enantiomers R‐43 and S‐43 and corresponding glum [33].
2.5 Supramolecular Systems
Recently, supramolecular self‐assembly of chiral AIEgens guided by relatively weak interactions is emerging as an efficient way to prepare CPL‐active materials. In 2012, Liu et al. introduced two mannose‐containing side chains into a tetraphenylsilole derivative and prepared a novel chiral AIEgen 44 (Figure 2.13) [34]. Further experiments demonstrated that the CPL performance of 44 was highly dependent on the self‐assembly conditions. Without a control over the self‐assembly morphology, no obvious CPL signals could be observed. On the contrary, the highly ordered self‐assembly structure formed in the microfluidic channels generated strong CPL with glum up to −0.32 (451 nm). This work pioneered the design of AICPL materials via supramolecular self‐assembly.
Figure 2.13 Molecular structures of chiral silole‐based AIEgens 44–47 and corresponding glum [34–37].
Coming down in one continuous line, several chiral AIEgens 45–47 based on silole derivatives were reported by Tang’s group (Figure 2.13) [35–37]. Compound 45 was comprised of a tetraphenylsilole luminescent core and several chiral phenylethanamine pendants and showed a typical AIE feature [35]. It was CD and CPL‐silent in solution or in a film probably due to the low efficiency of chirality transcription from the chiral side chains to the tetraphenylsilole core. However, after being mixed with chiral acids, such as R‐ or S‐mandelic acid, 45 revealed intense CPL signals centered around 500 nm with high |glum| of 0.01. This was attributed to the formation of ordered supramolecular structures. Compounds 46 and 47 were synthesized by combining an AIE‐active silole unit and various chiral pendants (valine‐ or leucine‐containing side chains) via click chemistry reactions. Compound 46 self‐assembled into helical structures after drying from a THF solution or from the THF/H2O mixtures and exhibited strong CPL with high glum of −5.0 × 10−2 (500 nm) [36]. For compound 47, similar helical self‐assembles were observed after drying from a CH2Cl2 solution and the micropatterned structure formed in the microfluidic channels showed CPL with glum of −1.6 × 10−2 (416 nm) [37]. In 2017, Tang’s group found that the unmodified HPS exhibited CPL with glum as high as −1.25 × 10−2 (550 nm) in a crystalline film due to the formation of well‐ordered helical self‐assembly [38].
As another commonly used AIEgen, TPE was also used to design novel chiral AIEgens. Monofunctionalized TPE‐based chiral AIEgens 48 and 49 and difunctionalized chiral AIEgens 50 and 51 have been reported by Tang’s group since 2014 (Figure 2.14) [39–42]. The TPE units were modified with valine‐ or leucine‐derived side chains via click chemistry reactions. With the help of the chiral side chains, compounds 48–51 formed helical aggregates. On the other hand, the AIE‐active TPE units endowed these chiral self‐assembles with strong blue luminescence as well as intense CPL. For CPL performance, the monofunctionalized chiral AIEgens exhibited CPL with high glum of +3 × 10−2 (445 nm) and +5 × 10−2 (450 nm) for 48 and 49, respectively. However, the difunctionalized TPE‐based chiral AIEgens 50 and 51 exhibited CPL with relatively lower |glum| of 3.2–5.3 × 10−3 (430–440 nm).
Figure 2.14 Molecular structures of chiral TPE‐based AIEgens 48–51 and corresponding glum [39–42].
In 2019, Zhang and Cheng et al. prepared four chiral AIEgens 52–54 combining a TPE unit and one or two chiral glutamic acid‐derived side chains (Figure 2.15) [43]. The enantiomers of monosubstituted molecule 52 exhibited CPL with glum up to ±2.0 × 10−2 (450 nm). As for disubstituted molecules, CPL signals with glum of −7.0 × 10−3 (500 nm) and −8.0 × 10−3 (500 nm) were observed for 53 and 54, respectively.
Figure 2.15 Molecular structures of chiral TPE‐based AIEgens 52–54 and corresponding glum [43].
In 2018, Zheng’s group reported a TPE‐based triangular macrocycle 55, which was decorated with three crown ether rings (Figure 2.16) [44]. The macrocycle was achiral itself, but exhibited CD and CPL signals after the addition of chiral acids and the |glum| (520 nm) was between 1.0 × 10−3 and 2.1 × 10−3. According to the proposed mechanism, the host–guest interaction between the crown ether rings and the chiral acids may render a single chirality of the TPE unit prevailing inside a macrocycle and hence lead to CD and CPL activities. Later in 2019, the same group prepared macrocycles 56 and 57 by connecting TPE dicycle or TPE unit with four chiral cholesterol groups [45]. CPL spectra showed that the dual cycle structure played an important role in single chirality induction. Thus, macrocycle 56 with such a structure showed higher CPL activity (|glum| = 3.0 × 10−3 at 450 nm) than macrocycle 57 (|glum| = 1.0 × 10−4 at 475 nm).
Figure 2.16 Molecular structures of triangular macrocycle 55, TPE dual cycle tetracholesterol 56 and TPE tetracholesterol 57, and corresponding glum [44, 45].
Besides the silole‐ and TPE‐based molecules, other chiral AIE‐active systems were also investigated in the supramolecular systems. In 2018, Huang et al. synthesized two chiral alanine‐containing Schiff base with AIE activities, which was self‐assembled into a helical structure and exhibited CPL with glum up to +1.3 × 10−2 (650 nm) [46]. Recently, Jiang and coworkers developed a gel system based on a novel chiral AIEgen 58, which was synthesized by connecting pyridine functionalized cyanostilbene with a chiral cholesterol unit through an ester linker (Figure 2.17) [47]. Due to the chirality transfer and amplification during the gelation process, 58 exhibited