Structural self-sorting of pseudopeptide homo and heterodimeric disulfide cages in water: mechanistic insights and cation sensing

Article abstract of DOI:10.1039/d1tc01445e

The molecular components of biological systems self-sortviadifferent mechanisms to act in a cooperative manner and to avoid interfering with each other. Herein we describe mechanistic insights and a versatile strategy for the synthesis of water-soluble, pseudopeptide molecular cages based on disulfide bonds. The use of trifunctional thiols led to a dynamic combinatorial library (DCL) of readily isolable, multi-component homo and hetero cage-like architectures showing a degree of self-sorting related to the symmetry and size of the trithiol. The work provided detailed kinetic studies and DFT molecular modeling giving original insights into the disulfide cages' properties. We also applied the selected cage system in the fluorometric detection of La³⁺cations, which led to the generation of a strongly luminescent metal-organic assembly.

Full text of DOI:10.1039/d1tc01445e

Journal of
Materials Chemistry C
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PAPER
Structural self-sorting of pseudopeptide homo
and heterodimeric disulfide cages in water:
mechanistic insights and cation sensing†
Cite this: J. Mater. Chem. C, 2021,
, 7607
9
ab
ab
c
Marcin Konopka, Piotr Cecot, Jack M. Harrowfield
and
ab
Artur R. Stefankiewicz
*
The molecular components of biological systems self-sort via different mechanisms to act in a
cooperative manner and to avoid interfering with each other. Herein we describe mechanistic insights
and a versatile strategy for the synthesis of water-soluble, pseudopeptide molecular cages based on
disulfide bonds. The use of trifunctional thiols led to a dynamic combinatorial library (DCL) of readily
isolable, multi-component homo and hetero cage-like architectures showing a degree of self-sorting
related to the symmetry and size of the trithiol. The work provided detailed kinetic studies and DFT
molecular modeling giving original insights into the disulfide cages’ properties. We also applied the
Received 29th March 2021,
Accepted 12th May 2021
DOI: 10.1039/d1tc01445e
3+
selected cage system in the fluorometric detection of La cations, which led to the generation of a
strongly luminescent metal–organic assembly.
rsc.li/materials-c
in isolating pure cages from post-reaction mixtures. Our objective
thus was to fill these gaps by developing an efficient synthesis of
Introduction
The synthesis of functional molecular cages continues to pose pure homo- and hetero-dimeric pseudopeptide, water-stable
an intriguing challenge due to the numerous applications of molecular cages, by using thio-amino-acid functionalized organic
1
2–4
such structures as chemical receptors,
nanocapsules for platforms. This was expected to provide a robust workshop to
5
–7
8
molecular transport,
or in molecular recognition. Despite explore the phenomenon of self-assembly and self-sorting in a
enormous progress in the development of this type of dynamic multi-disulfide system. We also attempted to draw some
9
–12
architecture,
soluble structures are still very scarce.
chemistry (DCC) of disulfides is an effective approach to generate calculations. Based on this, we propose a theoretical model that
examples of stable, yet dynamic and water- new insights into the structural and geometric factors crucial for
13–16
Dynamic combinatorial disulfide cage formation based on experimental data and DFT
17,18
artificial biocompatible systems in water.
Disulfides have could be applied for the effective design of next mono
19,20
21–25
been extensively studied with linear,
macrocyclic
but far less is known about disulfide cages. preliminary investigation of a strongly fluorescent metal–organic
To date, several examples have been reported of disulfide cages material obtained using a disulfide cage as poly-anionic ligand
or inter- and bicomponent disulfide cages. In addition, we describe a
26–31
locked topologies
32–35
based on trifunctional thiols,
although only a few are water- on La(III).
36–39
soluble.
There is a real lack of information regarding the
impact of the thiol component structure on the tendency of
dynamic system to favor exclusive formation of cage-like
structures through self-assembly and self-sorting in water-soluble
Results and discussion
Design of building blocks
40–42
systems.
Hetero-component cages seem to be particularly
understudied, a situation, which is probably due to the difficulties The three core platforms used were based on triamides formed
from aromatic tricarboxylic acids and L-cysteine. The use of
a
b
c
natural L-cysteine ensured the absence of diastereomeric
products and the formation of chiral cages with stable absolute
configurations. The set of aromatic platforms was selected to
gradually increase the size (diameter) of the rigid core. Each
was functionalised by amide formation with L-cysteine in order
to introduce –SH (for disulfide formation) and –COOH
(for water solubility) groups. All building blocks were obtained
Faculty of Chemistry, Adam Mickiewicz University, Uniwersytetu Pozna n´ skiego 8,
1-614 Pozna n´ , Poland. E-mail: ars@amu.edu.pl
6
Center for Advanced Technologies, Adam Mickiewicz University,
Uniwersytetu Pozna n´ skiego 10, 61-614 Pozna n´ , Poland
Institut de Science et d’Ing ´e nierie Supramol ´e culaires, Universit ´e de Strasbourg,
8
all ´e e Gaspard Monge, 67083 Strasbourg, France
†
Electronic supplementary information (ESI) available: Experimental details,
synthetic protocols, NMR, LC-MS, HPLC data. See DOI: 10.1039/d1tc01445e
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1
H NMR spectra of the respective components with those of
the cages and analysis of optimised structures revealed that
the aromatic proton signals of the cages are shifted upfield,
presumably as a result of close proximity of the aromatic
platforms in the cage structure. In contrast, the signals
2
originating from cysteine CH and CH groups were found to
be deshielded due to their position outside the cages (see ESI,†
Fig. S19, S24, S29 and S60–S65). Acidification to pH 2 of the
post-reaction mixtures yielded the three cages as the water-
insoluble hexa-carboxylic acids, which could be easily isolated
by filtration. However, down to the slightly acidic region of pH
5–6 the cages remained soluble, which extends the range of
their applications.
Synthesis of heterodimeric cages
Fig. 1 General scheme of the dynamic covalent DCL. Three building
blocks react together and give five different disulfide cages, three homo-
dimeric (1-1, 2-2 and 3-3) and two heterodimeric cage-like architectures
The next step was to investigate if the different building blocks
would react with each other and form the hetero-component
cage structures. Equimolar solutions containing pairs of
components were prepared at pH 8.0 and then stirred for five
days in air. Three different products in equal proportions were
detected in the post-reaction mixture derived from components
(1-2 and 2-3).
36
by a slightly modified three-step synthetic route (see ESI,† Fig. S1).
In the first stage, the reaction of each tricarboxylic acid with N-
hydroxysuccinimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl)
carbodiimide hydrochloride (EDCÁHCl) provided the corres-
ponding NHS activated esters. Next, formation of the amides
by reaction with L-S-trityl-cysteine followed by hydrolysis of the
trityl-moieties in acidic conditions led to the desired trifunctional
components 1, 2 and 3, which have planar sizes in the order
1
and 2 (Fig. 2). Based on the LC-MS analysis two homodimeric
cages, 1-1 and 2-2, were identified while the third product had
the composition expected for the heterodimeric cage 1-2.
Similar results were obtained for the mixture of 2 and 3, which
after the reaction contained cages 2-2, 3-3 and heterodimeric
cage 2-3. For the mixture of 1 and 3, only homodimeric
structures were found in the post-reaction mixture, with none
3
4 2 4 1 (Fig. 1).
Synthesis of homodimeric cages
Solutions of each component (5 mM) were prepared in an
aqueous 0.01 M NaOH solution and its pH was adjusted to
8.0 by addition of dilute HCl. The presence of thiolates in
solution is obligatory for their oxidation to disulfides as well as
for disulfide dynamic exchange. These solutions of the basic
components were then placed in loosely capped 2 mL vials and
allowed to oxidize and equilibrate for 5 days. To ensure that all
DCLs reached thermodynamic equilibrium, each sample was
further analysed by HPLC one week and one month after the
end of the 5 day reaction time. Control analyses showed no
quantitative or qualitative changes in any DCL. After this time,
LC-MS analysis was performed. This showed that each post-
reaction mixture contained only one product (more polar than
the substrate), which was identified by mass spectroscopy as
the monoprotonated species with a mass of twice that of the
reactant less the 6 hydrogen atoms corresponding to those lost
in formation of 3 disulfide links, i.e. the tris-disulfide dimeric
cage. The compound 1-1 is a known species, identified previously
1
37
by LC-MS and H NMR. However, we decided to include cage 1-1
in our studies for comparison and to establish its full Fig. 2 (a) HPLC chromatograms showing the formation of heterodimeric
cages (1-2 and 2-3) in the component pairs mixtures. For the pair of 1 + 3,
characteristics. Further details of the structure 1-1 were
1
cage 1-3 was not observed. Due to significant differences in molar
extinction coefficients of the components, the chromatograms do not
show the real amounts of the products. The dashed line shows the
actual content of each (see ESI,† Fig. S57 and S58), (b) comparison of
established from its H NMR spectrum, which showed the D
3
symmetry expected for the homodimeric product (see ESI,†
1
Fig. S19 and S20). The H NMR spectra of the dimeric cages, 2-2
and 3-3 showed the same symmetry. Comparison of the experimental and simulated mass spectra (ESI-MS) of all five cages.
7
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1
H NMR spectra of the hetero cage 1-2 and the components 1
and 2 is also in line with the cage structure (Fig. 3). The upfield
shifts of the doublets (cyan-green) from the triphenylamine
moieties and the singlet (blue) from the central phenyl ring
clearly indicate a strong interaction between aromatic hydrogens
and the proximity of organic platforms of 1 and 2. This explanation
is supported by analysis of the optimized structure of the 1-2 cage.
In this structure, all of the aromatic hydrogens are additionally
shielded by the accumulation of electron-rich sulfur (S–S bonds)
and oxygen (amides) atoms.
The hydrodynamic radii deduced from DOSY experiments
(
(
see ESI† for details) coincide closely with the calculated results
Table 1). Finally, we performed an experiment with an
equimolar mixture of all three components at once to check
if in this three component DCL any new architectures would
emerge. The resulting DCL contained an almost equimolar
mixture of all five cages, without any traces of new species
O, 298 K, 600 MHz) of hetero- (see ESI,† Fig. S44). As in previous experiments, no products with
1
Fig. 3 Comparison of H NMR spectra (D
2
dimeric cage 1-2 and components 1 and 2. Dashed lines show chemical
shifts of analogous signals from aromatic hydrogens from both organic
platforms.
a different architecture were found. This clearly established that
our system based on well-defined C3 symmetric components
spontaneously selects the stable cage-like structures from the
numerous possibilities of combinatorial connections between
of the possible cage 1-3. We observe here a kind of structural the three trifunctional components (see ESI,† Fig. S70).
self-sorting based on the components’ C symmetry and size Although we have obtained pure products, we have been
3
factors favoring the formation of the more symmetrical unable to obtain crystals suitable for X-ray structure determinations.
products in the final DCL. No linear or macrocyclic products For this reason, we decided to use computational tools to
were observed in any of the post-reaction mixtures. Each of the optimize the cages’ structures (Table 1 and ESI,† Fig. S60–
4
3–45
DCL samples was completely transparent before and after the S65).
Based on this it can be concluded that the cage
reaction and was not additionally filtered before LC-MS analysis. molecules prefer an ‘‘open’’ conformation, where a small
For this reason, we exclude the possibility that polymeric or internal space of the molecule is present and the disulfide
insoluble products have formed in the DCL that might not be groups are directed towards the inside of the molecule. The S–S
detected by the HPLC DAD detector. To isolate the heterodimeric torsion angles are typical for this binding in all optimized cages
cages, the experiment with pairs of components 1 + 2 and 2 + 3 and averaged 1071 (in the range 851 to 1361). One structure
was repeated on a larger scale and at a higher concentration with a ‘‘closed’’ conformation and disulfide groups directed
(10 mM). Heterodimeric cages were isolated by semi-preparative outwards turned out to be that of the cage 2-3, which can be
HPLC on a few milligrams scale and characterized by NMR explained by the significant differences in the size of aromatic-
1
spectroscopy. H and COSY NMR spectra confirmed their organic platforms that enforce such arrangement of cysteine
3
expected structures and C symmetry. Comparison of the arms and disulfide bonds. Also, we found that this structure is
Table 1 List of the 3D models and the comparison of experimental and calculated dimensions of cages
a
Cage
1-1
2-2
3-3
1-2
2-3
1-3
Calculated structure
Symmetry
D
3
D
3
D
3
C
3
C
3
C
3
n/a
n/a
10.22
4341
3.5
101.7
120.8
14.5
D [m s  10À10
2
À1
]
2.05
9.68
8.36
3971
7.9
107.0
95.1
4.9
1.80
11.02
10.35
5612
8.2
136.6
93.7
9.7
1.47
13.50
12.05
10 304
8.1
105.5
96.3
5.2
2.25
8.02
7.86
2873
7.5
105.4
106.9
8.1
1.93
10.28
9.13
4553
4.0
85.5
99.8
1.5
b
r
r
V
d
f
z
hyd [Å]
c
calc. [Å]
b
3
sph [Å ]
d
e
[
Å]
f
g
W
a
c
f
b
For cage 1-3 only calculated properties are available. Calculated from DOSY diffusion coefficients using the standard Stokes–Einstein equation.
d
e
Measured from optimised models using Gaussian software. d – distance between platforms. j – average disulfide bonds dihedral angle.
W – rotation angle between organic platforms. z – trapezoidal angle.
g
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additionally stabilized by stacking interactions, due to the very Kinetic studies
close proximity of the aromatic cores (3.0–4.0 Å). The cage 3-3
Because the calculations indicated that the formation of cage
composed of two large aromatic platforms should also be
stabilized in this way, since the angles between the arms and
platforms are close to 901, favoring extensive close overlap.
For this reason, we have defined two geometric factors that can
influence the structure.
1-3 is the least preferred for geometric reasons, but structurally
possible, we decided to supplement our work with kinetic
studies to find any additional reasons for the absence of 1-3.
We employed HPLC to monitor cage formation over time.
Reactions were performed in 2 mL vials at pH 8 in 5 mM
concentration in room temperature. Each reaction mixture was
observed for 30 hours and monitored every 30 minutes. The
One, termed the trapezoidal angle z (zeta), is the angle
between the platform plane and the cysteine arm, which
defines the relationship between the size of the platform and
the angle imposed on the binding arm. The second, W (theta), is
the degree of rotation between the platforms about the principal
z-axis. This angle is important for the measure of structural
stress caused by the differences in the size of the platforms and
for geometric considerations, e.g., steric hindrance. The angles
were determined for four points: centroid of the smaller
platform – carbon alpha of the arm of the smaller platform –
alpha carbon of the larger platform – centroid of the larger
platform. Centroids have been designated as the central nitrogen
atom in platform 2 or the geometric center of the benzene ring in
platforms 1 and 3. So, high values for both of these angles seem
to be universal factors for determining the geometric barriers to
formation of a given cage structure. For each homodimeric cage,
the values of those angles are very similar (W near 51 and z near
0
first HPLC injection (t ) for every sample was done immediately
after dissolving the component in basic water and setting the
pH to 8.0. Thus, we obtained a set of chromatograms illustrating
component decay and cage growth over time based on the
relative peak area (RPA). To ensure that the method was
quantitative, absorption spectra were recorded from the
solutions of each component and the corresponding cage at
the same concentration. The difference in registered
absorbance between the component/cage pairs did not exceed
5%. Each reaction was repeated and monitored three times to
ensure that results were the same. For the homodimeric cages,
the material distribution curves are shown in Fig. 5a–c.
These show that in mono-mixtures the time for conversion to
951) and those compounds are formed smoothly. This two-
angles factor is slightly more complicated for two observed
heterodimeric cages 1-2 and 2-3, but the values do not exceed
81 theta and 1151 zeta. Cage 1-3 is the only one not formed
during the experiments, and the reason for this may be the
difficulty involved in attaining the cage geometry. The zeta angle
takes on the highest value of 1211 due to the significant
difference between the sizes of the aromatic platforms of 1
and 3. The platforms can rotate about the z-axis to compensate
for this stretching effect but the optimal theta angle then turns
out to be large enough to stop the cage from forming (Fig. 4).
Fig. 4 Diagram of W (theta) and z (zeta) angles for the six optimized
dimeric cages (blue squares – homodimeric cages, triangles – hetero-
dimeric cages, green – observed cages, red indicates no cage formation
for 1-3).
Fig. 5 Normalized rate plots of mono-component DCL (a–c) and multi-
component DCL equilibration (d) 1 + 2 mixture, (e) 2 + 3 and (f) 1 + 2 + 3.
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100% product within experimental error was shortest for cage
3-3 (10 h), followed by 1-1 (20 h) and longest for 2-2 (26 h). We
attribute this to the influence of subtle effects such as the
differences in solvation of polar cysteine moieties, in surface
polarity and in the size of each component. Cooperativity
between these factors appears to result in the largest aromatic
4
6–48
species 3 having the fastest cage formation.
LC-MS measurements taken 60 minutes after the start of
reactions provided a deeper understanding of the cage formation
process. These measurements (ESI,† Fig. S45–S50) showed that
aside from unreacted trithiol, the dominant species present
corresponded to the linkage of two trithiol molecules through
one, two or three disulfide bonds. No species derived from more
than two trithiol units were detectable. This is consistent with an
initially intermolecular reaction to give the mono-disulfide
being followed by rapid intramolecular reactions giving the
Fig. 6 (a) normalized kinetics plot of 1 + 3 DCL equilibration (substrates
and products), (b) schematic representation of possible disulfide
exchanges in the 1 + 3 mixture.
More significantly, the homodimeric cages 1-1 and 3-3 form
macrocyclic bis(disulfide) and then the cage tris(disulfide). almost twice as fast as in the single component systems.
Competition by intermolecular processes after the first step Complete conversion of 3-3 takes about 4 h, while, once initiated,
appears to be ineffective. The fact that the cage dominates over complete formation of 1-1 takes about 10 h. Since the thiol
49,50
the macrocycle at a time when the mono-disulfide is still present oxidation is slower process than the disulfide exchange,
indicates that restrictions of motion in the macrocycle must formation of the first S–S linkage between two tri-thiols must be
51
lead to faster disulfide formation relative to that giving the the rate-determining step in the entire cage formation process.
macrocycle. Thus, in the first step, two molecules of trithiol What distinguishes both systems (single vs. mixed component) is
form open-chain mono-disulfide adducts (1A, 2A, 3A). These are the formation of intermediate species e.g. 1-3A (reaction III,
then further oxidized to the macrocyclic bis-disulfide products Fig. 6b), and we assume that the presence of the latter is related
(1B, 2B, 3B), which in the third step are finally oxidized to the to observed acceleration of cage formation. Thus, the nucleophilic
cages. (Intermediates are shown as gray and black lines in attack of thiolate on the 1-S–S-3 disulfide (1-3A), results in the
Fig. 5a–c). In the case of cage 2-2, the bis(disulfide) intermediate formation of linear S–S–S transition state, which decomposes to a
50
was more abundant than the mono-(disulfide) intermediate at more stable disulfide product. If 1 is a weaker acid than 3 then
À
all points where the reaction mixture was sampled (Fig. 5b). This under the same pH conditions, there must be a lower 1-S thiolate
is consistent with acceleration in rate due to the intramolecular concentration, which results in both a slower radical formation
nature of the second reaction step, which is largely dependent on during oxidation and a preferred attack of 3-thiolate on the
the chemical structure and shape of the component employed. intermediate 1-3A. In this case, the synthesis of 1-1 through either
This is supported by the molecular modeling results that showed the 1-3A path (reactions III and V) or path IV can be effectively
smaller conformational restrictions in the macromonocyclic stopped until the system is essentially drained of 3. However,
bis(disulfide) intermediate 2B than those found for 1B and 3B, considering the complexity of this system, identifying the exact
52
see ESI,† Fig. S66–S69). For cages 1-1 and 3-3, the observations cause of this phenomenon is very challenging.
are less complicated and the disappearance of the reactant in Where hetero-cage formation occurs as in reactions involving
both cases can be well fitted to a single exponential decay. 2, the kinetics become more complicated to analyze because
This implies that the reactant loss is a first-order process both reactants are involved in competitive processes. By LC-MS
(seemingly irreversible), a situation which might arise if the analysis we also observed all the expected hetero-component
rate-determining step of the reaction were to be the very intermediates, but at much lower concentrations (o5% RPA,
first step involving the trithiol and dissolved oxygen. Although LS-MS, ESI,† Fig. S51–S56). A schematic representation of
formally a second order process, if the agitation of the solution possible reactions between a pair of different components
and its contact with the normal atmosphere were sufficient to leading to a mixture of homo and heterodimeric cages is shown
maintain a constant concentration of dissolved oxygen, it would in Fig. 6a. We assume that for pairs 1 + 2 and 2 + 3, reactions IV,
obey pseudo-first-order kinetics. Once the first species is formed VII and XI occur with a similar probability, resulting in the
and then dimerizes, all subsequent reaction steps, as noted observed equimolar mixture of homo and hetero products.
above, would be intramolecular and probably greatly accelerated In the case of the pair 1 + 3, reaction intermediates III (1-3A)
as a result. Examination of the kinetics of cage formation in the and VIII (1-3B) were found at a low concentration (o2% RPA,
mixed 1 + 3 DCL (Fig. 6a) provided evidence for one of the LC-MS). Normalized concentration-time plots for the reactions
complications in these reactions. In this system cage 3-3 is 1 + 2 and 2 + 3 are shown in Fig. 5d and e. In the 1 + 2 system,
exclusively formed first, then 1-1. Formation of 1-1 shows an cage 2-2 forms most rapidly, then comes 1-2 and the slowest is
induction period lasting until the essentially complete conversion 1-1. The total reaction time is much longer than in the 1 + 3 DCL
of 3 to 3-3, and an intermediate species decaying to 3-3 is very and takes about 30 hours. 2-2 reaches 50% of the target conver-
rapidly formed in a substantial amount.
sion after 4 hours compared to 8 hours in the DCL with only 2.
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As in the 1 + 3 system, the products involving a geometrically
larger component (1-2 and 2-2) are created at higher rates.
This effect is even more visible in the 2 + 3 system (Fig. 5e).
Here, cages 2-3 and 3-3 are formed at almost the same rate and
the initial rate is much higher than in the 1 + 2 DCL. Only when
the DCL is dominated 80% by 2-3 and 3-3 (after about 4 hours),
does the rate of formation of 2-2 increase.
Observations on the system with all three components
present (1 + 2 + 3) were consistent with the results from the
two-component systems, once again showing an apparent size
effect on the cage formation (Fig. 5d–f). Products containing
the largest component 3 formed faster than those containing 2
and the slowest formed were those with 1. Cages built of 1 and
2
form preferentially only when the DCL has already consumed
most of the available 3.
3
+
Fluorometric detection of La cations and generation of a
strongly luminescent metal–organic material
While working with the cages, we noticed that both 2 and cage
2-2, due to the triphenylamine core, show strong fluorescence.
We decided to further explore this feature of 2-2 to preliminary
investigate some useful properties for potential application of
this particular cage. Comparison of the emission spectra of
2
and 2-2 shows almost four times stronger fluorescence of the
cage (max. 456 nm) than that of the component (max. 466 nm)
and the a blue-shift of the maximum emission of 2-2 by 10 nm
Fig. 7 Synthesis of a fluorescent 2-2-La material; (a) normalized emission
spectra of the cage 2-2 and 2 (10 –10 M, 1 Â 1 cm cuvette, ex: 345 nm,
water pH 8.0), (b) normalized solid-state emission spectra of 2-2 and 2-2-La
À5
À6
(
(
Fig. 7a). We assume that this is due to an AIE-like effect
aggregation-induced emission) because the closed cage
(
ex: 345 nm), images taken under 325 nm UV lamp, (c) TGA/DGA analysis of
structure prevents the chromophore NPh rings from rotating 2, 2-2 and 2-2-La, (d) SEM and SEM-EDS analysis of 2-2-La. See ESI† for
3
details (Fig. S71 and S72).
freely, which results in increased emission intensity. Moreover,
À
each cage is a polyanion with six CO
2
groups concentrated on
a small molecule area and available for coordination through
O-donors.
characterised, it would be expected that the organic parts of
Encouraged by these cage features, we decided to synthesize the materials would undergo progressive decomposition, probably
a metal complex based on 2-2. Mixing an aqueous solution of in multiple steps, to give a residue of La O plus, possibly, Na SO
4
2
3
2
2
-2 as sodium salt with aqueous La(NO3)3 results in the or Na CO from the 2-2-La and the sodium salts alone from 2-2, so
2 3
quantitative precipitation of highly insoluble, amorphous that the overall % changes in mass should be different. To get
metal–organic 2-2-La material (see ESI† for details). Taking into visual insights into the morphology of the 2-2-La complex, SEM
account the number and location of the cage chelating groups imaging was performed and revealed the amorphous state of the
3
+
as well as coordination preferences of La cation, formation of powdered material. The close-ups show a spongy porous structure
cross-linked coordination assembly was expected. The compo- with numerous pores, about 0.1 mm in diameter, presumably due
sition and purity of the generated material in the solid state was to the three-dimensional architecture of cage 2-2 with internal
confirmed by the elemental (NHCS) analysis, which indicated hydrophobic voids (Fig. 7d). Additionally SEM-EDS analysis was
1
: 1 metal : cage ratio (see ESI,† Fig. S72). This was further used to map the surface distribution of elements in the 2-2-La. The
supported by the ICP-MS analysis, which showed 10.6 wt% of obtained results confirmed the presence of all elements expected
La in 2-2-La for expected 12.6 wt% (see ESI,† page 45). in the investigated material (Fig. 7d and ESI† Fig. S71, S72). Since
Unfortunately, despite many attempts, the solubility of the in aqueous solution, 2-2 exhibits interesting fluorescence behavior,
obtained material prevented the mass spectrum to be registered. we decided to examine and compare this property with metal–
To determine the thermal stability of 2-2-La, TGA in the range of organic 2-2-La measured in the solid state. These measurements
30–600 1C was conducted and showed high stability of this show that described system is photo-responsive, which opens
material, which lost only 40 wt% with fairly linear weight loss. several avenues towards their application in various technological
In comparison, organic compounds 2 and 2-2 turned out to be fields. As presented in Fig. 7b, after excitation at 345 nm, the emission
much less stable and already at 250 1C showed a major weight spectrum of 2-2-La shows a broad band with its maximum at 445 nm,
loss of about 30 wt% (over the full range of heating lost of while in the emission spectrum of 2-2 apart from seemingly lower
65 wt% and 50 wt%, respectively was recorded; Fig. 7c). While intensity, there is a notable blue-shift (55 nm) in the band
the volatile products from thermal decomposition were not maximum = 500 nm (for absorbance spectra see ESI,† Fig. S59).
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The difference in solid-state fluorescence of both materials Knowledge Education Development. PC thanks the Poznan´
under excitation at 325 nm is also clearly visible to the naked Supercomputing and Networking Center (grant no. 401) for
eye as a significant difference in color (Fig. 7b insets).
granting super-computers access. ARS thanks the National
Science Centre (grant SONATA BIS 2018/30/E/ST5/00032) for
financial support. We thank Prof. Tomasz Grzyb for his help
with the solid-state fluorescence measurements.
Conclusions
We have described here a facile approach to the synthesis of
pure disulfide, water-soluble molecular cages based on trifunc-
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cages’ formation mechanism through careful analysis of every
intermediate product. As shown by the synthesis of 2-2-La in
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