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

in a THF solution (1.0 × 10⁻³ M). (d) CPL spectra of P‐33 and M‐33 in a THF/H₂O mixture (f_w = 90%).

      Source: Reproduced with permission [27]. Copyright 2016, American Chemical Society.

In 2017, Cao’s group prepared two chiral cubic cages 34 and 35 by reversible covalent condensation based on a TPE derivative (Figure 2.8) [28]. Each cage molecule was comprised of six TPE tetra‐aldehyde units and eight triaminoethylamine units. Unlike a single TPE molecule, the TPE units on each face of the cubic cage possessed stable configuration with P or M chirality. The rotation of the phenyl groups in the TPE units was restricted probably due to the strong steric hindrance from the neighboring TPE units. Therefore, the propeller‐shaped configurations of the TPE units were fixed in a cubic cage. By chiral HPLC, two pairs of enantiomers including homodirectional enantiomers (6P‐34 and 6M‐34) and heterodirectional enantiomers (2P4M‐35 and 4P2M‐35) were separated. Further characterizations showed that these cubic cages exhibited strong blue luminescence even in solution due to the rotation restriction of the benzene rings. Besides, the cage enantiomers exhibited mirror image CD and CPL spectra. The homodirectional cages (6P‐34 and 6M‐34) showed a higher CPL activity at 450 nm with g_lum of ±1.1 × 10⁻³, and the heterodirectional cages (2P4M‐35 and 4P2M‐35) revealed g_lum of ±9.3 × 10⁻⁴ (450 nm).

Figure 2.8 (a) Synthesis of organic cubes from six TPE tetra‐aldehyde units and eight triaminoethylamine units. Schematic representations of (b) homodirectional cubes 6P‐34 and 6M‐34, and (c) heterodirectional cubes 4P2M‐35 and 2P4M‐35, and corresponding g_lum.

      Source: Reproduced with permission [28]. Copyright 2017, American Chemical Society.

2.4 Metal Complexes and Clusters

Luminescent metal complexes also face the ACQ problem. Endowing metal complexes with AIE feature would greatly expand their applications in imaging and luminescent sensor. In 2018, Xiang’s group prepared a new class of chiral Pt(II)‐Salen complexes 36–40 (Figure 2.9) [29]. For all the complexes, nearly no emission was detected in dilute solutions of organic solvents such as CH₂Cl₂, benzene, MeCN, and THF, whereas they generated near‐infrared emission in THF/H₂O mixtures or in the solid state. With the absence of a traditional AIE‐active moiety, complexes 36–40 showed strong AIE activity. Theoretical calculations revealed that the binaphthyl unit quickly rotated in the solution and consequently caused luminescence quenching. In the aggregated state, the restriction of intramolecular motions (RIM) led to the near‐infrared emission. Complex 37 showed AICPL when excited at 460 nm with g_lum of ±5.0 × 10⁻³ (650 nm). Enantiopure 39 also exhibited CPL at 655 nm with lower g_lum of ±3.0 × 10⁻³. The same group also prepared a series of chiral binuclear Pt(II) complexes in 2019 [30]. With binaphthyl quinoline as the bridge ligand, these complexes possessed helical structure, and exhibited CPL with high g_lum up to ±1.0 × 10⁻² (653–683 nm).

Figure 2.9 Molecular structures of chiral Pt(II)‐Salen complexes 36–40 and corresponding g_lum [29].

Recently, several AICPL systems based on chiral clusters were also investigated. In 2017, Tang and coworkers synthesized a chiral Au₃ cluster with unique optical absorption activity (Figure 2.10) [31]. For an individual cluster 41, nearly no luminescence was observed. Interestingly, due to the strong CH/ π interaction, 41 self‐organized into nanocubes with a body‐centered cubic packing in a CH₂Cl₂/hexane mixture and exhibited intense CPL signal with g_lum of ±7.0 × 10⁻³ (583 nm).

Figure 2.10 Molecular structures of chiral Au₃ cluster enantiomers R‐41 and S‐41 and corresponding g_lum [31].

In 2019, Zang and coworkers prepared an atomically precise chiral copper(I) cluster 42 ([Cu₁₄(R/S‐DPM)₈](PF₆)₆) with a typical AIE feature and CPL activity from chiral ligands R‐DPM and S‐DPM (Figure 2.11) [32]. The PL intensity of 42 increased when the fraction of n‐hexane increased in the CH₂Cl₂/hexane mixtures. In a dimethyl sulfoxide (DMSO)/H₂O system, similar AIE effect was also observed. As for chiroptical properties, obvious CPL signals were observed with g_lum up to ±1.0 × 10⁻² (702 nm) for R‐42 and S‐42 when the fraction of n‐hexane exceeded 40%.

Figure 2.11 (a) Molecular structures of chiral ligands R‐DPM and S‐DPM. (b) Single crystal structures of R‐42 and S‐42, and corresponding g_lum in CH₂Cl₂/hexane mixtures.

      Source: Reproduced with permission [32]. Copyright 2019, Wiley‐VCH.

In 2020, a propeller‐like chiral trinuclear copper(I) cluster 43 was reported by Zang’s group (Figure 2.12) [33]. By introducing several flexible phenyl groups into the cluster, 43 was endowed with AIE activity following the RIM mechanism. CD spectra indicated that the chirality belonged to cluster 43 rather than the isolated chiral ligands. In the aggregated state (in DMSO/H₂O mixtures), remarkable CPL signal was observed with g_lum of ±2.0 × 10⁻² (610 nm).