Healable luminescent self-assembly supramolecular metallogels possessing lanthanide (Eu/Tb) dependent rheological and morphological properties

Article abstract of DOI:10.1021/ja511799n

Herein we present the use of lanthanide directed self-assembly formation (Ln(III) = Eu(III), Tb(III)) in the generation of luminescent supramolecular polymers, that when swelled with methanol give rise to self-healing supramolecular gels. These were analyzed by using luminescent and ¹H NMR titrations studies, allowing for the identification of the various species involved in the subsequent Ln(III)-gel formation. These highly luminescent gels could be mixed to give a variety of luminescent colors depending on their Eu(III):Tb(III) stoichiometric ratios. Imaging and rheological studies showed that these gels prepared using only Eu(III) or only Tb(III) have different morphological and rheological properties, that are also different from those determined upon forming gels by mixing of Eu(III) and Tb(III) gels. Hence, our results demonstrate for the first time the crucial role the lanthanide ions play in the supramolecular polymerization process, which is in principle a host-guest interaction, and consequently in the self-healing properties of the corresponding gels, which are dictated by the same host-guest interactions.

Full text of DOI:10.1021/ja511799n

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Article
Healable luminescent self-assembly supramolecular metallogels possessing
lanthanide (Eu/Tb) depended rheological and morphological properties
Miguel Martínez-Calvo, Oxana Kotova, Matthias E. Mobius, Alan P
Bell, Thomas McCabe, John J Boland, and Thorfinnur Gunnlaugsson
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja511799n • Publication Date (Web): 15 Jan 2015
Downloaded from http://pubs.acs.org on January 19, 2015
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Healable luminescent self-assembly supramolecular metallo-
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morphological properties
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‡†
1‡
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1
Miguel Martínez-Calvo , Oxana Kotova , Matthias E. Möbius , Alan P. Bell , Thomas McCabe ,
3
1*
John J. Boland , Thorfinnur Gunnlaugsson
1
School of Chemistry and Trinity Biomedical Sciences Institute, University of Dublin, Trinity College Dublin,
Dublin 2 (Ireland)
2
Sami Nasr Institute of Advanced Materials (SNIAM), School of Physics, University of Dublin, Trinity College
Dublin, Dublin 2 (Ireland)
3
Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) and School of Chemistry, University of Dub-
lin, Trinity College Dublin, Dublin 2 (Ireland)
KEYWORDS supramolecular gels, self-assembly, healing, lanthanides
ABSTRACT: Herein we present the use of lanthanide directed self-assembly formation (Ln(III) = Eu(III), Tb(III)) in the genera-
tion of luminescent supramolecular polymers, that when swelled with methanol give rise to self-healing supramolecular gels. These
1
were analysed by using luminescent and H NMR titrations studies, allowing for the identification of the various species involved
in the subsequent Ln(III)-gel formation. These highly luminescent gels could be mixed to give a variety of luminescent colours
depending on their Eu(III):Tb(III) stoichiometric ratios. Imaging and rheological studies showed that these gels prepared using only
Eu(III) or only Tb(III) have different morphological and rheological properties, that are also different to those determined upon
forming gels by mixing of Eu(III) and Tb(III) gels. Hence, our results demonstrate for the first time the crucial role the lanthanide
ions play in the supramolecular polymerization process, which is in principle a host-guest interaction, and consequently in the self-
healing properties of the corresponding gels, which are dictated by the same host-guest interactions.
cepts (such as hydrogen bonding and π−π interactions) with
metal coordination, by employing ligands that can bind to
INTRODUCTION
Supramolecular self-assembly formation of soft materials
such as gels have attracted the attention of researchers across
various disciplines due to the rich potential they can provide
lanthanides (Ln) and transition metal ions. Using lanthanide
ions, themselves possessing unique and highly desirable phys-
1
7
ical properties, we have demonstrated the formation of lumi-
nescent metallogels, where the coordination of ions such as
Eu(III), not only gives rise to unique luminescent properties,
but also, due to high coordination requirements of the Eu(III)
ions, aid in the formation of 3D structures with mechanically
enforced properties not seen for corresponding organogels (i.e.
1
–4
within different fields of applications.
The formation of
such materials is controlled by the interplay of self-
complementary and intermolecular supramolecular interac-
tions; these are further enforced through the generation of a
5–8
multidimensional matrix-structure. This makes the design of
18
new classes of small molecules that can give rise to gelation
with targeted properties often highly challenging as minor
modifications can greatly affect the macroscopic properties of
the resulting material in an unexpected manner. To date, the
generation of soft matter has been somewhat dominated by the
use of organic gelators which have been mainly serendipitous-
ly discovered. However, recently the concept of multi-
component supramolecular gels has been introduced, where,
for instance, ions in conjunction with gelators based on metal
in the absence of the ions). We also showed that upon aging,
these metallo(hydro)gels could be used as a platform for grow-
ing unique nano-structures, not previously seen, such as NaCl
1
9
and KCl nanowires. The concept of self-healing of soft mat-
ter is a current topical area of research within supramolecular
1
4,20–22
chemistry.
Through the use of molecular recognition
strategies (e.g. host-guest chemistry), self-healing supramo-
lecular polymers and gels have recently been formed and stud-
2
3,24
ied.
Capitalizing on the luminescent properties of the lan-
9
coordinating ligands have been used in order to gain better
thanides within supramolecular architectures has been of par-
2
5–28
control over structure (and hence, morphology), physical
ticular interest to our research endeavour.
With this in
9–11
properties and applications, etc. of such soft-materials.
This
mind, we set out to explore the potential of using lanthanide
ions as ‘guests’ in the generation of such self-healing soft-
matter in combination with structurally simple and robust or-
ganic polydentate ligands, functioning as ‘hosts’. As alluded to
above, the lanthanides have unique physical properties
concept has recently been elegantly demonstrated by Steed,
12–16
Maitra, Rowan, Yam, De Cola and co-workers,
to name
just a few. In recent work we have generated luminescent su-
pramolecular gels by combining classical self-assembly con-
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Scheme 1. Self-assembly formation of (A) Ln:H
2
L with the following formation of higher order metallo-supramolecular
polymers (B) and (C) structural formula of H L.
2
1
7
stemming from the f-electrons. Generally, Eu(III) and Tb(III)
will have similar coordination properties, while properties
such as luminescent efficiency, lifetimes, etc. can vary be-
Base hydrolysis of the protected 4 followed by adjusting the
pH to 2, resulting in the formation of H L in 82% yield, which
was fully characterised (see SI). X-ray quality crystalline
prisms were also obtained by slow evaporation of H L from
2
2
9,30
tween complexes formed from the same ligands.
Further-
2
more, we, and others, have demonstrated that the stability con-
ethanol under ambient conditions, which further confirmed the
formation of the desired product. The long-range interactions
in solid state consisted of both intermolecular and intramo-
lecular hydrogen bonding as well as π-π stacking interactions
stant for the formation of ternary complexes involving these
31,32
ions can often differ.
We thus proposed that these affinity
differences could potentially be exploited with a view of mod-
ulating the properties of metallo-supramolecular polymers and
metallogels such as their morphology and rheology. Herein,
we show that such a design strategy is achievable, using the
3
6,37
(see SI).
2
Thermogravimetric analysis (TGA) on H L (see
SI) showed a weight loss, assigned to loss of water molecules
form the solid sample of H L, occurred until ~100 °C after
2
pyridine-2,6-dicarboxylic acid (H
2
dpa) derivative H
2
L (4,4'-
which the compound remained stable until 260 °C, at which
(((pyridine-2,6-dicarbonyl)bis(azanediyl))bis(methylene))-
dibenzoic acid), Scheme 1 and Supporting Information (SI).
The ligand was designed to facilitate the initial formation of
three dimensional self-assembly units in either 1:2 or 1:3
temperature a decomposition started. The reaction of H L with
2
Ln(CF SO ) (Ln = Eu, Tb) in 3:1 stoichiometry in methanol
3
3 3
solution under microwave irradiation (75 °C, 20 min) resulted
in the formation of lanthanide complexes Ln(H L) (where Ln
= Eu or Tb; n= 2.50 and 2.05, respectively) as white powders
after precipitation out of CH OH using diethyl ether diffusion
see SI). The overall quantum yield of the lanthanide-centred
2
n
3
1
32–35
(
Ln:H
demonstrated the self-assembly of such H
luminescent ‘bundles’. We foresaw that these initial Ln:H
2
L) stoichiometries; in the past we and others
have
2
dpa ligands into
3
2
L
(
stochiometries would then form larger self-assemblies through
the formation of higher order metallo-supramolecular poly-
mers or gels by cross-linking of the initially pre-organised
complexes (via the terminal para carboxylic acids groups)
upon addition of further equivalents of Ln.
luminescence obtained for both Ln(III) complexes in methanol
ꢀꢁ
upon the ligand excitation (ꢀꢀ , %) is 4.9±0.1 % for the
38
Eu(III) complex and 7.2±0.5 % for the Tb(III) one.
The hydration state (the number of metal-bound water mol-
ecules or the q-value) of both complexes was determined by
We demonstrate that such gels can be formed reproducibly
and are luminescent (giving rise to red (Eu(III)) or green
Tb(III)) emission depending of the lanthanide ion used), and
possess self-healing properties. Moreover, we show that the
mechanical mixing of the same volumes of these two lantha-
nide gels results in the formation of a mixed gel with orange
emission, which can also self-heal, possessing unexpectedly
different rheological properties compared to ‘pure’ Eu(III) and
Tb(III) gels indicating that the self-recognition processes in
the individual gels is different to that of the mixed systems.
recording the lanthanide centred excited state lifetimes in both
39
2 2
H O and D O, respectively, upon excitation at 275 nm. In the
(
case of Eu(III) the excited state decay was best fitted to bi-
exponential decay (H O: τ = 0.306±0.003 ms (25%) and
= 1.21±0.01 ms (75%); D O τ = 1.10±0.02 ms (25%) and τ
2.72±0.01 ms (75%)) giving hydration states of q = 2.5±0.5
2
1
τ
2
2
1
2
=
1
and q
(
(
2
= 0.3±0.5, while for Tb(III) (H
25%) and τ = 1.48±0.03 ms (75%), D
25%) and τ = 2.12±0.02 ms (75%)) giving q
= 0.7±0.5. These results indicate the formation of both
Eu(H L) and Tb(H L) in solution as a major component with
the possible minor contribution from a second species, namely
Ln(H L) (H O) , the formation of which can be explained by
the dissociation process in the following manner: Ln(H L)
Ln(H L) (H O) + H L. These complexes were soluble in a
variety of solvents such as CH Cl , CHCl , CH CN, CH OH
2
O: τ
1
= 0.63±0.04 ms
1
= 0.97±0.03 ms
1
= 2.5±0.5 and
2
2
O τ
2
q
2
RESULTS AND DISCUSSION
2
3
2
3
Synthesis and structural characterisation of H
corresponding Eu(III) and Tb(III) complexes: The synthe-
sis of ligand H L was performed in three steps from commer-
2
L and its
2
2
2
3
2
3
→
2
2
2
2
3
2
cially available 4-(aminomethyl)benzoic acid 1 (see SI), by
converting it into the ester 2 and coupling the resulting product
with 2,6-pyridine-dicarboxylic acid 3 in 2:1 stoichiometry.
2
2
3
3
3
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–
6
Figure 1. Changes in (A) the Eu(III) centred emission spectra upon titrating H
2
L (7.80·10 M) with Eu(CF
B) experimental binding isotherms (•••) and their corresponding fits (−) for the titration with Eu(CF SO ; (C) changes in the Tb(III)
(0→6 equivalents), (D) experimental binding isotherms
in methanol at 298 K.
3 3 3
SO ) (0→6 equivalents),
(
3
3 3
)
–
6
centred emission spectra upon titrating H
2
L (7.70·10 M) with Tb(CF
SO
3 3 3
SO )
(•••) and their corresponding fits (−) for the titration with Tb(CF
3
3 3
)
and did not give rise to the formation of soft matter in these
solvents.
crease for both systems until the addition of ~0.75 equivalents,
after which the emission grew steadily. The appearance of the
strong contribution from the ΔJ = 0 band in the Eu(III) emis-
sion spectra would indicate that the 1:3 complex existed dom-
Photophysical analysis of the self-assembly formation
between H L and Tb(III) or Eu(III) in solution: The focus
2
here will be mainly on the identification of the species in-
volved in the gelation process reported in this work (see dis-
cussion below). We performed the self-assembly studies in the
1
7
3
inantly in C symmetry.
The changes in the ground and excited states for both titra-
tions were analysed by fitting the data using the nonlinear
regression analysis program SPECFIT. Factor analysis of the
overall changes observed from these titrations suggested the
formation of four species and the spectroscopic data changes
were satisfyingly fitted to 1:1, 1:2, 2:2 and 3:2 stoichiometries
solution between H
CH OH at low concentration. The changes in the absorbance
and luminescence spectra were monitored upon addition of
Ln(CF SO to H L solution in methanol (see SI). The ab-
sorption spectrum of H L in CH OH consisted of two main
bands centred at 235 nm (ε235 = 32070 M cm ) and a longer-
2 3
L and Ln(III) in both CH CN (see SI) and
3
3
3
)
3
2
2
3
of Ln:H L (see SI), being unable to get reliable fit for the
formation of 1:3 which is clearly also formed, Figure 1B and
2
–
1
–1
–
1
–1
wavelength band centred 275 nm (ε275 = 6935 M cm ) with a
D. In the case of the absorption data, it is necessary to take
into account the absorbance of the ligand itself, as it was not
possible to determine the difference between the presence of
1:1 and 2:2 species and thus 1:1 was omitted from the binding
model. The binding constants found by fitting the changes in
absorption spectra for the equilibria with the Eu(III) ion were
found as follows: logβ1:2 = 12.2±0.2, logβ2:2 = 18.9±0.4,
logβ3:2 = 25.2±0.4. At the same time very similar values were
found for the Tb(III) system: logβ1:2 = 12.4±0.4, logβ2:2
9.7±0.6, logβ3:2 = 24.0±0.6. Moreover, analysis of the lantha-
nide centred emission enabled the successful distinction be-
tween 1:1 and 2:2 stoichiometries for Eu(III), with logβ1:1
.7±0.1, logβ1:2 = 12.6±0.1, logβ2:2 = 18.5±0.1, logβ3:2
–
1
–1
shoulder at 285nm (ε285 = 4096 M cm ). The addition of
Eu(III) or Tb(III) to the solution of H L in methanol gave rise
2
to changes in the absorption spectra that were to a great extent
identical, where hyperchromism and a red shift were observed
for the 235 nm transition (see SI), while the bands at 275 nm
and 285 nm remained mainly unchanged. The evolution of the
Eu(III) and Tb(III)-centred emissions were monitored as well
upon excitation at 275 nm (Figure 1). Here the intensity in-
creased for both systems upon addition of the metal ions
demonstrating successful population of the excited states of
=
1
these ions upon coordination to H
2
L; the characteristic emis-
=
=
sion bands appeared at 580 nm, 595 nm, 616 nm, 650 nm and
6
2
5
7
695 nm for Eu(III) (assigned to D
0 J
→ F (J = 0–4) transitions),
3.3±0.1. Similar binding constants were obtained for the
Tb(III) assemblies: logβ1:1 = 6.6±0.1, logβ1:2 = 11.6±0.1,
logβ2:2 = 18.7±0.2, logβ3:2 = 22.7±0.2 (see SI).
while for Tb(III) these bands appeared at 490 nm, 545 nm, 583
5
7
4 J
nm and 620 nm (assigned to D → F (J = 6–3)), Figure 1. For
both the emission was gradually enhanced until the addition of
~0.3 equivalents of Ln(III). This was followed by a slight de-
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Figure 2. Eu(III) and Tb(III) gels (A) in day light and (B) their luminescence under UV light. (C) Luminescence of Eu(III), Tb(III) and
Eu(III)/Tb(III) gels on quartz plates. (D) Eu(III), Tb(III)- and (E) Eu(III)/Tb(III) mixed gels luminescence spectra at 25 °C
(λ
ex = 275 nm). (F–I) Healing experiment of Eu(III) gel where (F) is Eu(III) gel in the day light, (G) the same gel under UV light, (H)
gel after being cut in half and (I) self-healing properties of the gel (scale bars, 1cm).
For comparison, the titrations were also repeated in CH CN the binding mechanism between H
solution; here the solvent is of similar polarity to CH OH, but
3
2
L and Ln(III) ions and for
3
the purposes of elucidating the numbers of species involved in
1
aprotic and as such less competitive, also the lack of O–H
oscillators would result is less quenching. Here, absorption
the self-assembly process at higher concentrations, a H NMR
titration (400 MHz, CD
H
2
3
OD) was carried out (see SI) using
L (c = 1.1×10 M) and the diamagnetic ion La(III) (from
La(CF SO ). The results (see SI) demonstrated that even
–
1
–1
–3
bands were centred at 235 nm (ε235 = 65811 M ⋅cm ) and 275
–
1
–1
nm (ε275 = 13244 M ⋅cm ) with a shoulder 285 nm
3
3 3
)
–
1
–1
(
ε
285 = 7028 M ⋅cm ). Upon the addition of Ln(III), signifi-
cantly greater changes occurred to these seen above for
CH OH, where all the transitions were greatly affected up to
upon the addition of 0.10 equivalents of La(III), significant
broadening and downfield shifts were observed for all the
ligand protons (the aromatic resonances appearing at 7.45,
7.98, 8.20 and 8.34 ppm), signifying the self-assembly for-
mation, which is best explained as being in a fast exchange on
the NMR timescale. The overall titration profile can be viewed
in two parts; initially, between the addition of 0.1→0.3
equivalents of La(III), the protons assigned to the pyridyl unit
became particularly broadened, being shifted upfield, while
the protons of the benzoic acid were slightly shifted down-
field, and only became significantly broadened upon the addi-
tion of 0.3 equivalents. Between the addition of 0.3→1.0
equivalents of La(III), the overall changes were best explained
as being in slow exchange; where the pyridyl resonances be-
came more broadened and disappeared with growing in of new
broad signals assigned to the pyridyl protons further downfield
(Δδ ~ 0.5 ppm). Concomitantly, minor but clear changes were
seen for the benzoic acid protons (Δδ ~ 0.05 ppm). Further
additions of La(III) resulted in the formation of a spectrum
(after ca. 1.1 equivalents) of resolved signals for all the aro-
matic resonances, centred at 7.53, 8.03, 8.58 and 8.60 ppm.
During the titration, methylene bridge protons were in slow
exchange, being shifted from 4.71 to 4.92 ppm from 0→2
equivalents of La(III). These overall changes clearly indicate a
stepwise formation of a self-assembly, where the initial for-
mation of the 1:3 stoichiometry occurs, followed by formation
of a higher order self- assembly, involving the terminal car-
boxylic groups which consequently shifts the equilibrium to-
wards the formation of polymeric species. As such, these re-
sults confirm what was seen in solution at lower concentration
in the absorption and the emission titrations above.
3
the addition of ~0.3 equivalents, before plateauing (see SI) and
signifying the effect of the media on the self-assembly for-
mation, clearly demonstrating the formation of the expected
1
:3 stoichiometry. These changes in the absorption spectra
were subjected to non-linear regression analysis. Factor analy-
sis supported the formation of self-assembles, with stoichi-
ometries of 1:1, 1:2 and 1:3 (Ln:H
termined. It should be noted that H
2
L) (see SI) all being de-
L does not show any ki-
2
netic inertness towards lanthanide ions in solution compared to
the similar L8 ligand which has no carboxylic groups in its
3
2
40
structure. The stability constants for Eu(III) were deter-
mined as logβ1:1 = 6.7±0.1, logβ1:2 = 14.2±0.1 and logβ1:3
1.0±0.1, while for Tb(III) constants of logβ1:1 = 6.9±0.1,
logβ1:2 = 13.8±0.2 and logβ1:3 = 18.7±0.5. These results com-
plement those observed in CH OH quite well. For both, the
=
2
3
Eu(III) and Tb(III) centred emissions were also ‘switched on’
demonstrating the formation of the lanthanide directed self-
assembly in solution; with emission spectra similar to that
28,41
seen in CH
3
OH.
The analysis of the Ln(III)-centred emis-
sion changes also allowed us to determine the binding con-
stants (see SI). Moreover, here the 1:3 stoichiometry was ob-
served unlike in our analyses in CH OH.
3
2 3 3 3
NMR studies of H L with La(CF SO ) : The presence of
the 1:1 and 1:2 species, was confirmed by mass-spectrometry
studies of the complexes (see SI) while the 1:3 could not be
detected. However, the determination of the q values above
indicated the presence of both the 1:3 and the 1:2 species in
the solution, in the ratio of 75:25. With a view to understand
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chiometries through bridging of the terminal carboxylic acid
groups, as demonstrated schematically in Scheme 1. This in-
1
2
3
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8
9
volved preparing a ~10 mM solution of H
addition of the corresponding Ln(CF SO
in a 1:3 Ln:H
2
L followed by the
(Ln = Eu and Tb)
L stoichiometry under microwave irradiation at
3
3
)
3
2
75°C for 20 minutes (see SI), followed by cooling the solution
to room temperature. To this solution, various stoichiometric
ratios of lanthanide salts (relative to Ln(H
resulting in the formation of clear solutions in all cases, except
for Ln(CH COO) . Using Ln(CH COO) the formation of a
soft gel-like precipitate was observed at various (Ln:(H L)
Ln(CH COO) ) stoichiometries after these solutions were
subjected to further microwave irradiation at 75 °C Analysis of
the stoichiometric dependences of Ln(CH COO) , showed that
2 3
L) ) were added,
3
3
3
3
2
3
:
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60
3
3
3
3
upon full coordination (i.e. each of the six carboxylates coor-
dinate to a single Ln(III) ion) a clear solution was formed
while smaller ratios of Ln:(H L) : Ln(CH COO) enabled
2 3 3 3
formation of gel-like precipitates, the use of a 1:0.5 ratio giv-
ing rise to the formation of the most stable soft matter. These
initial soft self-assembly structures were then subjected to
centrifugation (3500 rpm, 10 min), which instantaneously lead
to the formation of robust gels that were resistant to inversion
tests as demonstrated for both the Eu(III) and Tb(III) gels
formed in Figure 2A. Similarly, such gels could be formed
upon simply storing the samples after microwave irradiation at
room temperature overnight; microstructural analysis of both
systems indicated the formation of gels with identical mor-
phology. Both gels were slightly opaque (to the naked eye)
and were resistant to inversion tests. Furthermore, both were
shown to be brightly luminescent upon exposure to UV-light
irradiation (Figure 2B). TGA analysis of these Eu(III) and
Tb(III) gels (see SI) showed the weight loss of 98.2 and 98.4
wt.% upon heating the samples from 20 °C until 85 °C, which
corresponds to the evaporation of the methanol (used to gel
these), after which the remaining complex was stable until
~
300 °C when the organic content of the complexes starts to
decompose. Thus, both of the obtained lanthanide gels contain
ca. 1.7±0.1 wt.% of the complex and ~98.3±0.1 wt.% of the
methanol.
As alluded to above, these gels were highly luminescent up-
on UV-light irradiation, Figure 2B; the green and red emission
observed was that of Eu(III) and Tb(III), respectively, under
excitation at 275 nm as seen in solution studies (Figures 1 and
2D). Both samples were prepared under identical experimental
conditions. In the spectra recorded for both samples (Figure
2D) characteristic sharp line-like emission bands were ob-
served at 579, 593, 616, 650 and 696 nm for Eu(III)-gel
5
7
Figure 3. SEM images of (A) Eu(III)-, (B) Tb(III)- and (C)
Eu(III)/Tb(III) mixed gels dried on the silicon plates (scale
bars, 500 nm).
(
D
0
→ F
J
, J = 0–4) and at 489, 545, 585, 622 nm for Tb(III)-
5
7
gel ( D
4
→ F
J
, J = 6–3). The emission spectra of the gels rec-
orded after transferring them to a quartz slide (Figure 2C)
were structurally similar to the emission spectrum obtained for
the self-assembly formation in the solution discussed above.
However, some minor notable differences were observed for
the Eu(III) emission such as the contribution from the ΔJ = 0
transition was much less intense, which is an indication of
changes in the symmetry of the Eu(III) centres upon gelation.
As discussed above, the luminescence quantum yields for both
complexes were shown to be similar for both the Eu(III) and
Tb(III) complexes. Consequently, the luminescence intensity
for their corresponding gels was similar.
2 3 3 3 3
Formation of lanthanide gels from Ln(H L) (CF SO ) :
Having demonstrated the formation of a higher order self-
assembly in solution, we next turned our attention towards
formation of such large supramolecular polymeric self-
assembles at higher concentrations, as these would be ex-
9
-11
pected to result in the formation of supramolecular gels.
1
Hence, as seen in the H NMR titrations, the further addition
of Eu(III) or Tb(III) (such as LnCl , Ln(ClO , Ln(CF SO
and Ln(CH COO) ) would enable us to generate polymeric
systems by ‘crosslinking’ the above 1:3 and 1:2 Ln:H L stoi-
3
4
)
3
3
3 3
)
3
3
2
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This enabled us to form a new gel by simply mixing (me-
of the ligand to sensitise the ions at the two different binding
units within H L (see SI). The difference in the emission from
these mixed gels can also be explained by mapping the colours
on to the CIE diagram (see SI), as above.
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chanically) equal volumes of the two Ln(III) gels, which re-
sulted in formation of a new yellow-orange luminescent
Eu(III)/Tb(III) gel (Figure 2C and E), the emission spectrum
of which showed the two main emission bands occurring at
545 (Tb(III) emission) and 616 nm (Eu(III) emission), Figure
2
The above supramolecular gels represent systems that are in
constant non-equilibrium self-assembly processes, which can
lead to features such as self-healing. Self-healing materials are
2
E. This colour corresponds to coordinates of (0.47, 0.47) on a
1
4,20-24
CIE diagram (see SI). Various experiments were carried out to
investigate the mixing of these two emissive gels and the for-
mation of a new luminescent gel. For instance, first forming
of great current interest across the scientific community,
particularly as such materials can mimic naturally occurring
biopolymers. While self-healing materials have been known
for the past few decades, they are generally made of cross-
Eu:H
with Tb(CH
2
L or Tb:H
2
L complexes in solution and treating these
and Eu(CH COO) , respectively, led to
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3
COO)
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linked polymers (such as hydrogels).
However, such sys-
the formation of luminescent gels, which unlike these seen for
the 1:1 mixing of the gels discussed above, had different emis-
sion colours. The latter indicated the contribution that the
bridging lanthanide ion (i.e. via the carboxylate groups) had
on the luminescent properties of the soft matter and the ability
tems often have limited lifespans, where unexpected damage
from constant strain and stress can result in degradation of the
materials. Since both the Eu(III) and Tb(III) gels are formed
through a dynamic process, where the recognition of 1:2, or
2
the 1:3 Ln:H L complexes by the second lanthanide ion at the
Figure 4. Oscillatory rheology measurements. Frequency dependence of the storage modulus G’ (square symbol) and loss modulus
G’’ (round symbol) at a strain amplitude of 0.1% are shown in panels (A) Tb(III), (C) Eu(III), and (E) Eu(III)/Tb(III). The
corresponding strain sweeps at f=1Hz for (B) Tb(III), (D) Eu(III), and (F) Eu(III)/Tb(III) are also displayed. Panel (G) shows the
recovery test for Eu(III)/Tb(III). By applying alternating strain amplitudes of 100% and 0.1% at f=1Hz the response changes from
liquid to solid like respectively. The moduli in these regimes remain unchanged.
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clearly demonstrated that the three gels are of different mor-
carboxylic acid terminus gives rise to a three-dimensional
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coordination network, as depicted in Scheme 1, we foresaw
that both could potentially function as self-healing materials.
As the formation of the gel is induced by the binding of
phology, as the Tb(III) gel SEM images are shown to be sig-
nificantly more tightly packed in comparison to that seen for
Eu(III). For the latter, the packing of the fibrous structure is
significantly more porous. This kind of imaging was carried
out on several different samples all of which had been aged
under identical conditions and all gave identical results. This
supports our earlier finding, that the macroscopic structure of
the two gels previously shown in Figure 2B is different, even
though they were made under identical experimental condi-
tions (i.e. same concentration, volume, etc.), but they clearly
have different volume, despite the fact that the solvent content
was determined to be of the same for both using TGA analy-
sis. This could indicate that for the two different systems, the
Eu(III) and Tb(III) supramolecular polymers formed, as de-
picted in Scheme 1B, interact differently with the solvent in
the gelation process. This was further confirmed by the SEM
images of the mechanically mixed Eu(III)/Tb(III) gel, as these
images show different morphology to these of the individual
gels, as for the mixed gel the packing and porosity seem to be
different. This result is in line with what would be expected
for the formation of new soft-matter from starting materials
possessing different morphological properties. It would also
support our observation above that the homo-supramolecular
polymers heal differently to these seen when the two were
spliced together. To probe this further we carried out rheologi-
cal analysis of these three gels.
2
Ln(III) to H L which follows fast kinetics, we anticipate that
the self-healing process will follow similar trend. Indeed, the
excess number of free carboxylic acid groups from the ligands
and high coordination number of the lanthanides will ensure
self-healing of the gel through coordination. The results from
our analysis are shown in Figures 2F–I, using the Eu(III) gel
formed as discussed above. The gel was cut in two, and re-
assembled instantaneously without external stimuli through
self-healing; the red emission from the gel clearly aiding in the
visual observation of the healing properties. Similarly, the
Tb(III) gels exhibited self-healing properties. The images
show that even to the naked eye (Figure 2G and I) look similar
despite the former being an example of an un-cut gel, while
the latter is the same gel after having been cut and then spliced
together again (c.f. Figure 2H). In fact, it is clear that the two
parts have interacted together in such a manner that the cut
itself is no longer visible, i.e. a splicing of two mechanically
and physically identical gel parts results in healing. This could
indicate that within the gel structure the diffusion of the sol-
vent and hence interaction of the lanthanide ions with the car-
boxylic groups between the edges of the cut is rapid. Conse-
quently, the healing process, which one would expect to be
taking place at the carboxylic terminus (as discussed above),
through host-guest interactions where the lanthanide is coordi-
nating to free carboxylates and as such functioning within this
system as supramolecular glue, is homogenous over the sur-
face of the cut. Similar effects were seen for the Tb(III) sam-
ples (see SI), where the cut was healed and not visible to the
naked eye. In contrast to this, we also spliced together the
Eu(III) and the Tb(III) gels. The images from that experiment
(see SI) also demonstrated that interaction between the two
gels occurred. However, it was not clear that inner diffusion of
ions between the two parts was occurring, as this would be
expected to give rise to an orange coloured gel as we had
demonstrated above by mechanically mixing the two. Never-
theless, a slight yellow emission could be seen within the bor-
der of the cut. This would suggest that, either this is due to
reflection of light, or that while the host-guest interaction is, in
principle, the same, then at molecular level the dynamic flow
of ions between the two materials is different, possibly being
hindered by macroscopic properties of the gels, such as its
density, fibrous nature, etc. and hence, their rheological prop-
erties. With a view to probing these self-healing properties
further, we next carried out both morphological and rheologi-
cal analysis of the three gel types shown in Figure 2C.
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Rheology: With the aim of knowing the mechanical proper-
ties of these novel metallogels, rheological studies (strain and
frequency sweeps) were performed, showing that the viscoe-
lastic properties of the three gels shown in Figure 4 are like
these commonly seen for a cross-linked polymer network.
Note that all the data shown in Figures 4A–F were averaged
over three consecutive runs on the same sample. There is little
frequency dependence of the moduli in the linear viscoelastic
regime for both the Tb(III) and Eu(III) gels (Figure 4A, C) and
their response is solid-like (G’>G’’). The mixed
Eu(III)/Tb(III) gel exhibits similar behaviour except that G’’
approaches G’ at high frequencies indicating a crossover to a
liquid like regime. The corresponding strain sweeps for the
three gels are shown in Figure 4B, D and F. At low strain am-
plitudes, the response is solid-like; the storage modulus re-
mains constant until the yield strain is reached at which point
the gel starts to flow and G’ decreases. While the overall be-
haviour is similar for the three gels, the Eu(III)/Tb(III) gel is
noticeably softer compared to the pure Tb(III) and Eu(III)
gels. The latter have storage moduli of 1.10 kPa and 0.41 kPa
in the linear regime, respectively, while G’ for the mixture is
0
.08 kPa – an order of magnitude smaller compared to the
Morphological properties of Eu(III) and Tb(III) gels:
The morphology of the gels was investigated using scanning
electron microscopy (SEM) that gave a high surface signal on
samples that were drop cast and dried on silica plates. The
results from this analysis are shown in A-C for the three gels
imaged (see more images in SI). The SEM images in Figure 3,
at first instance show gels possessing similar morphology. All
display fibrous nature with an average width of the fibres be-
ing ~30–50 nm, consisting of long intertwining bundles of
strings that are packed into strands, which overlap with each
other, giving rise to ‘cotton- or noodle-like’ material. This
would be expected, given the use of the same ligand for all
and the only difference between them being the nature of the
lanthanide. However, closer examination of these images
pure gels. Moreover, the yield strains for Tb(III) and Eu(III)
are around 0.5% while Eu(III)/Tb(III) gel yields at 1.5%. The
remarkable reproducibility is indicated by the small error bars
in Figures 4A-F. We also tested the recovery properties of the
gels by imposing alternating strain amplitudes of 100% and
4
4
0.1% at a constant 1 Hz oscillation frequency. As shown in
Figure 4G the Eu(III)/Tb(III) gel went from liquid-like
(G’’>G’) to solid-like (G’>G’’) behaviour almost instantly
with a quick recovery of the original values of the moduli in
these two regimes. The same behaviour was observed for both
the original Eu(III) and the Tb(III) gels (see SI). These results
point to the formation of stronger ‘pure’ Eu(III) and Tb(III)
gels if we compare G’ and G” values with those obtained for
the Eu(III)/Tb(III) mixture gel. This could be due to a self-
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recognition process occurring in the ‘pure’ gels that is not was significantly different to the mixed Eu(III)/Tb(III) gel
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happening in the same way when the gels are mixed which
could explain the lower values obtained in these experiments.
underlining the significance of the Ln(III) recognition pro-
cesses occurring during self-healing.
These are somewhat unexpected rheological results, given
the fact that the ligand is the same for all three cases, and that
Eu(III) and Tb(III) display binding affinity for the formation
EXPERIMENTAL SECTION
Materials and Methods. All solvents and chemicals were pur-
chased from commercial sources and used without further
purification. Deuterated solvents used for NMR analysis
2 2
of Eu:H L and Tb:H L complexes in solution that are almost
identical for both systems. Hence, the difference seen in the
rheology has to be due to the effect of the lanthanide ion itself,
and how these ions function as bridges in the formation of the
supramolecular polymer at higher concentration. This could
explain the differences seen in the SEM imaging above, where
the packing of the fibres in the Eu(III) gel is less dense then
that seen in the Tb(III) gel, where both are then different to
that seen for the mixed gel. It has to be noted that both Eu(III)
and Tb(III) gels heal. While healing is also observed for the
mixed gel the supramolecular interaction is different as indi-
cated by their rheological properties and in the analysis of
their morphology. Hence, these unexpected rheological results
demonstrate that slight differences in the electron configura-
tion of similar lanthanides ions can have significant effect on
the supramolecular level when the two gels interact with each
other.
3 3 3 2
(CD CN, CD OD, (CD ) SO) were purchased and used as
received.
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The H NMR spectra were recorded either at either 400 MHz
1
3
or 600 MHz instruments. All C NMR spectra were recorded
at either 100 or 150.9 MHz. Chemical shifts are reported in
ppm with the deuterated solvent as the internal reference. All
NMR spectra were carried out at 293 K.
Mass-spectrometry was carried out using HPLC grade sol-
vents using either electrospray mass spectrometer (ESI) or by
using Maldi-Q-Tof mass spectrometer. High resolution ESI
mass spectra were determined relative to a standard of leucine
enkephaline, while high resolution Maldi-Q-Tof spectra were
analyzed using Glu-Fib with an internal reference peak of m/z
1
570.6774.
Infrared spectra were recorded on a spectrometer equipped
with universal ATR sampling accessory.
CONCLUSIONS
Thermal gravimetric analysis was performed on an instrument
equipped with and ultra-micro balance with a sensitivity of 0.1
microgram. The temperature range is from 20-800 °C with a
scan rate 10 °C/min.
In this work we have developed a new method for the prepara-
tion of luminescent Ln(III) directed self-assembly gels from
the Eu(III) and Tb(III) complexes obtained with the dipicolinic
acid derived ligand H
Tb(III) and mixed Eu(III)/Tb(III) gels. The self-assembly stud-
ies of the individual complexes was explored in CH CN solu-
2
L, from which we formed Eu(III),
X-ray data (Table S1) was collected using a diffractometer
with a graphite-monochromated Mo-Kα radiation (λ = 0.71073
Å). A suitable crystal was selected and mounted on a 0.3mm
quartz fiber tip and placed on the goniometer head in a 150K
3
tion, from which we determined various species; but different
speciation was found in the same experiments when performed
in CH OH, where the role of the carboxylic acid groups be-
3
2
N gas stream. The data sets were collected using Crystalclear-
came apparent as these were involved in coordination to the
lanthanides, providing the platform necessary for the for-
mation of a coordinating polymer. The driving force for the
formation of the gel is the interaction between the ligand and
metal ions unlike in the case of the gels based on organic mol-
ecules only, which are obtained through hydrogen bonds, π-π
SM 1.4.0 software. Space group determination was obtained
using Crystalstructure ver.3.8 software. The structure was
solved by direct methods (SHELXS-97) and refined against all
F2 data (SHELXL-97). All H-atoms, except for N-H protons,
were positioned geometrically and refinied using a riding
model with d(CHaro) = 0.95 Å, Uiso = 1.2Ueq (C) for aro-
matic and 0.98 Å, Uiso = 1.2Ueq (C) for CH3. N-H protons
were found from the difference map and either freely refined
or fixed to the attached atoms with UH = 1.2UN. CCDC
4
5
1
–4
stacking and dipole-dipole interactions. Thus, the response
to the internal stimuli between these two classes of gels will be
different. Here we generated supramolecular polymers, by
using Eu(III) or Tb(III) complexes, formed under microwave
conditions in 1:3 stoichiometry, followed by treatment with
acetate salts of Eu(III) or Tb(III). This gave luminescent gels,
emitting with characteristic red or green lanthanide based
emission. Imaging of the gels by SEM showed similarity in
the morphology of both gels having a ‘cotton-like’ fibrous
microstructure, but with different porosity; the Tb(III) exhibit-
ed higher density of fibres packing. Similarly, a mixed
Eu(III)/Tb(III) gel was formed, which had different morpho-
logical features to the individual ‘pure’ gels. This fact was
confirmed by rheological studies, which showed that differ-
ences existed between all of the three gels, a consequence of
the supramolecular interactions between the lanthanide ions
and the complexes in the gelation process. These results clear-
ly demonstrated the important role of host-guest interactions in
such gel formation. These gels showed the characteristics of
self-healing, demonstrated by the recovery to the same value
of G’ as the one observed at the start within less than 30 s,
even in the case of a mixture of the original Eu(III) and Tb(III)
gels. Remarkably, the self-healing process of the ‘pure’ gels
1
019596 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Photophysical measurements. Otherwise stated, all measure-
ments were performed at 298 K in methanol (HPLC grade)
and acetonitrile (spectroscopic grade) solutions. The stock
2
solutions of H L in acetonitrile were prepared using 4% of
chloroform (spectroscopic grade). UV-visible absorption spec-
tra were measured in 1-cm quartz cuvettes. Baseline correction
was applied for all spectra. Emission (fluorescence, phospho-
rescence and excitation) spectra and lifetimes were recorded at
2
98 K by using a thermostated unit block. Phosphorescence
5 5
0 4
lifetimes of the Eu( D ) and Tb( D ) excited states were meas-
ured in both water and deuterated water and methanol and
deutarated methanol in time-resolved mode at 298 K. They are
averages of three independent measurements, which were
made by monitoring the emission decay at 616 nm and 543
5
7
0 2
nm, which corresponds to the maxima of the Eu(III) D → F
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4 J
→ F Tb(III) transitions respectively, enforcing a is available free of charge via the Internet at
5
7
and to the D
.1 ms delay.
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http://pubs.acs.org.
!",!
"#
The quantum yields (ꢀ! ) were measured by relative meth-
46,47
AUTHOR INFORMATION
Corresponding Author
gunnlaut@tcd.ie
od
(
using Cs
3
[Eu(dpa)
3
]·9H
2
O complex in 0.1 M Tris buffer
!"
"#
38
pH = 7.45) (ꢀ! = 24.0 ± 2.5 %) as a standard with
known quantum yield, to which the absorbance and emission
intensity of the sample are compared according to:
Present Addresses
!
!
!"#
",!
!!
!!
!!(!!)
!!(!!)
!!(!!)
!!(!!)
!!
ꢀ
= ! = !!
×
×
× !!!
(1),
†Departamento de Química Orgánica and Centro Singular de
Investigación en Química Biolóxica e Materiais Moleculares
!
where subscript r – reference and x – sample; E – integrated
luminescence intensity; A – absorbance at the excitation wave-
length; I – intensity of the excitation light at the same wave-
length, n – refractive index of the solution. The estimated error
for quantum yields is ±10%.
(
CIQUS), Universidade de Santiago de Compostela, 15782
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0
Santiago de Compostela (Spain).
Author Contributions
‡MMC and OK conducted all the experiments (contributed
equally), SEM experiments were done by AB and rheological
studies were carried out in collaboration with MM. MMC,
OK, MM and TG co-wrote the manuscript, all the authors
discussed the results and commended on the manuscript. TG
guided the research.
Spectrophotometric titrations and binding constants. The for-
mation of the luminescent (M:H
dipicolinic based dicarboxilic ligand) species was ascer-
tained by both UV-visible and luminescence titrations of a
2 2
L, where M = metal and H L
=
-
5
2 3 3 3 2
solution of H L (∼1·10 M) with M(CF SO ) ·6H O (M=
Eu(III), Tb(III)) (0→6 equivalents) at 298 K. The data were
fitted using the non-linear regression analysis program,
Each titration was repeated with evaluation of
the binding constants values for at least three times until data
convergence was achieved.
Funding Sources
®
40,48
SPECFIT .
Irish Research Council for Science, Engineering & Technolo-
gy (IRCSET; postdoctoral fellowship to OK), and Science
Foundation Ireland (SFI; RFP 2009, 2010 PI and 2013 PI
awards to TG and JJB).
Microscopy studies of the gels. To image the gel samples by
scanning electron microscopy (SEM), they were deposited
manually onto clean silicon samples with a thick silicon diox-
ide layer. The spatula and glass pipettes used for dosing and
silicon pieces used as substrates were all cleaned thoroughly
by sonication in HPLC grade acetone followed by HPLC
grade propan-2-ol. All components were dried in two steps
using a high pressure nitrogen gun and further dried under
ambient conditions. The gels were manually drop cast on to
the silicon at room temperature and dried during 5 days at
ambient conditions. The samples prepared for the imaging
using SEM did not have any additional conductive layer cover.
ACKNOWLEDGMENT
We thank University of Dublin and CRANN for their support.
This research was supported by the Irish Research Council for
Science, Engineering & Technology (IRCSET; postdoctoral
fellowship to OK) and Science Foundation Ireland (SFI). We
thank Dr. Steve Comby for the help with CIE diagrams and
Dermot Daly for the help with SEM images (AML, CRANN,
TCD).
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geometry. After placing the sample, the upper plate was slow-
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the measurements commenced. During the oscillatory meas-
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oscillatory strain sweep measurements were performed at a
frequency of 1 Hz while the frequency sweeps were performed
at a constant strain amplitude of 0.1%. All the data was aver-
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