anions were added, because the metal-ligand bonds were dissociated by the formation of Pt-Br complexes. The addition of AgOTf could remove the Br anion
by formation of AgBr precipitate, thus resulting in the reformation of the
discrete hexakis-pillar[5]arene metallacycles. Moreover, the competitive guest,
1,4-dicyanobutane, could also induce the gel-sol transition by destroying the hostguest interaction between the ditopic guest and the metallacycles. And the addition
of the native pillar[5]arene to include the competitive guest would lead to the
recovery of supramolecular gels.
Wang and coworkers reported rotaxane-based stimuli-responsive supramolecular
gels, in which the pseudorotaxane could be formed by the ditopic guest containing
the pyridine as the ending motifs and pillar[5]arene (Fig. 16) [20]. After the addition
of Pd(OAc) 2 , the metalla-supramolecular polymer could be formed with the rotaxane
as the main backbone. Thanks to the intermolecular hydrogen between the guest
molecules, the supramolecular gels could be obtained in a high concentration.
Due to the dynamic nature of hydrogen bond, this gel exhibited responsiveness to
the external temperature as well as the concentration of the gelators. Moreover, the
gel to sol transition could also be achieved by the addition of PPh 3 to destroy the
previous metal-ligand bonds. However, due to the occurrence of precipitate,
the reverse sol to gel transition could not be achieved.
Yang and coworkers reported a stimuli-responsive blue fluorescent supramolecular gel based on the pillar[5]arene-modified TPE derivatives and corresponding
guest molecules [33]. Firstly, they successfully synthesized the pillar[5]arene-modified TPE derivatives (H1) and the ditopic guest molecules (G2) containing the nitrile
motifs (Fig. 17a) [33a]. Similar to the above gel formation process, the 2D supramolecular polymer could be generated by mixing the host and guest molecules,
which could further aggregate into gels. The formation of large-scaled supramolecular polymer could restrict the rotation of TPE motif, affording the strong blue
fluorescence emission. This fluorescent supramolecular gel exhibited responsiveness
to the temperature and solvent. The increase in the temperature destroys not only the
host-guest interaction but also the aggregation manners, leading to the destruction of
supramolecular gels and the decrease in the fluorescence intensity. Moreover, they
also explored the influence of alkyl chain length on the formation of stimuliresponsive supramolecular gels (Fig. 17b) [33b]. Two sets of pillar[5]arene-modified
TPE derivatives (SH and LH) and neutral guests (SG and LG) with three binding sites
were synthesized; the only difference in these molecules was the alkyl chain length,
which could both form the supramolecular polymer by simply mixing corresponding
host and guest molecules. However, only the set with long alkyl chain could form the
supramolecular gels, resulting from the flexible structure induced looser supramolecular networks to include solvent molecules. And this supramolecular gel also
exhibited external stimuli to the change in temperature.
Zhang and coworkers synthesized a copillar[5]arene derivative containing one
1,4-bis(hexadecyl)benzene unit, which could form the supramolecular gels in the
CH 3 CN solution, driven by the C-HÁÁÁπ interaction and the van der Waals forces [34].
This gel showed the aggregation-induced enhanced emission at 450 nm. The addition
of the electron poor competitive guest molecule, hexadecylpyridinium chloride,
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X. Wu et al.
by formation of AgBr precipitate, thus resulting in the reformation of the
discrete hexakis-pillar[5]arene metallacycles. Moreover, the competitive guest,
1,4-dicyanobutane, could also induce the gel-sol transition by destroying the hostguest interaction between the ditopic guest and the metallacycles. And the addition
of the native pillar[5]arene to include the competitive guest would lead to the
recovery of supramolecular gels.
Wang and coworkers reported rotaxane-based stimuli-responsive supramolecular
gels, in which the pseudorotaxane could be formed by the ditopic guest containing
the pyridine as the ending motifs and pillar[5]arene (Fig. 16) [20]. After the addition
of Pd(OAc) 2 , the metalla-supramolecular polymer could be formed with the rotaxane
as the main backbone. Thanks to the intermolecular hydrogen between the guest
molecules, the supramolecular gels could be obtained in a high concentration.
Due to the dynamic nature of hydrogen bond, this gel exhibited responsiveness to
the external temperature as well as the concentration of the gelators. Moreover, the
gel to sol transition could also be achieved by the addition of PPh 3 to destroy the
previous metal-ligand bonds. However, due to the occurrence of precipitate,
the reverse sol to gel transition could not be achieved.
Yang and coworkers reported a stimuli-responsive blue fluorescent supramolecular gel based on the pillar[5]arene-modified TPE derivatives and corresponding
guest molecules [33]. Firstly, they successfully synthesized the pillar[5]arene-modified TPE derivatives (H1) and the ditopic guest molecules (G2) containing the nitrile
motifs (Fig. 17a) [33a]. Similar to the above gel formation process, the 2D supramolecular polymer could be generated by mixing the host and guest molecules,
which could further aggregate into gels. The formation of large-scaled supramolecular polymer could restrict the rotation of TPE motif, affording the strong blue
fluorescence emission. This fluorescent supramolecular gel exhibited responsiveness
to the temperature and solvent. The increase in the temperature destroys not only the
host-guest interaction but also the aggregation manners, leading to the destruction of
supramolecular gels and the decrease in the fluorescence intensity. Moreover, they
also explored the influence of alkyl chain length on the formation of stimuliresponsive supramolecular gels (Fig. 17b) [33b]. Two sets of pillar[5]arene-modified
TPE derivatives (SH and LH) and neutral guests (SG and LG) with three binding sites
were synthesized; the only difference in these molecules was the alkyl chain length,
which could both form the supramolecular polymer by simply mixing corresponding
host and guest molecules. However, only the set with long alkyl chain could form the
supramolecular gels, resulting from the flexible structure induced looser supramolecular networks to include solvent molecules. And this supramolecular gel also
exhibited external stimuli to the change in temperature.
Zhang and coworkers synthesized a copillar[5]arene derivative containing one
1,4-bis(hexadecyl)benzene unit, which could form the supramolecular gels in the
CH 3 CN solution, driven by the C-HÁÁÁπ interaction and the van der Waals forces [34].
This gel showed the aggregation-induced enhanced emission at 450 nm. The addition
of the electron poor competitive guest molecule, hexadecylpyridinium chloride,
362
X. Wu et al.