Aggregation-Induced Chirogenesis of Luminescent Polymers - ACS

Chapter 3

Aggregation-Induced Chirogenesis of Luminescent Polymers Downloaded by CORNELL UNIV on October 23, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1227.ch003

Michiya Fujiki* Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0101, Japan *E-mail: [email protected]

Herein we demonstrate aggregation-induced enhancement (AIEnh) in chiroptical signals, including circular dichroism (CD), optical rotation dispersion (ORD), and circularly polarized luminescence (CPL), of inherently emissive σand π-conjugated polymers in a homogeneous solution. This is in sharp contrast to the idea of aggregation-induced emission (AIE)-CPL materials utilizing non-emissive latent luminophores in a homogeneous solution. A restricted intramolecular rotation along C–C and Si–Si single bonds is a common idea of AIEnh- and AIE-CPL phenomena. To efficiently enhance the CD, ORD, and CPL signals, the choice of a surrounding fluidic medium with a tuned refractive index (RI) is critical because the chiral optofluidic effects play a key role in the AIEnh chiroptical effects. We showcase several AIEnh-CPL, AIEnh-CD, and AIEnh-ORD systems from optically active polymer aggregates obtained by: (i) optically active σ-conjugated polysilanes with chiral substituents; (ii) optically inactive polysilanes induced by limonene solvent chirality; (iii) optically inactive π-conjugated polymers induced by limonene solvent chirality; (iv) optically inactive photochromic π-conjugated polymer upon excitation of left- and right-handed circularly polarized light sources; and (v) optically inactive non-photochromic π-conjugated polymer upon excitation of left- and right-handed circularly polarized light sources. Our experiments should provide artificial models of an open-flow coacervates suspension in an optically tuned optofluidic medium in the photoexcited and ground states with © 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Downloaded by CORNELL UNIV on October 23, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1227.ch003

the aid of chirally shining AIEnh materials. The slowed leftand right-handed circularly polarized light of the structure suspension in a medium was assumed to be responsible for the enhanced CPL signals. Moreover, our photoexcited-induced aggregation-induced enhancement and inversion in CPL experiments shed light on open energy flow chiral systems by tuning the RI of the fluidic medium, regardless of the non-aggregated molecules, oligomers, and polymers in the ground state. AIE-CPL materials of several luminogens with large rotational freedom are further enhanceable with the help of achiral and chiral fluidic media with tuned RIs.

Introduction Historical Background Since the mid-19th century, the origin of homochirality on Earth has been one of the greatest mysteries in the modern scientific community (1–10). Living organisms are tempo-spatial and in a metastable state as a consequence of far-from-equilibrium systems (11) because life is a low-entropy open system (12). If life existed in the past on Earth, a curious question is whether stereogenic centers and/or stereogenic bonds were identical to those of our current life on Earth (1). However, answering this question remains difficult because there is a lack of fossil records and chiral molecular evidence. Moreover, the snowball earth hypothesis with the analysis of paleomagnetism asserts that Prokaryotes only inhabited Precambrian eras for approximately 2,000 million years (13, 14). Recent studies claimed that marked depletion of the 13C in 13C-/12C-isotopic ratio is direct evidence of the existence of methanogenic microbes in the Archaean era 3,500–3,800 million years ago (15–17), based on the fact that lighter 12C-containing substances in living organisms are enriched during their entire lifetime. So far, many scientists have invoked several plausible hypotheses from the primordial era to address the question. Scientists have long argued about the possibility that the circularly polarized radiation sources existing in our universe, such as γ-ray, X-ray, and vacuum UV, may become a trigger for the left-right selection of biomolecular substances (2–8). A subtle imbalance in L-/D-amino acids can catalyze the asymmetric generation of carbohydrates with a high ee, resulting in the homochiral biological world (18). A nearly racemic substance of 10–5 % ee can significantly amplify the ee value to reach nearly 100 % ee with the help of the Soai reaction (19). Coacervate Hypothesis – The Origin of Life From the 1920s–1930s, Oparlin (20) and Haldane (21) independently hypothesized that the cell-wall free coacervate might be a prototype of living cells during the chemical evolution of life. The coacervate refers to spherical-like aggregates surrounded by fluidic water. The diameter of the coacervate typically 64 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


ranges from 1 to 100 μm, almost identical to that of living cells. The aggregates made of stable organic substances were postulated to propagate with time, followed by spontaneous growth and metabolism. The hypothesis relies on spontaneous self-organization resulting from non-covalent interactions, including electrostatic, hydrogen bonding, and van der Waals forces. The hypothesis was, however, mostly abandoned because the coacervate did not evolve into living organisms. The reason why is that most coacervates are made of rigid hard particles surrounded by water in the dark. The confinement of external photon source energy and low-entropy chiral chemical substances in the coacervate are crucial for the chemical evolution of life, followed by propagation of life. If the coacervates are assumed to adopt a dynamic structure, this soft matter system might be adaptable to any changes by external chemical and physical biases. This idea led us to design a cell-wall free, semi-artificial coacervate model consisting of chain-like synthetic polymers surrounded by a mixture of chiral and achiral organic solvents. This open-flow system prompted us to investigate the tempo-spatial molecular chirality transcription of the soft aggregates. In particular, optically active polymer aggregates in the ground and photoexcited states are detectable by CD and CPL spectroscopies because photoluminescent aggregation, known as aggregation-induced emission (AIE), has become popular in recent years (22–25). Aggregation-Induced Emission (AIE) vs Aggregation-Induced Enhancement (AIEnh) in Chiroptical Signals In 2001, Tang et al. reported the first AIE effect of 1-methyl- 1,2,3,4,5pentaphenylsilole. The silole revealed an abrupt enhancement in the quantum yield (Φ) of photoluminescence (PL) from an ultraweak emissive state (≈ 0.06 %) in homogeneous ethanol solution to a highly emissive state (≈ 90 %) when the silole formed aggregate suspension in water–ethanol cosolvent (22). In 2003, this finding led to an important concept that AIE is caused by significantly restricted intramolecular rotations of multiple C–C bonds between the five peripheral phenyl rings and the silole core (23). The restricted motion is made feasible by an increase of solvent viscosity and a decrease of solution temperature. This concept was viable for polymer aggregates made of polyacetylenes substituted with 1,2,3,4,5-pentaphenylsilole in acetone-water cosolvents (24). In 2011, Tang et al. demonstrated the first AIE-circular dichroism (AIE-CD) and AIE-circularly polarized luminescence (AIE-CPL) of a silole derivative bearing two D-sugar moieties using n-hexane and dichloromethane as cosolvent (25). The optically active silole revealed a high Kuhn’s anisotropy in the CPL and CD signals in the UV-visible region. The magnitude of gCPL in a microfluidic channel reaches ≈ –0.32 at 500 nm (25). More recently, Tang et al. reported AIE-CPL of tetraphenylethylene bearing two L-valines in dichloroethane-methanol cosolvent (26). SEM/TEM images of the tetraphenylethylene suggested that the one-dimensional helical fibers are responsible for the AIE-CPL, which is of the order of gCPL ≈ –5×10–3. At approximately the same time, we reported the first aggregation-induced enhancement (AIEnh) in chiroptical signals, as proven by the gigantic enhanced 65 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


CD amplitudes of several σ-conjugated polysilanes (27–30), π-conjugated polymers, including chiral polythiophenes (31–33) and supramolecules of polyfluorene with polysaccharide (34), and cofacial π-π stacks of phthalocyanines (35–37). When used as building blocks, these polymers and molecules are inherently highly luminophoric and/or inherently chromophoric probes in the UV-visible region in homogeneous solutions. These results stimulated us to explore several AIEnh phenomena in CPL, CD, and optical rotation dispersion (ORD) since 2010. However, we did not fully understand the plausible reasoning for these AIEnh-CD, AIEnh-CPL, and AIEnh-ORD phenomena until recently. We assumed that the restriction of intramolecular and/or intermolecular rotations, known as ro-vibrational and translational modes, are commonly responsible for the AIE-CPL signal and AIEnh-CD/CPL/ORD signal amplitudes in the framework of the adiabatic relaxation process from Sn- (n = 1,2...) to S0-states. Chiral Optofluidics In 2006, optofluidics was coined as a new fusion concept of integrated optics and microfluidics (38–41). Optofluidics offers unique μm-scale liquid-based devices with great flexibility. Optofluidics is analogous to the corresponding solid-state devices with respect to the ability to (i) tailor several optical properties (particularly the refractive index (RI)) of the fluidic medium, (ii) obtain optically smooth interfaces between the media with immiscible droplets/aggregates, and (iii) easily confine photon energy into an optical cavity. Whiteside et al. demonstrated ultralow threshold multi-colored lasing action using rhodamine 560/rhodamine 640-doped microdroplets of benzyl alcohol (nD = 1.54, 20–40 μm in diameter) dispersed in fluorinated solvent (C7F15OC2H5, nD = 1.29) (42). This heterogeneous liquid device acts as an efficient optical cavity with whispering gallery mode (WGM) (43, 44). These noticeable advantages offer a great opportunity to more freely design the low-reflection-loss photoluminescent polymer aggregates with a high RI surrounded by a lower RI fluidic medium to fulfill a specific resonance condition. For example, the Fabry–Pérot cavity formed between Bragg grating reflectors in a planar microfluidic geometry revealed a resonantly enhanced transmission peak at a very specific RI value of the fluidic medium (45). It is expected that the photoluminescent aggregates with a high RI act as an optical cavity when surrounded by a fluidic medium with a lower RI that is properly tuned. Note that linearly polarized light is a superposition of pseudoscalar left (l-) and right (r-) circularly polarized light (CP-light) carrying an angular momentum of integer ±ℏ (46–48). Previously, Ghosh et al. (49) and Silverman et al. (50) independently indicated that CP-light signals can magnify the optical rotation of an isotropic chiral medium by several orders of magnitude when a coupled geometry of multiple prismatic cuvettes is filled with limonene, carvone, and camphorquinone-containing methanol. As an application of optofluidics, Mortensen et al. theoretically showed that the slow light of colloidal particles filled with a liquid medium allows for significant enhancement of the CD signal amplitude at the edges of the optical band gap (51, 52). This phenomenon is spectroscopically detectable as ORD signals in an RI-tuned fluidic medium. The 66 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


ORD spectrum detects differences in light speed between the l-CP and r-CP light of optically active aggregates as a function of the incident wavelength of the land r-CP light. Their surprising outcomes combined with the optofluidics and AIE stimulated us to investigate the chiral optofluidics of μm-sized photoluminescent polymer aggregates utilizing inherently highly photoluminescent σ- and π-conjugated polymers as the source of materials surrounded by optically inactive fluidic medium and/or by optically active fluidic media containing limonene (52–58). Herein, we employed a series of AIEnh in CPL, CD, and ORD experiments regarded as artificial models of open-flow coacervates dispersed in an optofluidic medium in the photoexcited and ground states with the aid of chirally shining AIE-related materials.

AIEnh-CD and AIEnh-CPL in the Ground and Photoexcited States Steady-State CD and CPL Spectroscopies Steady-state CPL and PL spectroscopic data provide information about the photoexcited but short-lived chiral species, whereas steady-state CD and UV-vis spectra dictate long-lived chiral species at ambient and/or lower temperatures. Based on a modified Jablonski diagram and Kasha’s rule of chiral luminophores (Figure 1), the short-lived chiral species (S1, S2 …) upon incoherent unpolarized photoexcitation are first generated by the Franck-Condon scenario of the order of 10–15 sec, followed by non-radiative relaxation process associated with ro-vibrational modes to the lowest vibronic state (S1-state with ν´=0) of the order of 10–11–10–12 sec, finally relaxing from the low-entropy chiral S1-state (ν´=0) to the high-entropy chiral ground S0-state (ν=0,1,2,3 ...) along with CPL radiation of the order of 10–9–10–6 sec. For example, an absolute temperature of 300 K equals 0.0259 eV (208 cm–1, 0.60 kcal mol–1, 0.00095 Hartrees). When luminescent molecules and polymers are excited at 400 nm (in a vacuum and in an air) by a light source, the 400-nm energy corresponds to 3.10 eV (25,000 cm–1, 71.5 kcal mol–1, 35,970 K, 0.114 Hartrees). CPL and CD spectroscopic methods can therefore detect different chiral information. CPL signals can dictate chiral species at 35,970 K, although CD signals provide chiral information about species at 300 K. When one can ensure the identity between the chiral photoexcited and ground states, an enantiopair of rigid luminophoric chromophores possessing very restricted intramolecular and/or intermolecular rotations at 35,970 K is needed. Chiral aggregate made of luminophores is one candidate to realize WGM-based chiroptical resonators with a pair of (+)- and (–)-sign AIEnh-CPL signals. This idea predicts that, although a non-rigid chiral luminophore possessing substantial ro-vibrational freedom in the ground state reveals weak CD signal and/or non-detectable CD signal (so-called cryptochirality), the luminophores may not emit CPL signals and solely emit unpolarized PL signals due to an equal probability of left- and right-handed photoexcited chiral structures. But, if a non-rigid chiral luminophore can produce a certain chiral aggregate in a 67 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


restricted translational freedom, one can detect this spatio-temporal, dynamic chiral aggregate as AIEnh-CPL signals. The lifetime of spatio-temporal species is on the order of ≈ 1 –10 nsec, which enables us to detect steady-state AIEnh-CPL signals.

Figure 1. Schematic Jablonski diagram, Kasha’s rule, and Franck-Condon rule of chiral luminophore without optical cavity effects. A representative example is CPL-active pyrene excimer systems. This phenomenon may be photoexcited induced aggregation-induced enhancement in CPL (PI-AIEnh-CPL). Recently, Imai, Fujiki, and coworkers have proved that several CD-silent and/or ultraweak CD-active pyrene-containing molecules and oligomers reveal intense pyrene excimer origin CPL signals on the order of |gCPL| ≈ 10–2, as discussed in a later section. However, reliable computer calculation to predict plausible photoexcited states of the chiral aggregates remains a major challenge. The underlying problem of the huge computational cost should be solved. An open question is whether optofluidics is actually valid in AIEnh-CD and AIEnh-CPL beyond an extension of AIE and the associated restricted intramolecular rotations. To address this question, whether the amplitudes in CD and CPL signals of the polymer aggregates are resonantly enhanced at very specific RI values of the surrounded medium had to be elucidated. Optically oriented film and/or flowing vortex conditions of the anisotropic chromophores and luminophores often cause unfavorable chiroptical signals, leading to an apparent sign inversion and alteration in the absolute magnitudes of the CD and CPL signals. To avoid these spectroscopic difficulties due to optically anisotropic film and/or vortex flowing conditions, the optically anisotropic aggregates suspended in optically isotropic fluidic medium due to the random orientation in the medium must be measured. 68 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Additionally, CPL/CD chiroptical signals (λ values, amplitudes, and signs), as well as PL/PL excitation (PLE) signals (λ values and amplitudes), are not significantly affected by Rayleigh scattering, which causes an unfavorable increment in the background UV-vis signals in proportion to λ–4 (λ: wavelength of light in a vacuum). Instrumental knowledge is one of the crucial factors to characterize an inherently chiral aggregate in the ground and photoexcited states. The RI under unpolarized incident light should be noted because incident light largely depends on the wavelength of the incident light. ORD spectroscopy can measure the difference in RI between left- and right-CP light. Unpolarized light is a superposition of left-CP (r-CP) and right-CP (l-CP) light. Thus, a fine tuning of the RI is crucial when chiral optofluidics plays a key role in AIEnh chiroptical signals. We showcased several typical examples in the following.

AIEnh-CD, AIEnh-ORD, and AIEnh-CPL Aggregates From Optically Active Polysilane with Chiral Substituents Herein, we show evidence that fine control of the RI value of the surrounding solvents is crucial to enhance AIEnh-CD, AIEnh-ORD, and AIEnh- CPL signals of helical polysilane aggregates bearing chiral substituents, including p-(S)-2-methylbutoxypheneyl-n-propyl-polysilane (1S) (27), poly(n- decyl-(S)3-methylpentylsilane) (2S) (29), poly(n-decyl-(S)-2-methylbutylsilane) (3S) (53), poly(n-dodecyl-(S)-2-methylbutylsilane) (4S) (53), and poly(n-dodecyl-(R)2-methylbutylsilane) (4R) (53) (Chart 1). In this section, we focused on mainly 3S aggregates as a function of the surrounding cosolvents to continuously tailor the RI value, whereas the RI value of the 3S aggregate was assumed to be ≈1.7 (53).

Chart 1. Chemical structures of semiflexible and rodlike polysilanes bearing chiral substituents. 69 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


The AIEnh-CD, AIEnh-ORD, and AIEnh-CPL spectra of the 3S aggregates (weight-average molecular weight (Mw) = 8.5×104, weight-averaged degree of polymerization (DPw) = 311) are given in Figure 2a–2c. From Figure 2a, the 3S aggregates clearly showed negative bisignate exciton couplet Cotton CD bands arising from the lowest Siσ–Siσ* transition at 323 nm. The magnitudes of the gCD values reach –0.31 at 325 nm and +0.33 at 313 nm. These gCD values correspond to 15.5 % left-circular polarization and 16.5 % right-circular polarization because the ideal left- and right-circular polarizations in the absorption of gCD are ±2.0, respectively.

70 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 2. (a) AIEnh-CD and UV spectra, (b) AIEnh-ORD and UV spectra, and (c) AIEnh-CPL and PL spectra of the 3S aggregates. (d) The gCD values of the 3S, 4S, and 4R aggregates vs the RI value of methanol-tetrahydrofuran (THF) cosolvent. Here, the AIEnh-CD and AIEnh-CPL spectra were normalized by dimensionless Kuhn’s anisotropy in the ground and photoexcited states (59, 60) (e) Schematic Jablonski diagram of chiral 3S aggregates. (f) Possible explanation for the AlEnh-CPL scenario by confining left- and right-circularly polarized light in chiral the 3S aggregates in an optofluidic medium (53). Reproduced with permission from ref. (53). Copyright 2011 American Chemical Society.

From Figure 2b, the 3S aggregates clearly showed negative bisignate Cotton ORD bands at the 323-nm transition. The speed of r-CP light at 330 nm is greatly slowed relative to that of l-CP light at 320 nm. Conversely, the speed of l-CP light at 320 nm is greatly slowed relative to that of r-CP light at 320 nm. Therefore, the RI values between r-CP light and l-CP light are strongly dependent on the wavelength. Higher RI values result in slower CP-light than other CP-light associated with wavelength shortening, while the wavenumber is unchanged regardless of the RI value. 71 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


In Figure 2c, the 3S aggregates clearly showed only a negative CPL band at the 330-nm PL band associated with a very small Stokes’ shift (5 nm, 466 cm–1). The small Stokes’ shift means that the structural alteration due to ro-vibrational modes of 3S aggregates in the photoexcited state is minimal. The magnitude of the gCPL value was –0.65 at 330 nm. This gCPL value corresponds to 32.5 % right-elliptical circular polarization, whereas the ideal left- and right-circular polarization in emission occurs at gCPL values of ±2.0, respectively. The AIEnh-CD and AIEnh-CPL characteristics of 3S are almost identical to those of 4S and 4R. However, these AIEnh-CD and AIEnh-CPL characteristics of 3S, 4S, and 4R strongly depend on the nD value of the methanol-THF cosolvent, as shown in Figure 2d. The gCD value of the 3S aggregates is resonantly enhanced when nD = 1.374. Similarly, the gCD values of the 4S and 4R aggregates are resonantly enhanced when RI = 1.359 for 4S and nD = 1.365 for 4R. These RIdependent resonance effects are a typical feature of chiral optofluidics, where a marked difference in speed between l- and r-CP light travelling in μm-scale colloidal particles dispersed in a tuned RI liquid medium causes enhanced CD signals in the absorption mode, followed by enhanced CPL signals in the emission mode (51, 52). Based on Figure 2a–2c, a modified Jablonski diagram of the 3S aggregates combined with the Kasha rule and exciton coupling theory is schematically given in Figure 2e (55). An optical cavity of l- or r-CP light due to a large difference in their RI values is crucial. This idea might be applicable to other polysilane aggregates carrying chiral substituents (27, 29). Figure 2f displays a possible explanation for the AlEnh-CPL signals in an optical cavity in WGM mode due to an efficient confinement of left- and rightcircularly polarized light in the chiral 3S aggregates with a high RI surrounded by optofluidic medium with a lower RI (53). The optically active aggregate acts as an optical cavity for l- and r-CP light separation. Firstly, l- and r-CP light (so-called natural light) at 317 nm (3.90 eV) simultaneously excite 3S aggregates with a high RI (n2) surrounded by a liquid with a lower RI. In this case, the RI value for l-CP light (n1(l)) at 317 nm is higher than that for r-CP light (n1(r)) at 317 nm. We hypothesized that n1(l) and n1(r) are 1.8 and 1.6, respectively, and that n2 is 1.4. In this case, the critical angles of refraction (θc) for l- and r-CP light by Snell’s law are estimated as 51° and 61°, respectively. The difference in the θc angles of l- and r-CP light acts as a chiroptical filter to sort l- and r-CP light sources at the polymer-liquid interface. Therefore, fine tuning the RI at the specific wavelength of the surrounding medium is a critical factor. This idea is valid for heterogeneous suspension systems but not for homogeneous solution systems. In other words, when the RI values of the aggregates and surrounding medium are identical, no refraction and no scattering at the polymer-liquid interface occurs. The heterogeneous system is apparently transparent. One of the examples is optically transparent TPX® (Mitsui Chemicals, poly(4-methyl1-penetene). The reason for the transparency is that the RI values of the crystalline and non-crystalline TPX® are almost identical. The l- and r-CP light in the polymer at 317 nm slow to 1.67 × 108 m sec1 and 1.88 × 108 m sec-1, respectively. However, the wavelength of 317 nm of 72 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


the incident l- and r-CP light in a vacuum becomes 176 nm and 198 nm in the polymer, respectively. As a result, the incident l-CP light at 317 nm becomes greatly slowed l-CP light of 176 nm. Similarly, the incident r-CP light at 317 nm become a slightly slower light of 198 nm in the polymer. If 176 nm l-CP light was employed, multiple total internal reflections will occur efficiently (12 times, for example) in the aggregates rather than the 198 nm r-CP light. Increasing the number of total internal reflections of CP light at the polymer-liquid interface increases the opportunity for light-matter interactions. The slowed l-CP light shifts to longer-wavelength, lower-energy r-CP light at 330 nm with a change in CD sign due to energy migration in the aggregates. The faster r-CP light at 317 nm migrates to the lower energy r-CP light at 330 nm without a change in CD sign. Due to the great suppression of the photoexcited aggregates at 330 nm, r-CP emission at 325 nm occurs from the lowest photoexcited S1 state with a minimal Stokes’ shift. This phenomenon is spectroscopically detectable as ORD signals in a RI-tuned fluidic medium. The ORD spectrum detects differences in light speed between the l-CP and r-CP light of optically active aggregates as a function of the incident wavelength of l- and r-CP light in a vacuum. From Optically Inactive Polysilanes Induced by Limonene Chirality Optically active polysilane aggregates are generated by adding poor solvent to a homogeneous solution of the corresponding polysilane carrying chiral substituents. However, this methodology requires expensive chiral source materials and multiple, time-consuming synthetic steps when chiral substituents are introduced to polysilanes. Herein, we demonstrate more facile, inexpensive, and environmentally friendly approaches to yield AIEnh-CPL and AIEnh-CD polymer aggregates (Chart 2). Previously, achiral 5 showed couplet-like AIEnh-CD with the help of (S)and (R)-1-phenylethyl alcohol and several other alkyl alcohols when the alcohol was used as solvent (29). It was noted that 5 in homogeneous solution adopts a CD-silent state (mirror symmetry) resulting from a racemic mixture of dynamic twisting between the left- and right-hand helices. During aggregation by adding a poor solvent (methanol), CD-silent 5 provided AIEnh-CD 5 as a consequence of mirror symmetry breaking, in which the CD sign is determined by the alcohol chirality. The chiral CH/O interaction is assumed to be responsible for the AIEnhCD (Chart 2) (29). This report prompted further testing of whether three CD-silent, rod-like dialkylpolysilanes (7, 8, 9) can provide the corresponding AIEnh-CD (7, 8, 9) and AIEnh-CPL (8, 9) in the presence of inexpensive terpenes, (S)- and (R)limonene (54). We assumed that non-covalent intermolecular chiral CH/π and van der Waals attractive interactions induce AIEnh-CD and AIEnh-CPL signals. In these cases, precise control of the RI of the surrounding solvents including limonene is critical as well. The AIEnh-CD and UV spectra of aggregates 7 and 8 are given in Figure 3a and 3b, respectively. For aggregate 7 in 10R-containing solvent, the magnitudes of the bisigned gCD values at 327 nm and 309 nm were +0.022 and –0.031, respectively. For aggregate 7 in 10S-containing solvent, the gCD values at 327 nm 73 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


and 309 nm were –0.021 and –0.033, respectively. Compared to the AIEnh-CD values of aggregate 7, the AIEnh-CD values of aggregate 9 were comparable, but aggregate 8 was decreased by one third. For aggregate 8 in 10R-containing solvent, the magnitudes of the bisigned gCD values at 331 nm and 317 nm were –0.007 and +0.010, respectively. In 10S-containing solvent, the gCD values at 331 nm and 317 nm were +0.005 and –0.007, respectively. The degree of circular polarization of 7, 8, and 9 was rather weak, on the order of 1.0–1.5 % circular polarization.

Chart 2. Chemical structures of CD-silent polysilanes and chiral solvents (1-phenylethyl alcohol and limonene).



Figure 3. AIEnh-CD and UV spectra of (a) aggregate 7 and (b) aggregate 8. (c) The gCD value of aggregate 7 vs Mw of 7. (d) AIEnh-CPL and PL spectra of aggregate 8. The gCD values of (e) aggregate 7 and (f) aggregate 8 as a function of the limonene-containing solvent. Reproduced with permission from ref. (54). Copyright 2012 Royal Society of Chemistry.

The absolute magnitudes of the AIEnh-CD values of 7, 8, and 9 had intense Mw dependence. A representative example of 7 is given in Figure 3c. When the Mw of 7 was 2.7×104, the gCD value was maximized; 8 and 9 had similar Mw-dependent AIEnh-CD effects (54). Only 8 and 9, with weaker gCD values, had AIEnh-CPL signals of ≈ 0.005 as the absolute gCPL values (Figure 3d). However, aggregate 7 did not have PL and CPL due to unresolved reasons. From Figure 3a–3b, it is evident that the CD sign of aggregate 7 is opposite of that of aggregate 8 when the same limonene chirality was employed as the solvent. However, the CD sign of 7 inverted at the specific RI value (nD = 1.36) of the limonene-containing solvent (Figure 3e). The gCD value resonantly enhanced twice at nD = 1.35 and 1.39. The CD sign of 8 was unchanged by the RI value of limonene-containing solvent but had an abrupt transition at nD = 1.41 (Figure 3f). Thus, subtle structural changes in the side chains of dialkylpolysilanes significantly affect the PL and CPL characteristics as well as the chiroptical sign.


From Optically Inactive π-Conjugated Polymers Induced by Limonene Chirality


Three CD-silent aggregates, 7, 8, and 9, in homogeneous solution provided the corresponding AIEnh-CPL (8, 9) and AIEnh-CD (7, 8, 9) in the presence of (S)and (R)-limonene (54). These results stimulated us to further test the possibility that achiral (CD-silent) π-conjugated photoluminescent polymers reveal AIEnhCD and AIEnh-CPL effects in the presence of (S)- and (R)-limonene (55, 56, 61–65).

Chart 3. Chemical structures of CD-silent π-conjugated polymers. Herein, we demonstrated the first successful limonene chirality transfer experiment of AIEnh-CD and AIEnh-CPL poly[(9,9-dioctylfluorenyl-2,7-diyl)alt-bithiophene] (PF8T2, 14) among several π-conjugated polymers (Chart 3). The AIEnh-CD and AIEnh-CPL characteristics of π-conjugated polymers (15, 17, 18, 20, 21, 22) are similar to those of 14. The AIEnh-CD and AIEnh-CPL characteristics (gCD and gCPL) of these aggregates were resonantly enhanced at the RI value of the limonene-containing solvent. The UV-vis and PL spectra of 14 in homogeneous chloroform are shown in Figure 4a. The PL spectrum has at least three well-resolved vibronic bands located at 500, 534, and 578 nm with ≈1350 cm–1 spacing. The corresponding UV-vis absorption band has a structureless broad band peaking at 457 nm associated with a weak shoulder at 478 nm. This indicates that 14 adopts a highly ordered πconjugated structure (a low entropy state) in the photoexcited state. Note that 14 is in a considerably disordered π-conjugated state (a high entropy state) in the ground state. These UV-vis and PL spectral features in homogeneous solution are typical characteristics of random coiled polysilanes (66, 67). The rotational freedom with an equal probability between left-and-right twisting modes of C–C, Si–Si, and Si–C single bonds in the ground state is responsible for the broadened 76 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


UV-vis absorption bands, resulting in no detectable Cotton CD band. Typically, the barrier heights of these single bonds are on the order of 1–2 kcal mol–1 based on our calculations (54, 55, 57, 63). During the aggregation process in the presence of limonene, 14 revealed AIEnh-CD and AIEnh-CPL spectra due to the loss of rotational freedom, as evident from Figure 4b and 4d. The magnitude of the bisigned gCD reached –0.085 at 510 nm and +0.042 at 394 nm (10R) and +0.114 at 510 nm and –0.041 at 394 nm (10S). Concurrently, the magnitude of the bisigned gCPL reached +0.012 at 489 nm and –0.058 at 511 nm (10R) and –0.010 at 489 nm and +0.056 at 511 nm (10S). In the presence of 10R, the (–)-sign 511 nm-CPL band originates from the (–)-sign 510 nm-CD band, while the (+)-sign 489 nm-CPL band originates from the (+)-sign 394 nm-CD band. The presence of 10S as the solvent provides the opposite chiroptical signs of 10R.



Figure 4. (a) UV-vis and PL spectra of 14 in homogeneous chloroform solution. (b) AIEnh-CD and UV spectra of unfiltered aggregate 14 in limonene-chloroform-methanol tersolvent. (c) AIEnh-CPL and PL spectra of unfiltered 14 in limonene-chloroform-methanol tersolvent. (d) The gCD value at 510 nm (the first Cotton band) as a function of the RI of the mixed solvent. (e) The gCD value of unfiltered 14 as a function of the limonene ee value. (f) The gCD value of 14 as a function of the aggregate size. Reproduced with permission from ref. (61). Copyright 2012 Royal Society of Chemistry. Similarly, the gCD value of 14 aggregates is resonantly enhanced at the nD = 1.44 of the limonene-containing solvent, as shown in Figure 4c. However, the gCD value varied extremely as a function of the ee value of limonene, exhibiting the socalled negative cooperative effect, as plotted in Figure 4e. This result indicates that enantiopure homochiral limonene needs the highest gCD value. We were aware that 78 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

the gCD value tends to decrease when the aggregate size decreases, as seen in Figure 4f. A larger size of aggregate may offer an advantage to facilitate a morphologydependent resonance condition in WGM-based chiroptical resonators, as expected by theory (68) and experiment (42).


From Optically Inactive Photochromic π-Conjugated Polymer Induced by Circularly Polarized Photon Chirality We demonstrated AIEnh-CD and AIEnh-CPL effects utilizing CD-silent σand π-conjugated polymers in the presence of (S)- and (R)-limonene. The gCD and gCPL signals are resonantly enhanced at the specific RI values of the optofluidic solvents. These results led us to further test the possibility of fully controlled absolute asymmetric synthesis (AAS) of two π-conjugated polymers as aggregate forms 14 (57) and 16 (58) by tuning the RI of the solvents (Chart 4).

Chart 4. Chemical structures of CD-silent π-conjugated polymers and alcohols used as a lower RI solvent with the aid of fully controlled AAS experiment using an r- and l-CP light source. Fully controlling refers to all chiroptical modes of chiroptical polarization, depolarization, inversion, retention, and switching. Historically, the possibility of AAS using an r- and l-CP light source was proposed independently by LeBel in 1874 and van’t Hoff in 1894 (3, 4). This conjecture was experimentally proven by Kuhn and Broun in 1929 (69). Their pioneering works led to many AAS studies over 150 years because specific, expensive chemicals may be not needed (70). However, researchers have long believed that an l-CP light source produces left-handed molecules preferentially or vice versa because the product chirality is determined solely by the handedness of CP light. In this section, we demonstrate the capability of r- and l-CP light-controlled chiroptical polarization, depolarization, inversion, retention, and switching of μm-sized aggregates made of 14 and 16 in achiral optofluidic media, as proven by the AIEnh-CD and AIEnh-CPL spectra. The AIEnh-CD and UV spectra of aggregate 16 upon excitation with an rand l-CP light source at 436 nm are given in Figure 5a. The gCD values at the first and second Cotton bands are –0.025 at 500 nm and +0.021 at 367 nm (r-CP light), whereas they are +0.025 at 509 nm and –0.027 at 364 nm (l-CP light). However, the AIEnh-CPL and AIEnh-PL signals were not feasible because 16 is not emissive due to an aggregation-caused quenching (ACQ) effect (22–26). Note that nonaggregate 16 and unsubstituted azobenzene in chloroform are weakly emissive. 79 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


The negative-couplet AIEnh-CD 16 generated by r-CP light at 436 nm turned into nearly zero AIEnh-CD upon l-CP light excitation at 436 nm for 5–10 min (Figure 5b). Further l-CP light excitation for 51 min led to an ideal, mirror-image, positivecouplet AIEnh-CD signal (Figure 5b). An alternative excitation between the r- and l-CP light source enables chiroptical inversion between the positive- and negativecouplet AIEnh-CD signals (57). The Arrhenius plots of aggregate 16 and unsubstituted azobenzene during thermal cis-to-trans isomerization indicated that the activation energy (Ea) from cis-16 to trans-16 in chloroform-methanol cosolvent is ≈22 kcal mol–1 (57, 71), which is slightly higher than that of azobenzene of ≈18 kcal mol–1 (Figure 5c). The Ea value of aggregate 16 may be responsible for the long-term thermal chiroptical stability at ambient temperature. From the activation enthalpy–activation entropy (ΔH‡_ΔS‡) relationship of the Eyring plot, the thermally excited (cis-to-trans) isomerization, possibly, the photoexcited (trans-to-cis) isomerization of 16 main chains in the aggregates, may obey the rotation mechanism of azobenzene moieties rather than the inversion mechanism (57). Regardless of 16, with considerably restricted rotational freedom, the photon chirality of the r- and l-CP light source should induce the generation, inversion, and retention of AIEnh-CD signals in the aggregate.



Figure 5. (a) AIEnh-CD and UV spectra of aggregate 16 upon excitation with an r- and l-CP light source at 436 nm. (b) AIEnh-CD of 16 initially generated by l-CP light at 436 nm, followed by l-CP light excitation at 436 nm. (c) The Arrhenius plots of 16 and unsubstituted azobenzene during thermal cis-to-trans isomerization. (d) Alcohol-dependent thermal stability of the CP-light source induced cis-16 aggregate at 25 °C. The gCD values as a function of the RI values of (e) non-branched alcohol and chloroform cosolvents and (f) isoalcohol and chloroform cosolvents. Reproduced with permission from ref. (57). Copyright 2013 Royal Society of Chemistry.

However, the degree of thermal chiroptical stability is greatly dependent on the nature of the alcoholic solvents. The gCD value of 16 in non-branched alcohols and isopropanol tends to diminish in two days from the half-life of the original gCD value (Figure 5d). In particular, methanol had a short lifetime of 4–5 hrs. The gCD value of 16 in isobutanol was notably unchanged for at least two days. Thus, the proper choice of alcohol is another crucial factor to generate and retain the chiroptical properties of the aggregates. More importantly, fine control of the RI value of the alcoholic chloroform solvents is important regardless of the nonbranched and branched alcohols (Figure 5e–5f). Regardless of the r- and l-CP light source excitation, the gCD values of aggregate 16 are resonantly enhanced at nF = 1.382 (methanol), 1.404 (ethanol), 1.410–1.412 (n-propanol), 1.418 (n-butanol), 1.426 (n-pentanol), 1.405–1.411 81 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

(isopropanol), and 1.415 (isobutanol). The resonance points of the gCD values are subtly altered by the nature of the alcohol, a longer alkyl chain tends to shift to a larger nF value.


From Optically Inactive Non-Photochromic π-Conjugated Polymer Induced by Circularly Polarized Photon Chirality The knowledge and understanding of photochromic, but non-emissive aggregate 16 led us to design a fully CP-light controlled AIEnh-CPL and AIEnh-CD polymer aggregates. To achieve this goal, highly photoluminescent but non-photochromic 14 was chosen for the AAS experiments using a solely wavelength-dependent r- and l-CP light source.



Figure 6. AIEnh-CD and UV spectra of aggregate 14 excited with an (a) r-CP light source at 546 nm and an l-CP light source at 365 nm and (b) an l-CP light source at 546 nm and an l-CP light source at 365 nm. (c) AIEnh-CPL and PL spectra of aggregates 14 excited with an r- and l-CP light source at 546 nm. (d) The gCD values as a function of the nD values of the cosolvents. (e) AIEnh-CD inversion of aggregate 14 excited with an r- and l-CP light source at 546 nm. (f) Thermal stability of aggregate 14. The gCD value as a function of solvent temperature. Reproduced with permission from ref. (58). Copyright 2015 Royal Society of Chemistry. The AIEnh-CD and UV spectra of aggregate 14 after excitation with an l-CP light source at 546 nm and 365 nm and an r-CP light source at 546 nm and 365 nm are given in Figure 6a–6b, respectively. The positive sign couplet CD spectrum induced by l-CP light source at 546 nm is completely inverted compared to that of the l-CP light source at 365 nm. Similarly, the negative sign couplet CD spectrum induced by the r-CP light source at 546 nm is completely inverted compared to that of the r-CP light source at 365 nm. CP light excitation at 313 nm and 405 nm gave similar trends as the 365-nm excitation, while CP light excitation at 436 nm and 577 nm had similar trends as 546-nm excitation. For the same l- (and r-) CP light, the choice of shorter (UV) and longer (visible) wavelengths of CP light switched the sign in the chiroptical polarization of aggregate 14. The handedness of CP-light, whether left or right, was not a deterministic factor for the AIEnh-CD sign of aggregate 14. 83 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


The AIEnh-CPL and AIEnh-PL spectra of aggregate 14 by excitation with an l- and r-CP light source at 546 nm are given in Figure 6c. A positive sign couplet CPL spectrum induced by r-CP light source at 546 nm and negative sign couplet CPL spectrum induced by l-CP light source at 546 nm can be observed. The magnitude of gCPL is weak and on the order of 10–3. Upon r-CP light excitation, aggregate 14 had weak positive-sign CPL at 570 nm arising from the positive-sign CD at 540 nm, while the aggregates showed weak negative-sign CPL at 518 nm originating from the negative-sign broad CD at 380 nm. The handedness of the CP light and its irradiating wavelength successfully allowed the generation of CPLactive 14 aggregates (quantum yield ≈ 8 %) on the order of |gCPL| = (2−4)×10–3 at 540 nm. The AIEnh-CD of aggregate 14 generated by r-CP light at 546 nm for 30 nm irradiation completely inverted from a positive-sign couplet to a negative-sign couplet by solely l-CP light at 546 nm for prolonged 120 nm irradiation, as shown in Figure 6d. Massless photon chirality carrying angular momentum is an efficient chiral physical force that enables chirality to be induced and inverted from the aggregates made of achiral substances at ambient temperature. By optofluidically tuning the RI of the cosolvents, the CP light-induced AIEnh-CD signals were resonantly enhanced at a specific nD =1.412, regardless of the r- and l-CP light source excitation (Figure 6e). The AIEnh-CD signals of aggregate 14 were thermally stable and unchanged at 25 °C for at least seven days (Figure 6f). For comparison, detectable CD signals of non-aggregate 14 in homogeneous CHCl3 solution before and after prolonged irradiation with r-CP light at 546 nm were confirmed. The restricted C–C rotational freedom along with efficient confinement of the CP light source in the aggregates as the optical cavity are critical factors in designing CP light-driven AAS experiments. Recent theoretical study (72) shows that a chiroptical enhancement is possible when an optically active aggregate with an ideal chiral sphere efficiently interacts with the surrounding chiral molecules. This means that the AIEnh-CPL and CD signals of chirally assorted, soft-matter aggregates may be further enhanced with the help of an optically tuned chiral fluidic medium. CP light plays a key role in the migration and delocalization of photoexcited energy in optically active macro-aggregates containing ~108 molecules of chlorophyll under excitation of incoherent unpolarized sunlight (73–76). This chiral aggregate requires the existence of three stereogenic centers at the peripheral positions of the chlorophylls and two chiral stereogenic centers in the long alkyl tail of the chlorophylls. Notably, the CP light-related photophysical and biological properties of the chiral aggregates are adaptable to any changes in osmotic pressure, Mg2+/K+ ions, sunlight intensity, and temperature (73–76), because the chiral macro-aggregates are surrounded by fluidic aqueous medium named stroma within the chloroplasts. The adaptability to external chiral chemical substances and/or chiral physical forces appears essential in the chemical evolution and propagation of life (76) by acquiring several sophisticated biological functions during the entire lifetime. Our experiments provide artificial models of an open-flow coacervates suspension in an optically tuned optofluidic medium in the photoexcited and ground states with the aid of chirally shining AIEnh materials (77). 84 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Photoexcited-Induced Enhancement and Inversion of CPL Signals Recently, we demonstrated several PI-AIEnh-CPL molecular systems (25–29) by using fluidic and non-fluidic media (Chart 5) (78–82). These rather simple molecules (25–29), however, can adopt 7,000–24,000 conformations due to free rotations along the C–C, C–O, and C–N single bonds with low rotational barriers in the ground state (80, 81). The system is an extension of the spatio-temporal, open-flow energy transition from low- to high-entropy states that should obey the arrow of time (second law of thermodynamics) (80). These conformationally labile molecular systems in the ground state did not have detectable CD signals and/or had ultraweak CD signals (80, 81). In particular, 29 revealed abrupt CPL signals during photoexcitation (80), and, more surprisingly, 25 and 26 had chiroptical sign inversion characteristics between the CPL and CD signals (81). These phenomena are applicable to designing elaborate photoexcited-induced AIE systems with restricted motion that are non-aggregated luminogens in the tuned RI of the fluidic medium without the addition of a precipitation solvent.

Chart 5. Chemical structures of CPL-active but CD-silent, ultraweak CD-active bi-pyrene-containing molecules in a low-viscosity solvent. 85 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Conclusion In this chapter, we demonstrated the aggregation-induced enhancement of the AIEnh-CD, AIEnh-ORD, and AIEnh-CPL signals of σ- and π-conjugated polymer aggregates. The starting polymers studied here are inherently highly emissive in homogeneous solution. The restricted intramolecular and/or intermolecular rotations are critical to AIEnh and aggregation-induced emission (AIE) phenomena in the ground and photoexcited states. Herein, we emphasized that, to efficiently enhance the CD, ORD, and CPL signals, the choice of surrounding fluidic medium with a tuned RI is a critical factor because the chiral optofluidic effects play a key role in these AIEnh chiroptical signals. We showcased several examples of optically active polymer aggregates obtained from (i) optically active alkylarylpolysilanes and dialkylpolysilanes with chiral substituents, (ii) optically inactive alkylarylpolysilanes and dialkylpolysilanes by solvent chirality, including limonene, (iii) optically inactive, polyfluorene analogs induced by limonene chirality, (iv) optically inactive, photochromic poly(fluorene-alt-azobenzene) induced by circularly polarized photon chirality, and (v) optically inactive, non-photochromic poly(fluorene-alt-bithiophene) induced by circularly polarized photon chirality. Our results suggest that the finely tuned RI of a fluidic medium in the ground and photoexcited states causes resonantly enhanced AIE-CPL signals by utilizing the ideally spherical chiral aggregates made of ultraweak emissive luminogens. Moreover, our recent PI-AIEnh-CPL experiments shed light on open energy flow chiral systems by tuning the RI of the fluidic medium, regardless of the non-aggregated molecules, oligomers, and polymers in the ground state.

Acknowledgments The author is grateful for the financial support from a Grant-in-Aid for Scientific Research (16655046, 21655041, 22350052, 23651092, 26620155, and 16H04155). The author expresses special thanks to his coworkers, including Dr. Yoko Nakano, Kana Yoshida, Yoshifumi Kawagoe, Dr. Yang Liu, Dr. Ayako Nakao, Dr. Nozomu Suzuki, Dr. Makoto Taguchi, Prof. Kotohiro Nomura, Dr. Mohamed Mehawed Abdellatif, Dr. Nor Azura Abdul Rahim, Abd Jalil Jalilah, Fumiko Ichiyanagi, Prof. Seiji Shinkai, Dr. Takao Noguchi, Dr. Masanobu Naito, Dr. Hiroshi Nakashima, Dr. Seiji Toyoda, Prof. Wei Zhang, Prof. Yonggang Yang, Prof. Julian R. Koe, and Prof. Yoshitane Imai.

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