**2. CD and CPL from Achiral and Racemic Molecules by Spontaneous Resolution**

Chiral organic luminophores are typically prepared via several steps from chiral molecules. Unfortunately, few chiral starting molecules are readily available. In addition, commercial chiral

molecules are generally more expensive than achiral or racemic molecules. To eliminate such concerns, it would be economically and industrially highly useful if a chiral organic luminophore could be produced via an achiral or racemic compound.

The origin and amplification of chirality, which has led to an overwhelming enantio-enrichment of organic molecules on Earth, has been a significant topic of interest in this field of science for many decades. One proposed hypothesis for the origin of chirality is the spontaneous resolution of one of the two possible enantiomers of chiral crystals from achiral or racemic molecules [21–30]. For example, 2-anthracenecarboxylic acid (**1**) is an achiral luminescent molecule, and racemic 1-phenylethylamine (*rac*-**2**) is a racemic molecule (an equimolar mixture of (*R*)- and (*S*)-1-phenylethylamine) (Figure 1). When a mixture of **1** and *rac*-**2** is crystallized from solution, the chiral supramolecular organic luminophores **I** or **I** can be preferentially obtained by spontaneous resolution. Chiral luminophores **I** and **I** are an enantiomeric pair and are composed of **1** and (*R*)-**2** for **I**, and **1** and (*S*)-**2** for **I**- . **I** or **I** can be obtained selectively by using the corresponding seed crystals.

**Figure 1.** Achiral and racemic component molecules for chiral organic luminophores.

Unlike many organic luminophores, chiral luminophore **I** (or **I**- ) is able to emit luminescence even in the solid state without aggregate quenching. The solid-state luminescences of **I** and **I** are shifted to a shorter wavelength (by 34 nm) relative to that of their component luminescent molecule **1**. The solid-state luminescent maximum (λPL) of **I** (or **I**- ) is 430 nm. The photoluminescence quantum yield (ΦF) increases from 4% for **1** to 20% for **I**. The solid-state CD spectra of **I** (indicated by the red line) and **I**- (indicated by the blue line) are mirror images (Figure 2). The CD signals derived from the fluorescent anthracene unit clearly appear between 330 and 450 nm. The circular anisotropy factor (*g*CD) of the major CD Cotton band (λCD <sup>=</sup> 404 nm) of **<sup>I</sup>** is approximately <sup>−</sup>0.6 <sup>×</sup> 10–3. This shows that the fluorescent anthracene units effectively exist in a chiral environment in the solid state.

**Figure 2.** Solid-state circular dichroism (CD) spectra of luminophores **I** (red line) and **I**- (blue line) in the solid state (KBr pellets).

As expected, X-ray analysis shows that luminophore **I** has a *P*21 chiral space group and a characteristic 21-helical network column (Figure 3a). This characteristic column is mainly formed via ionic and hydrogen bonds composed of the carboxylic acid anions of **1** and the protonated amine cations of (*R*)-**2** (**1**:(*R*)-**2** = 1:1 component ratio). Luminophore **I** is constructed via the aggregation of these 21-helical network columns by three types of edge-to-face interactions: anthracene–anthracene edge-to-face, benzene–anthracene edge-to-face, and anthracene–benzene edge-to-face interactions (Figure 3b). This suggests that the formation of a 21-helical network column is a key factor in the production of spontaneously resolved chiral organic luminophores [31].

**Figure 3.** Crystal structures of luminophore **I**. (**a**) 21-Helical columnar network structure along the *b*-axis. (**b**) Packing structure observed along the *b*-axis.

Chiral organic luminophore **I** (or **I**- ) was successfully prepared by combining an achiral fluorescent molecule and a racemic molecule as a spontaneous-resolution system. Thus, by using only achiral molecules, chiral organic luminophores were formed.

When a mixture of the achiral luminescent molecule 2-anthracenecarboxylic acid (**1**) and the achiral molecule benzylamine (**3**) (Figure 4) is crystallized from solution at room temperature, spontaneous resolution results in the chiral supramolecular organic luminophore **II** or **II**- **,** which are composed of **1** and **3** with opposite chirality. In this case, a small amount of **III,** another polymorphic luminophore, is also obtained. Luminophores **II** or **II** can also be selectively obtained by using the corresponding seed crystals.

**Figure 4.** Achiral component molecules for circularly polarized luminescence (CPL).

As expected, chiral luminophore **II** also has an anion–cation and hydrogen bonded 21-helical network column, similar to that of the spontaneously resolved chiral luminophore **I** (Figure 5a). The stoichiometry of components **1** and **3** in luminophore **II** is 1:1, and the space group is chiral (*P*21). Each network column is held in place through two types of edge-to-face interactions: anthracene–anthracene edge-to-face and benzene–anthracene edge-to-face interactions. The luminophore is created by the aggregation of this network column (Figure 5b).

**Figure 5.** Crystal structures of luminophore **II**. (**a**) 21-Helical columnar network along the *b*-axis. (**b**) Packing structure observed along the *b*-axis.

Chiral luminophores **II** and **II** show CD in the solid state (Figure 6). Characteristic mirror-image CD spectra of **II** and **II** are observed with a major peak around 416 nm (Figure 6). This peak originated from the fluorescent anthracene unit. The *g*CD value of the major CD Cotton band (λCD = 416 nm) of **II** is approximately <sup>−</sup>1.0 <sup>×</sup> <sup>10</sup>–3.

**Figure 6.** Solid-state CD spectra of luminophores **II** (red line) and **II**- (blue line) in the solid state (KBr pellets).

Luminophore **II** exhibits luminescence with a solid-state luminescent maximum (λPL) of 446 nm and a photoluminescence quantum yield (ΦF) of 16%, which is four times greater than that of the component luminophore **1** in the solid state. Interestingly, **II**, having a negative Cotton CD band, emits negative CPL with a circular anisotropy factor (Kuhn's dissymmetry ratio: *g*CPL) of approximately <sup>−</sup>1.1 <sup>×</sup> <sup>10</sup>–3, even though **II** is composed of achiral molecules (Figure 7). It can be concluded from the crystal structure and the theory of oscillator coupling that the CPL originates from the inter-columnar anthracene units between adjoining 21-helical columns.

**Figure 7.** Solid-state CPL and the unpolarized photoluminescence (PL) spectra of luminophore **II** in the solid state (KBr pellet).

In this system, the production of a small amount of polymorphic luminophore **III** is an issue. Notably, this problem can be solved by changing the crystallization method. When solid-state luminescent molecule **1** is left to stand in the vapor of liquid molecule **3** at room temperature, only **III** is selectively obtained. In contrast, when solid **1** and liquid **3** are directly mixed and ground using an agate mortar, only chiral **II** or **II**is produced.

Because many chiral molecules are not easily available and chiral molecules are more expensive than achiral or racemic molecules, chiral organic luminophores prepared from achiral or racemic molecules are preferred as industrial chiral luminophores. This study provides useful information for the creation of new spontaneously resolved CPL luminescent systems without using chiral factors [32].

#### **3. CPL from Optically Inactive Organometallic and Organic Luminophores under a Magnetic Field**

Organometallic luminophores have been a focus of CPL materials. Optically active lanthanide luminophores coordinated with chiral organic ligands in particular show an extremely narrow full-width at half-maximum CPL, with a high circular anisotropy factor from the visible to near-infrared regions [33–38]. In order to impart CPL characteristics to lanthanide luminophores, a chirality-inducible ligand causing antenna effects through ligand-to-metal charge transfer is inevitably needed. In recent

years, an external static magnetic field has been used to act as a versatile chirality-inducible physical force in the ground and excited states, and to perturb chiral electronic structures toward several optically inactive and achiral luminophores [39–46]. Such an external magnetic influence at the molecular level causes the optically inactive luminophore to emit MCPL. In this section, the MCPL capabilities of optically inactive lanthanide luminophores Eu(III)(hfa)3 and Tb(III)(hfa)3 (Figure 8) are discussed.

**Figure 8.** Optically inactive Eu(III)(hfa)3 and Tb(III)(hfa)3.

CPL was not observed for Eu(III)(hfa)3 in the absence of an external magnetic field. In contrast, Eu(III)(hfa)3 can emit MCPL and magnetic-field-induced unpolarized photoluminescence (hereafter denoted as PL, rather than MCPL) in CHCl3 and acetone solutions under a 1.6 T magnetic field, as shown in Figure 9 (blue lines for CHCl3 and red lines for acetone). The spectra under the N→S (N-up) magnetic field along the direction of the excitation light are shown via solid lines, and the spectra under the S→N (S-up) magnetic field along the direction of the excitation light are shown via dotted lines.

**Figure 9.** Magnetic circularly polarized luminescence (MCPL) (upper panel) and magnetic-field-induced unpolarized photoluminescence (PL) (lower panel) spectra of Eu(III)(hfa)3 in N→S (solid lines) and S→N (dotted lines) configurations under a 1.6 T magnetic field in CHCl3 (blue lines) and acetone (red lines) solutions (1.0 <sup>×</sup> <sup>10</sup>−<sup>3</sup> M).

The MCPL spectra (upper panel in Figure 9) of Eu(III)(hfa)3 in the two solutions are similar. The meaningful MCPL peaks (λMCPL) observed were ≈ 587, 596, 611, 621, 690 and 703 nm. They correspond to the 4f–4f transitions of Eu(III). The N→S magnetic field MCPL spectrum and S→N magnetic field MCPL spectrum are almost mirror images of each other. They show that the MCPL sign can be attributed to the direction of applied external magnetic field (N→S or S→N).

To discuss the quantitative MCPL efficiency, the *g*MCPL values are used. The *g*MCPL factor is defined as *g*MCPL = 2(*I*<sup>L</sup> <sup>−</sup> *I*R)/(*I*<sup>L</sup> + *I*R), and normalized by an external magnetic field (T–1). In this equation, *I*<sup>L</sup> and *I*<sup>R</sup> show the left- and right-handed apparent MCPL intensities, respectively, when photoexcited by unpolarized light. The <sup>|</sup>*g*MCPL<sup>|</sup> values of the main bands in CHCl3 were 0.81 <sup>×</sup> <sup>10</sup>−<sup>2</sup> <sup>T</sup>−<sup>1</sup> at 597 nm and 0.63 <sup>×</sup> <sup>10</sup>−<sup>2</sup> <sup>T</sup>−<sup>1</sup> at 611 nm. The <sup>|</sup>*g*MCPL<sup>|</sup> values of the two solutions are similar. These results indicate that the external static magnetic field can clearly induce CPL from optically inactive Eu(III)(hfa)3, or possibly a mixture of the Δ- and Λ-forms of *D*3-symmetric Eu(III)(hfa)3, in the solution state. The MCPL of

Eu(III)(hfa)3 was further investigated in three types of solid states—polymethylmethacrylate (PMMA) film, KBr pellet, and powder—under the same 1.6 T magnetic field as was used for the solution state measurements. The MCPL emitted from these solid-state luminophores was practically the same as that from the solution state.

Similarly, the 1.6 T magnetic field induces CPL in optically inactive Tb(III)(hfa)3 luminophores, or possibly a mixture of the Δ- and Λ-forms of *D*3-symmetric Tb(III) luminophores, in CHCl3 and acetone solutions. The N→S and S→N magnetic fields can control the sign of the MCPL of the Tb(III) luminophores, as shown in Figure 10 (blue lines for CHCl3 and red lines for acetone; solid lines for N→S and dotted lines for S→N configurations under 1.6 T). The λMCPL maxima and the |*g*MCPL| values of Tb(III)(hfa)3 in CHCl3 are 0.53 <sup>×</sup> 10–2 T−<sup>1</sup> at 484 nm, 0.46 <sup>×</sup> 10–2 T−<sup>1</sup> at 493 nm, 0.16 <sup>×</sup> 10–2 T−<sup>1</sup> at 538 nm, and 0.094 <sup>×</sup> 10–2 T−<sup>1</sup> at 553 nm, which correspond to the characteristic 4f–4f transitions of Tb(III) in CHCl3. The |*g*MCPL| values of the CHCl3 and acetone solutions are similar.

**Figure 10.** MCPL (upper panel) and PL (lower panel) spectra of Tb(III)(hfa)3 in N→S (solid lines) and S→N (dotted lines) magnetic directions under a 1.6 T magnetic field in CHCl3 (blue lines) and acetone (red lines) solutions (1.0 <sup>×</sup> <sup>10</sup>−<sup>3</sup> M).

Interestingly, the signs of the MCPL spectra of Tb(III)(hfa)3 between the CHCl3 and acetone solutions are partially inverted in the N→S and S→N magnetic fields. In CHCl3, the N→S magnetic field spectrum clearly shows positive(+)-/negative(−)-sign MCPL signals at 538/553 nm, derived from the 5D4→7F5 transitions. In contrast, the signs of the MCPL signals derived from the same 5D4→7F5 transitions are reversed in acetone, and are clearly negative(−)-/positive(+)-sign MCPL. This sign inversion may be caused by the different coordination environment around Tb(III) of lone pair electrons on the C=O oxygen of acetone. It appears from these results that the direction of rotation of the MCPL from Tb(III)(hfa)3 can be controlled by both the magnetic field direction and the nature of the solvent molecule. Similar to the chiroptical characteristics of Eu(III)(hfa)3, MCPL from Tb(III)(hfa)3 can also be emitted in solid states, such as a PMMA film, KBr pellet and powder, under a 1.6 T magnetic field [47].

Optically active π-conjugated organic luminophores demonstrating CPL are attracting attention in the fields of chiral photonic and optoelectronic materials science. Most organic CPL luminophores are prepared from chiral organic starting materials. Here, we describe CPL emitted from an achiral organic luminophore under an external magnetic field. Three typical π-conjugated luminophores—pyrene (**Py**), 1-pyrenol (**1-PyOH**) and 2-pyrenol (**2-PyOH**)—which do not possess any chiral sources were investigated (Figure 11).

**Figure 11.** Achiral luminophores **Py**, **1-PyOH**, and **2-PyOH**.

Surprisingly, these achiral pyrene luminophores clearly emit mirror-image MCPLs under a 1.6 T magnetic field in CHCl3 solution when Faraday-type N→S or S→N geometry is employed, similarly to lanthanoid luminophores, as shown in Figure 12 (solid lines for the N→S magnetic field and dotted lines for the S→N magnetic field). In addition, the signs of the MCPL can be controlled by the position (1- or 2-position) of the OH group on the peripheral pyrene ring under the same N→S or S→N geometry.

**Figure 12.** MCPL (upper panel) and PL (lower panel) spectra of **Py** (red lines), **1-PyOH** (green lines), and **2-PyOH** (blue lines) in N→S (solid lines) and S→N (dotted lines) magnetic directions under a 1.6 T magnetic field in CHCl3 solution (1.0 <sup>×</sup> 10–4 M).

In **Py**, luminescence peaks (λMCPL) can be observed at 374, 393 and 416 nm. These correspond to the 0–0 , 0–1 and 0–2 vibronic PL bands of the pyrenyl monomer species. The luminescence peaks of **1-PyOH** and **2-PyOH** are similar to those of **Py**. The |*g*MCPL| value of **Py** in CHCl3 is 0.82 <sup>×</sup> 10−<sup>3</sup> T−<sup>1</sup> at 374 nm. Pyrene has monomer and excimer luminescence. In the three pyrenes, the pyrenyl excimer-origin MCPL cannot be detected around 450–500 nm even at the high concentration of 1.0 <sup>×</sup> 10–2 M. This indicates that pyrene excimers in the excited state apparently do not contribute to MCPL.

By using this method, various magnetic circularly polarized luminophores can be prepared from optically inactive and achiral molecules. In the future, various achiral luminophores sandwiched by two permanent magnetic fields should be able to emit MCPL from the visible to near-infrared wavelength regions, and the sign of the MCPL could be modulated by the alternating current magnetic field [48].

#### **4. CPL from Cryptochiral Organic Luminophores**

In 1977, Mislow reported on cryptochirality [49,50], which means hidden molecular chirality. This novel and unique chiroptical phenomenon contributes to molecular cryptography, steganography and watermarks. The most reported chiral CPL luminophores are CD-active/CPL-active luminophores. To overcome this limitation, [(4*R*,5*R*)-2,2-dimethyl-1,3-dioxolane-4,5-diyl]bis-methanolyl-bis-1-pyrene [(*R*,*R*)-**4**] and its enantiomer [(*S*,*S*)-**4**] were designed (Figure 13). These luminophores can be prepared from the corresponding enantiomerically pure 2,3-*O*-isopropylidene-threitol and 1-pyreneacetic acid in a single step.

**Figure 13.** Cryptochiral dioxolane-bipyrene luminophores (*R*,*R*)-**4** and (*S*,*S*)-**4**.

A characteristic of these luminophores is that the chiral 2,2-dimethyl-1,3-dioxolane unit acts as a chiral inducer and conductor to induce chirality in the two distant fluorescent pyrene moieties. The two pyrenes are far from the central chirality of the 2,2-dimethyl-1,3-dioxolane unit and act as a hidden chiral excimer derived from photoexcitation, which is the heart of cryptochirality. Under the influence of this conductor, **4** can induce a preferential chiral twist of the fluorescent pyrenes upon an external bias on demand.

As shown in the lower panel of Figure 14, the UV–vis absorption spectrum of (*R*,*R*)-**4** (red line) (or (*S*,*S*)-**4** (blue line)) shows two major well-resolved vibronic π−π\* transitions of isolated pyrene in the 250–290 and 300–370 nm regions in CHCl3 solution. However, (*R*,*R*)-**4** (or (*S*,*S*)-**4**) does not provide any meaningful CD signals (Figure 14, upper panel). In contrast, (*R*,*R*)-**4** (red line) (or (*S*,*S*)-**4** (blue line)) can clearly emit CPL at 460 nm with an 18% Φ<sup>F</sup> value, originating from the excimer pyrene in CHCl3 solution (Figure 15, upper panel). Luminophores (*R*,*R*)-**4** and (*S*,*S*)-**4** provide nearly mirror-image CPL spectra. Their <sup>|</sup>*g*CPL<sup>|</sup> value is ~8.9 <sup>×</sup> <sup>10</sup>−<sup>4</sup> at approximately 460 nm in CHCl3.

**Figure 14.** CD (upper panel) and UV–vis absorption (lower panel) spectra of (*R*,*R*)-**4** (red lines) and (*S*,*S*)-**<sup>4</sup>** (blue lines) in CHCl3 solution (1.0 <sup>×</sup> <sup>10</sup>–4 M).

**Figure 15.** CPL (upper panel) and PL (lower panel) spectra of (*R*,*R*)-**4** (red lines) and (*S*,*S*)-**4** (blue lines) in CHCl3 solution (1.0 <sup>×</sup> <sup>10</sup>–4 M).

The most probable structures of (*R*,*R*)-**4** in both the ground and photoexcited states were calculated. From these calculations, a model of this mechanism was developed and is shown in Figure 16. The structure obtained for the ground state does not adopt face-to-face pyrenyl π-π stacking conformers or slip pyrenyl π−π dimer conformers, but adopts the T-shaped conformers of pyrenes. These unique pyrene conformations may be the reason for the nonexistent or weak CD signals in the ground state. In contrast, (*R*,*R*)-**4** adopts chiral pyrenyl π−π-stacked conformers in the photoexcited state. Thus, the CPL magnitudes may be enhanced by the chiral π−π pyrenyl stacks, providing chiroptically detectable signals [51].

**Figure 16.** Mechanism of cryptochiral CPL system.

Based on this mechanism, instead of using a central chiral 2,2-dimethyl-1,3-dioxolane unit that acts as a controller for the expression of chirality of two pyrenes, a cryptochiral CD-silent/CPL-active luminophore was designed using an axially chiral binaphthyl unit. Chiral luminophores (*R*)-**5** and (*S*)-**5** are composed of the corresponding chiral 5,5 ,6,6 ,7,7 ,8,8 -octahydro-1,1 -bi-2-naphthyl as the chiral unit, and two pyrenes as fluorescent units connected through flexible ester tethers (Figure 17).

**Figure 17.** Cryptochiral binaphthyl-bipyrene luminophores (*R*)-**5** and (*S*)-**5**.

To study the ground state chiroptical properties of **5**, the CD and UV–vis absorption spectra of (*R*)-**5** and (*S*)-**5** were measured, in the same manner as **4** (Figure 18). In the UV–vis absorption spectra, three main π-π*\** vibronic transitions (1La transitions) derived from two pyrene moieties were observed between 315 and 360 nm. On the other hand, the intensities of the CD signals of **5,** corresponding to these UV bands, were noticeably weak.

The PL and CPL spectra of (*R*)-**5** and (*S*)-**5** in CHCl3 are shown in Figure 19. From luminophore **5**, strong excimer PL is emitted at 468 nm (λPL) from the pyrenes. In addition, luminophores (*R*)-**5** and (*S*)-**5** emit clear CPL, and each CPL sign is positive and negative, respectively. The absolute *g*CPL value for (*R*)-**<sup>5</sup>** is +2.5 <sup>×</sup> 10−<sup>3</sup> at 454 nm. This suggests that because of the moderately controlled flexible framework, chiral luminophore **5** functions as a cryptochiral CPL system whose CD properties cannot be detected.

**Figure 18.** CD (upper panel) and UV–vis absorption (lower panel) spectra of (*R*)-**5** (red lines) and (*S*)-**5** (blue lines) in CHCl3 solution (1.0 <sup>×</sup> <sup>10</sup>–5 M).

**Figure 19.** CPL (upper panel) and PL (lower panel) spectra of (*R*)-**5** (red lines) and (*S*)-**5** (blue lines) in CHCl3 solution (1.0 <sup>×</sup> <sup>10</sup>–5 M).

This cryptochiral CPL system is similar to the cryptochiral dioxolane–bipyrene luminescent system **4**. The system simultaneously controls two chiral points (binaphthyl and pyrene units) in the photoexcited state through suitable choices of rotamer, linkers and fluorophores. In luminophore **5**, the two fluorescent pyrene units are in the conformation of an almost achiral T-shape in the ground state. Moreover, the θ value of the axially chiral octahydrobinaphthyl unit is approximately 80◦–90◦. The unique arrangements and conformations of the two pyrene and one binaphthyl units make the CD spectrum almost undetectable [52]. In the photoexcited state, however, the conformation of two pyrene units changes to a chiral π-stacked spatial arrangement. These configuration transfers are responsible for the cryptochiral properties, meaning the silent CD and active CPL derived from the excimer pyrene moiety [53].

#### **5. Conclusions**

In most CPL-emitting systems, a chiral luminophore derived from chiral molecules is essential. In addition, the selective emission of right- and left-handed CPL requires chiral organic or organometallic luminophores with opposite chirality. However, non-classical CPL (NC-CPL) systems that use no enantiomerically pure molecules have been reported. Such systems include: (i) a spontaneous-resolution CPL system, in which supramolecular organic luminophores prepared from achiral organic molecules can emit CPL without the use of any chiral auxiliaries; (ii) achiral or racemic luminophores that can emit CPL, viz. MCPL, by applying a 1.6 T external magnetic field in both the solution and solid states; and (iii) a cryptochiral CPL system, in which CD-silent organic luminophores can emit CPL in the photoexcited state. These systems indicate that CPL can be emitted from achiral or racemic molecules by suitable symmetry breaking.

**Funding:** This review was supported by a Grant-in-Aid for Scientific Research (nos. 18K05094, 19H02712, 19H04600, and 20H04678) from the MEXT/Japan Society for the Promotion of Science and the Research Foundation. This review was also supported by JST, CREST (JPMJCR2001), Japan.

**Acknowledgments:** Y.I. thanks Michiya Fujiki (Nara Institute of Science and Technology) and Reiko Kuroda (Chubu University) for helpful discussions regarding the origin of circularly polarized luminescence phenomena.

**Conflicts of Interest:** The authors declares no conflict of interest.
