Self-assembly formation of a healable lanthanide luminescent supramolecular metallogel from 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) ligands

Article abstract of DOI:10.1039/c5cc03139g

The synthesis of five new 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) ligands is described: the self-assembly behaviour of the tri-methyl ester, 1, with Eu(iii) showed the formation of a luminescent 1 : 3 Eu : btp complex, Eu1₃, which was studied in solution and in the solid state; while the tri-carboxylic acid, 2, formed a hydrogel and its corresponding complex Eu2₃, gave rise to a strongly red luminescent healable metallogel.

Full text of DOI:10.1039/c5cc03139g

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Self-assembly formation of a healable lanthanide
luminescent supramolecular metallogel from 2,6-
bis(1,2,3-triazol-4-yl)pyridine (btp) ligands
Cite this: DOI: 10.1039/x0xx00000x
Eoin P. McCarney,a Joseph P. Byrne,a Brendan Twamley,a Miguel Martínez-Calvo,b
Gavin Ryan,c Matthias E. Möbiusc and Thorfinnur Gunnlaugsson*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
The synthesis of five new 2,6-bis(1,2,3-triazol-4-yl)pyridine
(btp) ligands is described: the self-assembly behaviour of the
tri-methyl ester, 1, with Eu(III) showed the formation of a
luminescent 1:3 Eu:btp complex, Eu13, which was studied in
solution and in the solid state; while the tri-carboxylic acid, 2,
formed a hydrogel and its corresponding complex Eu23, gave
rise to a strongly red luminescent healable metallogel.
Fig. 1 Btp ligands fuctionalised at positions
Y and Z.
selective modification at Y and Z in an “orthogonal” manner. We
show that structures 1 and 2 form Eu(III) complexes in 1:3 (Eu:L)
stoichiometry, ligand 2 gives rise to a hydrogel while the Eu(III)
complex of 2, forms a metallogel in CH3OH solution endowed with
the healable and luminescent properties of the Eu(III) ion.
2,6-Bis(1,2,3-triazol-4-yl)pyridine (btp) based ligands are terdentate
pyridine-centred heteroaromatic structures that have been shown to
form stable coordination complexes with a variety of metal ions.1-5
While the use of the btp binding motif has grown significantly in
recent years,3a relatively few lanthanide (Ln(III)) complexes of btp
have been studied,1,2,3,5b some of the first examples of which showed
that btp can be used as a sensitising ‘antenna’ for ions such as
The synthesis of 1 (Scheme S1, ESI) demonstrates the ease of
formation and functionalisation of these btp ligands, 1-5 (all
syntheses and full characterisations are described in detail in the
ESI). Ligand 1 was formed in 85% yield, the key step being the
CuAAC 'click' reaction between protected bis-alkyne 9 and azide 11,
which had been prepared in situ from commercially available methyl
4-(bromomethyl)benzoate, 10 (see ESI). Ester hydrolysis of 1 in
aqueous NaOH gave 2 in high yield, 71%. The formation of Eu13,
and Eu23 was achieved by reacting the appropriate ligand with
Eu(OTf)3 in CH3OH in 1:3 metal:ligand (M:L) stoichiometry under
microwave irradiation at 80oC for 60 minutes followed by isolation
using precipitation from diethyl ether. Complexes Eu13, and Eu23
were characterised using conventional methods (see ESI). The
complexes of 3-5 will not be discussed in this communication.
Colourless, needle-like crystals of Eu13 were successfully grown
by diethyl ether diffusion into CH3CN from which the solid-state X-
ray structure was determined at 150 K. The resulting structure,
Eu(III).1a,5b The btp structure provides
a broad scope of
functionalisation through the use of one pot ‘click’ chemistry.3a,5
Recently, we developed the btp ligand 6, Figure 1, and investigated
its coordination, photophysical and electrochemical behaviour with
various d-metal ions.6 We showed that ester hydrolysis of 6,
followed by treatment with Ru(II) gave rise to the formation of a
metallo-supramolecular gel. Building on this, we have developed
Ln(III) bundles of 6 (and a range of its amino acid derivatives).3c
The Ln(III) ions possess remarkable photophysical and magnetic
properties1,2,7 with applications in a diverse range of areas.8-11 We,
and others, have used them to template the formation of self-
assembled supramolecular architectures from acyclic ligands.12 They
are ideal for template-directed generation of novel functional
nanomaterials, such as gels,13 work initiated by Rowan and co-
workers.14 Self-healing of supramolecular polymers is an
increasingly topical area of research within supramolecular
chemistry13a,15 with host−guest chemistry playing a significant role.16
Combining the ‘guest’ ability of Ln(III) ions with the versatile btp
‘host’ in the design of healable soft matter is our aim. With this in
mind, we designed the btp ligands 1-5, Figure 1, which possess
carbonyl groups at both the 4-pyridyl position (Y) and the benzyl
terminals (Z). This article will focus on 1 and 2, featuring three
methyl ester groups and three carboxylic acid groups, respectively.
Ligands 3-5 are currently being investigated in our lab for future
ŭ
shown in Figure 2, crystallised in the triclinic space group P̊,
demonstrating low symmetry in the Eu(III) complex. The structure
showed each of the ligands coordinating the Eu(III) ion via the
nitrogens of the pyridine and the two triazoles (through a syn-syn
orientation)1a,5b,17 giving an overall coordination environment of nine
(N9) for the Eu(III) ion. The crystal structure also showed that the
methyl benzoate arms adopted trans-like conformation about the
methylene linker with π-π interactions between the benzoate arms of
neighbouring tris-complexes; each one of them facing ‘outwards’
from the Ln(III) centre, which could be further used as ‘linking’
points in the generation of novel material such as gels.13a The H
1
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Fig. 3 Changes in Eu(III) centred luminescence spectra upon titration of
1
in CH3CN (c = 1x10-5M) against 0
→3.5 eq. of Eu(OTf)3 recorded at 25
oC. (Inset) Experimental binding isotherms at band wavelengths. (λex
325 nm).
=
Fig. 2 Ball and stick model of the X-ray crystal structure of Eu13
Hydrogen atoms and counter-ions are omitted for clarity.
.
present in 92.5% yield, with the ligand 1 becoming fully bound at
slightly higher eq. of Eu(III). The formation of the 1:1 and 1:2 M:L
species was also seen upon increasing the concentrations of Eu(III)
NMR (CD3OD, 400 MHz) analysis of Eu13 also indicated that the
same structure existed in solution (see ESI) as the geminal coupling
between the methylene protons (J = 14.6 Hz) was indicative of the with concomitant decrease in the abundance of the 1:3 M:L species
loss of chemical equivalency with each proton losing free rotation and decrease in overall luminescence.
about the methylene ‘hinge’. Furthermore, the significant upfield
shift of the pyridyl and triazolyl 1H and 13C resonances are indicative
of the paramagnetic nature of the Eu(III) core.
At low concentration the complexation between 1 and Eu(III)
results in the formation of discrete supermolecules in solution,
however, we are more interested in this process at higher
Having analysed the photophysical properties of Eu13 (see ESI) concentrations. Currently, there exists a great deal of interest in the
we next carried out titration studies to monitor the kinetic self- development of new supramolecular materials with various
functional properties that are different from their monomeric or
assembly behaviour of 1 with Eu(III) in CH3CN solution. Here,
specific aliquots of a stock solution of Eu(OTf)3 were added to a discrete molecular components.20,21 Having explored the various
solution of 1 (c = 1x10-5 M) and the changes in the UV-Vis photophysical properties of 1 upon addition of Eu(III), we set out
absorption and the emission spectra were monitored. Fluorescence investigating the possibility of using the btp ligands developed
herein for soft-material applications. For supramolecular gel
quenching and population of the Eu(III) excited state caused by
energy transfer from the btp antenna gave rise to metal centred formation to be successful, the use of weak intermolecular or metal
emission at 580, 595, 617, 650, and 695 nm assigned to the coordination interactions is of critical importance,3a,13a,22 hence, the
5D0→7FJ: J = 0-4 transitions (Figure 3). Analysis of these changes at methyl esters of 1 were hydrolysed to the tri-carboxylic derivative 2
as a potential candidate for a low molecular weight gelator.23 We
band wavelengths (inset) demonstrated that the emission enhanced
sharply with increasing equivalents (eq.) of Eu(III) ion added. foresaw that the acid moieties would function as ‘bridging points’
Thorough analysis of the Eu(III) spectral changes allowed for the through hydrogen bonding interactions (e.g. carboxylic
dimerisation). Initially, the use of the ligand 2 alone in the formation
stoichiometry in solution to be probed as the ꢀJ = 0 band at 580 nm
of soft matter was explored. The deprotonation of 2 in aqueous
NaOH solution (pH 8) fully solubilised ligand 2. Addition of dilute
HCl until pH 6 resulted in the formation of an off-white ‘cotton’ like
material that upon standing settled as a soft hydrogel material.
Alternatively, centrifugation of the above pH 6 solution allowed
recovery of the same hydrogel; the morphology was examined using
Scanning Electron Microscopy (SEM). The SEM imaging (Figure
4a) showed that the hydrogel displayed a fibrous network of
intertwined ‘spaghetti’ like strands of ca. 20−50 nm in diameter
arranged in a tightly packed formation. This material under UV lamp
excitation gave rise to blue coloured ligand based emission (Figure
4b). Thermogravimetric Analysis (TGA) was carried out on the
ligand hydrogel, showing a loss in mass of 98.07% due to water
content, giving it a weight percentage (wt. %) of 1.93 0.04 (see
ESI).
Having investigated the ability of ligand 2 to form hydrogels, we
then investigated the gelation of Eu23. We foresaw that the acid
moieties would assemble through the carboxylic dimerisation,
mentioned previously, or through the use of ‘bridging’ metal ions,
such as f-metal ions. This was done in a two step one-pot procedure
(see ESI). We treated a CH3OH solution of the Eu23 complex with
Eu(OAc)3. Upon solvation of the Eu(III) salt (3 eq.), a white fluffy
precipitate was generated. This mixture was irradiated again in the
corresponded strongly to the formation of the 1:3 species (Figure
S14, ESI). This, and the splitting pattern of the ꢀJ = 2 band, are
indicative of a single chemical species with C3 symmetry in
solution.18 Moreover, it is worth pointing out that the X-ray crystal
structure of Eu13 showed that the N9 coordination sphere formed a
distorted triaugmented trigonal prism (Figure S27, ESI),
a
polyhedron normally associated with D3h site symmetry. Such
variations between the solid and the solution state structures, having
previously been observed in our group,12a,12d,13a allow for the
symmetry adjustment (C3 is a subgroup of D3h) though, as expected,
the luminescence spectra of 1 at 0.33 eq. of Eu(III) and that of Eu13
in solution coincided closely (Figure S18, ESI).
Nonlinear regression analysis software (ReactLab Equilibria)19
was used to determine both the speciation distribution and the
stability constants for the formation of the Eu(III):1n (n=1-3) self-
assemblies in solution at 25 oC. The binding isotherms were best
fitted to 1:3, 1:2 and 1:1 M:L equilibria. The stability constants
(expressed as log βM:L) were determined as log β1:3 = 22.3 0.2; log
β1:2 = 16.2 0.2 and log β1:1 = 8.4 0.1 for these stoichiometries,
respectively. These compare well with those observed for related
systems3a,12a-d,k and are reasonably high. The calculated speciation
distribution diagrams from fitting the changes in the UV-Vis (Figure
S15) showed that at 0.33 eq. of Eu(III) the 1:3 M:L species was
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Fig. 5. Oscillatory rheology measurements of Eu(III) gel. (A) Frequency
sweeps at 0.1% strain amplitude of the storage modulus G ) and loss
modulus G ) are shown. (B) The corresponding strain dependence at
a f = 1 Hz. (C) Recovery test for Eu(III) gel with alternating strain
amplitudes of 20 % and 0.1 % at f = 1 Hz.
′(■
Fig. 4.(A) SEM images of hydrogel of
2
(C) SEM images of metallogel of Eu23 (scale bar 200 nm). (D)
Metallogel of Eu23 (in daylight and under UV lamp at λex = 365 nm).
(E) Eu(III) centred luminescence of Eu23 complex in CH3OH (red) and of
metallogel (black).
2 (scale bar 1 μm and inset at 200
nm). (B) Hydrogel of (in daylight and under UV lamp at λex =365 nm).
″(●
morphology to that of the hydrogel of 2, Figure 4c; clearly showing
the effect Eu(OAc)3 had upon the nano-structure. Here, the SEM
imaging showed a more condensed structure and lacked the enriched
fibrous network seen in the gelation of 2. TGA of the Eu23
metallogel showed a loss in mass of 95.15% arising from the
CH3OH content giving it a 4.85 0.08 wt. % (see ESI).
microwave for 30 minutes at 80 oC giving rise to the formation of an
off-white soft precipitate upon cooling. Upon centrifugation of this
precipitate a gel was obtained. The same gel could be formed upon
allowing the precipitate to settle over several hours. Furthermore,
this gel was highly luminescent to the naked-eye (Figure 4d) with
characteristic red Eu(III) emission. The gel was also shown to be
highly robust and easily applicable to surfaces; it was applied on a
quartz slide (Figure S20) which was then used for spectroscopic
analysis. The emission spectra closely matched that observed for
For better understanding of the mechanical properties of these
novel hydrogel and metallogel materials, rheological studies (strain
and frequency sweeps) were performed. The viscoelastic properties
of the metallogel compared to those commonly seen for a cross-
linked polymer system.14,24 Little frequency dependence of the
storage and loss moduli (G′ and G″ respectively) was observed in the
linear viscoelastic region for both the ligand (see ESI) and Eu(III)
gels (Figure 5a) and their response was solid-like (G′ > G″).
However, for the ligand gel G″ approached G′ at high frequencies
indicating a crossover to a liquid-like regime. The corresponding
strain sweeps for the gels were also carried out (Figure 5b). At low
strain amplitudes, the response was solid-like; the storage modulus
remained constant until the yield strain was reached at which point
the gel started to flow and G′ decreased. The Eu(III) gel had a
storage modulus of about 1.0 kPa in the linear regime while G′ for
the hydrogel was 0.8 kPa. The recovery properties of the gels were
tested by imposing alternating strain amplitudes of 20% and 0.1% at
a constant 1 Hz oscillation frequency. As shown (Figure 5c), the
Eu(III) gel went from liquid-like (G″ > G′) to solid-like (G′ > G″)
behaviour almost instantly with a quick recovery of within 50
seconds of the original values of the moduli in these two regimes,
consistently over consecutive runs. This demonstrated that the
Eu(III) gel is self-healing. The same behaviour was not observed for
the ligand gel (Figure S26); the moduli values were not being
recovered in the same smooth manner as for the Eu(III) gel. The
difference in the rheology behaviour is due to the effect of the
Eu(III) ion itself, acting as a cross-linker in the formation of the
extended 3D network. Incorporation of the Eu(III) ion results in the
formation of stronger materials, less subject to aging and more
healable. It can be concluded that the Eu(OAc)3, is responsible for
the ‘bridging’ of neighbouring tris-complexes through the
carboxylate ‘arms’ of Eu23. The nature of the unsaturated Eu(III)
‘bridges’ possessing vacant coordination sites meant that bond
Eu23 in solution for the
structurally identical, but of lower intensity (Figure 4e). Moreover,
unlike what was seen for Eu23 in solution, the J = 0 was observed
ꢀJ = 1, 2 and 3, while the ꢀJ = 4 was
ꢀ
in the gel-phase suggesting that a different Eu(III) coordination
environment was being added to the system.18 This was supported by
comparing the gel’s UV-Vis absorption spectrum to that of Eu23 in
solution (Figure S21). Band positions remained constant but a sharp
hypochromic effect on π→π* band indicated the Eu(III) interaction
with the carboxylate arms of Eu23 in the gel phase.
In order to characterise the newly formed Eu(III) environment
observed in the metallogel, lifetime studies at different stages of the
gelation process were conducted (see Table S3, ESI). Firstly, the
radiative decay of the Eu23 complex luminescence in CH3OH
solution was found to be monoexponential with a lifetime (τ) of 1.4
0.1 ms. To this was added 2 eq. of Eu(OAc)3, followed by
microwave irradiation and collection of the metallogel. This was
done in order to determine the lifetime of the still luminescent
supernatant which was also shown to be monoexponential with τ =
1.4 ms, indicative of Eu23. Further addition of 1 eq. Eu(OAc)3 was
required to collect the maximum amount of metallogel removing any
luminescent material from the supernatant. Lifetimes of the
metallogel showed biexponential radiative decay indicative of the
two Eu(III) environments. A long lived species (τ = 1.3 0.1 ms)
characteristic of the fully saturated Eu23 and a shorter lived species
(τ = 0.50 0.05 ms) indicative of the ‘bridging’ Eu(III) centres.
SEM imaging of the Eu23 metallogel demonstrated that
incorporation of the Eu(III) ions formed a gel with different
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DOI: 10.1039/C5CC03139G
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Journal Name
ChemComm
View Article Online
COMMUNICATION
DOI: 10.1039/C5CC03139G
Self-assembly formation of
a
healable lanthanide luminescent
supramolecular metallogel from 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp)
ligands
Eoin P. McCarney, Joseph P. Byrne, Brendan Twamley, Miguel
Martínez-Calvo, Gavin Ryan, Matthias E. Möbius and Thorfinnur
Gunnlaugsson
The self-assembly and rheological studies of self-healing Eu(III)
luminescent metallogels from a btp (2,6-bis(1,2,3-triazol-4-yl)pyridine)
ligand is described.
Upon 365 nm irradia on
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5