1.1.2 Hyperbranched polysiloxanes with terminal modifications
Different from skeleton variation, terminal modification provides a new
strategy to regulate molecular aggregation and manipulate localized TSC.
In 2016, our group reported that the modification of 20 wt% polyether
amine to the epoxy terminated HBPSi can improve their water solubility
obviously, and enhanced the QY from 4.61% to
7.30%.[35] The abundant oxygen atoms in polyether
promote the cluster of O atoms and localized TSC, and the grafted
polyether amine increases the molecular weights, thereby compact the
molecular conformation and suppress the chain rotation. Subsequently, we
discovered energy-transfer-induced enhanced emission mechanism in
L-glutamic acid modified HBPSis.[44] With the
increase of the content of L-glutamic acid, the fluorescent intensity of
HBPSi-GA is enhanced remarkably, and the QY reached 11.78%. As shown in
Figure 3A, HBPSi-GAs absorb UV energy at 238, 262, 300 and 360 nm, and
emit fluorescence at 380 and 450 nm. In concentrated solution, the
emission at 450 nm becomes stronger and that at 380 nm gets weaker. TEM
images demonstrate the change of morphology in concentrated solution and
DFT predictions reveal that the inter/intramolecular hydrogen bonds in
HBPSi-GA is much stronger than that in HBPSi, thus leading to enhanced
fluorescence emission. We speculated that the adsorbed energy
temporarily stored in free functional groups which emitted fluorescence
at 380 nm, and then transfered to the electron delocalizations in
relatively larger clusters via FRET, intensifying the fluorescence at
450 nm (Figure 3A and 3B). In diluted solutions, more small clusters are
formed, thereby promoting the emission at 380 nm. The introduction of
L-glutamic acid also helps to improve the biocompatibility of HBPSi-GA,
rendering it good cell imaging ability.[44]
In addition, the amphipathic effect is another driving force to regulate
the molecular aggregation. By replacing the L-glutamic acid with the
renewable oleic acid, the QY of HBPSi-OA reaches 28.57% and exhibits
multiemission at 330, 360 and 430 nm.[45] Compared
with HBPSi, the oleic acids distributed on the surface of HBPSi promote
the aggregation through hydrophobic effects. As depicted in Figure 3C,
the hydrogen bonds and interactions of O and N atoms are stronger in
HBPSi-OA, leading to intensified electronic communication among
hydroxyl, amine, ether, and -Si(O)3 groups in HBPSi-OA
aggregates. As a result, the fluorescence intensity and QY of HBPSi-OA
are obviously enhanced. Mechanism studies reveal that the multiemission
is generated by the heterogeneous electron delocalizations in the
self-assembled aggregates (Figure 3C and 3D), and the multiexcitation is
caused by the energy transfer between free functional groups, smaller
electron delocalizations, and the larger electron delocalizations.
Contrast to the L-glutamic acids and oleic acids with soft aliphatic
chains, β ‑cyclodextrin (β ‑CD) is a cage-like molecule with
abundant O atoms. The β ‑CD not only possess abundant O atoms that
greatly influenced the molecular aggregation (Figure 3E), but also have
rigidified macrocyclic structures that benefits the radiative decay,
thus the QY of HBPSi-CDs increases from 12.24% to
18.72%.[38] With the grafting of β ‑CD, the
intermolecular hydrogen bonds, O···O and O···N interactions, N → Si
coordination bonds, and O → Si interactions of HBPSi-CDs are much
stronger, leading to larger TSC and more compact conformations (Figure
3F). In addition, the energy gap of HBPSi-CD is much lower than that of
HBPSi. The enlarged TSC and rigidified conformations promote the
fluorescent intensity and redshift the emission maxima. These results
demonstrate the crucial impact of through-space electronic interactions
on the photophysical behavior of HBPSi, and provide the rational
molecular design of non-conjugated AIE luminophores. That is, by
incorporating electron-rich atoms, the TSC of the luminophore is
enhanced and the molecular conformations are rigidified. Consequently,
superior PL properties are achieved.
The length of dihydric alcohols also affects the molecular aggregation.
With shorter dihydric alcohols, the β ‑CD modified HBPSi
(HBPSi-β -CD) shows different PL properties. By adjusting the
distribution of electron-rich atoms and the grafting amount ofβ -CD, the HBPSi-β -CD shows different QY of 19.36%,
31.46%, 46.14% and 44.84% when excited by 360, 420, 450, and 550 nm
respectively (Figure 3G).[42]HBPSi-NH2 without β -CD terminal modification
shows excitation-dependent fluorescence with the emission colors ranging
from blue, green to red. However, its emission in the red region is
quite faint. In contrast, the red emission of HBPSi-β -CD is still
bright, indicating the truly multicolor. Compared with
HBPSi-NH2, the aggregates of HBPSi-β -CD are much
larger than that of HBPSi-NH2, thus generating larger
clusters and inhibiting the movement of molecular chains. Moreover,
HBPSi-β -CD has much stronger inter/intramolecular hydrogen bonds
than HBPSi-NH2, leading to more compact supramolecular
topology and larger electron delocalizations (Figure 3H). Thus, the
truly multicolor emission of HBPSi-β-CD is derived from their larger
electron delocalizations.[42]
The interactions of terminal functional groups and the density of
electron-rich atoms not only determine the fluorescence intensity and QY
of nonconventional polymers, but also affect their emission wavelength.
Instead of non-conjugated molecules, Bai et al. functionalized HBPSi
with aromatic amino acids.[46] By grafting
L-phenylalanine, L-tyrosine, and L-tryptophan on HBPSi, high
fluorescence intensity and QY were obtained in green, yellow, and red
emission regions (Figure 4B). Differ from the β -CD modified
HBPSi, HBPSi with π bonds has enhanced electronic communication among
conjugated π bonds and other functional groups, such as amine, hydroxyl,
and -Si(O)3 groups. Hydrogen bonds, high density of
functional groups, as well as amphiphilic effect, promote the
aggregation and clusterization of π bonds functionalized HBPSi (Figure
4A).
In addition to the enhanced electronic aggregation, the integration of S
atoms changes the TSC in HBPSi and forms non-conjugated D−A structures
in HBPSi-Cys. Recently, our group reported HBPSi-Cys with two distinct
emission peaks that refers to local excited (LE) and twisted
intramolecular charge transfer (TICT) emissions
respectively.[47] With the weak electron-accepting
S atoms, TICT regulated the solvatochromic emission with a large Stokes
shift of 213 nm. As can be seen in Figure 4C, the combination of
L-cysteine not only enhances the fluorescence intensity via increasing
electron density, but also leads to distinct spatial separation of HOMO
and LUMO distributions, which is favorable for the charge transfer
process. The TICT emission was experimentally confirmed by the
Lippert-Mataga plots (Figure 4E) and temperature-dependent fluorescence
spectra. Then, the DFT results further revealed that sulfhydryl groups,
amide groups, and adjacent delocalized electrons formed the acceptor
units, the amine and hydroxyl groups acted as the donor units, and the
Si atoms are used as bridges to mediate the charge transfer process in
polar solvents. Consequently, the S atoms regulated the inner electron
environment of HBPSi-Cys, and results in a new non-conjugated
hyperbranched polysiloxane with TICT
emissions.[47]
The PL behavior is also observed in hyperbranched polysiloxanes
consisted of Si-O-Si segments. In 2000, Lianos et al. synthesized
ureasils with end silicate groups using poly(ethylene oxide) or
poly(propylene oxide) as the polyether chain.[48]They found that ureasils emitted room-temperature luminescence in a
cluster-size-dependent manner, and the fluorescence cannot be detected
in diluted samples. They believed that the luminescence came from the
delocalized electron-hole recombination processes in the Si-containing
cluster induced by the aggregation of polymer chains. Feng et al.
reported siloxane−poly (amidoamine) (Si-PAMAM) dendrimers with several
generations by aza-Micheal reaction and amidation reaction in
2015.[37] The Si-PAMAM dispalyed strong blue
luminescence without addition of extra oxidizing reagent and the
emission intensity increased rapidly as the generation increased. The
tertiary amines are crucial to the luminescence, and the aggregation of
carbonyl groups that is generated by N→Si coordination bonds is
responsible for the enhanced fluorescence intensity.
1.2 Structural diversity of linearpolysiloxanes
The hyperbranched structures enables polysiloxanes with various PL
properties due to the aggregation of electron-rich atoms. However,
linear polysiloxanes, which undergo low spatial density of functional
groups, are usually nonemissive under ambient conditions. Compared with
hyperbranched polysiloxanes, linear polysiloxanes lacks
three-dimensional topology, leading to enlarged distance between chain
segments. Consequently, no fluorescence was observed. To enrich the
functions of linear polysiloxanes, substituents were covalently linked
to their backbones. Inert groups, such as methyl, phenyl and
trifluoropropyl groups, can reduce the surface energy of polymers and
active groups, such as hydrogen, vinyl and amino groups, are
conducive to their functionalization.
Thereby electron-rich atoms and fluorophores are used to endow linear
polysiloxanes with luminescent properties.