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be drawn from a concentrated solution of 2, characteristic for
the entanglement of linear supramolecular polymers forming
connected macrosized aggregates (see Figure S33 in the
Supporting Information).
It is noteworthy that 3 tends to form relatively large
polymeric assemblies compared to those of 2, as depicted by
the higher slope (1.83 for 2 versus 2.03 for 3 for viscosity
measurements) and lower diffusion coefficient values (1.48 ꢀ
10À10 for 2 versus 1.01 ꢀ 10À10 m2 sÀ1 for 3 for DOSY measure-
ments). The isodesmic model was then chosen to theoretically
calculate the average degree of polymerization, N, for the
resulting supramolecular polymers (Figure 3b and the Equa-
tion S3 in the Supporting Information). Briefly, when the
monomer concentration of 3 approaches 70 mmolLÀ1, N is
approximately 32 (total molar mass is 6.97 ꢀ 104 gmolÀ1),
whilst for 2 N is calculated to be 13 under the same
conditions.[1a,15,16] Hence, it suggests that different spacers on
the heteroditopic monomers influence the charge density of
the neighboring pyrene unit, thus leading to the stronger
tweezer/guest complexation in the case of 3.
Figure 4. Partial 1H NMR spectra (300 MHz, 298 K) of 2 (20.0 mm)
upon successive additions of 5a and 6: a) 0 equiv; b) 0.5 equiv;
c) 1.5 equiv; d) 2.6 equiv; e) 3.7 equiv; f) 5.2 equiv of 5a; g) 5.2 equiv
of 5a and 1.4 equiv of 6; h) 5.2 equiv of 5a and 3.6 equiv of 6;
i) 5.2 equiv of 5a and 5.1 equiv of 6; j) 5.2 equiv of 5a and 8.0 equiv of
6. Hpy, HAn, and Had denote the protons on pyrene unit, 5a, and 7a,
respectively.
Next, stimuli-responsive properties of the resulting supra-
molecular polymers 4 were investigated by taking advantage
of anthracene derivatives 5 as the chain-stopper moieties. The
recognition behavior between 5 and the monotopic receptor
1 was first tested, for which the Ka values were determined to
be (3.33 Æ 0.11) ꢀ 103 mÀ1 and (1.10 Æ 0.02) ꢀ 103 mÀ1 for 9-
methylanthracene (5a) and 9,10-dimethylanthracene (5b),
respectively (see Figure S26 in the Supporting Information).
The decreased binding affinity for 5b implies that two methyl
substituents on the anthracene impart steric hindrance for
tweezer/guest association. Subsequently, the influence of
anthracene additives on the depolymerization of the hetero-
ditopic monomer 2 was monitored by 1H NMR titration
experiments (Figure 4a–f). Upon progressive addition of 5a
to 2, the pyrene unit is gradually converted to the uncom-
plexed state, as manifested by the apparent upfield shift for
the proton Hpy (Figure 4a–f) and downfield shift for the
proton H13 (see Figure S34 in the Supporting Information),
which is in agreement with the tendency observed in Figure 2.
Meanwhile, protons H1,3-4 on the terpyridine unit shift upfield
considerably (Figure 4a–f). Such phenomena demonstrate
that 5a could act as the competitive guest to complex with the
tweezer unit, thus giving rise to the stoichiometry imbalance
between the molecular tweezer and pyrene moieties, and
thereby leading to the disruption of supramolecular polymers
4. The conclusion is further supported by the theoretical
calculations (see Figure S41 in the Supporting Information).
Particularly, when 1 equivalent of 5a is added to 2 at 70.0 mm,
the estimated degree of polymerization for the resulting
supramolecular polymers decreases significantly from
approximately 13 to 2. When 2 is replaced by 3, the calculated
value varies from approximately 32 to 3. Meanwhile, both
NMR titration experiments (see Figure S36 in the Supporting
Information) and theoretical calculations (see Figure S41 in
the Supporting Information) reveal that the degree of
polymerization value changes to a lesser extent with the
addition of 5b, and is consistent with its weaker binding
affinity towards the tweezer receptor when compared with
that of 5a.
We further tested the feasibility of reforming the supra-
molecular polymers 4, by making use of the facile room-
temperature Diels–Alder reaction between 5 and 6. It raises
the question as to whether the specific reaction could be
smoothly proceed in the presence of another arene moiety. To
address the issue, we first investigated the model system
comprising equimolar mixtures of pyrene, 5, and 6, and the
results clearly support the quantitative conversion of the
latter two compounds into the cycloaddition adduct without
the participation of pyrene unit (see Figures S38 and S39 in
the Supporting Information). On this basis, 6 was titrated into
the above mixtures of 2 and 5a (Figure 4 f), thus resulting in
the progressive signal conversion to the initial polymeric state
(Figure 4g–j). The original signals arising from 4 were
restored upon the addition of excess amounts of 6 (Figure 4i),
thus demonstrating that the Diels–Alder reaction is efficient
within the multicomponent system and leads to the reassem-
bly of supramolecular polymeric assemblies.
The interplay between the Diels–Alder reaction and
reversible supramolecular polymerization in the complex
systems could be monitored by DOSY experiments
(Figure 5). Practically, when 1.0 equivalent of 5a was added
into monomer 2 at 70 mm (linear polymeric species play
a prominent role at this concentration), the diffusion coef-
ficient values increase from 2.45 ꢀ 10À10 to 8.20 ꢀ 10À10 m2 sÀ1.
The significant size shrinking mainly correlates to the
disassembly of 4. Further addition of 3.5 equivalents of 6
induces the reassembly process, observed from the transition
of the coefficient value back to 1.84 ꢀ 10À10 m2 sÀ1.[17] Hence,
based on the 1H NMR titration and DOSY measurements, it
is evident that the supramolecular polymers 4 are highly
adaptive and capable of undergoing reversible transitions
triggered by Diels–Alder chemistry.
In summary, we have successfully constructed linear
supramolecular polymers based on the novel bis[alkynylpla-
tinum(II)] terpyridine molecular tweezer/pyrene recognition
motif. The head-to-tail supramolecular polymerization pro-
4
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!
Tian, Yu-Kui
