[47]. The neutral single crystal can emit bright green fluorescence. However, the symmetrical protonation state crystal obtained by adding H2SO4 shows a red‐shifted orange fluorescence (see BP4VA‐2H in Figure 2.4a), and the fluorescence of the asymmetrical protonation state crystal obtained from adding HCl is further red‐shifted to red (see BP4VA‐1H in Figure 2.4a). Figure 2.4b and c shows the photophysical properties of these 1‐2 molecular crystals. The different protonation states of crystals lead to various supramolecular interactions, different aggregation states, and even tunable optical properties. It provides a mechanism for understanding the role of protonation in organic conjugated molecules and a method for expanding the range of organic functional materials.
Figure 2.4 (a) The symmetrical and asymmetrical protonation states of compound 1‐2 (i.e. BP4VA in the figure) and fluorescence images of BP4VA, BP4VA‐1H, and BP4VA‐2H crystals; normalized (b) photoluminescence spectra and (c) absorption spectra of BP4VA, BP4VA‐1H, and BP4VA‐2H crystals [47].
Source: Reprinted (adapted) with permission from Ref. [47]. Copyright © 2017 American Chemical Society.
2.2.1.5 Multistimuli‐responsive Materials
Multistimuli response refers to the phenomenon that the fluorescence properties of molecules change reversibly under the action of various external stimuli (such as pressure, heat, light, solvent).
The fluorescence of DSA derivative powders 1‐2 changed from green to red after being fumed with HCl, and the fluorescence color could be returned to the initial state after being fumed with triethylamine [27]. Meanwhile, the fluorescence of the initial powders changed to red under pressure, and the fluorescence could also return to the initial state after heating. Compared with DSA derivative 1‐2, DSA derivative 1‐14 has two more benzene ring substituents, and it also has multistimuli response properties. The two polymorphs (G‐phase and O‐phase) based on compound 1‐14 were obtained by using a solvent evaporation method. They exhibit remarkable luminescence switching under the multistimuli such as grinding force, hydrostatic pressure, and acid and alkali treatments. The G‐phase with J‐type aggregation shows a green emission, while the O‐phase with H‐type aggregation presents an orange emission. The aggregation states of the two polymorphs changed under the grinding, resulting in red‐shifts of luminescence. Upon further heating the ground samples, both of them partly recovered their initial emissive colors. Furthermore, a structural transition from the O‐phase to G‐phase took place during grinding and subsequent heating processes. The O‐phase showed a more obvious red‐shift than the G‐phase. The molecular geometry of the O‐phase tended toward a more planar conformation than that of the G‐phase under the same hydrostatic pressure. It showed that the O‐phase has a higher sensitivity to the hydrostatic pressure than the G‐phase, which resulted from the different intermolecular interactions inside the two crystalline phases. The protonation–deprotonation of the two polymorphs showed that fumigation with HCl/diethylamine (DEA) vapor leads to the destruction and reconstruction of the noncovalent C–H⋯N bonds, resulting in more distinct luminescence switching in the G‐phase than in the O‐phase [48].
Two DSA derivatives, 1‐15 and 1‐16, have been synthesized and investigated. Both of them have AEE properties. 1‐15 with heteroatom N exhibits remarkable solvatochromism, reversible chromism properties, and self‐assembly effects. When increasing the solvent polarities, the green solution of 1‐15 turns orange with the fluorescence emission wavelength red‐shifting from 527 to 565 nm. Notably, 1‐15 shows reversible mechanochromism and thermochromism properties. The as‐prepared powders of 1‐15 emit a green fluorescence (λ = 525 nm) and the color can change to orange (λ = 573 nm) after grinding; further, the orange color can return to green color at high temperature. Based on these reversible chromism properties of 1‐15, a simple and convenient erasable board was designed. However, different from 1‐15, 1‐16 without heteroatom N has no obvious chromic processes. The investigation results obtained from X‐ray diffraction, differential scanning calorimetry, single‐crystal analysis, and theoretical calculations confirmed that the chromic processes depend on the heteroatoms N in 1‐15 [49].
DSA derivative 1‐17 containing alkyl chains of different lengths also has different degrees of response to various external stimuli [31]. When n ≥ 10, these compounds showed obvious piezofluorochromic properties. The crystal structure analysis found that supramolecular interactions have a great influence on its piezofluorochromic properties. At the same time, these compounds responded significantly to solvent vapor and temperature.
2.2.2 High Solid‐state Luminescent Materials
Organic luminescent materials with high solid‐state fluorescence quantum efficiency are widely applied in organic solid‐state lasers, organic fluorescence sensors, organic light‐emitting diodes, and other fields. It is of great significance to develop organic luminescent materials with high solid‐state luminescent efficiency. AIE provides a new way to achieve highly efficient solid‐state luminescent materials. Figure 2.5 shows some AIE organic molecules based on DSA that exhibit high efficient solid luminescence.
The crystal structures and photophysical properties of DSA and its three derivatives 2‐1, 2‐2, and 2‐3 (see Figure 2.5) were investigated. Their crystal structures present nonplanar conformations because of the supramolecular interactions, which lead to rigid molecules and relative tight stacking. All the four molecules have a typical AIE property. The investigation results of the relationship between the crystal structures and AIE properties of DSA and the three derivatives show that DSA moiety is the key factor of AIE property, and the AIE properties result from the restricted intramolecular torsion between the 9,10‐anthrylene core and the vinylene moiety [17].
The other three DSA derivatives 2‐4, 2‐5, and 2‐6 were synthesized and characterized. The crystal structures, structure–property relationships, and nanowire fabrication were reported. The investigation results of crystal structures show that the three DSA derivatives represent different molecular packing modes with varying substituents. Particularly, the introduction of a fluorine (F) substituent to generate weak intermolecular C–H⋯F interactions promotes the formation of intermolecular π–π stacking in 2‐5 and 2‐6 crystals. Photophysical studies and crystal structure analysis confirm that the high and blue‐shift fluorescence should result from the inhibition of vibrational relaxation in the aggregate state. Through controlling the experimental conditions, perfectly regular 1D nanowires of 2‐6 could easily be obtained. The weak intermolecular C–H⋯F interaction together with the effective π–π interaction plays a significant role in the nanowire formation of 2‐6. High‐quantum efficiency (75% for 2‐6) and regular 1D nanowires suggest that this kind of materials have potential applications in optoelectronic device [50].
In addition, Sun et al. demonstrated that the introduction of halogen atoms into DSA could influence the molecular packing and molecular geometries