Nonconventional aggregation‐induced emission polysiloxanes: Structures, characteristics, and applications

Nonconventional luminescent materials have been rising stars in organic luminophores due to their intrinsic characteristics, including water‐solubility, biocompatibility, and environmental friendliness and have shown potential applications in diverse fields. As an indispensable branch of nonconventional luminescent materials, polysiloxanes, which consist of electron‐rich auxochromic groups, have exhibited outstanding photophysical properties due to the unique silicon atoms. The flexible Si‐O bonds benefit the aggregation, and the empty 3d orbitals of Si atoms can generate coordination bonds including N → Si and O → Si, altering the electron delocalization of the material and improving the luminescent purity. Herein, we review the recent progress in luminescent polysiloxanes with different topologies and discuss the challenges and perspectives. With an emphasis on the driving force for the aggregation and the mechanism of tuned emissions, the role of Si atoms played in the nonconventional luminophores is highlighted. This review may provide new insights into the design of nonconventional luminescent materials and expand their further applications in sensing, biomedicine, lighting devices, etc.

Silicon is an important abundant element of group IV with no reported connate toxicity, which provides application potential in biological and medical.The observation of luminescence from silicon materials was first reported in 1996 when Cox et al. found that the oxidized porous silicon could generate blue PL in air. [31]Then Sailor et al. reported a series of PL silicate glasses in 1997. [32]The first reported organosilica luminescent material was synthesized by Lianos et al. in 1998. [33]They observed blue and yellow PL in the silica gel and attributed that to the carbon impurities and electronic transitions from the lone-pair electron in the amino group to the carbon impurities.In 1999, Carlos et al. connected the PL to larger and smaller local environments ascribed to a series of siliceous units. [34,35][36][37][38] In 2004, they further associated the recombination mechanism with oxygen-related defects which were believed to be the emitting centers. [37]In 2009, Uchino et al. confirmed that the luminescence center in siloxane-based organic-inorganic hybrid materials was a silicon/oxygenrelated defect rather than a carbon-and/or nitrogen-related defect. [39]In 2013, Yuan et al. reported CTE mechanism, and it opened a new avenue to the design of organosilica luminescent materials. [29]As an integral part of organic-inorganic hybrid polymers, hyperbranched polysiloxanes combine the advantage of polysiloxanes and hyperbranched structures, thereby processing wide temperature resistance, multifunctionality, low viscosity, etc. [40] As for now, the most popular methods of synthesizing hyperbranched polysiloxanes are hydrosilylation and hydrolysis polycondensation.However, the former needs expensive catalyst, and the latter suffers from poor control of the hydrolysis process.
In 2016, our group developed a new way to synthesize hyperbranched polysiloxanes, that is, transesterification polycondensation reaction. [26][43][44][45] Different from polysiloxanes consisting of Si-O-Si chain segment, the bond angle of Si-O-C chain segment is 120 • , which is less than conventional Si-O-Si segment (130 • ) and greater than C-O-C (110 • ) and C-C-C (109 • ) segments, thus renders HBPSis good flexibility and rigidity simultaneously.The good flexibility of organic chains contributes to the aggregation of HBPSis, and the rigidity of inorganic chains suppresses the rotation of chain segments, leading to AIE-active fluorescence.Due to the unique bond angle of Si-O-C chain segment, HBPSis exhibit extraordinary photophysical features.For example, most HBPSis can emit bright monochromatic or even multicolor fluorescence, due to the presence of abundant heteroatom-bearing groups and N → Si coordination bonds.
The unique silicon atom endows polysiloxanes with outstanding photophysical properties.Thus, the understanding of polysiloxanes, especially the role Si atoms played in these luminophores, would benefit in unveiling the emission mechanism of nonconventional luminophores and expanding their applications.In this review, we summarize recent progress in luminescent polysiloxanes with different topologies, including hyperbranched polysiloxanes, linear polysiloxanes, POSS-based polymers, etc. (Scheme 1), with a focus on the driving force for the aggregation and the mechanism of tuned emissions.

Photophysical properties of hyperbranched polysiloxanes
The observation of fluorescence from HBPSi can date back to 2016, when our group synthesized aliphatic tertiary amine-containing hyperbranched polysiloxanes (TAHPSis) through the polycondensation reaction of tetraethoxysilane, triethanolamine, N-methyldiethanolamine or diethylene glycol. [26,43]Contrary to the generally believed oxidation mechanism, TAHPSis can emit bright blue fluorescence under 365-nm UV light without further oxidation or acidification, and their fluorescence intensities were enhanced with increasing concentrations, showing typical AIE characteristics (Figure 1A).Investigations of TAHPSis reveal that the aggregation of hydroxyl groups is responsible for their luminescence.
The results of TAHPSis revealed that oxidation of aliphatic amines may not be the intrinsic interpretation of the luminescence mechanism, and we infer that the PL behavior of HBPSi strongly depends on the aggregation of hydroxyl groups.With the terminal hydroxyl groups, strong inter/intramolecular hydrogen bonds are formed, leading to aggregated electron-rich clusters, which serve as the luminescent center and generate strong PL under ambient conditions.To verify our hypothesis, HBPSis with terminal hydroxyl, amine, and epoxide groups were synthesized and their PL behaviors were systematically studied (Figure 1B). [46,47]We found that HBPSis with terminal hydroxyl groups are highly fluorescent, while almost no fluorescence is observed when the hydroxyl groups are blocked by t-butyl acetoacetate.The connection with t-butyl acetoacetate disturbed the hydrogen bonding of hydroxyl groups, leading to limited aggregation of HBPSi and the formation of the luminescent center.Epoxide groups could also promote the aggregation of HBPSi and enhance their luminescence.It works with hydroxyl group synergistically in HBPSi and makes a quantum yield (QY) of 4.61%.
Another critical factor that influences the molecular aggregation is the steric hindrance.To clarify the effect of steric hindrance, we synthesized two HBPSis with similar structures.With the same terminal hydroxyl and amine groups, the fluorescence intensity, lifetime and QY of HBPSi carrying neopentyl glycol moiety is higher than those of another HBPSi containing 2-methyl-1,3-propanediol moiety. [46,48]his could be attributed to the larger steric hindrance of neopentyl glycol.On the one hand, its larger steric hindrance would restrict the aggregation of HBPSi, going against the

S C H E M E 1 Molecular structures of aggregation-induced emission (AIE) polysiloxanes.
F I G U R E 1 (A) Synthetic route of TAHPSi with photographs of their solutions (20 mg/mL, 40 mg/mL, 60 mg/mL and pure sample).Reproduced with permission: Copyright 2016, The Royal Society of Chemistry. [43](B) Synthetic route of HBPSi with terminal hydroxyl and amine groups and photographs of their aqueous solutions.Reproduced with permission: Copyright 2015, John Wiley and Sons. [46] Reproduced with permission: Copyright 2022, Elsevier. [50]ormation of the luminescent center.On the other hand, it may restrict the molecular motion, favoring the fluorescence.With the hydrogen bonds promoted aggregation of terminal hydroxyl and amine groups, the larger steric hindrance of neopentyl glycol benefits more from the restricted molecular motion than the restricted aggregation, leading to enhanced PL properties.
With the empty 3d orbitals in Si atoms, coordination bonds can be formed between electron-rich atoms and Si atoms.In 2015, Feng et al. reported the N → Si coordination bonds in Si-assisted polymers. [49]Similarly, the N → Si coordination bonds can also promote the aggregation of HBPSi.X-ray photoelectron spectroscopy (XPS) revealed the existence of N → Si bonds in HBPSis, and density functional theory (DFT) further confirmed the molecular aggregation enhanced by N → Si coordination bonds. [44]Besides N atoms, electronrich atoms, such as S and O atoms, can form coordination bonds with empty 3d orbitals of silicon atoms, generating dd orbital splitting.The electron in split d orbitals can also absorb UV energy and generate radiative decay, which is beneficial to the fluorescence of HBPSis.
Theoretical calculations reveal that the bond angle of Si-O-C is about 120 • , which is between Si-O-Si (130 • ) and C-O-C (110 • ) (Figure 1C).As a consequence, the Si-O-C chain segment integrates the good flexibility of organic chains and the rigidity of inorganic chains, which further promotes the aggregation of molecules.Our group reported the long carbon chain integrated HBPSi which used 1,6-hexanediol as the reactant. [50]As shown in Figure 1D, the carbon chain in 1,6hexanediol is more flexible than previously reported dihydric alcohols, and it facilitates the formation of compact aggregates together with the Si-O-C chain segment.DFT results confirm that more hydrogen bonds and intra/intermolecular O⋅⋅⋅O and O⋅⋅⋅N interactions contribute to the overlap of electron clouds of electron-rich atoms, thus leading to the generation of through-space conjugation (TSC).With the satisfying flexibility and rigidity generated by Si-O-C chain segment and the long carbon chain in 1,6-hexanediol, the QY of the synthesized HBPSi reached 17.88%.
The above results revealed that the electron-rich atoms could accelerate the aggregation of HBPSis, leading to unique AIE emissions.DFT predictions suggest that not only hydrogen bonds, the interaction of O and N atoms and coordination bonds can also be found in the HBPSi aggregates.These inter/intramolecular interactions drive the aggregation of electron-rich atoms, making them clustered in close proximity.The electron clouds of these electron-rich atoms overlap with each other, generating "clustered chromophores" via TSC.With enriched energy levels, the energy gaps between HOMO and LUMO orbitals are reduced, thus benefiting excitation.The through-space interactions are also conducive to rigidifying the molecular conformation, thereby favoring radiative decay. [51]Guided by this idea, more attention was given to the exploration of structure-promoted aggregation at the molecular level, and multicolor HBPSis were synthesized afterward.
The empty 3d orbitals of silicon atoms can also facilitate charge transfer to fabricate nonconventional polymers with delayed fluorescence.Recently, our group has synthesized a series of hyperbranched polyborosiloxanes which employed hyperbranched polyborates as the core (Figure 1E), and they exhibited redshifted PL colors as the electron density increases on the monomer diol. [52]With a higher electron density on the monomer diol, P4 showed red emission and delayed fluorescence with a lifetime of 9.73 μs.The experimental results and DFT calculations revealed that significant heteroatom-induced electron delocalization and through-space O⋅⋅⋅O and O⋅⋅⋅N interactions were found in P4.The synergistic effect of the above factors greatly reduced the ΔE ST to 0.08 eV, and generated enhanced red delayed fluorescence.Our group also prepared another HBPSi with red emissions.By regulating the first excited state of HBPSi, enhanced energy transfer from HBPSi to Eu 3+ ion was observed with a distinct PL emission at 613 nm. [53]1.1

Hyperbranched polysiloxanes with local conjugations
The absence of aromatic structures generated unique advantages of HBPSis but also resulted in relatively low QY and short lifetime, and the emission bands are mainly centered in the blue region.Inspired by the classic luminescence theories, carbonyl groups were brought in to fabricate local conjugated HBPSis.56] One simple way to bring in carbonyl groups is to replace the dihydric alcohols with dicarboxylic acids.Together with the Si-O bond that exhibits partial double bond features, local conjugations were formed in HBPSi containing conjugated O=C-O-Si-C=C segment (marked as P1). [45]In comparison, HBPSis contain the O=C-O-Si-O-C=O segment (P2) and C-C-O-Si-C=C segment (P3) as reference.The three HBPSis all exhibit strong AIE characteristics, with a QY of up to 43.9% for P1.The largest conjugated segment O=C-O-Si-C=C renders P1 the highest QY among reported nonconjugated fluorescent polymers.The optimized structural parameters and topology results suggest that P1 molecules are aggregated with TSC due to the strong intermolecular H⋅⋅⋅O interactions, O → Si coordination bonds and overlapping between carbonyl and C=C groups.Thus, as depicted in Figure 2A, the energy levels are enriched and energy gaps are lowered with the increasing of molecular numbers, favoring the excitation of HBPSis.In the meantime, the strong H⋅⋅⋅O interactions and abundant O → Si coordination bonds also help to rigidify the molecular conformation, thereby promoting the radiative decay.DFT prediction also revealed that the C=O, C=C, and Si-O groups promote the formation of supramolecular TSC in P1, and coplanar TSC is generated when these groups are located in local conjugated positions (Figure 2B,C).The above factors lead to the highest QY of P1 jointly.
Recently, disulfide was integrated with the O=C-O-Si-C=C segment to further improve the QY of HBPSi.The electron-rich S atoms favor the formation of space electronic communication, and the dipole moment of the disulfide bond intensifies the distortion of the chain segment, which contributes to the molecular aggregation and restricted nonradiative decay (Figure 2D).With the increase in disulfide content, compact aggregates are formed, and a QY of 47.8% is achieved. [54] I G U R E 2 Schematic illustration of the possible fluorescence mechanism for P1, P2, and P3 (A) and molecular interactions in P1 (B).(C) Molecular structure of P1.Reproduced with permission: Copyright 2019, American Chemical Society. [45](D) Schematic illustration of the CTE mechanism in disulfide containing HBPSi.Reproduced with permission: Copyright 2022, John Wiley and Sons. [54]Molecular structure (E) and fluorescence microscope images (F) of HBPSi contains C=C-C=O segment.Reproduced with permission: Copyright 2020, The Royal Society of Chemistry. [56]other way to bring in carbonyl groups is to employ carbonyl-containing silanes.The methacryloxypropyltriethoxysilane and 1,3-propanediol were used to prepare a new HBPSi that contains a conjugated C=C-C=O segment. [56]In comparison, a linear structured polysiloxane was synthesized.Experiments suggest that the local conjugated HBPSi has a higher QY than the linear polysiloxane, which are 7.71% and 1.12% respectively.Under varying excitations, the HBPSi can emit distinct colors (Figure 2F).However, the reference linear polysiloxane can only emit weak blue fluorescence.DFT calculations reveal that HBPSis possess more compact aggregation and stronger oscillator strength than the linear polysiloxane due to the large number of terminal hydroxyl groups.As shown in Figure 2E, the multiring TSC is found in the ground state geometry of HBPSi, which leads to different electron delocalization systems in the supramolecular aggregates.Under different excitation wavelengths, the longwavelength emission may be generated by the big TSC rings consisting of the HBPSi main chain, and short-wavelength emission may be caused by the small rings that exist in their side chains.This conjecture was concluded as multiringinduced multicolor emission (MIE), and further supported by other non-conjugated AIE polymers.Conversely, it is more difficult for the linear polysiloxane to form strong multiring TSC, thus rendering it relatively low QY and weak blue emission.

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 enhance the QY from 4.61% to 7.30%. [47]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 compacting the molecular conformation and suppressing the chain rotation.Subsequently, we discovered an energy-transferinduced enhanced emission mechanism in L-glutamic acid modified HBPSis. [57]With the increase of the content of L-glutamic acid (GA), 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 a concentrated solution, the emission at 450 nm becomes stronger and that at 380 nm gets weaker.Transmission electron microscope (TEM) images demonstrate the change of morphology in concentrated solution and DFT predictions reveal that the inter/intramolecular hydrogen bonds in HBPSi-GA are much stronger than those in HBPSi, thus F I G U R E 3 Schematic illustration of the fluorescence mechanism (A) and the heterogeneous electron delocalizations (B) of HBPSi-GA.Reproduced with permission: Copyright 2020, American Chemical Society. [57](C) Schematic illustration of the mechanism for the self-assembled HBPSi-OA.(D) TEM image of HBPSi-OA aggregates in ethanol-water mixture.Reproduced with permission: Copyright 2021, John Wiley and Sons. [58]Schematic illustration of the self-assembly (E) and through-space conjugation (TSC) (F) of HBPSi-CD aggregate.Reproduced with permission: Copyright 2019, American Chemical Society. [44](G) Multicolor emission of HBPSi-NH 2 and HBPSi-β-CD.(H) Diagram of O⋅⋅⋅O and O⋅⋅⋅N interactions in HBPSi-β-CD aggregate.Reproduced with permission: Copyright 2022, American Chemical Society. [55]ading to enhanced fluorescence emission.We speculated that the adsorbed energy was temporarily stored in free functional groups which emitted fluorescence at 380 nm, and then transferred to the electron delocalizations in relatively larger clusters via FRET, intensifying the fluorescence at 450 nm (Figure 3A,B).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.

F I G U R E 4 Possible energy surfaces (A) and Commission
Internationale de l'Eclairage (CIE) plots (B) of aromatic amino acids modified HBPSi aggregates.Reproduced with permission: Copyright 2022, American Chemical Society. [59](C) Schematic diagram of the LE/TICT mechanisms and frontier molecular orbitals of HBPSi and HBPSi-Cys.(D) Emission spectra of HBPSi-Cys in different solvents.Insert: Fluorescence image of HBPSi-Cys.(E) Lippert-Mataga plot for HBPSi-Cys.Reproduced with permission: Copyright 2023, American Chemical Society. [60] addition, the amphipathic effect is another driving force to regulate the molecular aggregation.By replacing the Lglutamic acid with the renewable oleic acid (OA), the QY of HBPSi-OA reaches 28.57% and exhibits multiemission at 330, 360 and 430 nm. [58]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,D), and the multiexcitation is caused by the energy transfer between free functional groups, smaller electron delocalizations, and the larger electron delocalizations.
In 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 influence the molecular aggregation (Figure 3E), but also have rigidified macrocyclic structures that benefit the radiative decay, thus the QY of HBPSi-CDs increases from 12.24% to 18.72%. [44]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 conforma-tions are rigidified.Consequently, superior PL properties are achieved.
The length of dihydric alcohols also affects 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 of β-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). [55]BPSi-NH 2 without β-CD terminal modification shows excitation-dependent fluorescence with 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-NH 2 , the aggregates of HBPSi-β-CD are much larger than that of HBPSi-NH 2 , thus generating larger clusters and inhibiting the movement of molecular chains.Moreover, HBPSi-β-CD has much stronger inter/intramolecular hydrogen bonds than HBPSi-NH 2 , 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.
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 nonconjugated molecules, Bai et al. functionalized HBPSi with aromatic amino acids. [59]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).Differing 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 nonconjugated D−A structures in HBPSi-Cys.Recently, our group reported HBPSi-Cys with two distinct emission peaks that refer to local excited (LE) and twisted intramolecular charge transfer (TICT) emissions respectively. [60]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 the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (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 were 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 result in a new nonconjugated hyperbranched polysiloxane with TICT emissions.
The PL behavior is also observed in hyperbranched polysiloxanes consisting 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. [36]They found that ureasils emitted room-temperature luminescence in a cluster-size-dependent manner, and the fluorescence could not 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. [49]The Si-PAMAM displayed strong blue luminescence without the 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 are generated by N → Si coordination bonds is responsible for the enhanced fluorescence intensity.

Structural diversity of linear polysiloxanes
The hyperbranched structures enable 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 lack three-dimensional topology, leading to an 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.

Si-O-C consisted linear polysiloxanes
Generally, linear polysiloxanes consist of Si-O-C chain segment are nonemissive or faintly emissive due to the low local density of electron-rich atoms.To our delight, the insertion of disulfide bond makes it boosted luminescence owing to the chain distortion.The disulfide bond is a kind of dynamic covalent bond with low electronegativity.On the one hand, it shows weak binding capacity to outer shell electrons and is beneficial to reduce the HOMO-LUMO energy gap.On the other hand, the disulfide bonds can increase the distortion of polymer chains by the small bond angle.Our group has achieved multicolor fluorescence in linear polysiloxane by introducing disulfide bond and carbonyl groups.The synthesized linear polysiloxane can be excited by visible light (λ Ex = 508 nm) and emit multicolor fluorescence under different excitation wavelengths. [61]The emission behavior of the linear polysiloxane is further explained by DFT and TD-DFT.In Figure 5A, multiring TSCs with different sizes were found in disulfide bond-containing linear polysiloxane via Si-O and S-S bridges.The multicolor emission can be well deciphered in view of the MIE mechanism.The flexible Si-O-C chain and the disulfide bond can increase the distortion of main chains, thus shortening the distance between electron-rich groups.Then, the multiring TSC is generated owing to the decreased distance between Si-O, C=C, C(O)O, and S-S groups.The disulfide bond also reduces excitation energy, increases the dipole moment of linear polysiloxanes and rigidifies the conformation, thus favoring visible-light excitation and suppressing nonradiative transitions.By adjusting the distribution of local conjugated chains, our group further designed two linear polysiloxanes by introducing carbonyl (C=O) and vinyl groups (C=C) and proposed a new emission mechanism named "local conjugation enhanced multicolor emission". [62]In LPSi-1, the longer local conjugated chain segment (C=C-Si-O-C=O) is formed by the Si-O bond linked carbonyl and vinyl groups.The shorter local conjugation (Si-O-C=O) is fabricated in LPSi-2 as comparison.Spectra study demonstrated that the fluorescent intensity of LPSi-1 is much higher than that of LPSi-2, and LPSi-1 exhibited obvious polychromatic luminescence from blue, cyan, green to red.DFT further indicated that local conjugation plays a key role in their luminescence behavior.As shown in Figure 5D, the longer local conjugation leads to multiple TSC rings with different sizes in LPSi-1, which facilitated the multicolor emission.While the TSC rings formed in LPSi-2 are uniform with decreased throughspace electronic communications, therefore only weaker blue and cyan emissions were observed.

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 radiative decay.Consequently, functional groups are introduced F I G U R E 5 Schematic illustration of the multiring through-space conjugations (TSCs) (A) and frontier molecular orbital (B) of the linear polysiloxane contains disulfide bond.Reproduced with permission: Copyright 2021, American Chemical Society. [61]Schematic diagram of the possible fluorescence mechanism (C) and TSC (D) of linear polysiloxane contains C=C-Si-O-C=O segment.Reproduced with permission: Copyright 2022, The Royal Society of Chemistry. [62] fabricate luminescent linear polysiloxanes that are consisted of Si-O-Si chain segments.The flexible Si-O-Si chains offer multiple possibilities for the aggregation pattern of functional groups, contributing to the luminescence process.Feng et al. observed solvent polarity-driven selfassembly in polysiloxane-based ionic liquids (PNLs), which are synthesized from PMMS and imidazolium-Br. [63]When  [65] The flexible Si-O-Si chains of polysiloxanes play a crucial role in efficient electron transfer from triphenylamine to 4-nitrophenol in BpaP, causing fluorescence quenching and thereby achieving a 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 to coordinate with rare earth elements.[68][69][70][71][72][73][74] When polysiloxanes were excited, strong luminescence could be observed for efficient energy transfer from polysiloxanes to rare earth ions, leading to sensitized PL.For instance, Feng et al. reported luminescent silicone elastomers prepared by thiol-ene "click" chemistry of Eu 3+ coordinated functionalized polysiloxanes. [74]As shown in Figure 6A, the transparent elastomer with a strong fluorescence emission at 617 nm was obtained via the energy transfer from polysiloxanes to Eu 3+ .The luminescent elastomers exhibited 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 6B, two kinds of thermo-and photo-responsive F I G U R E 6 (A) Synthesis route and photograph of polysiloxane-based Eu 3+ elastomer.Reproduced with permission: Copyright 2014, John Wiley and Sons. [74](B) Chemical structure of DRPSs.Reproduced with permission: Copyright 2017, The Royal Society of Chemistry. [75](C) Schematic illustration of the silicone polymer-based conductive material.Reproduced with permission: Copyright 2019, John Wiley and Sons. [77](D) Chemical structure of poly(hydroxyurethane).Reproduced with permission: Copyright 2017, The Royal Society of Chemistry. [1]near polysiloxanes (DRPSs) with N-isopropyl amides and azobenzene/salicylideneaniline as side groups were designed by Feng et al. [75] The DRPSs exhibit lower critical solution temperature (LCST)-type phase separation, which can be controlled by temperature and UV light in aqueous solution.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.

Polysiloxanes with other structures
The luminescence of nonconventional organosilica 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. [33]In 2000, they synthesized six transparent nanocomposite gels based on poly(ethylene oxide) and poly(propylene oxide) chains with different lengths. [36]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 located 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 wavelengths and smaller clusters emit at shorter wavelength.Carlos et al. prepared an organic-inorganic hybrid by reacting three diamines with 3-isocyanatepropyltriethoxysilane. [34,35]The hybrids exhibit two emission peaks, blue and purplish-blue.In each hybrid, the inorganic coherent domains were combined with different proportions of siliconbased structures.They believe the emission band is associated 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 originated in the -NH 2 groups with electron-hole recombinations occurring in the siloxane nanoclusters. [37]he 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≡(CO 2 ).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 hybrid has a high QY of 19.2%, combined with the ability to tune the emission.By fabricating highly organized bilayer monoamidosil consisting 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. [38]oreover, the polysiloxanes have abundant noncovalent interactions; thus, polysiloxane hydrogels can be formed.Imae et al. reported the size-controllable nano-and microhydrogels consist of third-generation triethoxysilyl focal poly(amido amine) dendrons with hexyl spacer. [76]It formed a fiber-like texture at the 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 6C). [77]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.
Uchino et al. synthesized a series of n-octadecylsiloxanes containing end silicate groups, that is, R-SiO 3/2 , R-(CH 3 )SiO 2/2 , R-(CH 3 ) 2 SiO 1/2 (R = C 18 H 37 ). [39]Compared with the analogues with the same aliphatic chains, R-SiO 3/2 shows the highest emission intensity with a QY of 19 ± 0.5%.The silicon/oxygen-related defect species, instead of carbonrelated 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) derivativesbased AIE materials are another branch of polysiloxanes.Chang et al. reported blue PL of star poly(Nisopropylacrylamide)-b-polyhedral oligomeric silsesquioxane (PNIPAm-b-POSS) copolymer in water above LCST. [78]he 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, which lack common fluorescent units, through free radical polymerization or hydrazinolysis. [79]FTIR spectra confirmed the existence of dipole-dipole interactions and hydrogen bonding between the C=O groups and OH groups.The PL is caused by the crystallinity of poly(MIPOSS) and the clustering of locked C=O groups of POSS units.

Mechanism of nonconventional AIE polysiloxanes
The PL mechanism of nonconventional polysiloxanes can be well rationalized by the CTE mechanism with several unique features generated by Si atoms.With the empty 3d orbitals of the Si atom, coordination bonds can be formed with electron-rich atoms.For instance, N → Si, O → Si, and S → Si coordination bonds have been found in nonconventional polysiloxanes. [44,45,49,63]These coordination bonds are beneficial to the fluorescence of the polysiloxanes in two ways.On the one hand, they enhance the molecular aggregation, favoring the formation of clustered chromophores which show strong PL.On the other hand, they can generate d-d orbital splitting in Si atoms.The electrons in split d orbitals can absorb UV energy and generate radiative decay, leading to enhanced PL. [63] Moreover, compared with polymers that employed carbon backbones, the Si-containing backbones in nonconventional polysiloxanes exhibit superior flexibility, which further enhances the PL by promoting molecular aggregation. [45,50,54]With the abundant electron-rich atoms, not only hydrogen bonds, the interactions of O and N atoms and coordination bonds can be formed in aggregates of nonconventional polysiloxanes.These inter/intramolecular interactions drive the aggregation of electron-rich atoms, making them clustered in close proximity.The electron clouds of these electron-rich atoms overlap with each other, generating "clustered chromophores" via TSC.With enriched energy levels, the energy gaps between HOMO and LUMO are reduced, thus benefiting excitation.The through-space interactions are also conducive to rigidifying the molecular conformation of the nonconventional polysiloxanes, thereby favoring radiative decay.

APPLICATIONS
Organic AIE luminophores have been extensively used in OLEDs, sensing, and imaging due to their unique AIE features.Benefiting from the absence of large π conjugates and silicon-promoted luminescence, nonconventional polysiloxanes have shown promising applications in encryption, bioimaging, sensing, visualized drug delivery and controlled release owing to their water-solubility, biocompatibility and environmental friendliness.

Sensing applications
The good water-solubility and low toxicity of nonconventional polysiloxanes make them potential fluorescent probes in molecular biology, analytical chemistry, environmental monitoring and clinical diagnosis.The TSC, which is generated from the aggregation of electron-rich atoms, can be disturbed by electron-deficient compounds and exhibit stimulus responsiveness.For example, P1, which contains conjugated O=C-O-Si-C=C segment, is sensitive to Fe 3+ . [45]When adding the same amount of Ba 2+ , Na + , Ca 2+ , Hg 2+ , Cd 2+ , Al 3+ , Fe 3+ , Cu 2+ , Zn 2+ , Co 2+ , and Fe 2+ to the P1 solutions, the mixture of Fe 3+ and P1 showed quenched fluorescence, while other mixtures remain fluorescent (Figure 7A).Within a certain range of Fe 3+ concentrations, the fluorescence intensity of the P1 solution decreases along with the increase of Fe 3+ concentrations.A reasonable explanation is the ICT between the aggregated electron-rich atoms and Fe 3+ ions.Due to the chelating process between Fe 3+ and the electronrich atoms, a P1-Fe 3+ complex is formed when mixing Fe 3+ with P1.The oxidizing Fe 3+ can accept electrons and disturb the TSC of P1, resulting in charge transfer quenching in P1.
F I G U R E 7 (A) Fluorescent intensity change of P1 with various metal ions.Insert: linear relationship between ΔI/I 0 and the Fe 3+ .Reproduced with permission: Copyright 2019, American Chemical Society. [45](B) Color and fluorescent intensity change of HBPSi with Co 2+ .Reproduced with permission: Copyright 2022, American Chemical Society. [55](C) Fabrication of BpaP and BpaD sensors and detection for 4-nitrophenol.Reproduced with permission: Copyright 2020, Elsevier. [65]e addition of Na 2 EDTA can cooperate with Fe 3+ and disassembles the P1-Fe 3+ complex, restoring the fluorescence of P1.
Besides local conjugated HBPSi, our group reported that HBPSis contain no conjugates are also sensitive to metal ions, such as Fe 3+ , Cu 2+ and Co 2+ (Figure 7B), and they showed varied fluorescent quenching behavior according to the content and relative position of electron-rich groups. [55,60]he Fe 3+ responses were also observed in HBPSi synthesized from ethyl orthosilicate, [43] diethanolamine, [60] N-methyldiethyl alcoholamine, [44] and diethylene glycol, [57] which may be caused by the chelating process between lonepair electrons and Fe 3+ .The chelating effect between metal ions and electron-rich atoms could also generate obvious color change, as shown in Figure 7B.Thus, the HBPSi can be potential candidates for metal ion sensors.Intriguingly, Feng et al. reported the electron transfer from triphenylamine groups in BpaD and BpaP to 4-nitrophenol and developed simple and visualized paper sensors for 4-nitrophenol based on the 4-nitrophenol-caused fluorescence quenching. [65]By coating BpaD or BpaP on filter paper, paper sensors were obtained.Figure 7C shows the fluorescence quenching of BpaD or BpaP paper by the appearance of 4-nitrophenol.This method provides a portable and visual candidate for 4nitrophenol detection with limits of detection of 0.6 μM for BpaD and 0.23 μM for BpaP and a wide concentration range of 0-50 μM.

Anticounterfeiting and data encryption
Taking advantage of the fluorescent nature of the HBPSi, their derivatives can be used for latent fingerprint imaging and data encryption.Figure 8A illustrates the blue fluorescence imaging of latent fingerprints.The high-quality image with secondary minutiae features suggests the preferential adhesion of the polymer on the fingerprint ridge.Bai et al. used HBPSi "ink" to draw "XUST" and "NO" on a filter paper and only letters written in normal ink could be observed under daylight (Figure 8B).However, the blue colored "XUST" and "NO" were visualized by UV excitation, exhibiting a UV-triggered anticounterfeiting ability. [59]ased on the stimulus responses, polysiloxanes have great potential for data encryption.Our group reported an HBPSi with data encryption ability derived from the Fe 3+ quenching effect. [56]Using the Na 2 EDTA as the ink, HBPSi coated security paper was blue luminescent under UV light without diversity, thus the information was encrypted.The painting of a "key", Fe 3+ solution, can reveal the "AIE 20th" with a blue emission under UV light due to the Na 2 EDTA protected luminescence of HBPSi from Fe 3+ caused quenching.As can be seen in Figure 8D, the HBPSi serves as a convenient tool for data encryption without changing the paper's appearance.

Bioimaging
Compared with traditional AIE luminophores with π conjugations, nonconventional polysiloxanes enjoy unique superiority in bioimaging owing to their water-solubility and good biocompatibility.With the diverse reactive monomer, HBPSis exhibit varied cytotoxicity.For example, the HBPSi synthesized from diethylene glycol and 3-aminopropyltriethoxysilane is less toxic than that synthesized from 3-aminopropyltriethoxysilane and Nmethyldiethanolamine. [44,57]Besides, the cytotoxicity of Photographs of a latent fingerprint under 365-nm UV light.(B) Demonstration of the data encryption using HBPSi derivatives.Reproduced with permission: Copyright 2022, American Chemical Society. [59](C) Schematic representation of fluorescence quenching and recovery of HBPSi toward Fe 3+ .(D) Schematic illustration of the encryption and decryption utilizing the quenching effect of Fe 3+ .Reproduced with permission: Copyright 2020, The Royal Society of Chemistry. [56]PSis is proportional to their concentrations due to the positively charged amino groups induced cell death.The modification of L-glutamic acid or β-CD can reduce the cytotoxicity of HBPSi and the toxicity of HBPSi derivatives is related to the amount of L-glutamic acid or β-CD.For example, when the concentration of HBPSi-CD reaches 5 mg/mL, more than 60% cells remain alive.After incubation of HBPSi-GA with mouse osteoblast cells for 16 h at 37 • C, the cells exhibit blue luminescence.HBPSis and their derivatives can light up cells and demonstrate potent application in bioimaging.Lin et al. fabricated a polysiloxane-based fluorescent Schiff base (P1) for the imaging of the ferroptosis process. [64]he polysiloxane main chain emitted blue fluorescence with the rhodamine B in a "turn off" state.In the presence of Fe 3+ , the fluorescence of the polysiloxane main chain is quenched with the rhodamine B switched to a "turn on" state.The P1 selectively penetrated apoptosis HeLa cells and in situ monitored the Fe 3+ -induced apoptosis via switched fluorescence emission.

Visualized drug delivery and controlled release
Due to the excellent water-solubility, biocompatibility, and environmental friendliness of nonconventional polysiloxanes, they have gained increasing attention in biological applications, such as drug delivery and controlled release.Our group fabricated cell-targeting HBPSi-Apt by covalently linking the AS1411 aptamer with HBPSi and used it as a visual tracker for on-demand cell-targeting and intracellular drug release. [60]The AS1411 aptamer functionalized HBPSi selectively bound with nucleolin-overexpressed cancer cells than normal cells, confirming its on-demand drug delivery capability.When loaded with fluorescent DOX, intracellular drug accumulation and release were observed.As can be seen in Figure 9A, the HBPSi-Apt@DOX incubated cells display uniformly distributed blue fluorescence in the cell cytoplasm and time-dependent red fluorescence in the cell nucleus, which is consist with the drug release process.With the numerous amine groups in HBPSi and the disulfide bond in HBPSi-Apt, HBPSi-Apt@DOX showed pH and GSH dualresponsive drug release behavior (Figure 9B).The acidic and alkaline conditions affected the charge of HBPSi-Apt@DOX and formed different electrostatic interactions, thus accelerating the DOX release at pH 5.0.The presence of GSH cleaved the disulfide bond in HBPSi-Apt and exposed the HBPSi core, leading to an enhanced DOX release rate.
The terminal modification gives HBPSi controlled drug release capability.The hydrophobic cavity inside β-CD is an excellent hydrophobic drug carrier, thus the modification of β-CD improved the drug loading capacity of HBPSi from 80.0 mg/g to 160.0 mg/mg with a pH-responsive release behavior. [44]The connection with hydrophobic oleic acid also increases the ibuprofen loading capacity of HBPSi, which reaches 470 mg/g due to the hydrophobic core in the self-assembly structure. [58]The release rate of ibuprofen obviously accelerated in solution with a pH of 5.5 or 6.4 than that in solution at pH 7.4 (Figure 9D).The pH-controlled drug release behavior is caused by the electrostatic interactions between HBPSi derivatives and ibuprofen. [44]In an acidic buffer, both HBPSi derivatives and ibuprofen are positively charged, thus the charge repulsion promotes the drug release.On the contrary, the surface charge of ibuprofen changes to negative at pH 7.4, and the charge attraction suppresses its release.
The abundant terminal groups also make HBPSi an excellent candidate for responsive prodrug.With the successive linking with 3,3′-dithiodipropionic acid anhydride and 10hydroxycamptothecin, our group prepared a HBPSi prodrug (HBPSi-SS-HCPT) that displayed pH and redox dualresponsive drug release behavior. [50]The ester bond endows the HBPSi prodrug with pH responsiveness, and the disulfide F I G U R E 9 (A) Confocal laser scanning microscopy images of intracellular tracking of HBPSi-Apt@DOX.(B) pH and GSH dual-responsive DOX release of HBPSi-Apt@DOX.Reproduced with permission: Copyright 2023, American Chemical Society. [60]Schematic representation of ibuprofen loading (C) and the pH-controlled release (D) process of HBPSi-OA.Reproduced with permission: Copyright 2021, John Wiley and Sons. [58](E) Schematic illustration of the in vivo drug release behavior of HBPSi-Apt@DOX.(F) In vivo fluorescence imaging of tumor-bearing mouse after injection of HBPSi-Apt@DOX.Histological analysis of tumor tissues (G) and flow cytometric dot plots (H) of tumor cells harvested from control and experimental groups.Reproduced with permission: Copyright 2022, Elsevier. [50]nd in 3,3′-dithiodipropionic acid anhydride results in redox responsiveness.The pH and GSH work synergistically, thus making HBPSi prodrug stable under physiological conditions, and bursting drug release at intratumoral pH and GSH.In vivo antitumor experiments demonstrated efficient antitumor efficacy with a total tumor inhibition rate of 27% within 15 days (Figure 9G,H).Moreover, the fluorescent nature of HBPSi also enables visualized drug delivery and release in vivo (Figure 9F).

OLEDs
Generally, nonconventional luminescent materials possess relatively low QY due to the absence of large conjugations.Therefore, they are considered inapplicable in OLEDs, which require high PL performance.In recent years, nonconventional luminophores come into sight in OLEDs.Cheng et al. fabricated a three-layered OLED device with improved fluorescence purity and QY from 3,6-dipyrenylcarbazole-POSS hybrids (POSS-DPCz). [80]The POSS nanoparticles serve as a compatibilizer to facilitate the formation of 3D structures, suppress the aggregation and enhanced the color stability of the chromophore.Specifically, the three-layered OLED device exhibits a luminous efficiency of 1.4 cd/A with a maximum brightness of 8900 cd/m 2 at 450 nm.Zhang et al. designed a white LED based on a linear siloxane containing poly(hydroxyurethane).The flexible Si-O-Si segment benefits the aggregation of hydroxyurethane chromophores, leading to a high luminance of 8222 cd/m 2 . [1]The above OLED devices facilitate the realization of polysiloxane lighting devices with enhanced luminescence efficiency.

CONCLUSION
In The outstanding photophysical properties of polysiloxanes have attracted considerable attention in recent years, and remarkable progress has been made in variety enrichment, mechanism exploration, and application extension.However, nonconventional polysiloxanes with AIE behaviors still have several drawbacks, such as low QY, short fluorescent lifetime and indistinct PL mechanism.Therefore, the further developments of polysiloxanes may include several aspects.
(1) Developing polysiloxanes with good water-solubility, high QY and infrared luminescence plays an important role in expanding their applications.(2) Revealing the emission mechanism of polysiloxanes is critical to the design of luminescent nonconventional materials.So far, the emission mechanism of polysiloxanes is still under debate.Some believe that their luminescence comes from the electron rearrangement in the split 3d orbitals of Si atoms by N → Si and O → Si coordination bonds.While others suggest that the TSC generated from Si-O bonds and other auxochromic groups leads to the PL emissions.Emphasizing the role of Si atoms in the CTE mechanism may provide a new way to understand their emission behaviors.(3) To control the topologies of polysiloxanes precisely, exploring new synthesis methods is of central importance.Besides the frequently-used methods, such as hydrosilylation, hydrolysis polycondensation, and transesterification polycondensation reaction, atom transfer radical polymerization and proton transfer polymerization can also help to obtain nonconventional polysiloxanes.We believe the understanding of polysiloxanes would benefit in unveiling the emission mechanism of nonconventional luminophores and expanding the application of nonconventional luminescent materials.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interests.

R E F E R E N C E S
(C) Bond angle of Si-O-C, Si-O-Si, C-O-C, and C-C-C chain segments.(D) Intramolecular and intermolecular hydrogen bonds, and O⋅⋅⋅O and O⋅⋅⋅N interactions in HBPSi aggregates that contain 1,6-hexanediol chains.
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 orbitals in Si atoms.The electrons rearranged in split orbitals and caused d-d transitions, thus making PMMS fluorescent.Zhang et al. synthesized poly(hydroxyurethane) (Figure6D) 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 The flexible Si-O-Si chains also 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.[64]The imine-linked polysiloxanes exhibit blue fluorescence owing to the N → Si coordination bonds.The addition of Fe 3+ could switch the fluorescence emission of the probe from blue to red by destroying the N → Si bonds and regenerating rhodamine-B in an open-cyclic state.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.
summary, this review summarizes recent progress in nonconventional luminescent polysiloxanes without classic conjugated structures.The polysiloxanes generally consisted of Si-O-Si or Si-O-C chain segments with other electronrich auxochromic groups, such as hydroxyl, amino, carbonyl, ether, etc.The unexpected luminescence behavior is related to the aggregation caused by the flexible Si-O bonds and the electron delocalization generated by electron-rich atoms.With the decreased molecular distance in the polysiloxane aggregates, the electron cloud of electron-rich groups overlaps with each other and generates electron delocalization, lowering the excitation energy and rigidifying the molecular conformation, thus favoring the PL process.The empty 3d orbitals of Si atoms can generate coordination bonds like N → Si and O → Si in polysiloxanes, altering the electron delocalization of the material and resulting in radiative emissions.For HBPSis that contain Si-O-C chain segment, their unique bond angle renders HBPSis good flexibility and rigidity simultaneously, leading to enhanced luminescence, such as enhanced QY and multicolor fluorescence.
This work was financially supported by the Guangdong Basic and Applied Basic Research Foundation (program number: 2020A1515110540), the Key Research and Development Program of Shaanxi (program number: 2022SF-599), the National Natural Science Foundation of China (program number: 22175143), and Fundamental Research Funds for the Central Universities (program number: D5000230114).