The effect of the linker size in C2-symmetrical chiral ligands on the self-assembly formation of luminescent triple-stranded di-metallic Eu(iii) helicates in solution

Article abstract of DOI:10.1039/c8dt02753f

Chiral lanthanide-based supramolecular structures have gained significant importance in view of their application in imaging, sensing and other functional purposes. We have designed chiral C₂-symmetrical ligands (L) based on the use of two 2,6-

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O. Kotova, S. Comby, K. Pandurangan, F. Stomeo, J. E. O'Brien, M. Feeney, R. Peacock and C. McCoy,
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Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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The effect of the linker size in C2-symmetrical chiral ligands on the
self-assembly formation of luminescent triple-stranded di-metallic
Eu(III) helicates in solution
Received 00th January 20xx,
Accepted 00th January 20xx
Oxana Kotova,*a Steve Comby,a Komala Pandurangan,a Floriana Stomeo,a John E. O’Brien,a Martin
Feeney,a Robert D. Peacock,b Colin P. McCoyc and Thorfinnur Gunnlaugsson*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Chiral lanthanide-based supramolecular structures have gained significant importance in view of their application in
imaging, sensing and other functional purposes. We have designed chiral C2-symmetrical ligands (L) based on the use of
two 2,6-pyridine-dicarboxylic-amide moieties (pda), that differ from one another by the nature of the diamine spacer
groups (from 1,3-phenylenedimethanamine (1(S,S), 2(R,R)) and benzene-1,3-diamine (3(S,S), 4(R,R)) to much bulkier 4,4'-
(cyclohexane-1,1-diyl)bis(2,6-dimethylaniline) (5(S,S), 6(R,R))) between these two pda units. The self-assembly between L
and Eu(III) ions were investigated in CH3CN solution at low concentration whereby the changes in the absorbance,
fluorescence and Eu(III)-centred emission spectra allowed us to model the binding equilibria occurring in the solution to
the presence of [Eu:L2], [Eu2:L2], [Eu2:L3] assemblies and reveal their high binding constant values. The self-assembly in
solution were also studied at higher concentration by following the changes in the 1H NMR spectra of the ligands upon
Eu(III) addition, as well as by using MALDI-MS of the isolated solid state complexes. The chiroptical properties of the
ligands were used in order to study the structural changes upon self-assembly between the ligands and Eu(III) ions using
circular dichroism (CD) and circularly polarised luminescence (CPL) spectroscopies. The photophysical properties of [Eu2:L3]
complexes were evaluated in solution and showed the decrease of luminescence quantum yield when going from smaller
to bulkier linkers from ∼5.8% to ∼3.2-2.6 %. While mass-spectrometry revealed the possible formation of trinucler
assemblies such as [Eu3:L3] and [Eu3:L2].
ligands is crucial in order to achieve lanthanide complexes with
triple-stranded di-metallic helical motif and modifications of
ligand design have been recently proven successful in moving
Introduction
The design of self-assembled lanthanide (Ln) complexes with
triple-stranded di-metallic helical motif is inspired by
architecture, mechanics and structural features of biological
macromolecules and driven by possible application of such
systems thanks to unique magnetic and spectroscopic
properties of the metal centres.[1] The first di-metallic
lanthanide helicates were reported by Piguet et al.[2] and
since then, the structural and thermodynamic parameters of
such systems were investigated[3] moving towards nd-4f
helicates[4], polymeric helical structures[5], and water soluble
systems for imaging applications[6]. Such systems were also
developed as sensors for adenosine monophosphate,
fluoride[7] and chiral helical structures; which we also
investigate within our research group.[8] The design of the
from triple stranded helicates towards achieving 3D helical
structures,[9] pentanuclear lanthanide helicates,[10]
tetranuclear tetrahedrons and octanuclear cubes,[11] or cages
[12]. Recently, another interesting approach has been shown
where one can move from the use of dinuclear bis-
diketonate lanthanide complexes,[13] to D2-symmetrical
alternating circular helicate, by simply combining achiral bis-
β-
β
-
diketonate and chiral Pybox ligands within a single ligand
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structure.[14] In our research group, we have based our ligand the new ligands where shorter benzene-1,3-diamine (ligands
design on connecting two chiral half-helicate ligands either (S) Scheme 1) or much bulkier spacer such as 4,4'-
3–4,
or (R) 6-(1-(naphthalen-1-yl)ethylcarbamoyl)picolinic acid with (cyclohexane-1,1-diyl)bis(2,6-dimethylaniline) (ligands
5–6)
diamine spacers such as 4,4'-methylenedianiline,[8a] 3,3'- were utilised (Scheme 1).
methylenedianiline [8b] or smaller 1,3-phenylenedi-
The structural changes of the ligand upon self-assembly
methanamine.[8c] The interaction of these ligands with with Eu(III) in solution were accessed with the aid of
lanthanide ions in solution resulted in the formation of spectroscopic techniques by monitoring the changes in the
enantiomerically pure luminescent triple-stranded di-metallic absorbance, fluorescence and Eu(III)-centred emission of the
Ln(III) helicates. The use of 2,6- pyridinedicarboxamide (pda
)
ligand upon metal addition at low concentration, as well as
binding unit is very appealing for the formation of various circular dichroism (CD) and circularly polarised luminescence
types of Ln(III) bundles[8g] including highly novel systems such (CPL) spectroscopies, in addition to 1H NMR spectroscopy at
as molecular trefoil knots,[15a-15b] elegantly developed by high ligand concentrations.
Leigh and co-workers, that can be used in asymmetric
catalysis.[15c] This concept was recently expanded for Zn(II)
Results and Discussion
systems where trefoil knots were formed from trimeric circular
helicates,[15d] and by securing a supramolecular architecture
Synthesis of ligands 1-6 The ligands were synthesised in four steps
by tying a stopper in mechanically interlocked molecules[15e].
(Scheme S1, SI) starting from 2,6-pyridine-dicarboxylic acid through
In this work we continue our helicate endeavour with
protecting one of the carboxylic acids by reacting it with benzyl
Eu(III) and investigate the effect of the spacer size on the
bromide. The remaining carboxylic acid group, of resulting
formation of lanthanide self-assembly structures in solution by
compound 16 (see ESI, Scheme S1) was coupled with (S) or (R) 1-
comparing the system published earlier (ligands 1–2) [8c] with
(naphthalen-1-yl)ethanamine using N-(3-dimethylaminopropyl)-N′-
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Table 1. Binding constants (logβM:L) obtained by fitting the changes in absorption, fluorescence and Eu(III)-centred emission spectra of 3(S,S), 4(R,R), 5(S,S) or 6(R,R) upon
addition of Eu(CF3SO3)3 in CH3CN solution (25 °C). All titrations were done at least in triplicates.
Absorbance
logβ2:3
26.7±0.2
26.4±0.3
25.5±0.3
25.4±0.3
Fluorescence
logβ2:3
26.6±0.5
27.1±0.2
26.2±0.3
28.5±0.3
Eu(III)-centred emission
Eu(III)
logβ1:2
13.7*
logβ2:2
logβ1:2
13.3±0.4
13.2*
logβ2:2
logβ1:2
logβ2:3
logβ2:2
L = 3(S,S)
L = 4(R,R)
L = 5(S,S)
L = 6(R,R)
19.6±0.2
19.1±0.2
18.1±0.2
19.3±0.2
19.6±0.4
20.7±0.1
19.2±0.2
20.3±0.3
13.2±0.4
13.2±0.1
13.9±0.2
13.9±0.2
27.9±0.2
26.6±0.2
26.2±0.4
25.8±0.6
21.2±0.1
20.7±0.1
19.1±0.1
19.9±0.3
12.5±0.4
12.7±0.1
12.9±0.1
14.3±0.3
15.0±0.2
* - values were fixed to allow convergence of the experimental data to the proposed model
ethylcarbodiimide (EDCI), 1-hydroxybenzotriazole (HOBt) as
The self-assembly studies between the ligands 3–6 and Eu(III) ions
coupling reagents in dichloromethane resulting in the formation of in CH3CN solution were next monitored by following the changes in
compounds 12, 13 (Scheme S1, SI), followed by de-protection using the absorption, fluorescence and Eu(III)-centred emission spectra of
hydrogenation in the presence of or Pd/C as a catalyst. These so- each ligand upon gradual addition of metal ions (0→4 equivalents)
called “half-helicate” compounds (either 10 or 11, Scheme S1, SI) (Figure 1). The changes in the titration profiles for the pair of
were then coupled with the desired “spacer” diamines (Scheme 1) enantiomers 3(S,S), 4(R,R) and 5(S,S), 6(R,R) followed the same
using the same peptide coupling reagents as before, in the presence trends, and therefore, we will only discuss one of each sets of
of trimethylamine and 4-dimethylaminopyridine as catalyst. It has enantiomers herein. The changes in the absorption spectra of 4(R,R)
to be noted that 1,3-phenylenediamine linker (8) was obtained and 5(S,S) are shown in Figure 1A,B; the long absorption
from commercial sources while 4,4'-(cyclohexane-1,1-diyl)bis(2,6- wavelength being red shifted upon complexation of these ligands
dimethylaniline) linker was synthesised following the procedures with Eu(III). The changes observed for 3(S,S) and 6(R,R) upon
reported previously by Hunter et al.[17] Ligands 3–6 (Scheme 1) addition of Eu(III) (0→4 equiv.) were to greater extend identical;
were obtained in ~20–25 % yield and their composition was the main differences being the observation of a less decrease in the
confirmed by a combination of 1H, 13C and heteronuclear single absorbance centred at 222 nm between the additions of 0→0.50
quantum correlation (HSQC) NMR experiments along with equivalents of Eu(III), and higher hyperchromicity of the band
electrospray mass-spectrometry (ESI-MS), IR and elemental analysis centred at 334 nm in the case of 5(S,S) and 6(R,R) compared to
(see SI, Figures S1-S14).
3(S,S) and 4(R,R) (Figure 1A,B). This can reflect the differences in the
conformational changes between the self-assembly processes in
solution. In both cases the changes in the absorption bands of the
ligand occurred until addition of 0.67 equivalents of Eu(III) when the
plateau has been reached (Figures S16, S17, S21, S24).
Self-assembly studies between chiral ligands 3–6 and Eu(III) ions in
solution, formation of helical assemblies at low concentration
using UV-Vis and luminescence spectroscopy Initially, the
absorption spectra of the ligands developed herein, were evaluated
in CH3CN solution (Figure S15). The same trend was seen for all,
where two main absorption bands centred at ca. 223 nm and 280
nm were observed, these being assigned to π→π* and n→π*
transitions within ligands structure (ε280(3(S,S)) = 34553 M–1·cm–1,
Excitation into the naphthalene bands at λex = 281 nm resulted in
a weak ligand-centred emission with the maximum at ∼ 425-460 nm
for all the ligands (Figure S18, S19, S22, S25). Upon addition of
Eu(III) (0→0.67 equivalents) an enhancement was seen in the
ligand-centred emission of the benzene-1,3-diamine linker based
ligands 3(S,S) and 4(R,R), while after the addition of 0.67
equivalents of Eu(III), a gradual decrease in the ligand-centred
ε
ε
280(4(R,R)) = 34963 M–1·cm–1, ε280(5(S,S)) = 23260 M–1·cm–1 and
280(6(R,R)) = 23358 M–1·cm–1).
emission was observed. Concomitantly, the Eu(III)-centred emission
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was observed, indicating the successful population of the mono-exponential decays with the average values of ∼1.50–1.70 ms
lanthanide excited state, through sensitisation of the naphthalene suggesting the formation of fully saturated metal coordination
antennae, with the maximum of luminescence intensity being spheres (Figure S28).[8a,b; 19a,b]
achieved upon the addition of ca. 0.70 equivalents of Eu(III). Upon
The changes observed during the course of these titrations
further additions, a decrease in the lanthanide luminescence was were analysed by fitting the data using non-linear regression
observed until it reached plateau after the addition of ca. 1.25 analysis program SPECFIT.[20] In all the cases, the factor analysis
equivalents (Figure S18, S22). The case of increasing ligand centred suggested the formation of three additional species along with the
emission upon complexation with Eu(III) is unusual and suggests the initial ligand being present in solution. The stability constants were
presence of strong interaction between the ligands as well as determined (as logβML) for the binding models [Eu:L2], [Eu2:L2] and
decrease of vibration quenching effects within ligand structure [Eu2:L3] (Table 1). The values of the binding constants between the
occurring upon complexation.[16] In the case of 5(S,S) and 6(R,R) enantiomers of the same ligand closely corresponded to each other,
(with 4,4'-(cyclohexane-1,1-diyl)bis(2,6-dimethylaniline linker)[17] and were found to be higher for the ligands consisting of smaller
the weak ligand centred fluorescence decreased upon complexation linker groups such as (1(S,S), 2(R,R))[8c] and (3(S,S), 4(R,R))
with Eu(III). At the same time, the Eu(III)-centred emission appears; compared to (5(S,S), 6(R,R)) (Table 1). The speciation distribution
reaching maximum upon addition of 0.67 equivalents of Eu(III). diagram for these titrations also revealed that the yield of [Eu2:L3]
After this point, the ligand-centred emission is gradually enhanced, species at 0.67 equiv. of Eu(III) decreased from 80.2% for 1(S,S),
while Eu(III)-centred emission decreased until 4 equivalents of the 2(R,R)[8c] and 82.5% for 3(S,S), 4(R,R) to 72.2% for 5(S,S), 6(R,R),
ion added (Figure S19, S25). The changes in the delayed (0.10 ms) Figure S29.
Eu(III)-centred emission were following the same trend as these
Circular dichroism (CD) and circularly polarised luminescence (CPL)
observed in the fluorescence spectra earlier (Figure 2, S20, S23,
for [Eu2:L3] assemblies in solution The chiral nature of the ligands
5
S26). The ratio between the D0→5FJ transitions for these two sets
was probed using CD spectroscopy, and the Cotton effects in the
of diamine linkers varies significantly, suggesting discrepancy in the
spectra of corresponding pair of enantiomers (e.g. 3(S,S), 4(R,R) and
coordination environment of Eu(III) ion. Interestingly, the ratio
5(S,S), 6(R,R)) appeared as mirror images to each other, Figure 3.
between the species formed in solution also varies, which is a sign
For 3(S,S), three positive CD signals were observed with the maxima
that different self-assembly processes are occurring in solution for
at ca. 296, 229, 208 nm along with two negative CD signals at 271
these two types of ligands. However, in both cases, the presence of
5D0→7F0 band is observed, this suggesting the formation of species
where the Eu(III) ions are within a site symmetry of Cnv, Cn or Cs.[18]
230 and 209 nm, correspondingly, while showing only one negative
In contrast to these titrations, the titration profile for 3(S,S) and
and 219 nm. The signals in the CD spectra of 5(S,S) were similar to
these observed for 3(S,S) with the positive signals observed at 285,
signal at 223 nm. The shape of the CD spectra observed here closely
4(R,R) (with benzene-1,3-diamine linker) upon titration with Eu(III)
corresponds to similar structures previously developed.[8a,b]
corresponds closer to the behaviour previously observed for 1(S,S)
The corresponding [Eu2:L3] were then formed in solution through
and 2(R,R) (with 1,3-phenylenedimethanamine).[8c] In all the cases
self-assembly of 1 equiv. of the corresponding ligand with 0.67
the shape of the excitation spectra with λmax at 280 nm recorded
during the course of the titrations (λem = 616 nm; Figure S27)
confirmed that the population of the Eu(III) excited state occurs via
solution (Figure 3). Again the spectra corresponding to each pair of
to the “antenna effect” where the energy absorbed by the ligands is
equiv. of Eu(III). Each solution was then equilibrated for 15 minutes
to ensure the formation of the desired [Eu2:L3] helical assemblies in
enantiomers appeared as mirror image to one another suggesting
transferred onto the metal centres from where the characteristic
enantiomeric purity of the corresponding Eu(III) species (although
sharp line-like emission occurs. During the course of the titration
the lifetimes of the 5D0 level of Eu(III) were recorded upon the
addition of 0.67 eq. and these results were then best fitted to
this might also represent
a minor contribution from other
stoichiometries as outlined above). The CD spectra of the
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complexes appeared significantly different to those of the ligands. monitor the structural/conformational changes occurring upon
For example, for [Eu2:L3] with L = 3(S,S) (Figure 3A) the first negative complexation/self-assembly formation, but also used to evaluate
Cotton effect band was observed at 325 nm while second positive the binding affinities between the ligands and the ions in solution
Cotton effect band at 295 nm and an amplitude of 4.6 mdeg by fitting the data using non-linear regression analysis. With this in
showing negative chirality. The following positive Cotton effect mind, we monitored the changes in the CD spectra of the ligands L =
centred at 227 nm and negative Cotton effect at 214 appeared with 5(S,S) and 6(R,R), upon addition of Eu(III) (0→4 equiv.). From these
large amplitude of 115 mdeg. The presence of sinusoid-like shape of titrations, Figure 4, it was clear that
a significant structural
the CD spectra for [Eu2:L3] suggests the appearance of possible conformations occurred upon formation of the helicates in solution,
mixing between electric- and magnetic-dipole allowed transitions and the presence of a kinetic effects was also observed.[23] As
(most likely exciton coupling) between the chromophores that has previously observed for CD spectra of the ligands and their
been brought in one another close proximity due to the formation assemblies with Eu(III) (Figure 3) the titrations profile for 5(S,S) was
of the Eu(III) assemblies.[21] Similarly, for [Eu2:L3] complexes where a mirror image to that observed for 6(R,R) (Figure 4), and the
L = 5(S,S) (Figure 3B), the first negative Cotton effect was observed binding isotherms (Figure S30) reveal the main changes in the
at 340 nm, with the following positive Cotton effect being observed spectra happening until the addition of 0.67 equiv. of Eu(III) (Table
at 295 nm and the amplitude of 3 mdeg. The latter positive Cotton 1), supporting the formation of the bi-metallic triple-stranded
effect then evolved into the next positive CD band centred at 227 helicates in solution.
nm with the following negative Cotton effect at 212 nm and the
Finally, the CPL spectra of [Eu2:L3] assemblies were recorded in
amplitude of 245 mdeg. Similarities in the shape of the CD spectra
CH3CN solution, where the Eu(III) centred emission was monitored
between [Eu2:L3] (L = 3(S,S), 4(R,R); Figure 3A) and (L = 5(S,S), 6(R,R);
upon ligand excitation (Figure 5). This clearly revealed the chiral
Figure 3B) suggests similar geometrical character of the assemblies
nature of the Eu(III) centres; each pair of enantiomers resulted in
being formed in CH3CN.
CPL spectra that were mirror images opposite in signs and equal in
The changes in the CD spectra can be used to correlate the magnitudes. The absolute configuration of Eu(III) centres can be
nonlinear enhancement of the chiroptical response to the tentatively assigned by comparing these results to the CPL spectra
nonlinearity dependence on point chirality for tetrahedral Eu(III) of previously published mononuclear Eu(III) complexes, where the
chiral cage structures or used to monitor anion sensing events for absolute configuration was determined by using X-ray
different anions in solution.[7b,12c] Furthermore, in our previous crystallography.[19] As such, the [Eu2:L3] self-assemblies where L =
work, [22] we have shown that the changes in the CD spectra of 3(S,S), 5(S,S) can be assigned as being ꢀ,ꢀ around the two metal
chiral ligands titrated with Ln(III) ions can be used not only to ions, while for L = 4(R,R), 6(R,R), the assignment of the absolute
Table 2. Dissymmetry factor values (glum) determined from the spectra depicted on Figure 3.
Transition λ, nm glum (Eu2(3(S,S))3) glum (Eu2(4(R,R))3) glum (Eu2(5(S,S))3) glum (Eu2(6(R,R))3)
5D0→7F1
5D0→7F1
5D0→7F2
5D0→7F3
5D0→7F3
5D0→7F4
5D0→7F4
5D0→7F4
5D0→7F4
595
603
620
654
658
696
700
704
710
0.27
0.19
-0.27
0.43
0.57
-0.06
-0.08
-0.08
0.53
-0.27
-0.27
0.28
-0.50
-0.61
0.06
0.09
0.09
-0.52
0.17
0.18
-0.21
0.20
0.32
-0.03
-0.06
-0.07
0.45
-0.17
-0.16
0.20
-0.26
-0.32
0.05
0.05
0.10
0.46
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Table 3. Overall europium-centred luminescence quantum yields and sensitisation efficiencies for [Eu2L3] helicate complexes recorded in CH3CN.
Eu2L3
τ
obs, ms
Φ
tot, %
τR, ms
6.9±0.4
10.4±0.7
9.4±0.8
ΦLn
21.0±0.5
14.3±0.8
15.3±0.9
%
η
sens, %
27.6
22.4
17.0
Ln,
L = 1(S,S)
L = 4(R,R)
L = 5(S,S)
1.44±0.01
1.48±0.01
1.43±0.03
5.8±0.1
3.2±0.5
2.6±0.1
ηsens – antenna-to-ion energy transfer efficiencies, τobs – the observed luminescence lifetime, Φtot – the experimental overall luminescence quantum yield (λex
=
͆͢
279 nm), τ – the radiative lifetime, Φ
– the intrinsic quantum yield of lanthanide.
͆͢
͌
configuration is
Table 2 and represented in the g-value plots on Figure S31. The The values of the radiative lifetime (
5D0 7F1 transition these were determined to be of similar values ΦLnLn) and the antenna-to-ion energy transfer efficiencies (ηsens
observed for other reported mono-metallic complexes and di- calculated for Eu(III) assemblies using their emission spectra, the
metallic helicates [19a,b, 12c]. The exception, however, being the observed luminescence lifetime ( obs), and Φtot suggests that non-
Λ
,Λ
. The dissymmetry factors (glum) are listed in times higher for [Eu2(1(S,S))3] than [Eu2(4(R,R))3] and [Eu2(5(S,S))3].
τR), the intrinsic quantum yield
→
(
)
τ
bulkier ligands 5(S,S) and 6(R,R) which gave lower glum values. The radiative processes was most apparent for L = 1(S,S). However, as
5
glum values determined for the D0
→
7F2 transitions were also found the same system reveals the highest ΦLn and ηsens in comparison
Ln
to be similar to the mono-metallic Eu(III) “Trinity Sliotar” bundles, to the other two systems means that the radiative processes in this
previously developed in our group.[19a] In comparison to these case compete well with non-radiative ones. This can be due to the
results, the L = 3(S,S), 4(R,R) assemblies showed relatively high glum better sensitisation efficiency thanks to the ligand structure
5
values for the D0
→
7F3 transition; being 0.40–0.60, which is among allowing the position of triplet state level relative to Eu(III) levels as
the highest reported for this kind of structures. Additionally well as better geometrical arrangement of the ligand around the
reasonably high glum values were observed for the 5D0 7F4 metal ion.[1, 25]
transition; being 0.45–0.53 for [Eu2:L3] (L = 3(S,S), 4(R,R)) and 0.45
for [Eu2:L3] (L = 5(S,S), 6(R,R)) (Table 2). Such relatively high
glum(5D0 7F3) values might be a sign of strong J-mixing and a strong
crystal-field perturbation in these triple-stranded helicates.[18b]
→
∼
Mass-spectrometry analysis of [Eu2:L3] assemblies Mass-
spectrometry analyses were performed using [Eu2:L3] complexes
formed under thermodynamic control by reacting 1 equiv. of the
respective L with 0.67 equiv. of Eu(ClO4)3 (50 wt. % in water) in
→
Photophysical properties of [Eu2:L3] assemblies in solution The CH3OH at 65 °C under microwave irradiation for 2 hours. The
photophysical properties of the triple stranded di-metallic helicates complexes were precipitated out of solution using ether diffusion,
[Eu2:L3], were monitored in solution upon the self-assembly dried under vacuum before being subjected to matrix-assisted laser
formation in real time. These were seen to be close to identical for desorption/ionisation mass-spectrometry (MALDI-MS) with DCTB
each pairs of enantiomers, therefore we report herein, only the used as matrix (Figures 6, S34-S37). For the Eu(III) samples
photophysical results for one of each enantiomers (Table 3, Figure investigated in this work the presence of [Eu:L], [Eu2:L], [Eu:L2],
S32). For these complexes, the lifetimes were recorded in solution [Eu2:L2] species was observed. At the same time [Eu2:L3] species
by monitoring the decay of the Eu(III) 5D0 7F2 band upon ligand were only observed for the samples with ligands L = 3(S,S) and
→
excitation at 279 nm. In all the cases, the lifetime decay curves were 4(R,R). The presence of tri-metallic species was also observed for
best fitted to mono-exponential decay using Origin9.1®; the values complexes with L = 3(S,S) [Eu3:L3]. The samples with L = 6(R,R) also
being practically the same, ca. 1.45 ms, for all the complexes resulted in the assignment of a [Eu3:L2] species, suggesting possible
(Scheme 1), which also corresponds to the life-time values obtained formation of more complex systems, bus such higher order systems
for the addition of 0.67 equivalents of metal ion to the solution. have been reported in the literature by others[12], believed to be
This suggest the presence of one type of coordination environment due to supramolecular cooperativity effects.[23b, 26].
around Eu(III) ion in solution (Table 3, Figure S33). The total Unfortunately, we were unable to get reliable MS or HRMS results
quantum yield (Φtot) of the Eu(III) emission was also determined for when using samples from the various spectroscopic titrations
these self-assembly species. The results showed that Φtot was two discussed above, this most likely being due to low concentrations of
6 | J. Name., 2012, 00, 1-3
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these assemblies in these samples.
together with MALDI-MS data. In contrast to these results, then
upon addition of Eu(CF3SO3)3 to 6(R,R) significant broadening was
observed for most of the characteristic signals in the 1H NMR
spectrum beyond the addition of 0.44 equiv. of Eu(III), making it
impossible to distinguish between these signals (Figure S41).
However, similarly to the aforementioned titration it was possible
to follow the changes of the water peak where it started to shift
upon addition of 0.66 equiv. of Eu(III) due to the possible
encapsulation within [Eun:Lm] assemblies (Figure S42). The
broadening of the 1H NMR spectra during the self-assembly process
between 6(R,R) and Eu(III) occurs most likely due to the similar
exchange rate between metal and ligand during complexation
compared to the NMR time scale, which could be directly related to
the size of the central spacer unit.[27] The signals in the 1H NMR
1H NMR study of the self-assembly between L and Eu(III) We also
investigated the formation of these helicates in solution by
observing the changes in the 1H NMR of the ligands. Unlike
commonly seen for many other lanthanide complexes (such as
cyclen based macrocyclic complexes), the changes in the NMR of
the systems studied here upon addition of the lanthanides,
occurred over relatively narrow range, and with less signal
broadening, allowing for the formation of these assemblies to be
monitored in real-time. The solutions of 4(R,R) and 6(R,R) (c = 1.00
×
(0
10–3 M) were titrated against gradual addition of Eu(CF3SO3)3
1.99 or 0 1.11 equiv., correspondingly) in CD3CN. In case of
→
→
4(R,R) (Figure 7, S38), noticeable changes started to occur upon
addition of 0.22 equiv. of Eu(III), and these continued until the
addition of 0.66 equiv., after which no significant changes were
observed within the aromatic region. This confirming the successful
formation of the [Eu2:43] in solution at these concentrations.
Furthermore, we also observed that a resonance assigned to water,
that was present in the solution of 4(R,R) at 2.13 ppm, experienced
a significant upfield shift to 1.74 ppm (Figure S39) suggesting a
possible encapsulation of water within the helicate. As we
anticipate a full saturation around the two lanthanide ions (by the
three ligand strands), it is possible that the water molecule is
located within a cavity, that is generated at the centre of the
structure where the three bridges are located. However, in the
absence of a crystal structure of this [Eu2:L3] assembly, this cannot
spectrum of Eu(III) complex with
L = 5(S,S) made under
thermodynamic control revealed broad signals similarly to the
titration data and along with the exchange rate this can occur due
to the steric hindrance of the ligand during the assembly processes
(Figure S43). However, as the solution of the complex was heated
up to 40 °C and then cooled down to 22 °C it became more resolved
(Figure S44) as heating increased the reaction rate shifting the
equilibrium from L and Eu(III) towards the formation of highly
symmetrical [Eu2:L3] (L = 5(S,S)) species.[27]
Conclusions
1
In conclusion we have synthesised four new ligands by
coupling the diamine linker of various size with two chiral half-
helicate ligands. The titrations of these at low concentration
upon addition of Eu(III) in CH3CN allowed us to determine the
binding constants values for [Eu:L], [Eu2:L2] and [Eu2:L3] self-
be fully determined. The H NMR spectrum was recorded for Eu(III)
complex with L = 3(S,S) synthesised under thermodynamic control
and its shape closely resembles the spectrum observed for 0.66
equiv. addition of Eu(III) ion to the ligand during the titration (Figure
S40) therefore confirming the formation of [Eu2:L3] in both cases
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Journal Name
Pecharsky, Elsevier, Amsterdam, 2016 , vol. 50, ch. 287, pp.
141-176.
assembly species. Lower values of the binding constants were
found for the systems with the bulkier spacer
the resulting structures were most likely less rigid in
comparison to (S,S), (R,R). The conformational changes of
5(S,S), 6(R,R) as
2
3
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3
4
these chiral ligands upon complexation with Eu(III) ions were
monitored by following the changes in the CD spectra.
Recording CPL spectra for [Eu2:L3] allowed us to assign
absolute configuration of Eu(III) centres and calculate the
dissymmetry factor values.
4
The luminescence quantum yields between [Eu2:L3] with
(R,R) and (S,S) assemblies were surprisingly similar with
the highest quantum yield being observed for Eu(III) complex
with 1,3-phenylenedimethanamine linker ( (S,S)).
L
=
4
5
5
6
L = 1
The mass-spectrometry analysis of the solid complexes
shown the presence of [Eu2:L3] assemblies. However, the
results revealed the presence of
[Eu3:L2] and [Eu3:L3]
assemblies suggesting the presence of more complicated
species in the solid state at high concentration. 1H NMR
titration study performed at higher concentration than
spectroscopical studies confirmed the formation of highly
symmetrical [Eu2:L3] helical species in solution for the systems
7
8
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assemblies in the solid state was more complex. The diversity
of these systems allowed us to investigate their formation
using various techniques and help their development towards
materials for the applications in electronics for virtual reality
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Conflicts of interest
There are no conflicts of interest to declare.
12 (a) C.-L. Liu, L.-P. Zhou, D. Tripathy, Q.-F. Sun, Chem.
Commun., 2017, 53, 2459-2462; (b) L.-L. Yan, C.-H. Tan, G.-L.
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Acknowledgements
Authors would like to thank Dr. Ann Connolly (UCD) for
providing the results of elemental analysis and Science
Foundation Ireland (SFI) for financial support for a PI 2013
13/IA/1865 grant (TG).
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