English

• Chirality is ubiquitous in nature and plays essential roles in life science [1,2]. Chirality is also becoming increasing important in the design of advanced materials with functional properties [3–6]. Among synthetic chiral materials, circularly polarized luminescent (CPL) materials have drawn considerable attention owing to their various potential applications in 3D displays [7], optical data storage [8], asymmetric catalysis [9], and so on [10–13]. Organic π-conjugated chromophores are very appealing for developing CPL materials in terms of high luminescence efficiency, facile chemical modification and fine-tunable photophysical properties [14–17]. However, they mostly have planar achiral structures, and thus endowing them with chirality by chemical modification is essential for obtaining the CPL-active materials [18–21].

  Chiral macrocyclic hosts have confined chiral environment and excellent recognition properties, and their supramolecular assemblies allow for increased structural flexibility of achiral organic chromophores and exhibit CPL properties, which offer effective strategies for constructing CPL materials [22–27]. Pillararenes [28], as a fascinating macrocyclic host, can be extensively applied in the fields of supramolecular chemistry and materials science due to their inherent planar chirality [29–33]. By introducing organic chromophores into one unite or one rim of a pillar[5]arene, the resulting chromophore-substituted pillar[5]arenes afford CPL responses but generally exhibit rather modest CPL dissymmetric factors (g_lum) in the order of 10^–4~10^–3 [34–39]. Herein, we report hybrid chiral BINOL-Py functionalized pillar[5]arenes exhibiting strong CPL, which could be manipulable by aggregation state and the addition of guest.

  BINOL-Py-substituted pillar[5]arenes (Scheme 1) were synthesized according to Scheme S1 (Supporting information), and were characterized by nuclear magnetic resonance and high resolution mass spectroscopies (Figs. S1-S48 in Supporting information). These BINOL-Py-substituted pillar[5]arenes are well soluble in chloroform, and the molar extinction coefficients for compounds 2 and 4 at the wavelength of 350 nm are 2-fold larger than those for mono-substituted 2M and 4M (Fig. 1a). 2/4 and 2′/4′ gave more pronounced excimer fluorescence as result of the intramolecular π-π stacking (Fig. 1b and Fig. S49b in Supporting information), which is significantly affected by the linker length because the increasing length increased the freedoms of side chain and further reduced the conformational stability of intramolecular excimer. Also, the intramolecular π-π stacking of 2R/4R exhibited obvious solvent effect, affording varying excimer signals in different solvents. It is noteworthy that 2R and 4R showed increased pyrene excimer/monomer fluorescence ratios in highly polar methanol and ethanol than those in low-polarity solvents such as tetrahydrofuran and dichloromethane (Figs. 1c and d). These results demonstrate that the increased solvent polarity and short linker facilitate the intramolecular π-π stacking of pyrene unites.

  Scheme 1

  

  Scheme 1.  The chemical structures of hybrid chiral BINOL-Py derivatives and schematic diagram of π-π stacking in BINOL-Py-substituted pillar[5]arenes.

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  Figure 1

  

  Figure 1.  (a) UV–vis spectra of 2R, 4R, 2MR and 4MR (10 µmol/L) in CHCl₃. (b) Normalized fluorescence spectra of 2R, 4R, 2MR and 4MR (1 µmol/L) in CHCl₃ (λ_ex = 355 nm). Normalized fluorescence spectra of (c) 2R and (d) 4R (10 µmol/L) in various solvents (λ_ex = 355 nm).

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  The pyrene excimer fluorescence of 2R and 4R were greatly enhanced by increasing the amount of water in the solvent mixture of water-tetrahydrofuran. Thus, the excimer in 90% H₂O increased in intensity by 25-fold relative to those of the fluorescence intensity in 50% H₂O (Figs. S50a and b in Supporting information), and the aggregation of 4R was more apparent than 2R (Fig. S50c in Supporting information), revealing that the long spacers in BINOL-Py-substituted pillar[5]arenes cause favorable steric intermolecular interaction and supramolecular assembly. And the fluorescence spectra at low concentration (0.5 µmol/L) can further prove this conclusion (Figs. S52 and S53 in Supporting information). The aggregation also drastically increases the emission efficiencies, and 2R and 4R showed fluorescence quantum efficiencies Φ_f of 31% and 27% in 90% H₂O, much higher than in 50% H₂O (2%, Fig. S54 and Table S1 in Supporting information).

  2R/S and 4R/S bear two bulky subunits, and the flipping-induced inversion of planar chirality (FIIPC) [40] of 2R/S and 4R/S were completely suppressed to allow HPLC enantioseparation of their R_P and S_P conformers of pillar[5]arene (Figs. 2a and b). In contrast, the use of HPLC for the enantioseparation of 2M and 4M failed due to their rapid interconversion. The circular dichroism (CD) spectra at 310 nm were measured for assigning the absolute configuration of the first and second fractions of 2R/S and 4R/S, where the negative CD should be the S_P configuration [41]. In the CD spectra (Figs. 2c and d), the homochiral enantiomers (2RR_p and 2SS_p) showed stronger signals at 310 and 410 nm than heterochiral ones (2RS_p and 2SR_p), and similar phenomena could be observed for the enantiomeric conformers of 4R and 4S, verifying that more efficient transfer of CD signals in homochiral enantiomers. Moreover, the chiral transfer ability decreased with the increasing of chain length. Thus, the stronger CD signals at 410 nm could be obtained by short-tethered BINOL-Py-substituted pillar[5]arenes (2RR_p and 2SS_p) than tether-elongated ones (4RR_p and 4SS_p). Interestingly, the excimer fluorescence quantum efficiencies Φ_f for homochiral enantiomers are significantly stronger (ca. 3-fold) than the Φ_f for heterochiral enantiomers (Table 1, Fig. S55 in Supporting information), highlighting the critical role of synergistic effect of chirality on the intramolecular self-assembly. However, there is almost no difference between the excimer fluorescence intensity and Φ_f, respectively, for homochiral and heterochiral enantiomers in 90% H₂O, implying that synergistic effect of chirality exhibited almost no effects on the self-assembly (Figs. S56 and S57 in Supporting information). In addition, lifetime measurement results showed that the decay profile became a sum of two exponential functions with τ₁ = 4.68 ns and τ₂ = 19.43–19.51 ns, the latter of which was assigned to the excimer lifetime dominating with relative abundance of 95.1% and 84.7% at 520 nm for 2RR_P and 4RR_P, respectively (Figs. S60, S61 and Table S2 in Supporting information). However, it should be noted that the relative abundance of excimer for 4RR_P (83.4%) was much higher than 2RR_P (69.2%) in 90% H₂O. These results indicated that the short linker facilitates the formation of intramolecular excimer in solution, and the longer linker is more conducive to the formation of intermolecular excimer in aggregation state.

  Figure 2

  

  Figure 2.  Chiral-phase HPLC traces of (a) 2R and 2S and (b) 4R and 4S. CD spectra of the first and second fractions of (c) 2R and 2S and (d) 4R and 4S at 15 µmol/L in CHCl₃.

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  Table 1

  

  Table 1.  The photophysical property of 2RR_p, 2SS_p, 2RS_p, 2SR_p, 4RR_p, 4SS_p, 4RS_p and 4SR_p.

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  Compd	Φ (%) a	|glum| (10–3) a	Ɛ (L mol-1 cm-1) a	BCPL (L mol-1 cm-1) a	Φ (%) b	|glum| (10–3) b	Ɛ (L mol-1 cm-1) b	BCPL (L mol-1 cm-1) b
  2RRp	3.6	6.8	7.0 × 104	8.6	26.0	3.5	8.5 × 104	38.7
  2SSp	4.0	6.8	6.9 × 104	9.3	26.8	3.5	8.8 × 104	41.5
  2RSp	1.3	3.0	6.7 × 104	1.3	26.5	1.7	1.0 × 105	22.9
  2SRp	1.5	3.0	7.1 × 104	1.6	26.8	1.6	9.1 × 104	19.5
  4RRp	0.9	17	7.9 × 104	7.2	26.0	1.5	9.5 × 104	18.5
  4SSp	1.1	17	7.7 × 104	6.0	26.1	1.1	8.4 × 104	12.0
  4RSp	0.2	7.4	7.3 × 104	0.6	24.7	0.6	1.1 × 105	8.2
  4SRp	0.3	7.8	7.6 × 104	0.9	24.4	0.9	9.8 × 104	10.8
  a Measured in CHCl3. b Measured in 90% H2O/THF.

  CPL spectral studies revealed that both the homochiral and heterochiral enantiomers showed strong CPL signals (Fig. 3). Remarkably, the g_lum at 570 nm reached 0.017 for the homochiral enantiomers 4RR_P and 4SS_P (Fig. 3b), which is higher by a factor of 2.2 than those for the corresponding heterochiral enantiomers (Table 1). To the best of our knowledge, this is the highest value for the pyrenes induced by the planar chirality of pillar[5]arenes [42–44]. Moreover, intramolecular stacking facilities the excited chirality transfer [44,45], leading to the higher g_lum value of 4RR_p than that of 2RR_p. The CPL brightness (B_CPL: defined as B_CPL = εΦ_f|g_lum|/2) was further calculated to evaluate the chiral emission performance [46]. The B_CPL values of 2RS_p, 2SR_p, 4RS_p and 4SR_p in CHCl₃ were estimated to be 1.3, 1.6, 0.6 and 0.9 L mol⁻¹ cm⁻¹, respectively. That is 6 to 12 times less than the B_CPL value of the corresponding homochiral enantiomers. It is noted that the B_CPL values could be multiplied by easily tuning the chirality of pillar[5]arenes and BINOL to be identical. In addition, the B_CPL values in 90% H₂O were greatly enhanced by factors 2 to 18, compared with the corresponding values obtained in CHCl₃. The CD signal of 2RR_p at 420 nm was stronger than 2MR, 2′R and unenantioseparated 2R (Fig. S62a in Supporting information), which could be accounted for the more efficient chirality transfer of homochiral 2RR_p. Meanwhile, the CPL intensities of 4RR_p (g_lum = 0.017) were also higher than 4R (g_lum = 0.015), 4MR (g_lum = 0.002) and 4′R (g_lum = 0.010) at 570 nm in CHCl₃ (Fig. S64 in Supporting information). These results indicated that the planar chirality of pillararenes plays a highly important role in CPL chirality transfer.

  Figure 3

  

  Figure 3.  g_lum of 15 µmol/L (a) 2RR_p, 2SS_p, 2RS_p and 2SR_p and (b) 4RR_p, 4SS_p, 4RS_p and 4SR_p measured in CHCl₃.

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  In a previous paper, we demonstrated that the intermolecular interactions of hexanedinitrile and pillar[n]arenes can stabilize the cavity of pillar[n]arenes and inhibit the annulus rotation to increase the order of the ring units, resulting in the enhancement of CD signals [47]. We attempted to manipulate the CPL signals of pillar[n]arenes by the addition of hexanedinitrile. A Job plot of the change in the UV–vis absorption spectra gave a peak at the ratio of 0.5 (Fig. S65 in Supporting information), indicating the 1:1 complexation between the 2RR_p/4RR_p with hexanedinitrile. The association constants of hexanedinitrile with the 2RR_p and 4RR_p were calculated to be 2.34 × 10⁴ L/mol and 1.98 × 10⁴ L/mol, respectively (Fig. S66 in Supporting information). The addition of hexanedinitrile (30 equiv.) led to the enhancement of CD signals for 2RR_p and 4RR_p by a factor of 1.7 and 1.3, respectively (Fig. 4a). Intriguingly, attributed to the angle change between the pyrene stacking after the addition of hexanedinitrile [44], the CPL signals of the pillar[5]arenes with short and long spacer showed slight decrease and increase upon the addition of hexanedinitrile (Fig. 4b and Fig. S67 in Supporting information), providing a new avenue for guest-regulated CPL.

  Figure 4

  

  Figure 4.  (a) CD spectra and (b) g_lum of the enantiomers of 2 and 4 (15 µmol/L) in the absence and presence of hexanedinitrile (HAN) measured in CHCl₃.

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  In conclusion, we demonstrated that BINOL-Py-substituted pillar[5]arenes with short spacer facilitate the formation of intramolecular excimers, while an increase in solvent polarity or the formation of aggregates facilitate the formation of intermolecular excimers. Homochiral enantiomers showed stronger chirality transfer ability than heterochiral ones to afford high g_lum value up to 0.017, demonstrating the planner chirality of pillar[5]arenes as a critical role in chirality transfer. The aggregation formation and guest addition can efficiently manipulate the chiroptical properties, providing a new idea for regulating the CPL of pillararenes derivatives.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Xintian Qu: Writing – review & editing, Writing – original draft, Methodology, Formal analysis, Data curation. Zeen Liu: Writing – original draft, Methodology, Formal analysis, Data curation. Zhifan Wang: Validation, Data curation. Dongyan Yu: Writing – original draft, Methodology. Xueqiu Huang: Validation, Data curation. Jie Yang: Resources. Jiecheng Ji: Writing – review & editing, Writing – original draft, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization. Xueqin Wei: Writing – review & editing, Writing – original draft, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization. Cheng Yang: Writing – review & editing, Resources.

  Acknowledgments

  We acknowledge the support of this work by the National Natural Science Foundation of China (Nos. 22371055, 22201194 and 22001046), the Guangxi Natural Science Foundation (No. 2021GXNSFBA196053), the Science & Technology Department of Sichuan Province (No. 2024NSFSC1117), and the Guangxi Medical University Training Program for Distinguished Young Scholars.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111024.

  
  
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• 

  Scheme 1  The chemical structures of hybrid chiral BINOL-Py derivatives and schematic diagram of π-π stacking in BINOL-Py-substituted pillar[5]arenes.

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  Figure 1  (a) UV–vis spectra of 2R, 4R, 2MR and 4MR (10 µmol/L) in CHCl₃. (b) Normalized fluorescence spectra of 2R, 4R, 2MR and 4MR (1 µmol/L) in CHCl₃ (λ_ex = 355 nm). Normalized fluorescence spectra of (c) 2R and (d) 4R (10 µmol/L) in various solvents (λ_ex = 355 nm).

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  Figure 2  Chiral-phase HPLC traces of (a) 2R and 2S and (b) 4R and 4S. CD spectra of the first and second fractions of (c) 2R and 2S and (d) 4R and 4S at 15 µmol/L in CHCl₃.

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  Figure 3  g_lum of 15 µmol/L (a) 2RR_p, 2SS_p, 2RS_p and 2SR_p and (b) 4RR_p, 4SS_p, 4RS_p and 4SR_p measured in CHCl₃.

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  Figure 4  (a) CD spectra and (b) g_lum of the enantiomers of 2 and 4 (15 µmol/L) in the absence and presence of hexanedinitrile (HAN) measured in CHCl₃.

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  Table 1.  The photophysical property of 2RR_p, 2SS_p, 2RS_p, 2SR_p, 4RR_p, 4SS_p, 4RS_p and 4SR_p.

  Compd	Φ (%) a	|glum| (10–3) a	Ɛ (L mol-1 cm-1) a	BCPL (L mol-1 cm-1) a	Φ (%) b	|glum| (10–3) b	Ɛ (L mol-1 cm-1) b	BCPL (L mol-1 cm-1) b
  2RRp	3.6	6.8	7.0 × 104	8.6	26.0	3.5	8.5 × 104	38.7
  2SSp	4.0	6.8	6.9 × 104	9.3	26.8	3.5	8.8 × 104	41.5
  2RSp	1.3	3.0	6.7 × 104	1.3	26.5	1.7	1.0 × 105	22.9
  2SRp	1.5	3.0	7.1 × 104	1.6	26.8	1.6	9.1 × 104	19.5
  4RRp	0.9	17	7.9 × 104	7.2	26.0	1.5	9.5 × 104	18.5
  4SSp	1.1	17	7.7 × 104	6.0	26.1	1.1	8.4 × 104	12.0
  4RSp	0.2	7.4	7.3 × 104	0.6	24.7	0.6	1.1 × 105	8.2
  4SRp	0.3	7.8	7.6 × 104	0.9	24.4	0.9	9.8 × 104	10.8
  a Measured in CHCl3. b Measured in 90% H2O/THF.

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