Supramolecular Chirality in Achiral Polyfluorene: Chiral Gelation

Apr 21, 2016 - The CPL/PL spectra were recorded at 25 °C using a JASCO CPL-200 spectrofluoropolarimeter (Tokyo, Japan) equipped with a Peltier-contro...
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Supramolecular Chirality in Achiral Polyfluorene: Chiral Gelation, Memory of Chirality, and Chiral Sensing Property Yin Zhao,† Nor Azura Abdul Rahim,‡ Yijun Xia,† Michiya Fujiki,‡ Bo Song,† Zhengbiao Zhang,† Wei Zhang,*,† and Xiulin Zhu† †

State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China ‡ Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan S Supporting Information *

ABSTRACT: Producing supramolecular chirality from achiral π-conjugated polymers toward preferred chiral memory, effective circularly polarized luminescence, and chiral sensor is extremely important in design of functional chiral materials. Proposed herein is an effective protocol to generate and memorize the supramolecular chirality formed from achiral poly(9,9-dioctylfluorene) (PF8) induced by chiral solvation. The process of chiral supramolecular assembly was monitored by UV−vis spectroscopy, circular dichroism (CD), and fluorescent spectroscopy. Achiral PF8 dissolved in neat (R)(+)-limonene (1R) and (S)-(−)-limonene (1S) underwent chiral sol−gel transition at −20 °C. PF8 aggregates revealed intense CD and circularly polarized luminescence (CPL) signals due to β-phase, exhibiting absolute dissymmetry ratio of ≈2 × 10−3 at 430−470 nm. The supramolecular chirality of PF8 aggregates can be perfectly memorized in solid film even near decomposition temperature (300 °C), comparing favorably with that from chiral polyfluorene. Atomic force microscopy (AFM) inferred helically distorted PF8 aggregate motifs responsible for the CD and CPL functionality. Furthermore, the first chiral sensor to detect nonracemic limonene molecules employing achiral PF8 spin-cast film from CHCl3 solution was achieved.

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supramolecular chirality in solid polymer films are significant issues for the practical applications in chiroptical switch and memory, optical data storage, and detection of circularly polarized luminescence (CPL). To the best of our knowledge, there are only several reports of transfer and memory of chirality from chiral polymer film to film and from chiral polymer solutions to films, as reported by Fujiki et al.9a and Yashima et al.9b−d Chirality in π-conjugated polymers endows them with promising optoelectronic properties, for instance, CPL derived from the discrimination between left (L) and right (R) circularly polarized light.10 This unique property can be employed to construct the novel optoelectronic devices. Among them, chiral polyfluorene derivatives (PFs) have been intensely studied, owing to their easy control of chirality by various phase transitions (amorphous α, α′, β, and nematic phases) and relatively large dissymmetry factor of CPL.11 Scherf et al. reported the first circularly polarized electroluminescence (CPEL) from liquid-crystalline chiral PFs.10a

INTRODUCTION A scenario of hierarchical chirality spanning from elemental particles to atoms, molecules, supramolecules, polymers, and supramolecular polymers has long been a matter of curiosity among materials scientists.1−3 Recently, supramolecular chirality obtained from achiral sources is growing interest due to the appeal of avoiding of tedious synthesis of chiral polymers as well as the possibility of several chiroptical applications including memory and switching.4 To date, circularly polarized photon chirality,5 liquid crystal chirality,1e,g gelation,4a chiral solvation,6,7 and mechanophysical chirality8a−c are candidates to generate chiral polymers and supramolecular chirality when achiral π-conjugated polymers5b,c,e,f,7b,d,g−j and oligomers,7e σconjugated polymers,5a,d,7a,f vinyl polymers,7k,l and molecules4,7c,8 were employed as starting substances. The concept of chiral-solvation-induced chirality was first introduced to the polymer system by Green et al. 6 Subsequently, a series of achiral poly(n-hexyl isocyanate),6 πconjugated polymers, e.g., oligo(p-phenylenevinylene),7e polyfluorene analogues7b,c,g,h and polyacetylenes,7i,j σ-conjugated polysilanes,7a,f and side-chain polymers,7k,l have been shown to achieve supramolecular chirality driven by chiral solvation. However, the chiral transfer, amplification, and memory of © 2016 American Chemical Society

Received: February 21, 2016 Revised: April 14, 2016 Published: April 21, 2016 3214

DOI: 10.1021/acs.macromol.6b00376 Macromolecules 2016, 49, 3214−3221

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Macromolecules

2 nm, a scanning rate of 100 nm min−1, and a response time of 1 s. The CPL/PL spectra were recorded at 25 °C using a JASCO CPL-200 spectrofluoropolarimeter (Tokyo, Japan) equipped with a Peltiercontrolled housing using the SQ-grade quartz cuvette, a single accumulation, a path length of 1 mm, a bandwidth for excitation of 10 nm, a bandwidth for monitoring of 10 nm, a scanning rate of 100 nm min−1, a response time of 2 s, and a 360 nm excitation wavelength. The data were modified based on normalized Abs and PL at β-phase (around 435 nm/439 nm, responsible for β-phase). The UV−vis spectra were recorded on a Shimadzu UV-3150 spectrophotometer (Shimadzu China, Shanghai, China). Fluorescent spectra were measured on a PerkinElmer LS-50B spectrofluorometer with a scanning rate of 100 nm min−1, an excitation bandwidth of 2 nm, a monitoring bandwidth of 2 nm, a response time of 1 s, and 360 nm as excitation wavelength at 25 °C. Atomic force microscope (AFM) images were captured on a Multimode 8 microscope (Bruker Co.). Peak force quantitative nanomechanical mapping (QNM) in air scan mode with a ScanAsyst-air probe (nominal spring constants 0.4 N/m, a frequency of 70 kHz; Bruker) was adopted. The samples were prepared by immersing the mica plate into diluted polymer aggregates in neat 1R or 1S for 10 min, followed by drying in nitrogen flow for 4 h and then in a vacuum overnight. Thermal behavior and phase transition temperatures of PF8 were observed and obtained using a TA-Q100 DSC instrument. The temperature and heat flow were calibrated using standard materials (indium and zinc) at a cooling and heating rate of 5 °C min−1.

Meskers and co-workers realized for the first time that polymer photovoltaic cells made from a chiral PF copolymer are sensitive to the circular polarization of light.10c Indeed, it requires the induction of chirality in PF analogues by direct polymerization of chiral monomers prior to applications. The limitation of chiral substitute side-chain groups may restrict the application of chiral PFs. Therefore, it is significant to produce chiral PFs from achiral counterparts in a controlled manner. In this contribution, we present the construction of supramolecular chirality from the aggregation of achiral poly(9,9-dioctylfluorene) (PF8) in nonracemic limonene by cooling PF8 solution at relatively low temperatures (below −10 °C). This supramolecular chirality was then effectively transferred to PF8 solid film and perfectly memorized, which compared favorably with those formed by chiral PFs. It was further demonstrated that spin-cast achiral PF8 film from CHCl3 solution could be employed as a chiral sensor to detect nonracemic limonene molecules.

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EXPERIMENTAL SECTION

Materials. Tetrakis(triphenylphosphine)palladium(0) (97.0%, Tokyo Chemical Industry (TCI), Tokyo, Japan), 2,7-dibromo-9,9-din-octylfluorene (98.0%, TCI Shanghai Development Co., Ltd.), Shanghai, China), and 9,9-dioctylfluorene-2,7-diboronic acid (98.0%, J&K Scientific, Beijing, China) were used as received. (R)(+)-Limonene (1R, >95%, [α]24589 = +99.62°, Shanghai Development Co., Ltd., Shanghai, China) and (S)-(−)-limonene (1S, >95%, [α]24589 = −97.72°, Shanghai Development Co., Ltd., Shanghai, China) were used without further purification. All the other chemicals were obtained from Shanghai Chemical Reagents (Shanghai, China) and used as received. Poly(9,9-dioctylfluorene) (PF8) was synthesized according to the previously reported procedures.7g Mn = 29 880 g/mol, Mw/Mn = 2.42. 1H NMR (400 MHz, CDCl3): δ (ppm) 0.50−1.75 (brm, CH2 + CH3), 2.33−1.89 (brm, CH2), 7.50−8.10 (brm, ArH). Preparation of Poly(9,9-dioctylfluorene) (PF8) Gels. The solution of PF8 in 1R with concentration of 5 mg/mL was prepared by stirring their mixtures at 80 °C for 4 h until Tyndall scattering was not observed by passing the light through the solution. After being cooled down to room temperature, the solution was further filtered through 0.45 μm PTFE filter to remove the dust particles. Then the polymer solution was cooled at −20 °C for 20 h, and olive PF8−1Rgel was obtained. PF8−1S-gel was prepared with similar procedures. Preparation of Optically Active Poly(9,9-dioctylfluorene) (PF8) Aggregates. A solution of PF8 (0.20 mg/mL) in neat 1R or 1S was prepared previously in a 20 mL flask equipped with a stopcock by refluxing with vigorous stirring at 80 °C for 4 h to ensure the complete dissolution of PF8. After cooling to room temperature, the solution was further filtered through a 0.45 μm PTFE filter to remove the dust particles. Then a 0.30 mL aliquot of the polymer solution was transferred to a 0.30 mL quartz cell (optical length = 1 mm) equipped with a stopcock. After continuously cooling the solution for 40 h at −20 °C, green PF8 aggregates dispersed in 1R or 1S were obtained. The polymer aggregates at other temperatures were generated from the similar process (97, 3, and 2 h for −10, −30, and −40 °C, respectively). Preparation of Poly(9,9-dioctylfluorene) (PF8) Films. PF8 assemblies prepared in neat 1R and 1S at −20 °C (2.0 mg/mL) were transferred to quartz plates by spin-coating or drop-coating methods, followed by drying under nitrogen flow for 6 h and then drying in a vacuum for 12 h at 60 °C. The trace of limonene was estimated to be only 0.008% residual in PF8 film calculated by the 1H NMR spectrum as compared with that of the aggregate state, even which cannot be detected by infrared spectroscopy (IR). The thickness of spin-cast film measured by AFM is approximately 500 nm. Measurements. The CD spectra were recorded on a JASCO J-815 spectropolarimeter (JASCO China, Shanghai, China) at 25 °C equipped with a Peltier-controlled housing unit, using the SQ-grade cuvette, a single accumulation, a path length of 1 mm, a bandwidth of

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RESULTS AND DISCUSSION

Chiral Sol−Gel Behavior of PF8 in Neat Limonene. Condensed states of PF8 are known to adopt amorphous, α, α′, β, and nematic phases.11 These phases are susceptible to the nature of solvents and thermal processing.11a,12 This uniqueness prompts us to verify whether chiral sol−gel transition of PF8 in neat limonene will occur. This is an important step toward understanding chiroptical characteristics of PF8 thin film in the ground and photoexcited states directed toward several elaborate applications. First, we examined the sol−gel transition behavior of PF8 in enantiopure limonene as neat solvent. Figure 1 compares two photographs of 10 mg of PF8 dissolved in 2 mL of 1R at 80 °C placed in a test tube before and after cooling at −20 °C for 20 h. The formation of the PF8 gel is evident. Differential scanning calorimetry (DSC) analysis (Figure S1) indicates that the sol−gel transition temperatures of PF8 in 1R and 1S are 50 and 45 °C, respectively. PF8 in neat

Figure 1. (left) Illustration of α- and β-phase of poly(9,9dioctylfluorene) (PF8); structures of (R)-(+)-limonene (1R) and (S)-(−)-limonene (1S). (right) Photographs of PF8 solution and gel in neat 1R and 1S (5 mg/mL). 3215

DOI: 10.1021/acs.macromol.6b00376 Macromolecules 2016, 49, 3214−3221

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Macromolecules 1R and 1S underwent to gradually turn to gel state near −20 °C (Figure S1), which is similar to gelation behavior in toluene.11a,12a To our knowledge, this is the first example of chiral sol−gel transition of achiral PF8 in chiral solvents as neat state, which has been coined in π-conjugated small molecule systems.8b Chiroptical Properties of PF8 Aggregates. To trace the assembly process of PF8 in 1R and 1S in real time as functions of temperature and aging time, UV−vis, fluorescence, and circular dichroism (CD) spectroscopic measurements were employed (Figures 2, 3, and 4). Typically, PF8 in pure 1R or 1S

Figure 4. CD spectra employed to monitor the assembly process of PF8 in both 1R and 1S solutions as a function of time at (a) −10, (b) −20, and (c) −30 °C. Dependence of gCD values (443 nm) on the cooling time at different temperatures (d). The polymer concentrations are all 5 × 10−4 M (in repeating units).

the absorption maximum (385 nm) and a pronounced shoulder at a longer wavelength of around 436 nm, evidenced that π−π stacking took place. These spectral features indicate that PF8 chains are face-to-face π−π stacked with a preferential twist sense, likely oriented by the interaction between limonene molecules and n-octyl chains in F8 repeating units.7b,8b Additionally, the notable red-shifts in three vibronic PL bands (0−0′, 0−1′, and 0−2′ bands) were observed from the time-dependent fluorescence spectra at −20 °C (Figure 3), 0− 0′ PL band (from 424 to 450 nm), 0−1′ PL band (from 442 to 468 nm), and 0−2′ PL band (from 472 to 498 nm). These apparent red-shifts in vibronic PL bands indicate that in condensed plural phase of PF8 the lowest S1 state responsible for these PL bands dominantly arose from α-phase (higher energy state) to β-phase (lower energy state) due to photoexcited energy migration processes.11 Our results indicate that by applying prolonged cooling process at a specific lower temperature, α-phase-origin state turns to thermodynamically metastable β-phase with a higher β-fraction. This idea is compared to production of higher β-phase fraction of PF8 in homogeneous solution state.11 Our knowledge should provide a new strategy for obtaining a purely single β-phase of PF8 associated with conformationally uniform structure (Figure 1), highly delocalized π-electron system in the future. Purely βphase PF8 is inevitably needed to improve optoelectronic properties.13 CD spectral shape (magnitude and sign) of PF8 aggregates in 1R and 1S obtained at three different cooling temperatures (−10, −20, and −30 °C) show gradual increase in CD magnitudes exhibiting a nearly ideal mirror-image relationship (Figure 4). These results demonstrate the successful chirality transfer of limonene molecules solely to PF8 aggregates. The CD spectral characteristics (wavelength, magnitude, and sign) are almost identical to those generated in limonene-containing tersolvents reported previously,7b,g,h which was seriously affected by volume fraction of each solvent. Furthermore, particles only formed in previous tersolvents systems made it difficult to observe the real helical sense of aggregates. The absolute magnitudes in CD and gCD (443 nm) values were attained at 93 h (−10 °C), 40 h (−20 °C), and 3 h (−30 °C). The disappearance of the 443 nm CD signals at −40 °C is

Figure 2. Time dependence of UV−vis spectra of PF8 at −20 °C in (a) 1R or (b) 1S. The time dependence of β-phase content of PF8 assemblies formed in 1R or 1S at (c) −10 and −20 °C and (d) −30 and −40 °C. The polymer concentrations are both 5 × 10−4 M (in repeating units).

Figure 3. PL spectra employed to monitor the assembly process of PF8 in both 1R and 1S solutions as a function of time at −20 °C. The polymer concentrations are both 5 × 10−4 M (in repeating units).

at −20 °C initially shows a broader π−π* transition around 385 nm (Figures 2a and 2b), which is characteristic of α-phase.11 However, a new π−π* transition at 436 nm due to β-phase11 progressively propagated with the cooling time, as presented in Figures 2a and 2b. The level-off time of β-phase production highly depends on cooling temperature (Figures 2c and 2d), longer than 93 h at −10 °C, 40 h at −20 °C, 3 h at −30 °C, and 1.5 h at −40 °C. From a relative ratio in absorbance at the 385 and 436 nm bands,11f the fraction of β-phase was evaluated to be 20% (−10 °C), 33% (−20 °C), 25% (−30 °C), and 30% (−40 °C). The highest β-fraction was achieved at −20 °C among the four temperatures. At 80 °C, the polymer chain prevails as a nonaggregated state as proven by a broad α-phase absorption band (385 nm). Upon cooling, a decrease of the absorption band at 385 nm, concurrent with a slight red-shift of 3216

DOI: 10.1021/acs.macromol.6b00376 Macromolecules 2016, 49, 3214−3221

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Chiral Memory Property. To date, persistent chiroptical memory using supramolecular assemblies and aggregates has been rarely reported because of the difficulty in designing and controlling noncovalent dynamics.4,9 One successful example is helical poly(1-phenylacetylene) with help of chiral chemical dopants in homogeneous solution.9c,d In this case, achiral small molecules (e.g., amino alcohols) were needed to provide the chiroptical memory effect to poly(1-phenylacetylene). The successful induction of chirality to PF8 aggregates indicated the potential for demonstrating the persistent chiroptical memory effect with limonene. First, we obtained PF8 aggregate in 1R or 1S by cooling at −20 °C for 40 h. Second, thin film was fabricated onto quartz substrate by spin-coating the PF8 solution at room temperature. Finally, most of 1R or 1S in the PF8 film was removed at 60 °C under vacuum. The resulting PF8 films were kept for 3 months, and no marked changes in CD spectral characteristics were observed (Figure 7). The

attributable to the formation of precipitation arising from the rapid cooling process. Meanwhile, the absolute CD magnitude is almost linearly proportional to enantiopurity of 1R and 1S and their concentrations (Figure S2). CD spectra remained unchanged for one month when keeping the temperature under 25 °C (Figure S3). Conversely, elevating the temperature to 80 °C caused immediate disappearance of the β-phase origin 443 nm CD signals (Figure 5), though the 443 nm CD bands

Figure 5. UV−vis absorption and CD spectra monitored the assemblyand-disassembly process of PF8 in 1R and 1S. (a) The CD-active assemblies lose optical activity smoothly when elevating outside temperature to 80 °C. (b) The chiroptical switching property was investigated in five trials when assembling at −20 °C for 40 h and disassembling at 80 °C for 4 h. The polymer concentrations are both 5 × 10−4 M (in repeating units).

reappeared upon cooling run. The more fascinating issue is that the aggregation-and-disaggregation cycles permit high repeatability in the CD amplitude (Figure 5). This property thus enables the thermoresponsive chiroptical on−off switching function. AFM images clearly revealed the left- and right-handed twisted helical fibers induced by 1R and 1S chirality (approximately 20 nm in width) (Figure 6). The twisted helical fibers might be responsible for chirality of PF8. Possibly, these AFM images are the first visualization of how PF8 aggregates adopt the helical superstructure induced by limonene chirality. Similar morphology of PF8 aggregates was also observed by TEM (Figure S4). The formation of the PF8 nanofiber in toluene solution was also observed by Qiu et al., but without helical sense.12e

Figure 7. CD spectra of PF8 films spin-cast from PF8 assemblies in neat limonene.

concept of chiroptical memory in current system will provide another possibly excellent strategy to produce polymer chiral materials with a possibility of practically chiroptical applications, comparing favorably with that from chiral counterparts.10 Intriguingly, the fraction of β-phase PF8 in the obtained solid films was as high as 42%, which approaches the highest βfraction reported previously.11g The thermal stability of chirality in polymer film is another key issue for application in optoelectronic devices.10,14

Figure 6. Tapping-mode AFM images of PF8 assemblies formed at −20 °C for 40 h in (a) 1R and (b) 1S. 3217

DOI: 10.1021/acs.macromol.6b00376 Macromolecules 2016, 49, 3214−3221

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Therefore, facile generation and control of the CPL sign in the supramolecular chirality is a challenge as well as an efficient generation of supramolecules with a high dissymmetry ratio, glum, from entirely achiral sources. PF8 aggregates in 1R and 1S reveal relatively intense, vibronic circularly polarized luminescence (CPL) bands due to β-phase associated with gCPL of −1.78 × 10−3 at 442.5 nm (0−0′ band) for 1R and +1.21 × 10−3 at 442 nm (0−0′ band) for 1S (Figure 9a), respectively.

Considering the inherent instability of noncovalent interaction, the thermal stability of supramolecular chirality induced by limonene solvation in PF8 was studied. The PF8 films spin-cast from 1R or 1S solutions were treated for 5 min at various temperatures (from 25 to 300 °C) in a vacuum and were tracked by UV−vis and CD spectra (Figure 8). UV−vis spectra

Figure 9. PL and CPL spectra of PF8 aggregates by (a) cooling for 40 h at −20 °C and (b) corresponding spin-cast films.

On the other hand, another CPL 0−0′ band due to α-phase can be seen weakly in addition to the same sign of β-phase, gCPL of −0.74 × 10−3 at 417.5 nm (0−0′ band) for 1R and +0.69 × 10−3 at 414 nm (0−0′ band) for 1S (Figure 9a), respectively. Notably, regarding CPL signals of PF8, apparent CPL signs from PF8 film are the opposite of those of the aggregate state (Figure 9b). The origin of this anomaly is not well understood at the moment. In case of 1R, the negative-sign CPL band at the 0−0′ PL band at 435 nm in the gel state turned into a positive-sign CPL band at the 0−1′ PL band at 465 nm in the solid film, and vice versa for 1S. Regardless of the identical limonene chirality, the sign of CPL signals depends on aggregate and film states. We repeatedly confirmed that these CPL signs were not altered by solid film orientation and up/ back sides of the substrates. In the case of β-phase in the film, glum values at 0−1′ band (464 nm) are rather intense, +1.28 × 10−3 (1R) and −1.02 × 10−3 (1S), respectively, while glum values at the 0−0′ band (438 nm) are weak, −0.27 × 10−3 (1R) and +0.30 × 10−3 (1S), respectively. The weak 0−0′ and intense 0−1′ CPL bands appear to be an exciton couplet similar to bisignate characteristics. Analogous with of exciton couplet bisignate CD signals detecting the ground-state chirality, we assume a possibility of exciton couplet bisignate signals appeared in CPL spectra to detect the photoexcited state chirality in the thin films. This is not obvious in their CD spectra in the thin solid films. Similar CPL-sign inversion characteristics of chiral binaphthyl derivatives between solution and aggregation states were recently reported.16h,i In the aggregate (gel) state, limonene molecules may interstitially penetrate the side chains of weakly assorted aggregates. The loose aggregate state may be responsible for the absence of exciton-like spectra, but instead simply plus- or minus-sign CPL spectra. On the other hand, in the limonene-free highly assorted close-distance aggregate/film state, the close-distance aggregate is responsible for exciton coupling-like splitted CPL signals. This causes an apparent CPL sign inversion effect. We assume that CPL-sign inversion characteristics commonly occur, depending on homogeneity of the solution, gel, and condensed film states. Chiral Sensor Property of Achiral PF8 Film. The successful chiral transfer and memory of PF8 aggregates induced by limonene solvation motivate us to study the chiral sensing ability of achiral PF8 film to vapor of limonene

Figure 8. Normalized UV−vis absorption monitoring the phase change of PF8 films spin-cast from PF8 aggregates in (a) 1R and (b) 1S as a function of temperature. CD spectra were used to monitor the phase change of PF8 films spin-cast from PF8 aggregates in neat limonene from (c) room temperature (RT) to 80 °C and from (d) 125 to 300 °C.

(Figures 8a and 8b) showed that the typical absorption intensity of β-phase (436 nm) remains constant below 80 °C and gradually decreases above 80 °C (the critical phase transition temperature),15 which disappears at 160 °C. It was further supported by time-dependent FL spectra of PF8 films (Figure S5), where the emission peak (442 nm) of β-phase disappears and that of α-phase (424 nm) appears at 100 °C. The emission of nematic phase is observed at temperatures above 160 °C.15 The temperature-dependent CD spectra (Figures 8c and 8d) show that the β-phase peak at 443 nm decreases gradually as the temperature increases from 80 to 125 °C and disappears at 125 °C. Above 125 °C, the α-phase peak at 419 nm was mainly observed, which decreased with the temperature due to appearance of nematic phase and the gradual decomposition of PF8.15 These phase change behaviors were also verified by X-ray diffraction (XRD) (Figure S6) and DSC thermal analysis (Figure S7). No peaks are found in PF8 annealed films below 80 °C, indicating PF8 thin films by normal spin-coating from 1R and 1S solutions were amorphous. When the temperature was elevated above 80 °C, PF8 film changed from amorphous phase to a more orderly crystalline αphase. Once the temperature reaches 160 °C, PF8 film begins melting and recrystallizes to form α-phase when cooled to room temperature. CPL-Sign Inversion. Controlling CPL-sign in chiral materials, meaning switching chirality and helicity in the photoexcited state, has attracted considerable interest. Recently, CPL-inversion characteristics have been induced by external stimuli, including solvents,16a−d photoswitchable chiral dopants,16e counteranions,16f chiral nematic liquid crystal,16g and aggregation-and-disaggregation processes.16h,i Nevertheless, supplementary driving forces had to be employed for the above CPL-sign inversion in addition to chiral substances. 3218

DOI: 10.1021/acs.macromol.6b00376 Macromolecules 2016, 49, 3214−3221

Macromolecules molecules. It is noteworthy that the weak CD signals (Figure 10a), negative peak for 1R at 398 nm and positive peak for 1S

ACKNOWLEDGMENTS

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at 423 nm, of PF8 film spin-casted from CHCl3 solution were detected after exposure to 1R and 1S vapor. The corresponding peaks were also observed near 390 nm for both 1R (negative signal) and 1S (positive signal) in Figure 7. The absence of clear CD signals for β-phase observed is possibly attributed to the relatively low fraction of β-phase formed in PF8 films, as evidenced by the UV−vis spectra (Figure 10b). Meanwhile the β-phase peak in the UV−vis spectra showed different sensing to limonene chirality; for instance, the β-phase content induced by 1R (26.5%) was higher than that of 1S (23.5%). The β-phase content is 24.9% in the case of the 1R and 1S mixture (1R/1S = 1/4, v/v), in the range of values of 1R and 1S (Figure S8). It is notable that the current chiral-sensing film is the first example among π-conjugated polymers, presenting thermal stability as well as good mechanical and optical properties.14

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CONCLUSIONS To conclude, we have demonstrated the chiral sol−gel transition of achiral polyfluorene by heating−cooling its solution in neat limonene. The supramolecular chirality induced by limonene solvation was perfectly transferred and memorized in the film state, comparing favorably with that generated from chiral counterparts. The helically distorted PF8 aggregate with right-handness (1S) and left-handness (1R) was clearly observed by AFM. In view of the above results, the multilevel sensor (CD and UV−vis) for enantioselective recognition toward the limonene chirality was first achieved based on the achiral π-conjugated polymer film. Considering the potential application of chiral polyfluorenes in optoelectronic devices, this concept may provide a new way for designing and constructing chiral π-conjugated polymer materials. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00376. Figures S1−S8 (PDF)

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The authors are grateful for the financial support from the National Nature Science Foundation of China (21374072, 21374068, and 21574089), the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, and the Program of Innovative Research Team of Soochow University.

Figure 10. CD (a) and UV−vis (b) spectra of PF8 film spin-cast from CHCl3 solution (2 mg/mL) after exposure to 1R and 1S vapor.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (W.Z.). Notes

The authors declare no competing financial interest. 3219

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