Design and Synthesis of Polymers for Chiral Photonics

Sep 9, 2013 - To the best of our knowledge, we find the optical activity of one of the studied ... Xu Yang , Seogjae Seo , Chihyun Park , and Eunkyoun...
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Design and Synthesis of Polymers for Chiral Photonics Bruno Nowacki,†,‡ Heongsub Oh,† Cristiano Zanlorenzi,‡ Hongsub Jee,† Alexander Baev,† Paras N. Prasad,*,† and Leni Akcelrud*,‡ †

Institute for Lasers, Photonics, and Biophotonics, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States ‡ Laboratório de Polímeros Paulo Scarpa (LaPPS), Universidade Federal do Paraná, P.O. Box 19081, Curitiba 81531-990, Paraná, Brazil S Supporting Information *

ABSTRACT: Chiral photonics deals with enantioselective polarization control of linear and nonlinear optical functions and holds a great promise for a wide range of applications including optical signal processing, biosensing, and chiral bioimaging. Development of chiral materials with optical activity exceeding that of natural materials therefore becomes a prerequisite to realizing the full potential of this field. Herein, we report on a study of structure−property relations of two chiral polymers with ester functional groups in lateral branch. To enhance rotational strength, the macroscopic measure of chirality, we employ a fluorene−quinoxaline motif in the monomer unit. To the best of our knowledge, we find the optical activity of one of the studied polymers to be the highest reported so far. Theoretical calculations reveal a correlation between the topological conformations and the simulated CD spectra in agreement with the experimental measurements and help clarify the mechanism of optical activity enhancement that could create insight for further enhancement of chirality.

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INTRODUCTION The optical properties of anisotropic polymer films comprising aligned strands have been utilized for a wide variety of applications including the fabrication of anisotropic linear and nonlinear optical devices such as transistors, lasers and molecular strain sensors for polymer films, polarized-light emitting diodes, optical filters, infrared polarizers, and liquid crystal displays, to name a few examples.1−12 An efficient means to realize anisotropy is unidirectional alignment of conjugated species in a polymeric host by tensile drawing, as done previously with inorganic crystals.13,14 Recently, development of polymeric media mimicking the key properties of artificially nano-engineered nonpolymeric materials usually referred to as metamaterials, such as isotropic 3D bulk negative refractive index, and possessing low absorption loss at the same time, has emerged as an alternative avenue for a new class of flexible, lowcost materials.15−17 In this context chirality is the mostly sought after property with a great potential to realize negative or nearzero refractive index in true 3D bulk materials.18−20 Thermal realignment and self-organization of polymer fibrils in annealed films enhance optical activity in this case.18,19 Generally speaking, the whole area of chiral photonics is getting redefined with the advent of novel classes of flexible, low-cost chiral materials with optical activity non-accessible in naturally occurring materials. These materials hold great promise for use in metaphotonics (manipulation of electromagnetic fields in nano-engineered materials to control propagation of light) as well as for a much broader field of chiral photonics, which © XXXX American Chemical Society

involves, among others, chiral control of linear and nonlinear optical functions, biosensing, and chiral imaging. Enhancement of chirality and associated optical activity can be achieved in several ways such as the preparation of selfassembling structures and cholesteric liquid crystals, whereas in polymer systems this can be done by means of enantioselective interactions with small chiral molecules, doping polymeric films with plasmonic or excitonic nanoparticles, preparing nanocomposites, or initiating helical assembly of the polymeric fibrils.18,19,21−25 The latter could be sensitively controlled by solvent, temperature, pH, and the presence of solvent vapor.26−28 In solid state, changes in the chiroptical properties of large macromolecules can be induced by annealing.29 In this contribution, chirality was introduced into polymeric strands by building helicity into their backbone via incorporating asymmetric chiral centers (side chains) to the monomer units. This resulted in large circular dichroismthe main feature associated with the macromolecular configuration and conformation. Two polymers with a novel chiral side chain were synthesized. One is a polyfluorene homopolymer derivative (poly[9,9-bis(3-((S)-2-methylbutylpropanoate)fluorene]) (LaPPS61), and the other is an alternated copolymer of fluorene and quinoxaline, (poly[9,9-bis(3-((S)2-methylbutylpropanoate))fluorene-alt-5,8-(2,3-bis(phenyl)Received: August 19, 2013 Revised: August 22, 2013

A

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Scheme 1. Synthesis of Monomers M1, M2, and M3

range helical ordering occurring for instance in cholesteric phases.50−52 The optical properties of LaPPS61 and LaPPS62 were studied using circular dichroism, UV−vis, and photoluminescence spectroscopy along with density functional theory (DFT) calculations of electronic properties of interest.

quinoxaline)]) (LaPPS62), which is also a donor−acceptor structure (DA). The chiral site is located at the ester ramification at C9 in the fluorene moiety; this side chain also served to improve solubility. Chirality results in optical activity such as circular dichroism, circular birefringence, and circularly polarized luminescence. In chiral media the effective refractive index for circularly polarized light is given by neff =

ε′μ′ ± k

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(1)

EXPERIMENTAL SECTION

Materials and Methods. The chemicals 2,7-dibromofluorene, tertbutyl acrylate, tetrabutylammonium bromide, sodium borohydride, tetrakis(triphenylphosphine)palladium(0), [1,1′-bis(diphenylphosphino)ferrocene]dichloropalladium(II), 4,7dibromobenzo[c][1,2,5]thiadiazole), cobalt dichloride hexahydrate, benzil, bis(pinacolato)diboron, potassium acetate, trifluoroacetic acid, and the solvents water, toluene, N,N′-dimethylformamide, dichloromethane, ethanol, and chloroform were purchased from Aldrich. Magnesium sulfate was purchase from Fisher. Silica gel (60−200 μm) was purchased from BDH. Sodium hydroxide and potassium carbonate were purchase from J.T. Baker. Hydrochloric acid, sulfuric acid, and acetic acid were purchased from EMD. Deuterated chloroform and deuterated dimethyl sulfoxide were purchased from Cambridge Isotope Laboratories. All reagents and solvents were used as received without further purifications. 1 H and 13C NMR spectra were recorded on a Varian Inova-400 Instrument (399.65 MHz for 1H, 100.40 MHz for 13C). UV−vis and CD spectra were measured on a Shimadzu UV-3101PC spectrometer and a JASCO J-715 CD spectropolarimeter (scanning rate of 200 nm min−1, bandwidth of 1 nm, and response time of 1 s, using a single accumulation). For the temperature dependence studies all films were prepared by spin-coating at 1100 rpm from chloroform/chlorobenzene (10:1) 10 mg mL−1 solutions and annealed from room temperature (RT) to 200 °C in an argon atmosphere for 2 h. For the thickness dependence studies five different solutions were prepared: 5, 10, 20, 30, and 40 mg mL−1 using chloroform/chlorobenzene (10:1) as solvent. All films were prepared using these solutions by spin-coating at 1100 rpm and annealed at 150 °C in an argon atmosphere for 2 h. The thickness of films was measured from scanning electron microscopy (SEM) images. The equipment used was a Hitachi S4000 field emission microscope at an acceleration voltage of 25 kV. DSC analyses were performed with a TA Instruments Q200 system with an RCS-90 cooling device. All samples were heated from 20 to 200 °C at a rate of 10 °C min−1 in a nitrogen atmosphere and then

where μ′ and ε′ stand for the real parts of the electric permittivity and magnetic permeability, respectively, and κ is the chirality parameter, which is proportional to optical rotation (circular birefringence).30−32 This implies that the effective refractive index could be made negative, zero, or near-zero for one sense of circular polarization without achieving negative values of μ′ and ε′, if the chirality parameter is sufficiently large. In this context polyfluorene derivatives are an interesting group of conjugated polymers due to their optical activity in the visible and near-visible regions, good solubility, good processability, and high efficiencies of light absorption and emission combined with a variety of supramolecular organizations such as crystalline and noncrystalline phases that can increase the optical activity as already reported.33−39 Quinoxaline is a electron-acceptor unit usually applied in photovoltaic structures with a variety of possibilities of substitution at the 2,3-positions through the reaction of diamine and diketone groups.40,41 The poly(quinoxaline-2,3diyl) can assume a helical conformation that once formed is irreversible.42,43 The insertion of the ester functionality has been reported in similar cases, as for example in the fabrication of anionic polyfluorene focusing on its use in optoelectronic devices such as LEDs and photovoltaic cells as well as in biochemical sensors.44−47 Circular dichroism (CD) spectroscopy has been used to study the macromolecular organization of chiral π-conjugated polymers, including polyfluorenes.48,49 The magnitude and the sign of CD have been shown to be very sensitive to interchain electronic interactions as well as to longB

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Scheme 2. Synthesis of Polymers

cooled to 20 at 10 °C min−1. The glass transition temperatures, Tg, of the samples were determined from the midpoints of the transitions. Chemical Procedures. The chemical procedures for the preparation of the monomers and final polymers are illustrated in Schemes 1 and 2. 2,7-Dibromo-9,9-bis(3-(tert-butylpropanoate))fluorene (1). To a three-necked round-bottom flask, 2,7-dibromofluorene (5.00 g, 15.5 mmol), tetrabutylammonium bromide (125 mg, 0.445 mmol), and toluene (30 mL) were added under an Ar atmosphere. After 60 min, 12 mL of solution 50 wt % aqueous NaOH was added dropwise, and the solution was stirred for 60 min. After that, tert-butyl acrylate (7.9 g, 61.6 mmol) was added dropwise, and the mixture was stirred at 25 °C overnight. After this time, the reaction product was extracted with dichloromethane, washed three times with water, and dried with anhydrous MgSO4. After solvent removal the product was purified in a chromatographic column to give 6.55 g of white solid with a 73% yield. 1 H NMR (CDCl3, δ): 7.55−7.42 (m, 6H); 2.30 (t, 4H), 1.47 (t, 4H), 1.33 (s, 18H). 13C NMR (CDCl3, δ): 172.16; 149.89; 139.02; 130.00; 126.41; 122.01; 121.39; 80.41; 53.99; 34.35; 29.78; 27.96. 2,7-Dibromo-9,9-bis(3-(propionic acid))fluorene (2). To a threenecked round-bottom flask, 2,7-dibromo-9,9-bis(3-(tert-butyl propanoate))fluorene (1) (3.00 g, 5.19 mmol) and dichloromethane (10 mL) were added under an Ar atmosphere at 0 °C. After 40 min, trifluoroacetic acid (1.2 mL, 15.6 mmol) was added dropwise, and the mixture was stirred at 25 °C overnight. After this time, the mixture was dropped on basified water and impurities were extracted with ethyl ether. The aqueous layer was acidified, forming a white solid precipitate that was solubilized in ethyl acetate. The organic layer was washed with water three times and dried with MgSO4. The solvent was rotoevaporated and purified in a chromatographic column to give 2.1 g of white solid with a 87% yield. 1H NMR (DMSO-d, δ): 12.43 (s, 2H); 8.15−8.09 (m, 4H); 7.87−7.82 (m, 2H); 2.70 (t, 4H), 1.71 (t, 4H). 13C NMR (DMSO-d, δ): 173.64; 150.29; 138.82; 130.74; 126.48; 122.24; 121.33; 54.15; 33.36; 28.72. 2,7-Dibromo-9,9-bis(3-((S)-2-methylbutylpropanoate))fluorene (M1). To a two-necked round-bottom flask, 2,7-dibromo-9,9-bis(3(propionic acid))fluorene (2) (2.00 g, 4.28 mmol), (S)-2-methylbutanol (3.27 g, 37.1 mmol), and 2 drops of sulfuric acid concentrated were added under an Ar atmosphere and refluxed overnight. After that, the product was extracted with ethyl acetate and washed three times with cold water and one time with NaHCO3 aqueous solution and then dried with MgSO4. After, the solvent was evaporated under reduced pressure. The crude product was purified in a chromatographic column to give 2.16 g of white solid with a 83.7% yield. 1H NMR (CDCl3, δ): 7.57−7.47 (m, 6H); 3.81−3.66 (m, 4H); 2.38 (t, 4H); 1.60−1.54 (m, 6H); 1.38−1.03 (m, 4H); 0.86−0.81 (m, 12H). 13 C NMR (CDCl3, δ): 172.89; 149.49; 139.16; 131.18; 126.41; 122.00; 121.48; 69.20; 54.19; 34.42; 33.91; 28.82; 25.91; 16.36; 11.10. 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-bis(3((S)-2-methylbutylpropanoate))fluorene (M2). To a three-necked round-bottom flask, 2,7-dibromo-9,9-bis(3-((S)-2methylbutylpropanoate))fluorene (M1) (1.00 g, 1.65 mmol), potassium acetate (0.97 g, 9.9 mmol), bis(pinacolato)diboron (1.46 g, 5.77 mmol), and dry DMF (25 mL) were added under an Ar atmosphere. After 30 min, [1,1-bis(diphenylphosphino)ferrocene]dichloropalladium(II) (72.4 mg, 0.099 mmol) was added quickly, and

the reaction was stirred overnight at 60 °C. After this time, the product was extracted three times with ethyl ether. The organic layers were combined and washed three times with water and then dried with MgSO4. The solvent was removed, and the crude product was purified in a chromatographic column to give 650 mg of white solid with a 56.5% yield. 1H NMR (CDCl3, δ): 7.84−7.72 (m, 6H); 3.78−3.65 (ddd, 4H); 2.45 (t, 4H); 1.59−1.47 (m, 6H); 1.38 (s, 24H); 1.32− 1.02 (m, 4H); 0.85−0.79 (m, 12H). 13C NMR (CDCl3, δ): 173.52; 147.43; 143.80; 134.42; 128.92; 128.83; 119.78; 83.83; 68.88; 53.47; 34.48; 33.89; 28.88; 25.88; 25.00; 24.90; 16.30; 11.13. 5,8-Dibromo-2,3-bis(phenyl)quinoxaline (M3). To a one-necked round-bottom flask, 4,7-dibromobenzo[c][1,2,5]thiadiazole (700 mg, 2.38 mmol), cobalt dichloride hexahydrate (35 mg, 0.14 mmol), and dry ethanol (20 mL) were added under argon at 0 °C. After 20 min, sodium borohydride (2 g, 52.9 mmol) was added in four equal portions with interval of 20 min. The solution was heated to room temperature and stirred overnight without light. After that, the solvent was removed under reduced pressure, and the product was neutralized with 10% HCl (50 mL) and then extracted three times with dichloromethane. The organic layers were combined, and the solvent was removed under reduced pressure. The product 3,6-dibromobenzene-1,2-diamine (3) is unstable at light and air, and then it was used without purification. A mixture of 3,6-dibromobenzene-1,2-diamine (3) (600 mg, 2.2 mmol), benzil (4) (450 mg, 2.2 mmol), ethanol (50 mL), and acetic acid (15 mL) was refluxed overnight. The mixture was cooled to room temperature and dropped into 300 mL of water. The precipitated formed was filtrated and purified by recrystallization giving yellow crystals (650 mg, 61% yield). 1H NMR (CDCl3, δ): 7.93 (s, 2H); 7.69−7.64 (m, 4H); 7.45−7.34 (m, 6H). 13C NMR (CDCl3, δ): 154.13; 139.34; 137.93; 133.08; 130.26; 129.57; 128.36; 123.72. Poly(9,9-bis(3-((S)-2-methylbutylpropanoate))fluorene) (LaPPS61). To a three-necked round-bottom flask, 2,7-dibromo-9,9bis(3-((S)-2-methylbutylpropanoate))fluorene (M1) (176 mg, 0.29 mmol), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-bis(3((S)-2-methylbutyl propanoate))fluorene (M2) (205 mg, 0.29 mmol), 2 M K2CO3 solution (12 mL), and toluene (36 mL) were added under an Ar atmosphere. After 30 min, tetrakis(triphenylphosphine)palladium(0) (20 mg, 0.017 mmol) was added quickly, and the mixture was stirred at 90 °C for 60 h. Excess of 1-bromobenzene (endcap) was added, and the mixture was continuously stirred for 12 h. The product was extracted three times with chloroform, and the organic layers were combined and dried with MgSO4. After partial evaporation, the product was purified by chromatographic column to give 130 mg of pale gray-yellow solid with 51.2% yield. 1H NMR (CDCl3, δ): 7.91−7.61 (m, 6H); 3.70(m, 4H); 2.60(br, 4H); 1.73 (br, 4H); 1.53 (m, 2H); 1.35−0.99 (m, 4H); 0.79 (m, 12H). Poly(9,9-bis(3-((S)-2-methylbutylpropanoate))fluorene-alt-5,8(2,3-bis(phenyl)quinoxaline)) (LaPPS62). To a three-necked roundbottom flask, 5,8-dibromo-2,3-bis(phenyl)quinoxaline (M3) (198 mg, 0.45 mmol), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9bis(3-((S)-2-methylbutyl propanoate))fluorene (M2) (315 mg, 0.45 mmol), 2 M K2CO3 solution (12 mL), and toluene (36 mL) were added under an Ar atmosphere. After 30 min, tetrakis(triphenylphosphine)palladium(0) (20 mg, 0.017 mmol) was added quickly, and the mixture was stirred at 90 °C for 60 h. Excess of 1bromobenzene (end-cap) was added, and the mixture was C

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continuously stirred for 12 h. The product was extracted three times with chloroform, and the organic layers were combined and dried with MgSO4. After partial evaporation, the product was purified by chromatographic column to give 100 mg of green-yellow solid with 47.9% yield. . 1H NMR (CDCl3, δ): 8.10−7.88 (m, 6H); 7.76−7.51 (m, 6H); 7.42−7.27 (m, 6H); 3.79−3.57 (br, 4H); 2.78−2.52 (br, 4H); 1.99−1.77 (br, 4H); 1.05−0.86 (br, 6H); 0.80−0.67 (m, 12H). Theoretical Methods. To determine the topology of LaPPS61 and LaPPS62 helical oligomers, a sequence of steps was followed involving quantum chemical calculations. Initially, the geometry of each monomer was optimized with density functional theory (DFT), using B3LYP functional and 6-311G(d,p) split-valence basis set as implemented in the Gaussian 09 package.53,54 Based on the previous step and using the same level of theory, the geometry of LaPPS61 and LaPPS62 tetramers was optimized for two different conformations: R and S helix forms, related to clockwise or counterclockwise rotational sense. The electronic absorption and circular dichroism spectra were then calculated by using time-dependent DFT.54 The relative populations of each conformer were calculated based on the ratios of their statistic Boltzmann factors.

results have demonstrated that polyfluorene chains undergo conformational changes with annealing.36 Typically for the octyl-substituted homopolymer (PFO) an ordered conformation called β phase predominates up to 90 °C. This phase is characterized by fluorene units placed in an alternating up and down fashion, giving rise to a planar zigzag morphology with an extended conjugation length of about 30 repeating units, and its spectrum is red-shifted as compared to the amorphous PFO.37,55 The absorption at 425 nm was assigned to this type of morphology, which gradually disappears due to the interruption of the planar conformation induced by thermal displacements of chain segments. Accordingly, a blue-shift is observed due to the shortening of the effective conjugation length. With further raising the temperature, the free volume increases, and there is room for the formation of the more stable helical conformations. The DSC traces (Supporting Information) supports the assumption of disruption of the β phase in the region of the glass transition temperature (132 °C). No other transitions were detectable in the thermogram. Dependence of Chiroptical Properties on Film Thickness: LaPPS61. Figure 2 shows that absorption and the CD spectra

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RESULTS AND DISCUSSION Synthesis of Polymers. The molar masses of LaPPS61 and LaPPS62 as determined by GPC using polystyrene as standards and DMF as eluent were Mw = 18 700 g mol−1 with polydispersity (PDI) of 2.47 and Mw = 8200 g mol−1 with polydispersity (PDI) of 1.34, respectively. LaPPA62 shows a low molar mass due to the low solubility in DMF. Only the soluble fraction was used for further characterization. Also, as shown by Robert Abbel et al.49 molar masses around 8−10 kg mol−1 provide higher CD signals.49 The polymers showed good solubility in organic solvents such as CHCl3, THF, and dichloromethane. DSC showed a glass transition (Tg) of 132 °C for LaPPS61 and 167 °C for LaPPS62. The DSC traces are included in the Supporting Information. The thermal stability of similar structures containing the same ester pendant group polymer was studied using TGA, showing that this group is thermally stable until 200 °C under a nitrogen atmosphere.45 Spectroscopy (Absorption and CD). Dependence of Chiroptical Properties on Annealing Temperature: LaPPS61. Figure 1 displays the absorption and the CD spectra of

Figure 2. Film thickness dependence of CD and absorption spectra in thin films of LaPPS61.

turn gradually more intense with the increase in thickness, in a Lambert−Beer linear fashion (see Supporting Information), with no discernible deviation of such behavior up to 0.410 μm. This is a promising finding since it points out to the possibility of the preparation of self-supporting films without losing the high CD signals. Dependence of Chiroptical Properties on Annealing Temperature: LaPPS62. Figure 3 displays the absorption and the CD spectra of LaPPS62. The absorption profile has the typical “camel back” dual bands of a DA copolymer: the donor (fluorene which contains the chiral center) is placed in an alternate fashion with the acceptor (quinoxaline). As seen before with the homopolymer (LaPPS61), the CD signal increases with the temperature of annealing. It is noteworthy that the two signals of the Cotton effect of the copolymer (LaPPS62) are in the positive sense: the more intense at 425 nm and the less pronounced at 340 nm. On other hand, in the homopolymer (LaPPS61) spectrum which also consists of two bands, one is in the negative sense (stronger band at 390 nm) and the other in the positive sense (weaker band at 350 nm). A careful observation of the spectrum in the temperature range of 125−150 °C reveals a slight increase in absorption at around 475 nm. This finding

Figure 1. CD and UV−vis absorption spectra of annealed films of LaPPS61.

LaPPS61 annealed films at various temperatures for 2 h, showing a bisignate negative Cotton effect, with the intensity gradually increasing between room temperature and 100 °C, stabilizing at 150 °C, and then decreasing drastically between 150 and 200 °C. The negative peak at RT is centered at 395 nm and gradually blue-shifts upon heating to 200 °C. Reported D

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discussed. The new morphology developed with these changes as H aggregates, twisted or helical conformations are responsible for the increase in the CD signal observed at 100−150 °C.49 A lack of dependence of gabs on the thickness is observed at ∼400 nm (Figure 5b). This is the signature of true mesoscopic chirality, developed after annealing through formation of chiral aggregates.19 Concerning LaPPS62, no variation was observed in the spectral profile of gabs, whereas the intensity increased up to 150 °C, decaying afterward (Figure 5c). The exponential growth of CD with the thickness versus the linear one of the absorbance makes for a prominent dependence of the dissymmetry ratio on the film thickness (Figure 5d). The reason for this behavior is not completely clear and will be a subject of future study. One can speculate about possible propagation effects (pseudo-chiral dichroism)56 or contribution of linear dichroism due to perpendicular rather than parallel alignment of the polymer strands with respect to the incident light beam or even conformational changes. The findings described so far were discussed in terms of morphological changes. Conceptual comments on the subject have been made in the literature.56 Chirality Parameter. To obtain the dispersion of the chirality parameter, κ, we applied Kramers−Kronig transformation to the measured ellipticity (CD) spectrum with a subsequent algebraic transformation of the resulting optical rotation. The variation of the chirality parameter (κ) of the polymers with wavelength is shown in Figure 6. It was observed that the measured value for LaPPS62 at 455 nm was almost 3fold higher (1.02 × 10−2) than that of LaPPS61 (−3.5 × 10−3) at 400 nm. These results are among the highest observed so far for chiral polymers.18,19 Theoretical Analysis. In order to provide more insight into the possible conformations that could be adopted by the polymer chains, a theoretical study was undertaken. The study was initiated by performing a conformational search starting from the optimized geometry of dimers, varying the dihedral angle at the interconnect of one monomer unit to the other, and monitoring the energy, in order to find the minima associated with each conformation. The purpose of this step was to find the precursor geometries that originated the direction of the coiled conformations. It should be noted that the conformations in which the side groups attached to adjacent carbon 9 in the fluorene unit were placed in the syn conformation were not considered. A difference in energy of 1.84 and 4.10 kJ mol−1 was found between the R- and S-form conformers for LaPPS61 and LaPPS62, respectively. Based on these findings, the geometries of tetramers were optimized for both oligomers. The optimized geometries of the helical oligomers are depicted in Figure 7 for both R form (clockwise sense) and S form (counterclockwise sense). It was found that in all oligomers a large dihedral angle is acquired between the fluorene units (LaPPS61) and between fluorene and quinoxaline units (LaPPS62) due to steric hindrance between the hydrogen atoms of neighboring units forcing the polymeric structure to adopt a coil conformation to minimize the electrostatic repulsion. It is fair to assume that this angle which systematically appears among the units is the main reason for the formation of the helical structures. Figure 8 shows the calculated (as described in the Theoretical Methods section) circular dichroism (CD) spectra of each conformer of the oligomers corresponding to LaPPS61 and LaPPS62 as well as their topological projections (screw

could be ascribed to some degree of chain aggregation brought about by annealing, which could be playing a role in the inversion effect observed. The DSC trace of the pristine polymer (see Supporting Information) shows a glass transition temperature of 167 °C and an endotherm at 65−70 °C that could be ascribed to crystalline domains temperature (RT) up to 200 °C.

Figure 3. CD and UV−vis absorption spectra of LaPPS62 films that were annealed at room temperature (RT) up to 200 °C.

Dependence of Chiroptical Properties on Film Thickness: LaPPS62. The main conclusion that can be withdrawn from Figure 4 is that whereas the absorption intensity increases

Figure 4. Dependence of absorption and of the CD signal on film thickness of LaPPS62.

linearly with thickness, the CD signal grows exponentially (see Supporting Information), reaching the value of 2330 mdeg at 0.200 μm thickness. As compared to the homopolymer, LaPPS61, half of thickness of LaPPS62 yields a CD signal 3 times higher than that of its parent homopolymer, LaPPS61. Dissymmetry Ratio (gabs). The absorption dissymmetry ratio, gabs, is defined as 2(AL − AR)/(AL + AR), where AL and AR are the absorbances of left and right circularly polarized light, respectively. The dependence of the dissymmetry ratio gabs on annealing temperature and film thickness is shown in Figure 5. In the case of LaPPS61, the transition at 430 nm in Figure 5a is probably associated with the β phase which as noted increases up to 50 °C where it becomes the most prominent band in the spectrum. With further temperature increase the transition progressively decreases to a shoulder until its complete disappearance. This is accompanied by a blue-shift as before noted and E

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Figure 5. Dissymmetry ratio spectra, gabs, of LaPPS61 films that were annealed from room temperature (RT) up to 200 °C (a) and film thickness dependence (b). LaPPS62 films that were annealed at room temperature (RT) up to 200 °C (c) and film thickness dependence (d).

Figure 6. Chirality parameter spectra of LaPPS61 (blue line) and of LaPPS62 (red line).

Figure 7. Optimized geometries of the helical conformers of the tetramers of (a) LaPPS61-R, (b) LaPPS61-S, (c) LaPPS62-R, and (d) LaPPS62-S (hydrogen atoms omitted).

sense) figured out based on the molar weight as determined by GPC and corresponding number of coil turns based on calculated radius and pitch. Table 1 displays all the results obtained by the calculations performed. Figure 8 and Table 1 data show that conformers (S and R forms) related to each oligomer present great similarity in electronic aspects, as the low excitation energies, in very good agreement with the experimental results. Regarding the optimized geometries, LaPPS62(R,S) presents a higher number of turns per unit length: 75% higher for R form and 94% higher for S form in comparison with LaPPS61. This means that the pitch of the LaPPS61 helix is 2 times that of the LaPPS62 one. This fact helps understand the larger optical activity observed experimentally for LaPPS62 over LaPPS61 since the tighter the helix − the larger the optical activity. We have also calculated the electronic and CD spectra of the

LaPPS61 and LaPPS62 using the TDDFT methodology. A reasonable correlation was found between the calculated and the experimental results for the circular dichroism spectra of LaPPS61-R. This finding indicates that the polymeric chain stays mostly in its right-handed form. Moreover, taking into account the relative free Gibbs energy, this form represents 80% of the total population of both conformers, based on the Boltzmann distribution. This correlation does not hold well for LaPPS62, since the sign of the rotational strength at 450 nm for the R-conformer, representing 96% of the overall population according to our calculations, is opposite to the experimentally observed at 450 nm (the sign matches the experimentally observed at the shorter wavelength of ∼430 nm though). It is however known that the sign of the rotational F

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ASSOCIATED CONTENT

S Supporting Information *

Scanning electron microscopy (SEM) images of thin films, DSC analyses of polymers, absorbance and CD peak intensities vs thickness. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: pnprasad@buffalo.edu (P.N.P.). *E-mail: [email protected] (L.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a grant from the Air Force Office of Scientific Research (grant no. FA95500610398). B.N., C.Z., and L.A. are thankful to CNPq and CAPES.

Figure 8. Projected coiled structures of the polymers, based on the optimized geometries; electronic circular dichroism (ECD): experimental (solid line) and calculated (colored bars); dimensions: (a) width, (b) depth, (c) pitch size, and (d) length based on molar weight.

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strength is not uniequivocally reproduced with use of B3LYP functional.57

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REFERENCES

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CONCLUSION

To conclude, we studied both experimentally and by means of DFT calculations structure−property relations for two chiral fluorene-based polymers and found a reasonable correlation between the experimentally measured chiroptical properties and the computationally determined topological conformations. Helical self-organization of polymeric fibrils upon annealing of thin films resulted in the enhanced optical activitythe highest reported so far for the pure undoped polymers. The following two facts are noteworthy: (1) According to the calculated statistical weights the screw sense for both structures is right (R form). This does not correlate with the observed CD spectrum of LaPPS62 which indicates a left-handed form. (2) Apart from single molecule conformation, several aspects may play an important role in generating a particular CD spectrum such as macromolecular interactions (especially aggregation). In this case the calculations of short tetramers in a vacuum can only mimic a possible real situation. This study opens up a pathway to a novel class of flexible, low-cost polymeric materials for a wide range of applications in chiral photonics and metaphotonics.

Table 1. Results of Calculations of the Geometries, Electronic Structure, and Thermodynamics of the Oligomers LaPPPS61-R dihedral angle (deg) a (Å) b (Å) c (Å) no. of turnsb TDDFT Abs (nm) (f) exptl λmax Abs (nm) Gibbs energyc (kJ mol−1) Rvel tetramer (10−40 erg esu cm G−1) Rvel dimer (10−40 erg esu cm G−1) Boltzmann average a

LaPPPS61-S

38.33 17.50 6.68 105.18 4.30 386.6 (2.72) 388 0 −230.72 (385 nm) −90.27 (337 nm) 80

38.34 17.50 6.68 105.18 4.31 385.6 (2.70) 3.47 231.04 (385 nm) 82.25 (337 nm) 20

LaPPPS62-Ra

LaPPPS62-Sa

86.85 17.50 12.00 60.17 2.70 450 (1.84) 420 0 −216.40 (450 nm)

96.32 17.50 12.00 54.25 2.99 450.4 (1.84)

96

4

7.63 381.37 (450 nm)

Dihedral angle between fluorene−fluorene. bBased on dimensions. cRelative to ground state. G

dx.doi.org/10.1021/ma401731x | Macromolecules XXXX, XXX, XXX−XXX

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