432
Y. Kubo
12.7 Dynamic Covalent Bond
The use of dynamic covalent bonds is a potent methodology in the sense that they
combine the characteristics of covalent and non-covalent bonds (Wilson et al. 2014;
Rowan et al. 2002). Its reversible nature and “error-checking” characteristics allow
for the easy thermodynamic formation of ordered nanostructures, which create a
stable and covalently bound molecular system. Currently, functional systems based
on more than one type of dynamic covalent bond have received lots of attention
as “orthogonal dynamic covalent bonds” (Wilson et al. 2014). For the expansion,
“tunable orthogonal reversible covalent” chemistry (Reuther et al. 2019) allowed
for the construction of controlled molecular level motion (Bruns and Fraser Stoddart
2016). In principle, dynamic covalent reactions are roughly classified as the formation
of new dynamic covalent bonds and exchange reactions. The former case involves
imine condensation and aldol reactions. The covalent bonds known to be dynamic,
involve in C–C bond, C–N bond, C–O bond, C–S bond, S–S bond, and B–O bond.
Among them, imine bond/exchange (Vardhan et al. 2015; Belowich and Stoddart
2012) and boronic acid condensation (Kubo et al. 2015) are directional, which are
useful for the supramolecular organization.
As a typical example, multicolor emissive self-healing carbon dot/polymer gels
were reported (Bhattacharya et al. 2019). Chemical self-healing gels assembled
through dynamic covalent bond formation (Wei et al. 2014; Amaral and Pasparakis
2017) are well-known in the field of soft materials. In this study, Jelinek et al., utilized
carbon dots (C-dots) as luminescent nanoparticles, whose emission properties can be
tuned by the different carbonaceous precursors used (Miao et al. 2018). Figure 12.21a
shows the synthetic schemes and assembly of emissive C-dot/polymer gel. The three
aldehyde building blocks and experimental conditions gave the differently colored
C-dots; green emissive C-dots (G-C-dots) were prepared using glutaraldehyde at
150 °C for 2 h. Blue emissive C-dots (B-C dots) and yellow C-dots (CoAP-C-dots)
were obtained from benzaldehyde and polymeric cyclooctadiene-aldehyde, respectively. The synthetic strategy of the targeted gel was depicted in Fig. 12.21b. Mixing
the C-dots and polyethylhennkoueneimine (PEI) led to Schiff base reaction between
the aldehydes on their surfaces of the C-dots and a number of amines in the branched
PEI framework to give imine bonds, resulting in the formation of C-dots/PEI gel. The
dynamic covalent imine bonds are core structural elements that are responsible for
the self-healing properties (Fig. 12.19c). For the characterization, the high-resolution
transmission electron microscopy (HR-TEM) was used to reveal well-resolved lattice
planes confirming the formation of sp
2 graphitic cores. The average C-dot particle
sizes were determined to be 6 ± 3 nm, 3 ± 1 nm, and 7 ± 2 nm for G-C-dots, B-Cdots, and CoAP-C-dots, respectively. It is noteworthy that no aggregation-induced
fluorescence quenching was widely observed in C-dot systems in the solid phase,
strongly suggesting immobilization of the cross-linked C-dots within the gel matrix
to prevent their close proximity and concomitant self-quenching. The formation of
imine bonds was evidenced by FT IR and XPS measurements. A significant decrease
in 1700 cm
–1 due to C=O was observed, whereas the generation of imine bonds was
Y. Kubo
12.7 Dynamic Covalent Bond
The use of dynamic covalent bonds is a potent methodology in the sense that they
combine the characteristics of covalent and non-covalent bonds (Wilson et al. 2014;
Rowan et al. 2002). Its reversible nature and “error-checking” characteristics allow
for the easy thermodynamic formation of ordered nanostructures, which create a
stable and covalently bound molecular system. Currently, functional systems based
on more than one type of dynamic covalent bond have received lots of attention
as “orthogonal dynamic covalent bonds” (Wilson et al. 2014). For the expansion,
“tunable orthogonal reversible covalent” chemistry (Reuther et al. 2019) allowed
for the construction of controlled molecular level motion (Bruns and Fraser Stoddart
2016). In principle, dynamic covalent reactions are roughly classified as the formation
of new dynamic covalent bonds and exchange reactions. The former case involves
imine condensation and aldol reactions. The covalent bonds known to be dynamic,
involve in C–C bond, C–N bond, C–O bond, C–S bond, S–S bond, and B–O bond.
Among them, imine bond/exchange (Vardhan et al. 2015; Belowich and Stoddart
2012) and boronic acid condensation (Kubo et al. 2015) are directional, which are
useful for the supramolecular organization.
As a typical example, multicolor emissive self-healing carbon dot/polymer gels
were reported (Bhattacharya et al. 2019). Chemical self-healing gels assembled
through dynamic covalent bond formation (Wei et al. 2014; Amaral and Pasparakis
2017) are well-known in the field of soft materials. In this study, Jelinek et al., utilized
carbon dots (C-dots) as luminescent nanoparticles, whose emission properties can be
tuned by the different carbonaceous precursors used (Miao et al. 2018). Figure 12.21a
shows the synthetic schemes and assembly of emissive C-dot/polymer gel. The three
aldehyde building blocks and experimental conditions gave the differently colored
C-dots; green emissive C-dots (G-C-dots) were prepared using glutaraldehyde at
150 °C for 2 h. Blue emissive C-dots (B-C dots) and yellow C-dots (CoAP-C-dots)
were obtained from benzaldehyde and polymeric cyclooctadiene-aldehyde, respectively. The synthetic strategy of the targeted gel was depicted in Fig. 12.21b. Mixing
the C-dots and polyethylhennkoueneimine (PEI) led to Schiff base reaction between
the aldehydes on their surfaces of the C-dots and a number of amines in the branched
PEI framework to give imine bonds, resulting in the formation of C-dots/PEI gel. The
dynamic covalent imine bonds are core structural elements that are responsible for
the self-healing properties (Fig. 12.19c). For the characterization, the high-resolution
transmission electron microscopy (HR-TEM) was used to reveal well-resolved lattice
planes confirming the formation of sp
2 graphitic cores. The average C-dot particle
sizes were determined to be 6 ± 3 nm, 3 ± 1 nm, and 7 ± 2 nm for G-C-dots, B-Cdots, and CoAP-C-dots, respectively. It is noteworthy that no aggregation-induced
fluorescence quenching was widely observed in C-dot systems in the solid phase,
strongly suggesting immobilization of the cross-linked C-dots within the gel matrix
to prevent their close proximity and concomitant self-quenching. The formation of
imine bonds was evidenced by FT IR and XPS measurements. A significant decrease
in 1700 cm
–1 due to C=O was observed, whereas the generation of imine bonds was