Helix Induction to Polyfluorenes Using Circularly Polarized Light

Aug 27, 2018 - Chirality induction (preferred-handed helix induction) to poly(9,9-dioctylfluorene-2,7-diyl) (poly(DOF)), poly(9,9-didodecylfluorene-2,...
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Helix Induction to Polyfluorenes Using Circularly Polarized Light: Chirality Amplification, Phase-Selective Induction, and Anisotropic Emission Yue Wang,† Takunori Harada,§,∥ Le Quang Phuong,∥ Yoshihiko Kanemitsu,∥ and Tamaki Nakano*,†,‡

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†

Institute for Catalysis (ICAT) and Graduate School of Chemical Sciences and Engineering and ‡Integrated Research Consortium on Chemical Sciences (IRCCS), Institute for Catalysis, Hokkaido University, N21W10, Kita-ku, Sapporo 001-0021, Japan § Department of Integrated Science and Technology, Faculty of Science and Technology, Oita University, Dannoharu, 700, Oita City 870-1192, Japan ∥ Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan S Supporting Information *

ABSTRACT: Chirality induction (preferred-handed helix induction) to poly(9,9-dioctylfluorene-2,7-diyl) (poly(DOF)), poly(9,9-didodecylfluorene-2,7-diyl) (poly(DDF)), and poly(9,9-dihexylfluorene-2,7-diyl) (poly(DHF)) using circularly polarized light (CPL) was thoroughly investigated mainly using film samples. We found that the presence of the β-phase is indispensable in chirality induction to poly(DOF) which suggests the importance of interchain interactions and that chirality induction first occurs preferentially in the β-phase and then extended to the α-phase. Chirality induction only to the β-phase was attained by irradiating at the β-phase absorbance band (435 nm) or by irradiating the full wavelength range for a short time while pure α-phase chirality is induced for poly(DHF) having no β-phase using an achiral aid molecule. Chirality induction is feasible for polymers having octyl and dodecyl chains, poly(DOF) and poly(DDF), whereas it is not possible for the polymer having a hexyl chain, poly(DHF), unless an aid molecule is used. CPL-driven chirality induction occurs through enantiomerselective photoexcitation of either a right- and left-handed twisted diad composing the polymer chain into achiral, coplanar conformation, and deactivation of the coplanar conformation leading to right- and left-handed twisted diads. Chirality amplification takes place through biased deactivation of coplanar conformation preferentially to the hand which is the same as that of “preformed single-handed diad or longer chain” in the vicinity (“deactivation chirality amplification”). In addition, “ground-state chirality amplification” is also proposed on the basis of significant temperature effects in chirality induction. Chiral structure induced to the film samples is stable if they are stored at room temperature without irradiation with intense light. Optically active poly(DOF) films showed efficient circularly polarized luminescence.

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INTRODUCTION

sense-selective polymerization, helix-chirogenic polymerization) and supramolecular interactions between a polymer chain and external molecules.4−8 Regardless of whether the target structure is a polymer or a small molecule and whether the synthetic method is catalytic or noncatalytic, there has to be an

Chirality plays significant roles in material applications and also in life, and the development of synthetic methods for optically active compounds is hence an important aspect in chemistry. In this context, various techniques have been invented for the asymmetric synthesis of chiral small molecules and polymers.1−3 As for polymers, the most typical and widely studied chiral structure is helix, and it has been prepared mainly by the two major methods, namely, asymmetric polymerization (helix© XXXX American Chemical Society

Received: July 9, 2018 Revised: August 12, 2018

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Macromolecules optically active compound as the source of chirality in asymmetric syntheses. Such chirality-source chemicals are often natural compounds or their derivatives which are typically used as chiral ligands for organometallic species or as organic catalysts. Another potentially efficient source of chirality useful for chemical synthesis is circularly polarized light (CPL).9−15 CPL can be obtained by modulation of linearly polarized light (LPL) from artificial or natural light sources. The first ideas to connect CPL with chirality of chemicals emerged in 1874,16,17 and CPL has then been used for various chemical syntheses.9−12,18,19 As well as chiral small molecules, chiral polymer systems have been made optically active using CPL.18,19 As for such polymer systems, it should be pointed out that most of the examples of chirality induction using CPL are about polymers having azobenzene moieties in the main chain or the side chain.20−27 Though azobenzene transforms between trans- and cisconfigurations with light or heat as stimulus, it has no molecular chirality in either trans- or cis-configuration. The origin of the observed chiroptical properties for azobenzene-containing polymer induced by CPL is hence not clear in most cases including whether molecular chirality (helix) is involved or intermolecular chain alignment is responsible while there is an important example about a side-chain polymer where wellorganized azobenzene units have been shown to be responsible for chiroptical properties.27 We recently reported that preferred-handed helical structure is induced to poly(9,9-dioctylfluorene-2,7-diyl) (poly(DOF)) in a thin film form using CPL as the only source of chirality. This is the first example of molecular helix induction to a conjugated polymer using light and also the first example of chirality induction using light to a polymer having no photochromic moieties.28 Although poly(DOF) has no centers or planes of chirality, it has axes of chirality which coincide with the single bonds connecting the fluorene-2,7-diyl units in the main chain. While preferred-handed axial chirality of poly(DOF) cannot be maintained in solution, even if it is induced, due to extremely fast conformational dynamics, it can be stable in the solid state and can exhibit chiroptical properties. Unlike most of the aforementioned, azobenzene-bearing polymer systems where the origin of chiroptical properties and molecular mechanism of chirality induction are not necessarily clear, the mechanism of chirality induction to poly(DOF) is rationally proposed to be based on enantiomer-selective photo excitation of right- or lefthanded twist conformation of a dimeric sequence (diad). In addition, molecular helix formation leading to circular dichroism (CD) spectra is supported by theoretical calculations of full energy landscapes of poly(DOF) obtained by metadynamics simulations.29,30,12 In the present work, we thoroughly investigated full details of helix induction to polyfluorene derivatives, poly(DOF), poly(9,9-didodecylfluorene-2,7-diyl) (poly(DDF)), and poly(9,9dihexyl fluorene-2,7-diyl) (poly(DHF)) (Scheme 1) and clarified the effects of molar mass of polymers, solid-state morphology (α- and β-phases), wavelength of CPL, irradiation time, temperature, and chemical structure of polymer. These structural characters of polymers and irradiation conditions were found to play crucial roles in helix induction. In addition, we proposed chirality amplification mechanisms which are responsible for much higher anisotropy (chirality) of polyfluorenes than expected only on the basis of chirality of CPL. Also, stability of chirality induced by CPL was discussed. In

Scheme 1. Structures of Polyfluorene Derivatives and Fluorene as an Aid Molecule Used for CPL-Driven Chirality Induction Experiments and That of Optically Active DMBN Used for NPL-Based Chirality Induction

addition, we found that helical poly(DOF)s prepared using CPL emit circularly polarized luminescence at high efficiency.

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RESULTS AND DISCUSSION Reversible Chirality Induction by CPL Leading to Molecular Helix in the Solid State. We have briefly reported that poly(DOF) can be made optically active in a thin film form on irradiation with CPL.28 While polyfluorenes have no centers of chirality and they are hence generally recognized as achiral polymers, the single bonds connecting the fluorene units can be the axes of chirality because twists between neighboring fluorene units can be enantiomeric. Although the right- and left-handed twists are considered to be dynamically exchanged with each other in solution at ambient temperature due to fast molecular motions, they would be significantly stable in the solid state. It should therefore be possible to observe chiroptical properties if preferred-handed axial chirality, i.e., unequal populations of either right- and left-handed twists are induced in the solid state; CPL-driven chirality induction for poly(DOF) is indeed possible in film as well as in suspension (Figure 1A,B). Figure 1A indicates that a film sample prepared by drop-casting a solution of poly(DOF) (Mn 76700 (vs polystyrene)) on to a quartz glass plate exhibits mirror image CD spectra induced reversibly by alternating irradiation with L- and R-CPL’s (Figure 1A). As well as for the film, reversible chirality induction was observed for a suspension sample of poly(DOF) (Mn 99500 (vs polystyrene)) in a toluene−methanol (5/1, v/v) mixture (Figure 1B). These results indicate that in both the film and the suspension CPL’s with opposite sense lead to helices of poly(DOF) having opposite handedness in a reversible way. However, it should be pointed out that that CD intensity was much lower for the suspension sample irradiated with L-CPL for 30 min than for the film sample irradiated with L-CPL for 15 min (Figure 1A,B). These results suggest that interactions between glass surface and polymer chain has a role in chirality induction; such a possibility has been pointed out through theoretical studies.29 Considering that the film sample exhibited much more intense CD signals, in-depth examinations of chirality induction were conducted for film systems. A simplest case of CPL-driven asymmetric synthesis is a reaction where racemic source compound is enriched in one enantiomer where the chemical structure of the compound is untouched (reversible photoderacemization or photoresolution).9−12,31 The mechanism of such reaction may involve selective photoexcitation of one enantiomer of the racemic substrate. For the poly(DOF) systems herein discussed, chemical structures of the polymers are almost intact after irradiation as confirmed by spectroscopies and SEC analysis, and also chirality induction was performed in a reversible manner. The chirality induction for poly(DOF) by CPL is therefore considered to be a macromolecular photoderacemization process where preferential photoexcitation of right- or leftB

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Figure 1. CD/UV spectra of a poly(DOF) (Mn 76700, run 2 in Table 1) film (A) and a poly(DOF) (Mn 99500 run 1 in Table 1) suspension in toluene−methanol (5/1, v/v) (B) observed on alternative irradiation with L- and R-CPL’s.

Scheme 2. Chirality Induction of Polyfluorene Based on Enantiomer-Selective Photoexcitation by CPL (A), Chirality Amplification on Deactivation through Interchain Interactions (B), and Chirality Amplification on Deactivation through Intrachain Interactions (C)

handed twist between fluorene−fluorene diad in the main chain

The proposed mechanism involves a twisted−coplanar transition (TCT) of a fluorene−fluorene diad junction in the main chain as a key step; twisted and coplanar conformations are most stable in the ground state and in excited states, respectively. Such a transition through photoexcitation was first predicted for

leads to enrichment of either twist, resulting eventually in the formation of a macromolecular helix according to the proposed mechanism depicted in Scheme 2. C

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Table 1. Synthesis of Poly(DOF) via Yamamoto Coupling Using Ni(COD)2 in the Presence of Bpy and COD in a Toluene−DMF Mixture at 85 °Ca

MeOH-insoluble partc run 1 2 3 4 5 6

[M]0 (M) 0.08 0.05 0.05 0.05 0.05 0.05

[Ni]0 (M)

time (h)

0.120 0.075 0.050 0.050 0.050 0.050

72 12 6 1 0.75 0.5

convb (%)

yield (%) g

82 90 85 46 52 57

32 64 70 23 28 30

Mnd

Mw/Mnd

Mn(RALLS)e

af

99500 76700 50300 12200 11500 9400

2.36 2.08 1.97 1.35 1.29 1.21

20800 34900 16300 5100 4400 3000

0.91 0.95 0.94 0.74 0.75 0.72

a Monomer (2,7-dibromo-9,9-dioctylfluorene) = 0.100 g, Ni(COD)2 = 600 mg, DMF = 10 mL, toluene/DMF = 2/1 (v/v). bDetermined by 1H NMR analysis of crude mixture. cPurified by reprecipitation and collected with a centrifuge. dDetermined by SEC (vs standard polystyrenes). e Determined by SEC with right-angle laser light scattering (RALLS) detection. fMark−Houwink−Sakurada constant determined by SEC with online viscometric detection. gMeOH-isoluble, chloroform-soluble part.

that can be achieved with CPL is negligibly low (10−1−10−2 order) if there is no chirality amplification. Synthesis and Properties of Poly(DOF) with Various Molar Masses and Molar-Mass Effects on β-Phase Content. To study the effects of molar mass on chirality induction to poly(DOF), polymers having different molar masses were synthesized by polymerization of 2,7-dibromo-9,9dioctylfluorene in the presence of cyclooctadiene (COD) and 2,2′-bipyridine (bpy) using Ni(COD)2 as catalyst in a mixture toluene and N,N-dimethylformamide (DMF).38,39 Table 1 summarizes the conditions and results of polymerization. The polymerizations led to polymers having relative Mn’s from 9400 to 99500 estimated by size-exclusion chromatography (SEC) using standard polystyrene samples which correspond to Mn’s of 3000 to 20800 estimated by right-angle light scattering (RALLS) detected SEC. It seems that a higher molar mass of poly(DOF) is achieved at a higher monomer concentration and a higher Ni concentration (runs 1−3 in Table 1) and also that a longer reaction time tends to lead to a higher molar mass at constant monomer and Ni concentrations (runs 4−6 in Table 1). These results are reasonable for this type of polymerization. In addition, Mark−Houwink−Sakurada constants (a values in Table 1) of the polymers were in the range 0.72−0.91, and they tend to be higher for a longer chain, suggesting that the polymers have a rather rigid chain conformation in a solution of tetrahydrofuran (THF) and that rigidity of chain is higher for a longer chain.40−45 Figure 2 shows the UV absorption spectra of the poly(DOF)s in film. The spectral shape systematically varied depending on molar mass; in the wavelength range of 300−500 nm, the three samples having lower molar masses (Figure 2d−f) showed single peak centered at around 365 nm based on α-phase where the dihedral angle between fluorene units in the main chain is rather large. On the other hand, the three samples having higher molar masses (Figure 2a−c) showed, in addition to the α-phase band, three overlapped bands centered at around 435, 405, and 380 nm based on β-phase where the dihedral angle is rather small, and poly(DOF) chains adopt a planar zigzag or 2/1-helical conformation.46−49 The contents of β-phase of the polymers indicated in Figure 2 were estimated by waveform analyses of the UV spectra according the literature method47,49 (Figure S4) or through comparison of UV peak intensities at 435 and 380 nm.

biphenyl through theoretical calculations,32 and we recently reported that TCT can lead to racemization of a small molecular axially chiral compound33 and that of a helical polymer.34 When the photoexcitation is conducted by CPL, one of the enantiomeric, right- and left-handed twists is preferentially excited into the coplanar conformation, and the excited coplanar conformation is then deactivated into both right- and lefthanded twisted conformation. The relative population of the unpreferred twist hand thus increases to lead to an optically active system (Scheme 2A). However, the deactivation process may not lead to right- and left-handed twists at the same probability (“deactivation chirality amplification”) (Scheme 2B,C). If coplanar conformation in a chain is deactivated in the presence of pre-existing, single-handed twisted structure which has been preformed by earlier CPL excitation and deactivation events for other chains, deactivation may be biased for formation of the twist hand which is the same as that of the preformed twist by synergy effects through interchain interactions, leading to chirality amplification (Scheme 2B). In addition, deactivation of coplanar conformation in a chain can also be affected by a neighboring diad which has been preformed by earlier chirality induction, leading to biased deactivation through intrachain synergetic interactions (Scheme 2C). This aspect is discussed later in a separate, following section. The extent of polymer chirality induced by CPL is quantified in this work by Kuhn’s anisotropy factor (gCD) defined by the equation35−37 gCD = 2(εL − εR )/(εL + εR )

where εL and εR are molar absorptivities of one enantiomer toward L- and R- (left- and right-handed) CPL’s. In addition, enantiomeric excess (e.e.) of product that is achievable by CPL in reversible photoderacemization reaches a constant value at a photostationary state (γPSS), which is determined according to9−12,31 γPSS = 1/2 × gCD(100% e.e.) × 100 (%)

where gCD(100% e.e.) is the anisotropy factor of a compound at 100% e.e. Because gCD(100% e.e.) values of aromatic componds are generally very small (10−3−10−4 order), the e.e. of the product D

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corresponding to the former CD spectrum according to the equation33 n

relative irradiation energy sum =

∑ Δti × Absi i=0

where absorbance intensities at 365 nm (α-phase) and at 435 nm (β-phase) were considered, and the contribution of the βphase absorbance was normalized by a factor of 0.54 taking into account the emission intensity ratio of Hg−Xe lamp at the two wavelengths. As well as the CD spectra, UV spectra of the polymer films gradually changed where overall intensity and β-phase content decreased with an increase in relative irradiation energy sum (Figure S3). The decrease in overall UV spectral intensity is ascribed to hypochromism due to interchain π-stacking which is supported by the fact that the excimer emission is observed for the fluorescence of films after CPL irradiation (vide infra). Chain alignment may change along with helix induction through CPL irradiation to facilitate π-stacking between polymer chains. In addition, hypochromism may also arise partially from an increase in averaged dihedral angle in the main chain; an increase in dihedral angle of biphenyl molecule is known to lead to hyprochromism and blue-shifts.51 In addition, the decrease in the ratio of β-phase indicates that a part of the β-phase was transformed into the α-phase through CPL irradiation. This may be in line with the proposed increase in the dihedral angle. In addition, it is noteworthy that among the three poly(DOF)’s, a polymer having smaller Mn reached maximum gCD within a shorter duration of irradiation. This means that there is an optimal molar mass of poly(DOF) which is small enough to make the chirality induction fast enough and is long enough to generate the β-phase. Figure 4 shows the relation between gCD induced by L-CPL and initial β-phase content of the poly(DOF) samples. A clear, positive dependence of absolute gCD value at 435 nm on initial βphase content can be confirmed in the figure. Also, gCD at 380 nm appears to have a rough tendency to increase with an increase in initial β-phase content. The presence of the β-phase was thus found to half quantitatively affect chirality induction. Chirality Amplification. The gCD values of the poly(DOF)s attained by CPL and the relations between gCD and relative irradiation energy sum for the three poly(DOF)s (runs 1−3 in Table 1) strongly suggest that chirality is amplified through the CPL-driven helix induction. First, the maximum absolute gCD values were 3.1 × 10−3, 4.4 × 10−3, and 2.7 × 10−3 at 380 nm and were 1.4 × 10−2, 1.2 × 10−2, and 7.6 × 10−3 at 435 nm (negative sign) for the poly(DOF)s of Mn 99500, 76700, and 50300, respectively. As discussed earlier, for a reversible photoderacemization without any chirality amplification, the e.e. attained for aromatic compounds should be in the order of 10−1−10−2 order which are correlated to gCD values in the order of 10−6−10−8 because gCD(100% e.e.) (gCD at 100% e.e.) of aromatic compounds are in the order of 10−3−10−4.12 Nevertheless, the maximum absolute gCD values observed for the poly(DOF)s are comparable to those of small molecular biphenyl derivatives having high e.e.’s,52,53 suggesting that helices induced by CPL for poly(DOF)s are almost single handed. Such a high selectivity is only possible with chirality amplification. Second, the changes in gCD values support chirality amplification. The changes in CD spectra were quantitatively evaluated through the gCD (380 nm) vs relative irradiation

Figure 2. UV−vis absorption spectra of poly(DOF)s in film with ratios* of the α-phase to β-phase: (a) Mn 99500 (run 1 in Table 1), (b) Mn 76700 (run 2 in Table 1), (c) Mn 50300 (run 3 in Table 1), (d) Mn 12200 (run 4 in Table 1), (e) Mn 11500 (run 5 in Table 1), and (f) Mn 9400 (run 6 in Table 1). *The α-phase/β-phase ratios were estimated through waveform analysis of the spectra (Figure S4), and those in parentheses were calculated on the basis of the ratio of UV intensity at 380 nm and that at 435 nm.

The present results clearly indicate that a higher molar mass leads to a higher content of β-phase for the film samples whereas such an effect has been pointed out for a dilute solution system.50 Importance of β-Phase in Helix Induction. The polymers from runs 1−3 in Table 1 having higher molar masses were successfully made optically active through CPL irradiation to film (Figure 3 I−III). In a sharp contrast, the polymers from runs 4−6 in Table 1 having lower molar masses did not show clear responses to CPL irradiation (Figure S2). These results clearly indicate that β-phase is indispensable in chirality induction because the former polymers do and the latter polymers do not have the UV peaks at 435, 405, and 380 nm due to the β-phase. As chains are considered to be packed more closely in β-phase than in the α-phase, the present results may mean that interchain interactions are crucially important in effective chirality induction. Figure 3 indicates the changes in CD and UV spectra with irradiation time (Figure 3, I-A, II-A, and III-A) along with the plots of gCD at 380 nm against relative irradiation energy sum (relative absorbed dose) (Figure 3, I-B,C, II-B,C, and III-B,C) of the films of the polymers from run 1 (Mn 99500 (vs polystyrene)), run 2 (Mn 76700 (vs polystyrene)), and run 3 (Mn 50300 (vs polystyrene)) in Table 1. The CD spectra in Figure 3, I-A, II-A, and III-A, indicate that optical activity evolves with a progress of irradiation. Because a decrease in UV spectral intensity was observed in Figure 3 (I-A, II-A, and III-A), the “relative irradiation energy sum” (relative absorbed dose) was used rather than irradiation time for the quantity on the horizontal axis in Figure 3 (I-B,C, IIB,C, and III-B,C). It corresponds to the sum of irradiation time intervals (Δti) used to change a CD spectrum to the directly following CD spectrum multiplied by UV absorbance (Absi) E

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Figure 3. CD/UV spectra observed on L-CPL irradiation (A), gCD vs relative irradiation energy sum plot (B), and curve-fitting leading to a nonlinear function approximation of the gCD (380 nm) plot (C) for poly(DOF)s of Mn 99500 (I), 76700 (II), and 50300 (III).

for first-order chemical reactions which has an increasing form of exponential decay, gCD = A0(1 − e−(relative irradiation energy sum)). However, in a sharp contrast to a first-order chemical reaction, the gCD plots in Figure 3 (I-C, II-C, and III-C) are well approximated with a power function, gCD = a(relative irradiation energy sum)b, for the early stages of chirality induction (Figure 3, I-C, II-C, and III-C); the power exponent, b, was estimated to be 2.1, 1.5, and 1.7 for the polymers of Mn 99500, 76700, and 50300, respectively. The power functions that can approximate the gCD curves clearly indicate that chirality is amplified. The molecular mechanism of chirality amplification is proposed to involve interchain and intrachain interactions between excited, coplanarized diad and “preformed singlehanded, ground-state diad or a longer chain” in the vicinity which had already been made single-handed twisted through earlier-stage CPL-excitation and deactivation. Through such interactions, deactivation of a coplanar diad species may lead to the hand which is the same as the ground-state polymer chains; amplification is thus proposed to occur through synergy effects on deactivation (“deactivation chirality amplification”) (Scheme 2B,C). This proposal is in line with the fact that interchain

Figure 4. Relation between maximum gCD at 380 and 435 nm and βphase content. The β-phase content was estimated on the basis of waveform analyses of UV spectra (Figure S4).

energy sum plots (Figure 3, I-C, II-C, and III-C). If enantiomer selection (twist sense selection) occurs only on photoexcitation by CPL as indicated in Scheme 2A without any amplification of chirality, the gCD vs relative irradiation energy sum plots would have a curve shape similar to a conversion vs reaction time plot F

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positive, nonlinear relation between DMBN levels and gCD values. These results indicate that (1) “preformed singlehanded, ground-state diad” can indeed lead to biased deactivation of a coplanar diad into either right- or left-handed twist through synergy effects and (2) the extent of the bias on deactivation is greater at a higher level of “preformed singlehanded, ground-state diad”. The latter conclusion is completely in line with the fact that the slope of tangent line of the curves of gCD vs relative irradiation energy sum plots (panels C in Figure 3, I, II, and III) increased with an increase with a progress of irradiation. Morphology Selection in Chirality Induction: Effects of Irradiation Wavelength and Time. To identify effects of CPL wavelength on chirality induction, L-CPL irradiation to a film of poly(DOF) of Mn 76700 was conducted at 436 and 365 nm using band-pass filters. CPL’s at 436 and 365 nm correspond to the α- and β-phase absorption bands, respectively. CPL irradiation at both 365 and 436 nm effectively inducted chirality where it led to remarkably different CD spectral patterns (Figure 6A,B). Irradiation at 436 nm targeting the β-phase absorption band induced intense CD signals almost exclusively in the wavelength range of β-phase absorption range (>380 nm) and only much weaker signals corresponding to α-phase (Figure 6A). On the other hand, irradiation targeting at the α-phase absorption band induced CD signals in both α- and β-phase absorption ranges. These results may indicate that conformational transitions of polymer chains in the β-phase do not induce polymer chains in the α-phase to undergo structural changes (Figure 6B). This may mean that conformational transitions in α-phase can induce polymer chains in the β-phase to undergo conformation transitions while the opposite-direction induction (β-phase to α-phase) did not occur. Energy transfer from the αphase having higher excitation energy to the β-phase having lower energy may also be partially responsible for the results in Figure 6. It should be noted here that a polymer having the α-phase only cannot be made optically active while the α-phase coexisting with the β-phase can (Figure 3). However, it is clear from the above discussion that chirality of the β-phase does not induce chirality of the α-phase. Such a situation can be interpreted by proposing morphological variations of the αphase, i.e., (a) pure α-phase where interchain interactions are too weak for chirality induction by CPL and (b) α-phase associated with the β-phase where interchain interactions are significant enough for chirality induction. In connection with the discussion about morphological effects on chirality induction, it is noteworthy that CD spectral patterns

interactions are important in helix formation. In addition, “deactivation chirality amplification” is supported by an experiment where chirality was effectively induced in the presence of a small amount of chiral small molecule described in the following section. Chirality Induction by Chiral Additive: A Model Experiment of “Deactivation Chirality Amplification”. To support the proposed deactivation chirality amplification, ground-state chirality induction to poly(DOF) (Mn 76700) was examined using (R)-2,2′-dimethoxy-1,1′-binaphthyl (DMBN), an optically active, twisted small molecule, as a model of “preformed single-handed, ground-state diad”. The experiment was conducted in the following way. Toluene solutions of poly(DOF) (1 g/L) containing 5, 1, or 0.2 mol % of DMBN were heated to 100 °C to facilitate main-chain single-bond rotation of poly(DOF), which mimics the excited-state coplanar structure formation in the CPL-based experiments, and the solution was drop-casted onto a glass plate at room temperature, which mimics the deactivation from coplanar conformation to twisted conformation. Figure 5 shows the CD and UV spectra of

Figure 5. CD and UV spectra of poly(DOF) (Mn 76700) films containing 0.2 (a), 1 (b), and 5 mol % (c) of (R)-DMBN prepared by drop-casting of a toluene solution (1 g/L) heated at 100 °C onto a quartz glass plate at room temperature.

the obtained cast films; the films made at DMBN levels of 1 or 5 mol % exhibited intense CD spectra while the one at DMBN level of 0.2 mol % did not indicate clear CD responses. Absolute gCD values of the films at 410 nm were 9.83 × 10−4, 1.12 × 10−4, and nearly 0 at 5, 1, and 0.2 mol %, respectively; there is a

Figure 6. CD and UV spectra of poly(DOF) (Mn 76700) films irradiated using L-CPL with a bandpass filter at 436 nm (A) or 365 nm (B) for a 180 min total duration. The spectra were measured at 30 min intervals. G

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right- or left-handed diad twist into the coplanar conformation which is considered to be a trigger of efficient helix induction. The efficiency of enantiomer selection in photoexcitation should not be dependent on temperature as it is a photochemical process. As discussed earlier in connection with chirality amplification, selectivity in this process is generally very low, and it alone cannot explain the observed maximum gCD values. On the other hand, the chirality amplification is proposed to occur partially in the ground state in addition to “deactivation chirality amplification” discussed earlier. Although the initial phototransformation (twisted−coplanar transition) is only for the diad scale, it is proposed to be followed by greater-scale conformational transformation of a longer chain from a random structure to a single-handed helix in the ground state which may be facilitated at a higher temperature where molecular motion should be faster (“ground-state chirality amplification”). A rather short single-handed helical fragment as short as diad generated by CPL-based enantiomer-selective photoexcitation may behave as an internal chiral “seed” whose chirality is amplified through helix formation in the ground state. Regarding the results in Figure 8, it is notable that the difference between the gCD values attained at 100 and at 60 °C is much greater than the differences among the values attained at from −20 to 60 °C. The differential scanning calorimetry (DSC) profile of poly(DOF) (Mn 99500) (Figure 9) may account for

induced by full-wavelength CPL largely varied depending on irradiation time (relative irradiation energy sum) (Figure 3, I-A, II-A, and III-A). Figure 7 extracts the CD spectra induced for the

Figure 7. Normalized CD spectra of poly(DOF) (Mn 99500) in film observed after 6 min (A), 7 h (B), and 14 h (C) L-CPL irradiation.

polymer of Mn 99500 by irradiation with L-CPL for 6 min, 7 h, and 14 h. The earliest-stage CD spectrum (6 min irradiation) is characterized by two positive signals centered at around 430 and 410 nm while the latest-stage spectrum (14 h irradiation) shows two negative signals centered at around 435 and 415 nm and a positive signal centered at 370 nm. The spectrum obtained by 7 h irradiation appears to be just in between the two spectra observed at after 6 min and 14 h irradiation. The spectral pattern of the earliest-stage CD spectrum is very similar to that induced by CPL irradiation selectively at 436 nm showing chirality almost only in β-phase (Figure 6A), and the spectral pattern of the latest-stage CD spectrum rather resembles that induced by CPL irradiation selectively at 365 nm showing chirality in both α- and β-phases. These results mean that in full-wavelength CPL irradiation chirality induction in the β-phase takes place faster than that in the α-phase. The faster conformational transition in the β-phase may occur through more effective interchain interactions than in the α-phase. Temperature Effects on Chirality Induction. Temperature effects on chirality induction were studied for early stages of chirality induction by irradiating the films with L-CPL at different temperatures in the range from −20 to 100 °C. Figure 8

Figure 9. DSC profile of poly(DOF) of Mn 99500 (first heating run).

this observation. The DSC curve indicates that the polymer undergoes successive, multistep endothermic phase transitions starting at around 25 °C probably based on glass transition and melting up to ca. 70 °C, which is generally in line with the fact that helix induction is faster at a higher temperature. The DSC curve further shows that the aforementioned endothermic transitions are followed by a clear exothermic transition with a peak at 97 °C which may be ascribed to chain ordering or crystallization. These results may mean that chain ordering at attained at 100 °C significantly facilitates helix induction by CPL. This conclusion again emphasizes the importance of interchain interactions during the helix induction by CPL. Side-Chain Length Effects. Although poly(DOF) has been exclusive discussed so far, effects of chemical structure of polyfluorene on chirality induction would be important from a view of generalizing the CPL method. From this view, we studied chirality induction using CPL for poly(9,9-didodecylfluorene-2,7-diyl) (poly(DDF)) (Mn 23200) and poly(9,9dihexylfluorene-2,7-diyl) (poly(DHF)) (52900) having a longer and shorter side chains at the 9-position of the fluorene unit, respectively, in comparison with poly(DOF) (Figure 10, I and II). While a film of poly(DDF) prepared by solution-casting of poly(DDF) as prepared showed only negligible intensity of the

Figure 8. gCD at around 380 nm of poly(DOF) (Mn 76700) films observed on L-CPL irradiation for 30 and 60 min durations at −20, 0, 23, 60, and 100 °C.

shows the relation between attained gCD values of poly(DOF) (Mn 76700) films at 380 nm and irradiation time up to 60 min; chirality induction was faster at a higher temperature. This result clearly suggests that helix induction to PDOF is based not only on excited-state processes but also on ground-state processes. As indicated in Scheme 2A, CPL excitation selectively transforms H

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derivatives since interchain interactions and ordering are important in efficient chain conformational transitions. Completely amorphous polymers are therefore difficult targets for chirality induction. To overcome this limitation, we recently devised a method that uses an external, achiral small molecule reinforcing interchain interactions (“aid molecule method”). With this method, a completely amorphous, star-shaped oligofluorene having no chain ordering in the solid state was made optically active in the presence of an achiral aid molecule, fluorene or phenanthrene.54 The aid molecule method was herein applied for poly(DHF); a film was fabricated using poly(DHF) (Mn 52900, Mw/Mn 2.41) in a molar ratio of DHF residue to fluorene of 1.0/3.3 of fluorene as an aid molecule. As shown in Figure 11A, in the presence of fluorene, the poly(DHF) film was efficiently made optically active through L-CPL irradiation where gCD reached 1.7 × 10−3 at 380 nm. It should be noted that the aid molecule did not induce β-phase formation at all which can be confirmed by the complete absence of a peak to 435 nm in the UV spectra. The aid molecule is hence considered to reinforce interchain interactions only in the αphase. The CD spectral shapes induced by L-CPL for poly(DHF) are different than those observed for poly(DOF)s and poly(DHF) in lacking a trough or a negative Cotton splitting corresponding to the β-phase absorbance at around 435 nm. The induced CD spectra may therefore represent pure αphase chirality. Similar spectral patterns have been reported for polyfluorenes having centers of chirality in the side chain.55,56 In addition, the gCD vs relative irradiation energy sum plot is well approximated by a power function, gCD = 2.96 × 10−5 × (relative irradiation energy sum)1.69, for the early stages of induction, indicating that chirality amplification is responsible in the chirality induction to poly(DHF) in the presence of fluorene as an aid molecule (Figure 11B,C). Stability of Chirality Induced by CPL. Although chirality of films is immediately lost through conformational randomization when the film is dissolved in solvent, it is stable as far as the film is kept solid at ambient temperature in the dark for years. Here, we tested stability of induced chirality of film against LPL which is achiral light and against heating. Chirality was induced to the sample films made using poly(DOF) (Mn 76700) by LCPL irradiation for 6 h. Figure 12A indicates the changes of CD and UV spectra of optically active films on irradiation with LPL at 23 °C. CD spectral intensity of the film gradually decreased on LPL irradiation, suggesting that single-handed twisted conformation was excited into coplanar conformation and formed both right- and left-handed twisted conformation leading to total racemization or randomization of helix. At the same time, the UV spectral intensity also decreased; LPL irradiation may rearrange chain alignment in the film as well as CPL irradiation discussed earlier. Figure 12B shows the results of heating an optically active film at 100 °C. The change in CD spectra on heating was much faster than that on LPL irradiation. It is noteworthy that the change in UV spectra on heating was much less obvious than that on LPL irradiation with only clear decrease in intensity of the β-phase band at 435 nm which was not the case at 23 °C. This may mean that heat-induced, ground-state racemization at 100 °C which transforms β-phase into α-phase but does not rearrange chain alignment. It has been reported that films with much lower gCD values having chirality mainly in the β-phase prepared by shorter-time irradiation undergo similar loss of chirality on LPL irradiation and heating.28 These results indicate that stability of chiral

Figure 10. CD/UV spectra observed on L-CPL irradiation (A), gCD vs relative irradiation energy sum plot (B), and curve-fitting leading to a nonlinear function approximation of the gCD (380 nm) plot (C) for an annealed film of poly(DDF) (Mn 23200) (I) and CD/UV spectra of poly(DHF) (Mn 52900) film observed on L-CPL irradiation (II).

β-phase signal at 435 nm in UV spectra, β-phase content remarkably increased to 32% on thermal annealing at 65 °C under hexane vapor stream (Figure S5). Chirality was efficiently induced to the annealed film on L-CPL irradiation where the maximum absolute gCD value was 2.13 × 10−3 at 380 nm, 2.34 × 10−3 at 400 nm, and 5.79 × 10−3 at 440 nm which is comparable to those of poly(DOF)’s. The gCD vs relative irradiation energy sum plot was well approximated by a power function, gCD = 2.86 × 10−6 × (relative irradiation energy sum)2.03, for the early stages of chirality induction as well as those for poly(DOF)s. These results mean that chirality amplification occurs in helix induction to poly(DDF). In a sharp contrast, the poly(DHF) film did not indicate the βphase signal at all in UV spectra even after thermal annealing, and the intensity of the induced CD spectra on L-CPL irradiation was only 5.13 × 10−5 at 380 nm. Thus, the formation of the β-phase was found to depend not only on the main chain length (molar mass) but also on the structure of side chain. In addition, the importance of the β-phase in chirality induction may be applied for a wide range of polyfluorenes having various side-chain structures. Pure α-Phase Chirality Induction to Poly(DHF) Using an Achiral Aid Molecule. As so far discussed, the presence of β-phase is mandatory in chirality induction to polyfluorene I

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Figure 11. CD/UV spectra observed on L-CPL irradiation (A), gCD vs relative irradiation energy sum plot (B), and curve-fitting leading to a nonlinear function approximation of the gCD (380 nm) plot (C) for poly(DHF) (Mn 52900).

Figure 12. CD and UV spectral changes on LPL irradiation to poly(DOF) (Mn 76700) film for 3−30 min at 23 °C (A) and heating at 100 °C for 3−30 min (B) after inducing chirality by L-CPL irradiation for 6 h.

conformation induced to poly(DOF) does not depend on whether it is in the α-phase or β-phase. Circularly Polarized Luminescence of Optically Active Poly(DOF) Film. Chiral polymers find various applications among which circularly polarized luminescence may be expected for poly(DOF). Several polyfluorenes and related polymers having chiral side chains or a terminal group have been reported to show circularly polarized luminescence.55−61 Prior to circularly polarized luminescence measurements, unpolarized emission was measured before and after chirality induction (Figure 13A,B). Unpolarized emission spectral shape largely changed through chirality induction as shown in Figure 13A. While the film without chirality indicated two, rather sharp emission peaks at around 430 and 475 nm as main signals, a broad and intense signal centered at around 540 nm emerged after chirality induction. The broad, lower-energy signal may be ascribed to interchain excimer emission, which is in line with the earlier conclusion that interchain π-stacking leading to hypochromicity is enhanced through chirality induction by CPL. In addition, time-resolved fluorescent measurements were conducted for the optically active film to determine the decay lifetimes of the emission bands located at 430−454 nm (band a),

454−482 nm (band b), and 482−540 nm (band c) (Figure 13B). The fluorescence decay curves of the three emission bands composed of two exponential decays. The averaged decay times of emission bands a, b, and c were estimated to be 59, 68, and 156 ps, respectively (Table S1). These decays of the fluorescent emission of the film were faster than those reported for similar polymers in solution;62 similar results were observed for poly(pphenylenevinylene)s.63 Chain aggregation may accelerate interchain quenching. The fact that emission band c had a longer lifetime than those of the shorter-wavelength emission bands (bands a and b) strongly supports that the broad emission band centered at around 540 nm in Figure 13A arises from interchain excimer. Longer lifetimes for excimer than for corresponding monomer have been known for small molecular chromophores.64−67 These results confirm that the proposed formation of π-stacking between chains through chirality induction is reasonable. Circularly polarized luminescence measurements were performed for two poly(DOF) (Mn 76700) films showing mirror-image CD spectra (Figure S8) which had been prepared by L- and R-CPL irradiations for 6−7 h; gCD values at 380 nm were 1.75 × 10−3 for the film irradiated with L-CPL for 7 h and J

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that the optically active poly(DOF) films have mirror-image single-handed helical conformations in excited states as well as in the ground state. In addition, the maximum absolute glum values of these films were as high as about 6 × 10−3 (Figure 14C) while the polymers’ maximum absolute gCD values were about 2 × 10−3. The fact that glum was much greater than gCD may mean that poly(DOF) chains have higher anisotropy in excited states than in the ground state. Changes in polymer conformation or chain alignment may occur in excited states where chirality is amplified. In addition, absolute glum had a rough tendency to be greater at a shorter wavelength; isolated, unstacked chains may have greater anisotropy in excited states.

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CONCLUSIONS Chirality induction (preferred-handed helix induction) to polyfluorenes using CPL was thoroughly investigated mainly using film samples under various conditions for polymers having different molar masses (β-phase contents) and chemical structures; the results shed light on the mechanisms of helix induction through enantiomer-selective photoexcitation of either right- and left-handed twisted and deactivation chirality amplification through inter- and intrachain interactions. The extents of chirality induced by CPL irradiation were much higher than expected for a reversible photoderacemization process at the photostationary state, and almost single-handed helicity was considered to be attained. This is possible through chirality amplification which takes place through biased deactivation of coplanar conformation preferentially to the hand which is the same as that of “preformed single-handed, ground-state diad or a longer chain” in the vicinity (“deactivation chirality amplification”). This amplification mechanism is supported by chirality induction experiments in the ground state using optically active (R)-DMBN as a model for the “preformed single-handed species”. In addition, chirality induction to poly(DOF) was faster at a higher temperature, indicating the contribution of “ground-state chirality amplification”. Conformational transitions of sequences longer than diad leading to a long helix may be facilitated may by heat. Also, chain ordering enhanced at a higher temperature led to efficient chirality induction, confirming the importance of interchain interactions in chirality induction. In addition, the importance of solid-state morphologies was demonstrated for poly(DOF), i.e., (1) the presence of the βphase leading to strong interchain interactions was indispensable in chirality induction and (2) chirality induction first occurred preferentially in the β-phase and was then extended to the α-phase. Pure β-phase chirality was induced by irradiation at 435 nm or by irradiation to the full wavelength range for a short time while pure α-phase chirality was induced for poly(DOF) while pure α-phase chirality was induced for poly(DHF) using an achiral aid molecule. Moreover, chemical structure of polymers had a significant impact on CPL-based chirality induction. Chirality induction was feasible for polymers having octyl and dodecyl chains, poly(DOF) and poly(DDF), but not for the polymer having hexyl chain, poly(DHF), which required an aid molecule to assist chirality induction. Furthermore, chirality induced to the film samples was stable over years if they are stored at room temperature without irradiation with intense light; however, the extent of chirality largely decreased on irradiation with intense LPL at room temperature and in the dark at an elevated temperature.

Figure 13. Unpolarized emission spectra of poly(DOF) (Mn 76700) film before (a) and after (b) chirality induction with L-CPL for 10 h (A) (λex = 385 nm) and fluorescent decay for the film after chirality induction (B).

−2.01 × 10−3 for the film irradiated with R-CPL for 6 h. The extent of anisotropy in circularly polarized luminescence was evaluated in terms of glum according to the equation glum = 2(IL − IR )/(IL + IR )

where IL and IR are emission intensities of left- and right-handed CPL. Figure 14 shows circularly polarized luminescence (A), total photoluminescence (B), and glum (C) spectra of the poly(DOF) films showing mirror-image CD spectra. The two enantiomeric films exhibited clear circularly polarized luminescence spectra (Figure 14A) which were mirror images to each other, indicating

Figure 14. Circularly polarized luminescence (A), total luminescence (B), and glum (C) spectra of optically active poly(DOF) film (Mn 76700). Chirality induction was performed through L-CPL irradiation for 7 h and with R-CPL irradiation for 6 h. K

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Macromolecules Finally, optically active poly(DOF) films emitted efficient CPL with rather high gCPL values. This is the first example of preparation of efficient CPL-emitting material using CPL as the chirality source. CPL-driven chirality induction to polyfluorene derivatives thus can lead to optically active polymeric materials with practical functions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01453.

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Experimental details, SEC profiles of poly(DOF)s, CD/ UV spectra of poly(DOF)s of lower molar masses, change in β-phase content on irradiation, determination of βphase content by waveform analysis, annealing effects on UV spectrum of poly(DDF), DSC profiles of poly(DHF), poly(DOF), and poly(DHF), contour representation of time-resolved fluorescence spectra, tabulated lifetime data for fluorescence, CD/UV spectra of poly(DOF) films used for circularly polarized luminescence measurements (PDF)

AUTHOR INFORMATION

Corresponding Author

*(T.N.) E-mail [email protected]. ORCID

Yoshihiko Kanemitsu: 0000-0002-0788-131X Tamaki Nakano: 0000-0002-7843-4146 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.N. acknowledges financial support from the Mitsubishi Foundation. This work was supported in part by the MEXT program of Integrated Research Consortium on Chemical Sciences (IRCCS). We thank Mr. Zhaoming Zhang for his support in preparation of the manuscript. Technical Division of Institute for Catalysis, Hokkaido University is acknowledged for technical support for experiments.

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DOI: 10.1021/acs.macromol.8b01453 Macromolecules XXXX, XXX, XXX−XXX