Generation of Circularly Polarized Luminescence by Symmetry Breaking

: Circularly polarized luminescence (CPL) has attracted signiﬁcant attention in the ﬁelds of chiral photonic science and optoelectronic materials science. In a CPL-emitting system, a chiral luminophore derived from chiral molecules is usually essential. In this review, three non-classical CPL (NC-CPL) systems that do not use enantiomerically pure molecules are reported: (i) supramolecular organic luminophores composed of achiral organic molecules that can emit CPL without the use of any chiral auxiliaries, (ii) achiral or racemic luminophores that can emit magnetic CPL (MCPL) by applying an external magnetic ﬁeld of 1.6 T, and (iii) circular dichroism-silent organic luminophores that can emit CPL in the photoexcited state as a cryptochiral CPL system. 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 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.

CPL generally requires chiral organic or organometallic luminophores. In organometallic luminophores, chiral organic ligands coordinating with optically active metal ions induce chirality in the luminescent complex. In such a CPL-emitting system, a chiral organic molecule is indispensable. In addition, in practical applications of CPL, both right-and left-handed CPL are used, and their selective emission requires chiral organic or organometallic luminophores with opposite chirality. Chiral organic luminophores that can be prepared from achiral or racemic molecules are preferred as efficient and industrially useful chiral luminophores [19,20].
In this review, three types of non-classical CPL (NC-CPL) systems, including symmetry breaking CPL (SB-CPL) systems, are reported. The first is a spontaneous-resolution CPL system using achiral or racemic molecules. The second is a magnetic circularly polarized luminescence (MCPL) system based on achiral or racemic molecules under an external magnetic field. Finally, a cryptochiral CPL system based on CD-silent molecules in the photoexcited state is discussed.

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][22][23][24][25][26][27][28][29][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. 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 (λ PL ) of I (or I ) is 430 nm. The photoluminescence quantum yield (Φ F ) 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 (λ CD = 404 nm) of I is approximately −0.6 × 10 -3 . This shows that the fluorescent anthracene units effectively exist in a chiral environment in the solid state. As expected, X-ray analysis shows that luminophore I has a P2 1 chiral space group and a characteristic 2 1 -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 2 1 -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 2 1 -helical network column is a key factor in the production of spontaneously resolved chiral organic luminophores [31]. 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. As expected, chiral luminophore II also has an anion-cation and hydrogen bonded 2 1 -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 (P2 1 ). 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). 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 (λ CD = 416 nm) of II is approximately −1.0 × 10 -3 . Luminophore II exhibits luminescence with a solid-state luminescent maximum (λ PL ) of 446 nm and a photoluminescence quantum yield (Φ F ) 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 −1.1 × 10 -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 2 1 -helical columns. 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].

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][34][35][36][37][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][40][41][42][43][44][45][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. 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 CHCl 3 and acetone solutions under a 1.6 T magnetic field, as shown in Figure 9   The MCPL spectra (upper panel in Figure 9) of Eu(III)(hfa) 3 in the two solutions are similar. The meaningful MCPL peaks (λ MCPL ) 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 L − I R )/(I L + I R ), and normalized by an external magnetic field (T -1 ). In this equation, I L and I R show the left-and right-handed apparent MCPL intensities, respectively, when photoexcited by unpolarized light. The |g MCPL | values of the main bands in CHCl 3 were 0.81 × 10 −2 T −1 at 597 nm and 0.63 × 10 −2 T −1 at 611 nm. The |g MCPL | 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 ∆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 ∆and Λ-forms of D 3 -symmetric Tb(III) luminophores, in CHCl 3 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   Interestingly, the signs of the MCPL spectra of Tb(III)(hfa) 3 between the CHCl 3 and acetone solutions are partially inverted in the N→S and S→N magnetic fields. In CHCl 3 , the N→S magnetic field spectrum clearly shows positive(+)-/negative(−)-sign MCPL signals at 538/553 nm, derived from the 5 D 4 → 7 F 5 transitions. In contrast, the signs of the MCPL signals derived from the same 5 D 4 → 7 F 5 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). Surprisingly, these achiral pyrene luminophores clearly emit mirror-image MCPLs under a 1.6 T magnetic field in CHCl 3 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. In Py, luminescence peaks (λ MCPL ) 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 CHCl 3 is 0.82 × 10 −3 T −1 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 × 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].
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   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]. 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). 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 ( 1 L a 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 CHCl 3 are shown in Figure 19. From luminophore 5, strong excimer PL is emitted at 468 nm (λ PL ) 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)-5 is +2.5 × 10 −3 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.  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].

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.