Induced circular dichroism of an optically active polyfluorene derivative in phase-separating solutions

Abstract

When methanol, a nonsolvent, was added to dilute tetrahydrofuran solutions of an optically active polyfluorene derivative, liquid–liquid phase separation occurred, and circular dichroism (CD) was induced at a low temperature. The polymer concentration of the minor separating phase, estimated by light scattering, was very high (∼0.4 g cm⁻³). The CD induction occurring in that concentrated phase was temperature sensitive. When the phase-separating solution was quenched from 40 to 15 °C, the CD increased according to first-order reaction kinetics, and it was a rather slow process (the rate constant was 2.5 × 10⁻⁴ s⁻¹). The intermolecular chiral interaction in the concentrated phase may be responsible for the CD induction or non-racemization of this helical polyfluorene derivative in phase-separating solutions.

Introduction

Polyfluorene, a π-conjugated polymer, has interesting electrical and optical properties and attracts increasing interest as a candidate for organic light-emitting diodes with high quantum yield and high hole mobility.^{1, 2} Circular dichroism (CD), optical activity and circular polarized luminescence are additional optical and electro-optical properties that enable polyfluorene to be used in optical devices or sensors. Several researchers^{3, 4, 5, 6, 7} were interested in these properties and investigated optically active polyfluorene derivatives bearing chiral side chains.

Molecular modeling on the basis of ab initio molecular orbital calculations indicated that the energetically favorable conformation of the polyfluorene backbone is a 5/2 or 5/1 helix,^{8, 9} and this helical conformation was supported by electron and x-ray diffraction from the spin-coated film and fiber of a polyfluorene derivative,^{8, 10} as well as from a solution study.⁶ However, dilute solutions of optically active polyfluorene derivatives usually exhibit little CD.^{6, 7} This implies that the optically active side chain hardly differentiates between the energies of the right- and left-handed helical conformations of the polyfluorene backbone. On the other hand, strong CD, circular selective reflection and circular polarized luminescence were observed for optically active polyfluorene derivative films, showing that chiral discrimination arises from intermolecular interaction in the condensed phase.^{3, 4, 11}

Some π- and σ-conjugated polymers, for example, polythiophene and polysilylene derivatives, were reported to induce strong CD in dilute solutions by aggregation or phase separation.^{12, 13, 14, 15} In these aggregation- and phase-separation-induced CDs, the intermolecular chiral interaction among conjugated polymer chains may have an important role. However, the detailed mechanism of these phenomena has not been elucidated.

Recently, Wu and Sato⁷ reported aggregation-induced CD in dilute tetrahydrofuran (THF) and toluene solutions of two optically active polyfluorene derivatives on cooling to as low as −80 °C. The observed exciton-coupling signal of CD arising from the polyfluorene main chain was inverted by changing the solvents (THF and toluene) or through tiny differences in the side-chain chemical structure. Owing to the difficulty in the characterization of polyfluorene aggregates in solution at −80 °C, the detailed mechanism of the aggregation-induced CD was not investigated in the previous study.

In this study, we have investigated the solvent condition in which an optically active polyfluorene derivative exhibits CD in a dilute solution near room temperature. Poly(2,7-[9,9-bis((S)-citronellyl)]fluorene) (PCF, Scheme 1) was chosen as the test polymer, and phase-separation-induced CD was found in dilute THF solutions of this polymer near room temperature by adding a nonsolvent, methanol. To elucidate the detailed mechanism of the CD induction, we have investigated the kinetics of CD induced on cooling in the THF–methanol solution, as well as the separated droplet phase, by light scattering measurements. On the basis of those experimental results, we have proposed a model for CD induction in phase-separating solutions of helical polymers.

Scheme 1

Repeating unit of PCF.

Experimental procedure

Polymer samples

The dibromofluorene monomer was polymerized in a hot mixture of toluene and N,N-dimethylformamide using a zero-valent nickel reagent by the Yamamoto coupling reaction.⁵ The polymer was divided into seven fractions by fractional precipitation using toluene as the solvent and methanol as the precipitant. Two fractions, PCF2-1 and PCF2-3, were chosen for the following experiments. The solvents THF and methanol were distilled under calcium hydrate as a desiccant.

Turbidity

Methanol was added dropwise under stirring to THF solutions of the fraction PCF2-1 with different polymer concentrations, and the turbidity of each solution was observed at room temperature by eye to construct the ternary phase diagram of PCF, THF and methanol. In what follows, the composition of the ternary system is expressed in terms of the volume fraction φ_MeOH of methanol at the mixing of THF and methanol and the PCF mass concentration c in the total solution.

CD and UV-VIS absorption

CD and ultraviolet-visible light (UV-VIS) absorption spectra were measured on phase-separating solutions of fraction PCF2-1 with φ_MeOH=0.5 and different c values using a JASCO J-720WO spectropolarimeter (JASCO, Tokyo, Japan) at 40 and 15 °C. A quartz cell with a 10-cm optical pass length and a thermostat jacket was used for the measurements.

Light scattering

Static light scattering measurements were carried out on THF and methanol-added THF solutions of samples PCF2-1 and PCF2-3 using a Fica 50 light scattering photometer with vertically polarized incident light of 546 nm and without an analyzer. The light scattering systems were calibrated using toluene as the reference material.

For THF solutions of the two PCF samples, the Rayleigh ratio R_θ excess over that of the solvent obtained was analyzed by the conventional Berry plot to determine the weight-average molar mass M_w, the second virial coefficient A₂, and the z-average square radius of gyration 〈S²〉 using the equations¹⁶

where K is the optical constant and k is the magnitude of the scattering vector. The results of M_w, A₂ and 〈S²〉^1/2 for samples PCF2-1 and PCF2-3 in THF are listed in Table 1.

Table 1 Molecular characteristics of the PCF samples used

Light scattering from the polymer in a mixed solvent is affected by preferential adsorption. Owing to this effect, equation (1) should be replaced by the following equation,¹⁷

where ${(\partial c/\partial c\text{)}}_{{\mu }_1}$ is the increment of the mass concentration c₁ of the secondary solvent (methanol in our system) with increasing c at a constant solvent chemical potential, and ${\text{(∂}ñ/\text{∂}c\text{)}}_{c_1}$ and (∂ñ/∂c₁)_c are the specific refractive index increments of the polymer and the secondary solvent components, respectively.

The specific refractive index increment ${\text{(∂}ñ/\text{∂}c\text{)}}_{c_1}$ at constant solvent composition, included in K, was measured at 30 °C using a differential refractometer of the modified Schulz-Cantow type. Figure 1 shows the results of ${\text{(∂}ñ/\text{∂}c\text{)}}_{c_1}$ at φ_MeOH (c₁)=0 (0 g cm⁻³), 0.2 (0.21 g cm⁻³) and 0.35 (0.38 g cm⁻³), represented by filled circles. The value of ${\text{(∂}ñ/\text{∂}c\text{)}}_{c_1}$ at φ_MeOH (c₁) = 0.5 (0.57 g cm⁻³) was obtained by extrapolation (unfilled circle in Figure 1).

Figure 1

Specific refractive index increments of PCF in THF and THF–methanol mixtures at 30 °C.

Results and discussion

Phase diagram

Figure 2 illustrates the phase diagram of the ternary system of sample PCF2-1, THF and methanol at room temperature. Here, c is the polymer mass concentration, φ_MeOH is the volume fraction of methanol at the mixing of THF and methanol, and the unfilled and filled circles indicate single-phase and biphasic states, respectively. This is a typical ternary phase diagram of a polymer, solvent and nonsolvent.¹⁸ CD and UV-VIS absorption, as well as light scattering measurements, mentioned below, were made in the two-phase region mostly at fixed φ_MeOH=0.5.

Figure 2

Phase diagram of the ternary system PCF, THF and methanol at room temperature; unfilled circles, one-phase region; filled circles, two-phase region. A full color version of this figure is available at the Polymer Journal online.

CD and UV-VIS absorption

Figure 3 shows CD and UV-VIS absorption spectra of sample PCF2-1 in the THF–methanol mixture of φ_MeOH=0.5 (c=6.0 × 10⁻⁷ g cm⁻³). Although the solution is CD inactive at 40 °C, a bisigned signal grows on cooling of the solution to 15 °C. Although the peak height of the UV-VIS absorption decreases with the cooling time, that of the CD spectrum increases, except at 300 min. The diminishment of the absorption spectrum may arise from the increase of scattering from the solution; the scattered light cannot contribute to the absorption. (Although not shown, the baseline of the original UV-VIS absorption curve was considerably dependent on the wavelength λ according to Rayleigh's λ⁻⁴ law¹⁶ and increased with time at 15 °C. This baseline was subtracted from the original absorption curve to obtain A, shown in Figure 3). Similar CD and UV-VIS absorption spectra were obtained at different c values (=2.0 × 10⁻⁷ and 4.0 × 10⁻⁷ g cm⁻³). These CD inductions were almost reversible, that is, when the quenched solutions were heated to 40 °C, the induced CD vanished again.

Figure 3

(a) CD and (b) UV-VIS absorption spectra for a phase-separating solution of sample PCF2-1 (φ_MeOH=0.5, c=6.0 × 10⁻⁷ g cm⁻³) at 40 °C, quenched to 15 °C. A full color version of this figure is available at the Polymer Journal online.

In Figure 3, the UV-VIS absorption peak seems to show a slight red shift. If this red shift reflects the transformation to the β-phase of polyfluorene chains,¹⁹ the peak should return to the original position on heating to 40 °C. However, the red shift was not reversible. The UV-VIS absorption experiment for the phase-separating solutions is affected by the wavelength-dependent scattering, and the baseline subtraction may not be enough to perfectly correct this scattering effect. Owing to this experimental uncertainty, we do not argue the origin of the small peak shift here.

The induced CD must arise from the separating concentrated phase, and the molar CD Δɛ_c of the concentrated phase is calculated from the observed ellipticity θ by

where c_c and Φ are the mass concentration and the volume fraction (in the total solution) of the concentrated phase, respectively, M₀ is the molar mass of the PCF repeating unit and l is the path length; Φl represents the average path length of the concentrated phase. On the other hand, the average molar extinction coefficient ɛ_c of the concentrated phase is calculated from the observed absorbance A using

where c_d is the mass concentration of the coexisting dilute phase. Thus, the Kuhn dissymmetry factor of the concentrated phase may be proportional to the following quantity:²⁰

Here, θ_m and A_m are the peak heights of ellipticity and absorbance, respectively. The value of c_d was estimated by light scattering (cf. Figure 6).

Figure 4 displays the time evolution of g_c of the solutions shown in Figure 3 after quenching from 40 to 15 °C. The data points can be fitted to single-exponential functions (solid curves in the figure), although the initial and final g_c values are slightly different at each polymer concentration c. Therefore, the CD induction obeys first-order reaction kinetics (cf. the discussion at the end of this section). From the fitting curves, the reaction rate constant was estimated to be 2.5 × 10⁻⁴ s⁻¹. Thus, the CD induction is a rather slow molecular event.

Figure 4

Time evolution of the Kuhn dissymmetry factor for the concentrated phase in the phase-separating solution shown in Figure 3 after quenching from 40 to 15 °C; solid curves, single-exponential fitting results (cf. equation (15)).

Light scattering

Figure 5 compares (Kc/R_θ)^1/2 at θ → 0 for sample PCF2-3 in THF and a THF–methanol mixture with φ_MeOH=0.2. The disagreement of the intercepts comes from preferential adsorption, and using equation (2) we can estimate the degree of preferential adsorption ${(\partial c/\partial c\text{)}}_{{\mu }_1}$ to be −0.025. [At φ_MeOH=0.2, (∂ñ/∂c₁)_c=0 = −0.089 cm³ g⁻¹]. Neglecting the solvent composition dependence of ${(\partial c/\partial c\text{)}}_{{\mu }_1}$, we may calculate the optical constant K^* in equation (2) at φ_MeOH=0.5 using (∂ñ/∂c₁)_c=0 = −0.11 cm³ g⁻¹ and ${\text{(}\partial ñ/\partial c_1\text{)}}_{c_1}$ = 0.326 cm³ g⁻¹ (cf. Figure 1).

Figure 5

Concentration dependences of (Kc/R_θ)^1/2 at θ → 0 for sample PCF2-3 in THF and a THF–methanol mixture with φ_MeOH=0.2.

At φ_MeOH=0.5, R_θ is high enough, even for very dilute solutions, because droplets of the minor concentrated phase possess very strong scattering power. In Figure 6, (R_θ/K^*)^1/2 at θ → 0 for sample PCF2-1 at φ_MeOH=0.5 and 40 °C is plotted against c. From this plot, R_θ seems to vanish at ∼3 × 10⁻⁸ g cm⁻³. This critical concentration can be regarded as the polymer concentration c_d of the coexisting dilute phase. In what follows, we are interested in droplets of the minor concentrated phase in the solution, which is responsible for the CD induction. The mass concentration of the concentrated phase droplets in the solution is given by c–c_d. Strictly speaking, c_d slightly depends on c, but within the dilute c range we examined c_d may be approximated to be 3 × 10⁻⁸ g cm⁻³, as determined in Figure 6, irrespective of c.

Figure 6

Concentration dependence of (R_θ/K^*)^1/2 at θ → 0 for sample PCF2-1 at φ_MeOH=0.5 and 40 °C.

Figure 7 compares [K^*(c−c_d)/R_θ]^1/2 for the concentrated droplet phase in a solution of sample PCF2-1 (φ_MeOH=0.5, c=6.0 × 10⁻⁷ g cm⁻³) at 40 °C and on cooling to 15 °C. The scattering intensity and thus the size and amount of the droplet phase in the solution change little on cooling. Similar temperature-insensitive light scattering results were also obtained for solutions of sample PCF2-1 with φ_MeOH=0.5 and different c values (2.0 × 10⁻⁷ and 4.1 × 10⁻⁷ g cm⁻³). This is in sharp contrast to the CD induction shown in Figure 3, indicating that phase separation may be a necessary condition, but not a sufficient one, for CD induction.

Figure 7

Angular dependences of [K^*(c–c_d)/R_θ]^1/2 for the concentrated droplet phase in a solution of sample PCF2-1 (φ_MeOH=0.5, c=6.0 × 10⁻⁷ g cm⁻³) at 40 °C and on cooling to 15 °C; solid curves, calculated by equations (6, 7, 8, 9 and 10) with the fitting parameters listed in Table 2. A full color version of this figure is available at the Polymer Journal online.

Let us assume that the droplets of the concentrated phase are polydisperse spherical particles obeying a log-normal distribution. The log-normal distribution^{21, 22} is expressed in terms of the weight fraction w(M) of the molar mass M given by

where

with weight- and number-average molar masses M_w and M_n. The particle scattering function P(k) of the sphere with a molar mass M is given by²³

where R is the radius of the sphere related to M by

with Avogadro's constant N_A and mass concentration c_c of the concentrated phase.

Polymer concentrations of the solutions investigated are so dilute (<10⁻⁶ g cm⁻³) that the interparticle interference effect may be neglected in the scattering intensity. In such a case, K^*(c−c_d)/R_θ can be calculated by^{16, 24}

Figure 8 shows fitting results of K^*(c−c_d)/R_θ for phase-separating solutions of sample PCF2-1 with φ_MeOH=0.5 and three different c values at 40 °C. The solid curves, drawn using equations (6, 7, 8, 9 and 10) with the fitting parameters listed in Table 2, almost fit to the experimental data points. The weight-average molar mass M_w of the concentrated droplet phase is of the order of 10¹⁰, indicating that each droplet consists of ∼10⁵ PCF chains. The molar mass distribution of the droplet phase is wide, ranging from 10⁷ to 10¹². The concentration of the concentrated droplet phase is ∼0.4 g cm⁻³, and we can expect a strong intermolecular interaction among PCF chains in the concentrated phase. The solid curves in Figure 7 are also theoretical curves calculated by equations (6, 7, 8, 9 and 10) with the fitting parameters listed in Table 2. The fitting parameters indicate that the molar mass distribution of the droplet phase becomes slightly wider and the smaller droplet phase increases on cooling.

Figure 8

Angular dependences of [K^*(c–c_d)/R_θ]^1/2 for concentrated droplet phases in solutions of sample PCF2-1 with φ_MeOH=0.5 and three different c values at 40 °C; solid curves, calculated by equations (6, 7, 8, 9 and 10) with the fitting parameters listed in Table 2.

Table 2 Fitting parameters characterizing the concentrated droplet phase in solutions of sample PCF2-1 with φ_MeOH=0.5

It was verified that the anisotropic light-scattering intensity is almost zero for the phase-separating solution of PCF with c∼10⁻⁶ g cm⁻³ and φ_MeOH=0.5. This indicates that the concentrated phase may not be the liquid-crystalline phase.

The concentration c_c is much higher than the overlap concentration of sample PCF2-1 (∼10⁻³ g cm⁻³; cf. Table 1), and the PCF chains are highly entangled with each other in the concentrated phase. Therefore, the helical sense conversion in PCF chains is rather difficult, which may be responsible for the slow CD induction shown in Figure 4.

Intermolecular chiral interaction and CD induction

According to the molecular orbital calculation and molecular modeling,^{8, 9} the polyfluorene chain favorably takes a 5/2 or 5/1 helical conformation. However, PCF did not exhibit CD in a dilute THF solution near room temperature.^{6, 7} This indicates that PCF chains take right- and left-handed helical conformations with equal probability. On the other hand, in phase-separating THF solutions of PCF by adding methanol, the right- and left-handed helical conformations are discriminated to induce CD because of polyfluorene main-chain absorption. This chiral discrimination may come from some chiral interaction among PCF chains in the separating concentrated droplet phase.

Applying McLachlan's general theory^{25, 26, 27, 28} of dispersion interaction, Osipov^{29, 30, 31} formulated the chiral attractive interaction potential w^* between two chiral objects as follows:

where a and a′ are unit vectors parallel to the principal axes of the two objects, p is the distance vector between the centers of mass of the two objects, and J^* is the interaction strength calculated by

Here, ℏ is the Dirac constant, υ is the segment volume, ω is the angular frequency, ɛ_m, ɛ_∣∣ and ɛ_⊥ are the solvent permittivity and the longitudinal and transverse components of the segment permittivity, respectively, and g_∣∣ and g_⊥ are the longitudinal and transverse components of the segment gyration tensor, respectively. w^* is a short-range interaction proportional to p⁻⁷. Equation (12) may govern the solvent dependence of the induced CD.⁷

The right- or left-handed helical conformation of the PCF chain provides the chirality. Therefore, helical segments of the PCF chain can be regarded as chiral objects, and the chiral interaction w^* given by equation (11) is expected among the segments. Although w^* should be zero if the segment orientation is perfectly isotropic, we may expect non-zero w^* because of the local anisotropic orientation of the segments due to the anisotropic interaction at close approach.

From symmetry, w^* between right-handed helical segments must be of the same magnitude as, and opposite in sign to, that between left-handed helical segments; also, w^*=0 between right- and left-handed helical segments. Thus, in the solution of PCF in which the fraction of the right-handed helical segment is f_P, the average w^* should be proportional to 2f_P−1 of the solution.

Including the entropic term, the difference ΔG_h in the free energy between right- and left-handed helical segments may be written in the form

where κ is the proportional constant, and its sign is determined by the intrinsic chirality of the PCF chain, that is, its chiral side chain. Lifson et al.³² calculated f_P on the basis of the one-dimensional Ising model. Their theory includes three fitting parameters, ΔG_h, the free energy ΔG_r of the helix reversal and the number of segments N₀ per chain. Now, we choose the repeating unit of PCF as the segment; N₀=280 for sample PCF2-1. Although we have no information about ΔG_r for the PCF chain, here we choose a value of 10 kJ mol⁻¹ for ΔG_r. This is the typical ΔG_r value for helical polymers such as polyacetylene or polyisocyanate derivatives.^{20, 33} Figure 9 displays the temperature dependence of 2f_P−1 for three different κ-values on the basis of the Ising model. It can be seen that 2f_P−1 sharply increases with decreasing temperature below some critical value. The strong temperature dependence of the induced CD in Figure 3 seems to be consistent with this theoretical result.

Figure 9

Temperature dependence of 2f_P–1 calculated for three different κ-values on the basis of the Ising model for helical polymers 32 with N₀=280, ΔG_r=10 kJ mol⁻¹, and equation (13).

When the helical sense conversion of the PCF segment occurs through the segment–segment interaction, there are four elementary reaction processes:

where P and M represent the right- and left-handed helical states of each segment, and k_P,P, k_P,M, k_M,P and k_M,M are rate constants of the elementary reactions. If those elementary reaction rates are determined only by the energies of the initial and final states, we have the relations k_P,P=k_P,M (≡k_P) and k_M,P=k_M,M (≡k_M) from the above symmetry argument on w^*. Using these relations, we finally obtain the following kinetic equation of the first-order reaction

This is consistent with the time evolution of g_c shown in Figure 3.

Conclusion

We have investigated the induced CD in phase-separating solutions of an optically active polyfluorene derivative (PCF). In the phase-separating solutions, the polymer concentration of the minor concentrated phase was very high (∼0.4 g cm⁻³). The CD induction occurring in that concentrated phase was temperature sensitive and obeyed first-order reaction kinetics in the quenched solution. This was a rather slow process (the rate constant was 2.5 × 10⁻⁴ s⁻¹).

The above experimental results can be explained by the following molecular mechanism. In the separating concentrated phase, each PCF segment feels strong chiral interactions from surrounding PCF segments. These interactions may bring about a helical sense conversion, followed by the non-racemization of PCF chains that induces CD. The helical sense conversion by the segment–segment interaction originates from the four elementary reaction processes given by equation (14), which provide first-order reaction kinetics under a certain condition. The temperature sensitivity of the CD induction can be explained on the basis of the Ising model for helical polymers. The slow process of CD induction may be due to high entanglements among PCF chains in the concentrated phase.