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:
Source: Reprinted from Ref. [71] (Copyright 2012 Royal Society of Chemistry).
The aforementioned photoactivation based on quenching groups is irreversible, while photoredox and photocyclization reactions are often used to design reversible photochromic molecules. However, the fatigue resistance of these photochromic systems is usually not desirable. Hou's group reported a class of reversible photochromic molecules 67 that link SSB and tetraphenylethene (TPE) together with both AIE and ESIPT [73]. An SSB moiety can be converted from enol to keto under the irradiation of UV light to achieve the photochromic process (Figure 3.27d). Molecule 67 remained as yellow solid with an enol form, which had strong fluorescence at 545 nm and did not absorb at 550 nm. After UV light irradiation, 67 turned red solid with a trans‐keto structure, and the fluorescence intensity was significantly reduced while absorption at 550 nm is extremely enhanced. After removing the UV light, 67 gradually returned to the previous state. Elevated temperature or visible light irradiation can promote the conversion rate of trans‐ketone to cis‐ketone, thereby increasing the rate of discoloration (Figure 3.27f, g). Due to the reversibly photocontrollable luminescence of 67, it was applied in photopatterning materials with erasable properties (Figure 3.27e).
Figure 3.27 (a) Chemical structures of compounds 55–66 and the scheme of photouncaging 56–64 to yield 65–67, which are fluorescent at different wavelengths (colors) by UV irradiation at different wavelengths. (b) The multicolor fluorescence enhancement or change upon irradiation at 365 or 300 nm for 56 (green), 57 (yellow), 58 (orange), 61 (light orange), and 63 (from blue to white purple) in their solid and aggregate (colloid solution) states. (c) Stepwise photoactivating a multiple‐color fluorescent image of flowers (blue, orange, and white purple) and leaves (green) made of 56, 58, and 63 as solids by sequential UV irradiations at 365 and 300 nm.
Source: Reprinted from Ref. [72] (Copyright 2015 John Wiley and Sons).
(d) Proposed mechanism for the color change of 67 upon UV irradiation. (e) Generating different patterns on the same film of 67. (f) The thermal fading kinetics of 67 at different temperatures. (g) The dotted lines are the fluorescence intensity of 67 at 545 nm before and after excess UV light irradiation. The scatterplot is the fluorescence intensity of UV‐irradiated 67 at 545 nm exposed in light with different wavelengths for one minute.
Source: Reprinted with permission from Ref. [73] (Copyright 2017 Royal Society of Chemistry).
Upon external stimuli such as heat, force, solvent, temperature, etc., the arrangement of the material molecules can be varied among their polymorphisms. Therefore, the conformation, planarization, and intra/intermolecular interaction of the molecules change, which induces the fluorescence switching. Tong's group reported a class of reversible thermochromic SSB 68 (Figure 3.28a) showing polymorph‐dependent AIE and ESIPT fluorescence [74]. Two fluorescent colors of 68 single crystals 68‐Crys. (YG) and 68‐Crys. (G) were obtained from crystallization in concentrated ethyl acetate solution (Figure 3.28b). The results of X‐ray crystal structure analysis showed that the dihedral angle between the benzene ring and the Schiff base plane in the two crystals was different, which led to the difference in molecular aggregates in the two crystals. In addition, according to differential scanning calorimetry (DSC) and powder X‐ray diffraction (PXRD), it is known that 68‐Crys. (G) undergoes phase transformation to the aggregate of 68‐Crys. (YG) by annealing at 231 °C, and 68‐Crys. (YG) converted to 68‐Crys. (G) by ablation treatment at 236 °C. During multiple annealing/ablation treatments, reversible switching of solid fluorescent color was obtained (Figure 3.28c).
Figure 3.28 (a) Molecular structure of 68. (b) Polymorphic single crystals of 68 (68‐Crys. (G) and 68‐Crys. (YG)) under the illumination at 365 nm. (c) Peak position versus thermal treating cycle.
Source: Reprinted from Ref. [74] (Copyright 2013 American Chemical Society).
(d) Molecular structure of 69. (e) Photographs of polymorphous single crystals of 69 (69(G), 69 (YG), and 69(Y)). (f) Schematic illustration of the relationship between slip‐stacking modes and an emission wavelength of 69. (g) Reversible vapor‐ and thermo‐responsive fluorescence printing and erasing by using 69.
Source: Reprinted from Ref. [75] (Copyright 2015 John Wiley and Sons).
By introducing rotary benzene rings into the symmetric positions of salicylaldehyde azine to increase the conformational flexibility thus achieving thermochromic switch provides a new idea for the design of stimulus‐responsive AIE materials. Based on this design principle, Tong's group modified 68 with long alkyl chains and reported the single crystals of 69 exhibiting three different fluorescence colors (Figure 3.28e), which also showed reversible stimuli‐responsive fluorescence switching (Figure 3.28d) [75]. X‐ray crystal structure analysis shows that the great differences existed in the molecular conformation and arrangement of the three crystals due to the presence of long alkyl chains, especially the small interplanar spacing of 69 (Y) with intermolecular p–p interactions, which resulted in a red‐shift in emission wavelength (Figure 3.28f). In addition, under the solvent fumigation of dichloromethane, the yellow form of 69 (Y) changed to its green form 69 (G) due to the molecular rearrangement from relatively close interplanar spacing and intermolecular p–p interaction therein to “monomer”‐like packing evidenced by DSC, polarized light microscopy, and PXRD. Annealing operations recovered an orange fluorescence from the green form with molecular arrangements similar to “dimers” (Figure 3.28f). Such reversibly stimuli‐responsive characteristics of molecule 69 were further applied to fluorescence printing and erasing in response to organic vapor and thermal stimuli (Figure 3.28g).
Mechanical stimuli include pressure, stress, shear, friction, and pulverization; such stimuli‐responsive solid fluorescent materials have a wide range of applications in pressure sensing, memory devices, and optical recording due to their controllable fluorescent properties. Tong's СКАЧАТЬ