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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 g_lum 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 g_lum [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 |g_lum| 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 g_lum 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 g_lum of −1.6 × 10⁻² (416 nm) [37]. In 2017, Tang’s group found that the unmodified HPS exhibited CPL with g_lum 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 g_lum 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 |g_lum| of 3.2–5.3 × 10⁻³ (430–440 nm).

Figure 2.14 Molecular structures of chiral TPE‐based AIEgens 48–51 and corresponding g_lum [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 g_lum up to ±2.0 × 10⁻² (450 nm). As for disubstituted molecules, CPL signals with g_lum 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 g_lum [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 |g_lum| (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 (|g_lum| = 3.0 × 10⁻³ at 450 nm) than macrocycle 57 (|g_lum| = 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 g_lum [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 g_lum 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 intense blue CPL with high g_lum of −3.0 × 10⁻² (480 nm) and −1.7 × 10⁻² (480 nm) for the gel and xerogel film, respectively. More interestingly, reversible luminescence modulation was realized by protonation and deprotonation of the pyridine group. The color was highly tunable between blue and orange (480–530 nm) by varying the exposure of trifluoroacetic acid, without any significant change of g_lum.

Figure 2.17 (a) Molecular structure of chiral AIEgen 58. (b) Scanning electron microscope image of xerogel dried from DMSO. (c) Proposed molecular packing of chiral AIEgen 58 for blue CPL. (d) Multicolor CPL upon exposure of the xerogel films to different amount of TFA.

Source: Reproduced with permission [47]. Copyright 2020. Wiley‐VCH.

In contrast to the above‐mentioned pure organic CPL‐active systems, Ikeda et al. prepared a Pt(II) complex 59 with chiral alkyl side chains, which revealed solvent‐sensitive self‐assembly and CPL activity (Figure 2.18) [48]. Guided by the Pt–Pt, π–π stacking, and dipole–dipole interactions, complex 59 only self‐assembled into a non‐helical structure in chloroform, but formed helical fibers after a slow self‐assembly process (c. 200 minutes) in toluene. Interestingly, the UV–vis, PL, CD, and CPL performance were highly related to the chiral self‐assembly process. For an individual complex, no obvious CD and CPL signals were observed. After self‐assembly, however, 59 exhibited both AIEE and AICPL features in toluene, with high g_lum of ±1.0 × 10⁻² (535 nm).

Figure 2.18 (a) Molecular structures of R‐59 and S‐59 and schematic illustration of their self‐assembly in chloroform and in toluene. (b) CD spectra of R‐59 (dotted line) and S‐59 (solid line) after self‐assembly in toluene at 25 °C. (c) CPL spectra of R‐59 (dashed line) and S‐59 (solid line) after self‐assembly in toluene at 25 °C.

Source: Reproduced with permission [48]. Copyright 2015, The Royal Society of Chemistry.

Recently, natural and/or artificial chiral templates are used in the fabrication of AICPL materials. In 2019, Ding and coworkers utilized the co‐assembly of DNA (chiral template) and carbazole‐based biscyanine (molecule 60, achiral luminophore) to fabricate CPL‐active materials (Figure 2.19) [49]. In contrast to the conventional DNA‐binding dyes, the AIE‐active luminophore 60 showed highly enhanced fluorescence after binding to the minor groove of DNA due to the restriction of intramolecular rotation. Induced CPL signals with |g_lum| of 1.7 × 10⁻³ (550 nm) were observed for the DNA‐cyanine complexes, and the sign of CPL could be tuned by the chirality of DNA templates. Interestingly, the DNA‐cyanine complexes showed variable CPL upon multiple cycles of annealing. Recently, Zheng’s group and coworkers prepared a series of TPE macrocycle diquaternary ammoniums 61–64 (Figure 2.20a and b), which are associated with DNA via electrostatic interactions and exhibited chiroptical properties [50]. Their CPL performance was highly dependent on the position (cis‐ or gem‐position) of the macrocycle. The mixture of cis‐compounds and DNA exhibited strong CPL around 530 nm with g_lum up to +2.8 × 10⁻², whereas the gem‐compounds/DNA complexes showed nearly no CPL signals (Figure 2.20c and d). The results demonstrated that the restriction of double‐bond rotation via cis‐position cyclization played an important role in the generation of induced CPL.

Figure 2.19 Schematic illustration of DNA‐biscyanine hybrid CPL‐active materials.

Source: Reproduced with permission [49]. Copyright 2019, American Chemical Society.

Figure 2.20 Molecular structures of TPE macrocycle diquaternary ammoniums (a) cis‐61–62 and (b) gem‐63–64. Schematic illustration of the generation of CPL for (c) cis‐62 and (d) gem‐64.

Source: Reproduced with permission [50]. Copyright 2020, American Chemical Society.

In addition to the AIEgen‐DNA complexes, in 2017, Liu et al. reported a co‐assembly system with promising CPL activity based on a chiral gelator 65 and several achiral AIEgens (Figure 2.21) [51]. Compound 65 self‐assembled into nanotube‐like structures in a DMSO/H₂O mixture (1 : 1). Without the addition of AIEgens, no emission can be observed due to the lack of chromophores. After doping with the achiral AIEgens, the resulting gel exhibited tunable CPL (425–595 nm) with |g_lum| in the range of 0.2–1.7 × 10⁻³. In 2019, Yin et al. adopted a similar strategy to construct CPL‐active materials by the cogelation of a chiral glutamic acid‐containing gelator 66 and an achiral AIEgen 67 (Figure 2.22) [52]. Under D‐66/67 = 100 : 1, left‐handed CPL signal around 510 nm was observed with g_lum of +1.1 × 10⁻². However, the CPL signal inversed (g_lum = −1.5 × 10⁻², 520 nm) at D‐66/67 = 16 : 1, mainly due to the competition of distinct packing modes in the hydrogen bonding‐driven co‐assembly.

In 2020, Liu’s group developed a CPL‐active supramolecular system based on cyclodextrin‐metal‐organic framework ( γ CD‐MOF) and achiral luminophores, including traditional organic dyes and AIE luminophore [53]. Through host–guest interactions, the chiral space of γ CD‐MOF could be utilized for CPL induction of achiral luminophores. The authors emphasized the importance of the ordered packing of the achiral luminophores in γ CD‐MOF for the induction of CPL, since the amorphous luminophores and γ CD mixture exhibited nearly no CPL signals. For the crystalline luminophores@ γ CD‐MOF materials, the result was quite different. As for the AIE luminophore@ γ CD‐MOF system, CPL centered at 500 nm was observed with g_lum of −2 × 10⁻³. In 2020, Tang et al. developed a novel CPL‐active system comprised of achiral AIEgen and chiral crystalline poly(L‐lactide) (PLLA) [54]. The confinement of AIEgen in the semicrystalline PLLA film endowed the resulting film with CPL activity with |g_lum| of 1.6 × 10⁻³ (540 nm).

Figure 2.21 (a) Schematic illustration of the co‐assembly processes. (b) Molecular structure of chiral gelator 65. (c) Molecular structures of achiral AIEgens.

Source: Reproduced with permission [51]. Copyright 2019, Wiley‐VCH.

Figure 2.22 Schematic illustration of the co‐assembly of molecules 66 and 67 under different molar ratio.

Source: Reproduced with permission [52]. Copyright 2019, The Royal Society of Chemistry.

In 2019, Tang’s group reported a unique CPL‐active supramolecular system based on the co‐assembly of a chiral BINOL‐derived gold complex 68 (Figure 2.23a) and three achiral luminophores (Figure 2.23b) [55]. Chiral complex 68 formed helical fibers through a long‐time self‐assembly process, and the chirality was controlled by the chirality of the gold complex. R‐68 formed M‐type helical fibers, while S‐68 formed P‐type helical fibers. However, no obvious CPL signal was observed for these helical fibers. Interestingly, the co‐assembly systems comprised of the chiral gold complex and achiral luminophores exhibited strong CPL signals with different colors (410–560 nm) and relatively high g_lum up to 5 × 10⁻³ (Figure 2.23c). In 2019, Han and coworkers prepared CPL‐active AIEgen‐silica hybrid hollow nanotubes through a spontaneous chiral self‐assembly process in the absence of symmetry‐breaking agent [56]. These nanotubes revealed a helical structure and exhibited CPL around 450 nm with |g_lum| up to 2 × 10⁻².

Figure 2.23 (a) Molecular structures of chiral gold complex enantiomers R‐68 and S‐68. (b) Molecular structures of achiral luminophores TPE, 2,3,5,6‐tetrakis(4‐methoxyphenyl)pyrazine (TPP‐4M), and 9,10‐bis(phenylethynyl)anthracene (BPEA). (c) Schematic illustration of the hierarchical self‐assembly and co‐assembly processes.

Source: Reproduced with permission [55]. Copyright 2019, American Chemical Society.

The development of chiral AIE‐based supramolecular systems greatly improved the CPL performance and expanded the family of AICPL materials. Besides, the properties of the resulting CPL‐active materials were highly tunable via modulation of the supramolecular interactions.