Molecular Design Principles for Achieving Strong Chiroptical

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Molecular Design Principles to Achieve Strong Chiroptical Properties of Fluorene Copolymers in Thin-Films Chidambar Kulkarni, Martin H.C. van Son, Daniele Di Nuzzo, Stefan C.J. Meskers, Anja R.A. Palmans, and E.W. Meijer Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00601 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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Chemistry of Materials

Molecular Design Principles to Achieve Strong Chiroptical Properties of Fluorene Copolymers in Thin-Films# Chidambar Kulkarni,1,† Martin H.C. van Son,1,† Daniele Di Nuzzo,2 Stefan C.J. Meskers,1 Anja R.A. Palmans,1 and E.W. Meijer1,* 1. Institute for Complex Molecular Systems, Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands 2. Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, United Kingdom Email: [email protected] † These authors contributed equally to this work # This paper is dedicated to Jean-Luc Brédas for his 65th birthday, his groundbreaking science in the understanding of the structure-property relationship of π-conjugated materials and a lifelong friendship.

ABSTRACT: Understanding the molecular level origin of chiroptical properties in enantiopure π-conjugated polymers is essential to tailor these materials for application in organic light emitting diodes and photonic devices. Here we have studied poly(9,9ʹ-dialkylfluorene-alt-2,5dialkoxyphenyl)s as prototypical copolymers to investigate their structure-chiroptical property relationship. The effect of a systematic variation of the location and configuration (S or R) of chiral and achiral side chains in the repeating units on the liquid crystalline ordering and its relationship to chiroptical properties was investigated. The results clearly indicate that enantiopure side chains on the fluorene units are critical to obtain the cholesteric liquid crystalline ordering and thereby strong chiroptical properties in both absorption and photoluminescence in annealed thin-films. Finally, solution processed organic light emitting diodes constructed from a liquid crystalline polymer show strong emission of circularly polarized light, demonstrating the potential application of these systems.

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■ INTRODUCTION π-Conjugated oligomers and polymers have been extensively employed as active layers in various organic electronic devices such as light-emitting diodes and solar cells.1 Recently they are also used in energy storage devices such as redox-flow batteries.2 Among a plethora of organic conjugated semiconductors, fluorene based homo- and copolymers have been widely used in various devices due to their high photoluminescence (PL) quantum yield, thermal stability, efficient charge-transport and wide-band-gap.3–11 Besides the chemical structure, the supramolecular organization has been identified as an important factor in dictating the device performance.12 Therefore, understanding and controlling the organization of π-conjugated polymers in thin-film from both an experimental and a theoretical standpoint is an actively pursued research topic,13,14 with new insights emerging at a rapid pace. Side chain engineering is an easily conceivable, synthetically straightforward and widely used tool to influence the supramolecular organization of solution-processable conjugated polymers.15 The high reactivity of 9,9՛-position of fluorenes renders it amenable to substitution by various alkyl/aryl chains. Thus a large number of polyfluorene homo- and copolymers have been reported comprising n-alkyl,16,17 branched alkyl,18–21 spirocyclic,22,23 or even aryl based side chains.24–26 Side chain variation has also been investigated for poly(fluorene-alt-phenyl) type alternating copolymers that show liquid crystalline behavior.27–32 Our group introduced the concept of using optically active π-conjugated polymers to study the morphology and state of aggregation already in 1995 by using regioregular polythiophenes with enantiomerically pure side chains.33 The optical activity of electronic transitions is expressed as the degree of circular polarization (termed g-value), defined as,


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𝑔=

𝐼𝐿 ― 𝐼𝑅 1 (𝐼 + 𝐼𝑅) 2 𝐿

where, IL and IR denote the intensity of left and right circularly polarized light (in absorption, photoluminescence, and electroluminescence), respectively. For most conjugated polymers in solution and in thin films, the g-value in absorption (gabs) are generally low (10-2 to 10-3) and independent of film thickness.34–37 Typically, circular dichroism spectroscopy is used to analyze these thin-films and the π – π* transition is often exciton coupled. The analysis of this exciton coupling is highly supported by theoretical models and with the great support of Jean-Luc Brédas, we were able to determine the handedness of the helicity of the aggregates formed.38 Many studies followed these initial reports.39,40 More recently, optically active π-conjugated polymers are of high interest to be applied in displays, to LEDs with preferred emission of circular polarized light.41,42 Circularly polarized light emitting diodes can replace polarizers used in LCD back lighting or active-matrix OLED display technologies leading to compact devices with enhanced contrast. Moreover, the chiral organization of polymers can lead to very interesting phenomenon such as the chiral induced spin-selectivity (CISS) effect to realize all-organic spin-filters.43 In contrast to most of the polymers, annealed films of chiral polyfluorene and poly(pphenyleneethynylene) have been shown to exhibit much higher gabs (> 0.1), where the former is film-thickness dependent.44,45 To control and better understand the high gabs observed for polyfluorenes, several studies have been carried out with structural changes to chiral polyfluorenes and these studies reveal that the gabs critically depends i) on the location of the stereogenic center on the side chain,19,46,47 i.e., as the distance between the conjugated backbone and the stereogenic center on side chain increases, the gabs diminish dramatically and ii) on the ratio of chiral to achiral side chains in the copolymers.21,44,48 For thin films of polyfluorenes or polythiophenes a



dependence of gabs on film thickness has been found. Generally, thicker films give higher gabs.45,49– 51

The high gabs and thickness dependence found for polyfluorenes is attributed to its cholesteric

(chiral nematic) ordering in annealed thin-films.52 Our group has previously shown that molecular weight of the polymer53 and addition of supramolecular plasticizers54 both strongly influence the magnitude of the gabs in fluorene copolymers. In spite of the detailed understanding into the origin of high gabs in polyfluorenes, it is still challenging to design polymers with a combination of desired photophysical properties (e.g., band gap) and high gabs (> 0.1). In this work, we perform a systematic study to elucidate the structure–property relationship of chiral thin-films made from enantiopure fluorene alternating copolymers. A set of poly(9,9ʹdialkylfluorene-alt-2,5-dialkoxyphenyl) copolymers was chosen as the model system with variation in the location and configuration (S or R) of chiral and achiral side chains in the repeating units. A combination of differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray scattering was used to study the effect of the side chains (both position and configuration) on the thermal and phase behavior in bulk. In addition, the chiroptical properties in the thin-films were investigated by circular dichroism and spectroscopic ellipsometry. The results show that enantiopure side chains on the fluorene unit is essential to achieve high g-values (> 0.1) both in absorption as well as in photoluminescence through cholesteric ordering of polymer chains. The utility of these cholesteric polymers as active components in organic light emitting diodes which emit circularly polarized light is shown.


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■ RESULTS AND DISCUSSION We have chosen dialkoxyphenyl as a comonomer with 9,9ʹ-disubstituted fluorene to systematically study the influence of side chains on the chiroptical properties. The chiral copolymer set studied here can be classified into three categories, namely – i) constitutional isomeric polymers (P1 and P2, Figure 1), ii) diastereomerically related polymers (P3 and P4, Figure 1), and iii) polymers with a systematic variation of the side chains on dialkoxyphenyl unit while keeping the (S)configuration of side chain on the fluorene (P2 – P4). All four polymers were synthesized by following the standard Suzuki-polycondensation reaction (see Supplementary Information). Furthermore, the polymers were treated with a palladium scavenger (14 – 25 eq. with respect to the amount of palladium catalyst used without the ligands, see supporting formation for details) to remove trace levels of palladium catalyst impurities, which can adversely affect the photoluminescence properties.55 The chemical structures of the polymers were verified by 1HNMR, 13C-NMR, FT-IR, and elemental analysis (SI, Figure S1 – S16). The molecular weights of the polymers was determined by size exclusion chromatography (SEC, SI Figure S17) using polystyrene as the standard (Table 1).



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R 2O

R1 R1

OR2

P1

(S)

P2

(S)

R2 =

R1 =

P3 P4

(S)

(S)

(S)

(R)

Figure 1. Molecular structure of poly(9,9ʹ-dialkylfluorene-alt-2,5-dialkoxyphenyl) copolymers P1 – P4 studied.

We have first studied the thermal properties of all polymers in bulk using DSC (Table 1, SI Figure S18). Polymers P2 – P4 showed a glass transition (Tg) around 92 – 94 °C and additionally a second transition (T2) was observed (140 – 155 °C). With the help of polarized optical microscopy (POM), the region between Tg and T2 is attributed to a liquid crystalline phase for P2 – P4 as clear birefringence was observed under crossed polarizers (SI Figure S19). At and beyond the second transition (T2) for P2 – P4, the melt viscosity decreased, however when viewed under a POM the birefringence was not completely lost even on annealing at 200 °C (SI Figure S20). Based on these observations, T2 could be a plausible order-disorder transition. In stark contrast to the liquid crystallinity observed for P2 – P4 (with chiral side chains on the fluorene unit), P1 (with achiral side chains on fluorene) showed only an isotropic transition (Figure S18a, Table 1), without any liquid crystalline ordering as observed by POM (Figure S19a and S21).

Table 1. Comparison of the molecular weights, thermal and photophysical properties of polymers (P1 – P4).

Polymer

Mn

Đa

(kg/mol)a P1

23.5

2.04

Tg

Tm

T2

λmax, abs

λmax, em

(°C)b

(°C)b

(°C)b

(nm)c

(nm)c

-

110

-

369

415


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P2

25.5

1.74

93

-

140

369

416

P3

24.5

1.56

94

-

150

369

418

P4

19.5

2.09

92

-

155

368

414



a)

Molecular weight (Mn) and dispersity (Đ) of the polymers as determined by SEC in THF using polystyrene

standards; b) Thermal transitions as obtained from DSC with heating rate of 2 K min-1 for P1, 5 K min-1 for P2, and 40 K min-1 for P3 and P4; c) Absorption and emission maximum in chloroform solution (c = 0.06 mg mL-1; λexcitation = 330 nm)

In order to investigate the contrasting thermal characteristics of P1 with those of P2 – P4, X-ray scattering measurements were performed on all the four polymers. X-ray scattering profiles of P2 – P4 show a rather broad scattering peak centered around 0.35 Å-1 (corresponding to a dspacing of 16.1–18.8 Å, Figure 2a, see Figure S22 b-d), whereas P1 shows a very sharp peak centered at 0.362 Å-1 (d-spacing of 17.4 Å, Figure 2a and Figure S22a). The d-spacing of 16–18 Å matches well with the length of fully-stretched alkyl-chains on the fluorene unit and it corresponds to the separation between two consecutive layers of polymer chains.56 The rather sharp X-ray scattering peak observed for P1 indicates a strong crystallization of the linear n-octyl chains on the fluorene unit. This is reminiscent of the β-phase of polyfluorenes which exhibits strong crystallization with n-octyl chains on fluorene.57 On the other hand, the branched chiral chains on the fluorene lead to weaker interaction between the chains leading to a broad scattering profile for P2 – P4. Thus, the crystalline packing of P1 only results in an isotropic thermal transition, whereas the weakly interacting chiral side chains lead to liquid crystalline phases for P2 – P4.


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Figure 2. a) X-ray scattering profile of the polymers in a glass capillary measured at room temperature after annealing above the Tg (120 °C, 1 hour) of polymers P1 – P4 (see supporting information for sample preparation and full scattering profile). b) Normalized UV-vis absorption and photoluminescence (λex = 330 nm) spectra of P2 in chloroform solution (dashed line, c = 0.06 mg mL-1, l = 10.00 mm) and in annealed thin film (solid line, d = 68 nm).

Having established the bulk phase ordering of P1 – P4, we have further studied their photophysical

and

chiroptical

properties

in

annealed

thin-films.

The

UV-vis

and

photoluminescence spectra of P2 in solution (CHCl3) and in annealed thin-film exhibit similar absorption and photoluminescence maximum with a tailing at the longer wavelength region (Figure 2b). This indicates a lack of preferred ordering in the aggregation in thin-film, otherwise either blue or red-shifted UV-vis spectra would be the result. Similar UV-vis spectra were observed for polymers P1, P3 and P4 (Table 1, SI Figure S23). The UV-vis spectra of P1 – P4 are similar to their achiral counterparts reported in literature.31



Figure 3. a) Circular dichroism spectra and b) corresponding gabs plots for annealed (100 – 120 °C, 15 min) thin films (71 – 74 nm) of P1 – P4. For (b) the wavelength region where no absorption occurs (λ > 400 nm) is omitted. All the spectra were recorded at room temperature.

The chiroptical properties in thin-film were investigated by circular dichroism (CD) spectroscopy and generalized ellipsometry. The as spin-coated samples gave low to zero Cotton effects for all the polymers (SI, Figure S24). However, on thermal annealing at elevated temperature (>Tg, 100 – 120 °C, 15 min), and then measuring the CD at room temperature, a pronounced negative Cotton effect (> 600 mdeg) was observed for P2 – P4 (Figure 3a). In contrast, polymer P1 showed a very low Cotton effect (< 20 mdeg) even after annealing. The corresponding maximum gabs for annealed films with comparable thickness were +0.001 (λ = 376 nm), -0.06 (λ = 376 nm), -0.09 (λ=390 nm) and -0.11 (λ = 384 nm) for polymers P1, P2, P3, and P4, respectively (Figure 3b). This indicates that annealing into the liquid crystalline phase for P2 – P4 significantly improves the chiroptical properties in thin-films, whereas the gabs for P1, which lacks such an ordered phase, remains low despite thermal annealing.


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Figure 4: Film-thickness dependence of maximum gabs in annealed thin-films of P1 – P4. Lines in (a) indicate the general trend of thickness independent gabs for P1 - P4 till 200 nm. The complete thicknessdependence of gabs including the thicker films is shown in (b). The data points marked with gray circle indicates the maximum gabs observed for each of the polymer.

For fluorene-based polymers, gabs in annealed thin-films depends on film thickness,45 therefore high gabs value (> 0.1) are thought to arise from a non-local mechanism.58,59 Thus we have investigated the film-thickness dependence of gabs for all the four polymers to shed light on the origin of this intriguing chiroptical property. A monotonic increase of gabs (> 0.1) as a function of film-thickness is indeed observed for annealed films of P2 – P4 (d < 200 nm, Figure S25-S26), whereas for P1 the gabs was observed to be very low (~10-3) and independent of film-thickness (Figure 4a). For thick annealed films of P2 – P4 (d > 200 nm), the dependence of gabs on film thickness was much weaker and in particular for P4 even a decrease in gabs was observed at d > 200 nm (Figure 4b). This might be due to the longer annealing times required for the thicker films, but other explanations cannot be ruled out. The highest gabs of -0.82 (~ d = 250 nm) was observed for polymer P4 that comprises of side chains with (S)- and (R)-configuration of side chains on the fluorene and dialkoxyphenyl unit, respectively.



Figure 5. a) Degree of linear polarization in the extinction of vertically and horizontally polarized light through aligned annealed films of P2 oriented in line with the rubbing direction (rub V, black) and rotated clockwise +45° (rub cw 45°, orange). Solid lines show the data fitted to a modelled cholesteric with a pitch of 720 nm. Cartoons in the inset show the orientation of polymer chains in the vertical and +45° configurations. b) Unpolarized transmission data of P2 (d = 10 μm) as measured (black solid line) and modelled using the Good-Karali formalism for a cholesteric with pitch of 720 nm (black dotted line). The corresponding degree of circular polarization in the transmission of vertically and horizontally polarized light (orange) indicates a left handed cholesteric arrangement.

The thickness dependence of gabs most probably arises from a cholesteric ordering of polymers P2 – P4 in annealed thin-films.52 To understand this in more detail, we have employed a recently developed method that combines linear dichroism measurements with modelling to determine the characteristic parameters of the cholesteric order in annealed aligned polymer thin films (d < 1 μm).60 For this method annealed films were prepared on a planar polyimide alignment layer (SI). The pitch and handedness of the cholesteric order is then determined by measuring the degree of linear polarization as a function of layer thickness and orientation. Figure 5a shows the linear polarization in extinction for aligned annealed films of P2 and corresponding fitting to a model of cholesteric arrangement with pitch of 720 nm. The initial decrease in linear polarization (d < 180 nm) indicates a left-handed cholesteric arrangement. The linear dichroism results for polymers P2 – P4 are summarized in Table 2 (See SI, Figure S27). Having obtained the pitch of


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the polymers P2 – P4, we have re-examined the evolution of gabs as a function of fraction of pitch (film thickness/pitch), as this normalized quantity captures both the effect of film-thickness and pitch.61 Once again the gabs shows a monotonic increase with increase in fraction of pitch for filmthickness < 200 nm (Figure S28). This is in accordance with the linear film-thickness dependence of gabs as discussed above (Figure 4). Another and more commonly employed method to determine the pitch of cholesterics in small molecular/polymeric networks is studying the transmission of thick films (~ few µm). However, such an approach is difficult with conventional polymers due to the high melt-viscosity. Interestingly, we prepared thick films of P2 and P3 (d = 10 μm) by sandwiching the polymer between two clean glass-slides with the distance between them dictated by an epoxy glue with glass beads (See supporting information for detailed procedure). The thick annealed films of P2 and P3 (d = 10 μm) showed a clear dip in the transmission spectra of unpolarized light (stop band) around 1100 nm and 2300 nm, respectively (Figure 5b, SI Figure S29). To the best of our knowledge, this is the first time a stopband is observed in π-conjugated polymeric films. By modelling the transmission of light through the cholesteric liquid crystalline films of P2 and P3 (Figure S30) using the Good-Karali model,62 the pitch for different polymers were determined. These values are in good agreement with the pitches obtained by the linear dichroism method (Table 2). The left-handed arrangement of the cholesteric is further confirmed by a preferential transmission of right circularly polarized light at the wavelength of the stopband (Figure 5b). For polymer P4 no clearly discernable stopband was obtained, only a gradual decrease in transmission was observed (SI Figure 29). This is thought to arise from strong scattering effect in very thick samples,63 thus masking the presence of a stopband.



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Table 2. Summary of the chiroptical properties (pitch, handedness, maximum gabs and gPL) of polymers P1 – P4. Polymer

Pitch

Handedness gabs, max

(nm)

dmax(gabs)

gPL, max

(nm)

dmax(gPL) (nm)

P1

-a

-a

0.0013

94

-

-

P2

720

Left

-0.696

749

-0.33

138

P3

1500

Left

-0.699

592

-0.62

333

P4

850

Left

-0.825

258

-0.30

130

a) No cholesteric liquid crystalline order was found. d: film-thickness

Figure 6. a) Thin film photoluminescence (λex = 370 nm, d = 160 nm) spectra of P2 (blue trace). Also the corresponding degree of circular polarization in photoluminescence (gPL) spectra of P2, P3 and P4 in annealed films. b) Film-thickness dependence of gPL for P2 – P4. For gPL spectra λex = 313 nm and a 380 nm cut-off filter was used. In (a) the film-thickness of P2, P3, and P4 are 138 nm, 175 nm, and 130 nm, respectively.

With the observation of high gabs and an understanding into its origin through various spectroscopic studies, we have further investigated if the high gabs values would also reflect in the photoluminescence, which is relevant for device applications. The circularly polarized photoluminescence spectrum of an annealed thin-films of P2 – P4 shows good match with its corresponding steady-state photoluminescence spectrum with a maximum at 415 nm (Figure 6a). The negative sign of gPL indicates preferential emission of right circularly polarized light. The


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magnitude of gPL observed for a film thickness of 130 – 200 nm corresponds very well with the magnitude of gabs (Figure 6a). However for thicker films (> 200 nm) the magnitude of gPL either decreased or remained more or less constant for P2 – P4 (Figure 6b and Figure S31). This is similar to the trend observed in gabs (Figure 4b). We attribute the lack of linear increase in magnitude of gPL with film thickness to pronounced scattering effects. The locally generated circularly polarized light needs to travel a longer distance in thicker films before coming out of the film and reaching the detector. Since P2 – P4 form multidomain cholesterics and emit in the blue region of the spectrum, the scattering undergone by the emitted light is much stronger compared to polymers emitting in the longer wavelength, thus adversely affecting the magnitude of gPL.

Figure 7: a) Device architecture used for OLED fabrication. P* is the different polymers (P2 – P4) used for the device fabrication. Also the energy levels of polymer P2 determined by using cyclic voltammetry and UV-vis absorption is shown in the right panel. b) Electroluminescence spectra of an OLED based on annealed P2 and c) corresponding dissymmetry factor of electroluminescence (gEL). The 100 nm layer of



P2 was annealed at 100 °C for 15 minutes. The gEL spectrum is an average over twenty individual experiments. The OLEDs were driven with a square-wave pulsed voltage, having amplitude and peak voltage of 10.5 V, 10 KHz repetition rate and 10 s pulse duration.

Polymers P2 – P4 showed excellent photoluminescence quantum yield of 20 – 30% in annealed thin-films (100 nm) (Figure S32). This prompted us to construct OLEDs to explore the applicability of our materials in light-emission technologies. The OLED structure and the energy levels of the different layers are shown in Figure 7a. The HOMO and LUMO levels of P2 were estimated from cyclic voltammetry and UV-vis absorption spectrum. The P2 layer functioned both as transporting and emitting layer and was sandwiched between PEDOT:PSS for hole-injection and Lithium fluoride/Aluminium for electron injection. The P2 layer was 100 nm thick and was annealed at 100 °C for 15 minutes. The current-luminance-voltage characterization of the OLEDs constructed from P2 – P4 are reported in the Supporting Information (Figure S33). In Figure 7b the electroluminescence spectra shows that an excess of right-handed polarization of electroluminescence (EL) emitted by the diode. The dissymmetry in electroluminescence (gEL) as function of wavelength, recorded while operating the OLED under pulsed voltage at 10 KHz repetition rate, with pulses of 10.5 V amplitude and peak. The pulsed-mode operation was chosen to limit degradation during the measurements. The recorded gEL is negative through most part of emission. This is accordance with the negative sign observed in gPL. The gEL spectrum peaks at ca. 400 nm, with absolute values around 0.2, thus similar to that of gPL recorded on films of similar thickness described above; Similar device studies on P3 and P4 also show an excess of righthanded polarization of electroluminescence (Figure S34). However, the absolute values of gEL are lower, particularly for P4. The discrepancy between the gEL peak values of the three polymers might be due to small differences in film thickness in the OLEDs and to differences in the position of the recombination zone due to not identical charge mobilities. In general, we note that the


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generation of circularly polarized electro-luminescence takes place in a very different optical structure compared to thin films, likely involving multiple reflections at metal-polymer interfaces. This makes the comparison with the absolute values of gPL challenging.

■ CONCLUSIONS In summary, the structure-property relationship of chiral poly(9,9ʹ-dialkylfluorene-alt-2,5dialkoxyphenyl) copolymers have been systematically investigated by varying the location and configuration (S or R) of the chiral side chains. Juxtaposing the liquid crystalline ordering and the chiroptical properties of P1 – P4, it was noted that enantiopure side chains on the fluorene unit are essential in rendering a cholesteric liquid crystalline phase to annealed thin-films which consequently gives rise to high gabs and gPL of the order of 0.1-0.8. On the other hand, the n-octyl side chains on the fluorene unit tend to crystallize, thereby impeding cholesteric liquid crystalline organization, resulting in low chiroptical properties as seen for polymer P1. Irrespective of the linear n-octyl or chiral side chain substituents on dialkoxyphenyl unit, polymer P2 – P4 (bearing enantiopure side chains on fluorene unit) exhibit high gabs and gPL as these polymers adopt a cholesteric organization. It is worth noting that; i) the handedness of the cholesteric organization is solely governed by the configuration of enantiopure side chains on the fluorene unit and ii) polymer P4 bearing (S)- and (R)-configuration side chains on fluorene and dialkoxyphenyl unit, respectively shows lower helical pitch and the highest gabs among the polymer P1 – P4, presumably due to the tighter packing enabled by the opposite configuration of side chains. These observations clearly indicate the side chain configuration on the co-monomer of fluorene is an additional handle to modulate the chiroptical properties.



In retrospect, the above stated principle that enantiopure side chains on fluorene leads to strong chiroptical properties is indeed valid for many examples reported in literature.48,50,51,53,58 However, there are a few cases were the gabs is apparently low (~10-3) in spite of the enantiopure side chains on the fluorene unit.54 For such cases, we suggest that the co-monomeric unit of fluorene plays an important role in dictating the chiroptical properties. If the co-monomer is a bulky or large aromatic unit with strong aggregation tendency, the reorganization of the polymer into cholesteric liquid crystalline phase is significantly prohibited upon thermal annealing. Thus we suggest that enantiopure side chains on fluorene unit is a necessary but not a sufficient condition to achieve strong chiroptical properties in fluorene copolymers. For systems which exhibit low gabs in spite of possessing enantiopure side chains on fluorene, an external plasticizer additive54 might be utilized to improve the ordering and the chiroptical properties. We envisage that the molecular level insights gained from this study could be used to achieve both strong chiroptical response and desired optical bandgap simultaneously by appropriate choice of fluorene co-monomers. Acknowledgments: Authors thank financial support from NWO (TOP-PUNT Grant 10018944) and the Dutch Ministry of Education, Culture, and Science (Gravitation program 024.001.035). D.D.N. acknowledges financial support from the Engineering and Physical Sciences Research Council of the UK (EPSRC). C.K. acknowledges the Marie Skłodowska-Curie postdoctoral fellowship (704830) for financial support. We thank Brigitte Lamers for X-ray measurements and Profs. René Janssen and Richard Friend for fruitful discussions.

Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx


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Synthesis and molecular characterization, DSC thermograms, polarized optical microscopy images, X-ray scattering profile, and additional CD spectra of polymers P1 – P4. Details of pitch determination, transmission spectra of thick-films and OLED characteristics of polymers P2 – P4 (PDF).

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Table of contents graphic:




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Molecular Design Principles for Achieving Strong Chiroptical

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