Solvent-to-Polymer Chirality Transfer in Intramolecular Stack Structure

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Solvent-to-Polymer Chirality Transfer in Intramolecular Stack Structure Daehoon Lee,† Young-Jae Jin,† Hyojin Kim,† Nozomu Suzuki,‡ Michiya Fujiki,*,‡ Toshikazu Sakaguchi,§ Seog K. Kim,∥ Wang-Eun Lee,† and Giseop Kwak*,† †

Department of Polymer Science, Kyungpook National University, 1370 Sankyuk-dong, Buk-ku, Daegu 702-701, Korea Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan § Department of Materials Science and Engineering, Graduate School of Engineering, University of Fukui, Bunkyo, Fukui 910-8507, Japan ∥ Department of Chemistry, Yeungnam University, Kyoungsan City, Kyoung-buk 712-749, Korea ‡

S Supporting Information *

ABSTRACT: Solvent-to-polymer chirality transfer was examined using conjugated polymer with intramolecular stack structure (IaSS). When achiral poly(diphenylacetylene)s (PDPAs) dissolved in limonene, the solvent chirality was successfully transferred to the side phenyl stack structure, leading to intramolecular axial chirality. The phenyl−phenyl IaSS was under thermodynamic control to readily undergo asymmetric changes in chiral limonene, leading to optical activity in the isotropic structure between the main chain and resonant side phenyl rings. The axial chirality was significantly affected by the chain length and substitution position of the side alkyl groups. The longer alkyl chains and bulkier alkyl group prevented direct intermolecular interactions between the side phenyl rings and the chiral limonene molecules. PDPA with sterically congested, highly stable, and regulated IaSS was not favorable for efficient solvent-to-polymer chirality transfer.

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INTRODUCTION Optically active, conjugated polymers have attracted considerable attention for potential applications to circular polarization-based optical and electrical device materials.1−6 Chirality transfer from a chiral small molecule to an achiral host polymer may be one of the most elegant nature-mimicking approaches to generate optical activity in a conjugated skeleton.7−17 Akagi et al. successfully demonstrated the unique synthesis of an artificial gigantic figure with helical structure. They elegantly synthesized a superhelical fibril and bundle of polyacetylene in the chiral nematic liquid crystal reaction field and emphasized the importance of molecular information on asymmetric reaction fields, such as the helical pitch, twisting power, and molecular alignment, which affected the morphology of the final film product significantly.18−26 Yashima et al. also showed the excellent performance of the guest−host chemistry on chiral recognization, based on the noncovalent acid−base interactions between achiral acidic poly(phenylacetylene) and chiral basic molecules.27−32 Several research groups have reported that aggregation is a critical factor in the solventinduced chirality of the π-conjugated polymers and oligomers. Fujiki et al. examined solvent-to-polymer chirality transfer between conventional conjugated polymers, such as polysilanes and polyfluorene derivatives, and chiral solvents, by utilizing very weak intermolecular interactions, such as hydrogen bonding and π−π interactions. Although the weak interactions © 2012 American Chemical Society

were not sufficient for the effective generation of optical activity in the dispersed state of the solution, the chirality of solvents could be amplified significantly by aggregation in the presence of a nonsolvent.33−36 Moore et al. also demonstrated similar aggregation effects of solvent-induced transition from a random, disordered conformation in good solvents to an ordered, putative helical conformation in poor solvents using πconjugated phenyleneethynylene oligomers.37−44 In principle, the aggregation of such conjugated systems is based on the intermolecular stack structure (IeSS) due to the planar geometry and chain rigidity. Therefore, an intramolecular stack structure (IaSS) should also be the subject for chirality transfer. In particular, if certain achiral polymers have IaSS, in which the conformation is under thermodynamic control (i.e., the stack structure undergoes a conformational transformation at normal temperatures in solution), the IaSS might be affected by chiral solvent molecules in solution to transform into the asymmetric form without aggregation. Many polymers with stable and regulated IaSS in the side chains, including the well-known polyvinylcarbazole (PVK), have been reported.45−49 Such chain conformation significantly influenced their electrical and optical properties.50−55 Recently, Received: May 15, 2012 Revised: June 10, 2012 Published: June 19, 2012 5379

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poly(diphenylacetylene)s (PDPAs) were found to have side phenyl−phenyl IaSS with different degrees of stack regulation and stability according to the chain length and substitution position of the side alkyl groups.56−58 PDPAs have unique qualities including an ultrahigh molecular weight, excellent mechanical strength, chemical and thermal stability, good solubility in organic solvents, and film-forming ability.59−67 As one of the most interesting features of PDPA derivatives in association with IaSS, these polymers are intensively fluorescent in the visible region due to intramolecular excimer emission, which originates from the highly efficient exciton confinement between the bulky side phenyl rings.68−72 These polymers show rapid gas/liquid diffusion due to the intrinsic amorphous nature and the extremely large fractional free volume (FFV).73 When placed in contact with various organic solvents, such as alcohol and hydrocarbon liquids, these polymer films show significant swelling-induced emission enhancement (SIEE).74−78 The SIEE phenomenon is now believed to be due to the diffusion of solvent molecules into the polymer film through the many microvoids, which loosen the entangled polymer chains while simultaneously relaxing the phenyl−phenyl IaSS, which results in an increase in the electronic π*−π transition energy and an increase in oscillator strength.56,57 This means that the conformation of the IaSS is under thermodynamic control. If the polymers dissolve or swell in certain chiral solvents, the side phenyl rings may undergo an asymmetric rearrangement in the IaSS, leading to a preferred-handed helicity due to the variable stack structure of side phenyl rings in solution. If this solventto-polymer chirality transfer can be achieved, it will expand the research field of optically active PDPAs not only in synthesis but also in circular polarization-based optical applications. The first aim of this study was to transfer the chirality of the solvent to achiral PDPAs in solution. The next goal was to determine the origin of optical activity in PDPAs to comprehensively understand the chiroptical properties in association with the IaSS. A pair of limonene enantiomers is nonploar, inexpensive, and naturally occurring biomass enantiomers (Scheme 1). Owing to the nonpolar characteristic, this solvent well dissolved the all polymers tested in this study. Thus, this terpene compound was chosen as the chiral solvent. Using several achiral PDPAs containing different alkyl side chains via a silylene linkage (Scheme 1), this study examined how the achiral PDPAs interact with chiral limonene in solution by circular dichroism (CD), ultraviolet−visible (UV−vis), circularly polarized luminescence (CPL), and photoluminescence (PL) spectroscopy. Eventually, the chiral source molecules were found to stimulate the side phenyl rings leading to axial chirality in the side phenyl−phenyl IaSS. The chain length and substitution position of the side alkyl groups influenced chirality transfer significantly. Details on this will be described later.

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Scheme 1. Chemical Structures of Achiral PDPAs and Chiral Solvents Used in This Study

Measurements. The weight-average molecular weight (Mw) and number-average molecular weight (Mn) of all polymers were evaluated by gel permeation chromatography (GPC, Shimadzu A10 instruments, Polymer Laboratories, using PLgel Mixed-B (300 mm in length) as a column, and HPLC-grade tetrahydrofuran as eluent at 40 °C) calibrated with polystyrene standards. The CD/UV−vis spectra of the solutions were measured simultaneously at 25 °C on JASCO J-715, J-725, and J-820 spectropolarimeters equipped with a Peltiercontrolled housing unit an SQ-grade cuvette with a path length of 1 mm (with a scanning rate of 100 nm min−1, a bandwidth of 1 nm, and a response time of 1 s, using a single accumulation). UV−vis spectra of the sheared film were measured independently on a JASCO UV-550 spectrophotometer at 25 °C. The sheared film was prepared by manually rubbing the polymer gel with a critical concentration of approximately more than 2 wt % in toluene. The CPL/PL spectra were recorded on a JASCO CPL-200 spectrofluoropolarimeter with a path length of 1 mm at room temperature, whereas the instrument was designed to obtain a high S/N ratio by adjusting the angle between the incident and traveling light to 0° with a notch filter (scanning rate of 100 nm min−1, a slit width for excitation at 3000 μm, a slit width for monitoring at 3000 μm, and a response time of 1 s). The PL spectra were recorded independently on a JASCO ETC-273 spectrofluorometer at 25 °C. The photoluminescence quantum yields were determined relative to the quinine sulfate solution in 1 N H2SO4.83,84

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RESULTS AND DISCUSSION Figure 1 shows the CD and UV−vis spectra of p-C1 in (R)-/ (S)-limonenes. The p-C1 in the chiral solvents showed a marked chirality-dependent strong CD signal at 383 nm as a first Cotton band, whereas this polymer naturally showed no CD signals in achiral solvents, such as toluene. The second and third Cotton bands appeared at 338 and 318 nm, respectively. Previously, Aoki, Masuda, and Tang independently synthesized PDPA derivatives containing chiral pinanyl and menthyl groups in a side chain.85−88 Their polymers showed similar CD spectra with an extremum at ∼383 nm to those of p-C1 in (R)-/(S)limonenes. One of the interesting findings by the Aoki and Masuda research groups was that the PDPA containing a pinanyl group showed a strong CD effect as well as a large

EXPERIMENTAL METHODS

Materials. The syntheses of p-C1, p-C8, p-C18, p-(C2)3, m-C1, and p-tBuC1 were reported elsewhere.79−82 The polymers used in this study have high weight-average molecular weights (Mw) of 5.23 × 106, 7.54 × 106, 4.18 × 106, 6.02 × 106, 1.40 × 106, and 3.60 × 106 g/mol, respectively, and polydispersity indices (PDI = Mw/Mn) of 3.2, 1.8, 2.5, 4.2, 5.4, and 2.6, respectively. The MEH-PPV (Mw: (1.5−2.5) × 105; PDI: 5.0) was purchased from Sigma-Aldrich and used as received. The (R)- and (S)-limonenes were obtained from Wako (Tokyo, Japan) and used as received. (R)-Limonene: [α]25589 = +100.78° (neat), >99.0% ee; (S)-limonene: [α]25589 = −100.97° (neat), >99.0% ee. 5380

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Figure 1. CD and UV−vis spectra of p-C1 in (R)-/(S)-limonenes (c = 5.0 × 10−4 mol L−1). Red: (S)-limonene; blue: (R)-limonene. Figure 2. Polarized UV−vis absorption spectra of p-C1 in a sheared film as a function of the polarizer angle against the shearing direction.

optical rotation, whereas the corresponding disubstituted polymers with only one phenyl ring showed very weak CD and a quite small optical rotation.87 The significant difference in the Cotton effect between the PDPA derivative and the corresponding other types of disubstituted polymers was an unexpected result because the chiral side group and geometric structure of the corresponding polymers were the same as those of the PDPA derivative. Although the reason has not been described in detail and is still unclear, this strongly suggests that the intramolecular phenyl−phenyl stack structure is favorable for the generation of optical activity in disubstituted acetylene polymers. The magnitude of the circular polarization at the ground state is defined as gCD = 2(εL − εR)/(εL + εR), where εL and εR denote the extinction coefficients for left and right circularly polarized light, respectively.89 Experimentally, the value of gCD is determined by Δε/ε at the wavelength with a CD maximum intensity. The gCD value of p-C1 at 383 nm was as large as 0.38 × 10−3, indicating the optical activity of an isolated polymer chain in a molecularly dispersed solution. On the other hand, the p-C1 in (R)-/(S)-limonenes showed a dual absorption band at approximately 370 and 430 nm, which is characteristic of PDPA derivatives. The 383 nm CD band did not match the dual UV absorption wavelengths precisely. To discuss the solvent-induced chirality in p-C1, it is important to discuss the origin of the 383 nm CD band, particularly the origin of the optical activity. According to a previous study, PDPA derivatives have a very unusual biaxial conjugation structure, resulting in dual absorption bands.90 That is, the 370 nm band is the result of absorption due to a π−π* transition of localized electrons in the resonance structure between the side phenyl rings attached in a direction perpendicular to the molecular axis of the main chain, whereas the 430 nm band is caused by absorption due to a π−π* transition of the conjugation structure extended to a direction parallel to the main chain. In the polarized UV absorption spectra of p-C1 in the sheared film, an isosbestic point appears at 380 nm, as shown in Figure 2. This indicates that the polymer has the same absorbance at 380 nm in both directions parallel and perpendicular to the molecular axis. This isosbestic point almost matches the 383 nm wavelength of the CD extremum, suggesting that the optical activity occurs mainly in the isotropic structure between the main chain and resonant side phenyl rings. On the other hand, this polymer does not possess any stereogenic center. This means that a certain asymmetric

change in the axial structure between the bulky side phenyl rings and stiff main chain is responsible for the effective solventto-polymer chirality transfer in a completely isotropic state of a molecularly dispersed solution. In theoretical calculations, the side phenyl rings of p-C1 were coarsened in a discontinuous arrangement.91 This suggests that the phenyl−phenyl IaSS is under thermodynamic control to readily undergo asymmetric changes in chiral limonene by the attack of solvent molecules, leading to optical activity in the isotropic structure between the main chain and resonant side phenyl rings. That is, the side phenyl rings rearranged in a helical manner with a one-handed screw-sense along with an axis of the backbone. This helical IaSS of p-C1 in limonene was also reflected in the PL emission properties. The emission quantum efficiency (Φem) of p-C1 was slightly lower in (R)-limonene (∼30%) than in racemic limonene (∼35%). The effects of the refractive index and solvent viscosity on the PL properties can be excluded completely because the chiral and racemic limonenes have the same chemical and physical properties. Therefore, the lower Φem in chiral limonene should be due to the IaSS rearranged in a helical manner because the Φem generally decreases with increasing degree of stacking. Figure 3 shows a schematic diagram of the proposed solvent-induced helical IaSS in chiral limonene compared to a fully relaxed random IaSS in racemic limonene. Although the axial chirality of such PDPA derivatives might be strongly dependent on the molecular stereoregularity of the backbone, i.e., whether the molecular configuration has a trans or cis structure,92−94 the IaSS should be a key factor in producing the optical activity of PDPA derivatives. The solvent-induced CD revelation of p-C1 in molecularly dispersed solution is a very unusual event because the solventinduced CDs of other conjugated polymers have been revealed exclusively in aggregate form.33−44 This is important in terms of the molecular design of achiral polymers suitable for efficient solvent-induced chirality transfer. Thus, this paper provides further insight into correlation of CD and chain conformation of the polymers in chiral solvents. Conventional conjugated polymers, such as poly(thiophene), poly(fluorene), poly(phenylene), poly(phenylenevinylene), and poly(phenyleneethynylene) derivatives, are composed of coplanar aromatic rings in the main chain.96−103 These polymer backbones are generally stiff, rigid-rod-like chain molecules 5381

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Figure 3. Schematic illustration of the proposed IaSS of p-C1 in chiral and racemic limonenes. Trimethylsilyl groups are omitted for viewing clarity.

with planar geometries and a high barrier energy for conformational transformations. Ultimately, this leads to highly cofacial chain packing in the solid state, resulting in high crystallinity. Therefore, conventional conjugated polymers are not favorable for solvent-to-polymer chirality transfer in a molecularly dispersed state because the intermolecular weak interactions between the polymer and chiral solvents cannot overcome the high conformational transition energy barrier needed to induce chirality. As one example of conventional conjugated polymer, a poly(phenylenevinylene) derivative, poly[2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV), was tested for the solvent-to-polymer chirality transfer in this study. The polymer showed no Cotton effect in the chiral limonene solution (Figure S1 in Supporting Information). In contrast, the PDPA derivatives have a highly twisted main chain because of the steric repulsion between the bulky side phenyl rings, and its backbone is essentially nonplanar with the side phenyl rings.104 Because of the extremely long polymer chains (>1 μm in length), as expected for fully extended polymer chains in theoretical calculations, the PDPA derivatives should be wormlike chain molecules with a relatively weak interchain interaction and nonplanar geometry, which leads to an unstacked chain conformation in the solid state. In particular, the side phenyl−phenyl stack structure in pC1 is remarkably variable in solution and swollen states.74−78 Indeed, the intramolecular excimer emission due to the side phenyl−phenyl stack structure varies significantly according to the viscosity of the solvent media.57,77 This strongly supports the idea that the phenyl−phenyl stack structure can be relaxed in a chiral solvent and subsequently undergo asymmetric changes during rearrangement through scissoring and/or torsional motion, leading to axial chirality in the isotropic structure between the main chain and side phenyl rings. Because limonene contains alkyl groups and π-moieties within the chemical structure, the intermolecular interaction between p-C1 and limonene in solution should be very weak forces, such as van der Waals (∼1 kcal/mol), π−π (∼1 kcal/mol), and CH−π (∼0.5 kcal/mol) interactions.105−108 Therefore, when the chiral stimuli is addressed to the IaSS through weak interactions, direct stress should be essential for the efficient induction of chirality from limonene molecules to the side phenyl rings. To confirm this, CD spectra of longer alkyl chaincontaining PDPAs (p-C8 and p-C18) and a bulkier triethylsilyl group-containing PDPA (p-(C2)3) in (R)-/(S)-limonenes were measured. As shown in Figure 4, both p-C8 and p-C18 showed CD intensities approximately one-fourth that of p-C1, and the p-(C2)3 exhibits no CD peak. This suggests that the longer alkyl chains and bulkier alkyl group prevent direct intermo-

Figure 4. CD spectra of p-C1, p-C8, p-C18, and p-(C2)3 in (R)-/(S)limonenes (c = 5.0 × 10−4 mol L−1). Black: p-C1; red: p-C8; blue: pC18; green: p-(C2)3; plus signal at 383 nm: (S)-limonene; minus signal: (R)-limonene.

lecular interactions between the side phenyl rings and chiral molecules. The common feature of the chemical structure of the present polymers is that these polymers have a silylene linkage between the phenyl and alkyl groups. Despite the intermolecular Si−π interaction being ultraweak (∼0.001 kcal/ mol) and much weaker than the other intermolecular interactions mentioned above, it should not be ignored. Therefore, the CD spectra of another PDPA derivative (ptBuC1in Scheme 1), in which a tert-butyl group instead of an alkylsilyl groups is attached directly to a phenyl ring, was measured. The p-tBuC1 showed a slightly lower gCD value at 383 nm (0.19 × 10−3) than that of p-C1, indicating that the effect of intermolecular Si−π interactions on chirality transfer in the present polymer systems is quite small but not negligible (Figure S2 in Supporting Information). The p-C1 showed large helicity via an axially asymmetric rearrangement of the side phenyl rings in chiral limonene. As mentioned previously, the IaSS should be variable in solution to achieve solvent-to-polymer chirality transfer. Therefore, the conformational freedom and the stack degree of the side phenyl rings of PDPAs, i.e., thermodynamic stability and regularity of the IaSS, may be critical for effective chirality transfer. To clarify this idea, CD spectra of the m-substituted PDPA (m-C1) in (R)-/(S)-limonenes were measured and compared with p-C1. As shown in Figure 5, m-C1 exhibits no CD peak, which is in sharp contrast to the large CD signal of p-C1. This can be 5382

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C1 in limonene. The m-C1 shows almost completely quenched PL emission, whereas p-C1 shows quite intense emission. This indicates that the side phenyl rings of m-C1 are sterically congested, probably due to the meta-substitution of trimethylsilyl groups. The cross-sectional phenyl−phenyl stack area of m-C1 is greater than that of p-C1, resulting in a nonradiative electronic structure in the polymer chain even in the dispersed state of the dilute solution.58 From a viewpoint of molecular kinetics, the conformational freedom of the side phenyl rings for an axially asymmetric change is strictly restricted. Accordingly, the highly stable and regulated IaSS of m-C1 is responsible for the lack of a CD signal in the chiral solvents. This strongly suggests that the stability and regularity of the IaSS should be a critical factor in chirality transfer. As illustrated in Figure 3, the IaSS of p-C1 spontaneously undergoes an asymmetric change in chiral solvents. At this stage, molecular reorganization of the IaSS should be essential for a highly ordered secondary structure with a more optimal conformation. Specifically, the conformational transformation of IaSS may be accelerated by heating at a higher temperature. Indeed, the CD signal of p-C1 in (R)-/(S)-limonenes increased significantly with increasing thermal annealing time. Figure 7 shows the change in the CD spectra of p-C1 in (R)-/(S)limonenes upon annealing at 100 °C. The gCD value of p-C1 at 383 nm increased to 1.40 × 10−3 after annealing for 2 h, which is ∼4 times that of the initial value before annealing. The conformational freedom became greater at higher temperature because the side phenyl rings of p-C1 are highly variable due to the loosely stack structure. Consequently, thermal annealing gave a more regulated IaSS via a disorder-to-order rearrangement of the side phenyl rings, leading to a significant increase in the CD signal of p-C1. At the same time, the PL intensity of pC1 decreased slightly after annealing (Figure S3 in Supporting Information). These results suggest that the axially chiral IaSS

Figure 5. CD spectra of p-C1 and m-C1 in (R)-/(S)-limonenes (c = 5.0 × 10−4 mol L−1). Red: p-C1; blue: m-C1; plus signal at 383 nm: (S)-limonene; minus signal: (R)-limonene.

explained by the significant difference in IaSS between m-C1 and p-C1. Theoretical calculations using the 10-mer model compounds of both polymers showed that the side phenyl rings of m-C1 are arranged continuously in a helical manner, whereas those of p-C1 are arranged discontinuously in a zigzag pattern.58 Naturally, the helical structure of m-C1 is equally populated by right- and left-handed helices due to the absence of a chiral source. In general, the aromatic stack structure leads to remarkably attenuated PL emission with a quantum efficiency that decreases with increasing degree of stacking. The difference in the degree of stacking of side phenyl rings between p-C1 and m-C1 could be confirmed by PL measurements. Figure 6 shows the significant difference in the PL emission spectra and photographs between m-C1 and p-

Figure 6. PL emission spectra and photographs of p-C1 and m-C1 in (R)-limonene (excited at 383 nm, c = 1.0 × 10−5 mol L−1). Solid line: p-C1; dotted line: m-C1. 5383

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the 383 nm CD band, and the axially chiral structure between the backbone and side phenyl rings is responsible for this CPL band.

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CONCLUSION Solvent chirality was successfully transferred to achiral PDPA with IaSS simply by dissolving the polymer in a chiral solvent. The chiral source molecules stimulated the side phenyl rings leading to intramolecular axial chirality in the aromatic stack structure. The chain length and substitution position of the side alkyl groups significantly affected the solvent-to-polymer chirality transfer. This approach promises a very unique and facile preparation of various optically active polymers. The present polymer may also provide chiroptically functional films and fibers simply by dissolving the polymer in chiral solvents and subsequently casting or spinning the solution. Therefore, this type of achiral polymer would be a very useful host body not only for unique syntheses of but also for a wide range of applications. Such polymers should also be regarded as a typical chirality-responsive polymer and provide the basis to construct a novel chirality-sensing system to determine the absolute configuration and enantiomeric excess (ee) of biologically important chiral molecules. These findings provide some design rules that may allow applications to other types of IaSS-based polymers.

Figure 7. Variation of CD spectra of p-C1 in (R)-/(S)-limonenes (c = 5.0 × 10−4 mol L−1) upon annealing at 100 °C. Black: before annealing; blue: after annealing for 1 h; red: after annealing for 2 h; plus signal at 383 nm: (S)-limonene; minus signal: (R)-limonene.

becomes more stable and more regular by thermal annealing. On the other hand, as expected in the highly dense and sterically congested phenyl−phenyl stack structure, m-C1 showed no changes in both CD and PL intensities and still remained optically inactive, even after thermal annealing at the same temperature for prolonged heating times (>5 h) (Figures S4 and S5 in Supporting Information). Figure 8 shows the CPL and PL spectra of p-C1 in (R)-/(S)limonenes. The p-C1 in the chiral solvents shows a CPL

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ASSOCIATED CONTENT

S Supporting Information *

CD spectrum of MEH-PPV (Figure S1), CD and UV−vis spectra of p-tBuC1 (Figure S2), PL spectra of p-C1 (Figure S3) upon thermal annealing, CD spectra (Figure S4) and PL spectra (Figure S5) of m-C1 upon thermal annealing. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +82-53-950-7558; fax +82-53-950-6623; e-mail gkwak@ knu.ac.kr (G.K.), [email protected] (M.F.). Notes

The authors declare no competing financial interest.

■ Figure 8. CPL and PL spectra of p-C1 in (R)-/(S)-limonenes (c = 5.0 × 10−4 mol L−1). Blue: excited at 383 nm; red: excited at 420 nm; solid circle: (S)-limonene; open circle: (R)-limonene.

ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grants funded by the Korea government (MEST) (2012-0000633, 20120001714, 2012-0005143). This work was also supported by Grant in-Aid for Scientific Research (B) (22350052) from MEXT, Japan.

maximum at 520 nm, which matches the PL band. The sign of the CPL signals at 520 nm was identical to that of the 383 nm CD band. The magnitude of circular polarization at the excited state is defined as gCPL = 2(IL − IR)/(IL + IR), where IL and IR indicate the output signals for left and right circularly polarized light, respectively. Experimentally, the value of gCPL is determined by ΔI/I at the wavelength of the CPL maximum intensity.89 The absolute magnitude of the gCPL value (∼0.6 × 10−3, excited at 383 nm) at 520 nm was similar to that of the gCD value (∼0.38 × 10−3) at 383 nm. These results suggest that the CPL band at 520 nm was derived from the same source as

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