Abstract
The addition of methanol to dilute THF solutions of chiralachiral random copolymers of fluorene derivatives and the chiral homopolymer showed thermoreversible circular dichroism (CD) induction in the mainchain fluorene absorption region. This finding demonstrated the uneven population of the right and lefthanded helical conformations in the polymer chains. From the sign of the induced CD, two helical screwsense inversions were found by changing the chiral monomer content. The Ising model for chirally interacting chiralachiral random copolymers can explain the double screwsense inversions.
Introduction
Recently, π and σconjugated polymers, which possess interesting electrical and optical properties, have attracted much interest as candidates for organic lightemitting diodes^{1, 2, 3} and chemical sensors.^{4} Chiroptical properties, such as circular dichroism (CD), optical rotation, and circular polarized luminescence, enrich the applications of these organic semiconductors as optical devices or sensors.^{5, 6, 7, 8, 9} Optically active side chains may introduce chiroptical properties into conjugated polymers, but some polymers exhibit these properties only in films or phase separating solutions,^{10, 11, 12, 13} which indicates that intermolecular chiral interactions are necessary to induce the chiroptical properties in such conjugated polymers.
Polyfluorene derivatives are πconjugated polymers. From electron and Xray diffraction,^{14, 15} as well as molecular modeling,^{14, 16} these polymers are known to take a helical conformation, but chirality introduced to the side chain of the polymers was not found to induce circular dichroism (CD) on their backbone chain in dilute solution. This finding probably results from weak intramolecular chiral interactions that cannot discriminate against the handedness of the backbone helical conformation.
In a previous study,^{17} the CD induction of an optically active homopolymer of fluorene derivative, poly(2,7[9,9bis((S)citronellyl)]fluorene) (PCF), in dilute THF solutions was found by the addition of a nonsolvent methanol. Although the methanoladded solutions were almost transparent because of low polymer concentrations (∼10^{−6} g cm^{−3}), static light scattering measurements indicated a liquidliquid phase separation to form concentrated droplet phases, and it was concluded that this induction arises from the intermolecular chiral interaction of polyfluorene chains in the concentrated droplet phases separated from the dilute solution.
Helical chiralachiral random copolymers often exhibit interesting chiroptical properties, for example, sergeantsandsoldiers behavior,^{18, 19, 20} compositiondriven helical screwsense inversion,^{21, 22} and so on. Therefore, the CD induction study in phase separating solution was recently extended to a helical chiralachiral random copolymer of fluorene derivatives, poly(2,7[9,9bis((S)citronellyl)]fluorenerandom9,9dinoctyl fluorene), as shown in Scheme 1, and double screwsense inversions were found by changing the chiral monomer content x. To the best of our knowledge, this is the first observation of compositiondriven double sense inversions in helical chiralachiral random copolymers. In this paper, this finding is reported along with the theoretical argument of its origin.
Experimental procedure
Samples
Mixtures of the chiral and achiral dibromofluorene monomers of different compositions were copolymerized in a hot mixture of toluene and N,Ndimethylformamide (DMF) using a zerovalent nickel reagent by the Yamamoto coupling reaction.^{7} Each copolymer sample was divided into three fractions by fractional precipitation using THF as the solvent and methanol as the precipitant, and the middle main fraction was chosen for the following experiments.
The chiral monomer content x of five middle copolymer fractions was estimated from the ratio of the integrated intensities of the proton signals of the chiral side chains and aromatic rings. The results, listed in Table 1, are very close to the initial compositions fed at the copolymerization (the numbers in the parentheses in the second column of Table 1). The weight average molecular weights, M_{w}, and the ratios of M_{w} to the number average, M_{n}, of the five fractions were determined by sizeexclusion chromatography with a multiangle light scattering detector (SECMALS).^{23} Table 1 lists the results and weight average degree of polymerization N_{0,w} calculated from the M_{w} determined. The values of M_{w}/M_{n} indicate the relatively narrow molecular weight distributions of the fractions used in this study. One fraction of the chiral homopolymer PCF prepared in the previous study was added to the copolymer fractions for the following CD induction study, and its M_{w} was determined by static light scattering (SLS).^{17} The values of N_{0,w} of all the fractions were within the range of ca. 100–300.
Measurements
Each fraction was first dissolved in THF (8 × 10^{−7} g cm^{−3}), and then, methanol was added to the THF solution at room temperature. The methanol volume fraction φ_{MeOH} in the mixed solvent and the final polymer concentration c in the THFmethanol solution were adjusted to 0.5 and 4 × 10^{−7} g cm^{−3}, respectively. CD and SLS measurements were conducted using the THFmethanol solutions prepared using a JASCO J720WO spectropolarimeter and a Fica 50 light scattering photogoniometer with 546 nm incident light, respectively, at 40 and 15 °C.
Results
Static light scattering
Figure 1 shows the light scattering profiles of THFmethanol solutions of fractions PC5O5 (x=0.50) and PCF (x=1) at 40 °C and 15 °C (300 min after quenching from 40 °C). Here, K is the optical constant, c is the polymer mass concentration, R_{θ} is the excess Rayleigh ratio at the scattering angle θ, and k^{2} is the square of the scattering vector. All of the profiles exhibit very strong angular dependences, demonstrating the existence of large particles in the solutions. The phase diagram obtained previously^{17} for the methanoladded THF solution of PCF indicates that the solutions may be in the biphasic region, and the large particles are the concentrated droplet phases separated from the dilute solutions.
For dilute solutions of polydisperse spheres, (Kc/R_{θ})^{1/2} can be calculated by^{17}
where R is the radius of the sphere, w(R) is the weight fraction of a sphere of radius R, c_{c} is the polymer mass concentration in the spherical concentrated phase, and N_{A} is the Avogadro constant. Assuming that the size distribution w(R) obeys the lognormal distribution,^{17} equation (1) was fitted to the experimental results to obtain the weightaverage radius R_{w}, the ratio of R_{w} to the numberaverage R_{n} of the droplet phase and c_{c}. The solid and dotted curves in Figure 1 show the fitting results, and the parameters obtained are listed in columns 5–8 in Table 1. Droplet sizes are on order of 100 nm. The concentrations of the droplet phases are as high as 0.6 g cm^{−3} for both fractions and almost independent of temperature. The results of c_{c} for fractions PC5O5 and PCF indicate the similar affinities of the chiral and achiral monomer units to the mixed solvent.
Circular dichroism induction
None of the copolymer fractions exhibited CD in dilute THF solution, as in the case of the chiral homopolymer PCF,^{17} which indicates that the intramolecular chiral interaction between the main and side chains of the copolymers is too weak to energetically discriminate the right and lefthanded helical conformations in the fluorene main chain.
Figure 2 shows UVvisible absorption and CD spectra of the phase separating solution of PC5O5 quenched from 40 to 15 °C. In Panel a, the main peak of the UVvisible absorption, arising from the π–π^{*} transition in the fluorene main chain, is essentially unchanged over time since quenching, but the peak height decreases, the peak wavelength slightly increases, and a new sidepeak appears at 426 nm. On the other hand, a bisign CD signal is induced in the fluorene mainchain absorption region by quenching in Panel b, just as in the case of fraction PCF reported previously.^{17} The induced CD disappeared upon heating to 40 °C and reappeared by quenched again to 15 °C, which is similar to PCF and demonstrates the thermal reversibility of the CD induction. The phaseseparating solutions were verified to have no optical anisotropy,^{17} which means that the induced CD does not arise from a liquid crystal phase. The polyfluorene chain is known to take 5/2 or 5/1 helical conformations favorably^{14, 16} so that the CD induction indicates uneven population of the right and lefthanded helical conformations of the copolymer mainchain. The Kuhn dissymmetry factor g_{c}≡Δɛ/ɛ at 410 nm (the CD peak position) reaches the asymptotic value at approximately 60 min (cf. Insert of Figure 1), which is slightly faster than g_{c} of the PCF solution.^{17}
The new side peak at 426 nm in Panel a corresponds to the socalled ‘βphase’ observed for poly(9,9dinoctylfluorene) films,^{24, 25} which is assigned to an almost planar conformation of the fluorene main chain with the torsional angle ≈160° stabilized by the noctyl side chains.^{26} A more pronounced sidepeak was observed in the methanoladded THF solution of fraction PC2O8, indicating that some of achiral monomer units take similar planer conformations in the concentrated droplet phase. Interestingly, a sidepeak also grows in the CD spectrum (Panel b) at the same wavelength, which may arise from the achiral monomer unit in the copolymer chain taking the ‘βphase’ conformation.
In the methanoladded THF solution with the same φ_{MeOH} and c, a similar CD induction was observed for PC6O4 quenched from 40 to 15 °C, but a CD signal appeared for PC2O8 even at 40 °C and did not change with time after quenching to 15 °C. The CD induction experiments were examined twice for fraction PC2O8, and nearly the same CD spectra were obtained, confirming the reproducibility of the CD induction for this fraction.
Figure 3 shows the asymptotic CD absorption spectra for all chiralachiral random copolymers and the chiral homopolymer PCF in methanoladded THF solutions (φ_{MeOH}=0.5, c=4 × 10^{−7} g cm^{−3}). The bisign CD main signals induced in PCF and PC2O8 solutions are positive in the longer wavelength region but opposite in PC6O4 and PC5O5 solutions, and PC8O2 exhibits almost no CD. The sign of the side peak of PC2O8 (at 429 nm) is also opposite to that of PC5O5 (at 426 nm). The insert in Figure 3 shows the x dependence of the Kuhn dissymmetry factor at the CD peak in the region of 400–420 nm. These results demonstrate that the helical screw sense of the chiralachiral random copolymer is inverted twice with changing x. To the best of our knowledge, this is the first observation of the double sense inversions in helical chiralachiral random copolymers.
Discussion
Consider the origin of the double sense inversions in helical chiralachiral random copolymers. For simplicity, assume that the mainchain bond (for example the internal rotation angle) of the helical polymer chain takes P or Mstates (a twostate model). The Phelix (Mhelix) means the sequence of mainchain bonds taking the Pstate (Mstate). In a concentrated solution, a bond taking the Pstate (Mstate) feels the chiral molecular field w̄^{*}(P) (w̄^{*}(M)) generated by neighboring polymer molecules.^{27} If the solution contains P and Mstate bonds of fractions f_{P} and f_{M}, respectively, the molecular fields, w̄^{*}(P) and w̄^{*}(M), may be given by
where w^{*}_{PP}, w^{*}_{MM}, and w^{*}_{PM} are the chiral interactions between two pairs of adjacent monomer units connected by the bonds both taking the Pstate, both taking the Mstate, and taking the P and Mstates, respectively. Because the molecular field may include both enthalpic and entropic contributions, w̄^{*}(P) −w̄^{*}(M) can be regarded as the free energy difference 2ΔG_{h} of a bond when taking the Pstate and Mstate in the concentrated solution. From equation (2), ΔG_{h} is a linear function of the enantiomer excess 2f_{P}−1, for example, ΔG_{h}=κ(2f_{P}−1) + λ where κ and λ are parameters related to the chiral interactions. In the previous study,^{17} it was assumed that λ=0, but w^{*}_{PP} and w^{*}_{MM} are not necessarily identical unless the interacting monomer units are both achiral.
In the case of a chiralachiral random copolymer solution, ΔG_{h} may depend on the kinds of adjacent monomer units connected by the bond under consideration:^{28}
where the subscripts C and A denote the chiral and achiral monomer units, respectively. Because the achiral homopolymer should be racemic at 2f_{P}−1=0, λ_{AA} must be zero. Helical polymers must have long sequences of the onesense helical state along the main chain, or the helix reversal must be a rare event. Thus, the free energy ΔG_{r} of the helix reversal, where the adjacent bonds take the opposite helical state, must be quite high.^{29} In what follows, the dependence of ΔG_{r} on the kinds of adjacent monomer units connected by the bond under consideration is not considered, which should have a minor effect on the helical screw sense inversion.
The enantiomer excess 2f_{P}−1 of the chiralachiral random copolymer chain can be calculated by the matrix method for the Ising model.^{28, 30, 31} The matrix includes the statistical weights of the P and Mstates and the helix reversal, which are respectively written as
where the subscripts i(k) (1 ⩽ k ⩽ N), taking C (the chiral monomer unit)) or A (the achiral monomer unit), specify the copolymer sequence and RT is the gas constant multiplied by the absolute temperature. (This R should be distinguished from the radius R in equation (1)) Generating 100 sequences of chiralachiral random copolymers with a given N and mole fraction x of the chiral unit on a computer, 2f_{P}−1 can be calculated numerically for the given values of ΔG_{h,CC}, ΔG_{h,CA}, ΔG_{h,AA}, and ΔG_{r} in the routine procedure.
The calculated 2f_{P}−1 must fulfill equation (3) for ΔG_{h,CC}, ΔG_{h,CA}, and ΔG_{h,AA}. This selfconsistent calculation can be performed as follows. From equation (3), the relations among ΔG_{h,CC}, ΔG_{h,CA}, and ΔG_{h,AA} are obtained:
A trial value of ΔG_{h,CC} is chosen first, and ΔG_{h,CA} and ΔG_{h,AA} are calculated from equation (5) using a given set of the parameters κ_{CA}/κ_{CC}, κ_{AA}/κ_{CC}, λ_{CC}, and λ_{CA}. Then, 2f_{P}−1 is calculated by the matrix method using those values of ΔG_{h} and a given value of ΔG_{r}, and the resulting 2f_{P}−1 is substituted into the first equation of equation (3) to check the equality. The selfconsistent value of ΔG_{h,CC} that fulfills the equation is sought.
The previous result indicated that 2f_{P}−1 for PCF in the phaseseparating solution is close to zero at 40 °C but takes a positive finite value at 15 °C.^{17} To reproduce these results, κ_{CC}=40 J mol^{−1} and λ_{CC}=2 J mol^{−1} were selected (using these parameters, 2f_{P}−1 for PCF becomes 0.09 at 40 °C and 0.38 at 15 °C). Furthermore, N=220 (the average value of our five polymer fractions) and ΔG_{r}=10 kJ mol^{−1} (a typical value for helical polymers such as polyacetylene or polyisocyanate derivatives^{32, 33}). The remaining parameters in equation (2), κ_{CA}, κ_{AA}, and λ_{CA}, were taken as adjustable parameters. (The κ and λ parameters may depend on the polymer concentration c_{c} in the concentrated phase, but here it is assumed that these parameters are independent of x because c_{c} may be insensitive to x due to similar affinities of the chiral and achiral monomer units to the mixed solvent, mentioned above.).
Figure 4 shows the results of 2f_{P}−1 at 15 °C as a function of ΔG_{h,CC} and x for κ_{CA}/κ_{CC}=−2.5, κ_{AA}/κ_{CC}=3.5, λ_{CC}=2 J mol^{−1}, and λ_{CA}=−4 J mol^{−1}. Slight fluctuations in the calculated 2f_{P}−1 values arise from statistical errors in the chiral monomer content generated. The straight line in Figure 4 represents the linear relation of 2f_{P}−1=(ΔG_{h}−λ_{CC})/κ_{CC} with κ_{CC}=40 J mol^{−1}. The intersecting point of this line and the curve for each x fulfills equation (3). At large x, the curve intersects with the line once, but, in the small x region, there are three intersecting points.
Figure 5 shows the selfconsistent solution of 2f_{P}−1 at 15 °C, obtained from Figure 4, as a function of x. At large x (> 0.227), the calculation gives the unique selfconsistent solution of 2f_{P}−1. When x decreases from unity, 2f_{P}−1 changes from positive to negative (the solution of branch 1), corresponding to the first helical screw sense inversion. However, one negative and two positive solutions appear at x <0.227. Among these solutions, the positive one reaching the origin (labeled as the branch 2) should be the real solution because the achiral homopolymer must be racemic. Therefore, a transition from branch 1 to 2 is expected with decreasing x, which corresponds to the second helical screw sense inversion. This discontinuous transition is an interesting phenomenon. The CD induction at x between 0.2 and 0.5 will be studied experimentally in more detail in the near future.
Conclusions
Methanoladded THF solutions of chiralachiral random copolymers of fluorene derivatives with different chiral monomer content x were studied. The addition of methanol to dilute THF solutions of the copolymers and the chiral homopolymer induced a liquidliquid phase separation that produced concentrated phase droplets with a polymer concentration as high as 0.6 g cm^{−3} and a size of the order of 100 nm. In the concentrated phase, the copolymers and homopolymer exhibited mostly thermoreversible circular dichroism (CD) induction after quenching in the mainchain fluorene absorption region, demonstrating the uneven population of the right and lefthanded helical conformation in the polymer chains. From the sign of the induced CD, two helical screwsense inversions were found by changing x. The Ising model for chirally interacting chiralachiral random copolymers can explain the double screwsense inversions.
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Acknowledgements
Y Sanada thanks the Global Center of Excellence Program, the ‘Global Education and Research Center for BioEnvironmental Chemistry’ of Osaka University. This work was partly supported by a Grant in Aid for Scientific Research on Priority Area ‘Soft Matter Physics.’
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Sanada, Y., Terao, K. & Sato, T. Double screwsense inversions of helical chiralachiral random copolymers of fluorene derivatives in phase separating solutions. Polym J 43, 832–837 (2011). https://doi.org/10.1038/pj.2011.75
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Keywords
 chiralachiral random copolymer
 circular dichroism induction
 helical screwsense inversion
 phase separation
 polyfluorene
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