Inorganic Chemistry
Communication
III
The Eu -directed self-assembly of 1 and 2 was then
evaluated by a series of spectroscopic measurements in situ in
order to establish their ability to form the 2:3 helicate in
solvents of varying competitiveness. Measurements entailed
titrations of 1 × 10− M solutions of either 1 or 2 against
increasing [Eu(CF SO ) ] and then analysis of the changes in
following the titration of 1 with Eu(CF SO ) in CH OH
3
3 3
3
5
3
3 3
III
the absorption, fluorescence, and Eu -centered emission of the
3
resulting solutions. Studies were performed in CH CN,
the presence of three absorbing species in solution including
ligand 1 and the 2:2 and 2:3 species. The predominant species
in solution is the Eu ·1 dimetallic triple-stranded helical
3
CH OH, and CH OH/H O solvent systems (50:50 and
3
3
2
8
0:20). Studies carried out in CH CN for both ligands were
3
2
3
species, with it being formed in approximately 54% at 0.6 equiv
3
III
S14). The absorption spectrum of ligand 2 recorded in
of Eu (log β = 25.7 ± 0.3). The Eu ·1 species is formed in
2
3
2
2
III
CH OH was shown to consist of two characteristic bands, as
approximately 98% yield upon the addition of 3 equiv of Eu
3
was observed in CH CN [λ = 226 and 281 nm (ε = 16981
(log β = 19.8 ± 0.1). Similar results were displayed following
3
max
22
−
1
−1
M
cm )], which presented a less hyperfine structure than
fitting of the changes in the absorption spectra of ligand 2 and
luminescence titration data.
was observed in CH CN. The overall changes in the
3
absorption spectrum of ligand 2 upon the addition of
The self-assembly of 1 and 2 in CH OH/H O (50:50 and
3
2
Eu(CF SO ) in CH OH (Figure S17) shows a loss in the
80:20), in a constant ionic strength, and in a 2-[4-(2-
hydroxyethyl)piperazin-1-yl]ethanesulfonic acid buffered sol-
little, slow, or no conformational changes in solution upon
3
3
3
hyperfine structure of the band centered at λ = 281 nm, while
an enhancement in absorption was observed at λ = 250 nm
max
III
up until the addition of ∼1 equiv of Eu , with subsequent
additions resulting in an absorption plateau. The changes
III
observed are much less pronounced in CH OH because of the
interaction with Eu . In contrast, significant luminescence
3
III
enhanced competitive nature of the environment.
A gradual enhancement in the characteristic linelike
emission bands located at 595, 615, and 695 nm is depicted
changes indicated complexation and subsequent Eu sensitiza-
tion. The trend observed in CH CN and CH OH was not
3
3
observed in this more competitive environment, making
nonliner regression analysis of these data inconclusive.
Therefore, a number of kinetic measurements were conducted
where the system was allowed to reach a state of equilibrium.
However, the overall changes in the luminescence spectra after
1
indicating that the self-assembly processes between Eu and
III
1
/2 in such a highly competitive media give rise to the
formation of identical species, which could involve other self-
assembly processes that are not seen in CH CN/CH OH.
3
3
While we were unable to structurally analyze these, Law and
3e
co-workers have elegantly been able to structurally character-
ize the formation of 4:4 cage systems using chiral picolinate
ligands, indicating the possibility of other higher-order systems
being formed in situ in the case of 1 and 2.
The assembly of Eu ·1 into higher-order architectures was
2
3
next investigated using scanning electron microscopy (SEM).
Prior to this, the stability of both Eu ·1 and Eu ·2 complexes
2
3
2
3
III
III
Figure 3. Overall changes in the Eu -centered phosphorescence
spectra upon titration of 2 (1 × 10− M) against Eu(CF SO ) (0 →
(synthesized) was measured by monitoring the Eu -centered
5
3
3
3
emission exhibited from the solid dissolved in 80:20 CH OH/
3
5
equiv) in CH OH at room temperature.
3
H O. Gratifyingly, no significant decrease in the emission was
2
stable (Figure S37). In a 50:50 solution, however, a precipitate
the EuIII 5D excited state to F states (where J = 1, 2, and 4).
7
0
J
The binding isotherm graph (Figure 2, inset) demonstrates
that, upon the addition of 0 → 0.6 equiv of Eu , a rapid
increase in the emission intensity was observed, similarly for
that seen in CH CN, followed by a sharp decrease up until the
appeared after a short period of time. Samples of Eu ·1 (0.2%,
2 3
III
w/v) in these media were drop-cast onto silica plates and dried
in air before the morphologies of the resulting samples were
analyzed using SEM. While a thin film was observed from
CH CN, samples drop-cast from CH OH and CH OH/H O
3
III
III
addition of ∼1 equiv of Eu . Additional aliquots of Eu result
in an eventual plateau in the luminescence intensity because
initially the Eu ·2 species is formed, after which Eu ·2
2
3
3
3
2
mixtures all demonstrated the formation of hierarchical self-
assemblies, displaying microsphere morphologies. Spherical
2
3
2
becomes the dominant species in solution. The fluorescence
aggregates produced from CH OH (Figure 4) were typically
3
S18), showing the band centered at λmax = 400 nm
monodispersed, displaying a mean particle size of 0.681 μm. In
a similar manner, using Eu ·A , spherical aggregates were also
3
2
3
experiencing a moderate quenching effect after the addition
3
III
of ∼0.65 equiv of Eu (47% and 56% for 1 and 2 in CH OH,
particle size of 0.531 μm (Figure S41). In contrast, while also
generating monodisperse aggregates, those formed from Eu ·1
3
respectively). The overall changes in the absorption,
fluorescence, and Eu -centered phosphorescence spectra
2
3
III
in CH OH/H O mixtures displayed a dramatic decrease in the
3 2
C
Inorg. Chem. XXXX, XXX, XXX−XXX