Handbook of Aggregation-Induced Emission, Volume 3. Группа авторов

the most recently reported AICPL systems and classified them into six categories according to the different features: (i) SOMs, (ii) macrocycles and cages, (iii) metal complexes and clusters, (iv) supramolecular systems, (v) polymers, and (vi) LCs.

2.2 Small Organic Molecules

SOMs play an important role in CPL materials. However, most of them suffer from a severe ACQ effect, which largely limits the further applications. To solve the ACQ problem in the aggregated state, several strategies have been applied in designing novel CPL‐active molecules. Among all the strategies, introducing AIE‐active moieties (such as tetraphenylethene, TPE) into chiral luminophores is one of the most useful methods. In 2014, Moya’s group reported a CPL‐active organic molecule with a novel skeleton, in which BODIPY acted as the luminophore moiety and 1,1′‐bi‐2,2′‐naphthol (BINOL) functioned as the chirality perturbing moiety [14]. Later in 2015, covalent modification based on the luminescent BODIPY–BINOL skeleton by introducing two TPE units via Sonogashira reaction (molecule 1) was reported by Zhu et al. (Figure 2.1) [15]. Photoluminescence (PL) spectra in CH₂Cl₂/hexane mixtures demonstrated that molecule 1 has a typical AIE feature where the luminescence intensity strongly increased following the addition of hexane. In addition, upon varying the hexane fraction, CPL of molecule 1 remained nearly constant with g_lum of c. ±2 × 10⁻³ (630 nm). In 2017, Cheng and coworkers synthesized another AICPL molecule 2 based on a diboron core and two TPE pendants [16]. Molecule 2 showed interesting photophysical properties in the different mixed solvents upon aggregation. For instance, 2 exhibited aggregation‐induced emission enhancement (AIEE) effect in CH₂Cl₂/hexane mixtures, and |g_lum| (absolute value of g_lum) varied in the range of 2.7–5.4 × 10⁻⁴ (550 nm). Meanwhile, it revealed an ACQ feature in tetrahydrofuran (THF)/H₂O mixtures, and |g_lum| was lower than 5.5 × 10⁻⁴ (550 nm). The above two examples demonstrated a simple molecular design using chiral luminophores and AIEgens linked together by π ‐conjugation. However, this strategy is limited by the tedious synthesis of the chiral chromophores and the relatively low |g_lum|. In 2018, Tang and coworkers synthesized molecule 3 by incorporating TPE units and a BODIPY dye through a single bond [17]. By adding water into a THF solution, 3 exhibited tunable emission from green to yellow. Besides, mirror image CPL signals with |g_lum| up to 4.1 × 10⁻³ (525 nm) were observed.

Figure 2.1 Molecular structures of chiral AIEgens 1–13 and corresponding g_lum (f_w indicate the fraction of water in the solvent mixture) [15–20].

      Incorporating less emissive chiral moieties and AIE luminophores appeared to be an alternative and more flexible pathway to AICPL due to the facile synthesis and broader source of chiral precursors. In 2015, two pairs of chiral 1,8‐naphthalimide chromophores 4 and 5 with AIE activities were prepared by Cheng and coworkers [18]. In contrast to the previous examples (1–3), the chiral moiety and chromophores of 4 and 5 were connected by flexible alkyl chains rather than rigid conjugated junctions. PL spectra demonstrated the AIE effect of 1,8‐naphthalimide in THF/H₂O mixtures. In a THF solution, nearly mirror image CPL signals centered at 462 nm could be observed with g_lum of −6.1 × 10⁻³ and +5.5 × 10⁻³ for R‐4 and S‐4, respectively. In THF/H₂O mixtures, CPL centered at 490 nm could be observed with lower g_lum of +2.8 × 10⁻³ and −2.2 × 10⁻³ for R‐4 and S‐4, respectively. It was found interestingly that the CPL signals became reversed in a pure THF solution and in THF/H₂O mixtures. The CPL spectra of 5 exhibited similar phenomenon where the CPL signal reversed at various states with lower g_lum than 4. Calculation results demonstrated that the CPL signals may result from different conformations and dihedral angles at distinct states. Besides, the difference between 4 and 5 indicated that the length of alkyl chain may also influence the CPL performance.

      In 2019, Jiang et al. prepared two boron difluoride complexes (6 and 7) with red emission [19]. In spite of a small difference in structure, 6 and 7 exhibited dramatically different photophysical properties. The luminescence intensity of 6 was not sensitive to the aggregation process, while 7 showed typical AIEE effect in the aggregated state. In a dichloromethane (DCM) solution, 6 showed CPL signals with |g_lum| in the range of 1.3–1.6 × 10⁻³ (600 nm). Complex 7 exhibited weak CPL with |g_lum| of c. 10⁻³ (609 nm) in a THF solution. As a contrast, enhanced CPL with high |g_lum| up to 1.6 × 10⁻² (653 nm) was observed in the aggregated state. Besides, it was found interestingly that 7 could be used as a CPL switcher by reversible protonation of the N,N‐dimethylaminium groups. In 2019, Zhao et al. reported a series of R‐BINOL‐derived CPL‐active AIE molecules 8–13, which exhibited tunable luminescent properties [20]. With the various modifications on the BINOL skeleton, the corresponding CPL could be tuned with |g_lum| in the range of 0.6–10 × 10⁻³ (518–617 nm).

CPL‐active AIE SOMs 1–5 were prepared by incorporating chiral binaphthyl moieties with AIEgens. This principle of molecular design successfully endowed the SOMs with both the CPL and AIE activities. However, the |g_lum| of these molecules was still limited in a low range (<6 × 10⁻³), presumably due to the insufficient association between the chiral moieties and AIEgens. Later in 2020, Qiu’s group synthesized a series of CPL‐active AIE molecules 14–25 based on helicenes (Figure 2.2) [21]. The molecular structures were varied in linkage position, conjugation, length of linkage, and number of substituent. It was found that the linkage position played a dominating role in CPL activities. According to the CPL spectra of the suspension, all the 2‐ (or 2,15‐) substituted AIE‐helicene adducts (compounds 14–19) exhibited obvious CPL signals, while all the 4‐ (or 4,13‐) substituted molecules (compounds 20–25) appeared to be nearly CPL silent. Theoretical calculations indicated that this phenomenon was probably due to the different distribution of various rotational conformers in the aggregated state. The existence of favored conformations for 2‐ (or 2,15‐) substituted AIE‐helicene adducts leads to CPL‐active properties. On the contrary, the coexistence of various rotational conformers for 4‐ (or 4,13‐) substituted analogs results in a cancellation of CPL signals. Controlling the conjugation between the helicene moieties and AIE luminophores could easily tune the emission color. Additionally, shortening the length of linkage or increasing the number of substituents were proved to be efficient ways to enhance the CPL performance, especially for |g_lum| (up to 1.5 × 10⁻²).

Figure 2.2 Molecular structures of chiral AIEgens 14–25 and corresponding g_lum [21].

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