1.1.2 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 enhanced the QY from 4.61% to 7.30%.[35] 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 compact the molecular conformation and suppress the chain rotation. Subsequently, we discovered energy-transfer-induced enhanced emission mechanism in L-glutamic acid modified HBPSis.[44] With the increase of the content of L-glutamic acid, 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 concentrated solution, the emission at 450 nm becomes stronger and that at 380 nm gets weaker. TEM images demonstrate the change of morphology in concentrated solution and DFT predictions reveal that the inter/intramolecular hydrogen bonds in HBPSi-GA is much stronger than that in HBPSi, thus leading to enhanced fluorescence emission. We speculated that the adsorbed energy temporarily stored in free functional groups which emitted fluorescence at 380 nm, and then transfered to the electron delocalizations in relatively larger clusters via FRET, intensifying the fluorescence at 450 nm (Figure 3A and 3B). 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.[44]
In addition, the amphipathic effect is another driving force to regulate the molecular aggregation. By replacing the L-glutamic acid with the renewable oleic acid, the QY of HBPSi-OA reaches 28.57% and exhibits multiemission at 330, 360 and 430 nm.[45] 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 and 3D), and the multiexcitation is caused by the energy transfer between free functional groups, smaller electron delocalizations, and the larger electron delocalizations.
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 influenced the molecular aggregation (Figure 3E), but also have rigidified macrocyclic structures that benefits the radiative decay, thus the QY of HBPSi-CDs increases from 12.24% to 18.72%.[38] 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 conformations are rigidified. Consequently, superior PL properties are achieved.
The length of dihydric alcohols also affects the 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 ofβ -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).[42]HBPSi-NH2 without β -CD terminal modification shows excitation-dependent fluorescence with the 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-NH2, the aggregates of HBPSi-β -CD are much larger than that of HBPSi-NH2, thus generating larger clusters and inhibiting the movement of molecular chains. Moreover, HBPSi-β -CD has much stronger inter/intramolecular hydrogen bonds than HBPSi-NH2, 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.[42]
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 non-conjugated molecules, Bai et al. functionalized HBPSi with aromatic amino acids.[46] 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). Differ 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 non-conjugated D−A structures in HBPSi-Cys. Recently, our group reported HBPSi-Cys with two distinct emission peaks that refers to local excited (LE) and twisted intramolecular charge transfer (TICT) emissions respectively.[47] 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 HOMO and 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 are 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 results in a new non-conjugated hyperbranched polysiloxane with TICT emissions.[47]
The PL behavior is also observed in hyperbranched polysiloxanes consisted 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.[48]They found that ureasils emitted room-temperature luminescence in a cluster-size-dependent manner, and the fluorescence cannot 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.[37] The Si-PAMAM dispalyed strong blue luminescence without 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 is generated by N→Si coordination bonds is responsible for the enhanced fluorescence intensity.
1.2 Structural diversity of linearpolysiloxanes
The hyperbranched structures enables 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 lacks three-dimensional topology, leading to 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.