodisc®
trations ~10–7 M the experimental values syringe filter (Supplier: Pall Corporation)
of extinction coefficients depart from the before injection. Injection volume was 2
values expected by the isodesmic model. mL. Detection was made by UV-Visible
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[12] For two recent examples see a) Q.-Y. Yu, J.-Y.
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This would be the first experimental ob-
absorption at a wavelength of 254 nm.
Kuang, C.-Y. Su, J. Phys. Chem. C 2011, 115,
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servation of the higher energy of dimeriza-
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Han, X. Li, New J. Chem. 2015, 39, 3736.
[13] a) T. F. A. de Greef, M. M. J. Smulders, M.
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NMR spectra were recorded on a
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In the broader context of dyes for solar Brucker Avance III 400 (1H 400 MHz, 13C
cell applications, the results could provide 101 MHz) instrument and are reported rel-
a very simple explanation for the trends of- ative to the residual deuterated solvent as
ten observed that, at a given concentration, the internal standard (CDCl3: δ[1H] = 7.26
dyes with longer alkyl chains have higher ppm; δ [13C] = 77.16 ppm ; DMSO–d6 :
extinction coefficients. We would attribute δ [1H] = 2.50 ppm; δ [13C] = 39.52 ppm,
this mainly to the formation of aggregates 74.40 ppm). Coupling constants are ex-
of different size in solution. Indeed minute pressed in Hz.