To understand the chiroptical responses of polyfluorene–phenylene (PFP) films, circular dichroism (CD), surface-enhanced Raman scattering (SERS), attenuated total reflection (ATR), and spectroscopic ellipsometry studies were performed. The pristine PFP films exhibited significant CD at room temperature, and the chirality increased by thermal annealing at 120 °C (above the glass transition temperature). SERS and ATR spectra indicated that the chirality enhancement was due to the rearrangement of the polymer backbones on the glass substrate by thermal annealing and/or self-organization. In addition, comparison of PFP and polyfluorene polymers indicated that molecular structure of polymer backbone plays an important role in the chiroptical responses of the chiral polymers.
© 2016 Optical Society of America
25 February 2016: A correction was made to the acknowledgments.
Organic and polymer light-emitting devices have attracted considerable attention because of their high luminescence quantum yields, wide wavelength tunability, and easy processability. There have been many efforts to understand the underlying mechanism of electro-/photo-luminescence and to enhance the luminescence performance [1–5]. When light, from a light source or to a photodetector, propagates through optical components and materials, its propagation depends on its polarization and chirality. Therefore, control of the polarization and chirality of light is critical in device applications; this control can be achieved using various chiral media such as helical structures, natural chiral media, and synthesized chiral polymers. Traditionally, chiroptical properties of biomaterials and organic small molecules are often observed in limited condition such as liquid solution and self-assembled structure/gelatin [6–8]. In addition, chiral polymers can be fabricated as thin film using solution-processing techniques and these films exhibit high CD value [9–11]. Among these polymers, chiral polyfluorene (PF) derivatives exhibit significantly enhanced chirality in thin-film. Furthermore, chiral PF copolymers can provide wide wavelength tunability and easy processability. Many research groups studied device applications and chirality enhancement in PF films. The Neher group studied circularly polarized electroluminescence in liquid-crystalline chiral PFs [12, 13], and the Prasad group investigated the chirality and nonlinear optical properties of chiral PF polymers [14, 15]. In addition, the Bradley group reported linearly polarized luminescence in oriented PF films , and investigated the optical constants of PF films using spectroscopic ellipsometry . Scherf and List studied the structure–property relationships of PFs . The Meijer group demonstrated the formation of a chiral liquid crystalline phase in PF films using CD spectroscopy and polarized optical microscopy, and they studied the effect of molecular weight on the mesoscopic order of chiral fluorene homopolymer and copolymer films . Finally, the Friend group studied the factors that affect the thin-film morphology of PF-based electroluminescent polymer blends using atomic force microscopy, micro-Raman spectroscopy, and X-ray photoelectron spectroscopy, and investigated the dependence of device performance on the phase separation of electroluminescent polymers . However, to the best of our knowledge, systematic research on chiral polyfluorene-phenylene films using several optical spectroscopic methods such as CD, Raman spectroscopy, ATR, and spectroscopic ellipsometry has been limited.
In this study, we investigated the chiroptical properties of poly[(9,9-di((R)-3,7-dimethyloctyl)fluoren-2,7-diyl)-alt-(1,4-phenylene)] (PFP) films. After synthesizing the PFP polymer with its pendant alkyl groups, thin films were spin-coated on glass or gold substrate, and annealed above the glass transition temperature (). Their chiroptical properties were measured by optical absorption and CD spectroscopy. The origin of the chirality enhancement in the solid film was investigated using spectroscopic ellipsometry, ATR, and surface-enhanced Raman spectroscopy. To determine the effect of the molecular structure of the polymer backbone on the chirality of the PFP film, PF polymer was synthesized, and thin films were prepared on glass or gold substrate, and annealed above . The chiroptical properties of the PF films were compared with those of the PFP films.
2. Materials and experimental methods
The chiral PFP polymer was synthesized via the Suzuki coupling polymerization of 2,7-dibromo-9,9-bis((S)-3,7-dimethyloctyl)fluorene with 1,4-bis (4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene , which is modified method described in the literature [21–23]. The synthetic procedure for PFP is as follows (Fig. 1): 2M K2CO3 (5 mL), Pd(PPh3)4 (22.8 mg, 20.0 µmol), and a trace amount of Aliquat 336 were added to a solution of 2,7-dibromo-9,9-bis((S)-3,7-dimethyloctyl)fluorene (0.25 g, 0.4 mmol) and 1,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene (0.132 g, 0.4 mmol) in toluene (15 mL) under argon atmosphere. The mixture was stirred at 100 °C for 24 h. The cooled mixture was added to vigorously stirred methanol (200 mL) and the resulting precipitate was filtered. The crude product was purified by Soxhlet extraction with acetone, hexane, and chloroform successively. The chloroform fraction was concentrated and then precipitated into methanol to yield a light yellow solid (0.15 g, 74%. number average molecular weight (Mn) = 9.2 kDa, polydispersity index (PDI) = 1.74). Anal. Calcd for (C37H50S2)n: C, 84.35; H, 9.57; S, 6.09. Found: C, 84.13; H, 9.81; S, 6.32. The chiral PF (Mn = 7.7 kDa, PDI = 1.98) as reference polymer was also synthesized according to literature .
Chiral PFP films were prepared on glass or gold substrate. The chiral polymer films were spin-coated on the substrate from a solution of the PFP in a mixture of chloroform and chlorobenzene (9:1 v/v %). The PFP films were annealed on a temperature-controlled hot plate under ambient conditions. For the gold-coated plates, a gold film (50 nm) was deposited on the glass plate using thermal evaporation at a pressure of 10−6 torr.
UV-visible absorption spectra were obtained using a diode-array spectrophotometer (Agilent, 8453, USA), and the CD spectra were measured using a spectropolarimeter (Jasco, J-815, Japan). The absorption and CD spectra were observed in the UV and visible regions. Raman spectra were measured at room temperature using a confocal Raman microscope (WITec, Alpha 300R, Germany) with 633-nm light emitted from a He–Ne laser. In order to obtain the Raman spectra, the laser beam was focused on the chiral polymer film using a microscope objective (100). The optical constants were measured using variable-angle spectroscopic ellipsometry (Woollam, M-2000U, USA) and fixed angle small-spot ellipsometry (Woollam, RC2-XF, USA). Spectroscopic ellipsometry measurements were performed over a wavelength range from 250 to 1000 nm. The incidence angles were 60°, 65°, 70° and 75°. The optical constants of the PFP films were obtained from the measured ellipsometric angles. To observe the surface plasmon resonance effect, the ATR spectra were measured using variable-angle spectroscopic ellipsometry. The incidence angle was fixed at 45°, and the ATR spectra were obtained over a wavelength range from 300 to 1800 nm.
3. Results and discussion
The prepared PFP polymer with its pendant alkyl groups, synthesized as in Fig. 1(a), exhibited Mn and PDI values of 9.2 kDa and 1.74, respectively, which were determined by gel permeation chromatography (Waters GPC, Waters 515 pump, Waters 410 RI, 2 × PLgel Mixed-B) using polystyrene as the standard and chloroform as the eluent at 35 °C. of the PFP polymer was estimated to be 65 °C by differential scanning calorimetry (Fig. 1(b)). To find a suitable thickness for the efficient generation of the CD effect in the PFP film, three films were prepared at different thicknesses by adjusting the rotating speed of the spin coater. The PFP1, PFP2, and PFP3 films were prepared on glass plates by spin coating at 5000, 2500, and 1000 rpm, respectively. The film thicknesses of the PFP1, PFP2, and PFP3 samples were estimated to be 37 nm, 53 nm, and 67 nm, respectively, from spectroscopic ellipsometry. Thermal annealing at 120 °C for 15 min was performed to improve the chiroptical properties of the PFP films.
Figure 2(a) presents the CD spectra of the PFP films. The CD spectra exhibit a strong negative band at 381 nm and weak positive bands at 331 /412 nm. For the negative peak, the magnitudes of the dips in the CD spectra are 47, 98, and 419 mdeg for PFP1, PFP2, and PFP3, respectively, with a larger dip in PFP3 than in PFP1 or PFP2. Figure 2(b) presents the CD spectrum of the annealed film (PFP3A). Notably, the magnitude of the dip in the CD spectrum of the annealed film (PFP3A) is much larger than that for the pristine films (PFP1, PFP2, and PFP3), although the peak absorbance is lower in the former than the latter. In general, the CD properties of a chiral polymer depend on several factors, such as the chirality and spatial arrangement of the polymer backbone, and the film thickness. Circular dichroism arises because of the different absorption by the chiral sample by left (L)– and right (R)–handed circularly polarized light. If the material is optically active, the transmitted wave will exhibit differential absorption between the left and right circularly polarized light. The ellipticity (θ) is defined as the arctangent of the ratio of the minor axis to the major axis,Eqs. (1)-(4), λ is the wavelength, and is the path length. In Eq. (1), () and () are the amplitudes and light intensities of the left- (right-) circularly polarized light, respectively. In Eq. (4), AL and AR are the absorbance of the left- and right-circularly polarized light, respectively. For small CD values [(4π/λ) ∆k << 1], θ and ∆A are related by the following linearized equation:
Figure 2(c) presents the g-value spectra of the pristine films (PFP1, PFP2 and PFP3). The spectra exhibit a characteristic CD signature with a negative band at 383 nm and positive bands at 316 /422 nm. The relative ratios of the peak-to-valley CD variations in the g-value spectra of PFP1, PFP2, and PFP3 are 1.0, 2.1, and 8.6, respectively. The peak-to-valley CD variation is larger in PFP3 than PFP1 or PFP2. Figure 2(d) presents the g-value spectra of the annealed films (PFP2A and PFP3A). The peak g-values of the PFP2A and PFP3A films are increased to 23 and 91 times that of the pristine PFP1 film by thermal annealing. Strong chirality is observed in the solid film; however, this chirality disappears in the liquid phase. Thus, the room-temperature chirality of the pristine films (PFP1, PFP2 and PFP3) is thought to be related to the self-organization of the fluorene-phenylene backbones on the glass substrate. Thermal annealing can accelerate the rearrangement of the fluorine-phenylene backbones, such that the annealed PFP film exhibits strong chirality. The room temperature CD of the PFP polymers can be utilized in polarized light-emitting devices and polarization-dependent optical sensors.
Figure 3(a) presents the optical absorption spectra of the pristine and annealed PFP films. The absorption band between 300 and 420 nm is due to the π–π∗ transition of the conjugated main chains. In addition, the absorption band between 200 and 250 nm is due to the π–π∗ transition of the side chains. Absorption peaks of the main chains for PFP1, PFP2, and PFP3 are observed at 375, 374, and 372 nm, respectively, clearly showing a slight blue-shift in the wavelength as the film thickness increases. At the absorption peak, relative peak absorbance ratios of 1.0, 1.2, and 1.5 were observed for PFP1, PFP2, and PFP3, respectively. The peak absorbance is higher in PFP3 than PFP1 or PFP2. To determine the effect of annealing on the absorption spectrum, we also measured the optical absorption spectrum of the annealed film (PFP3A). The peak absorbance is lower in the annealed film (PFP3A) than the pristine film (PFP3). The absorption peak of the annealed film (PFP3A) is observed at 357 nm, and is significantly blue-shifted compared with the pristine film (PFP3). Figure 3(b) presents the optical absorption spectrum of the PFP polymer in chloroform solution as well as those of the PFP films. The absorption maximum of the pristine PFP1 film is similar to that of the PFP solution, and that of the pristine PFP3 film is slightly blue-shifted. In addition, the absorption peak of the annealed film (PFP3A) is significantly blue-shifted compared with the PFP solution or the pristine film. The blue shift is attributed to the rearrangement of the polymer backbones by thermal annealing or self-organization.
To further study the effects of the molecular structure and spatial arrangement of the polymer backbones on the chirality of the polymer film, we examined the Raman spectra of the films. To avoid sample damage and/or local heating by the excitation light, we used SERS method. Figure 4(a) presents the SERS spectrum of the annealed PFP3A film. In this experiment, the SERS spectrum was obtained with an excitation power of 0.3 mW. For Raman measurement of the chiral polymer film on the glass substrate, an excitation power of 3.0 mW was required to obtain a Raman scattering intensity comparable to that obtained from the chiral polymer film on the gold layer using the SERS method. The SERS spectrum of the annealed PFP3A film exhibited many Raman peaks corresponding to specific functional groups of the PFP backbone. The strong Raman peak at 1605 cm−1 arises from the in-plane stretching mode of the phenylene rings in the polymer backbone [24, 25]. The peaks at 1282, 1302, and 1342 cm−1 have been mainly assigned to C−C stretching modes between phenylene rings [24, 25]. The 1133 cm−1 Raman peak is attributed to the C−H bending mode, which occurs in the plane of the phenylene ring [24, 25]. The weak Raman peak at 1418 cm−1 corresponds to a combined C−H bend and C−C stretch motion between adjacent monomers . Figure 4(b) presents the SERS spectra of the pristine and annealed films (PFP1, PFP2, PFP3, and PFP3A) on the gold layer. The Raman peak intensities of PFP backbones are stronger in the annealed film (PFP3A) compared to the pristine films (PFP1, PFP2, PFP3), and are larger in PFP3 than in PFP1 or PFP2. This result supports that the rearrangement of the fluorine-phenylene backbones by thermal annealing should contribute to the chirality enhancement. Grell et al. proved that there exists the liquid crystalline ordering of polymer backbones in polyfluorene film using X-ray diffraction experiments . The Meskers group demonstrated that left- and right-handed cholesterically ordered domains exist in a polyfluorene film , and the intensive chiroptical effect in annealed films could arise from a cholesteric-like arrangement of the fibrillar aggregates in the annealed films . Therefore, Raman enhancement and chirality enhancement were thought to be related with liquid-crystalline arrangement of the fibrillar aggregates in the annealed films. In our PFP polymer, the alkyl side chains can help to form a helical arrangement of the polymer backbones. According to previous theoretical research on the effect of the side chains on the backbone conformation of chiral polyfluorenes [30, 31], the β-phase backbone conformation (a semicrystalline phase) is stabilized by the adoption of an anti-gauche-gauche (agg) side-chain conformation, and the γ-phase backbone conformation (a crystalline phase) is stabilized by the adoption of an all-anti (aaa) side-chain conformation. In general, thermal annealing can improve the crystallization of amorphous materials [32, 33]. Thus, it is possible to achieve chirality enhancement by thermal annealing at a temperaure higher than .
To further study the chiroptical properties of the PFP films, we performed spectroscopic ellipsometry measurements of the PFP film. The optical constants of the PFP can be determined from the ellipsometric measurements. The ellipsometric angles of chiral polymer films and conjugated polymer films are better fitted by the anisotropic model than by the isotropic model . In the anisotropic model, the chiral polymer film is described by the real (ne, no) and imaginary (ke, ko) parts of the complex refractive index for both ordinary (in-plane) and extraordinary (out-of-plane) polarizations. Figure 5(a) presents the refractive index and extinction coefficient spectra of the pristine PFP3 film. In the wavelength region near the absorption peak, the peak absorption of the ordinary (in-plane) polarization wave is lower than that of extraordinary (out-of-plane) polarization wave. As expected from Kramers-Kronig relations, the extraordinary refractive indices (ne) are different from the ordinary refractive indices (no), respectively, as observed in Fig. 5(a). Figure 5(b) shows the refractive index and extinction coefficient spectra of the annealed PFP3A film. In the wavelength region near the absorption peak, the peak absorption of the ordinary (in-plane) polarization wave is higher than that of extraordinary (out-of-plane) polarization wave. According to Kramers-Kronig relations, the ne values are different from the no values, respectively, as observed in Fig. 5(b). Principally, the anisotropic optical constants can be related to the ordering of polymer main chains. In the π–π∗ transition of conjugated polymers, the anisotropic absorption can be produced by the alignment of main chains. The angular distribution of polymer chains is described by 
To understand the Raman enhancement by the PFP films on gold, we calculated the extinction spectrum of the PFP film on gold using Mie theory .Figure 6(a) shows the extinction spectrum of the PFP films on gold calculated using Mie theory. Thus, we can expect the SPR in the visible region.
To confirm the SPR effect of the gold substrate, we performed ATR experiments for the gold. For the ATR experiment, we used the Kretschmann configuration with [prism–metal–chiral polymer–air] structure. A p-polarized light of 300~1800 nm is incident at 45° in the prism and the spectrum of reflected light is measured in the ATR system. Figure 6 presents the ATR spectra of the p-polarized light for the bare gold and the PFP films on gold. First, we consider the SPR effect of bare gold. The SPR wavelength of bare gold was observed at 610 nm. The width of SPR bands was 120 nm. When the Raman excitation wavelength is on or near resonance with the SPR wavelength of the gold substrate, local electromagnetic fields are amplified by factors of |E|2, leading to intense SERS signals . Since the SPR wavelength of the gold at 610 nm is close to the Raman excitation wavelength (633 nm) and the optical field is confined near the gold surface, it seems that the gold substrate in the Raman scattering experiment may play a role as a hot plate for SERS signals of the PFP films, indicating that the SERS detection is based on the optical field confined near the gold surface.
Next, the SPR wavelengths of bare gold, PFP1/gold, PFP3/gold, and PFP3A/gold were observed at 610, 1090, 1410, and 1475 nm, respectively. The widths of SPR bands were 120, 145, 148, and 190 nm for gold, PFP1/gold, PFP3/gold, and PFP3A/gold, respectively. Therefore, the quality factors () were 0.20, 0.13, 0.11, and 0.13 nm for gold, PFP1/gold, PFP3/gold, and PFP3A/gold, respectively. SPR wavelength shift to the longer wavelength in Fig. 5 is attributed to the increase of the film thickness, consisting with the film thicknesses from the ellipsometric measurements. This result may suggest that the SERS effect can be observed at the SPR wavelength for the PFP film on gold. However, the SERS experiment used confocal microscope geometry, different from using the Kretschmann configuration for the ATR experiment. To use the gold substrate as a hot plate for SERS signals of the PFP films, it is efficient to use the SPR wavelength of the bare gold due to the optical field confinement near the gold substrate. Finally, we consider the reflectance values in the ATR spectra. At the SPR, the reflectance values of p-polarized light for gold, PFP1/gold, PFP3/gold, and PFP3A/gold were attenuated from total reflection due to the SPR effect of gold. The minimum reflectance (Rmin) of SPRs in PFP1/gold, PFP3/gold, and PFP3A/gold increased from zero reflectance (Rmin = 0) of bare gold. It is known that Rmin can be increased from 0 by adsorption of absorptive or chiral/anisotropic materials [36–39]. Since our PFP films are transparent in the 300~1800 nm wavelength regime, we can exclude the increase in Rmin by absorption effect. Because the polarization direction of the incident wave can be changed by the reflection at an anisotropic film–metal interface and the PFP films exhibit significant CD, the increase in Rmin might be attributed to the chiral film on gold.
Next, we investigated the effect of the fluorene–phenylene repeating unit on the chiroptical properties of the PFP. For this purpose, we synthesized a PF polymer with pendant alkyl groups (Fig. 7). The PF film was prepared on glass or gold substrate by spin coating at 1000 rpm, respectively. The film thickness of the PF sample was estimated to be 54 nm from spectroscopic ellipsometry. The chiroptical properties of the PF polymer films were measured using optical absorption and CD spectroscopic techniques. Figures 7(a) and 7(b) present the optical absorption and CD spectra of the PF films, respectively. A comparison of Figs. 3 and 7 reveals that the spectra of the annealed PFP film are similar to those of the annealed PF film. Figure 7(c) presents the SERS spectrum of the annealed PF film. A comparison of Figs. 4 and 7 shows that the Raman spectrum of the annealed PF film is similar to that of the annealed PFP film, except that an additional Raman peak at 1195 cm−1 is observed in the PFP film. These results suggest that the spatial rearrangement of the polymer backbones plays an important role in the chiroptical responses of the PFP and PF polymers. Figure 7(d) presents the SERS spectra of the pristine and annealed PF films on the gold layer. The Raman peak intensities of PF backbones are stronger in the annealed film than in the pristine films. These results suggest that chirality enhancement in PF is attributed to the rearrangement of the polymer backbones by thermal annealing at a temperature higher than . In addition, the CD spectra of the pristine PF films are different from those of the pristine PFP films. That is, the pristine PFP3 film exhibited significant CD at room temperature; however the pristine PF film did not exhibit significant CD at room temperature. A possible reason for the room-temperature chirality in the PFP film is the reduction in steric hindrance, which is attributed to the molecular structure of the fluorene–phenylene repeating unit in the polymer backbone. The molecular structures of PFP and PF differ from each other. As illustrated in Fig. 8, the optimized geometries of PFP and PF polymers indicate that there is more available space in the PFP compared with the PF. Thus, the steric effect of PFP is weaker than that of PF. The reduced steric hindrance facilitates the self-organization of the polymer backbones in the PFP film, resulting in the significant chirality at room temperature.
The chiroptical properties of the PFP polymer films were investigated using CD, SERS, ATR, and spectroscopic ellipsometry. The PFP polymer with pendant alkyl groups was synthesized, and thin films were spin-coated on glass or gold substrate. The film thicknesses of the PFP1, PFP2, and PFP3 samples were estimated to be 37 nm, 53 nm, and 67 nm, respectively, from spectroscopic ellipsometry. The pristine PFP films exhibited significant CD at room temperature, and the chirality increased by thermal annealing at 120 °C. The room-temperature chirality of the pristine film is thought to be related to the self-organization of the fluorene–phenylene backbones on the glass substrate. Thermal annealing can accelerate the rearrangement of the fluorene–phenylene backbones such that the annealed film exhibits strong chirality.
SERS analyses supported the hypothesis that the rearrangement of the fluorine-phenylene backbones by thermal annealing should contribute to the chirality enhancement. The SERS spectra of the PFP polymer films on gold exhibited many Raman peaks corresponding to the specific functional groups of the PFP backbone. The Raman peak intensities were stronger in the annealed film than in the pristine film. In addition, comparison of PFP and PF polymers revealed that the molecular structure of the polymer backbone plays an important role in the chiroptical responses of the chiral polymers.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Korean government [Grant numbers 2015R1D1A1A01057501 (Lee), NRF-2010-0027963 (PBRC)], and in part by Kwangwoon University in 2015. This research was supported by the National Research Foundation of Korea (NRF2012R1A2A1A01008797).
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