Читать книгу Handbook of Aggregation-Induced Emission, Volume 2 - Группа авторов - Страница 47

Organic solid fluorescent materials apply widely in organic light‐emitting diodes (OLEDs), photovoltaic devices, organic semiconductor lasers, fluorescent sensors, data storage, security printing, and anticounterfeit materials. Most conventional fluorescent molecules undergo fluorescence quenching in their aggregated state, and improvement of emission quantum yield and brightness are limited when designing for solid fluorescent materials. In contrast, the fluorescence enhancement of AIE molecules in the aggregated state has promoted their development in the field of solid fluorescent materials.

SSB molecules exhibit the characteristics of ESIPT. On the one hand, their large Stokes shift weakens the self‐quenching effect and results in high quantum yields [68]. On the other hand, the ESIPT process can occur rapidly even at low temperature [69], which shows powerful advantages of these SSB molecules as solid fluorescent materials. Furthermore, SSB molecules usually show dual‐color emission and the fluorescence is susceptible to the foreign stimuli factors such as light, heat, mechanical forces, and organic vapor fumigation due to the variation of stacking mode and molecular arrangement in the solid sates, so it has great potential as stimulus‐responsive fluorescence sensing materials.

Figure 3.25 (a) Chemical structures of 46 and 47, and a schematic illustration of 46 for intracellular LD staining in healthy cell and 47 for the detection and membrane staining in apoptotic cells. (b) Confocal images of HeLa cells stained with 20 μM 46 and costaining with 0.5 μM Nile Red. c Confocal images of early‐stage apoptotic HeLa cells induced by 1 μM staurosporine for two hours, followed by incubation with 20 μM 47 for 30 minutes at 37 °C and stained with propidium iodide.

Source: Panels (a–c) are adapted with permission from Ref. [63] (Copyright 2015 American Chemical Society).

(d) Schematic illustration of 47 for selective targeting, imaging, and killing of bacteria over mammalian cells. (e) CLSM images of cells and bacteria incubated with 20 μM 47.

Source: Adapted with permission from Ref. [64] (Copyright 2015 John Wiley and Sons).

A series of p‐carboxyl‐N‐salicylideneaniline derivatives with different solid morphologies and emission colors were reported in 2011 by Tong's group [70] (Figure 3.26a). With different substituents of salicylaldehyde, the fluorescence of these derivatives in their crystalline states shows green to orange emission (λ_em = 518 nm/556 nm). X‐ray crystal structure analysis reveals one‐dimensional microrods obtained with carboxyl substitution on the para‐position of aniline due to the promoted formation of intermolecular hydrogen bonds compared with chlorine substitution (Figure 3.26b–e). The microrods also show good optical waveguide property owing to the orderly arrangement and transparency. No matter where the location site of excitation is, the transmission of the excited fluorescence to both ends in a one‐dimensional direction was observed (Figure 3.26f).

Yang's group reported a class of molecules 54 with AIE and ESIPT properties in 2012 [71]. The fluorescence intensity of 54 in water or powder is significantly enhanced compared to THF solution (Figure 3.26g). X‐ray crystal structure analysis showed that 54 exhibited a J‐type aggregate, and the N⋯π interaction of the N atom in the thiophene ring with the adjacent thiophene ring of another molecule stabilizes this aggregation form (Figure 3.26h). In addition, the cis–trans tautomerism of the keto structure is hindered when the molecules are closely packed. Based on these properties, 54 was used as a light‐emitting layer to form a simple three‐layer nondoped OLED device with higher color purity and lower efficiency roll‐off.

Photochromic or photoactivatable molecules, which achieve color or fluorescence switching under specific light radiation, have great application potential in the fields of molecular switches, molecular logic gates, photocontrollable materials, anticounterfeiting, and photolithographic patterns. A series of wavelength‐selective photoactivatable multicolored SSB molecules are shown in Figure 3.27 [72]. Under the irradiation of certain UV light, the substituted quenching group on the hydroxyl of SSB is removed, and the ESIPT and AIE of the molecule are restored (Figure 3.27a). Different caging groups endow the selectivity of activation wavelength for the SSB fluorophores, and the different substituents on the benzene ring structure can adjust the fluorescence color (Figure 3.27b). In particular, when the caging group itself is a fluorescence emissive 7‐methoxycoumarin, the molecule shows a blue fluorescence of coumarin before activation and emits a mixed color of coumarin and SSB fluorophore after light activation. Photocontrolled fluorochromism is thereby achieved. These photoactivatable SSB had also been successfully applied to photolithographic patterns (Figure 3.27c).

Figure 3.26 (a) Molecular structures of 48–53. (b–d) SEM images of 48–50 1D microrods generated by vacuum evaporating from ethyl acetate solution of corresponding compounds, respectively. Insertions were their fluorescence microscopy images. (e) Photograph of crystals for 51–53 with centimeter size under a 365‐nm ultraviolet (UV) light. (f) Fluorescence microscopy images obtained by exciting identical microrods at three different positions without artificial staining.

Source: Reprinted from Ref. [70] (Copyright 2011 Elsevier B.V.).

(g) Photographs of 54 under UV illumination at 365 nm in THF solution, water, and powder from left to right, respectively. Insertion was the molecular structure of 54. (h) N⋯π interaction of 54.

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 group reported an SSB 70 in 2011 (Figure 3.29) [76]. When the crystal of 70 was annealed by heating to 115 °C or by grinding, the solid fluorescence color changed from yellow to green with significant emission enhancement. In the structure of 70, a local dipole presents with N,N‐diethylamino as the electron donor (D) and salicylaldehyde Schiff base structure as the electron acceptor (A). Dipole coupling of molecules between adjacent sheets stabilizes the crystal structure; the p–p interaction enhances the delocalization of excited state, revealed by single‐crystal X‐ray structural analysis, PXRD, and DSC. When the crystal is annealed or ground, the molecular arrangement changes and p–p interaction is therefore weakened resulting in the blue‐shift and enhancement of emission. Planarized fluorophores and push–pull electron groups play significant roles in the construction of solid materials with luminescence conversion properties.

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 (λ_ex = 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).