1.3 Polysiloxanes with other structures
The luminescence of nonconventional siloxanes was first reported in
1998. Lianos et al. synthesized a fluorescent material through the
interaction of (3-aminopropyl)-triethoxysilane with acetic acid under
oxygen-free conditions. The synthesized material can emit bright
fluorescence at 450 and 560 nm, and the fluorescence lifetime and QY are
9.9 ns, 5.8 ns, 21% and 12% respectively.[31] In
2000, they synthesized six transparent nanocomposite gels based on
poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) chains with
different lengths.[48] Through urea bridges, the
end silicate groups are linked to the polyether chains. These gels can
emit luminescence at room-temperatures and the enhancement of
luminescence intensity was observed in gels with shorter polyether
chains or doped with cations of large atomic number. The longer
polyether chains can form dispersed silicon and urea groups and increase
defects on emitting clusters, thus decreasing the luminescence. The
attachment of heavy cations to the silica cluster surface can increase
their luminescence by eliminating surface defects. Thus, they inferred
that the emitting centers are ocated on the surface of silica clusters
with concentrated -NH- and C=O groups and the luminescence was generated
by the recombination of delocalized electron-holes. Larger clusters emit
luminescence at longer wavelength and smaller clusters emit at shorter
wavelength.[48]
Carlos et al. prepared organic-inorganic hybrid by reacting three
diamines with
3-isocyanatepropyltriethoxysilane.[64,65] The
hybrids exhibit two emission peaks, blue and purplish-blue. In each
hybrid, the inorganic coherent domains were combined with different
proportions of silicon-based structures. They believe the emission band
is associate with larger and smaller siliceous units. The recombination
of electron-holes is responsible for their luminescence. In 2004, they
found the PL of sol-gel-derived siloxane-based hybrids was originated in
the -NH2 groups with electron-hole recombinations
occurring in the siloxane nanoclusters.[66] The
white light emission is generated by the radiative recombination of the
donor-acceptor pairs. EPR results revealed oxygen-related defects in
siliceous nanodomains, in which Si was coordinated to one carbon and two
other oxygen atoms, namely •O-O-Si≡(CO2). They believe
the defect-related siliceous nanodomains are the emitting centers for
sol-gel-derived siloxane-based hybrids. According to Carlos et al., this
kind of organic-inorganic hybrids have a high QY of 19.2%, combined
with the ability to tune the emission. By fabricating highly organized
bilayer monoamidosil consisted of 2D
siliceous domains, they uncovered
that their PL was closely related to the annihilation/formation of the
hydrogen bonded amide-amide array during their phase transition between
order-disorder.[67]
Moreover, the polysiloxanes have
abundant noncovalent interactions, thus polysiloxane hydrogels can be
formed. Imae et al. reported the size-controllable nano- and
microhydrogels consists of third generation triethoxysilyl focal
poly(amido amine) dendrons with hexyl spacer.[68]As depicted in Figure 7D, it formed fiber-like texture at high
concentration and emitted fluorescence which was stronger in
base-catalyzed condition than in acid-catalyzed condition. The emission
intensity depends on the growth of PAMAM generations. Feng et al.
introduced imidazolium into the silicone polymers to construct
conductive silicone materials with oxalic acid as a crosslinking agent
(Figure 7C).[69] The prepared silicone polymers
exhibit AIE character with a yellow-green fluorescence due to the
aggregation of imidazolium moieties. The entangled polymer chains and
the intermolecular interactions in the solid silicone materials
decreased molecular vibrations and formed new conjugated structures by
electron overlapping, thus leading to obvious fluorescence emission. The
same group further prepared two elastomers by incorporating TPE
derivatives into polydimethylsiloxane networks through dynamic covalent
cross-linking (Figure 7E).[70] The connection of
TPE derivatives can activate the fluorescence of the elastomers by
inhibiting the nonradiative relaxation.
Uchino et al. synthesized a series of n-octadecylsiloxanes containing
end silicate groups, that is, R-SiO3/2,
R-(CH3)SiO2/2,
R-(CH3)2SiO1/2 (R=
C18H37).[71]Compared with the analogues with the same aliphatic chains,
R-SiO3/2 shows highest emission intensity with a QY of
19±0.5%. The silicon/oxygen-related defect species, instead of
carbon-related species, are responsible for their luminescence. The
experiments indicated that the number of oxygens attached to Si atoms
determines the luminescence behavior, and organic groups kinetically
hinder the relaxation of metastable defect pairs generated from the
condensation of silanol groups.
Polyhedral oligomeric silsesquioxane (POSS) derivatives-based AIE
materials are another branch of polysiloxanes. Chang et al. reported
blue PL of star poly(N-isopropylacrylamide)-b-polyhedral oligomeric
silsesquioxane (PNIPAm-b-POSS) copolymer in water above a lower critical
solution temperature (LCST).[72] The PL of
PNIPAm-b-POSS is generated by the constrained geometric freedom and
relatively rigid structure due to the abundant intramolecular hydrogen
bonding. Kuo et al. fabricated POSS-containing polymers (Figure 7F),
which lacks common fluorescent units, through free radical
polymerization or hydrazinolysis.[73] FTIR spectra
confirmed the existence of dipole-dipole interactions and hydrogen
bonding between the C=O groups and OH groups, and the PL is caused by
the crystallinity of poly(MIPOSS) and the clustering of locked C=O
groups of POSS units.
Carbon dots doped with nitrogen and silicon (SiN-CDs) can also exhibit
PL emission by the hydrothermal reaction of HBPSi (Figure
7G).[74] According to the PL spectra, the SiN-CDs
have three excitation bands (240, 300 and 360 nm) and only one emission
band (440 nm). The HBPSi used in preparing SiN-CDs has two emission
peaks at 350 nm and 440 nm generated by -OH, -NH2, and
aggregated fluorescent groups. The SiN-CDs show only one emission band
at 440 nm, indicating the SiN-CDs and HBPSi share the same emission
center. In SiN-CDs, the multi excitation is caused by the π-π*
transition of C=C in the sp2 core (240 nm), n-σ* transition of the
isolated groups (300 nm) and n-π* transition of the aggregated groups
(360 nm) respectively. The observed emission peak at 440 nm
(λ ex=360 nm) is caused by the
energy transfer from the absorbed
energy in sp2 core and isolated functional groups to the aggregated
fluorescent groups, leading to blue PL emission as the radiative decay.