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

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