Handbook of Aggregation-Induced Emission, Volume 2. Группа авторов
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Название: Handbook of Aggregation-Induced Emission, Volume 2
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119642961
isbn:
Figure 3.29 Schematic diagram of the molecular structure of 70 and its mechanical/thermal stimulus response.
Source: Adapted with permission from Ref. [76] (Copyright 2011 American Chemical Society).
Laskar's group reported a reversible piezochromic molecules 71 (Figure 3.30a) in 2016 [31]. The molecular arrangement of 71 is J‐shaped in a solid or aggregate state, which reduces p–p interaction and promotes the ESIPT process with a strong yellow fluorescence emitting. After grinding 71 solid, the intermolecular interaction weakens, resulting in a reversible blue‐shift of fluorescence from yellow to green, and returns to the state before grinding after a period of time (Figure 3.30b, c). Another type of mechanoresponsive molecule 72 (Figure 3.30d) was also reported by this group in 2017 that undergoes fluorescent discoloration under different external stimuli including shear force (grinding), axial pressure (hydraulic press), and temperature [21]. When a shearing force and an axial force are applied to 72, fluorescent color changes from blue to green. Particularly, the process is irreversible under the action of shearing force, while the molecule can slowly return to the initial state after axial force is applied. When 72 is under low temperature in liquid nitrogen, the fluorescence color becomes blue‐green and the fluorescence gradually returns to blue after replacing at ambient conditions (Figure 3.30e, f). Crystal structure analysis reveals that every two molecules form an antiparallel molecular pair via hydrogen bonding; the adjacent molecular pairs are then connected to form a chain, and the adjacent chains are then laterally connected to form a sheet structure. By comparing with the crystal data in liquid nitrogen, the molecular pair structure retained at low temperature, and there existed four kinds of intermolecular interactions when the molecular pair linked into a chain, accompanied by the electron distribution variation, which resulted in fluorescent color change (Figure 3.30g).
Figure 3.30 (a) Molecular structure of 71. (b) Normalized emission spectra of 71 before and after grinding. (c) Switching of emission wavelength (∼553–530 nm and vice versa) after and before grinding, respectively.
Source: Reprinted from Ref. [31] (Copyright 2016 Elsevier B.V.).
(d) Molecular structure of 72. (e) Luminescence images of 72 under various conditions (λex = 365 nm). (f) Luminescence images of the as‐synthesized and ground samples of 72 (photographs taken under a 365‐nm UV illumination). (g) (a and c) Packing diagrams of 72 at room temperature, and (b and d) packing diagrams of 72 at liquid N2 temperature (interactions shown in the figure are in Å).
Source: Reprinted from Ref. [21] (Copyright 2017 Royal Society of Chemistry).
3.3.2 Nanoparticles
SSB derivatives are also designed to form NPs, which will allow the fluorescent material much more stable with multifunctionalities applicable for material science, bioimaging, and PDT. Two common design principles for SSB NPs are generally used. One way is growing a transparent shell such as silica out of the SSB fluorogens' aggregate core. As Figure 3.31a and b shows, SSB fluorescent aggregates encapsulated in silica nanoparticles (SiNPs) were reported according to the Stober standard method [77]. Three SSB‐based AIEgens 73, 74, and photoresponsive compound 75 were noncovalently embedded into SiNPs during the polymerization of silicate ester monomers to prepare AIE luminogen‐embedded fluorescent SiNPs (AIE‐FSNPs‐1–3). Compared with the conventional ACQ fluorophore fluorescein embedded in SiNPs prepared in the same way, AIE‐FSNPs exhibit an ~10‐fold fluorescent intensity; therefore, they show much higher sensitivity in further analytical application. Additionally, AIE‐FSNPs display satisfactory stability to external environmental variations. After experiencing multiple washings or under different pH buffers, the fluorescent spectra of AIE‐FSNPs show no obvious change. By covalently modifying AIE‐FSNPs with DNA aptamer AS1411, Apt‐AIE‐FSNPs were prepared and showed specific binding to nucleolin overexpressed on the surface of various cancer cells (MCF‐7, HeLa, etc.), thereby distinguishing cancer cells from normal cells in cell imaging. As shown in Figure 3.31c, Apt‐AIE‐FSNPs‐1 emitted a strong green fluorescence under a 405‐nm laser excitation after incubating with human breast cancer cells MCF‐7 but exhibited no obvious fluorescent signal with normal cells MCF‐10A, indicating their perfect performance in specific cancer cell recognition. More interestingly, as shown in Figure 3.31d, since the fluorescence of 75 was caged by photolabile group o‐nitrobenzyl, 75‐encapsulated Apt‐AIE‐FSNPs‐3 initially emitted no fluorescence incubated with MCF‐7. After irradiation with a 365‐nm UV light, the o‐nitrobenzyl group was left to recover the orange fluorescent signal. Such photoactivatable characteristics give Apt‐AIE‐FSNPs‐3 unique advantages in the selective imaging of target cells at a specific location of interest by controlling the site of UV irradiation at the desired time.
Another way to produce SSB‐based AIE NPs is to initiate self‐assembly of the monomer. By means of designing SSB fluorophores into the main chain of a polymer [78] or mixing with other small molecules [57], SSB dyes self‐assembled into substable NPs with fascinating functions in imaging or cancer therapy. As Figure 3.32a illustrates, Tang's group reported a dual‐organelle‐targeted NPs with synergistic chemo‐PDT functions [57]. Via self‐assembly of AlPcSNa4 and AIE‐Mito‐TPP(39) through electrostatic, hydrophobic, and π–π interactions, the formed AIE‐Mito‐TPP/AlPcSNa4 NPs almost did not show fluorescence due to the fluorescence resonance energy transfer (FRET) process between 39 and AlPcSNa4 and the self‐quenching of π‐planar AlPcSNa4 in the aggregation state. After uptake by cancer cells through endocytosis, NPs decompose rapidly in acid lysosomes, which releases 39 and AlPcSNa4 in cytoplasm and subsequently light up mitochondria and lysosome in green and red fluorescence, respectively (СКАЧАТЬ