Cooperative self-assembly of platinum(ii) acetylide complexes

Article abstract of DOI:10.1039/c3cc44997a

Rod-like platinum(ii) acetylide complexes have been demonstrated to form one-dimensional helical supramolecular polymers by the cooperative growth mechanism, leading to supramolecular gels by bundling single fibrils into entangled networks.

Full text of DOI:10.1039/c3cc44997a

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Cooperative Self-Assembly of Platinum(ІІ) Acetylide Complexes
Yu-Jing Tian,a E. W. Meijer,b and Feng Wang*a,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
intriguing to self-assemble platinum(ІІ) acetylide complex into
50 ordered supramolecular architectures in a cooperative manner.
Herein, we integrate platinum(ІІ) acetylide unit into the rod-
like monomers 1 and (S)-2 with two amide groups (Scheme 1).
By virtue of the synergistic hydrogen bonding and π–π stacking,
it is anticipated to form highly ordered one-dimensional
55 supramolecular polymeric assemblies with cooperative
mechanism. Introduction of (S)-3,7-dimethyloctyl groups into the
peripheral side chain of compound (S)-2 could induce a helical
bias for the resulting assemblies.10 Consequently, supramolecular
polymerization process of the two designed monomers could be
60 quantitatively probed with the combination of spectroscopic
measurements and the corresponding mathematical analysis.
The two monomers 1 and (S)-2, resulting from the efficient
coupling reaction between trans-[PtCl2(PEt3)2] and alkynes 4a–b
(Figure S1), were fully characterized with multi-nuclear NMR
65 (1H, 13C, 31P) and ESI-MS spectra (Figure S6–13). 31P NMR
spectra for both 1 and (S)-2 exhibit one signal at 10.9 ppm,
indicating the equivalence of the two phosphorus atoms and
thereby demonstrating trans configuration. Differential scanning
calorimetry (DSC) measurements reveal liquid crystallinity in the
70 bulky state for both compounds 1 and (S)-2 (Figure S14).
5
Rod-like platinum(ІІ) acetylide complexes have been
demonstrated to form one-dimensional helical
supramolecular polymers with cooperative growth
mechanism, leading to supramolecular gels by bundling single
fibrils into entangled networks.
10
In-depth understanding of self-assembly processes is crucial
for the applications of one-dimensional organic nanostructures in
the field of photovoltaics, sensors, and biomaterials.1 Based on
the energy evolution of the self-assembling systems, two different
types of mechanisms could be divided, namely isodesmic and
15 cooperative (nucleation–elongation) mechanisms.2 Cooperative
growth mechanism, resembling the process adopted by natural
systems,3 exhibits superior properties in comparison to the
isodesmic mechanism in many cases. For example, it avoids the
formation of small oligomers and leads to highly organized
20 aggregates with well-defined sizes and shapes. Up to now a
variety of supramolecular assemblies have been developed with
cooperative growth mechanism.4 It is worthy to note that multiple
non-covalent interactions with more than two nearest neighbors
should be embedded in the monomer structure to induce such
25 cooperativity. Therefore, judicious choice of the building blocks,
as well as profound understanding of the interplay between
different types of non-covalent interactions, would be of
paramount importance.5
In view that metal–ligand coordination interactions possess
30 highly directional and strength adjustable properties,6 it is
fascinating to achieve metallo-supramolecular assemblies with
cooperative growth mechanism, which would take full
advantages from both sides (highly ordered superstructures from
cooperative mechanism, as well as significantly optical, redox,
35 electrochemical, catalytic and magnetic properties from metal–
ligand coordination interaction). However, up to now cooperative
metallo-supramolecular systems have been far-less exploited.7
The challenge primarily lies in the steric hindrance exerted by the
metal–ligand complex, which hinders effective coupling with
40 other non-covalent interactions. Within this context, platinum(ІІ)
acetylide complex is an appealing building block, since it assigns
unique properties to the resulting supramolecular assemblies.8 For
example, square planar geometry of the d8 transition metal ion,
Pt(ІІ), facilitates the combination with other non-covalent bonds,
45 which potentially imparts cooperativity to the resulting self-
assembly systems. Moreover, platinum(ІІ) acetylide chromophore,
due to its intriguing optical property, demonstrates promising
applications in electronics and photonics fields.9 Therefore, it is
75
80
Scheme 1. Schematic representation for the construction of one-
85 dimensional helical supramolecular polymers 3 from monomers 1 or (S)-
2.
Compound 1 could gelate hydrocarbon solvents such as
heptane and hexane at room temperature (critical gelation
90 concentration: 37 mmol/L in heptane). It should be noted that
similar platinum(ІІ) acetylide systems without amide groups fail
to gelate any non-polar solvents,8b definitely supporting the
significant role of intermolecular hydrogen bonding for the
gelation capability. Heating the supramolecular gel leads to a
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fluid solution, while subsequent cooling to room temperature 60 previously reported platinum(ІІ) acetylides, lacking amide groups
restores the gel state, demonstrating thermo-responsive sol-gel
transition behavior (Figure 1a). Although platinum(ІІ) acetylide
complex 1 is non-emissive in solution, it exhibits a single
excitation peak centred at 437 nm, along with an emission peak
on the monomer structures, prefer to adopt H-type aggregation.8a-
b
Such phenomena highlight the huge impact of lateral hydrogen
bonds on the spatial arrangement of platinum(ІІ) acetylide
chromophores. On the other hand, in contrast to the gel/film state,
5
fixed at 485 nm in the gel state (Figure 1b). The apparent broader 65 it is non-fluorescent emission under UV illumination in MCH,
emission tail in the longer wavelength region is ascribed to the
scattering effect induced by gelation. Moreover, enhanced
fluorescence emission could also be observed from the doped
suggesting that non-radiative decay process still takes place for
the self-assembled aggregates in solution.
2.0
323 K
313 K
180
120
60
323 K
313 K
303 K
293 K
283 K
273 K
263 K
a
b
303 K
293 K
283 K
273 K
263 K
10 poly(methyl methacrylate) (PMMA) film state (Figure S16). It is
speculated that reduced conformational flexibility in the gel or
film state significantly suppresses the non-radiative decay process,
leading to an enhancement of fluorescence.11
1.5
1.0
0.5
0.0
70
75
80
0
-60
-120
-180
1.0
a
15
b
250
300
350
/ nm
400
450
250
300
350
/ nm
400
450
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.5
1.0
0.5
0.0
c
d
20
0.0
300
400
500
600
700
Wavelength (nm)
Figure 1. a) Thermo-responsive sol-gel transition; b) Normalized
emission (blue line, λex = 437 nm) and excitation spectra (red line, λem
25 485 nm) of the supramolecular gel from monomer 1.
=
270
280
290
300
310
320
0
20
40 60
Fraction of (S)-2
80
100
T/ K
85 Figure 2. a) Variable-temperature UV spectra of (S)-2 from 50 °C to -10
°C in MCH at the monomer concentration of 5 × 10-5 M; b) Variable-
temperature CD spectra; c) Net helicity Φ as a function of temperature for
(S)-2 in MCH, λ = 373 nm; d) Net helicity Φ as a function of the sergeant
(S)-2 fraction for the achiral 1 and chiral (S)-2 coassemblies, T = 10 °C, c
90 = 5 × 10-4 M in MCH, λ = 373 nm.
Supramolecular gels commonly originate from the bundling
of fibril structures into entangled networks. Hence, we further
investigated the aggregation of rod-like platinum(ІІ) acetylide
30 complex 1 and (S)-2 into one-dimensional supramolecular
polymers. As we know, chlorinated solvent is benign for the π-
conjugated systems in solution, whereas in the aliphatic solvent
such as cyclohexane, both hydrogen bonding and π–π stacking
would be more favourable. Therefore, solvent and temperature
To get more insights into the supramolecular polymerization
process, temperature-dependent CD spectroscopy was further
performed in MCH for the chiral monomer (S)-2 (c = 5 × 10-5 M).
No Cotton effect is present above room temperature, whereas
95 decreasing the temperature below 10 ºC leads to strong bisignate
Cotton effects, with a positive maximum at 354 nm and a
negative maximum at 373 nm (Figure 2b). Hence, it is apparent
that peripheral (S)-3,7-dimethyloctyl groups on (S)-2 express
chirality at the supramolecular level to afford single-handed one-
100 dimensional helical stack. Monitoring CD intensity at 373 nm
versus temperature leads to a non-sigmodial melting curve,
characteristic for cooperative supramolecular polymerization
mechanism (Figure 2c). Moreover, UV cooling curve exhibits a
higher onset temperature than that of the CD cooling curve
105 (Figure S19), indicating the presence of small and disordered
oligomers in the nucleation stage before elongating into stable
one-dimensional aggregates.
1
35 dependent H NMR spectra of compound 1 at the concentration
of 2 mM were initially performed. The aromatic protons in d12-
cyclohexane become significantly broad at room temperature
(Figure S17), suggesting the involvement of π–π stacking for the
formation of high-molecular-weight aggregates. Warming to 333
40 K results in sharpened signals, similar to the phenomena observed
in d-chloroform at room temperature, indicating the existence of
monomeric platinum(ІІ) acetylide chromophore under these
1
conditions. Moreover, for the concentration-dependent H NMR
measurements of compound 1 in d-chloroform (Figure S18),
45 significant downfield shifts are observed for the amide proton
upon increasing the concentration from 0.15 to 64.6 mM,
illustrating that hydrogen bonding interaction also plays a vital
role for the supramolecular polymerization process.
Next, self-assembly behaviour was investigated by UV
50 absorption spectroscopy in methylcyclohexane (MCH) (c = 5 ×
10-5 M). When the temperature is above 20 ºC (Figure 2a), UV
spectrum of the chiral compound (S)-2 exhibits molecularly
dissolved platinum(ІІ) acetylide spectral feature, for which the
maxima absorption wavelength of the long-axis polarized π–π*
55 and Pt–π* transitions locates at 348 nm (Figure S15). In contrast,
the absorption spectra below 10 ºC exhibit remarkable
To acquire the thermodynamic parameters for the
supramolecular polymerization process, normalized CD cooling
110 curve of (S)-2 is analyzed by applying the van der Schoot
mathematical model (Figure S20).4a-b Based on the non-linear
least-squares analysis, the curve fits very well with the model
over the whole temperature range, for which the enthalpy release
he value is –68 kJ mol-1, while the elongation temperature Te
115 value is 288 K. Notably, the he value is more negative than that of
the previously reported benzene-1,3,5-tricarboxamides (BTAs)
system containing three amide groups,4b-c supporting the
bathochromic shifts (λmax
= 356 nm), representing the
overlapping of the platinum(ІІ) acetylide chromophores to form
edge-to-edge J-type aggregates. It should be emphasized that
2
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DOI: 10.1039/C3CC44997A
a
Key Laboratory of Soft Matter Chemistry, Department of Polymer
50 Science and Engineering, University of Science and Technology of China,
Hefei, Anhui 230026 (P. R. China). Fax: (+86) 551 3606 095; E-mail:
drfwang@ustc.edu.cn
participation of π–π stacking for the cooperative self-assembly
process. Moreover, the quantitative Ka value (the equilibrium
constant of the nucleation step) is determined to be 3.79 × 10-4,
reflecting a moderate degree of cooperativity. The fitting results
are further validated by performing CD cooling curves at three
different concentrations, which exhibit independent he and Ka
values (Table 1 and Figure S20). On the other hand, the gradually
increased Te values suggest the earlier formation of one-
dimensional helical aggregates at higher monomer concentration.
b
Institute for Complex Molecular Systems, Eindhoven University of
5
Technology, PO Box 513, 5600 MB Eindhoven (The Netherlands).
55 † Electronic Supplementary Information (ESI) available: [synthesis,
1
characterization, H NMR, CD, UV data and other materials]. See DOI:
10.1039/b000000x/
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10 Table 1. Self-assembly thermodynamic parameters of compound (S)-2
derived from van der Schoot mathematical model.
2
Concentration (M)
Te (K)
he (kJ mol-1)
Ka
1 × 10-4
293
–65.5
3.46 × 10-4
3.88 × 10-4
3.79 × 10-4
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7.5 × 10-5
291
–66.4
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70
Due to the dynamic properties of the resulting cooperative
supramolecular assemblies, chiral amplification study was further
15 investigated by mixing monomers 1 and (S)-2 together at 10 ºC
(sergeants-and-soldiers experiment). Upon gradual addition of
small amounts of (S)-2 to an MCH solution of 1 at a total
concentration of 5 × 10−4 M, the shape of the CD spectra remains
unchanged (Figure S21). The CD effect at 373 nm is then plotted
20 as a function of sergeant fraction, the intensity of which exhibits a
small deviation from linearity (Figure 2d). Considering that helix
reversal penalty is very high, such a weak sergeants-and-soldiers
amplification effect could probably be attributed to the rather low
mismatch penalty between the sergeant (S)-2 and soldier 1
25 species at 10 ºC (close to Te).12
75
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85
Conclusions
In summary, square planar geometry of the platinum(ІІ)
acetylide complex facilitates the cofacial aggregation of rod-like
monomers 1 and (S)-2, resulting in the formation of one-
30 dimensional helical supramolecular polymers with cooperative
mechanism. The fibril structures could further aggregate into
entangled 3D networks in non-polar solvents, exhibiting
gelation-induced enhanced emission behaviour. Detailed
mechanistic studies illustrate that both intermolecular hydrogen
35 bonding and π–π stacking exert significant effects on the
supramolecular polymerization process. A rather weak sergeants-
and-soldiers amplification behaviour is observed for the achiral 1
and chiral (S)-2 coassemblies. These findings shows that deeper
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40 development of platinum(ІІ) acetylide complex as novel types of
π-functional materials with tailored properties.
90
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Acknowledgements
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This work was supported by the National Natural Science
Foundation of China (21274139, 91227119) and the Fundamental
45 Research Funds for the Central Universities. The authors thank
Dr. Anja R. A. Palmans for the valuable discussion and Mr. Yu-
Kui Tian for the artwork.
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Notes and references
This journal is © The Royal Society of Chemistry [year]
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