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Synlett 2024; 35(12): 1361-1374
DOI: 10.1055/a-2158-8820
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Stereogenic π-Conjugated Macrocycles: Synthesis, Structure, and Chiroptical Properties Including Circularly Polarized Luminescence

Masashi Hasegawa∗
,
Yasuhiro Mazaki

This work was financially supported by the Japan Society for the Promotion of Science (JSPS KAKENHI grants JP21K05043 and JP22K05070) and the Core Research for Evolutional Science and Technology (JST, CREST grant JPMJCR2001).
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Dedicated to the memory of Professor Masahiko Iyoda, who passed away on September 21, 2023.

Abstract

Highly symmetrical and aesthetically pleasing molecules have attracted the attention of organic chemists. We synthesized new highly symmetric stereogenic π-conjugated macrocycles with planar or axial chirality. Macrocyclic oligomers synthesized by Yamamoto coupling or Suzuki–Miyaura cross-coupling from the π-unit containing chirality. These cyclization reactions gave multiple oligomers in relatively high yields. We then elucidated their structures and investigated their chiroptical properties, including circular dichroism (CD) and circularly polarized luminescence (CPL). Because of the selection rule for rigid and symmetric structures, these macrocycles exhibit a high dissymmetry factor (g abs or g lum) for circularly polarized light in CD or CPL. Several rigid cyclic compounds retain a highly symmetric structure in the excited state and exhibit higher g lum values than common chiral organic compounds. This Account provides a brief background regarding chiroptical properties, followed by a summary of the various macrocycles synthesized in this study. We are glad if this Account will be a source of ideas not only for chemists working with π-conjugated compounds, but also for synthetic chemists working with chiral compounds, especially those engaged in asymmetric synthesis.

1 Introduction

2 Brief Description of Chiroptical Properties

3 Stereogenic Macrocycle Based on [2.2]Paracyclophane

3.1 Stereogenic Double-decker Oligothiophene

3.2 Stereogenic Biselenophene Macrocycle

3.3 Helical Oligophenylene Linked with [2.2]Paracyclophane

4 Stereogenic Macrocycle Based on Binaphthyl

4.1 Cyclic Oligomer of Chiral Binaphthyl

4.2 Doubly Twisted Binaphthyl Dimer

4.3 Cyclic Oligomer of Binaphthyl Extended with Paraphenylene

4.4 Curved Helical Paraphenylene Anchoring Chiral Binaphthyl

4.5 Binaphthyl-Hinged [5]-Helicene

5 Summary


#

Key words

macrocycle - chirality - circularly polarized luminescence - [2.2]paracyclophane - binaphthyl - oligothiophene - paraphenylene - helicene

Biographical Sketches

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Masashi Hasegawa was born in Tokyo, Japan, in 1977. He earned his PhD in 2004 from Tokyo Metropolitan University (TMU) under the supervision of Professor Masahiko Iyoda. He then served as a JSPS postdoctoral fellow with Professor Timothy M. Swager at MIT and Professor Yohji Misaki at Ehime University, Japan. Then, he was appointed a faculty member at the School of Science, Kitasato University from September 2007, and his subsequent research has been recognized with the Daicel Chemical Industry Award in Synthetic Organic Chemistry (2010), Mazda Foundation Award (2009), and BCSJ Award (2011). His research focuses on the synthesis and development of novel chiral emitter for chiroptical materials.

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Yasuhiro Mazaki was born in Akita, Japan in 1958. He earned his PhD in 1987 from Tohoku University under the supervision Professor Shô Itô. Then, he was appointed a faculty member at the University of Tokyo (Prof. Keiji Kobayashi group). In 1996, he moved to Kitasato University, where he became a professor in 2007. His research interests cover organic redox active compounds.

1

Introduction

Chiral compounds that contain a single chiral carbon belong to point group C 1, which has the lowest symmetry. Therefore, the word ‘asymmetric’ is often used interchangeably with the word ‘chiral’. However, a molecule is described as ‘chiral’ if its mirror image is nonsuperimposable. Strictly speaking, a chiral molecule does not have an improper axis S n.[1] Thus, a chiral molecule can have a symmetric structure and still be chiral if it is composed of multiple chiral centers or a stereogenic structure with planar or axial chirality. A chiral macrocycle composed of twisting units belongs to this class. Shape-persistent macrocycles composed of terminal-less π-conjugated systems sometimes induce intrinsic molecular distortions, resulting in global stereogenic structures without chiral carbons. This unusual molecular design strategy for the assembly of simple and aesthetically pleasing symmetric macrocycles using several chiral units is fascinating from the viewpoint of structural organic chemistry. We were interested in the synthetic challenges of such chiral architectures and their chiroptical properties. A π-conjugated macrocycle composed of aromatic rings unavoidably involves inherent distortion. In addition, cyclization reactions compete with polymerization, so the formation of macrocycles with distortion is often difficult. Chiroptical properties are optical properties resulting from the interactions between chiral compounds and polarized light; they include optical rotation, circular dichroism (CD), and circularly polarized luminescence (CPL).[2] In particular, chiral organic compounds that exhibit CPL have recently attracted considerable attention because of their potential applications in spintronics, optoelectronic devices, quantum computing, and biological probes.[3] [4] [5] [6] [7] [8] Although the number of excellent organic CPL emitters has increased in the past decade, molecular design guidelines for chiral materials that exhibit superior chiroptical properties remain unclear.[9–11] Hence, revealing the structure–chiroptical property relationship of such functional materials is of considerable interest to structural organic chemists. Elucidation of these relationships will enable rational design of organic materials with distinctive chiral properties, bringing us closer to the realization of applications in the future.

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Figure 1 Chiral compounds with symmetric structures. (a) Chiral macrocycles exhibiting exceptional chiroptical properties reported by Diederich et al. (b) Acyclic chiral compounds containing tetrathiafulvalene(s).

When we started our research, the synthesis of a number of chiral π-conjugated macrocycles had already been achieved. In 2010, Diederich et al. synthesized a chiral macrocycle 1 with D 4 symmetry (Figure [1a]).[12] This macrocycle comprised alleno-acetylene units and exhibited a remarkable Cotton effect in its CD spectrum owing to its high symmetry. Although the reason for the amplified CD was not clearly stated in this paper, it is understood that the arrangement of the transition dipole moments (magnetic and electric) resulting from the symmetrical structure enhanced the chiroptical properties because of the selection rule in a certain electronic transition. We have also reported several acyclic chiral compounds with C 2-symmetric tetrathiafulvalene (TTF) connected to allenes 2,[13] [14] [15] [2.2]paracyclophane ([2.2]PC, 3),[16] and spiro carbons 4;[17] these compounds exhibit chiroptical and redox properties (Figure [1b]). The oxidized species of these compounds are all stable, and the electronic spectra including CD were measured. Thus, the design of symmetrical molecules for chiroptical materials is an important strategy that will later be applied to superior CPL emitter. However, controlling the molecular structure in the excited state is more challenging than that in the ground state.[18]

To address this issue, we focused on macrocyclic structures to develop a key strategy for structural control. Shape-persistent macrocycles have a rigid molecular structure and contribute to conformational control in both the ground and excited states. Our macrocycles contained [2.2]PC or 1,1′-binaphthyl units as inherent chiral structures. These planar or axial chiral elements cause chiral perturbations in the macrocycle, leading to global chirality. We have synthesized cyclic compounds from halogen derivatives of the corresponding chiral π-conjugated units in one-pot using Yamamoto coupling with Ni(cod)2 or Suzuki–Miyaura cross-coupling. This approach has afforded cyclic compounds in relatively higher yields than previous syntheses. In this Account, we summarize our recent progress in the molecular design, synthesis, structure, and chiroptical properties of π-conjugated stereogenic macrocycles.


# 2

Brief Description of Chiroptical Properties

Contrary to absorption and emission spectra, chiroptical properties are not intuitive for organic chemists. In this section, we provide a brief description of chiroptical properties, especially CD and CPL. Organic compounds in chiral circumstance may absorb or emit predominantly left- or right-handed circularly polarized light. The absorption is observed in CD spectra and the emission is observed in CPL (Figure [2]).

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Figure 2 (a) Schematic illustration of the CD (left) and CPL (right) phenomena. (b) Electronic transition diagram of CD (left) and CPL (right).

The difference between the left and right circularly polarized lights is expressed as the dissymmetry factor g (g abs for CD and g lum for CPL), which is defined in Equation 1.

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Equation 1

Here, I L and I R refer to the left- and right-handed intensities of absorption and emission, respectively. In the case of absorption, I L and I R correspond to ε R and ε L, respectively. The value of g indicates the degree of excess circularly polarized light. For example, when g = 2.0, it indicates 100% left-handed circular polarization. When g = –0.02, it implies 1% excess of right-handed circular polarization. Most of chiral organic compounds exhibit g values in the range of 10–4 to 10–3.

If we consider the electronic transition from state i to j, g can theoretically be a function of the rotational strength R ij and dipole strength D ij.

Equation 2 is determined using the electronic transition dipole moment (μ), magnetic transition dipole moment (m), and the cosine of the angle between them. Equation 2 is expressed as shown in Equation 3.

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Equation 2
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Equation 3

For most chiral organic compounds, μ is much larger than m; therefore, Equation 3 can be written as as shown in Equation 4.

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Equation 4

The dissymmetry factor is governed by the dipole moments of the electronic transitions during absorption or emission. Hence, g lum reflects the structure in the excited state, while g abs reflects the structure in the ground state. Equation 4 also shows that g is proportional to m and inversely proportional to μ. Therefore, a high g can be achieved by decreasing μ and enhancing m. In addition, g is a function of cosθ. This implies that the maximum value of g can be obtained when |cosθ| = 1. Here, the relative angle between μ and m is either 0° or 180°.

The nature of an electronic transition is described by the selection rule based on the molecular orbital (MO) symmetry of the structure. Moreover, the symmetry of the MO determines the relative orientations of μ and m. For chiral macrocycles, the topology of MO in a symmetrical structure results in a better combination of μ and m and provides their optimal angle.


# 3

Stereogenic Macrocycle Based on [2.2]Paracyclophane

[2.2]PC consists of two paraphenylenes bridged by a diethylene (–CH2CH2–) moiety and is widely employed as a building block for stacked π-conjugated structures.[19] [20] The two phenylene units face each other and cannot rotate because of steric hindrance. Therefore, [2.2]PC with substituents at specific positions on the phenylene rings have planar chirality. Additionally, [2.2]PCs are often used as chiral ligands for asymmetric synthesis,[21–23] chiroptical materials,[20] , [24–28] and stereogenic units in supramolecular chemistry.[29] [30] The optical resolution of planar chiral [2.2]PC can be achieved via the diastereomeric crystallization method developed by Morisaki et al.[31] or via the recycling HPLC method, which uses a chiral stationary phase, i.e., a chiral column. We chose pseudo-ortho-substituted [2.2]PC as the stereogenic scaffold because of its attractive chiral architecture. Scheme [1] illustrates the synthesis of an important chiral precursor, 4,12-diboromo[2.2]PC. The regioselective halogenation of [2.2]PC (5) is very difficult, but pseudo-para-dibromo-[2.2]PC 6 can be separated because of its low solubility.[32] Subsequently, the pseudo-ortho isomer 7 can be obtained through radical cleavage at high temperatures and the removal of pseudo-para-dibromo-[2.2]PC.

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Scheme 1 Scalable synthesis of racemic pseudo-ortho-diboromo[2.2]paracyclophanes[10]
3.1

Stereogenic Double-decker Oligothiophene

Oligothiophenes and their derivatives have been extensively used in organic electronic devices, such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFET), and photovoltaic cells.[33] [34] [35] The facile oxidation of oligothiophenes affords radical cations (polarons) and dications (bipolarons), which often form intermolecular π-dimers.[36–38] This molecular association is essential for electrical conduction in conducting polymers, and several studies have investigated the electronic structures of π-dimers using model molecules. We designed a stereogenic double-decker oligothiophene 7 to study the electronic structure of oxidized species with intrinsic chirality (Figure [3]).[39]

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Figure 3 Molecular structures of (R p,R p)-8 and (S p,S p)-8.
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Scheme 2 (a) Synthesis and optical resolution of 8. (b) Molecular structure of reference compound of 9.

Scheme [2] shows the synthetic route for (R p,R p)-8 and (S p,S p)-8 and their optical resolution. Racemic 8 was converted into iodide derivative 9 in the literature.[40] The Suzuki–Miyaura coupling reaction of 9 with a bithiophene derivative afforded 10. The enantiomers of 10 were separated using preparative recycling chiral HPLC. The absolute configuration of each enantiomer was elucidated using CD spectroscopy and time-dependent density functional theory (TD-DFT) calculations. The reactions of (R p)-10 and (S p)-10 with ICl afforded (R p)-11 and (S p)-11, respectively. Finally, the Ni(cod)2-mediated Yamamoto coupling reactions of (R p)-11 and (S p)-11 afforded the target macrocycles (R p,R p)-8 and (S p,S p)-8, respectively, in 40–46% yield. A small amount of trimer was also obtained in this reaction. Because the coupling reaction was carried out with a single enantiomer, no meso products or higher oligomeric products with R- and S-[2.2]PC units were formed. X-ray crystallography revealed that the quarter thiophenes were twisted with respect to [2.2]PC because of the steric repulsion caused by the hydrogen atoms on the –CH2CH2– moieties on the linker. Consequently, an intense Cotton effect derived from the distorted quarter thiophene was observed in the CD spectra.

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Figure 4 (a) Electronic spectra of oxidized species in CH2Cl2. (b) CD spectra of (R p)-isomers in CH2Cl2. The sharp noise at 750 and 1700 nm is due to the photo multimeter artifact. (c) Schematic illustration of chiral π-dimer in 8 2+.
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Scheme 3 Synthesis of (R p,R p)/(S p,S p)-16. The molecular structures of (R p)-isomers are only shown for clarity.

(R p,R p)/(S p,S p)-8 undergo a multistep redox reaction during cyclic voltammetry (CV). The peak current was estimated using a rotating disk electrode. This estimation indicated that compound 8 was oxidized to form 8 •+, 8 2+, and 8 4+. The first redox potential E 1 of 8 was found to be lower than those of reference compounds 12 and 13, indicating a through-space interaction between the two quarter thiophenes in 8. The chemical oxidation of 8 using magic blue (p-BrC6H4)3N[SbCl6] produced 8 •+ and 8 2+. In the electronic spectra, singly oxidized 8 •+ exhibited absorption maxima at λ = 426, 768 (P 1 transition), and 1444 nm (P 2 transition, Figure [4a]). In contrast, the spectrum of 8 2+ showed characteristic absorption bands for the π-dimers of oligothiophene at λ = 678 (D 1), 1008 (D 2), and 1219 (D 3) nm. As the spectrum was recorded in a dilute solution, these π-dimeric bands are attributed to the intramolecular face-to-face transannular interactions (Figure [4c]). Thus, the CD analysis, including the short-wave infrared region (SWIR 700–1800 nm), showed a clear Cotton effect. In particular, a bisignate response at 1550 (Δε > 0) and 1217 nm (Δε < 0) was observed in the chiral π-dimer of (R p,R p)-82+ . To the best of our knowledge, this is the first study to report the chiroptical properties of a π-dimeric radical cation in the SWIR region. These findings on the electronic structures of oxidized active species may help researchers elucidate the conduction mechanism of spintronic-devices-based chiral organic conductors and spin-polarized materials.


# 3.2

Stereogenic Biselenophene Macrocycle

We prepared a stereogenic macrocycle using biselenophene and [2.2]PC 16 (Scheme [3]).[41] Selenophene is the selenium analogue of thiophene; however, fewer examples of π-extended selenophenes have been reported than that of thiophenes.[42] The change from sulfur to selenium can alter the molecular structure and electronic state of the compound and enhance its intermolecular interactions in the solid state.[43] [44] [45] Similar to 8, compound 16 was synthesized by the homocoupling cyclization of the enantiomeric precursor of 15 (Scheme [3]). The yield of 16 was lower than that of 8 because of the low thermal stability of the selenophene framework. The X-ray crystal structure showed that 16 adopted a C 1-symmetric structure with cisoid and transoid biselenophenes. In the CD spectra, 16 exhibited a pronounced Cotton effect resulting from the stereogenic biselenophene moiety. CV measurements revealed a stepwise redox process, indicating attractive interactions between the biselenophene moieties in a face-to-face arrangement.


# 3.3

Helical Oligophenylene Linked with [2.2]Paracyclophane

Next, stereogenic cyclic oligophenylenes were synthesized using a combination of biphenyl and [2.2]PC units (Scheme [4]).[46] The benzene rings of cyclophane and biphenyl form a p-quarterphenyl (p-QP) moiety, and chiral cyclic oligomers form a symmetrical structure with D n. A phenyl group was introduced into [2.2]PC using pseudo-ortho-dibromo[2.2]PC via an improved Suzuki–Miyaura coupling reaction in the presence of Pd2(dba)3 and [t-Bu3PH]BF4 salts.[47] This reaction afforded the desired product in good yield even with the dibromo derivative, which is relatively less reactive than the diiodo derivative 9. Treatment with ICl in the presence of AgOTf afforded the diiodo derivatives of 18, which were easily separated into two enantiomers by chiral HPLC. Yamamoto coupling reactions of (R p)/(S p)-18 using Ni(cod)2 produced oligomers of cyclic dimers (R p,R p)/(S p,S p)-[2]-19 and cyclic trimers (R p,R p,R p)/(S p,S p,S p)-[3]-19. Interestingly, [3]-19 was obtained in a higher yield despite having more reactive points; DFT calculations were performed assuming a homodesmotic reaction. The strain energy for [3]-19 (10.3 kcal/mol) was found to be lower than that for [2]-19 (21.1 kcal/mol).

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Scheme 4 Synthesis of (R p,R p)/(S p,S p)-[2]-19 and (R p,R p,R p)/(S p,S p,S p)-[3]-19

The X-ray structure of (S p,S p)-[2]-19 showed that this molecule adopts a double helical structure anchored by two chiral [2.2]PC (Figure [5]). The [2.2]PC of the S p-isomer formed a right-handed double helix, similar to that in DNA. Steric repulsion between phenylene and methylene in [2.2]PC induced the distortion, which is the starting point of the double helix. In contrast, no obvious through-space interactions occur between the two helical p-QP moieties, and the benzene rings are free to rotate in solution, even at low temperatures. The cyclic trimer (R p,R p,R p)-[3]-19 formed an isosceles triangular structure. Two of the three p-QPs were formed in an almost straight shape and linked the outer plane of the [2.2]PC to the inner plane of another [2.2]PC. The other p-QP was twisted to link the outer plane of one [2.2]PC to the outer plane of the another [2.2]PC.

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Figure 5 X-ray structure of (a) (S p,S p)-[2]-19 and (b)(R p,R p,R p)-[3]-19. (CCDC # 2069162 and 2069163). Reproduced with permission from ref. 46. Copyright (2021) John Wiley & Sons, Inc.

Figure [6] shows the photophysical (UV and PL) and chiroptical (CD and CPL) spectra of (R p,R p)-[2]-19 and (R p,R p,R p)-[3]-19. The electronic spectra showed that the maximum absorption wavelength and adsorption edge of [3]-19 were red-shifted compared with those of [2]-19. This suggests that [3]-19 is less torsional than [2]-19 and that the QP moiety is effectively conjugated. Compounds [2]-19 and [3]-19 were highly emissive in solution. Compounds [2]-19 and [3]-19 showed high quantum yields (φ PL) of 70% and 83%, respectively. The CD and CPL spectra were in the same range as the UV and PL spectra, respectively. The dissymmetry factors in the CD and CPL spectra were calculated to be |g abs| = 2.3·10–3 for [2]-19, 2.7·10–3 for [3]-19, and |g lum| =1.6·10–3 for both [2]-19 and [3]-19. In general, |g lum| depends on the structure of the excited state and is usually significantly lower than |g abs|. However, in the chiral macrocycles of [n]-19, |g abs| and |g lum| are only slightly lower because of the shape-persistent structure.

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Figure 6 Optical properties of (R p,R p)-[2]-19 and (R p,R p,R p)-[3]-19. (a) UV (dashed line) and PL (solid line) spectra. (b) CD (dashed line) and CPL (solid line) spectra. Reproduced with permission from ref. 46. Copyright (2021) John Wiley & Sons, Inc.

#
# 4

Stereogenic Macrocycle Based on Binaphthyl

When 1,1′-binaphthyl and its derivatives are substituted at the 2- and 2′-positions, they exhibit atropisomerism because of axial chirality and a rotational barrier. Axially chiral binaphthyl is a common chiral building block and is often employed as a metal ligand in asymmetric synthesis,[48] [49] a chiroptical material,[50–55] a chiral dopant in liquid crystals,[56–58] and an important component in new π-conjugated chiral compounds.[59] [60] [61] [62] Several enantiomeric derivatives of 1,17-binaphthyl, such as chiral 1,1′-bi-2-naphthol (BINOL), are commercially available. As these derivatives can be easily modified at the 2-, 3-, and 6-positions by halogenation or lithiation, many studies have reported on chiral compounds, extended at these positions. However, only a few examples of chiral compounds extended at the 7- and 7′-positions of binaphthyl (Scheme [5a]) have been reported; this is because the precursor enantiomers are not commercially available and hence not easily accessible.

Spada et al. reported that diastereomer 22, including the (R)-configured binaphthyl moiety obtained from the reaction of 21 with menthyl chloroformate, could be separated by recrystallization.[63] We modified this method and successfully purified the filtrate using chiral HPLC to obtain the enantiomers of 23 (Scheme [5b]). Subsequent reactions with NaOH afforded optically pure (R)- and (S)-24 in multigram quantities. We then synthesized π-extended shape-persistent macrocycles using this skeleton as a starting material.

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Scheme 5 (a) (S)- and (R)-BINOL; (b) modified method for the optical resolution of 7,7′-dibromo-1,1′-bi-2-naphthol on a multigram scale
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Scheme 6 (a) Synthesis of [n]-26 (n = 2–5); (b) optimized molecular structures of [n]-26 (n = 2–5) in (R)-configuration, calculated by B3LYP/6-31G(d,p). Reproduced from ref. 64 with permission from the Royal Society of Chemistry.

1,1′-Binaphthyls form s-cis and s-trans conformers when the dihedral angles between the two naphthalene rings are θ < 90° and θ > 90°, respectively (Figure [7a]). Chiroptical properties such as intensity and sign (plus/minus) vary with the dihedral angle θ. The first Cotton effect in the CD spectrum is associated with the 1 L b transition, which is caused by the electronic transition along the long axis of the naphthalene ring (Figure [7b]). For the (R)-isomer, this band shows a positive response when the dihedral angle θis acute and a negative response when θis obtuse (Figure [7b]). The CD spectrum reflects the ground-state structure, whereas the CPL spectrum reflects the excited structure. Therefore, the CD and CPL spectra of chiral compounds containing binaphthyl can be used for studying the electronic structures of molecules.

4.1

Cyclic Oligomer of Chiral Binaphthyl

In 2019, we prepared a series of symmetric cyclic oligomers based on (R)- and (S)-binaphthyl. The one-pot Yamamoto coupling of (R)- or (S)-25 using Ni(cod)2 afforded the corresponding chiral macrocycles [n]-26 (n = 2–5, Scheme [6]).[64] The coupling of each enantiomer afforded chiral macrocycles with binaphthyl in the same configuration. Surprisingly, small cyclic compounds of [2]-26, which appear to have the highest distortion among the chiral homologues, were formed.

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Figure 7 (a) The s-cis and s-trans conformer for a 1,1′-binaphthyl framework; (b) theoretical CD spectra of 2,2′-dimethoxy-1,1′-binaphthyl in different dihedral angles calculated by TD-B3LYP/6-31G(d,p) level of theory

These macrocycles exhibit size-dependent chiroptical properties in their CD and CPL spectra (Figure [8]). In the CD spectra of the (R)-isomers of [n]-26 (n = 2–5), a positive response was observed in the first Cotton band corresponding to the 1 L b transition. In contrast, the intensity of [5]-26 was much lower and showed almost cryptochiral features with a small sign inversion to negative (Figure [8a], inset). According to DFT calculations, the dihedral angleθ in [5]-26 is obtuse, where the sign reverses. In these homologues, torsion between the naphthalene moieties was observed at the 7,7′-linkage; however, it did not significantly affect the 1 L b transition. Figure [8b] shows that the CPL of the (R)-isomer was positive for [n]-26 (n = 2–5) and negative for the cyclic pentamer [5]-26. In the excited state, the dihedral angle θis expected to be larger in [5]-26. Hence, the ring size affects the dihedral angle between the binaphthyls and affects the CD/CPL sign and intensity. Thus, we found that the ring size constrained the dihedral angle between the binaphthyls and affected their chiroptical properties. However, the magnitude of the dissymmetry factor |g lum| in the CPL spectra showed a negligible correlation with ring size. This is because the dihedral angle of the binaphthyl group does not change uniformly in the excited state; hence, the molecules are not expected to exhibit D n symmetry.

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Figure 8 (a) CD and (b) CPL spectra of [n]-26 (n = 2–5) with the (S)-configuration. Reproduced from ref. 64 with permission from the Royal Society of Chemistry.

# 4.2

Doubly Twisted Binaphthyl Dimer

Because the one-pot Yamamoto coupling reaction produces a cyclic dimer, we decided to investigate this small, highly distorted cyclic compound. For cyclic homologues, the sign of g lum depends on the ring size without a quantitative correlation. Next, to study the interactions of the local binaphthyl moieties in the excited state, we prepared cyclic binaphthyl dimers 27a–c (Figure [9a]).[65] The dimer comprises four naphthalene rings, forming a figure-eight shape. In recent years, π-extended macrocycles with figure-eight shapes have attracted considerable attention, and the syntheses and properties of these impressive compounds have been reported.[66] [67] [68] [69] [70] [71] [72] [73] These compounds were synthesized in relatively high yields from the precursor via a stepwise synthesis, although the chiral macrocycle possesses inherent distortion. Optical resolution was then achieved by chiral columns.

We found that the coupling reaction with tethered, i.e., linked with –O(CH2)nO–, binaphthyls selectively afforded the dimer in a better yield than that of [2]-26, possibly because of the restricted dihedral angle of the two naphthalene rings in the tethered system. X-ray crystallography and theoretical calculations show that the dihedral angle of naphthalene varies with the length of the bridge, forming a figure-eight shape. Overall, the π-conjugation is doubly twisted and appears to be an ‘impossible figure’ when viewed for the first time (Figure [9b]).

Compounds 27a–c in the (R)-configuration were prepared from the enantiopure tethered precursors of (R)-28 (Scheme [7a]). The Yamamoto coupling reaction afforded the corresponding cyclic compounds in 14–19%, along with a small amount of cyclic trimer. In earlier syntheses, [2]-26 was produced in only 3% yield when the structurally more flexible precursor was used as the starting material. However, these macrocycles were produced in higher yields. X-ray crystallography revealed that (R,R)-27a exhibits a doubly twisted molecular structure with a dihedral angle θ 1 of 52.5° (Scheme [7b], right). The optimized structures had θ 1 values in the following order: 27a (51.6°) < 27b (58.3°) < 27c (63.4°). The θ 1 values depend on the length of tethering. This trend in dihedral angles was also evident in the CD spectra of the solutions. The intensity of the first Cotton effect (Δε) was in the order 27a > 27b > 27c, while the intensity of the molar coefficient (ε) at the longest absorption band in the UV/Vis spectra varied modestly. Thus, the dissymmetry factor g abs of the absorption, determined by Δε/ε, is of the same order as the intensity of the Cotton effect. The maximum |g abs| values for 27a and 27b were 1.4·10–2 and 9.0·10–3, respectively (Table [1]).

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Figure 9 (a) Molecular structures of doubly twisted 27a–c; (b) examples of chiral impossible figure
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Scheme 7 (a) Synthesis of (R,R)-27a–c. Only (R)-isomers are shown. (b) X-ray analysis of (S,S)-27a (CCDC #1988602). Reproduced with permission from ref. 65. Copyright (2021) John Wiley & Sons, Inc.

Table 1 Fluorescence Properties, Lifetimes, and Dissymmetry Factors of |g abs| and |g lum| of [2]-26 and 27a–c a

Compd

PL λmax (nm)

φ PL (%)

τ (ns)

Maximum |g abs|b

Maximum |g lum|

27a

400

12

3.0

1.7·10–2

1.6·10–2

27b

368

28

4.7

0.90·10–2

1.0·10–2

27c

382

36

4.7

0.42·10–2

0.31·10–2

[2]-26

388

31

2.1

0.29·10–2

0.36·10–2

a Measured in CHCl2 at 25 °C.

b Calculated from Δε/ε for the first Cotton effect.

In the CPL spectra, pronounced chiroptical properties were observed for 27a and 27b, with maximum |g lum| values of 0.014 and 0.016, respectively. In contrast, the maximum |g lum| value of compound 27c, which contains a longer tethering units, was 3.4·10–3, which is close to that of [2]-26, which does not contain a tethering unit. The observed |g lum| values of 27a and 27b are much higher than those of ordinary chiral organic compounds. In addition, for most chiral compounds, |g abs| > |g lum|, however, in the present system, these two values are comparable. In other words, the tight tethering of 27a and 27b suppresses the structural relaxation in the excited state, which may lead to high |g lum| values. This was also evidenced by the short fluorescence lifetime and the small difference in the optimized structures of the excited and ground states (Table [1]). In addition, DFT calculations of the excited states showed that in 27a and 27b with shorter tethering, the MO is spread over the entire molecule (Figure [10]). In other words, the excited states are delocalized throughout the molecule; hence, the magnetic transition dipole moment m is longer, leading to a higher g lum value.

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Figure 10 Calculated electronic transition of excited state (S 1 to S 0) and MOs of 27a–c estimated by TD-CAM-B3LP/6-31+G(d,p). The isovalues are set as 0.015 a.u. Reproduced with permission from ref. 65. Copyright (2021) John Wiley & Sons, Inc.

# 4.3

Cyclic Oligomer of Binaphthyl Extended with Paraphenylene

The small figure-eight chiral macrocycles 27a–c showed excellent CPL properties; however, their quantum yields were low and unsatisfactory. The brightness of organic emitters exhibiting CPL is important for future applications such as chiral organic light-emitting diodes (OLEDs). Recently, metrics of brightness for CPL (B CPL) have been proposed to estimate emission performance.[74] These metrics include ε, φ PL, and |g lum|. However, g lum and φ PL are inversely correlated because these parameters are associated with the lengths of the electric (μ) and magnetic (m) transition dipole moments in the electronic transition (Equation 4). Therefore, a molecular design that maintains a fine balance between molecular torsion and π-conjugation length is currently considered to be an effective strategy. Accordingly, we designed π-extended macrocycles anchored by chiral tethered binaphthyl [n]-28 (n = 2–4, Figure [11]).[75] The substituted binaphthyl at the 7,7′-position endows it with excellent CPL properties, thus compensating for the reduction in g lum value because of the π-extension.

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Figure 11 Molecular structures of [n]-28 with S-configuration

These cyclic oligomers were prepared by the Suzuki–Miyaura cross-coupling reaction using the chiral tethered binaphthyl (R)/(S)-30 and 1,4-dibromobenzene (Scheme [8]). Cyclic dimers, trimers, and tetramers with (R)-configurations were obtained in 15%, 8%, and 3% yields, respectively. (S)-Isomers were obtained in similar yields. These yields are not very low for cyclization reactions by cross-coupling, although unidentifiable polymers were formed at the same time.

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Scheme 8 Synthesis of [n]-28 (n = 2–4)

Preparation of single crystals for the X-ray crystallography of these chiral macrocycles was unsuccessful. Instead, the two enantiomers were mixed in equal amounts and recrystallized as racemic compounds to obtain the single crystals of [2]-28 and [4]-28. Compound [2]-28 has a figure-eight shape extended by the phenylene spacer (Figure [12a]). The naphthalene–phenylene–naphthalene (NPN) moieties are slightly distorted. In contrast, the distance between the two phenylenes is sufficiently long to allow free rotation. In compound [4]-28, the binaphthyl units with a small dihedral angle distort the macrocycle structure, and two of the four NPN units curve to form a concave shape (Figure [12b]). Therefore, chiral macrocycles do not have sufficient cavities for guest molecules. The phenylene unit of the straight NPN moiety is disordered, indicating that it can rotate freely.

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Figure 12 ORTEP drawing of (a) racemic [2]-28 (top and side view) (CCDC #2174423) and (b) racemic [4]-28 (CCDC #2174424). Reproduced with permission from ref. 75. Copyright (2022) John Wiley & Sons, Inc.

As expected, compounds [n]-28 were emissive, and their φ PL values were found to be in the 60–79% range. In addition, all cyclic compounds exhibited intensive CPL. The |g lum| values of [2]-28, [3]-28, and [4]-28 were 5.2·10–3, 3.9·10–3, and 4.9·10–3, respectively. Although these values are lower than those of the directly connected dimer 27b, they are still relatively higher than those of conventional chiral organic compounds. The B CPL values of [2]-28, [3]-28, and [4]-28 were 131, 107, and 151, respectively. These values are also higher than those reported for CPL emitters (typically B CPL < 100).[74] The high B CPL values are attributed to the large ε, the high φ PL, and |g lum| derived from the chiral macrocyclic structure with the tethering system.


# 4.4

Curved Helical Paraphenylene Anchoring Chiral Binaphthyl

Recently, several stereogenic molecules with curved paraphenylenes (PPs) units have been reported.[76] [77] [78] [79] [80] Tanaka et al. reported the chiroptical properties of a curved PP system connected to an axially chiral ethynylbinaphthyl or ethynylbiphenyl moiety.[81] Casado and Stępień reported the synthesis and CPL properties of a lemniscate-shaped PP.[82] In addition, Šolomek et al. developed a Möbius aromatic structure using a stereogenic curved PP unit linked to helicene.[83] In 2020, our group reported the synthesis and CPL properties of a curved PP (R)/(S)-31, anchoring a chiral binaphthyl scaffold at the 7,7′-positions.[84] Helicity of the PP units was induced by a chiral binaphthyl scaffold.

Compound 31 was prepared as shown in Scheme [9]. We used racemic 32 and 33 to produce the acyclic precursor 34 via Suzuki coupling. An intramolecular ring-closure reaction using Ni(cod)2 afforded compound 35. After the deprotection of triethylsilyl substituents from 1,4-cyclohexene diol, a reductive aromatization reaction using SnCl2 afforded racemic 31 in 78% yield. Optical resolution by chiral HPLC yielded enantiomers of (R)- and (S)-31.

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Scheme 9 Synthesis and optical resolution of (R)/(S)-31

X-ray analysis indicated that the PP unit was distorted by anchoring the chiral binaphthyl at the 7,7′-positions. The isomers containing R- and S-binaphthyl lead to M and P helicity in the PP moiety, respectively. These enantiomers are highly emissive in solution, powder (dispersed in KBr pellets), and poly methyl methacrylate (PMMA) films (Figure [13a]). Although the φ PL in the solid state (15%) and CH2Cl2 solution (17%) were comparable, the φ FL in the PMMA film was higher (47%) owing to the suppression of molecular motion in the polymer matrix. In the CD spectra, 31 with the (R)-configuration exhibited a negative sign in the first Cotton effect region. This band corresponds to the electronic transition from the HOMO to the LUMO, where both MOs are mainly located at the helical PP moiety. While similar CPL spectra were observed for the solution and PMMA films, the spectra of the powder were red-shifted compared to these spectra. The g lum values in CH2Cl2 solution and PMMA film were comparable (4.3·10–3 to 4.4·10–3) but higher than those in the solid state (1.5·10–3, Figure [13b]). Notably, the PMMA film improved the quantum yield while maintaining |g lum|. The optimized structure in the excited state by DFT calculations (B3LYP/6-31G(d,p)) exhibited tight curvature in the PP units compared to that in the ground state (Figure [13c]). In addition, the MOs of π and π* orbitals are spread over the curved PP moiety, and hence the excitons are delocalized over them (Figure [13d]).

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Figure 13 (a) Photograph of (S)-31 in CH2Cl2 solution (1.2 mM), powder, PMMA film under irradiation of 345 nm. (b) CPL spectra of (S)-31. (c) Superposition of calculated molecular structure in the ground (grey) and excited (green) states. (d) MOs of π (HOMO, #202) and π* (LUMO, #203) in the optimized structure in the excited state (S 1) calculated by TD-CAM-B3LYP/6-31+G(d,p). Reproduced with permission from ref. 84. Copyright (2021) John Wiley & Sons, Inc.

# 4.5

Binaphthyl-Hinged [5]-Helicene

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Figure 14 Molecular structures of (R)-/(S)-31a,b

Recently, we synthesized chiral macrocycles 37a,b, in which helicenes were combined with tethered binaphthyl, based on the fact that binaphthyl induces chirality (Figure [14]).[85] [5]-Helicene slowly racemizes at room temperature.[86] [87] In 2004, Marinetti et al. reported that a chiral 1,3-pentandiol tether at the end of [5]-helicene prevents epimerization and induces chirality in the helicene moiety.[88] However, except for optical rotation, the chiroptical properties of these stabilized helicenes have rarely been investigated. Our molecular design of compounds 37a,b shows that the tethered/untethered binaphthyl group induces chirality in helicene. Furthermore, the C 2-symmetric structures in the ground and excited states are expected to have high asymmetry factors (g abs and g lum).

Each enantiomer was synthesized individually, as shown in Scheme [10]. The Suzuki–Miyaura cross-coupling of (R)-28b with boronic acid afforded (R)-38. Subsequently, a ring-closure Wittig reaction under high-dilution conditions using a syringe pump afforded (R)-39 as the major product. Photocyclization of tethered (R)-39 under a high-pressure Hg lamp (450 W) afforded (R)-37a in 71% yield. In contrast, the synthesis of (R)-37b, which is an untethered binaphthyl, via a similar route was unsuccessful when the methoxy derivative of 38 was used as the starting material. Hence, the –CH2CH2– moiety of (R)-37a was removed using BBr3 and methylated to yield (R)-37b.

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Scheme 10 Synthesis of (R)-37a,b

X-ray crystallography of 37a was performed in the racemic form after mixing both enantiomers. As expected, the chirality of the binaphthyl group produced a distinct helicity. For instance, (R)-binaphthyl afforded a [5]-helicene with M-helicity, and (S)-binaphthyl afforded a [5]-helicene with P-helicity (Figure [15]). The structure of the helicene moiety was shrink compared to the parent [5]-helicene owing to anchoring by the binaphthyl unit.

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Figure 15 X-ray analysis of 37a. Side view (left) and top view with dihedral angles θ 1–θ 4 (right). Only (R)-isomers are shown. (CCDC #2219716). Angles and distance: θ 1 = –61.5, θ 2 = 104.5°, θ 3 = –29.2°. Reproduced with permission from ref. 84. Copyright (2023) John Wiley & Sons, Inc.

Figure [16] shows the UV/Vis, CD, PL, and CPL spectra of (R)-37a. The absorption spectrum of 37 exhibited a weak band and the corresponding Cotton effect. This band was characteristic of [5]-helicene derivatives. This absorption was attributed to the electronic transition from the HOMO to the LUMO+1. These molecular orbitals were mainly located in the helicene region. The g abs of this band was 7.1·10–3, almost similar to that of (R)-37b (Table [2]), but higher than that of common organic compounds. Compound 37 displays fluorescence in CHCl3, and its fluorescence lifetime is in the range of 1.2–8.2 ns, which is much shorter than that of [5]-helicene (26 ns). This is caused by the rigid structure of 37, which suppresses structural relaxation. Similar behavior was observed for the binaphthyl dimer described above. Both 37a and 37b exhibited intense CPL spectra within the same range as the fluorescence spectra. The absorption maximum of (R)-37a was observed at 412 nm with g lum = 6.0·10–3. The highest g lum value of (R)-37a was found to be 7.4·10–3 at 402 nm. These observed dissymmetry factors are larger than those of typical helicenes and [5]-helicenes (|g lum| = 2.7·10–3).

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Figure 16 Optical properties of (R)-37a in CHCl3: (a) UV (dash line) and PL (solid line) spectra; (b) CD (dash line) and CPL (solid line) spectra. Reproduced with permission from ref. 84. Copyright (2023) John Wiley & Sons, Inc.

Table 2 Fluorescence Properties, Lifetimes, and Dissymmetry Factors of |g abs| and |g lum| of (R)-37a and (R)-37b a

Compd

PL λmax (nm)

φ PL (%)

τ (ns)

Maximum |g abs|b

Maximum |g lum|

(R)-37a

412, 431

3.3

1.2, 8.2

7.1·10–3

6.0·10–3

7.4·10–3 c

(R)-37b

412, 431

3.0

1.4, 6.9

7.3·10–3

6.8·10–3

8.0·10–3 c

a Measured in CHCl3 at 25 °C.

b Calculated from Δε/ε for the first Cotton effect.

c Maximum g lum value.

The molecular structure of the excited state calculated at the ωB97XD/6-31+G(d,p) level of theory is shown in Figure [17a]. This structure was not remarkably different from that observed in the ground state. The C 2 axis is placed in the long-axis direction; hence, the electric and magnetic dipole moments are parallel to each other (Figure [17b]). This enhanced the g lum value in the CPL spectra. Studies using this molecular design strategy to develop new CPL emitters are underway in our laboratory.

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Figure 17 (a) Superposition of calculated molecular structure of (R)-37a in the ground (grey) and excited (orange) states. (b) Magnetic (m) and electric (μ) transition dipole moment in (R)-37a calculated by ωB97XD/6-31+G(d,p). Reproduced with permission from ref. 84. Copyright (2023) John Wiley & Sons, Inc.

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# 5

Summary

In summary, several studies have shown that stereogenic macrocycles composed of π-conjugated units have aesthetically pleasing structures and exhibit excellent chiroptical properties (CD and CPL). These macrocycles have a highly symmetrical and shape-persistent structure, making this class of compounds the most suitable for evaluating the structure–property relationships in chiroptical properties. For chiral macrocycles containing multiple planar chiral [2.2]PCs, the interactions between the torsional π-conjugated units were evaluated using CD spectroscopy. Substitution at the pseudo-ortho positions in [2.2]PC afforded a twisted structure in the π-conjugated unit. This structure also exhibited chiroptical properties derived from the π-conjugated moieties. In addition, the structures of binaphthyl cyclic oligomers vary with ring size, which is useful for the qualitative evaluation of molecular structures in both the ground and excited states using CD and CPL spectra, respectively. Tightly linked binaphthyl dimers with figure-eight structures achieved high g lum values of 10–2 orders of magnitude owing to the suppression of structural relaxation. We also applied this strategy to synthesize π-extended chiral macrocycles that exhibited excellent CPL properties. Macrocycles connected to paraphenylenes (PP) provide chiroptical properties derived from the helical structure of the PP unit. In a macrocycle comprising helicene and binaphthyl, the helicity of the helicene moiety is induced by the chirality of binaphthyl. This macrocycle exhibited relatively high |g abs| and |g lum| values because of its C 2-symmetric structure in both the ground and excited states.

As described in this Account, our recent studies were motivated by the synthetic challenges and developing chiroptical materials. The synthesis of chiral macrocycles containing inherent distortion could be overcome by one-pot synthesis, although it competes with polymers. However, for applications such as OLEDs, scalable synthesis of multigram quantities is required. Selective synthesis will be necessary by designing precursors that are suitable for cyclization reactions. Thanks to the rigid and symmetrical structure, some macrocycles showed higher g abs and g lum values than common organic chiral compounds. This means that the polarization of circularly polarized light is increased in both absorption and emission. However, further research on chiroptical properties is essential to provide rational guidance for molecular design. We believe that chiral macrocycles with excellent chiroptical properties will emerge in the future, leading to breakthroughs in materials science.


#
#

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

M.H. would like to express my sincere gratitude to ‘chiral team’ in Kitasato University. We would also like to thank Dr. Yuki Nojima, Dr. Kosuke Kobayakawa, Wanli Xiao, Hikari Kawashima, Chika Hasegawa, Yuki Ishida, Kenta Sato, Sumire Ishioka, Yuta Okiyama, Hiroaki Sasaki, Masataka Osono, Miku Mizoe, Sae Nibu, Takehiro Shimada, and Momoko Abe for their dedicated work and Dr. Masafumi Ueda for his helpful discussion. We also thank our collaborators: Prof. Dr. Yoshitane Imai (Kindai University), Prof. Dr. Ken-ichi Sugiura (Tokyo Metropolitan University), Prof. Dr. Kazunori Tsubaki (Kyoto Prefectural University), Prof. Dr. Kazuteru Usui (Showa Pharmaceutical University), and Dr. Tomohiko Nishiuchi (Osaka University). All the calculations were performed at the Research Center for Computational Science (Okazaki, Japan).


Corresponding Author

Masashi Hasegawa
Department of Chemistry, Graduate School of Science, Kitasato University
Sagamihara, 252-0373, Kanagawa
Japan   

Publication History

Received: 13 August 2023

Accepted after revision: 22 August 2023

Accepted Manuscript online:
22 August 2023

Article published online:
16 October 2023

© 2023. Thieme. All rights reserved

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Figure 1 Chiral compounds with symmetric structures. (a) Chiral macrocycles exhibiting exceptional chiroptical properties reported by Diederich et al. (b) Acyclic chiral compounds containing tetrathiafulvalene(s).
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Figure 2 (a) Schematic illustration of the CD (left) and CPL (right) phenomena. (b) Electronic transition diagram of CD (left) and CPL (right).
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Equation 1
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Equation 2
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Equation 3
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Equation 4
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Scheme 1 Scalable synthesis of racemic pseudo-ortho-diboromo[2.2]paracyclophanes[10]
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Figure 3 Molecular structures of (R p,R p)-8 and (S p,S p)-8.
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Scheme 2 (a) Synthesis and optical resolution of 8. (b) Molecular structure of reference compound of 9.
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Figure 4 (a) Electronic spectra of oxidized species in CH2Cl2. (b) CD spectra of (R p)-isomers in CH2Cl2. The sharp noise at 750 and 1700 nm is due to the photo multimeter artifact. (c) Schematic illustration of chiral π-dimer in 8 2+.
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Scheme 3 Synthesis of (R p,R p)/(S p,S p)-16. The molecular structures of (R p)-isomers are only shown for clarity.
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Scheme 4 Synthesis of (R p,R p)/(S p,S p)-[2]-19 and (R p,R p,R p)/(S p,S p,S p)-[3]-19
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Figure 5 X-ray structure of (a) (S p,S p)-[2]-19 and (b)(R p,R p,R p)-[3]-19. (CCDC # 2069162 and 2069163). Reproduced with permission from ref. 46. Copyright (2021) John Wiley & Sons, Inc.
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Figure 6 Optical properties of (R p,R p)-[2]-19 and (R p,R p,R p)-[3]-19. (a) UV (dashed line) and PL (solid line) spectra. (b) CD (dashed line) and CPL (solid line) spectra. Reproduced with permission from ref. 46. Copyright (2021) John Wiley & Sons, Inc.
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Scheme 5 (a) (S)- and (R)-BINOL; (b) modified method for the optical resolution of 7,7′-dibromo-1,1′-bi-2-naphthol on a multigram scale
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Scheme 6 (a) Synthesis of [n]-26 (n = 2–5); (b) optimized molecular structures of [n]-26 (n = 2–5) in (R)-configuration, calculated by B3LYP/6-31G(d,p). Reproduced from ref. 64 with permission from the Royal Society of Chemistry.
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Figure 7 (a) The s-cis and s-trans conformer for a 1,1′-binaphthyl framework; (b) theoretical CD spectra of 2,2′-dimethoxy-1,1′-binaphthyl in different dihedral angles calculated by TD-B3LYP/6-31G(d,p) level of theory
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Figure 8 (a) CD and (b) CPL spectra of [n]-26 (n = 2–5) with the (S)-configuration. Reproduced from ref. 64 with permission from the Royal Society of Chemistry.
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Figure 9 (a) Molecular structures of doubly twisted 27a–c; (b) examples of chiral impossible figure
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Scheme 7 (a) Synthesis of (R,R)-27a–c. Only (R)-isomers are shown. (b) X-ray analysis of (S,S)-27a (CCDC #1988602). Reproduced with permission from ref. 65. Copyright (2021) John Wiley & Sons, Inc.
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Figure 10 Calculated electronic transition of excited state (S 1 to S 0) and MOs of 27a–c estimated by TD-CAM-B3LP/6-31+G(d,p). The isovalues are set as 0.015 a.u. Reproduced with permission from ref. 65. Copyright (2021) John Wiley & Sons, Inc.
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Figure 11 Molecular structures of [n]-28 with S-configuration
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Scheme 8 Synthesis of [n]-28 (n = 2–4)
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Figure 12 ORTEP drawing of (a) racemic [2]-28 (top and side view) (CCDC #2174423) and (b) racemic [4]-28 (CCDC #2174424). Reproduced with permission from ref. 75. Copyright (2022) John Wiley & Sons, Inc.
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Scheme 9 Synthesis and optical resolution of (R)/(S)-31
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Figure 13 (a) Photograph of (S)-31 in CH2Cl2 solution (1.2 mM), powder, PMMA film under irradiation of 345 nm. (b) CPL spectra of (S)-31. (c) Superposition of calculated molecular structure in the ground (grey) and excited (green) states. (d) MOs of π (HOMO, #202) and π* (LUMO, #203) in the optimized structure in the excited state (S 1) calculated by TD-CAM-B3LYP/6-31+G(d,p). Reproduced with permission from ref. 84. Copyright (2021) John Wiley & Sons, Inc.
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Figure 14 Molecular structures of (R)-/(S)-31a,b
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Scheme 10 Synthesis of (R)-37a,b
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Figure 15 X-ray analysis of 37a. Side view (left) and top view with dihedral angles θ 1–θ 4 (right). Only (R)-isomers are shown. (CCDC #2219716). Angles and distance: θ 1 = –61.5, θ 2 = 104.5°, θ 3 = –29.2°. Reproduced with permission from ref. 84. Copyright (2023) John Wiley & Sons, Inc.
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Figure 16 Optical properties of (R)-37a in CHCl3: (a) UV (dash line) and PL (solid line) spectra; (b) CD (dash line) and CPL (solid line) spectra. Reproduced with permission from ref. 84. Copyright (2023) John Wiley & Sons, Inc.
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Figure 17 (a) Superposition of calculated molecular structure of (R)-37a in the ground (grey) and excited (orange) states. (b) Magnetic (m) and electric (μ) transition dipole moment in (R)-37a calculated by ωB97XD/6-31+G(d,p). Reproduced with permission from ref. 84. Copyright (2023) John Wiley & Sons, Inc.