Читать книгу Handbook of Aggregation-Induced Emission, Volume 2 - Группа авторов - Страница 32

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 in the crystals and endow them with different photophysical properties. Moreover, they also found that depending on the amount and position, F substitution can tune the structures and photophysical properties of DSA effectively [51, 52].

The intrinsic photophysical process of DSA and four derivatives 1‐1, 2‐2, 2‐7, and 2‐8 upon excitation were studied by steady‐state and ultrafast spectroscopy. It was confirmed that the intramolecular rotation around the vinyl moiety plays the vital role in the whole photophysical process besides the electronic properties of the peripheral substituents. In dilute solutions, these molecules have twisted structures in the ground state, which can relax to planar structures within picoseconds. The fluorescence process is dominated by the relaxed excited state, and the quantum yield is affected by the competition between the nonradiative and radiative deactivations. The enhanced fluorescence of the molecules in aggregated states originates from the optically allowed S₁–S₀ transition as well as the suppressed nonradiative deactivation via molecular stacking. The results furnish an in‐depth understanding of the origin of the aggregation‐enhanced emission process [53].

Two DSA derivatives 2‐9 and 2‐10 were designed and synthesized, and their photophysical properties and crystal structures were investigated by Tian et al. In contrast to DSA, the maximum photoluminescent emission peaks of 2‐9 and 2‐10 had a red‐shift both in tetrahydrofuran (THF) solution and in crystal. The two crystals of 2‐9 and 2‐10 emitted strong fluorescence with high efficiencies of Φ_F = 45 and 33%, respectively. The strong fluorescence originated from inapparent π–π interactions between molecules inside the crystals, which is due to the large distance between the two adjacent molecules and the nonoverlapping central anthracene planes. The investigation results of crystal structures of 2‐9 and 2‐10 revealed that the molecules in both of the two crystals adopt the same conformation and orientation, i.e. uniaxially oriented packing. And this arrangement of the molecules originates from the supramolecular interaction of CH/π in the two crystals and additional C–H⋯N interactions in 2‐9 [54].

Figure 2.5 Small molecules and macromolecules of DSA derivatives that exhibit high solid‐state luminescence.

The electronic structures and charge transport properties of DSA and three DSA derivatives 2‐2, 2‐9, and 2‐10 were investigated by density functional theory. The results indicate that DSA and the derivatives have high charge mobility and high solid‐state fluorescent efficiency. The introduction of one electron‐withdrawing group, cyano group, to DSA decreases the reorganization energy of holes, which is conducive to the high hole mobility of 2‐9. The hole mobility of 2‐9 is of the same order of magnitude as that of DSA. However, the electron mobility of 2‐9 is about 30 times lower than that of DSA owing to its larger reorganization energy and disadvantageous transfer integral of electrons. As to the electron‐donating substituted molecules, 2‐2 and 2‐10, they exhibited lower charge mobility compared to DSA because of the steric hindrance of the substituents. However, both of them tend to exhibit balanced transport properties, which primarily results from the balanced values of transfer integrals for both hole and electron [55].

DSA derivative 2‐7 has a unique cross‐dipole stacking in the crystalline state owing to the butterfly‐like structure. Then, a significant optical coupling between the ground state and the lowest excited state in the solid state can be obtained, and therefore, the cross‐dipole stacking is highly conducive to the fluorescence properties. As shown in Figure 2.6, the most intriguing structural feature of 2‐7 crystal is the cross‐stacking of molecules in the formed 1D molecular column along the b axis. The wing‐like phenyls are fixed by intermolecular interactions, and each molecule is rotated to its neighbor. Two crystallographically independent conformations of 2‐7 molecules exist in the crystal, and both are highly twisted. The two conformations are alternately packed into the column and rotate relative to the other by an angle of c. 67° around the b axis. In view of the high solid‐state fluorescence efficiency due to the intriguing structure, fascinating optical properties such as amplified spontaneous emission (ASE) with a low threshold and efficient electroluminescence were acquired by using 2‐7 [56].

A methyl‐substituted DSA derivative, 2‐11, was synthesized, and the photophysical properties were investigated by Tian et al. It was found that the tight intermolecular stacking due to the supramolecular interactions in the crystal gave rise to the strong fluorescence emission with a high fluorescence efficiency of 35%. Moreover, the tight intermolecular stacking induced the formation of large size and high‐quality needle‐like single crystal of 2‐11. ASE property with a low threshold value of 10 μJ/pulse was obtained [18]. In addition, the pressure‐induced remarkable luminescence‐changing behavior of DSA, 2‐9 and 2‐11, was also investigated [57].

Figure 2.6 Single crystal of 2‐7 (i.e. 9,10‐bis(2,2‐diphenylvinyl) anthracene [BDPVA] in the figure) under UV light (365 nm), and the perspective view of unit cell structure of 2‐7 along the b axis [56].

Source: Reproduced with permission from Ref. [56].

A uniaxially oriented crystal from 2‐12, which exhibited an excellent waveguide and polarization performance, was prepared by Tian et al. It was found that the low loss coefficient (2.75 cm⁻¹) and the high polarization contrast (0.72) result from the uniaxially oriented packing and layer‐by‐layer molecular structure in the 2‐12 crystal. Furthermore, ASE was observed from the 2‐12 crystal with a low threshold of 265 μJ/cm², and the gain coefficient was 52 cm⁻¹ at a peak wavelength of 509 nm. These properties of the 2‐12 crystal indicate that it has potential application in the field of optical waveguides and organic solid‐state lasers [58].

Tian et al. prepared two polymorphs (the block‐like crystal C1 and needle‐like crystal C2) of a supramolecular cocrystal by self‐assembly with an AEE‐active luminogen 1‐2 as the luminescent host molecule and an aromatic molecule 1,3,5‐trifluoro‐2,4,6‐triiodobenzene (FIB) as the guest molecule. The two polymorphs of C1 and C2 were obtained by slow solvent evaporation at room temperature under the rigorous exclusion of light. The self‐assembly behavior, molecular stacking structures, and photophysical properties of C1 and C2 were investigated. The block‐like crystal C1 packed in segregated stacking with strong π–π interactions between the host and guest molecules. It showed weak green emission with a low efficiency (Φ_F) of 2%. However, the needle‐like crystal C2, packed in segregated stacking with no obviously strong intermolecular interactions, showed a bright yellow emission. In addition, C1 exhibited obvious mechanochromic behavior, and the fluorescence of C1 showed a red‐shift after grinding and recovered to the initial state by itself after 24 hours at room temperature. Specifically, the emission peak of C1 changes from the initial 510 nm to the final 546 nm under grinding, whereas it returns to 515 nm after 24 hours [59].

In addition, Yang et al. introduced the DSA molecule to bridge two pillarenes to form a dimeric host, which can assemble into a linear supramolecular polymer upon cooperatively binding to a neutral guest linker and then achieving a yellow fluorescence emission in solution and solid states [60].

Besides the small molecules with AIE properties exhibiting high solid‐state luminescence, some AIE oligomers and macromolecules also have high solid‐state luminescence. Two DSA oligomers 2‐13 and 2‐14 containing phenothiazine groups were synthesized, and their intramolecular charge transfer, as well as AIE properties, was investigated. The results indicated that both oligomers show typical AIE properties and solvent polarity‐dependent emission. Time‐resolved fluorescence spectra confirmed that the twisted intramolecular charge transfer state formed in polar solvents accounts for the weak emission with large Stokes shifts, and the interactions between solvent and solutes facilitate the nonradiative decay. The restriction of intramolecular torsion induced by supramolecular interactions in aggregates can eliminate the charge transfer state, thus resulting in efficient AIE [61].

Three conjugated oligocarbazoles in 2‐15 (molecules with n = 1, 2, 3 named as Cz2, Cz4, and Cz6, respectively) with a DSA core were synthesized by Tian's group. It was found that the molecules in 2‐15 exhibited the transition from AIE to AEE behavior with increasing the conjugation length. Self‐assembly of the Cz4 molecule gave rise to nanorings with a high fluorescent efficiency (see Figure 2.7) [19].

An effective strategy to achieve large two‐photon action cross sections in the solid state was developed by designing a new series of dendrimers 2‐16 based on DSA and triphenylamine. When the dendrimer molecules aggregate in the mixed solutions or thin films, the intramolecular rotations are restricted and the molecules become more planar due to the stacking forces and intermolecular interactions, which causes an increase in the emission efficiency and a red‐shift in the emission color. These dendrimers presented enhanced fluorescence in the nanoaggregate or solid state, as well as AIE activity and large two‐photon action cross sections. The two‐photon action cross‐sectional values of these dendrimers increase with the generation number increasing [62].

Figure 2.7 Aggregation emission properties and self‐assembly of conjugated oligocarbazoles 2‐15[19].

Source: Reproduced from Ref. [19] with permission from the Royal Society of Chemistry.

From the above investigation results, it can be observed that the specific molecular packing plays important roles in achieving enhanced fluorescence emission for these AIE molecules in the aggregation state. Radhakrishnan et al. quantitatively assessed the relevance of molecular packing in terms of intermolecular energy transfer. They reported quantitative explorations of the correlation between molecular assembly and fluorescence efficiency enhancement [63–67].