Synthesis of optically active π-stacked compounds based on planar chiral tetrasubstituted [2.2]paracyclophane

Abstract

Optically active X-shaped compounds based on planar chiral [2.2]paracyclophane were synthesized. These compounds composed of two different π-electron systems stacked at central aromatic rings through chemoselective Sonogashira–Hagihara coupling. The optical and chiroptical properties of the obtained compounds were investigated, which exhibited large molar absorption coefficients, good photoluminescence quantum efficiency, and intense circularly polarized luminescence (CPL) with large dissymmetry factors on the order of 10⁻³, indicating that the compounds are excellent CPL emitters.

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Introduction

[2.2]Paracyclophane is a unique molecule consisting of face-to-face benzene rings in proximity; the distance between the two benzene rings is approximately 3.0 Å.¹ [2.2]Paracyclophane has been utilized as a scaffold to construct π-stacked structures; various π-electron systems can be stacked in a molecule. 4,12-Disubstituted and 4,7,12,15-tetrasubstituted [2.2]paracyclophanes are representative scaffolds, as shown in Fig. 1.² For example, when using 4,12-disubstituted [2.2]paracyclophane to construct a π-stacked structure, the terminal benzene rings of the π-electron systems are stacked (Fig. 1a). Using 4,7,12,15-tetrasubstituted [2.2]paracyclophane as a scaffold, the central phenylenes of the π-electron systems are stacked (Fig. 1b). Thus, the π-stacked position of the π-electron system can be tuned by the choice of the [2.2]paracyclophane scaffold.

Fig. 1 [2.2]Paracyclophane scaffolds to construct π-stacked structures.

[2.2]Paracyclophane compounds with substituent(s) exhibit planar chirality due to the restricted rotational motion of the π-stacked benzene rings, depending on the substitution positions.³ 4,12-Disubstituted and 4,7,12,15-tetrasubstituted [2.2]paracyclophanes are also planar chiral compounds. Recently, we have developed a practical optical resolution of 4,12-disubstituted [2.2]paracyclophane^4,5 and the first optical resolution of 4,7,12,15-tetrasubstituted [2.2]paracyclophane,^6,7 leading to enantiopure scaffolds to synthesize optically active compounds with higher-ordered chiral structures. In that study, optically active V-shaped,⁴ X-shaped,⁸ 8-shaped,^6a,b and double-helical^6c compounds were synthesized which emit excellent circularly polarized luminescence (CPL) by photo-excitation.

The optical resolution of 4,7,12,15-tetrasubstituted [2.2]paracyclophane is carried out with 4-hydroxy-7,12,15-tribromo[2.2]paracyclophane (rac-1) using a chiral auxiliary, and then, the hydroxyl group is converted to the trifluoromethylsulfonyl group to afford enantiopure compound 2 (Fig. 2).^6a The catalytic system, Pd₂(dba)₃/P(t-Bu)₃/CuI, was found for chemoselective Sonogashira–Hagihara coupling;⁹ only bromo groups reacted, and the trifluoromethylsulfonyl group remained (Fig. 2).^6a This chemoselective coupling makes it possible to synthesize new types of X-shaped compounds with different π-electron systems stacked in molecules (Fig. 2). In this study, we demonstrate the stacking of different π-electron systems (p-phenylene ethynylenes) consisting of three benzene rings at the central phenylene units. The synthetic routes and optical properties of the obtained π-stacked compounds are also shown.

Fig. 2 Strategy to obtain X-shaped compounds with different π-electron systems.

Results and discussion

Schemes 1 and 2 show the synthetic routes to the target X-shaped compounds with different π-electron systems stacked; the synthetic routes from (R_p)-2^6a are also shown. Chemoselective Sonogashira–Hagihara coupling of (R_p)-2 with trimethylsilylacetylene in the presence of catalytic amounts of Pd₂(dba)₃ (dba = dibenzylideneacetone), (t-Bu)₃P·HB₄, and CuI afforded triyne (R_p)-3 in 86% isolated yield, as shown in Scheme 1. Sonogashira–Hagihara coupling of (R_p)-3 with p-methoxyphenylacetylene using PdCl₂(dppf)·CH₂Cl₂ (dppf = 1,1′-bis(diphenylphosphino)ferrocene) and CuI afforded the corresponding tetrayne (R_p)-4 in 77% isolated yield. The trimethylsilyl group could be readily removed by reaction with K₂CO₃ in MeOH to obtain (R_p)-5 in 89% isolated yield. Finally, Sonogashira–Hagihara coupling of (R_p)-5 with iodobenzene using Pd₂(dba)₃, dppf, and CuI afforded the target compound (R_p)-6 in 74% isolated yield, as shown in Scheme 1.¹⁰

Scheme 1 Synthesis of (R_p)-6.

Scheme 2 Synthesis of (R_p)-9.

The same synthetic strategy produced the other target compound (R_p)-9 (Scheme 2). Triyne (R_p)-3 was reacted with phenylacetylene using PdCl₂(dppf)·CH₂Cl₂ and CuI to obtain tetrayne (R_p)-7 in 48% isolated yield. Its trimethylsilyl groups were also removed by K₂CO₃ in MeOH to obtain (R_p)-8 in 93% isolated yield. Sonogashira–Hagihara coupling of (R_p)-8 with p-iodoanisole using Pd₂(dba)₃, dppf, and CuI afforded compound (R_p)-9 in 97% isolated yield, as shown in Scheme 2.¹⁰

The optical properties of compounds (both enantiomers) 6 and 9 were evaluated by ultraviolet (UV) and photoluminescence (PL) spectroscopies. Fig. 3 shows the UV and PL spectra of (R_p)-6 and (R_p)-9 in their dilute CHCl₃ solutions (1.0 × 10⁻⁵ M). Both compounds exhibited similar optical behaviours. Their peak tops in the UV spectra were at 352 nm and 357 nm, respectively, and those in the PL spectra appeared at 420 nm and 424 nm, respectively. The absolute PL quantum efficiencies (Φ_lum) of (R_p)-6 and (R_p)-9 were estimated to be 0.71 and 0.73, respectively. PL lifetime decay curves were fitted with a single exponential equation, and their lifetimes (τ) were calculated to be 3.57 ns and 3.00 ns, respectively. In addition, we have prepared two kinds of optically active X-shaped compounds: without a methoxy group^8a and with four methoxy groups^8b (Table S1 in the ESI†). As the number of methoxy groups increased, the spectra were slightly red-shifted, Φ values improved, and τ values decreased. The PL spectra of (R_p)-6 and (R_p)-9 were broad without any vibronic structure, and the τ values were relatively long (>3 ns). It is known that the excited state is delocalized in the whole molecule in the case of close energy levels between through-bond and through-space states.^2b,d,e,i,j Thus, the emission mechanisms of (R_p)-6 and (R_p)-9 are identical to those of previous studies, and there are no essential differences among these compounds.

Fig. 3 UV and PL spectra of (A) (R_p)-6 and (B) (R_p)-9 in CHCl₃ (1.0 × 10⁻⁵ M, excited at each absorption maximum for the PL spectrum).

The chiroptical properties of both enantiomers of 6 and 9 were investigated using circular dichroism (CD) and CPL spectra. The CD and the absorption spectra of both enantiomers of 6 and 9 in dilute CHCl₃ (1.0 × 10⁻⁵ M) are shown in Fig. 4A and B. In both cases, the absorption peaks around 350 nm are the typical π–π* transition bands of p-phenylene-ethynylenes, and clear Cotton effects were observed in the absorption bands. As shown in the CD spectra, chirality was induced in the arylene–ethynylene moieties by the planar chiral [2.2]paracyclophane moiety.

Fig. 4 CD and UV spectra of (A) enantiomers of 6 and (B) enantiomers of 9 in CHCl₃ (1.0 × 10⁻⁵ M).

Fig. 5 shows the CPL and PL spectra of (S_p)- and (R_p)-6 and 9 in dilute CHCl₃ (1.0 × 10⁻⁵ M), and the g_lum spectra are also included. The g_lum is given by the following equation: 2(I_left − I_right)/(I_left + I_right), where I_left and I_right indicate the luminescence intensities of left- and right-handed CPL, respectively.^11,12 Both compounds 6 and 9 exhibited mirror image spectra. Positive and negative signals were observed for (S_p)- and (R_p)-isomers, respectively. These behaviours were the same as those reported in previous studies (Table S1 in the ESI†). The positive/negative signals were identical to those of the longest wavelength in the corresponding CD spectra, indicating the same orientations of dipole moments between electric and magnetic transitions in the ground and excited states. The absolute g_lum values between the enantiomers of 6 and 9 were estimated to be approximately 1.5 × 10⁻³ and 1.7 × 10⁻³, respectively. Compounds 6 and 9 exhibited relatively large ε and good Φ_lum (>0.7), in addition to the large g_lum value on the order of 10⁻³. Therefore, optically active X-shaped compounds are promising CPL materials.

Fig. 5 CPL and PL spectra, including g_lum spectra, of (A) enantiomers of 6 and (B) enantiomers of 9 in CHCl₃ (1.0 × 10⁻⁵ M, excited at 280 nm).

Conclusions

In summary, planar chiral [2.2]paracyclophane-based optically active X-shaped compounds were synthesized. Different π-electron systems could be stacked at the central phenylene units by the chemoselective Sonogashira–Hagihara coupling using a Pd₂(dba)₃/(t-Bu)₃P/CuI catalytic system. Although essential differences between compounds consisting of different π-electron systems and the same π-electron systems were not observed, the compounds exhibited good emission behaviours, resulting in an excellent CPL emitter with a large ε, good Φ_lum, and a large g_lum value on the order of 10⁻³. The strategy in this study can stack two different π-electron systems; thus, our next target is the synthesis of optically active π-stacked compounds consisting of strong donor and acceptor π-electron systems. In addition, the systematic synthesis of planar chiral [2.2]paracyclophane-based optically active compounds by changing the stacking position^4c and orientation of the stacked-π-electron systems will be studied to elucidate the CPL emission mechanism of the planar chiral [2.2]paracyclophane.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by Grants-in-Aid for Scientific Research on Innovative Areas (No. 17H05165 for Y. M. and 17H05148 for T. O.) “π-Figuration” from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The financial support from the Nagase Science and Technology Foundation is also acknowledged. The authors are grateful to Professor Naoto Tamai (Department of Chemistry, School of Science and Technology, Kwansei Gakuin University) for PL lifetime measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details including synthetic procedures, characterization, and NMR spectra. See DOI: 10.1039/c7qm00613f

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