Hierarchical Emergence and Dynamic Control of Chirality in a

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Spectroscopy and Photochemistry; General Theory

Hierarchical Emergence and Dynamic Control of Chirality in a Photoresponsive Dinuclear Complex Yuichiro Hashimoto, Takuya Nakashima, Miku Yamada, Junpei Yuasa, Gwénaël Rapenne, and Tsuyoshi Kawai J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00690 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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The Journal of Physical Chemistry Letters

Hierarchical Emergence and Dynamic Control of Chirality in a Photoresponsive Dinuclear Complex Yuichiro Hashimoto,† Takuya Nakashima,*,† Miku Yamada,† Junpei Yuasa,‡ Gwénaël Rapenne,†,§ and Tsuyoshi Kawai*,†,§ †

Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara

630-0192, Japan. ‡

Department of Applied Chemistry, Tokyo University of Science, Kagurazaka, Shinjuku, Tokyo

162-8061, Japan §

NAIST-CEMES International Collaborative Laboratory, CEMES-CNRS, 29 rue Jeanne Marvig,

BP94347, 31055 Toulouse, France AUTHOR INFORMATION Corresponding Authors *T. N.: E-mail: [email protected] *T. K.: E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Chiroptical photoswitches are of interest from a viewpoint of applications in advanced information technologies. Herein, we report dynamic on-off photoswitching of circularly polarized luminescence (CPL) in a binuclear europium complex system. Two coordination units are arranged closely in a chiral fashion by a photoresponsive ligand with a one-handed helical structure. The chirality in the helical scaffold is hierarchically transferred to the chirality in ninecoordinate complex sites. The chiral close-arrangement of complex units induces the enrichment of a specific chiral coordination structure in the nine-coordinate europium sites. The chiral arrangement of complex units is switched in conjunction with the photoinduced helix-non-helix structural change in the photoresponsive framework, demonstrating on-off switching of CPL with high contrast.

TOC GRAPHICS


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Circularly polarized luminescence (CPL) is of increasing interest as an important chiroptical phenomenon, which provides the differential emission intensity of right- and left-circularly polarized light.1-3 CPL technique is expected to find potential applications in sensors, displays and advanced information technologies including anti-counterfeiting labeling.4 Recent researches have made efforts on the manipulation and modulation of CPL activity to further develop these technologies.5-17 High-speed switching of CPL in noninvasive and remote manners by light16,17 or electric stimuli18,19 should be of particular interest for the application in cryptographic communication as a technology for information security.20 We have recently reported a simultaneous pursuit of intense photoluminescence property and reversible stereospecific photoswitching in a chiral photochromic scaffold bearing two pyrene units, demonstrating dynamic CPL modulation.17 However, most chiral organic fluorophores afford CPL with a limited dissymmetry value (glum < 0.05),21 which is determined by the equation glum = 2(IL - IR) / (IL + IR), where IL and IR are the left- and right-circularly polarized emission intensities, respectively. Photoswitches capable of modulating the CPL activity with high contrast (∆glum) still remain an important challenge. Unlike organic fluorophores, large glum-values as high as 1.45 are obtained for the magnetic dipole transition in europium(III) complexes with chiral coordination geometries.22-30 Chiral EuIII-complexes have intrinsic CPL capability of large glum values associating with the significant contribution of magnetic transition dipole moment in the 5

D0 → 7F1 band at 595 nm. They also possess specific diversity and flexibility in coordination

structures, which motivates us to introduce a specific strategy for the remote control of their chiroptical properties. In the present study, we introduce terpyridine (terpy) ligands at both end of the helical tetrathiazole scaffold17,31,32 (D/L-1, Scheme 1a) to retain tris(β-diketonate)EuIII complexes


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(Eu(terpy)(tta)3), thus forming a helical photochromic dinuclear complex. In the previous reports, the same Eu(terpy)(tta)3 unit was combined with photochromic diarylperfluorocyclopentenes in covalent33 and non-covalent34 manners. However, the optical activity could not be induced in these photochromic lanthanide complexes even though the ninecoordination structure of Eu(terpy)(tta)3 is considered to have intrinsic chirality. There are several approaches to inducing optical chirality in lanthanide complex systems including the use of ligands with chiral structures,22-30 optical resolution35-37 and chiral discriminative emission quenching38,39 in racemic mixtures. Chiral chelating solvents35 or a chiral antimony complex36,37 were employed as a resolving agent to enrich or separate an enantiomeric complex in a racemic mixture of inherently optically active structures. Herein, we expect that the chiral helical geometry of the tetrathiazole framework31,32 can serve as a sort of “resolving agent” to bring about the optical activity in the binuclear complex system, thus expressing hierarchical chirality. The point chirality of amino-acid spacer in 1(Eu)-o is effectively transferred to the photoresponsive main framework as a one-handed helical secondary structure via the directional intramolecular hydrogen bonding interactions,17 bringing EuIII-complex sites close together in a chiral arrangement. Chiral arrangement of the Eu(terpy)(tta)3 unit with a close contact may transfer chiral information to the inherently chiral coordination unit with asymmetric tta (thenoyltrifluoroacetone) ligands in a remote manner, leading to an enriched population of an enantiomeric coordination structure (Scheme 1b). Furthermore, the photoinduced helical to nonhelical transformation of the main framework switches off the close arrangement of the coordination units. The labile coordination character of lanthanide ions40,41 should negate the enrichment of enantiomeric coordination structure, diminishing CPL activity. The present helical photochromic EuIII dinuclear complex therefore achieves not only the photoswitching of


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emission intensity owing to the change in electronic structure of ligands, which has been demonstrated in various photochrome-metal complex systems,33,34,42-44 but also the modulation of CPL activity in conjunction with the change in chiral coordination structures.

Scheme 1. (a) Photoswitching Reaction of D-1 and D-1(Eu). (b) Schematic Illustration of Hierarchical Chirality Emergence and Its Reversible Modulation in the Photoresponsive Chiral Dinuclear Complex.

The helical tetrathiazoles having two terpy groups (D-,

L-1-o)

were synthesized by

condensation of two terpyridine carboxylates and an amine-functionalized tetrathiazole precursor17 with a chiral phenylalanine linker (see the Supporting Information for details). Conformational behavior of 1-o in chloroform was investigated with 1H NMR and circular


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dichroism (CD) spectra. Variable temperature (vt) 1H NMR measurement and CD spectra showed a very similar behavior to that of pyrene-functionalized tetrathiazole.17 An upfield shift for the amide protons and a slight down-field shift in the aromatic region on increasing temperature suggested intramolecular hydrogen bonding interactions between the amide groups and π-π stacking of terpy units, affording a folded compact conformation for 1-o (Figure S4). CD spectra of 1-o enantiomers gave mirror-image profiles in the region of π-π* transitions, demonstrating the preferential formation of one-handed helix in the folded conformation (Figure 1b). The positive first Cotton effect observed for the D-1-o suggests the formation of a righthanded helix and vice versa for L-1-o. The preferential handedness in the helices was considered to be determined by the intramolecular hydrogen bonding interactions of chiral diamide spacers introduced between the terpy and tetrathiazole units.17 Thus the conformational study supports the chiral arrangement of terpy units with close contact in 1-o, which should assure the chiral close-arrangement of two coordination units in the dinuclear complex as shown in Scheme 1. The substitution of pyrene group with a terpy unit brought about no change in their photoreaction performance.17 The closed-ring isomers (1-c) with a visible absorption band over 500 nm were formed by UV irradiation, which was also confirmed by 1H NMR spectral change (Figure 1a and Figures S5, 6). D- and L-1 showed apparent quantum yields of 50% and 3% for photo-cyclization and -cycloreversion, respectively, with a high conversion ratio of 94% at the photostationarystate (PSS) achieved by the irradiation at 365 nm. It should be noted that the optical absorption at terpy units cannot contribute to the photocyclization reaction, apparently decreasing the photocyclization reaction quantum yield compared to those of typical tetrathiazole photochromic molecules.31,32 Chiral induction of helix-handedness in D- and L-1 successfully led to a diastereoselective photoreaction proceeding in a conrotatory manner, giving a broad CD


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band in the visible region (Figure 1b inset).17,32 The right- and left-hand helices of D- and L-1 gave the closed-ring isomers with chiral (R, S)- and (S, R)-forms, respectively, for the 5,6substituted cyclohexa-1,3-diene core (Scheme 1a).32

Figure 1. Absorption spectral change of D-1 upon photoirradiation (black: before photoirradiation; broken line: at the PSS; green solid line: D-1-c, [D-1] = 10 µM). (b) CD spectra of photochromic ligand 1 (blue lines: D-1; red lines: L-1) before and after UV irradiation at the PSS (inset).

To gain an insight into the preferential coordination geometry in the EuIII-complex sites in Dand L-1(Eu), the single crystal structure of Eu(terpy)(tta)3 (Scheme 1a) was investigated. The reference complex Eu(terpy)(tta)3 gave a single crystal composed of racemic coordination


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structures with enantiomeric ligand orientations (Figure 2, Figure S7 and Table S1). Whereas the –

terpy ligand distorted in the same direction, giving a chiral crystal with a space group of P1 (Table S1), one of the β-diketonate ligands (tta) coordinates to EuIII in an opposite orientation in these complexes (orange and green ones in Figure 2b). In fact, the compound is possible to provide eight patterns of enantiomeric coordination structures in terms of ligand orientations (Figure S8). Among them, a pair of enantiomeric coordination patterns were adopted in the single crystal of Eu(terpy)(tta)3 (Figure 2). No energy difference between those two enantiomeric coordination structures without any chiral perturbation provided the mixture of near mirror-image enantiomeric complexes.

Figure 2. X-ray crystal structure of Eu(terpy)(tta)3. (a) Packing structure in a unit cell. (b) Schematic coordination forms in the single crystal. Terpyridine ligands are depicted in blue color. β-diketonate ligands arranged in the same orientation are shown in red color and orange (left) and green (right) β-diketonate ligands are a mirror image of each other.


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The enantiomeric 1-o ligands were reacted with two equimolar of tris(tta)-EuIII complex (Eu(tta)3) to form D- and L-1(Eu)-o. Since 1(Eu) hardly gave a single crystal suitable for X-ray crystallographic analysis, the formation of dinuclear complexes was first evaluated by mass spectrometry (MS) using the electrospray ionization (ESI) method, which gave an m/z value corresponding to a dicationic complex losing a tta ligand from each complex site during the ionization process (Figure S9). Complexation of 1-o ligand with tris(tta)-lanthanide complex was investigated by 1H NMR, wherein the paramagnetic EuIII was replaced with nonmagnetic LaIII using La(tta)3 for the clearer evaluation. Each proton was assigned on the basis of peak shifts of terpy and tta ligands on the formation of La(terpy)(tta)3 (Figures S10). The shifts observed for the signals corresponding to terpy-protons in 1-o in the presence of La(tta)3 support the complexation of 1-o and La(tta)3 to form the dinuclear complex 1(La) (Figure S11). Furthermore, the ratio of 1-o and tta ligands was confirmed to be 1:6 by comparing the peak integral values corresponding to the methyl protons of 1-o and α-protons in tta (Figure S11), assuring the composition ratio in the dinuclear complex. 1(Eu)-o exhibited bright red emission from EuIII-centered f-f transitions with an apparent emission quantum yield (Φlum) of 0.20 (λex = 360 nm) and an emission lifetime of 0.55 ms (Figure S12) in CDCl3. Given the optical absorption of tetrathiazole main framework at 360 nm cannot sensitize the EuIII-emission, the practical emission quantum yield should be higher than the measured value. The photoluminescence spectrum (Figure 3a) gave emission peaks at 580 nm (5D0 → 7F0), 595 nm (5D0 → 7F1), and 620 nm (5D0 → 7F2). Meanwhile, mirror-image CPL signals were obtained for the enantiomer pair of 1(Eu)-o complexes at the bands of 5D0 → 7F1 and 5D0 → 7F2 transitions corresponding to magnetic- and electronic-dipole ones, respectively (Figure 3b). The magnetic dipole transition (5D0 → 7F1) satisfies the magnetic-dipole selection


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rule, ∆J = 0, ±1 (except 0↔0), which often shows the larger circular polarization compared to that of the electronic dipole one.1,29 Since we used a diode laser with the fixed wavelength (λ = 375 nm) for the excitation of the light-harvesting ligands (tta and terpy units), the photoisomerization reaction could also take place during the CPL measurement simultaneously. To minimize the effect of this isomerization, relatively concentrated solutions (0.5 mM) were used with the measurement range of 570-650 nm and the data accumulation period of 15 min. Even though this restriction, the emergence of CPL signals clearly suggested the induction of optical activity in the dinuclear EuIII complexes. We obtained |glum| values of 0.1 at the magneticdipole transition band, which are ten times larger than that of the pyrene-containing analog17 and reasonably in the range of typical values for chiral mononuclear EuIII complexes.22-30 The complex sites in 1(Eu)-o is essentially chiral as demonstrated by the structure of Eu(terpy)(tta)3 (Figure 2). Unlike Eu(terpy)(tta)3 with no chiral groups, the chiral structure in the main framework arranges the complex sites closely in a chiral manner, which should give a chiral perturbation to the complex sites in 1(Eu)-o, leading to a preferential enrichment of an enantiomeric nine-coordination structure.


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5D →7F 0 2

5D

7 0→ F0

5D →7F 0 1

1

(a)

5D →7F 0 1

2

CPL (IL - IR)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity/ a.u.

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5D →7F 0 2

(b)

0 -1 -2 580

600 620 Wavelength/ nm

640

Figure 3. (a) Photoluminescence (50 µM) and (b) CPL spectra of D-(blue) and L-1(Eu)-o (red) (0.5 mM) in CHCl3.

To further confirm that the enantiomeric enrichment was indeed achieved in the LnIIIcomplex sites, we investigated the chiroptical property in the inner shell electron transitions (f-f transitions). Because of the very small absorption coefficient of EuIII ion and the limited solubility of 1(Eu)-o in common organic solvents, we replaced the EuIII by NdIII ion which has a larger absorption coefficient (εmax > 20 for tris(β-diketonate) complexes45) to form 1(Nd)-o using Nd(tta)3 for CD measurement. While the ligand 1-o does not show any absorbance band over 500 nm, 1(Nd)-o exhibited several sharp absorption bands corresponding to the f-f electronic transitions in NdIII ion (Figure 4a). The enantiomers of 1(Nd)-o gave mirror-image CD signals for those absorption bands (Figure 4b). This result strongly supports that the origin of optical activity in the dinuclear complex system includes the coordination chirality in the LnIII-complex sites. Furthermore, the CD signal intensity was dependent on the temperature (Figure S13) and increased upon decreasing temperature. This result could be described by a change in the


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population ratio (enantiomeric excess) between the major and minor chiral coordination structures in the complex sites depending on the temperature.

4I

ε / M-1cm-1

9/2 → 2G 4 7/2 + G5/2

(a)

100 4I 9/2 → → 4F 2 5/2 + H9/2 4S 4F + 3/2 7/2 4I

50

0 0.04

∆ε / M-1cm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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9/2

4I

9/2 → 2G 4 7/2 + G5/2

(b)

0.02 0 -0.02 -0.04 500

600 700 Wavelength/ nm

800

Figure 4. (a) Absorption and (b) CD spectra of

D-(blue

lines) and

L-(red

line) 1(Nd)-o

complexes (0.5 mM in CHCl3).

In order to obtain an insight into the interaction between the complex sites in 1(Eu)-o, we performed 19F NMR measurement. For this purpose, the use of non-magnetic LaIII-complex was not necessary. 19F NMR spectra were compared for 1(Eu)-o and the reference Eu(terpy)(tta)3. While the free tta ligand showed a sharp peak at -75.5 ppm in CDCl3, the peak shifted to -80.4 ppm with a slight broadening in Eu(terpy)(tta)3 (Figure S14). The CF3 signal further broadened in 1(Eu)-o. This signal also appeared to split into two peaks, suggesting unequal electromagnetic environment for CF3 groups in the dinuclear complex. This broad peak was separated into two peaks and the peak integral ratio was found to be about 1:2 (Figure S14b). A model structure of 1(Eu)-o was constructed using a molecular mechanics simulation with the


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Universal force field (Figure S15).46 An enantiomeric coordination structure was extracted from the eight possible coordination geometries (Figure S8) on the basis of X-ray crystal structure (Figure 2) to construct the model structure to reduce the calculation cost. We thus here limit our discussion into the working principle of present molecular system without accurate consideration for the chiral coordination structure preferentially adopted in the dinuclear complex. The model clearly suggested the close contact of complex sites in the helically folded conformation. A tta ligand in a local complex site is located at the contact face and the remaining two tta ligands are placed outward. The unequal electromagnetic environment for tta ligands suggested by the

19

F

peak splitting could be thus explained by this close-contact of complex sites. The close-contact in a chiral manner should induce the energy differences between enantiomeric coordination structure patterns. We then varied β-diketonate ligands in the dinuclear complex system to figure out the impact of ligand structure on the induction of optical activity. Various symmetric and asymmetric β-diketonate ligands were employed to form dinuclear complexes and photoluminescence and CPL properties were investigated (Chart 1, Table 1). The EuIIIcomplexes with asymmetric β-diketonate ligands having a trifluoromethyl (CF3) and an aromatic units showed better emission efficiency compared to those with symmetric β-diketonate ligands. While the electric dipole transitions in EuIII ion are known as forbidden transitions, the use of asymmetric ligands could enhance the radiation probability (kr).47 The EuIII-complexes with those asymmetric ligands gave similar CPL activity with the |glum| values around 0.1 (Figure S16). Unfortunately, 4(Eu)-o and 5(Eu)-o with symmetric β-diketonate ligands substituted by CF3 and thiophene groups, respectively, were not so emissive with the small emission quantum yields (< 0.03) to study the CPL property. We therefore employed a β-diketonate ligand with


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perfluoroethyl (C2F5) groups (6(Eu)-o), affording better emission property with Φlum of 0.06. The enantiomeric complexes D- and L-6(Eu)-o showed the suppressed CPL activity with the |glum| value of 0.01. This result further confirms that the enriched population of an enantiomeric coordination structure in the EuIII-complex sites with the asymmetric β-diketonato ligands plays the primary role in the induction of optical activity in the dinuclear complex system. However, smaller but non-zero CPL activity observed in 6(Eu)-o with a symmetric β-diketone suggests the additional contributions by inter-ligand exciton coupling48 and/or chiral torsion in the ninecoordination sites.49

Chart 1. Chemical Structures of Dinuclear Complexes(D-Isomers).


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Table 1. Emission Properties of Dinuclear Eu-Complexes in CDCl3.

Φlum

τ (ms)

kr (s-1)a

|glum|b

D-1(Eu)-o

0.20

0.55

3.6 × 102

0.1

D-2(Eu)-o

0.14

0.30

4.7 × 102

0.08

D-3(Eu)-o

0.13

0.58

2.2 × 102

0.1

D-4(Eu)-o

0.03

0.74

41

-

D-5(Eu)-o

-

-

-

-

D-6(Eu)-o

0.06

0.92

65

0.01

a

Radiative rate constant (kr = Φ/τ). b The values at λlum= 590 nm.

UV irradiation to a chloroform solution of 1(Eu)-o resulted in the emergence of an absorption band in the visible region due to the formation of closed-ring isomer in the photochromic framework (Figure 5a). The photoreaction also induced a decrease in the emission intensity in the whole spectral region and the Φlum value dropped to 0.03 at the PSS with a conversion ratio of 85% (Figure 5b). The emission lifetimes at the PSS also decreased to 0.39 ms (Figure S12). The decrease in the emission intensity and lifetime were attributed to the Förster resonance energy transfer (FRET) mechanism from the excited state of EuIII to the photochromic center.33,34,42,43 It should be also noted that the emission profile in the electronic dipole transition band at 620 nm slightly changed, suggesting a modulation in the ligand field of the complex sites by the photochromic reaction.42 The photocyclization reaction of the photochromic backbone of 1(Eu) also promoted a decrease in CPL intensity (Figure 5c). Given the enantiomeric structure induced in the EuIII complex sites is maintained after the photoreaction, the glum value should be unchanged even the CPL intensity decreases. However, the |glum| value decreased to less than


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0.01, affording a large modulation amplitude in CPL dissymmetry, |∆glum| > 0.09. The decrease in the dissymmetry factor should be attributed to the cancellation in the enrichment of one-hand chiral coordination structure in the EuIII complex sites with aid of labile coordination character.40,41 The on–off photoswitching behavior was observed for around 10 cycles with alternate UV and visible irradiations (Figure 5d).


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Figure 5. (a) Absorption (6.3 µM), (b) photoluminescence (50 µM) and (c) CPL (0.5 mM) spectral change of D-(blue lines) and L-1(Eu) (red lines) in CHCl3 before (broken lines) and after photoirradiation to achieve PSS under irradiation at λ = 365 nm (solid lines). (Inset in b: picture of emission change) (d) Reversible changes of CPL intensity at 595 nm of L-1(Eu) in CHCl3 with UV (365 nm) and visible light (> 480 nm) irradiations.


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Meanwhile, UV irradiation to D-1(Nd)-o gave rise to an emergence of a broad negative CD signal over 500 nm due to the diastereoselective formation of the closed-ring isomer (D-1(Nd)c),17,32 while the 4I9/2 →4G5/2 band remained at 580 nm as a positive feature in the CD spectrum after 1 min irradiation (Figure 6b). The apparent feature of this positive signal at 580 nm in the broad negative CD band diminished as the photoreaction proceeded after 5 min irradiation, whereas the absorption bump was maintained in the absorption profile at the PSS (Figure 6a). The same behavior with the opposite sign response in CD spectra was also observed for L-isomer (red traces in Figure 6b). This result suggests that the preferential formation of enantiomeric coordination structure in the LnIII-complex sites was effective only in the helical structure, wherein the close contact between the complex sites is operative (Scheme 1b).

Figure 6. (a) Absorption and (b) CD spectral change of 1(Nd) before (broken lines) and after UV irradiation (solid lines) for 1 min and 5 min in CHCl3 (0.8 mM). D-isomers (blue lines); Lisomers (red lines).

We have demonstrated a high-contrast CPL switching between glum-values of 0.1 and

Hierarchical Emergence and Dynamic Control of Chirality in a

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