Construction of supramolecular hyperbranched polymers via the tweezering directed self-assembly strategy

Article abstract of DOI:10.1039/c4cc03158j

A bis-alkynylplatinum(ii) terpyridine tweezer-alkynylgold(iii) diphenylpyridine guest is shown to maintain the specific complexation in the presence of a B21C7-secondary ammonium salt recognition motif, which facilitates the formation of supramolecular hyperbranched polymers via the tweezering directed self-assembly strategy. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03158j

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Construction of supramolecular hyperbranched
polymers via the ‘‘tweezering directed self-assembly’’
strategy†
Yu-Kui Tian,a Zhi-Shuai Yang,a Xiao-Qin Lv,a Ri-Sheng Yao*b and Feng Wang*a
Cite this: Chem. Commun., 2014,
50, 9477
Received 28th April 2014,
Accepted 29th May 2014
DOI: 10.1039/c4cc03158j
www.rsc.org/chemcomm
A bis-alkynylplatinum(II) terpyridine tweezer–alkynylgold(III) diphe-
nylpyridine guest is shown to maintain the specific complexation in
the presence of a B21C7–secondary ammonium salt recognition
motif, which facilitates the formation of supramolecular hyper-
branched polymers via the ‘‘tweezering directed self-assembly’’
strategy.
The integration of donor and acceptor chromophores in a mixed
array is a convenient approach to construct nano-sized optoelec-
tronic devices.1 Due to the non-specific and non-directional
properties of donor–acceptor interactions, a variety of erratic
stacking modes (such as phase segregation, random or alter-
nating mixing) could exist,2 which is disadvantageous for
device performance. In this respect, the precise organization of
p-conjugated donor and acceptor moieties via supramolecular
control is of paramount importance. We have recently developed
the ‘‘tweezering directed self-assembly’’ strategy for the construc-
tion of well-defined donor–acceptor arrays.3 Considering that the
two electron-deficient alkynylplatinum(II) terpyridine pincers on the
molecular tweezer unit facilitates the encapsulation of electron-rich
guests such as pyrene,4 the resulting AB-type monomers could
efficiently assemble into linear supramolecular polymers in CHCl3
solution.3 It is worthy of note that such a strategy overcomes the
non-directional complexation problems for the conventional
Scheme 1 Schematic representation of the construction of supramole-
cular hyperbranched polymers 5.
donor–acceptor interactions, thus leading to the exact positioning (B21C7)–secondary ammonium salt moieties,5 is implemented
of alternating donor and acceptor chromophores. and employed as the branching unit for the construction of
To expand the scope of the ‘‘tweezering directed self-assembly’’ supramolecular hyperbranched polymers 5 (Scheme 1). Due to
strategy, the compatibility of the tweezer–guest recognition the limited solubility of the secondary ammonium salt in CHCl3,
motif should be extensively examined in the presence of other more polar solvent mixtures such as CHCl3/CH3CN (2 : 1, v/v) were
functional groups. Based on this consideration, herein another required to accomplish the supramolecular polymerization
type of non-covalent recognition motif, namely benzo-21-crown-7 process. Therefore, emphasis should be placed on the tolerance
of the tweezer–guest recognition moieties to the modified external
a Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and
Engineering, University of Science and Technology of China, Hefei, Anhui 230026,
P. R. China. E-mail: drfwang@ustc.edu.cn; Fax: +86 551 6360 6095
b School of Medical Engineering, Hefei University of Technology, Hefei,
Anhui 230009, P. R. China. E-mail: Rishengyao@163.com
conditions.
Our preliminary study revealed that the previously utilized
bis-alkynylplatinum(II) terpyridine molecular tweezer–pyrene
recognition motif3 is not a desirable candidate for the fabrication
of 5, since its binding affinity is rather weak in CHCl3/CH3CN
(2 : 1, v/v) solution (manifested by the very slight absorption
† Electronic supplementary information (ESI) available: Synthesis, characteriza-
tion, UV-Vis titration data and other materials. See DOI: 10.1039/c4cc03158j
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changes in the UV/Vis titration experiments, Fig. S34, ESI†). inset).4 Upon progressive titration of 7 to 6 in CHCl3/CH3CN
Therefore, in this communication pyrene was replaced by the (2 : 1, v/v), the intensity of the MLCT–LLCT absorption bands
alkynylgold(III) diphenylpyridine unit, which has been proven to gradually decreased. Non-linear curve-fitting of the UV/Vis
exhibit enhanced complexation towards the molecular tweezer absorption data obtained at 460 nm (eqn (S1), ESI†) provided
moiety.4a,b Specifically, heteroditopic monomers 1 and 2 were the association constant (Ka = (3.33 Æ 0.39) Â 104 MÀ1) for 6/7
designed (Scheme 1), both of which bear B21C7 units on one side. (Fig. 1b). The strong tweezer–guest complexation capability is
The bis-alkynylplatinum(II) terpyridine tweezer and alkynylgold(III) also supported by the UV/Vis titration experiments for the
diphenylpyridine guest were decorated on the other side. If the complexes 6/2 (bearing one B21C7 unit) and 1/2 (bearing two
highly specific complexation of the tweezer–guest recognition B21C7 units), which gave association constant values of (1.43 Æ
motif is maintained, the mixing of equivalent amounts of 1 and 0.61) Â 104 MÀ1 and (6.66 Æ 1.11) Â 103
M
À1, respectively
2 should afford the dimeric complex 4. Subsequent addition of (Fig. S36 and S37, ESI†). Based on the above three calculated
the homotritopic secondary ammonium salt monomer 3 to 4 was values, it is evident that pendant crown ether units exert
anticipated to furnish the desired A2B3-type supramolecular negative effects on the tweezer–guest binding affinity, which
hyperbranched polymers 5 with stimuli-responsive character.6,7
could be ascribed to the following two factors. One is due to the
The synthetic routes towards the desired monomers 1–3 direct encapsulation of the o-dioxybenzene group located on
were quite straightforward (Scheme S1–S3, ESI†). The purpose the B21C7 unit in the cavity of the molecular tweezer, whilst
of introducing long alkyl chains on 1–3 was to increase the another indirect effect arises from the B21C7-induced polarity
solubility of the self-assembled monomers. In addition, these increase of the microenvironment.9
flexible spacers were expected to decrease the conformational
To pursue more convincing explanations for the B21C7-
entropy of the multiple non-covalent complexation processes.8 induced decrease of the tweezer–guest binding affinity, we chose
The structures of the synthetic compounds were validated by monotopic 6, 7, the B21C7 ester and a secondary ammonium
NMR and ESI-MS spectra (Fig. S1–S33, ESI†).
salt as the model compounds and carried out four-component
The non-covalent complexation was first studied between molecular recognition studies (Fig. S38, ESI†). It is well-known
the monotopic molecular tweezer 6 and alkynylgold(III) diphenyl- that slow-exchanging B21C7–secondary ammonium salt recogni-
pyridine guest 7 (Scheme 1). When an equivalent amount of tion motif features 1 : 1 binding stoichiometry.5 Hence, after
7 was added to 6 in CDCl3/CD3CN (2 : 1, v/v), the originally mixing equivalent amounts of the four monotopic compounds
broad 1H NMR spectrum of 6 (mainly resulting from the together in CDCl3/CD3CN (2: 1, v/v), the resulting spectrum
irregular self-complexation of 6, Fig. S4, ESI†) converted to (Fig. S38a, ESI†) was found to almost overlap the 1H NMR spectra
well-defined signals (Fig. S35, ESI†), illustrating the preferential of the corresponding 6/7 and B21C7–secondary ammonium salt
sandwiching of 7 into the cavity of 6. The fast-exchanging complexes (Fig. S38b–e, ESI†), supporting the non-interfering
complexation property of the tweezer–guest binding motif complexation model for these two non-covalent recognition
was also confirmed, as demonstrated by the presence of only motifs.
one set of 1H NMR signals. Based on the molar ratio plot
Besides the above monotopic model system, the designed
(Fig. 1a) derived from the 1H NMR titration experiment monomers 1–3 could also demonstrate highly specific complexa-
(Fig. S35, ESI†), the binding stoichiometry between 6 and 7 tion behavior via a step-wise self-assembly pathway (Fig. S39,
was determined to be 1 : 1. Moreover, the tweezer–guest binding ESI†). Practically, after mixing equivalent amounts of 1 and 2
affinity could be quantitatively evaluated by UV/Vis spectro- (9 mM for each monomer) together, specific tweezer–guest
scopy, as a result of the fascinating optical properties of the complexation takes place, as visualized by the significant upfield
alkynylplatinum(II) terpyridine units on 6. Briefly, the MLCT shift of the H18 signal (from 7.332 to 7.198 ppm). Meanwhile, the
and LLCT absorption bands of 6 were located predominately signals corresponding to the H20–22 protons belonging to the
between 400 and 500 nm in the UV/Vis spectrum (Fig. 1b, B21C7 unit hardly changed, implying that the preferential
tweezer–guest complexation occurred without the involvement
of the crown ether unit (Fig. S39c, ESI†). On this basis, subse-
quent addition of 3 (6 mM) led to the splitting of protons H20–22
belonging to B21C7, suggesting its favorable association with the
secondary ammonium salt moiety (Fig. S39b, ESI†). Interestingly,
unlike the above-mentioned monotopic systems, herein two sets
of proton signals were present in the NMR spectrum for the
fast-exchanging tweezer–guest moieties (such as H1 and H3),
corresponding to the B21C7-complexed and B21C7-uncomplexed
species, respectively.
Notably, all of the above NMR experiments give clear evidence
that the possibility for the direct complexation of the o-dioxy-
Fig. 1 (a) Molar ratio plot for monomers 6 and 7 derived from a 1H NMR
titration experiment (monitoring the chemical shift changes of Ha on 7); (b)
change in the UV/Vis absorption spectra at 460 nm upon stepwise addition
benzene group on B21C7 with the tweezer unit could be totally
of gold(III) alkynyl complexes 7 to the molecular tweezer 6: the red line was
excluded. Hence, the decreased tweezer–guest binding strength
primarily results from the presence of B21C7 ethyleneoxy chains,
obtained from non-linear curve-fitting. Inset: UV/Vis absorption changes
at 5.00 Â 10À5 M in CHCl3/CH3CN (2 : 1, v/v).
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which increase the polarity of the microenvironment. Conse-
quently, the electropositivity of the alkynylplatinum(II) terpyri-
dine unit could be potentially dispersed, leading to a weakening
of the donor–acceptor interactions.
After elucidating the highly specific non-covalent recogni-
tion behavior, the supramolecular polymerization process was
performed via the ‘‘one-pot’’ mixing of the three monomers 1–3
at different concentrations. As shown in Fig. 2, for the 3 : 3 : 2
mixture of 1, 2 and 3 at low monomer concentrations, the
1H NMR spectra exhibit signal splitting for both the fast-
exchanging (such as for H1,3,5 on the tweezer–guest moieties)
and slow-exchanging (such as for H24,25 on the B21C7–secondary
ammonium salt moieties) recognition motifs, suggesting the
coexistence of supramolecular oligomeric and polymeric species.
Fig. 3 DOSY spectra (400 MHz, CDCl3, 298 K) of the supramolecular
polymers (linear or hyperbranched architectures) at different concentra-
tions of 1: blue, 5 mM and red, 70 mM.
Increasing the monomer concentration favors the formation of 5.83 Â 10À10 to 8.49 Â 10À11 m2 sÀ1 under the same conditions
supramolecular hyperbranched polymers, as manifested by the (Fig. 3 and Fig. S41c and d, ESI†). Such results illustrate that the
fact that the oligomeric signals progressively decrease, whereas self-assembling architectures (hyperbranched vs. linear) exert a
the polymeric ones considerably strengthen (Fig. 2e and f). In significant impact on the size of the resulting supramolecular
addition, the apparent peak broadening phenomenon also assemblies.
reflects the tendency to form high-molecular-weight polymeric
aggregates at high monomer concentration.
The stimuli-responsive properties of the resulting hyper-
branched polymers 5 were further exploited, via the manipula-
Two-dimensional diffusion-ordered NMR spectroscopy tion of the dynamic B21C7–ammonium salts recognition motif.
(DOSY) is a convenient technique to probe the size of the As we know, the metal ion K+ exhibits a higher affinity toward
dynamic aggregations. As the concentration of 1 increased B21C7 than the ammonium salt unit.5e After the addition of
from 5.00 mM to 70.0 mM, the measured diffusion coefficients KPF6 (1 equiv.), the intensities of the complexed H23–25 signals
for the mixture of 1–3 decreased remarkably from 3.75 Â 10À10 noticeably decreased in the 1H NMR spectra (Fig. S42, ESI†),
to 5.12 Â 10À12 m2 sÀ1 (Fig. 3 and Fig. S41a and b, ESI†). Based suggesting that K+ competitively complexes with B21C7 to
on the Stokes–Einstein equation,6c the hydrodynamic radii for destroy 5. Subsequent addition of 2 equiv. of 18-crown-6-
the resulting hyperbranched polymers 5 were determined to be ether, which exhibits a higher affinity toward K+ than B21C7,
1.15 and 84.5 nm, respectively. The significant size variation recovered the disrupted supramolecular hyperbranched poly-
behavior is highly consistent with the above concentration- mers, as visualized by the reappearance of the original 1H NMR
dependent 1H NMR results. Additionally, when homotritopic signals (Fig. S42, ESI†).
monomer 3 was replaced by the corresponding homoditopic
In summary, we have demonstrated that, for the bis-
secondary ammonium salt 8 (Fig. 3), representing the for- alkynylplatinum(II) terpyridine tweezer–alkynylgold(III) diphenyl-
mation of linear supramolecular assemblies for the mixtures pyridine recognition motif, although the binding strength is
of 1, 2 and 8, the measured diffusion coefficients changed from influenced by the microenvironment changes, it still maintains
a highly specific complexation capability in the presence of a
B21C7-based host–guest system. With the implementation of
these two non-covalent recognition motifs, A2B3-type dynamic
supramolecular hyperbranched polymers were successfully
constructed. Therefore, the current work expands the scope of
the ‘‘tweezering directed self-assembly’’ strategy, aiding the
further manipulation and application of such hierarchical
supramolecular assemblies.
This work was supported by the National Natural Science
Foundation of China (21274139, 91227119), and the Fundamental
Research Funds for the Central Universities (WK2060200012,
WK2060200012).
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