1.2.2 Si-O-Si consisted linear polysiloxanes
In general, the good flexibility of Si-O-Si chain segments helps to form
aggregates. However, the powerful flexibility is adverse to rigid
molecular conformation, leading to nonemissive or weak emissive
polysiloxanes due to the radiative decay. Consequently, luminophores are
introduced to fabricate luminescent linear polysiloxanes that are
consisted with Si-O-Si chain segments. The flexible Si-O-Si chains offer
multiple possibilities for the aggregation pattern of luminophore,
contributing to the luminescence process. By incorporating anthryl
groups to the polysiloxanes, blue/green luminescence was observed in
PAPMS-1. The UV-triggered dimerization between anthracene groups breaks
the π-π interactions, thus weakened the ACQ effect and enhanced the
emission.[51] Feng et al. observed solvent
polarity-driven self-assembly in polysiloxane-based ionic liquids
(PNLs), which is synthesized from PMMS and
imidazolium-Br.[52] When dispersed in a nonpolar
solvent, PMMS forms a nonpolar shell around the imidazolium-Br group
owing to the difference in the polarity of imidazolium-Br group and PMMS
main chain. The apparent electrostatic interactions within the
imidazolium-Br group cause the aggregation of side-chains of the PNLs.
They proposed that the fluorescence of PMMS is generated by the S → Si
coordination bond induced splitting of 3d orbital in Si atoms. The
electrons rearranged in split orbitals and caused d-d transitions, thus
make PMMS fluorescent.
As previously discussed, on the one
hand, polysiloxanes have flexible chains that are beneficial to the
aggregation. Taking advantage of the flexible polysiloxanes, AIE
luminophores are covalently linked to polysiloxanes to fabricate
luminescent polymers. Lin et al. combined the stability and
biocompatibility of polysiloxane, good fluorescence QY of naphthalimide
and the polar sensitivity of triphenylamine (TPA), and obtained a
polysiloxane-based fluorescent polar probe TR-1 (Figure
6A).[53] The intramolecular charge transfer (ICT)
betwee TPA (donor) and amide (acceptor) in TR-1 facilitates
solvatochromic effect and selective detection of cancer cells. Liang et
al. developed an AIE featured 1,2,4,5-tetraphenylbenzene functionalized
polysiloxane (TPB-P) (Figure 6D) through Diels–Alder
reaction.[54] Influenced by the π-π stacking of
TPB groups and the flexibility of Si-O-Si chains, TPB-P tends to
aggregate into porous nanoparticles, thus favoring the absorbance of
small molecules. The flexible Si-O-Si chains and π-π interactions
enhanced the electron transfer between electron-deficient picric acid
and electron-rich fluorophore, resulting in fluorescence quenching.
Zhang et al. synthesized poly(hydroxyurethane) (Figure 6B) from carbon
dioxide, siloxane (Si-O-Si)-containing bisepoxide and diamine, and it
shows a strong emission intensity with a QY of 23.6% owing to the
hydrogen bonding induced intramolecular n-π* interactions of HO···C=O
and C=O···C=O.[1] The emission property of
poly(hydroxyurethane) can be regulated by precise control of the
hydrogen bonding interactions by adjusting the content of the -OH group.
The hydrogen bonding is facilitated by the hydrophobic and flexible
Si-O-Si linkage induced aggregation of hydrophilic hydroxyurethane
groups. The poly(hydroxyurethane) film could be used to fabricate a
white LED with white light emitting behaviors.
Besides AIE luminophores, fluorophores with π conjugations are also used
to construct luminescent linear polysiloxanes. Manners et al. prepared
fluorescent block copolymers by linking fluoroboron dipyrrole (BODIPY)
dyes to side chains of linear polysiloxanes.[55]By the sequential addition of unimer at ambient temperature, precisely
controlled 2D platelet micelles and block co-micelles were built from
these crystalline-coil copolymers (Figure 6C). They also synthesized a
series of fluorescent block copolymers by connecting fluorophores with
BODIPY core to their side chains (Figure 6F), and further assembled into
multicompartment micelles with controllable
emissions.[56] Liang et al. reported the
fluoranthene-modified linear polysiloxane via Diels–Alder reaction
(Figure 6E).[57] The flexible Si-O-Si chains
suppress the π-π stacking between fluorophore units and result in strong
fluorescence emission in the aggregation state. Lin et al. designed a
fluorescent ratiometric probe by covalently connecting imine-linked
polysiloxanes and rhodamine-B (Figure 6G).[58] The
imine-linked polysiloxanes exhibit blue fluorescence owing to the N → Si
coordination bonds. The addition of Fe3+could switch
the fluorescence emission of the probe from blue to red by destroyed the
N → Si bonds and regenerated rhodamine-B in open-cyclic state.
On the other hand, the Si-O-Si chains can also disturb the aggregation
of luminophore. Feng et al. attached tetraphenylethylene (TPE) to
polydimethylsiloxane via aza-Michael addition and synthesized
n-TPE-AP-PDMS with TICT effect.[59] The TICT
behavior between dimethylamino (donor) group and acryloxy (acceptor)
group in n-TPE was enhanced as the increase of solvent polarity. With
the TICT effect and suppressed aggregation caused by the flexible
aminopropyl polydimethylsiloxane, the n-TPE-AP-PDMS exhibits ACQ
behavior in THF/water mixture and AIE phenomenon in THF/hexane mixture.
The same group also obtained 7, 11-diphenyl-8H - acenaphth[1, 2-f ] isoindole-8, 10 (9H )-dione functionalized
polysiloxane (PMDF) by Diels–Alder reaction.[60]The siloxane structure suppresses the intermolecular π-π stacking of
luminophores and make the PMDF an AIE material.
The empty 3d orbitals of Si atoms tend to form electron delocalization.
Basing on that, Feng et al. synthesized two types of fluorescent
polysiloxanes via Heck reaction, those are linear-shaped BpaP with the
backbone of polysiloxane and BpaD with the backbone of
siloxane.[61] The flexible Si-O-Si chains of
polysiloxanes plays a crucial role in efficient electron transfer from
triphenylamine to 4-nitrophenol in BpaP, causing fluorescence quenching
and thereby achieved lower detection limit for 4-nitrophenol (0.6 μM for
BpaD and 0.23 μM for BpaP). The excellent permeability and low surface
energy caused by flexible Si-O-Si moiety also make polysiloxane an ideal
candidate for the combination with rare earth elements. Using thiol-ene
“click” chemistry between functionalized polysiloxanes and
EuIII, as shown in Figure 7A, transparent elastomer
with a strong fluorescence emission at 617 nm were
formed.[62] The luminescent elastomers exhibit
desired mechanical flexibility and their color can be tuned by
incorporating suitable lanthanide ions. Flexible Si-O-Si backbones can
also reduce the conformational energy. As shown in Figure 7B, two kinds
of thermo- and photo-responsive linear polysiloxanes (DRPSs) with
N-isopropyl amides and
azobenzene/salicylideneaniline as
side groups were designed by Feng et al.[63] The
DRPSs exhibit tunable phase separation, which can be controlled by
temperature and UV light, and a lower critical solution temperature
(LCST) in aqueous solution, which depends on the content of incorporated
side groups and their isomerization state. Upon irradiation, higher
values of LCST were observed due to the higher
polarity caused by the isomerization
of the side groups. Thereby reversible solubility change was achieved in
the LCST range before and after
irradiation. The sensitive phase
separation is attributed to the reduced conformational energy caused by
the flexible Si-O-Si backbones.