Contrasting self-assembly and gelation properties among bis-urea- and bis-amide-functionalised dialkoxynaphthalene (DAN) π systems

Article abstract of DOI:10.1002/chem.201002208

Gelled together: Self-assembly and gelation studies of bis-urea- and bis-amide-functionalised dialkoxynaphthalene chromophores (DAN-U and DAN-A, respectively) revealed contrasting J- and H-type aggregation properties in solution, respectively (see scheme)

Full text of DOI:10.1002/chem.201002208

DOI: 10.1002/chem.201002208
Contrasting Self-Assembly and Gelation Properties among Bis-urea- and
Bis-amide-Functionalised Dialkoxynaphthalene (DAN) p Systems
Anindita Das and Suhrit Ghosh*[a]
Precise control over the spatial arrangement of various
functional chromophores[1] plays an important role in modu-
lating the inter-chromophoric interactions and consequently
the photophysical properties. The motivation to study these
properties stems from the desire to understand the effect of
molecular-level interactions on macroscopic properties be-
cause of the relevance in organic electronic device applica-
tions.[2] In the recent past, there have been many efforts
aimed towards developing suitable supramolecular strategies
for achieving long-range order in the self-assembled struc-
ture of various p systems, such as p-[3] and n-type[4] semicon-
ductors, push–pull-type chromophores,[5] and donor–accept-
or charge-transfer systems.[6] The design of the building
blocks in most of these systems ensure that self-assembly
can be facilitated by gaining the synergistic effect of several
non-covalent forces, such as p–p interactions, hydrogen
bonding, dipole–dipole interactions, and the hydrophobic
effect. In most cases, the role of hydrogen-bonding function-
alities has been restricted only to strengthen the propensity
for self-assembly, except for a few recent examples in which
the nature of the chromophore arrangements could be dras-
tically changed (H- vs. J-type aggregation) by varying the
peripheral functional groups[7] or the hydrogen-bonding
motif.[8]
action sites, they only differ in terms of providing differen-
tial stability to the self-assembled structure or are they also
capable of altering the mode of the chromophoric arrange-
ments? To answer this question, we have synthesised two
derivatives of the dialkoxynaphthalene (DAN) p system,
namely, DAN-U and DAN-A (Scheme 1), which differ only
in amide and urea functionalities. Herein, we report the con-
trasting solution self-assembly and gelation behaviour of
DAN-U and DAN-A and our studies related to understand-
ing the correlation between these two phenomena.
Firstly, we studied the self-assembly in solution by sol-
vent-dependent absorption spectroscopy. In a recent report
we have shown that DAN-A forms H-type p stacks in non-
polar medium such as methylcyclohexane (MCH)/CHCl3
(95:5).[9] However, DAN-U was found to be insoluble in
CHCl3 at room temperature and so to avoid ambiguity in
the comparative self-assembly studies in solution, herein we
have used a THF and MCH solvent composition.
In Figure 1a, we show the UV-visible absorption spectral
changes of DAN-A as a function of solvent composition.
With gradual addition of non-polar solvent MCH to the so-
lution of the chromophore in THF, there was almost no
change in the absorption spectra until around 80% MCH/
THF (v/v), which suggested no p stacking. Further addition
of MCH caused drastic changes in the absorption spectra;
the intensity of the major bands at 313 and 326 nm reduced
by ꢁ60% with a slight blueshift and the peaks became sig-
nificantly broader, suggesting formation of H aggregates
(Figure 1a, inset) as a result of hydrogen bonding and p
stacking. When similar experiments were performed with
DAN-U, to our surprise, we noticed distinctly different spec-
tral changes. In THF, which is a polar, hydrogen-bonding
competing solvent, the absorption spectrum was almost
identical to that of DAN-A, suggesting presence of mono-
meric chromophores. However, with an increasing amount
of MCH, the major absorption bands at 326 and 313 nm ex-
hibited strong hyperchromic shifts (absorption intensities at
326 nm increased by almost 200% from THF to 95:5 MCH/
THF) along a with small bathochromic shift of about 3 nm
for both peaks and the modified spectrum became more
Amide and urea are the two most common self-comple-
mentary hydrogen-bonding motifs and have been extensive-
ly used for various dye assemblies.[3–5] However, to the best
of our knowledge there have not been any studies to exclu-
sively compare the influence of these two functionalities in
deciding the chromophoric arrangements in the self-assem-
bled structure. The question we asked is whether, due to the
ꢀ
ꢀ
mismatch in number of hydrogen-bonding ( NH···O ) inter-
[a] A. Das, Dr. S. Ghosh
Polymer Science Unit
Indian Association for the Cultivation of Science
Kolkata, 700032 (India)
Fax : (+91)33-2473-2805
E-mail: psusg2@iacs.res.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002208.
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Scheme 1. Structure of the chromophores.
the UV-visible spectra of two extreme cases (monomeric
and fully aggregated) for both chromophores in Figure 2a.
It can be noted that in THF, in which both chromophores
remain as monomers, the absorption spectra (indicated by
dotted lines) are very similar in nature and the extinction
coefficients (e) for the lowest energy band at 326 nm are
6000 and 6900mꢀ1 cmꢀ1 for DAN-A and DAN-U, respective-
Figure 1. Solvent-dependent absorption spectra of DAN-A (a) and DAN-
U (b): Arrows indicate direction of spectral changes from THF to MCH.
Inset: schematic representation of the proposed mode of chromophore
assembly (hydrogen bonding is shown with a dashed bond). Concentra-
tion of the chromophore=0.1 mm and temperature=258C.
sharp (full width at half maxima reduced to 3 nm from
6 nm) compared with the monomeric spectrum in THF.
These spectral changes clearly suggest that DAN-U forms J
aggregates[10] in non-polar media wherein the chromophores
are arranged with a longitudinal displacement along the
long axis (Figure 1b, inset).
Figure 2. Solvent-dependent absorption (a) and emission (b) spectra
(lex =295 nm) of DAN-A (blue) and DAN-U (red). Dotted lines indicate
spectra recorded in THF and solid lines indicate spectra recorded in
MCH/THF (95:5). Concentration=0.1 mm, Temperature=258C.
Just to highlight the fact that the mode of self-assembly of
DAN-A and DAN-U are completely different, we compared
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S. Ghosh and A. Das
ly. Going from THF to MCH/THF (95:5) the two spectra
changed completely in opposite directions. There was a hy-
pochromic shift along with a blueshift (indicated by the blue
arrow in Figure 2a) for H-aggregated DAN-A, whereas a
hyperchromic shift along with a redshift (indicated by the
red arrow) for DAN-U was observed. As a result, the value
of e for the lowest energy band in the aggregated state was
found to be 13400mꢀ1 cmꢀ1 for DAN-U, which is astonish-
ingly high relative to that for DAN-A (2900mꢀ1 cmꢀ1).
The self-assembly of the two chromophores were also
probed by fluorescence spectra. In Figure 2b, the emission
spectra of the two chromophores were compared in the
monomeric (in THF) and aggregated (MCH/THF) states.
Going from THF to MCH, for DAN-U the emission intensi-
ty increased by 2.5 times with peak sharpening, which is a
well-known signature for J aggregates.[10] In contrast, the
fluorescence of DAN-A was found to be almost quenched
with peak broadening as a result of aggregation, which fur-
ther supports that the mode of self-assembly in these two
chromophores is significantly different.
Apart from the difference in amide and urea functionality,
the two chromophores also differ in the number of atoms
(n) in between the DAN chromophore and the 3,4,5-trial-
koxy benzene ring (n=4 and 5 for DAN-A and DAN-U, re-
spectively), which may influence in the packing of the two
chromophores due to the change in the conformational
angle between the two aromatic units. We chose these two
systems because we wanted to keep the number of methyl-
ene units the same between the DAN chromophore and the
hydrogen-bonding functionalities to avoid confusion regard-
ing the odd–even effect[11] of the spacer moiety. However, to
examine this effect we synthesised another amide-function-
alised control molecule, DAN-A’ (Figure 3)[12] in which n=
5. Solvent-dependant UV-visible studies (Figure 3) revealed
that going from polar (THF) to non-polar solvents (MCH),
there was a significant reduction (the intensity of the band
at 326 nm reduced to 57%) in the peak intensity in conjunc-
tion with peak broadening. These spectral variation are simi-
lar to those observed for DAN-A, and thus suggests similar
aggregation properties for both amide-containing chromo-
phores.
From this experiment, it appears that the difference in
number of atoms between the aromatic units is not responsi-
ble, at least directly, for the observed difference in the mode
of self-assembly between DAN-A and DAN-U. However,
that does not confirm that the amide versus urea functional-
ity is the only reason for contrasting self-assembly because
there are also other structural differences between these
two molecules that cannot be completely decoupled. For ex-
ample, the carbonyl functionality, involved in hydrogen
bonding, is not located in identical chemical environments
with respect to the aromatic units in both cases. In case of
DAN-A, it is the terminal unit that is directly linked to the
benzene ring, whereas that is not true for DAN-U. Thus, al-
though the experiments indicate the amide versus urea func-
tionality plays a crucial role in deciding the mode of self-as-
sembly, one cannot completely rule out the possibility of ad-
ditional influences of other structural differences as de-
scribed above on the contrasting self-assembly between
these two molecules.
Now we estimated the propensity for self-assembly in
these two systems by calculating the mole fraction of aggre-
gate (aagg) values as a function of solvent composition from
the variation in their absorption spectra (Figure 1a and b)
by using Equation (1):[7]
AmixꢀAmon
aagg
ꢁ
ð1Þ
AaggꢀAmon
The absorbance at 326 nm at the lowest and highest
MCH/THF ratios were taken as the values for Amon and
Aagg, respectively. Amix is the absorbance at 326 nm at a
given solvent mixture. It can be clearly seen in Figure 4 that
for DAN-U the aggregate formation starts at a much lower
MCH/THF ratio, suggesting the relatively high tendency to
form aggregates compared with DAN-A. From these plots
50
agg
the a (solvent composition at which aagg =0.5) values were
Figure 3. Solvent-dependent (c: THF, g: MCH/THF (95:5)) absorp-
tion spectra of DAN-A’. Concentration of the chromophore=0.1 mm and
temperature=258C.
&
Figure 4. Plot of aagg as a function of solvent composition for DAN-U ( )
!
and DAN-A ( ).
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Self-Assembly and Gelation Properties
COMMUNICATION
estimated to be 68 and 88% MCH/THF for DAN-U and
DAN-A, respectively. This significant difference was attrib-
uted to the stronger hydrogen-bonding interaction in the
urea functionality than in the amide.
To ascertain that hydrogen bonding is indeed responsible
for self-assembly, we examined the effect of MeOH, a hy-
drogen-bonding competing solvent, in the absorption spectra
of both chromophores (Figure 5). It was observed that even
observed difference may not only be due to stronger hydro-
gen bonding, but the slipped geometry of the chromophores
in the J-type assembly could also contribute to less electro-
static repulsion among the adjacent chromophores com-
pared with that for the face-to-face p stacking in the H-type
assembly.
Self-assembly often leads to gelation at higher concentra-
tions.[15,3–6] Gelation with chromophoric building blocks such
as ours provides the opportunity to study self-assembly sep-
arately by spectroscopic methods at a concentration below
the critical gelation concentration (CGC) and understand
the effect of the nature of inter-chromophoric interactions
on gelation.[16] Thus, we were curious to know the effect of
the remarkable differences in the self-assembly of DAN-U
and DAN-A on their gelation properties. Gelation of these
two chromophores was studied in ten common organic sol-
vents of different natures and the results are presented in
Table 1. J-aggregated DAN-U was found to be an excellent
Table 1. Gelation data for DAN-A and DAN-U.[a]
Solvent
DAN-U CGC [wt%] DAN-A CGC [wt%]
toluene
G
G
G
G
GP
G
G
G
G
P
0.06
0.07
0.05
0.32
–
0.40
0.30
0.29
1.20
–
G
S
S
G
G
S
S
G
S
1.39
–
–
0.46
1.14
–
–
0.76
–
benzene
1,2-dichlorobenzene
MCH
cyclohexane
TCE[b]
CHCl3
CCl4
THF
dioxane
S
–
[a] G=gel, GP=gel-like precipitate, P=precipitate, S=soluble.
[b] TCE=tetrachloroethylene.
Figure 5. Effect of gradual addition of MeOH on the absorption spectra
of the self-assembled structure of DAN-A (a) and DAN-U (b) in 95%
MCH/THF. Concentration=0.1 mm, Temperature=258C.
gelator for various types of solvents, such as aliphatic
(MCH), aromatic (toluene, benzene, 1,2-dichlorobenzene),
chlorinated (chloroform, TCE, tetrachloromethane) and
even hydrogen-bonding solvents (THF). The CGC values in
all of the solvents were found to be very low (<1.0 wt%
except for THF) and particularly in aromatic solvents[17]
they were found to be extremely low (<0.1 wt%). It is note-
worthy that gelators with such low CGC values are often
called super-gelators[18] to emphasise their extraordinary ge-
lation ability.
in the presence of very little MeOH, the assembled structure
was completely destroyed and the monomeric spectrum
could be recovered. For DAN-A the monomeric spectrum
could be regenerated in the presence of only about 1% (v/
v) MeOH in 95:5 MCH/THF, whereas to obtain the same
for DAN-U, about 11% (v/v) MeOH was required. These
differences corroborate well with the conclusion drawn from
In contrast, H-aggregated DAN-A showed gelation in
only a few cases of all tested solvents (Table 1). Such con-
trasting gelation properties can be attributed to the vast dif-
ferences in the nature (J- vs H-aggregation) and the propen-
sity for self-assembly among DAN-U and DAN-A. Please
note that strikingly similar results for J- and H-aggregated
perylenebisimide (PBI) organogelators have been recently
reported by Wꢁrthner and co-workers.[19] To compare the
thermal stability of the gels, we determined the gel-to-sol
melting temperatures for DAN-U and DAN-A in MCH at a
fixed concentration (4.0 mm) and they were found to be 94
and 598C, respectively, which clearly revealed much higher
thermal stability in the former case. The morphology of the
50
the observed differences in their a values and clearly sug-
agg
gest stronger self-assembly for J-aggregated DAN-U.
The stabilities of these two types of self-assembled struc-
tures could also be compared by variable-temperature UV-
visible studies.[13] For DAN-A, the aggregated spectrum
could be reversibly converted to the monomeric species at
relatively low temperatures (408C), suggesting a very weak
nature of the assembly, which is in accordance with the ob-
servations made by Iverson and Cubberley for DAN chro-
mophores.[14] However, for J-aggregated DAN-U, there was
negligible change in the absorption spectra even at 658C, re-
iterating the fact that stronger self-assembly occurred. The
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S. Ghosh and A. Das
self-assembled fibres responsible for gelation was examined
by AFM studies. The AFM images of DAN-A and DAN-U
are shown in Figure 6. In both cases the formation of micro-
Figure 6. AFM height images of DAN-A (top) and DAN-U (bottom) in
90% MCH/THF (concentration=0.05 mm). In the right-hand side the
cross-sectional analyses from a–b are shown for the respective images.
Figure 7. Rheological data for DAN-A (a) and DAN-U (b) gels in MCH.
*
*
: G’, : G’’. Concentration=3 wt%, and temperature=258C. Insets
show respective gel (4.0 mm) pictures. In 7a, for G’ no points are visible
after the yield stress because the value became negative and thus in the
log plot it is not visible.
meter long fibres is clearly visible, as commonly observed
for gelation. The heights of the smallest fibres were found to
be 3.3 and 4.3 nm, for DAN-U and DAN-A, respectively.
From a cursory observation it appears that the extent of
cross-linking among the DAN-U fibres is less than those for
DAN-A.
We also examined the flow behaviour of the two types of
gels by comparing their rheological properties (Figure 7).[20]
It can be seen for both systems that initially G’ (storage
modulus) was higher than G’’ (loss modulus), suggesting the
presence of a gel phase. With a gradual increase in applied
stress, both G’ and G’’ remained invariant and then beyond
a certain stress they deviated from linearity.
The stress at which deviation occurred is defined as the
yield stress and gels begin to flow above this point.[20] The
yield stress for DAN-A and DAN-U were estimated to be
11.7 and 2.6 Pa, respectively. The ratio of G’/G’’ was also
found to be higher for DAN-A (11.2 Pa) than that for
DAN-U (6.7 Pa). To confirm that this result is not just one
exception, we also compared the rheological data for both
gels in another solvent, toluene, and even in this case similar
trends were observed.[13] Thus, the rheological data clearly
revealed better mechanical stability for DAN-A gels, which
contradicts the rest of the observations in which we have
shown stronger self-assembly, more versatile gelation and
higher thermal stability for the DAN-U gelator. To under-
stand the reason behind this apparent anomaly, we proposed
the following hypothesis, which is shown in Figure 8: The
building block self-assembles to generate elongated fibres
(step 1), which at higher concentrations entangle and trap
the solvent molecules to show gelation.[13f] Various physical
properties of the gel thus depend on both the strength of
self-assembly as well as the nature of the self-assembled
fibres. Since DAN-U has much stronger self-assembly prop-
erties than DAN-A, it forms elongated fibres in various sol-
vents at lower concentrations and also the thermal stability
is much higher.
Thus gelation in this case happens in a wide range of sol-
vents with higher thermal stability and lower CGC values.
However, due to strong hydrogen-bonding interaction, the
DAN-U fibres are expected to be stiffer and thus cannot un-
dergo extensive entanglement.[21] On the other hand, DAN-
A forms more coil-like flexible fibres due to weaker hydro-
gen-bonding interaction and thus formation of an entangled
network structure is more facile in this case. Consequently,
with increasing stress it is easier to disintegrate the compa-
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Self-Assembly and Gelation Properties
COMMUNICATION
confirm that the observed in-
crease in the anisotropy values
for DAN-U is indeed due to
self-assembly, we added MeOH
to the solution and found a
sharp decrease in the value
(Figure 9, inset). Previously, we
have shown that in the presence
of MeOH disassembly occurs
(Figure 5b) and thus this ex-
periment undoubtedly demon-
strates that the increase in the
anisotropy is indeed due to the
self-assembly.
In conclusion, we have dem-
onstrated distinct differences in
the mode of self-assembly be-
tween bis-amide- and bis-urea-
Figure 8. Schematic presentation of the proposed hypothesis regarding the steps of gelation for the two sys-
tems.
ratively less-entangled DAN-U gel network than that of
DAN-A.
In an attempt to correlate this hypothesis with molecular-
level interactions, we carried out solvent-dependent fluores-
cence anisotropy studies[22] for both systems (Figure 9). It
functionalised DAN chromophores. The urea-functionalised
system formed J aggregates in solution with a superior self-
assembling propensity and thermal stability than the H-ag-
gregated amide-functionalised system. The contrasting self-
assembly behaviour resulted in significantly different gela-
tion properties for the two systems. The urea-functionalised
system showed gelation in versatile organic solvents with
much greater thermal stability and lower CGC, whereas the
amide-functionalised DAN building block formed gels with
higher mechanical stability. The observed anomaly was ex-
plained by comparing the difference in rigidity of the two
types of self-assembled fibres. To the best of our knowledge,
such a drastic difference in self-assembly (H vs. J aggregate)
of amide- and urea-functionalised chromophores are not
known.[23] We believe that the present study, which aims to-
wards understanding the relationship between molecular
structure and macroscopic properties, will add valuable in-
formation to the ongoing effort to study the supramolecular
assembly of various other functional p systems in the con-
text of organic electronic device applications.[24] Currently,
we are exploring the utility of this structural variation in
tuning the self-assembling properties of other DAN-related
chromophores[25] and also other types of functional p sys-
tems.
Figure 9. Solvent-dependent (MCH/THF) fluorescence anisotropy studies
&
*
of DAN-A ( ) and DAN-U ( ) at 258C. Inset: effect of MeOH on the
anisotropy values of DAN-U at 95:5 MCH/THF. Concentration=0.1 mm,
lex =295 nm, lem =344 nm.
can be seen that for both DAN-U and DAN-A, the aniso-
tropy values are very close to zero in THF, suggesting the
presence of freely tumbling monomeric chromophores.
However, when the relative amount of MCH was increased
gradually, beyond about 65% MCH/THF, the anisotropy
value for DAN-U started increasing significantly and finally
reached a value of 0.11. It is noteworthy that the inflection
point in this plot is almost identical to that observed for the
solvent dependent aagg plot (Figure 4), which indicates that
the enhanced anisotropy is directly related to self-assembly.
For DAN-A there was also a certain increase in the aniso-
tropy value beyond a certain solvent composition. However,
the significantly higher anisotropy value (0.11) of DAN-U
than DAN-A (0.03) clearly suggests a more rigid chromo-
phore assembly in the former case than the later. To further
Acknowledgements
We thank the Department of Science and Technology (DST), New Delhi,
India, for financial support (project no. SR/FT/CS-039/2008). We also
thank Anshupriya Shome and Prof. P. K. Das, Biological Chemistry De-
partment, IACS, Kolkata, for the fluorescence anisotropy measurements.
A.D. thanks CSIR, India, for a research fellowship.
Keywords: dialkoxynaphthalene · gels · hydrogen bonds ·
organogels · self-assembly
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S. Ghosh and A. Das
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Received: August 2, 2010
Published online: November 9, 2010
13628
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