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Ambidextrous Chirality Transfer Capability from Cellulose Tris(phenylcarbamate) to Non-helical Chain-like Luminophores: Achiral Solvent Driven Helix-helix Transition of Oligo- and Polyfluorenes Revealed by Sign Inversion of Circularly Polarized Luminescence (CPL) and Circular Dichroism (CD) Spectra Sibo Guo, Hiroki Kamite, Nozomu Suzuki, Laibing Wang, Asuka Ohkubo, and Michiya Fujiki Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.7b01554 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017

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Ambidextrous Chirality Transfer Capability from Cellulose Tris(phenylcarbamate) to Non-helical Chain-like Luminophores: Achiral Solvent Driven Helix-helix Transition of Oligo- and Polyfluorenes Revealed by Sign Inversion of Circularly Polarized Luminescence (CPL) and Circular Dichroism (CD) Spectra Sibo Guo,a Hiroki Kamite,a Nozomu Suzuki, a,b Laibing Wang,a Asuka Ohkubo, a and Michiya Fujikia* a

Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara, 630-0192, Japan

b

Department of Chemistry, College of Science, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan

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ABSTRACT.

We

investigated

whether

helicity

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and/or

chirality

of

cellulose

tris(phenylcarbamate) (CTPC) can transfer to non-charged, non-helical oligo- and polyfluorenes (F3, F5, F7, PF6, and PF8, Chart 1) when CTPC was employed as a solution processable homochiral platform of a D-glucose-skeletal polymer. Noticeably, CTPC revealed the solventdriven, ambidextrous intermolecular helicity/chirality transfer capability to these fluorenes. The chiroptical inversion characteristics of circularly polarized luminescence (CPL) and the corresponding CD spectra were realized by solely choosing a proper achiral solvents and/or achiral cosolvents. When the solution of PF6 and CTPC in tetrahydrofuran (THF) was casted on a quartz substrate, the dissymmetry ratio of CPL (gCPL) from the polymer film showed gCPL = +2.1 × 10–3 at 429 nm. Conversely, when dichloromethane (DCM) was used as the solvent, the CPL sign was inverted to gCPL = –2.4 × 10–3 at 429 nm. The dissymmetry ratio of Cotton CD band (gCD) from the THF solution was gCD = +3.2 × 10–3 at 392 nm, conversely, from the DCM, the CD sign inverted to gCD = –0.8 × 10–3 at 371 nm. The sign and magnitude of the gCD values were interpreted to a London dispersion term (δd) of Hansen solubility parameter (δ) of the casting solvents rather than a dipole-dipole interaction term (δp) and a hydrogen bonding interaction term (δh) of the δ values and dielectric constant (ε). Analysis of solvent-driven changes in FT-IR spectra, wide-angle X-ray diffraction profiles, and differential scanning calorimetry diagrams indicated that solvent driven on-off switching of multiple hydrogen bonds due to three urethane groups of CTPC play the key for the inversion. Intermolecular CH/π and

π-π interactions among phenyl rings and alkyl groups were assumed to be crucial for helicity/chirality transfer capability based on molecular mechanics and molecular dynamics simulations of PF6–CTPC hybrids. These chiroptical inversion characteristics arose from rather


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solvent-driven order–disorder transition characteristics of CTPC helix than a helix-helix transition of CTPC itself.


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INTRODUCTION In 1919, Eligio Perucca observed optical rotatory dispersion (ORD) from colored chiral NaClO3 crystal grown from a water solution containing a propeller-like cationic triarylmethane dye, extra China blue.1,2 The dye may be an ORD-silent atropisomer possessing with a dynamic chirality. This would be the first crystal chirality controlled mirror-symmetry-breaking experiments. NaClO3 exerts as an inorganic crystal chirality platform for catalyst-free, enantioselective physisorption and inclusion. Recently, this concept is developed to various dyeinclusion-crystals, which are a realm of supramolecules consisting of dyes and inorganics.2-4 On the other hand, helical motifs are ubiquitous in animate and inanimate worlds.5,6 The motifs play crucial roles in several functionalities, including enantiomer recognition,7 nonlinearly boosted chiroptical response in the ground state (GS) and photoexcited state (ES),8 enzymatic reaction,9 light-harvesting photosynthesis,10 photoscissable scaffolding,11 and command surface.12 However, regardless of the presence of stereogenic centers, the helix sense often inverts to the opposite one when external chemical and physical biases are applied. This phenomenon is called helix-helix (HH) transition and/or stereomutation. These transitions are detectable as chiroptical inversion characteristics utilizing ORD, circular dichroim (CD), circularly polarized luminescence (CPL), vibrational circular dichroim, and Raman optical activity spectroscopy.13 The HH transition is classified to intra- and inter-molecular events. The former is likely to occur by a cooperative manner of multiple stereogenic single/double/triple bonds, whilst the latter requires a great reorganization between chiral/helical and achiral/non-helical structures. The representative intramolecular HH transition is BZ transition of GC-pair oligo-DNA and GC-rich calf-thymus DNA.14–16 Certain protein also reveals HH transition from a right-hand α-


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helix to a left-hand β-sheet.17,18 The nature of the substituents in several cellulose alkyl esters and cellulose aromatic carbamates cause an inversion of helix preference in solution and lyotropic liquid crystalline phase.19,20 Until now, many artificial polymers and supramolecular stacks, e.g., poly(L-aspartate

β-ester)s,21

polyisocyanates,22,23

polyacetylenes,24-26

polysilanes27

polyquinoxaline,28 polyguanidine,30 and others,31-33 undergo intramolecular HH transition and/or chiroptical inversion in dilute solutions, as aggregates in fluidic solvents, and in the solid film states. Contrarily, intermolecular HH transition systems are relatively rare. The nature of achiral solvent molecules affects the ES and GS chirality of luminophores. Solvent quantity of chiral molecules (e.g., alcohols, limonene, and α-pinene), a small amount of chiral additives, and achiral solvents cause sign inversion in CD and/or CPL spectra when π-/σ-conjugated polymers are employed.34-39 Simple chiral molecular luminophores in homogeneous solutions invert their CPL and CD signs by the nature of achiral solvent molecules.40-42 Recently, cellulose in the forms of microcrystalline nanofibers and solution-processable derivatives are a growing interest.43-49 Among several semi-synthetic cellulose derivatives, cellulose tris(phenylcarbamate) (CTPC) coated onto a porous silica gel exhibits an excellent enantioselectivity as a chiral stationary phase (CSP).45-48 A small diastereomeric energy difference on the order of ~0.1 kcal mol–1 at molecular surface of CTPC is responsible for the enantioselectivity of aromatic compounds.46,47 Molecular mechanics (MM) and molecular dynamics (MD) simulations indicate an importance of π–π interactions between CTPC and aromatic enantiomers. Maintaining intra-and inter-molecular hydrogen bonds between O-C=O and H-N groups of three urethane groups at CTPC ensures the CSP performance.46,47 To buildup these hydrogen bond networks of CTPC, tetrahydrofuran (THF) is empirically used as a


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coating solvent to the silica due to unresolved reasons.45-48 A poorer cosolvent consisting of isopropanol and n-hexane is chosen as an eluent of CSP.45-48 In a previous paper,49 we reported that cellulose triacetate (CTA) and cellulose acetate butyrate (CABu) exert as non-charged solid polymer chirality platforms that provide helicity and/or chirality transfer capabilities to non-charged oligo- and polyfluorenes (F3, F5, F7, PF6, and PF8, Chart 1) in a catalyst-free, zero-step synthesis at ambient temperature. These fluorenes reveal clear CPL and CD spectra with a considerably high dissymmetry ratio of ~10–3 in UVvisible region. These oligo- and polyfluorenes are inherently CD-silent ones because of restricted rotations along multiple C–C bonds between fluorene rings due to C-H/H-C repulsion (Chart 1). Curiously, the induced CD and CPL signs were inverted in response to an opposite helix preference of CTA (left) and CABu (right) regardless of a common D-glucose framework. Intermolecular C–H/O=C interactions between fluorenes and cellulose alkyl esters are responsible for inter-molecular helicity/chirality transfer capability. In this paper, we demonstrate the first ambidextrous intermolecular helicity/chirality transfer capability of CTPC to F3, F5, F7, PF6, and PF8. Although CTPC obtained by one-step synthesis from unsubstituted cellulose is a single polymeric chirality platform, these unique inversion characteristics in CPL and CD spectra were realized by choosing a proper solvent among THF, dichloromethane (DCM), 1,2-dichloroethane (DCE), pyridine (Py), chloroform (CHCl3), and THF-DCM cosolvents. Solvent driven on-off hydrogen bonding state of the urethane groups played the key for the inversion.


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Chart 1. Chemical structures of cellulose triphenylcarbamate (CTPC), 9,9-di-n-hexylfluorene trimer (F3), 9,9-di-n-hexylfluorene pentamer (F5), 9,9-di-n-hexylfluorene heptamer (F7), poly(9,9-di-n-hexylfluorene)

(PF6),

poly(9,9-di-n-octylfluorene)

(PF8),

poly(9,9-di-n-

octylfluorene-alt-ethynylene) (PF8E), poly(9,9-di-n-octylfluorene-alt-bithiophene) (PF8T2), and poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (PF8BT). Restricted C–C bonds between fluorene rings due to C-H/H-C repulsion are responsible for latent CD-signals.

RESULTS AND DISCUSSION The CD spectra of CTPC film onto a quartz substrate showed commonly (-)-sign of the Cotton effect at 200–260 nm due to π-π* and n-π* transitions of the phenylcarbamates regardless of THF and DCM as spin-coating solvents. On the other hand, CTA and CABu films casting from their CHCl3 solutions showed (-)-sign and (+)-sign CD spectral profiles (Figure S3a, supporting


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information (SI), arising from that CTA prevailing right-hand helix and CABu prevailing lefthand helix in solutions,19,20 respectively. CTPC is a single polymer chirality platform but has bistability of helix sense in lyotropic liquid crystalline phase.19 The helix sense is inverted by the nature of solvent molecules (alkyl ether, alkyl ketone, and alkyl ester).19 CTPC thin films prepared from THF and DCM solutions showed commonly similar (-)-sign CD spectral profiles at 200–260 nm, although the amplitude of (-)-CD signals greatly depends on THF and DCM (Figure S3b, SI). The (-)-CD magnitudes from the THF coating is stronger than that from the DCM coating, suggesting a well-ordered helical state from THF but an ill-ordered helical state form DCM. This great chiroptical difference in the (-)-CD signals associated with certain structural alteration should lead to the solvent-driven inversion in CD- and CPL-spectra of F3, F5, F7, PF6, and PF8. The chirality transfer capability endowed with CTPC was proven by their clear CD/UVvisible and CPL/PL spectra (Figure 1). The chirality efficiency slightly depended on the fluorene ring number (n) in CTPC films (Figure 2). The chiroptical inversion characteristics are interpreted with physicochemical parameters of solvents (Figure 3), changes in IR frequencies of functional groups (Figure 4), changes in heart enthalpy by differential scanning calorimetry (DSC) (Figure 5), and two major reflections of wide-angle X-ray diffraction (WAXD) (Figure 6) of CTPC prepared by THF, DCM, and Py solutions.

Chiroptical characteristics of oligo- and polyfluorenes in CTPC Figure 1a-1b and Figure S4a-S4d (SI) show UV−visible and CD spectra of F3, F5, F7, PF6, and PF8 in the range of 300−420 nm, that are assigned to π−π* transitions of the fluorenes.50 In these hybridized films, F3, F5, F7, PF6, and PF8 reveal an intense positive-sign CD signal at the


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first Cotton band in the 360−400 nm range when THF is employed as solvent. Conversely, F3, F5, F7, PF6, and PF8 have an intense negative-sign CD band at 360−400 nm when DCM is used as solvent. According to our preliminary tests, CTPC did not induce CD spectra to other polyfluorene derivatives (PF8T2, PF8E, and PF8BT, Chart 1) because of a non-restricted rotational freedom along multiple C–C single bonds of the backbones due to lack of C-H/H-C repulsion.


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Biomacromolecules

3

(e) /10–3

2

CPL

at ≈ λ

em

1

0

-1

g

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

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-2

-3 0

20

40

60

80

100

THF in THF-CH Cl cocolvent (%, v/v) 2

2

Figure 1. CD and UV-visible spectra of F3, F5, F7, and PF6 embedded in CTPC film prepared from (a) THF and (b) DCM solutions, respectively. CPL and PL spectra of F3, F5, F7, and PF6 embedded in CTPC film prepared from (c) THF and (d) DCM solutions, respectively. (e) The gCPL value of PF6 embedded in CTPC film, prepared from pure THF and DCM solvents and THF-DCM cosolvent, as a function of volume fraction of THF. (f) The changes in the gCPL values monitored at CPL λext wavelengths of F3 (red), F5 (orange), F7 (green), and PF6 (blue) in CTPC films at 293 K on a quartz substrate under unpolarized light excitation. Excited at 345 nm for F3, at 355 nm for F5, and at 360 nm for F7 and PF6. The raw data were obtained using a built-in program of the JASCO CPL-200 spectrofluoropolarimeter and plotted as gCPL = ellipticity (in mdeg)/[32980/ln10]/PL (DC in Volts). This is equivalent to gCPL = (IL – IR)/[1/2•(IL + IR)], where IL and IR stand for the recorded intensities of left- and right-handed CPL under the excitation of non-polarized light, respectively.


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These CD profiles of F3, F5, F7, and PF6 show commonly bisignate nature in these spectral range, that is ascribed to the allowed π–π* transition from a long axis of twisted fluorene geometry at a longer wavelength of 380–400 nm and the forbidden π–π* transition from a short axis of the fluorene at a shorter wavelength of 300–350 nm.49 However, the bisignate features are definitively opposite when THF and DCM are employed as solvents. Herein, the dissymmetry ratio in the GS of the hybridized films, gCD, is characterized as gCD = ∆Abs/Abs = (AbsL – AbsR)/[1/2(AbsL + AbsR)], where AbsL and AbsR are absorbance for left- and right-circularly polarized light, respectively.51 Among five fluorene derivatives, PF6 affords the greatest gCD value at the first Cotton bands, reaching gCD = +3.2 × 10–3 at 392 nm from THF and gCD = –0.8 × 10–3 at 371 nm from DCM (Figure 1a and 1b, Figure S4a-S4d, SI). 9,9-Di-n-hexylfluorene oligomers and polymer in the films preferentially adopt P-helix49 in THF, whilst M-one49 in DCM. The (+)-sign CD signal at the first Cotton band possibly comes from Phelicity of the fluorene, conversely, the (-)-sign CD signal at the first Cotton band will be from M- helicity. Although the solvent-dependent bisignate CD band profiles of the optically active luminophores in the films are somewhat puzzling, the corresponding CPL sign radiating from the lowest S1 state is a direct evidence to characterize the helix sense of the chiral luminophores at the S0 and lowest S1 states, according to a contemporary Kasha’s rule.52 Figure 1c-1d and Figure S5a-S5d in SI display solvent-inversion CPL spectra and the corresponding PL spectra of F3, F5, F7, and PF6 in the range of 400−600 nm, that are assigned to photoemission from the lowest π−π* ES of the chiral luminophores. These fluorenes are excellent emitters with high quantum yields in solutions and in the solid films.50 Evidently, F3, F5, F7, and PF6 reveal an intense (+)-sign CPL signal associated with two or three vibronic


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bands with (+)-sign in the 400−550 nm range when THF is employed as the solvent. Conversely, F3, F5, F7, and PF6 have intense (-)-sign CPL bands associated with two or three vibronic bands with (-)-sign in the 400−550 nm range when DCM is used as the solvent. From THF as the solvent, (+)-CPL sign is same of the (+)-CD sign at the first Cotton band, whilst, from the DCM solvent, (-)-CPL sign is same of the (-)-CD sign at the first Cotton band. Noted that these CPL spectral profiles between THF and DCM are nearly in a mirror-image relationship. These CD and CPL spectral characteristics of fluorenes are ascribed to emerging helical main chains endowed with an inherent nature of CTPC concomitant with certain natures of solvent molecules. The solvent associated with CTPC is a determining factor of a helix preference for the fluorenes, as detectable by their chiroptical signs in the GS and ES. However, CTPC is still keeping the same helix sense regardless of THF and DCM, as revealed the same (-)-sign CD bands in far-UV region (Figure S3, SI). The dissymmetry ratio of the hybridized films in the ES, gCPL, is expressed as gCPL = (ΙL –

ΙR)/[1/2(ΙL + ΙR)), where IL and IR are the intensities with left- and right-handed circularly polarized emissions, respectively.51 Among the four di-n-hexylfluorenes, PF6 in CTPC film affords the greatest gCPL value, reaching gCPL= +2.1 × 10–3 at 429 nm from THF and gCPL= –2.4 ×10–3 at 429 nm from DCM (Figure 1c and 1d, Figure S4d in SI). CPL and PL spectral profiles of F3, F5, and F7 are similar to those of PF6 (Figure S5, SI), excepting that PL peak wavelength, λem, redshifts in the order of F3, F5, F7, and PF6. The CPL spectral inversion characteristics are also possible by tailoring cosolvents. Figure 1e and Figure S6 (SI) show the gCPL values of PF6 in CTPC as a function of THF volume function in THF-DCM cosolvents. The gCPL value almost linearly alters in response to the volume fraction of the cosolvents. However, it should be noted that the gCPL sign inverts at a specific volume


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fraction at ~60 % (v/v) of THF in the cosolvents. Regardless of the volume fractions of the cosolvents, a lower volume fraction of THF induces the (-)-sign CPL, whilst a higher volume fraction of THF induces the (+)-sign CPL. We can thus generate ambidextrous chiroptical signs from five CD-active/CPL-active fluorenes embedding in CTPC film by a choice of a pure solvent (DCE or THF) and their cosolvents in a minute at room temperature in the absence of chiral catalysts and enantiopair of specific chiral substituents. Whether the helical fluorenes in the ES can stably exist or not remains an open question. To address this question, we designed the photoinduced racemization experiment of CPLactive/CD-active F3, F5, F7, and PF6 in CTPC (0.2 cm2 in area) under aerobic conditions under continuous irradiation of monochromated unpolarized light source with a bandwidth of 10 nm (excited at 345 nm for F3, 355 nm for F5, 360 nm for F7, and 360 nm for PF6) utilizing Xe-Hg arc lamp of CPL-200 spectrometer according to our previous experiment.49 Figure 1f displays the changes in the gCPL values of F3, F5, F7, and PF6 in CTPC as a function of the photoirradiation time of the unpolarized UV-light source. The photoinduced racemization is not obvious and the helical fluorenes exist stably in the ES. The solvent-driven CPL sign and its prefer helix sense even in the ES are unchanged for at least 60 min. The prolonged UV-irradiation experiment teaches that CPL sign is solely determined by the chosen solvent (THF or DCM). The photoinduced stability in the CPL sign and the magnitudes of F3, F5, F7, and PF6 in CTPC is independent of the solvent, THF or DCM. However, the absolute gCPL value, |gCPL|, of F3, F5, F7, and PF6 in CTPC prepared from DCM slightly tends to increase in the order of PF6, F7, F5, and F3, whilst the |gCPL| values of F3, F5, F7, and PF6 in CTPC from THF are almost constant in the range of 1.5–2.2 × 10–3 at λem. The CPL sign and


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magnitudes of these fluorenes are unchanged with a time and memorized with the help of CTPC solid films prepared from DCM and THF solutions for at least one-hour upon even by nonpolarized focused irradiation beams at near-UV region.

Photophysical characteristics of oligo- and polyfluorenes in CTPC The next question is photophysical behaviors of left- and right-handed helical oligo- and polyfluorenes regarded as semiconducting molecular and macromolecular wires50,53 confined into non-fluidic CTPC films. These films are solution processible, ambidextrous π-conjugated helical quantum wires in the helix-inducible polymer platform. We compared these behaviors of oligo- and polyfluorenes in CHCl3, and less polar CTA and CABu films.49 Investigation of the behaviors in non-fluidic CTPC regarded as a solid dipolar solvent is a curious issue.


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Figure 2. (a) The gCD and gCPL (b) values at the first Cotton band of CD and of CPL spectra as a function of fluorene ring number (n) in CTPC films after dissolved in THF and DCM.

When π-electron cloud in one-dimensional molecular wires delocalizes, the S0→S1 transition energy decreases.49 The degree of twisting in the π-conjugated wires with a lower optical band gap (Eg,opt) is responsible for the charge carrier mobility. This tendency can be seen by an apparent red-shift of the PL and UV–visible bands. The Eg,opt values obtained from λmax and λem values in the four dihexylfluorenes in CTPC films decreased almost linearly as a function of the reciprocal fluorene ring number (n−1) (Figure 2b). The Stokes’s shift between Eg,opt from λmax and Eg,opt from λem decreased as the n value increased. A very minimal reorganization of the fluorene main chain in CTPC films occurs. On the other hand, the absolute magnitudes of gCD and gCPL values, |gCD| and |gCPL|, are a measure of the degree of photoexcited twisted e-h pair in helical one-dimensional wire in the ES and GS. The |gCD| and |gCPL| values are largely dependent of n (Figure 2a). When the value of n increases, the absolute magnitudes of gCD and gCPL increase and level off at n = 47. The |gCD| value, meaning the degree of helicity in the GS, from DCM is smaller than that in THF solution, conversely, the |gCPL| value, meaning the degree of helicity in the ES, from DCM solution is larger than from THF solution. The greatest |gCD| and |gCPL| values reaching approximately (2.4–3.2) × 10–3 suggested49 that PF6 in CTPC may adopt a well-ordered helical conformation prevailing one-handed helix sense in the GS and ES.


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Interpretation between solubility parameters of CTPC and the gCD values of PF6 To discuss the origin of the solvent dependent CD-/CPL-inverting behaviors, we focus on physicochemical parameters of solvents54 by plotting gCD value of PF6 embedded in CTPC films prepared from five pure common solvents (THF, DCM, CHCl3, Py, and DCE).


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Figure 3. (a) Normalized CD and UV-visible spectra of PF6 in CTPC film from five different casting solvents (DCM, CHCl3, THF, DCE, and py). The gCD value of PF6 in CTPC film as functions of (b) dielectric constants (ε), (c) Hansen’s solubility parameter (HSP, δ in SI unit), (d) London dispersion term of HSP (δd in SI unit), (e) dipole-dipole interaction term of HSP (δp in SI unit), and (f) hydrogen bonding interaction term of HSP (δh in SI unit),

Figure 3a compares UV−visible and CD spectra of PF6 in CTPC films when THF, DCM, CHCl3, Py, and DCE are employed as casting solvent. Evidently, CD spectra from THF and Py have commonly (+)-sign, whilst those from CHCl3 and DCM are commonly (-)-sign and no obvious CD signal from DCE is observed. Among the five solvents, the λmax value of PF6 from Py has the longest π-π* transition. To discuss of what kinds of physicochemical parameters is crucial for the chiroptical inversion, we consider dielectric constants (ε) and Hansen’s solubility parameter (HSP, δ).54,55 HSP is a modern variable based on Hildebrand’s solubility parameter (SP) to evaluate a cohesive energy including behaviors of hydrocarbon solvents, hydrogen bond and polarity of solvents.55,56 The δ value is a sum of London dispersion (δd), dipole-dipole


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interaction (δp), and hydrogen bonding interaction terms (δh). The δd value is equivalent to SP value. Either HSP or SP is widely utilized in polymer science and coating engineering when one predicts complex behaviors of various polymers in solutions as well as a miscibility among solvents. Among the gCD–ε, gCD–δ, gCD–δd, gCD–δp, and gCD–δh plots (Figure 3b–3f), only the gCD–δd relationship obeys a simpler correlation curve that is convex downward. The gCD–δd curve shows the minimum gCD when δd = 19.8 (Figure 3d). Other gCD–ε, gCD–δ, gCD–δp, and gCD–δh plots do not obey simpler curves (Figure 3b, 3c, 3e, 3f) because THF and Py afford an intense (+)-sign and a weak (+)-sign CD signals, respectively, appear out of the simple correlation curves. All

δ, δd, δp, and δh should take in an account of miscibility of a complex ternary system consisting of PF6, CTPC, and these solvents. One possible reasoning why gCD value at DCM with at δd = 19.8 goes to negative value. We have to think about the ternary system, but it seems difficult to answer. Disregard of this difficulty, the δd value is likely to satisfactorily give a plausible explanation for the present solvent-driven CPL- and CD-sign inversion behaviors. In case of homochiral CTPC, intermolecular CH-π and π-π interactions among phenyl rings and alkyl chains may be one of the most crucial factors for the emergence and inversion of CD and CPL signals from PF6, PF8, and other oligofluorenes.

Solvent-dependence of urethane-origin vibrational IR bands Regarding hydrogen bond characteristics due to three urethane (O-C=O-NH•••O=C-) moieties per glucose unit, we observed noticeable changes in FT-IR spectra of CPTC films prepared from their DCM and THF solutions at room temperature, as shown in Figure 4 and Figure S7 (SI).


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Figure 4. Change in FTIR spectra of CPTC films after treated by two solutions at room temperature. Spectra are shown in (a) ν(NH) and (b) δ(NH) and ν(C=O) bands. Blue curves refer to the film casting from DCM solution and red curves refer to the film from THF solution.

The broader asymmetric and symmetric two ν(N-H) band in the 3200–3450 cm–1 region and two δ(N-H) bands in the 1500–1550 cm–1 region57 are considerably sensitive to the solvents used. When solvent changes from DCM to THF, the ν(N-H) band at 3322 cm–1 greatly redshifts to 3301 cm–1 and the ν(N-H) band at 3390 cm–1 greatly weakens. Conversely, the δ(N-H) band at 1527 cm–1 slightly blueshifts to 1540 cm–1, whilst ν(O=C, urethane) band at 1732 cm–1 redshifts to 1746 cm–1. These changes in frequencies and intensity of three ν(N-H), δ(N-H), and ν(O=C) bands57 lead to an idea that THF behaves as the hydrogen bond forming solvent, conversely, DCM acts as the hydrogen bond breaking solvent. The hydrogen bond forming solvent induces well-ordered helix motifs of CTPC, leading the helicity oriented transfer ability. However, the hydrogen bond breaking solvent induces illordered helical motifs. This implies a possible scenario that switching occurs from CTPC main chain helicity transfer to a glucose local chirality transfer. In the oligo- and polyfluorene in


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CTPC films, the sign and magnitude of CD- and CPL-signals greatly depend on the degree of these intra- and inter-hydrogen bonds in three phenylcarbamates of CTPC and their packing states.

Solvent-dependence of DSC curves To verify our hypothesis, we characterized the intra- and inter-hydrogen bond interactions of urethane origin N-H/O=C-O bonds in CPTC films prepared from THF, DCM, and Py solutions by differential scanning calorimetry (DSC). The solvent dependent change in enthalpy (∆H) decreases in the order of 9.85 kcal mol–1 (THF), 2.86 kcal mol–1 (Py), and 1.88 kcal mol–1 (DCM) (Figure 5). The difference in the ∆H value reflects from dissociation energy of NH/O=C-O bonds. The ∆H value of N-H/O=C-O interaction is 4−15 kcal/mol.58,59 The dissociation of two urethane origin hydrogen bonds derived from two secondary N-H/O=C-O groups (glucose-O-C=O/H-N-Ph) directly attached to the glucose rings of CTPC is attributable to the endothermic signal near 100 °C. One urethane-origin hydrogen bond (glucose-CH2-OC=O/H-N-Ph) derived from a primary N-H/O=C-O group linked via single methylene spacer is ascribed to the endothermic signal in the vicinity of 50–70°C. The intense N-H/O=C-O interactions (named on-state) in THF and weaker N-H/O=C-O interactions (named off-state) in DCM are schematically illustrated in Figure 5. The strong (+)-sign CD signals of the fluorenes (Figure 1a) are induced by the well-ordered structure in the side chains of CTPC suggesting from the intense (-)-sign CD signal of CTPC (Figure S4, SI). On the other hand, the weaker (–)-sign CD signal of the fluorenes (Figure 1b) are related to the ill-ordered structure in the side chains of CTPC suggesting from the weak (-)-sign CD signal of the CTPC side chains (Figure S4, SI). The ill-ordered state may result from the


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primary N-H/O=C-O due to a great rotational freedom of the methylene spacer. Since Py had a lower ∆H value, Py may be crucial as the hydrogen bond breaking solvent to synthesize CTPC from microcrystalline cellulose.

Figure 5. (a) Raw DSC diagrams of pulverized CPTC powders obtained from THF (red) and DCM (blue), and Py (green) solutions under heating rate of 10 deg min–1. (b) Schematic on-andoff states of intra- and inter-molecular urethane-based hydrogen bonds regulated by the nature of solvent molecule.

The noticeable changes in the DSC diagrams confirm the occurrence of solvent-driven structural changes of CTPC, resulting in the helix inducibility, while maintaining the chiral structure in the side chains. The three urethane origin N-H/O=C-O bonds per D-glucose unit casting from THF solution are hard to dissociate upon heating because of the large ∆H value.


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The one or two urethane origin N-H/O=C-O bonds per glucose ring casting from DCM solution are easily dissociated suggesting from the small ∆H value. Solvent dependency of WAXD profiles The solvent sensitive intra- and inter N-H/O=C-O interactions reflect packing structures of and CTPC solids. Previously, molecular structure and liquid-crystalline behaviors of chitosan phenylcarbamate was reported.60 Figure 6 compares two WAXD profiles of CTPC solids prepared from DCM and THF solutions, respectively. Two major d-spacings of 15.983 Å and 4.505 Å from CTPC solids prepared from DCM solution are shifted to 13.253 Å and 4.637 Å, respectively. The d-spacings of 15.983 Å and 13.253 Å are ascribed to interchain distance of CTPC chains, while the d-spacings of 4.505 Å and 4.637 Å may be a helix pitch of CTPC chains. The shorter d-spacing in THF indicates a densely packed CTPC chains due to wellordered side chain structures by enhanced N-H/O=C-O interactions.60 The longer d-spacing in DCM indicates a loosely packed CTPC chains due to ill-ordered side chains by very limited networks of intra- and inter-interactions by the N-H/O=C-O groups.


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(b)

Figure 6. (a) WAXD profiles of CPTC films deposited onto an Si substrate obtained with THF (red) and DCM (blue) solutions. (b) Schematic packing model of CPTC.

Computer-assisted modeling of PF6 embedded in CTPC In the previous sections, we showed the helicity/chirality transfer capability of CTPC to the several fluorenes, leading to emerging CD and CPL signals and solvent-driven inversion in their signs. We focus on the origin of helicity/chirality mechanisms depending on the conformational changes in multiple side chains of CTPC and possible intermolecular interaction between the fluorenes and CTPC. Molecular dynamics (MD) simulations61 using a Materials Studio Forcite module with a universal force field (UFF)62 allow us to discuss these interactions. Three phenylcarbamate groups per D-glucose ring of CTPC can contribute to the N-H/O=C-O hydrogen bonds. The hydrogen bonds should have lead to certain chiral and helical structures to the fluorenes.


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Aiming at ensure to interpret possible intermolecular interactions existing in a realistic, lesscrystalline hybridized film, we designed a hybridized system consisting of CTPC with twelveunits and PF6 with twelve-units. Numbering the backbone and side-chain atoms is indicated in Figure 7. To optimize by an energy minimization scheme, the CTPC-PF6 hybrid system was annealed at high temperature by Forcite anneal dynamics, and the model was finally equilibrated at 300 K and 1 atm for 150 ps using the NPT method via classical MD method.49 To consider the effects of the helicity-and-chirality transfer capabilities as well as twisted PF6, CTPC backbone carries three unique N-H/O=C-O bonds of the urethane groups in the backbone. Among three urethanes, two bonds are derived from secondary OH groups of glucose ring and one bonding is derived from primary OH group with one methylene spacer. These interactions lead to the induction of helical and/or chiral motifs to PF6, as shown in Figure S8 (SI). The optimized structure of CTPC with twelve-units and PF6 with twelve-units in an amorphous cell are obtained with Forcite geometry optimization algorism. The two structures are connected to form the hybridized polymers.


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Figure 7. Zoom-in hybridized model of PF6 and CTPC taken from Figure S8 (SI). Simulated C-H/O=C interactions (blue dotted line), CH/π interactions (green dotted line), and π/π interactions (red dotted line) between PF6 and CTPC.

From the comprehensive tables of van der Waals radii provided by Bondi63 and by RowlandTaylor,64 the MD simulations led us to discuss the existence of three major intermolecular CH/O=C, C-H/π, and π/π interactions between CTPC and PF6 (Figure 7). The distance between H and phenyl groups ranging from 2.74 to 3.00 Å is slightly shorter than a sum of van der Waals contacts of 2.88–3.12 Å (Table S1, SI). The distance between phenyl groups ranges from 2.93 Å to 3.39 Å is considerably shorter than a sum of van der Waals contact of 3.54–3.59 Å (Table S2, SI). However, the distance between H and O atoms ranges from 2.77 Å to 2.99 Å (Table S3, SI), that slightly elongates than a sum of van der Waals contacts of 2.46–2.68 Å, suggesting that the C-H/O=C interactions may be rather repulsive than attractive interactions. Subtle balance between the attractive intermolecular CH/π and π/π interactions and the repulsive intermolecular C-H/O=C interactions is possible to explain the solvent dependent chirality inversion. However, a huge number of three intermolecular and intramolecular interactions could efficiently contribute transfer their helicity and/or chirality to CD-silent PF6, following the induction of left–right imbalance and inversion, as detectable by clear CD and CPL signals (Figures 1 and 2).

Perspectives Recently, aiming at obtaining high-performance CPL-functional materials directed toward with gCPL = ±2 from several chemical resources available, several sophisticated approaches beyond


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the contemporary stereochemistry, synthetic chemistry, and materials chemistry are newly proposed.65-76 Without use of enantiopair of D- and L-sugar molecules,77 homochiral D-sugar origin cellulose derivatives including CTPC, CTA, and CABu20,78-80 and molecular sugars81 can be utilized as solution processable solid chirality platforms to efficiently generate both (+)- and ()-sign CPL-/CD-active luminophores starting from the corresponding achiral and/or non-helical luminophores in a minimal synthetic way. Our proposal is operational under a milder condition using common volatile organic solvents within 1–2 min in the absence of additional enantiopure chiral sources and chiral catalysts. The work of whether this approach is applicable to other achiral/ π-conjugated emitters is in progress.

CONCLUSION Non-charged cellulose tris(phenylcarbamate) (CTPC), a homochiral polymer constituting Dglucose only in the backbone, revealed ambidextrous intermolecular helicity/chirality transfer capability to non-charged, non-helical oligo- and polyfluorenes (F3, F5, F7, PF6, and PF8). Nearly ideal CPL and CD spectral inversion characteristics were achieved by choosing a proper solvent among five pure volatile organic solvents and their cosolvents. The dissymmetry ratio in CPL of PF6 in CTPC film obtained with tetrahydrofuran (THF) solution attained gCPL= +2.1 × 10–3 at 429 nm, conversely, with dichloromethane (DCM) solution, inverted to gCPL = –2.4 × 10–3 at 429 nm. The dissymmetry ratio in CD with the THF was gCD = +3.2 × 10–3 at 392 nm, whilst, with the DCM, inverted to gCD = –0.8 × 10–3 at 371 nm. The chiroptical inversion characteristics were assumed to result from solvent-driven order–disorder transition characteristics of CTPC helix, but not due to helix-helix transition of CTPC helix-sense. Solvent dependence of FT-IR spectra, WAXD profiles, and DSC diagrams indicated that the solvent driven on-off switching


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state of hydrogen bonds due to urethane groups of CTPC plays the key for the inversion. The sign and magnitude of the gCD values were interpreted to the δd value of HSP of the casting solvents. This idea was supported from MM/MD simulations, suggesting existence of intermolecular CH-π and π-π interactions between CTPC and PF6. Our results should shed light on a new aspect among contemporary stereochemistry, asymmetric synthesis, and chiroptics in molecular, supramolecular, and polymer chemistry and materials science.

ASSOCIATED CONTENT Supporting Information    Additional text including Experimental Section; 19 figures showing gCD values, CD and UV−visible spectra, FT-IR spectra, computer generated PF6-CTPC hybrid, 3 tables summarizing distances between selected C–H and phenyl groups, between selected phenyl groups, and between selected C–H and O=C groups, several and chemical structures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR Information Corresponding Authors *(M.F.) [email protected]

ORCID Nozomu Suzuki: 0000-0003-3716-735X Laibing Wang: 0000-0002-3380-7826


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Michiya Fujiki: 0000-0002-3139-9478

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS SG sincerely acknowledges Profs. Jun-ichi Kikuchi, Masao Tanihara, and Hiroko Yamada for valuable comments and discussion to his doctoral course work. We thank Kazuki Yamazaki (GPC), Shohei Katao (WAXD), Fumio Asanoma (NMR), and Masahiro Fujihara (DSC) for technical assistance throughout this work.

Author Contributions MF, SG, NS, and LBW co-proposed and co-designed all of the experiments. SG, HK, LBW, AO, and MF measured and analysed the CD, UV-visible, PL, PLE spectral characteristics of several emitters in CTPC, and, for comparison, in several soluble celluloses. SG, LBW, and MF measured and analysed FT-IR spectra and DSC charts. SG measured and analysed WAXD data. SG and HK synthesized CTPC. SG and NS performed MM/MD calculations. MF, SG, NS, and LBW co-wrote the paper. All authors discussed the data and commented on the manuscript.

Funding Sources This work was operated by financial support from JSPS KAKENHI (16H04155).


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ToC used only


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Ambidextrous Chirality Transfer Capability from ... - ACS Publications

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