Circularly Polarized Luminescence (CPL) Based on Planar Chiral [2.2]Paracyclophane

Abstract

In this chapter, chiroptical properties, especially circularly polarized luminescence (CPL) properties of optically active molecules based on planar chiral [2.2]paracyclophane are mainly introduced. In addition, practical optical resolution methods of disubstituted and tetrasubstituted [2.2]paracyclophane molecules are also focused on. The enantiopure [2.2]paracyclophane compounds have been used as chiral building blocks to synthesize the optically active molecules by means of optical resolution. The [2.2]paracyclophane-based molecules are π-stacked molecules, which construct optically active second-ordered structures, such as V-, X-, triangle-shaped, and one-handed double helical structures, due to the orientation of stacked π-electron systems. Photoexcitation allows them to emit bright CPL with good photoluminescence (PL) quantum efficiencies and large dissymmetry factors (g_lum values). Thus, planar chiral [2.2]paracyclophane is the ideal scaffold to achieve excellent CPL properties.

1 Introduction: Circularly Polarized Luminescence (CPL)

Circularly polarized luminescence (CPL) is a chiroptical (chiral + optical) property that is responsible for the difference in luminescence intensity between left-handed and right-handed emissions. Circular dichroism (CD) is another chiroptical property that is responsible for the difference in molar extinction coefficients between left- and right-polarized light. CPL and CD spectroscopy provide important information about the orientation of luminophores and chromophores, respectively (Berova et al. 2000). Many optically active molecules exhibit CPL and CD in the excited and ground states, respectively, but these phenomena cannot be always observed even if the molecules possess a chiral source.

CPL is evaluated by the anisotropic factor (dissymmetric factor), referred to as g_lum value (by Berova et al. 2000; Riehl and Richardson 1986; Riehl and Muller 2012), which can be calculated as the following Eq. (10.1):

$$ g_{\text{lum}} = \Delta {\text{I}}/{\text{I}} $$

(10.1)

∆I = (emission intensity of left-handed CPL) − (emission intensity of right-handed CPL) and I = emission intensity

The g_lum value can also be expressed as

$$ g_{\text{lum}} = 4\left| {\varvec{\upmu}} \right|\left| {\mathbf{m}} \right|\cos\uptheta/\left( {\left| {\varvec{\upmu}} \right|^{2} \left| {\mathbf{m}} \right|^{2} } \right) $$

(10.2)

Here, μ and m represent electric and magnetic transition dipole moments, respectively, and the θ represents the angle between them (Berova et al. 2000; Riehl and Richardson 1986; Riehl and Muller 2012). Thus, the maximum absolute g_lum value is calculated to be |g_lum| = +2.

Ideally, forbidden electric transitions and allowed magnetic transitions are preferred. Generally, the g_lum values of chiral lanthanide complexes are much larger than those of chiral organic molecules. The emission of lanthanides is derived from the Laporte forbidden f-f transition (Muller 2014); therefore, the μ is small and m is large. It is reported that the chiral Eu(III) complex, tetrakis(3-heptafluoro-butylryl-(+)-camphorato) Eu(III), has a g_lum value of +1.38 (Lunkley et al. 2008).

Many of g_lum values of organic molecules reported are within the range 10⁻³–10⁻⁵ order due to the much smaller m. Therefore, Eq. (10.2) can be simply converted to the approximated Eq. (10.3).

$$ g_{\text{lum}} = 4\left| {\mathbf{m}} \right|\cos \theta /\left| {\varvec{\upmu}} \right| $$

(10.3)

Considering the photoluminescence quantum efficiency (Φ_PL) and emission intensity, an allowed electronic transition is preferable. It is a challenging task to achieve a large m and small μ, and their linear orientation (their angle θ = 0° or 180°) in the organic molecules. Organic CPL dyes are attractive luminescent materials due to their light-weight, as well as the ease of fabrication, functional group modification, and emission color control. These dyes are expected to be promising candidates for next-generation emissive materials such as light-emitting diodes for three-dimensional displays, security inks, light for plant growth, bioimaging materials, etc.

A wide variety of organic CPL emitters have been reported thus far, and the last five years have seen a marked increase in the number of related manuscripts because of the widespread use of CPL spectrometers (Maeda and Bando 2013; Sánchez-Carnerero et al. 2015; Tanaka et al. 2018; Chem and Yan 2018). The CPL-emitting small organic molecules in the dispersed state are briefly introduced below. In 1967, CPL from organic molecules was reported for cyclic ketones with a central chirality (Emeris and Oosterhoff 1967); a representative example is shown in Fig. 10.1. Because of the forbidden n-π* transition, the molecule emitted CPL with a very large g_lum value of +3.5 × 10⁻². This value was long considered the champion value for a long time in the small organic molecules in a diluted solution, although the Φ_PL was quite low (Φ_PL = 1 × 10⁻⁵). Various CPL emitters with the central chirality have been reported to show large g_lum values. As the representative examples, naphthyl (Amako et al. 2014) and naphthalimide (Sheng et al. 2016) emissive units were connected on chiral cyclic scaffolds, which showed CPL emission from the intramolecular aggregate (excimer) of the aromatic units with |g_lum| values of 9.4 × 10⁻³ (Φ_PL = 0.02) and 1.4 × 10⁻², respectively. In 2017, the champion g_lum value was updated for the cylindrical molecule 4 with cylinder helicity (Fig. 10.2), and it was on the order of 10⁻¹ (|g_lum| = 1.5 × 10⁻¹ and Φ_PL = 0.80) (Sato et al. 2017). Theoretical studies revealed a large m, as well as linearly oriented m and μ (θ = 180°).

Fig. 10.1

Representative examples of CPL emitters with central chirality

Fig. 10.2

Cylindrical molecules with an excellent CPL profile

Biaryl-based axial chirality is an easily accessible chiral scaffold for CPL emission. Figure 10.3 shows the binaphthyl-based molecules. In 2007, perylenebiscarboxydiimide-containing binaphtylene (S)-5 was prepared (Kawai et al. 2007). This compound showed intense emission with Φ_PL of 0.88, and the g_lum was estimated to be approximately 3 × 10⁻³. Simple biaryl-based molecules (open- and closed-type molecules (S)-6^open and (S)-6^close, respectively) were synthesized (Kimoto et al. 2012). Despite their similar photoluminescence behavior and identical absolute configuration, these molecules showed different CPL signals. The values for (S)-6^open and (S)-6^close were calculated to be +1.0 × 10⁻³ (Φ_PL = 0.19) and −1.4 × 10⁻³ (Φ_PL = 0.25), respectively. Optically active naphthalene oligomers were synthesized; the representative molecule (R,R,R,R,R,R,R)-7 consisting of eight naphthyl moieties is also shown in Fig. 10.3 (Takaishi et al. 2017). This molecule showed bright CPL emission with g_lum = +2.2 × 10⁻³ and Φ_PL = 0.90. Compound (S)-5 emitted from the perylenediimide moieties, whereas (S)-6 and 7 showed emission from the oligoaryl scaffolds.

Fig. 10.3

Representative examples of oligoaryl-based CPL emitters with axial chirality

Helical chirality is also employed for CPL emission. Generally, Φ_PL of a helical molecule is not very high; however, its g_lum value is very large, often on the order of 10⁻². Figure 10.4 shows the helicene derivatives (M)-8 (Field et al. 2003) and (M)-9 (Sawada et al. 2012) as representative examples. Molecule 8 is the first helicene emitting CPL in dilute solution. This molecule was obtained as a diastereomer due to the attachment of a camphanate unit as the chiral auxiliary. The |g_lum| value was calculated to be 1.1 × 10⁻³. A very large |g_lum| value of 3.2 × 10⁻² (Φ_PL = 0.30) was obtained from molecule 9, which was prepared by the Rh-catalyzed enantioselective [2 + 2 + 2] cycloaddition. Molecule 10 is a stimuli-responsive compound used for capturing halogen anions to form a helix (Haketa et al. 2012). The chiral counter cation is allowed for the introduction of one-handed helicity in dilute solution. The resulting ion pair was emissive, and the |g_lum| value was calculated to be 1.3 × 10⁻² with Φ_PL of 0.33.

Fig. 10.4

Representative examples of helical CPL emitters

Recently, planar chirality has also been applied to CPL emission. However, there are fewer CPL emitters based on planar chirality as compared to those based on axial and helical chiralities. Unique optically active molecules have been prepared by taking advantage of the planar chirality of a [2.2]paracyclophane scaffold, and large g_lum values, as well as high Φ_PL were achieved (Morisaki and Chujo 2019). Various interesting second-ordered structures were also constructed. This chapter focuses on planar chirality and CPL emission in the mono-dispersed state from the optically active small organic molecules based on the planar chiral [2.2]paracyclophanes.

2 Introduction: [2.2]Paracyclophane and Its Planar Chirality

Cyclophane is a cyclic compound containing at least one aromatic ring in the main chain skeleton. It is a well-known cyclic compound and have been used particularly in the field of synthetic organic chemistry and organometallic chemistry (Vögtle 1993; Gleiter and Hopf 2004). The typical cyclophane is [2.2]paracyclophane (Fig. 10.5), in which two phenylenes are stacked in proximity (the distance between two benzene rings is approximately 3.0 Å). [2.2]Paracyclophane was first synthesized in 1949, by pyrolysis of para-xylene (Brown and Farthing 1949); subsequently, a direct synthesis from 1,4-bis-bromomethylbenzene by Wurtz-type intramolecular cyclization was reported in 1951 (Cram and Steinberg 1951). Since then, various synthetic routes have been developed to prepare a wide variety of [2.2]paracyclophane molecules. [2.2]Paracyclophane has a unique π-stacked structure, and there have been many studies on its physical properties within the field of organic chemistry (Gleiter and Hopf 2004; Brown and Farthing 1949).

Fig. 10.5

Structure of [2.2]paracyclophane and the corresponding planar chirality

Although [2.2]paracyclophane is a molecule that has generally been used in the field of organic chemistry and organometallic chemistry, its utilization is not widespread in the fields of polymer chemistry and materials chemistry (Hopf 2008; Morisaki and Chujo 2006, 2008, 2009, 2011, 2012; Mizogami and Yoshimura 1985; Guyard and Audebert 2001; Guyard et al. 2002; Salhi et al. 2002; Salhi and Collard 2003; Jagtap and Collard 2010; Weiland et al. 2019). In terms of the through-space conjugated system, [2.2]paracyclophane-based π-stacked molecules have been systematically prepared, and their electronic communication between the stacked π-electron systems have been known since 1998 (Oldham et al. 1998; Bazan et al. 1998; Bartholomew and Bazan 2001; Bazan 2007). In 2001, thiophene-substituted [2.2]paracyclophanes were polymerized electrochemically; however, the polymer was deposited on the electrode and was insoluble in solvents (Guyard and Audebert 2001). In 2002, soluble π-stacked polymers consisting of [2.2]paracyclophane as a repeating unit in the main chain were synthesized (Morisaki and Chujo 2002). Since then, various π-stacked polymers have been prepared (Morisaki and Chujo 2006, 2008, 2009, 2011, 2012). The structures have been well-characterized, and the properties, such as optical properties, have been shown due to their solubility in organic solvents. Since their optical properties changed continuously depending on the number of the stacked π-electron systems, they were called “through-space conjugated polymers”. Highly efficient unidirectional fluorescence resonance energy transfer (FRET) (Morisaki et al. 2013, 2014a, 2014b, 2017), as well as through-space electron transfer (Molina-Ontoria et al. 2011; Wielopolski et al. 2013) was achieved by precisely designing the stacked π-electron systems. [2.2]Paracyclophane-based through-space conjugated polymers and oligomers can act as single molecular wires.

As described above, [2.2]paracyclophane consists of two phenylene units fixed in proximity; therefore, the rotational movement of the benzene rings is suppressed. By introducing a substituent at appropriate positions on the benzene ring(s), the corresponding [2.2]paracyclophane becomes a planar chiral molecule (Fig. 10.5) (Cram and Allinger 1955; Rozenberg et al. 2004; Rowlands 2008; Gibson and Knight 2003; Aly and Brown 2009; Paradies 2011). The planar chirality of [2.2]paracyclophane is well-known in the fields of organic chemistry and organometallic chemistry, and planar chiral [2.2]paracyclophanes have been utilized as chiral auxiliaries and chiral ligands. However, until recently, the planar chirality of [2.2]paracyclophane has been ignored in the fields of polymer chemistry and materials chemistry until recently. In 2012, a new optical resolution method was developed for pseudo-ortho-disubstituted [2.2]paracyclophane, and the transformation and polymerization of the enantiopure [2.2]paracyclophanes were reported (Morisaki et al. 2012a, b). The resulting optically active polymer emitted CPL (Morisaki et al. 2012).

This chapter highlights and introduces the recent results on the synthesis of enantiopure disubstituted and tetrasubstituted [2.2]paracyclophanes. The preparation of optically active molecules based on the [2.2]paracyclophanes for their application in the fields of polymer and materials chemistry as the CPL emitters is also described.

3 Synthesis of Enantiopure Disubstituted [2.2]Paracyclophane and Optically Active π-Stacked Molecules

Optical resolution routes of mono-substituted [2.2]paracyclophanes were developed, and various enantiopure ortho-, pseudo-geminal-, and syn-latero-disubstituted [2.2]paracyclophanes were prepared (Cram and Allinger 1955; Rozenberg et al. 2004; Rowlands 2008; Gibson and Knight 2003; Aly and Brown 2009; Paradies 2011). In addition, several synthetic routes to the syntheses of enantiopure disubstituted [2.2]paracyclophanes, e.g., pseudo-ortho-disubstituted [2.2]paracyclophanes, have been reported (Pye et al. 1997; Rossen et al. 1997; Zhuravsky et al. 2008; Jiang and Zhao 2004; Jones et al. 2003; Pamperin et al. 1997; Pamperin et al. 1998; Braddock et al. 2002), as shown in Fig. 10.6. A representative example is the synthesis of enantiopure pseudo-ortho-bis(diarylphosphino)[2.2]paracyclophane (Ph-PHANEPHOS) (Fig. 10.6a) (Pye et al. 1997). The resulting (S_p)- and (R_p)-Ph-PHANEPHOS are the commercially available chiral ligands for the transition metal-catalyzed asymmetric reactions. This PHANEPHOS makes it possible to produce enantioenriched pseudo-ortho-dibromo[2.2]paracyclophane (Fig. 10.6b) by kinetic resolution (Rossen et al. 1997). Synthesis of optically active 4-bromo-12-hydroxy[2.2]paracyclophane (Zhuravsky et al. 2008), pseudo-ortho-dihydroxy[2.2]paracyclophane (PHANOL) (Jiang and Zhao 2004), and pseudo-ortho-dihydroxymethyl[2.2]paracyclophane (Jones et al. 2003) were successfully synthesized by using chiral camphanic acid chloride as the chiral auxiliary; for example, Fig. 10.6c shows the optical resolution of PHANOL racemate. The enzyme-promoted kinetic resolutions of pseudo-ortho-disubstituted [2.2]paracyclophanes have also been developed (Pamperin et al. 1997, 1998; Braddock et al. 2002).

Fig. 10.6

Examples of optical resolution of pseudo-ortho-disubstituted [2.2]paracyclophanes

A practical route to the optical resolution of pseudo-ortho-dibromo[2.2]paracyclophane was reported in 2012, in which (1R,2S,5R)-(-)-menthyl (S)-p-toluenesulfinate was used as the chiral auxiliary (Fig. 10.7) (Morisaki et al. 2012). Racemic pseudo-ortho-dibromo[2.2]paracyclophane rac-11 was reacted with n-butyllithium (n-BuLi). Then, the reaction with (1R,2S,5R)-(-)-menthyl-p-toluenesulfinate afforded the diastereomers (R_p,S)- and (S_p,S)-12, which could be separated by using SiO₂ column chromatography. The isolated diastereomers were reacted with t-BuLi to form dilithiated intermediate 13; not only the lithium-halogen exchange reaction, but also the lithium-sulfur exchange reaction occurred (Rowlands 2008; Clayden 2002; Hitchcock et al. 2005; Parmar et al. 2010). The subsequent reaction with various electrophiles produced enantiopure pseudo-ortho-disubstituted [2.2]paracyclophanes (Fig. 10.7). (R_p)- and (S_p)-Diformyl[2.2]paracyclophanes 14 could be separated by chiral column chromatography; thus, their chromatographic optical resolution was also possible (Morisaki et al. 2012). This synthetic route can be used to produce new PHANEPHOS; for example, cyclohexyl groups could be introduced on phosphorus atoms to produce Cy-PHANEPHOS 16 (Fig. 10.7).

Fig. 10.7

Optical resolution of pseudo-ortho-disubstituted [2.2]paracyclophane and transformations

Pseudo-ortho-diformyl[2.2]paracyclophane 14 could be converted to the corresponding diethynyl[2.2]paracyclophane 17 using the Ohira-Bestmann reagent (Fig. 10.8) (Ohira 1989; Müller et al. 1996), which was used as a monomer to synthesize poly(p-arylene-ethynylene)s (poly-PAEs) via Sonogashira-Hagihara coupling (Tohda et al. 1975; Sonogashira 2002) polymerization. The treatment of 17 with 1,4-diiodobenzene derivative 18 affords the optically active through-space conjugated polymers (R_p)- and (S_p)-19 (Morisaki et al. 2012). Chiroptical properties (chiral optical properties) of 9 showed the intense CPL in the emission region (Fig. 10.9). In addition, its absolute photoluminescence (PL) quantum efficiency (Φ_PL) was approximately 80% and the |g_lum| value was 2.2 × 10⁻³ in dilute CHCl₃ solution. These results indicated that the optically active polymer 19 is a good CPL emitter.

Fig. 10.8

Synthesis of optically active polymer

Fig. 10.9

CPL and PL spectra of (R_p)- and (S_p)-19 in CHCl₃ (10 × 10⁻⁵ M) excited at 290 nm for CPL and λ_abs,max for PL

The chiroptical properties of PAE oligomers were investigated (Morisaki et al. 2014). As shown in Fig. 10.10, the optically active PAE-type π-stacked dimer (R_p)-20, trimer (R_p)-21, tetramer (R_p)-22, and cyclic trimer (R_p)-23 were prepared from (R_p)-15 (shown in Fig. 10.7) and (R_p)-17 as the key chiral building blocks.

Fig. 10.10

Optically active PAE-type π-stacked oligomers (R_p)-20–22 and cyclic trimer (R_p)-23

Figure 10.10 includes the optical and chiroptical data. The absolute anisotropic factors of absorbance (g_abs values = [(molar absorption coefficient of left-handed circular polarized light) – (molar absorption coefficient of right-handed circular polarized light)]/(molar absorption coefficient) at $\lambda$_abs,max) were calculated from the CD spectra of the optically active oligomers, and the |g_lum| values at $\lambda$_PL,max calculated from CPL spectra. The |g_abs| values of 20–22 were substantially constant at 3.0 × 10⁻³, regardless of the number of stacked π-electron systems. Linearly π-stacked structures of 20–22 in the ground state can adopt various conformations in dilute solution, leading to the constant g_abs values. In other words, the constant g_abs values reflect only the chirality of optically active V-shaped skeleton of the cyclophane moiety. A g_abs value is a normalized value; therefore, the g_abs values of 20–22 were constant independent of the number of stacked π-electron systems.

In contrast, the g_lum values of 20–22 increased depending on the number of stacked π-electron systems. It is speculated that some sort of chirality is induced in addition to the optically active V-shaped structure of the cyclophane moiety in the excited state. The comparison of linear trimer 21 with cyclic trimer 23 gave the answer to this question. The g_abs values of 21 and 23 were different, and their g_lum values were identical. The cyclic structure is a fixed chiral triangle structure (fixed optically active second-ordered structure), whereas the linear π-stacked structure adopts various conformations in the ground state. Thus, chiralities of both the second-ordered structure (chiral triangle structure) and the V-shaped structure affect the larger g_abs value of 23 (3.7 × 10⁻³) more than that of 21 (3.0 × 10⁻³). On the other hand, the g_lum values of 21 and 23 were identical, indicating that 21 and 23 adopt a similar structure in the excited state. PAEs, poly(p-arylenevinylene)s (PAVs), and poly(p-arylene)s (PAs) are known to form planar structures in the excited states, because of the contribution of the quinoid moiety in the excited state. Considering the conformation of 21 in the excited state, each of the three π-electron systems of 21 assumes a planar structure, producing either a zigzag structure or a helical structure (foldamer) in the excited state. The g_lum values of 21 and 23 were identical; therefore, 21 forms a structure similar to a cyclic one, i.e., a one-handed helical structure (optically active second-ordered structure), as shown in Fig. 10.11.

Fig. 10.11

Plausible conformations of the monomeric unit and linear trimer in the ground and excited states

Optically active through-space conjugated polymer 26 consisting of enantiopure planar chiral 4,12-disubstituted [2.2]paracyclophane and quaterthiophene was synthesized by Suzuki-Miyaura coupling (Miyaura et al. 1979) between 24 and 25 (Fig. 10.12) (Morisaki et al. 2015). Moderate CPL was observed (Fig. 10.13), and the Φ_PL and |g_lum| values were estimated to be 0.07 and 5 × 10⁻⁴, respectively.

Fig. 10.12

Synthesis of optically active polymer consisting of [2.2]paracyclophane and quaterthiophene

Fig. 10.13

CPL and PL spectra of (R_p)- and (S_p)-26 in CHCl₃ (10 × 10⁻⁵ M) excited at 380 nm for CPL and λ_abs,max for PL

Chromatographic optical resolutions of pseudo-ortho-disubstituted [2.2]paracyclophanes were reported by Lützen (Meyer-Eppler et al. 2013) and Hasegawa (Kobayakawa et al. 2014; Hasegawa et al. 2017, 2019; Ishioka et al. 2019), and several enantiopure π-stacked molecules have been produced. Figure 10.14 shows the (R_p)-isomers prepared by Hasegawa and coworkers (Kobayakawa et al. 2014; Hasegawa et al. 2017; Hasegawa et al. 2019; Ishioka et al. 2019). Although the CPL properties of these compounds were not reported, their interesting chiroptical and electrochemical properties were shown. Compound 27 was used as a chiral dopant for nematic liquid crystals with a helical twisting power of around 10 m⁻¹ (Kobayakawa et al. 2014), and 28 (Hasegawa et al. 2017) and 29 (Hasegawa et al. 2019) exhibited unique redox behaviors owing to the closely stacked quaterthiophenes and biselenophenes, respectively. Recently, PA-type π-stacked molecules 30 and 31 have been reported, in which terphenylenes and quaterphenylenes were stacked at the terminal benzene rings (Ishioka et al. 2019), respectively, as shown in Fig. 10.14. Intense CPL was observed from 30 with Φ_PL of 0.20 and |g_lum| of 4.2 × 10⁻³ as well as 31 with Φ_PL of 0.64 and |g_lum| of 1.5 × 10⁻³.

Fig. 10.14

Pseudo-ortho-disubstituted [2.2]paracyclophane-based chiral molecules

4 Synthesis of Enantiopure 4,7,12,15-Tetrasubstituted [2.2]Paracyclophane and Optically Active π-Stacked Molecules

In 2014, an optical resolution method of 4,7,12,15-tetrasubstituted [2.2]paracyclophane was reported (Fig. 10.15) (Morisaki et al. 2014). The racemate 4,7,12,15-tetrabromo[2.2]paracyclophane rac-32 was synthesized by Chow and coworkers (Chow et al. 2005). One of the bromo groups of rac-32 was converted to a hydroxy group to obtain rac-33, and the reaction with (1S,4R)-(-)-camphanic chloride as a chiral auxiliary resulted in the diastereomers (R_p,1S,4R)-34 and (S_p,1S,4R)-34. The absolute configuration could be determined by X-ray crystallography. Separation of diastereomers was readily carried out in gram-scale by simple SiO₂ column chromatography.

Fig. 10.15

Optical resolution of 4,7,12,15-tetrasubstituted [2.2]paracyclophane

The chiral unit was easily removed by hydrolysis to afford the optically active phenol 33 (Fig. 10.16). The treatment of 33 with trifluoromethanesulfonic anhydride afforded the optically active 4,7,12,15-tetrasubstituted [2.2]paracyclophane 35. Subsequent Sonogashira-Hagihara coupling between the optically active cyclophane 35 with trimethylsilyl (TMS) acetylene proceeded smoothly to afford triyne 36 with good yield. Interestingly, Pd₂(dba)₃/P^tBu₃/CuI catalyst system (dba = dibenzylideneacetone and P^tBu₃ = tri(t-butyl)phosphine) led to chemoselective coupling with bromo groups. Then, the remaining trifluoromethylsulfonyl group could be reacted with TMS acetylene using a PdCl₂(dppf) catalyst (dppf = 1,1′-bis(diphenylphosphino)ferrocene) to give the optically active tetrayne 37. The TMS groups were removed to give the optically active 4,7,12,15-tetraethynyl[2.2]paracyclophane 38.

Fig. 10.16

Synthesis of optically active 4,7,12,15-tetrasubstituted [2.2]paracyclophane building blocks

Optically active cyclic molecules 39–41 were synthesized with 38 serving as a chiral building block (Fig. 10.17), and their chiroptical properties were revealed. The large chirality was induced in both the ground state and the excited state. For example, the specific rotation of 39 was approximately 1,500, and the molar ellipticity reached 3,000,000 deg cm² dmol⁻¹. Cyclic molecule 39 exhibited excellent CPL profiles (Fig. 10.18), and the |g_lum| value at $\lambda$_PL,max was on the order of 10⁻² with Φ_PL of 0.45. In addition to the planar chirality of the [2.2]paracyclophane skeleton, the chiral second-ordered structure, namely, the chiral two blades propeller-shaped structure contributed to the chiral induction in the excited state. The propeller-shaped molecules 40 (Gon et al. 2015) and 41 (Gon et al. 2016) showed a large molar absorption coefficient, good Φ_PL, and large CPL g_lum value. Especially, propeller-shaped molecule 40 was also an excellent CPL emitter with a large g_lum value and good Φ_PL of 0.60.

Fig. 10.17

Chiral propeller-shaped molecules based on optically active 4,7,12,15-tetrasubstituted [2.2]paracyclophane

Fig. 10.18

CPL and PL spectra of (R_p)- and (S_p)-39 in CHCl₃ (10 × 10⁻⁶ M) excited at 314 nm for CPL and λ_abs,max for PL

Optically active X-shaped molecules 42–44 were synthesized from 4,7,12,15-tetrasubstituted [2.2]paracyclophane 38 (Fig. 10.19) (Gon et al. 2015). Various aromatic units such as benzene, naphthalene, and anthracene could be introduced to the 4,7,12,15-tetrasubstituted [2.2]paracyclophane skeleton. All compounds emitted CPL with good Φ_PL, and large CPL g_lum value; as a representative example, PL and CPL spectra of 43 are shown in Fig. 10.20.

Fig. 10.19

Structures of chiral X-shaped molecules

Fig. 10.20

CPL and PL spectra of (R_p)- and (S_p)-43 in CHCl₃ (10 × 10⁻⁵ M) excited at 300 nm for CPL and λ_abs,max for PL

Figure 10.21 shows the 3rd generation chiral dendrimer (Gon et al. 2016) consisting of a chiral X-shaped core and Fréchet-type dendrons that were prepared by the convergent method (Hawker and Fréchet 1990; Fréchet 1994). The 2nd to 4th generation dendrimers exhibited good film-forming ability, and the thin films could be obtained by the spin-coating approach. The thin film emitted bright CPL from the chiral core unit upon excitation of the benzene rings of dendrons. Energy transfer occurred from the dendrons to the chiral core, and intense PL was emitted. Due to the dendrons, aggregation-caused quenching was completely suppressed, and thus, the bright CPL was observed; CPL and PL spectra are shown in Fig. 10.22. The Φ_PL of the thin films were estimated to be 65%, and the g_lum value at $\lambda$_PL,max was approximately 2 × 10⁻³.

Fig. 10.21

Structure of chiral dendrimer with a chiral X-shaped [2.2]paracyclophane core

Fig. 10.22

CPL and PL spectra of the thin film of (R_p)- and (S_p)-45 excited at 279 nm for CPL and λ_abs,max for PL

An optically active X-shaped molecule 46 consisting of a more extended π-conjugation system was prepared from 38 as a chiral building block (Fig. 10.23) (Gon et al. 2017). The X-shaped molecule 46 emitted CPL with Φ_PL of 87% in the dilute solution with a good |g_lum| value of 1.2 × 10⁻³. In addition, this molecule showed good film formability. The g_lum value of the thin film formed by the spin-coating method exhibited the positive CPL signal with the g_lum value of +2.1 × 10⁻² (Fig. 10.24), which was larger by one order of magnitude compared with that the dilute solution. The annealed spin-coated film exhibited a negative g_lum value of –0.25 (Fig. 10.24), which was a very large value on the order of 10⁻¹. The optically active higher-ordered structure was constructed in the thin film by π-π interactions among the extended π-conjugation systems, as well as the van der Waals force of the C₁₂H₂₅ chains. It seems that the thermodynamically stable higher-ordered structure was formed by the heating protocol.

Fig. 10.23

Structure of chiral X-shaped molecule (R_p)-46 and the CPL profiles

Fig. 10.24

CPL and PL spectra of the spin-coated and the annealed film of (R_p)-46

Chemoselective Sonogashira-Hagihara coupling using Pd₂(dba)₃/P^tBu₃ catalyst system was utilized to stack different π-electron systems (Sasai et al. 2018). The triyne 36 mentioned above was used as the substrate to produce chiral X-shaped π-stacked dimers 48 and 50 consisting of different π-electron systems as shown in Fig. 10.25. Normal X-shaped molecule 51 was also prepared from 36 via 37 (Gon et al. 2017; Kikuchi et al. 2019). The optical, as well as chiroptical properties of 48 and 50, were almost the same as those of the corresponding X-shaped molecule 51 (Table 10.1). The number of methoxy groups increased, spectra were redshifted and Φ_PL and g_lum values were increased. Thus, they were excellent CPL emitters.

Fig. 10.25

Synthesis of X-shaped π-stacked dimers

Table 10.1 Optical and chiroptical data of 48, 50, and 51 in the diluted solutions

Optically active p-arylene-vinylenes (PAVs)-stacked X-shaped molecules 52 and 53 were also prepared (Gon et al. 2017), and the structures are shown in Fig. 10.26. Racemic molecule 52 was prepared and their properties were investigated in detail (Bazan et al. 1998; Bartholomew and Bazan 2001; Bazan 2007; Morisaki and Chujo 2002). Molecule 52 exhibited CPL with a good Φ_PL of 0.78 and |g_lum| value of 3.7 × 10⁻³, while the PL of the aggregates quenched to exhibit Φ_PL of 0.03 with good |g_lum| = 4.3 × 10⁻³. Molecule 53 showed moderate PL profiles both in solution (Φ_PL = 0.58) and in the aggregated state (Φ_PL = 0.24), and moderate CPL properties with |g_lum| in the order of 10⁻⁴ in solution, as well as in the aggregation state were observed. The optical and chiroptical properties varied drastically by attaching phenyl groups to the ethene moieties.

Fig. 10.26

Structures of chiral X-shaped molecules consisting of PAVs

5 Synthesis of Enantiopure Bis-(Para)-Pseudo-Ortho-Tetrasubstituted [2.2]Paracyclophane and Syntheses of Optically Active π-Stacked Molecules

In 2016, a new type of enantiopure 4,7,12,15-tetrasubstituted [2.2]paracyclophane was produced. Racemic bisphenol rac-54 was used as a starting material and reacted with (1S,4R)-(-)-camphanic chloride as a chiral auxiliary to obtain the diastereomers (R_p,1S,4R)-55 and (S_p,1S,4R)-55 (Figs. 10.6c, 10.27) (Jiang and Zhao 2004). The diastereomers were reacted with bromine using iron without separation (Fig. 10.27). Regioselective bromination proceeded with iron to obtain 4,7,12,15-tetrasubstituted [2.2]paracyclophanes (R_p,1S,4R)-56 and (S_p,1S,4R)-56 (Kikuchi et al. 2019; Morisaki et al. 2016); this tetrasubstituted isomer is called bis-(para)-pseudo-ortho-tetrasubstituted [2.2]paracyclophane (Vorontsova et al. 2008). The diastereomers were separated by simple column chromatography using SiO₂. Removal of the chiral auxiliary groups with KOH formed chiral bisphenol 57, and the successive reaction with trifluoromethanesulfonic anhydride resulted in enantiopure bis-(para)-pseudo-ortho-type tetrasubstituted [2.2]paracyclophane building blocks 58 (Morisaki et al. 2016).

Fig. 10.27

Optical resolution of 4,7,12,15-bis-(para)-pseudo-ortho-type tetrasubstituted [2.2]paracyclophane and the transformations

Chemoselective Sonogashira-Hagihara coupling was available for 58. The treatment of 58 with TMS acetylene in the presence of a catalytic amount of Pd₂(dba)₃/P^tBu₃/CuI afforded the diyne 59 selectively, as shown in Fig. 10.28. The coupling reaction between 59 with triisopropylsilyl (TIPS) acetylene using a Pd₂(dba)₃/dppf catalyst system enabled us to obtain the corresponding tetrayne 60. K₂CO₃/MeOH allowed the chemoselective removal of the TMS group from 60 to obtain diyne 61. Then, Sonogashira-Hagihara coupling of 60 with p-iodoanisole produced 62. The TIPS group was easily removed by Bu₄NF, and the reaction with m-diiodobenzene provided enantiopure cyclic molecule 63 consisting of two optically active cyclophanes, in which boomerang-shaped arylene-ethynylene containing five benzene rings are stacked at the second and fourth phenylene moieties. This cyclic molecule forms a one-handed double helical structure; for example, the (R_p,R_p)-isomer construct left-handed double helices. This double helical molecule also emitted intense CPL with Φ_PL of 0.62 and |g_lum| = 1.6 × 10⁻³; the PL and CPL spectra are shown in Fig. 10.29.

Fig. 10.28

Synthesis of one-handed double helical molecule

Fig. 10.29

CPL and PL spectra of (R_p)- and (S_p)-63 in CHCl₃ (10 × 10⁻⁵ M) excited at 300 nm for CPL and λ_abs,max for PL

Enantiopure bisphenol 57 was reacted with K₂CO₃ and MeI to obtain a new chiral building block (R_p)-64 in high yield (Fig. 10.30) (Kikuchi et al. 2019). The reaction of (R_p)-64 with TMS acetylene in the presence of Pd₂(dba)₃/P^tBu₃/CuI catalyst system proceeded smoothly to obtain the cross-coupling product (R_p)-65. The TMS groups of (R_p)-65 were easily removed by K₂CO₃/MeOH to obtain diyne (R_p)-66. The Sonogashira-Hagihara reaction of (R_p)-66 with iodotoluene derivative resulted in the corresponding V-shaped molecule (R_p)-67 (Fig. 10.30). In molecule 67, π-electron systems are stacked at the terminal benzene rings, whereas the same π-electron systems are stacked at the central benzene rings in the X-shaped molecule 51 (Fig. 10.25). Their CPL spectra are shown in Fig. 10.31, indicating the opposite CPL signs in spite of having the same absolute configuration. Incidentally, Fig. 10.31 shows the spectra of (S_p)-isomers, because the simulation was carried out for them (vide infra).

Fig. 10.30

Synthesis of chiral V-shaped molecule

Fig. 10.31

CPL spectra of (S_p)-67 and 51 in CHCl₃ (10 × 10⁻⁵ M)

The electronic transition dipole moments and magnetic transition dipole moments were simulated for V-shaped 67 and X-shaped 51 (Fig. 10.32) (Kikuchi et al. 2019); the simulations were carried out for (S_p)-isomers. Theoretically, a g_lum value is expressed in the following equation: 4|μ||m|cosθ/(|μ|² + |m|²) as mentioned. The μ and m represent electric and magnetic transition dipole moments, respectively, and the θ represents the angle between the μ and m. The angle θ between μ and m of (S_p)-67 was estimated to be 144°, and that of (S_p)-51 was estimated to be 87°. Simulation results predicted opposite CPL signs for 67 (negative due to θ = 144°) and 51 (positive due to θ = 87°), which was supported by the experimental results (Fig. 10.31). Thus, the appropriate construction of the intermolecular orientations of π-electron systems is important not only for CPL intensity, but also for CPL signs. In other words, negative and positive signs of CPL can be controlled by the orientation of the π-electron systems.

Fig. 10.32

Simulation results: the transition dipole moments of (S_p)-67 and (S_p)-51 in the excited states by using the TD-DFT calculations at the ωB97XD/6-31G(d,p) level of theory with following options; TD = (NStates = 10, Root = 1), SCRF = (SOLVENT = Chloroform). Lengths of the dipole moments on the structures are based on the atomic units

6 Synthesis of Enantiopure Bis-(Para)-Pseudo-Meta-Tetrasubstituted [2.2]Paracyclophane

In 2018, enantiopure bis-(para)-pseudo-meta-tetrasubstituted [2.2]paracyclophane derivatives were prepared as new chiral building blocks by means of diastereomeric optical resolution (Sawada et al. 2018). On the other hand, Lützen and coworkers had already reported the chromatographic optical resolution of disubstituted analogs in 2014 (Meyer-Eppler et al. 2014). Racemic 4,7,12,15-tetrasubstituted [2.2]paracyclophane rac-33 was used as the starting material, which was reacted with BuLi to form the dilithiated intermediate rac-68 (Fig. 10.33). After the formation of lithium phenoxide, the next lithium-halogen exchange occurred at pseudo-meta-position due to the electronic effect, leading to bis-(para)-pseudo-meta-type dilithiated [2.2]paracyclophane. Addition of B(OMe)₃ to the intermediate and the treatment with H₂O₂/NaOH resulted in dibromodihydroxy[2.2]paracyclophane rac-69. Diastereomer approach using (1S,4R)-(-)-camphanic chloride enabled the optical resolution of the bis-(para)-pseudo-meta-type tetrasubstituted [2.2]paracyclophane (Fig. 10.33). The resulting diastereomers (R_p,1S,4R)-70 and (S_p,1S,4R)-70 were separated by simple column chromatography using SiO₂. The reaction of 70 with KOH afforded the bisphenol and the next reaction with trifluoromethanesulfonic anhydride produced enantiopure bis-(para)-pseudo-meta-type tetrasubstituted [2.2]paracyclophanes 71. These enantiomers are also used as the chiral building blocks for various CPL emitters.

Fig. 10.33

Optical resolution of 4,7,12,15-bis-(para)-pseudo-meta-type tetrasubstituted [2.2]paracyclophane and the transformations

7 Conclusion

In summary, the authors have focused on the planar chirality of [2.2]paracyclophane and the intense CPL emission from the enantiopure [2.2]paracyclophane-based molecules in this chapter. Their optical resolution methods basically consisted of diastereomer resolution methods using the common chiral auxiliaries such as (1R,2S,5R)-(-)-menthyl-p-toluenesulfinate and (1S,4R)-(-)-camphanic chloride instead of chromatographic resolution by chiral columns. A wide variety of optically active π-stacked molecules, cyclic molecules, oligomers, and polymers were prepared using the obtained enantiopure disubstituted and tetrasubstituted [2.2]paracyclophanes as chiral building blocks. They were used to construct the chiral higher-ordered structures such as V-shaped, X-shaped, propeller-shaped, triangle-shaped, and double helical structures both in the ground and excited states due to the structurally stable planar chiral cyclophanes. This is important to show excellent CPL performance. [2.2]Paracyclophane-based molecules exhibit good PL property with high Φ_PL. Their ε values are large because of the extended π-electron systems. Generally, it is difficult to produce the materials emitting intense CPL with high Φ_PL and large g_lum. The planar chiral [2.2]paracyclophane skeleton seems to be an ideal scaffold for developing materials exhibiting strong CPL.