Circularly Polarized Light with Sense and Wavelengths To Regulate

Article pubs.acs.org/JACS

Circularly Polarized Light with Sense and Wavelengths To Regulate Azobenzene Supramolecular Chirality in Optofluidic Medium Laibing Wang,†,‡ Lu Yin,‡ Wei Zhang,*,‡ Xiulin Zhu,‡ and Michiya Fujiki*,† †

Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China

‡

S Supporting Information *

ABSTRACT: Circularly polarized light (CPL) as a massless physical force causes absolute asymmetric photosynthesis, photodestruction, and photoresolution. CPL handedness has long been believed to be the determining factor in the resulting product’s chirality. However, product chirality as a function of the CPL handedness, irradiation wavelength, and irradiation time has not yet been studied systematically. Herein, we investigate this topic using achiral polymethacrylate carrying achiral azobenzene as micrometer-size aggregates in an optofluidic medium with a tuned refractive index. Azobenzene chirality with a high degree of dissymmetry ratio (±1.3 × 10−2 at 313 nm) was generated, inverted, and switched in multiple cycles by irradiation with monochromatic incoherent CPL (313, 365, 405, and 436 nm) for 20 s using a weak incoherent light source (≈ 30 μW·cm−2). Moreover, the optical activity was retained for over 1 week in the dark. Photoinduced chirality was swapped by the irradiating wavelength, regardless of whether the CPL sense was the same. This scenario is similar to the so-called Cotton effect, which was first described in 1895. The tandem choice of both CPL sense and its wavelength was crucial for azobenzene chirality. Our experimental proof and theoretical simulation should provide new insight into the chirality of CPL-controlled molecules, supramolecules, and polymers.

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INTRODUCTION Mirror-symmetry breaking (MSB), which is a process of emerging chirality detected by chiroptical signals originating from optically inactive substances, has been one of the most topical phenomena in the realm of molecular chirality for over a century.1−5 MSB usually requires the intervention of chemical influences (i.e., chiral reagents and catalysts) and external physical sources, such as circularly polarized light (CPL) in the UV−visible region.6−12 However, an open question that has long been a point of contention is whether a CPL source is crucial for the origin of biomolecular handedness and, subsequently, the evolution of life.13 CPL-driven chiral molecular synthesis, which was later termed absolute asymmetric synthesis (AAS), was proposed by LeBel in 1874 and van’t Hoff in 1894.14,15 In 1929, Kuhn and Braun16 confirmed their ideas and performed the first CPL destruction experiments involving racemic compounds in alcoholic solution at 280−300 nm. In the 1970s, Calvin and co-workers17,18 reported photochemical synthesis of [8]-helicene in toluene, including the structural and wavelength dependence of the induced optical yields. With a racemic precursor as a starting source, a CPL source followed by a ring-closure reaction with iodine led to the synthesis of optically active helicene in 0.5−2% enantiomeric excess (ee). Kagan and co-workers19 also demonstrated CPLdriven photochemical destruction (290−370 nm) reaching 20% © 2017 American Chemical Society

ee when racemic camphor in solution was employed. Kagan and co-workers19expounded upon the importance of the anisotropy factor, gCD, of enantiopure substances with the aid of a detailed theoretical analysis. Recently, Vlieg and co-workers20 produced amino acid derivatives in high ee by a mechanical grinding process triggered by the CPL sense. This approach overcomes an inherent problem of CPL-driven AAS with a small ee.21,22 These results have encouraged scientists to elucidate the potential of CPL sources in controlling the AAS of chiral molecules and polymers. Indeed, several researchers have verified emerging circular dichroic (CD) signals in various achiral macromolecules in the form of thin films upon irradiation by a single-wavelength CPL source.23−31 The chiroptical sign and degree of magnitude can be efficiently controlled by the CPL sense alone. However, the CPL wavelength (i.e., photon energy) dependence of AAS has been rarely studied because most researchers concur that the choice of CPL sense (r or l) determines the chirality (r or l) of helical or chiral structures. Recently, Meinert et al.32 achieved the photon energy-dependent MSB photodestruction of amorphous rac-alanine films at 200 nm (6.19 eV) and 184 nm (6.74 eV) using CP synchrotron radiation, which resulted Received: July 29, 2017 Published: August 28, 2017 13218

DOI: 10.1021/jacs.7b07626 J. Am. Chem. Soc. 2017, 139, 13218−13226

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Journal of the American Chemical Society

Figure 1. gCD value at 313 nm and ellipticity as a function of DCE−MCH cosolvent, measured for PAzoMA2 aggregates in mixed solvents exposed to 365 nm l- and r-CPL irradiation. Mn,GPC = 7700 g·mol−1; polydispersity index = 1.12; [concn]0 = 0.1 mg·3 mL, path length = 1.0 cm in SQ rectangular cuvette. The volume fraction of DCE/MCH (v/v) ranged from 0.9/2.1 (v/v, no detectable CD signal) to 0.2/2.8 (v/v) including DCE/ MCH = 0.9/2.1, 0.8/2.2, 0.7/2.3, 0.6/2.4, 0.5/2.5, 0.4/2.6, 0.3/2.7, and 0.2/2.8 (v/v).

at nD = 1.425 [DCE/MCH = 0.5/2.5 (v/v)], and a value of ±1.0 × 10−2 was attained by applying l- and r-CPL sources. The CPL confinement effect within the 200 nm polymer aggregates with higher nD values (estimated to be 1.7−1.8) is responsible for this tendency, as proven by several examples.33,41,44−46 A similar gCD−nD value tendency was confirmed, regardless of the DPn of PAzoMA and the use of a poor solvent with a low nD value. Among the four samples, PAzoMA2 (middle Mn) gave the greatest gCD value, which was dependent on the DPn value. We also tested the poor solvent dependency of the gCD value using PAzoMA2 aggregates. Among the six poor solvents (methanol, ethanol, 2-propanol, cyclohexane, MCH, and noctane), MCH was the best poor solvent and resulted in the greatest gCD value (Figure S2). Subsequently, we applied DCE/ MCH = 0.5/2.5 (v/v) as a fixed fraction in the following CPLAAS experiments with PAzoMA2. Chiroptical Property. Bisignate Cotton bands at the π−π* transitions of azobenzene are observed as intense couplets in CD spectra of PAzoMA2 aggregates upon l- and r-CPL (365 nm) irradiation (Figure 2a). Bisignate Cotton CD bands emerged at 360−390 and 310 nm. A negative sign at the first Cotton band (390 nm) and a positive sign at the second Cotton band (310 nm) appeared when l-CPL was employed. Conversely, a positive sign at 390 nm and a negative sign at 310 nm appeared when r-CPL was used. The 310 and 360−390 nm bands are associated with π−π* transitions due to azobenzene moieties with chirally assorted side chains. However, as shown in Figure 2b, the gCD value increased nonlinearly with time and reached a maximum value of ±1.0 × 10−2 (150 s). More prolonged irradiation resulted in a zero gCD value (750 s). This decrease is ascribed to the production of cis-azobenzene moieties with a bent structure, which prevents efficient transazobenzene stacks and leads to an ill-ordered structure.47−51 It should be noted that CPL irradiation at 365 nm selectively excited the first Cotton CD band at 360−390 nm directly. Conversely, CPL at 313 nm (32 μW·cm−2), which is the second Cotton CD band, alternatively induced a bisignate Cotton CD band (Figure 2c). A bisignate CD band at approximately 350 nm was induced, whereas no Cotton CD band was confirmed in the UV−visible region before irradiation. Figure 2c shows the CD and UV−visible spectra of PAzoMA2 aggregates irradiated with l- and r-CPL sources at

in the opposite ee value, regardless of whether the CPL sense was the same. More recently, our group33 reported the first CPL photon energy-dependent chiroptical swapping experiments of π-conjugated polymer aggregates; the chiroptical sense of the aggregates during CPL source irradiation was determined by the CPL source energy (CPL wavelength), regardless of whether the CPL sense was the same. However, a more important question remains to be resolved: are CPLdriven photodestruction and photogeneration in these systems very special cases, or are they generalizable ? To verify the CPL wavelength dependence of photogeneration, photoresolution, and photoswitching modes, we chose azobenzene with flexible linkers attached to chainlike polymethacrylate.34 This polymer is a CD-silent, hidden chiral polymer, and it can be dispersed as micrometer-sized aggregates in an optofluidic medium with a tuned refractive index (nD). This optofluidic system35−41 permits resonant boosting of CD amplitudes of the aggregates in CPL-driven AAS, leading to efficiently shortened photochemical reactions.34 Herein, we prove that the choice of CPL wavelength (313 nm and 365/405/436 nm) is the determining factor for photogeneration, photoresolution, and photoswitching of the azobenzene polymer aggregates, regardless of whether the sense of the CPL source is the same. Moreover, efficiently irradiating the aggregates with a CPL source at 365 nm induces nearly optically pure chiral azobenzene aggregates with a maximum dissymmetry ratio of gCD ≈ ±1.0 × 10−2 at 313 nm, as suggested by Zerner’s intermediate neglect of differential overlap (ZINDO) calculation42,43 of the corresponding model oligomers.

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RESULTS AND DISCUSSION Refractive Index Dependence of Circularly Polarized Light-Driven Absolute Asymmetric Synthesis. Figure 1 shows the CD ellipticity (in millidegrees) and gCD value at 313 nm of PAzoMA2 aggregates (DPn = 19; hydrodynamic diameter ∼200 nm) as a function of the nD value of the cosolvent by mixing 1,2-dichloroethane (DCE, nD = 1.444) and methylcyclohexane (MCH, nD = 1.422) upon irradiation with l- and rCPL at 365 nm (30 μW·cm−2) for 60 s. Similarly, the DPn dependence of the gCD value in DCE−MCH cosolvents was also tested (Figure S1). Evidently, the gCD value was maximized 13219


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Figure 2. (a) CD and UV−vis spectra of PAzoMA2 aggregates exposed to 365 nm l-CPL and r-CPL for 150 s (2.5 min). (b) gCD value (313 nm) and ellipticity (313 nm) of PAzoMA2 aggregates in DCE/MCH (0.5/2.5 v/v) under 365 nm CPL irradiation vs irradiation time. (c) CD and UV−vis spectra of PAzoMA2 aggregates after 313 nm l-CPL and r-CPL irradiation for 360 s (6 min). (d) gCD value (313 nm) and ellipticity (313 nm) of PAzoMA2 aggregates in DCE/MCH (0.5/2.5 v/v) under 313 nm CPL irradiation vs irradiation time. (e) CD and UV−vis spectra of PAzoMA2 aggregates exposed to 436 nm l-CPL and r-CPL for 360 s (6 min). (f) The gCD value (313 nm) and ellipticity (313 nm) of PAzoMA2 aggregates in DCE/MCH (0.5/2.5 v/v) with 436 nm CPL irradiation with irradiation time.

313 nm for 360 s (6.0 min). A bisignate Cotton CD band at 360−390 and 310 nm emerged but was clearly upset, regardless of the same-sense CPL source. The difference is attributed solely to the irradiation wavelength: a positive sign at 390 nm and a negative sign at 310 nm were observed for l-CPL, whereas a negative sign at 390 nm and a positive sign at 310 nm were observed for r-CPL. Figure 2d shows that the gCD amplitude (at 313 nm) asymptotically increases with irradiation time before leveling off at ±0.4 × 10−2 (600 s). The formation of trans-azobenzene stacking in side chains is responsible for the supramolecular chirality of azobenzene moieties under l- and r-CPL irradiation. To validate these results, PAzoMA2 aggregates were irradiated with visible CPL (436 nm) to photoisomerize the cis to trans forms.51

Figure 2e shows that the bisignate Cotton CD bands at 360− 390 and 310 nm are similar to those obtained via 365 nm CPL irradiation. Figure 2f shows that the gCD amplitude at 313 nm increases asymptotically with the irradiation time before leveling off at ±1.3 × 10−2 (550 s). The 436 nm band is a tail of the 390 nm π−π* transition of azobenzenes. Similarly, the CPL-AAS experiment at 405 nm (Figure S3) is almost identical to those at 365 and 436 nm. The 405 nm band is a tail of the 390 nm π−π* transition of azobenzenes. However, no detectable effects were observed for 254 nm (15.5 μW·cm−2) and 546 nm (30.0 μW·cm−2) CPL sources, as presented in Figure S4. The 546 nm band can directly excite the n−π* transition of azobenzene moieties, while the 254 nm band originates from the shorter axes of azobenzene aromatic moieties. CPL-AAS experiments can efficiently excite the 13220


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Journal of the American Chemical Society π−π* transition. CPL-AAS experiments of PAzoMA2 aggregates in MCH−DCE under irradiation with nonpolarized light at 365 nm did not induce any detectable Cotton CD signals (Figure S5). Furthermore, CPL-AAS experiments of PAzoMA2 with irradiation at 365 nm in mixed solvents with relatively higher volume fractions of DCE did not induce any detectable Cotton CD signals (Figure S6). For comparison, CPL-AAS experiments of monomer aggregates in MCH-DCE under rCPL irradiation at 436 nm were performed and did not induce any detectable Cotton CD signals (Figure S7). These results lead us to conclude that the bisignate Cotton CD band produced by the 313 nm CPL source is definitively the opposite of those resulting from the other three CPL sources (i.e., 365, 405, and 436 nm). This difference arises from PAzoMA2 aggregates (∼200 nm) with trans-azobenzene stacks having restricted conformational freedom. Meanwhile, no detectable effect was confirmed for conformationally labile PAzoMA 2 in homogeneous solution. Pristine PAzoMA 2 aggregates exhibit CD-silent or hidden chirality before CPL irradiation. To support our conclusion, we designed chiroptical polarization, restoration, inversion, switching, and memory experiments involving PAzoMA2 aggregates in tuned DCE− MCH medium using r- and l-CPL sources at 313, 365, and 436 nm. The changes in hydrodynamic diameter and shapes of PAzoMA2 aggregates were monitored in real time by dynamic light scattering (DLS; Table S1 and Figure S8) and fiber-optic microscopy (FOM; Figure S9). Notably, the spherical aggregate size (125−300 nm) was only slightly decreased after irradiation with wavelengths of 250−600 nm, which is responsible for the excellent (chir)optical stability of the aggregates. Chiroptical Switching Properties. Due to the distinctive chiroptical properties, we further demonstrated that PAzoMA2 aggregates were induced by the same handedness using two different CPL sources (r/l-CPL [313 nm] and r/l-CPL [436 nm]), as shown in Figure 3. Evidently, the gCD value changed

of l-CPL (313 nm) and l-CPL (436 nm) sources was employed (Figure 3). For comparison, Figure S10 shows the changes in the gCD value of PAzoMA2 aggregates induced by r-CPL (365 nm) and nonpolarized CPL (365 nm). Nonpolarized CPL led to chiroptical restoration and switching between zero and negative gCD values with the help of an r-CPL source. We also investigated the chiroptical switching characteristics by alternating l- and r-CPL sources. Figure 4 shows the changes in the gCD value (at 313 nm) of PAzoMA2 aggregates resulting from alternating l- and r-CPL sources at 313, 365, and 436 nm as a function of irradiation time. In this case, we employed a CPL source with a single wavelength. Upon irradiation at 313 nm, the gCD value changed from zero to a positive value (lCPL) and then inverted to a negative value (r-CPL). The induced gCD value mirrors the sense of the 313 nm CPL source. Conversely, upon irradiation at 365 and 436 nm, the gCD value changed from zero to a negative value (l-CPL) and then inverted to a positive one (r-CPL). The induced gCD value completely followed the sense of the 365 and 436 nm CPL sources. Thus, the CPL-induced gCD value can be set to zero simply by controlling the irradiation time. The chiroptical polarization, restoration, inversion, and switching processes can be controlled by choosing a single CPL source with an appropriate irradiation wavelength, irradiation time, and sense. Chiroptical Memory. We also tested long-term thermal stability based on the potential use of persistent chiroptical memory at room temperature. Figure 5a shows the time-course CD and UV−vis spectra of PAzoMA2 aggregates initially generated by irradiation with the 365 nm CPL source for 90 s. The optically active aggregates were left at room temperature. No obvious changes in the CD and UV−vis spectra of optically active PAzoMA2 aggregates were detected, regardless of whether the l- or r-CPL source was used. Figure 5b plots the CD ellipticity (in millidegrees) and gCD values at 313 nm of optically active PAzoMA2 aggregates as a function of the time they were held at room temperature and confirms that they exhibited the same tendency as the chiroptical memory. On the basis of these results, optically active PAzoMA2 aggregates have long-term chiroptical stability at room temperature for at least 7 days, regardless of whether an l- or r-CPL source is used. The chiroptical stability of the optically active PAzoMA2 aggregates slightly depends on temperature. As shown in Figure S11, the memory function of the CD-active polymer aggregates was kept in the range of −10° and 40 °C. The memory function, however, abruptly was lost at 60 °C. Possibly, DCE molecules penetrated to the interior of the aggregates, leading to swollen (loose) aggregates. Chirally assorted transazobenzene stacks were racemized. Zerner’s Intermediate Neglect of Differential Overlap Calculations. Additionally, to verify the origin of the bisignate Cotton CD bands of PAzoMA2 aggregates, we generated a racemo sequence of six repeating units bearing six azobenzene side chains as realistic models of ordered and slightly disordered PAzoMA2 in a vacuum by optimizing with a universal force field followed by ZINDO calculations. Figure 6 and Figure S12 present representative ordered and slightly disordered models of AzoMA2 hexamers. The most favorable assorted structures of azobenzene side chains tend to adopt chirally tilted π−π stacking motifs. These chiral motifs are assumed to be responsible for the bisignate Cotton CD band that was observed experimentally. However, the π−π* dipole transition moments in these motifs have an orthogonal relationship. The π−π* dipole transition at the long axis is

Figure 3. Chiroptical switching of the gCD values (at 313 nm) of PAzoMAS aggregates in DCE/MCH (0.5/2.5 v/v) under alternating 313 and 436 nm CPL irradiation, using CPL sources with the same sense.

from zero to a positive value upon 313 nm r-CPL irradiation but recovered to a value of nearly zero upon 436 nm r-CPL irradiation. Eventually, this value was inverted to a negative one by prolonged 436 nm r-CPL irradiation. The chiroptical polarization, restoration, inversion, and switching processes can be controlled by simply choosing two CPL sources with appropriate irradiation wavelengths, irradiation time, and handedness. A symmetrical change was observed when a pair 13221


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Figure 4. Chiroptical switching of gCD values (313 nm) of PAzoMA2 aggregates in DCE/MCH (0.5/2.5 v/v) under l- and r-CPL alternating irradiation: (a) 313 nm, (b) 365 nm, and (c) 436 nm.

Figure 5. Long-term stability as persistent chiroptical memory. (a) Dependence of CD spectra of PAzoMA2 aggregates on different days after initial irradiation with 365 nm CPL for 90 s. (b) Corresponding dependence of CD and gCD values (313 nm) of PAzoMA2 aggregates.

Cotton CD sign inversion depends on the x- and y-axes of pseudo-C2 symmetry, as predicted by Snatzke.52 The simulated |gCD| value at 400 nm is ∼(1−2) × 10−3, which is nearly a tenth of the experimental gCD value of ±1.3 × 10−2 (436 nm r- and lCPL for 550 s). We assume that the high experimental gCD value is responsible for the almost ideal chiral assortment of azobenzene side chains occurring in the aggregate states. Subtle Changes in Fourier Transform Infrared Spectra before and after CPL Irradiation. Finally, Fourier transform infrared (FT-IR) investigations were performed to explore the changes in intermolecular forces under CPL irradiation. Figure 7 presents the FT-IR spectra of PAzoMA2 aggregates before and during irradiation with an r-CPL source at 365 nm. Herein, strong absorption bands near 3000 cm−1 were observed, corresponding to the stretching vibrations of carbon−hydrogen bonds in azobenzene rings. The absorption bands at approximately 1730 cm−1 are characteristic of the stretching vibrations of ester CO bonds. Additionally, other ether bands near 1250 and 1150 cm−1 were observed for asymmetric and symmetric stretching vibrations, respectively (Figure S13).53 Using these three vibrational bands [v(C−H), v(CO), and v(C−O−C)], we discuss the changes in side-chain structure. Notably, no change in the FT-IR spectra of PAzoMA2 aggregates after irradiation with an r-CP UV light source for 150 s was observed. However, after prolonged irradiation for 600 s with r-CP UV light, we noted slightly significant red shifts of all three bands, v(C−H), v(CO), and v(C−O−C). These changes are ascribed to the trans−cis photoisomerization of azobenzene moieties.54,55 On the basis of FT-IR and CD spectra, this photoisomerization should break the ordered stacking structure of PAzoMA2 (365 nm r-CPL irradiation for 0 and 150 s), resulting in a direct chiroptical response, reflecting a

Figure 6. (top) Six slightly twisted trans-azobenzene side chains. (bottom) Simulated CD and UV−vis spectra of the model (half-width at half-height = 0.3 eV).

located at a longer wavelength (∼400 nm), while the π−π* dipole transition at the short axis is located at a shorter wavelength (∼300 nm). Note that the Cotton bands at the long axis and short axis are opposite. For example, the CD sign at 300 nm is positive, whereas the CD sign at 400 nm is negative. Regardless of the identical chirally assorted structure, the 13222


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Figure 7. (a) FT-IR spectra of PAzoMA2 aggregates produced in DCE/MCH (0.5/2.5 v/v) with 365 nm r-CPL irradiation for different irradiation times. (b) Magnified spectra in the range from 3200 to 2600 cm−1.

Figure 8. Proposed scheme of long- and short-axis-dependent twisted stack led by wavelength-dependent l- and r-CPL sources.

significantly diminished CD signal (365 nm r-CPL irradiation for 600 s). Scenario for Circularly Polarized Light WavelengthDependent Swapping of Azobenzene Chirality. Light, which is a massless electromagnetic wave, conveys angular momentum (s = ±ℏ) and orbital angular momentum (OAM) with mℏ (m = 0, ±1, ±2, ...).56 These momenta generate optomechanical torque to matter. In 1909, Poynting57 hypothesized that the r/l-CPL source reveals mechanical shaft action. In 1936, Beth58 used a Cavendish-type torsion balance to experimentally confirm that the r/l-CPL source at 1200 nm allows a quartz-made waveplate (6 mm in diameter) to rotate in clockwise (CW) and counterclockwise (CCW) directions, respectively, because the r/l-CPL source has angular momentum. A subtle difference in the refractive index (Δn) on the order of 10−4, that can generate a tiny rotation torque (T) on the order of 10−9 dyne·cm (10−16 N·m),58 in the x−y axes of the quartz plate was responsible for this rotation. In 1992, Allen et al.59 theoretically predicted light with OAM, followed by experimental proof that a 2-μm-size poly(tetrafluoroethylene) (PTFE) particle rotates in CCW direction at double speed and rests the rotational mode by adding and subtracting OAM to angular momentum.60 This work led to an idea of vortex light enabling tweezing of 7-μmsized droplets composed of nematic liquid crystal in water.61

Light with OAM is now called optical spanner, toroidal light, twist light, vortex light, helical light, and so on.60−67 The optomechanical torque of the r/l-CPL source is transferable to an optically anisotropic trans-azobenzene stack that possesses long and short axes within submicrometer-sized PAzoMA aggregates. If the Δn value of the trans-azobenzene stack is assumed to be 0.1, the T value should be on the order of 10−6 dyne·cm (10−13 N·m). To efficiently store the CPL source as an optical cavity in the near-UV region, a minimal aggregate size might be on the order of 100−300 nm. Molecularly dispersed PAzoMA in homogeneous solution does not fulfill this condition. The activation energy (Ea) of PAzoMA2 aggregates in the ground state (S0) to cause CPL-driven chiroptical generation, swapping, and inversion is considerably large, as demonstrated by the long-term chiroptical memory at room temperature for a week. Actually, the Ea value of an azobenzene−fluorene alternating copolymer aggregate is estimated to be approximately 20 kcal·mol−1.44 The π−π stacking forces in the aggregates become loose due to lowering of Ea in the photoexcited states (S1, S2, ...) only when the optomechanical torque by CPL source is operational. The torque is transferred to the nonhelical or CD-silent π−π stacks of trans-azobenzene pendants, followed by the emergence of preferable handed helical π−π stacks in CW or CCW rotation. The degree of helical stacks in the ground state 13223


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Figure 9. Schematic Jablonski diagram with the coupled oscillator concept68 of twisted azobenene stacks induced by wavelength-dependent irradiation of r-CPL source.

ments using high-energy polarized light source with CPL and OAM of UV-Synchrotron Orbital Radiation Facility (UVSOR, Okazaki, Japan)67 is one candidate to further prove these ideas.

is detectable by the CD spectrum, followed by chiroptical analysis as the gabs value. Swapping the helix sense of the π−π stacks in the aggregates is possible by solely changing the CPL handedness from l to r or vice versa. However, the origin of the wavelength dependence of CPL-driven chiroptical generation, swapping, and inversion of the PAzoMA aggregates is not understood, and a theoretical elucidation is needed. Previously, Snatzke52 noted that pseudo-C2 symmetrical and helically assorted motifs reveal the coexistence of the opposite chirality sense, depending on the x- and y-axes of the motifs. This situation should afford the opposite Cotton CD bands at different wavelengths. The x- and y-axis selective responses induced by the optomechanical torque could result in the apparent wavelength-dependent CPL-driven chiroptical swapping phenomenon. Bisignate Cotton CD bands are commonly observed in the present PAzoMA system and in four previous examples.17,32,33,44 A proposed scenario and a schematic Jablonski diagram associated with the coupled oscillator concept68 of the twisted azobenene stacks induced by wavelength-dependent irradiation of CPL sources are illustrated in Figures 8 and 9, respectively. The next challenge will be to detect the light−matter interaction between optically inactive substances and a polarized light source with OAM66,67 as a function of irradiation wavelength in an optofluidic medium. This OAM should provide an ultimate MSB system under a controlled far-fromequilibrium open-flow system, imparting photogeneration, photoresolution, and photoswitching modes to optically inactive substances. Solvent-free chiral polymer aggregates (named xero-aggregates) dispersed in nonfluidic UV-transparent silicone polymers with a lower nD are one candidate.68 However, the OAM-driven optomechanical responses to these chiral aggregates and xero-aggregates may break the classical Kasha’s rule and may obey anti-Kasha’s rule.68,69 Asymmetric photosynthesis, photodestruction, and photoresolution experi-

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CONCLUSION

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ASSOCIATED CONTENT

We demonstrated an enhanced bisignate Cotton effect of azobenzene side chains with restricted conformational freedom in aggregates irradiated by r- and l-CPL sources at four wavelengths (313 and 365/405/436 nm). Regardless of whether the CPL source had the same sense, the choice of wavelength (313 or 365/405/436 nm) was the determining factor of the bisignate Cotton effect. Our results suggested that CPL wavelength-dependent swapping of the product chirality is generalizable, although this phenomenon has been reported in only four systems over the past 90 years.17,32,33,44 This scenario is similar to the so-called Cotton effect, which was first described in 1895.70,71 Irradiating the aggregates with a CPL source at 365 nm induced the highest dissymmetry ratio: gCD ∼± 1.0 × 10−2 at 313 nm. On the basis of this finding, we demonstrated elaborate chiroptical functions, in which chiroptical polarization, restoration, inversion, switching, and long-term memory of the PAzoMA nanoaggregates can be more efficiently achieved by tuning an optofluidic medium via irradiation for 10−20 s with even very weak incoherent UV− visible light (i.e., on the order of 30−40 μW·cm−2) and using CPL sources with multiple wavelengths rather than a singlewavelength CPL source. Our results describe a novel and efficient approach for CPL-induced supramolecular chirality that has enormous potential in the MSB field.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b07626. 13224


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Additional text including Experimental Section; 15 figures showing gCD values, CD and UV−vis spectra, DLS plots and optical microscopic images, chiroptical switching, simulated CD and UV−vis spectra, FT-IR spectra, GPC curves, and chemical structures; two tables listing particle sizes and molecular weight characteristics (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(W.Z.) [email protected] *(M.F.) [email protected] ORCID

Laibing Wang: 0000-0002-3380-7826 Michiya Fujiki: 0000-0002-3139-9478 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.F. and L.W. acknowledge financial support from JSPS KAKENHI (16H04155). L.Y., W.Z., and X.Z. acknowledge financial support from the National Nature Science Foundation of China (21374072 and 21574089), the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, and the Program of Innovative Research Team of Soochow University. L.W. and M.F. thank Kazuki Yamazaki (for GPC), Yoshiko Nishikawa and Shohei Yoshimoto (for DLS), and Dr. Sibo Guo (for FT-IR) at Nara Institute of Science and Technology for technical assistance throughout this work. M.F. thanks Professors Masahiro Kato and Masaki Fujimoto at the Institute for Molecular Science (Okazaki, Japan) for stimulating discussion regarding angular and orbital angular momenta of light.

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