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.