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.