uril (CB[n]) depending on the number of units of glycoluril. They contain a
hydrophobic cavity lining the urea carbonyl and can enclose a series of neutral
or positively charged guests with very high binding constants, providing an opportunity for the development of new supramolecular polymers. The first member
of this circular family is hexamer CB[6], and since then CB[n] has evolved to
include smaller or larger homologs (CB[5]-CB[10]). In this family, CB[6] and
CB[8] have poor water solubility, while CB[5] and CB[7] have moderate solubility.
In this regard, Mock, J. Fraser Stoddart, Kimoon Kim, Lyle Isaacs, etc. have done a
lot of exploratory work [17].
However, due to the difficulty in the modification of CBs, supramolecular
polymers based on CB[5], CB[6], and CB[7] are rarely reported. The particularity
of CB[8] is that its larger cavity can entrap two specific guest molecules. Therefore,
in the CB family, CB[8] is the most promising host molecule for the construction
of supramolecular polymers. However, limited by the poor water solubility
(<1 mM) of CB[8], in most cases, dimerization and oligomerization cyclization
dominate in diluted aqueous solutions compared to linear polymerization.
16.3.1 Adjustable Dynamic Photophysical Properties Based on CB
In 2016, Tao et al. (2016) [18] reported a class of supramolecular methods based
on host–guest function, which greatly reduced the synthesis steps to construct a
system with adjustable dynamic photophysical properties. As shown in Fig. 7,
due to the new electron distribution of the chromophore guest molecule in the
CB[8] rigid hydrophobic cavity, the luminescent color of the system can achieve
blue-green, yellow, green, and white changes by simply adding the host molecule
in the aqueous solution, accompanied by an increase in fluorescence lifetime and
quantum yield. This work realizes the stimuli response adjustment of color in
a single solution combined with a single organic fluorescent object and a single
body, which provides a new idea for the development of new luminescent materials.
16.3.2 Supramolecular Polymer with Photoisomerism Based on CB
Scherman et al. (2013) [19] displayed control over the stoichiometry of a series of CB
[8] complexes through photoirradiation. The E-Z isomerism of azobenzene-containing
guest molecules for CB[8] was modified in the presence of the macrocycle, Fig. 8. A
remarkable stabilization of the Z-azobenzene moieties of guests 2–4 was achieved by
CB[8] complexation. Host–guest complexes that can be remotely switched between
highly thermostable photostationary states in aqueous media are valuable molecular
systems in biological and materials sciences and are also of interest in information
storage technologies and for the construction of molecular machinery. Furthermore,
guests 3 and 4 in the presence of CB[8] can form supramolecular polymers, which
were characterized both in solution and, for the first time, in the solid state and whose
structures can be externally controlled by light irradiation.
460
F. Gu and X. Ma
hydrophobic cavity lining the urea carbonyl and can enclose a series of neutral
or positively charged guests with very high binding constants, providing an opportunity for the development of new supramolecular polymers. The first member
of this circular family is hexamer CB[6], and since then CB[n] has evolved to
include smaller or larger homologs (CB[5]-CB[10]). In this family, CB[6] and
CB[8] have poor water solubility, while CB[5] and CB[7] have moderate solubility.
In this regard, Mock, J. Fraser Stoddart, Kimoon Kim, Lyle Isaacs, etc. have done a
lot of exploratory work [17].
However, due to the difficulty in the modification of CBs, supramolecular
polymers based on CB[5], CB[6], and CB[7] are rarely reported. The particularity
of CB[8] is that its larger cavity can entrap two specific guest molecules. Therefore,
in the CB family, CB[8] is the most promising host molecule for the construction
of supramolecular polymers. However, limited by the poor water solubility
(<1 mM) of CB[8], in most cases, dimerization and oligomerization cyclization
dominate in diluted aqueous solutions compared to linear polymerization.
16.3.1 Adjustable Dynamic Photophysical Properties Based on CB
In 2016, Tao et al. (2016) [18] reported a class of supramolecular methods based
on host–guest function, which greatly reduced the synthesis steps to construct a
system with adjustable dynamic photophysical properties. As shown in Fig. 7,
due to the new electron distribution of the chromophore guest molecule in the
CB[8] rigid hydrophobic cavity, the luminescent color of the system can achieve
blue-green, yellow, green, and white changes by simply adding the host molecule
in the aqueous solution, accompanied by an increase in fluorescence lifetime and
quantum yield. This work realizes the stimuli response adjustment of color in
a single solution combined with a single organic fluorescent object and a single
body, which provides a new idea for the development of new luminescent materials.
16.3.2 Supramolecular Polymer with Photoisomerism Based on CB
Scherman et al. (2013) [19] displayed control over the stoichiometry of a series of CB
[8] complexes through photoirradiation. The E-Z isomerism of azobenzene-containing
guest molecules for CB[8] was modified in the presence of the macrocycle, Fig. 8. A
remarkable stabilization of the Z-azobenzene moieties of guests 2–4 was achieved by
CB[8] complexation. Host–guest complexes that can be remotely switched between
highly thermostable photostationary states in aqueous media are valuable molecular
systems in biological and materials sciences and are also of interest in information
storage technologies and for the construction of molecular machinery. Furthermore,
guests 3 and 4 in the presence of CB[8] can form supramolecular polymers, which
were characterized both in solution and, for the first time, in the solid state and whose
structures can be externally controlled by light irradiation.
460
F. Gu and X. Ma