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Spontaneous Formation of Left- and Right-Handed Cholesterically Ordered Domains in an Enantiopure Chiral Polyfluorene Film Matteo Savoini,†,|| Paolo Biagioni,† Stefan C. J. Meskers,‡ Lamberto Duo,† Bert Hecht,§ and Marco Finazzi*,† †
CNISM - Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy Molecular Materials and Nanosystems, Eindhoven University of Technology, P.O. Box 513, NL 5600 MB Eindhoven, The Netherlands § Nano-Optics & Biophotonics Group, Department of Experimental Physics 5, R€ontgen Research Center for Complex Material Systems (RCCM), Physics Institute, University of W€urzburg, Am Hubland, D-97074 W€urzburg, Germany ‡
ABSTRACT: Thermally annealed chiral polyfluorene films are studied by circular differential optical microscopy. We observe the presence of micrometer-sized domains displaying circular dichroism of opposite sign. Our findings suggest the spontaneous occurrence of left- and right-handed cholesterically ordered domains from an enantiopure precursor. The role of enantiopure stereo centers in the side chains of the polymer is to lower the energy of the left-handed cholesteric domains with respect to the righthanded ones. SECTION: Molecular Structure, Quantum Chemistry, General Theory
C
ollective phenomena leading to chiral organization of molecules are of fundamental interest in physics and chemistry, and might have important implications for biology and life sciences. For example, the puzzling occurrence of homochirality in biomolecules has been stirring a lively debate in the scientific community.1 Chirality effects in the mesoscopic self-organization of polymeric materials have also been the subjects of many studies addressing, for instance, spontaneous chiral symmetry breaking.24 Isotactic polypropylene, to give an example, can crystallize in a β-phase in which homohelical chains cluster into isochiral domains5,6 despite containing an equal number of R and S stereo centers. Chiral symmetry-breaking phenomena have also been observed in the smectic phase of molecules having an inversion symmetry plane.7 A further case of an intramolecular cooperative phenomenon leading to dramatic chiral effects is described by the so-called “majority rule”. It is observed in copolymers consisting of a mixture of opposite enantiomers, which respond sharply to slight differences in the enantiomer concentrations. The emerging optical activity in such systems is undistinguishable from the one exhibited by enantiopure samples, fully determined by whichever enantiomer is in the majority.8,9 The mentioned examples highlight the profound effect of intermolecular interactions and long-range organization on the mesoscopic chiral properties of a polymer compound. Here we report a novel phenomenon, namely, the spontaneous formation of domains exhibiting opposite chiroptical properties within an enantiopure polyfluorene polymer film. In the following we describe how optical absorption allows for direct imaging of the mesoscopic domains in the enantiopure polyfluorene film. We observe the coexistence of two types of laterally defined domains displaying a similarly large circular dichroism (CD) r 2011 American Chemical Society
amplitude but with opposite sign interpreted in terms of opposite-handed long-range supramolecular chiral order. In order to address this issue, we employ scanning CD microscopy. This technique provides a powerful tool to image macromolecular organization and mesoscopic chiral ordering with spatial resolution on the submicrometer length scale, e.g., to investigate the macromolecular organization of DNA.1013 Chiral polyfluorene films show exceptionally large degrees of CD in the blue region of the visible spectrum after thermal annealing.14 Diffraction studies on polyfluorene with branched side chains indicate that the lowest energy conformation of its backbone is helical, and thus chiral, even in the absence of enantiopure stereo centers.15,16 The sample is a film of chiral poly[9,9-bis((3S)-3,7-dimethyloctyl)2,7-fluorene] (1, inset of Figure 1). The enantiomeric excess of the chiral side chains is larger than 95%, as specified by the supplier (Sigma-Aldrich). Given the Mn of the polymer (20 kg/mol), each individual chain could incorporate a few (up to ∼3) RS fluorene units. A small fraction of all chains (0.1) may have one RR fluorene unit. In solid films, this compound shows a pronounced absorption line in the blue region of the visible spectrum, originating from the lowest-excited singlet state positioned 3 eV above the ground state.14 A solution of 1 has been prepared according to refs 17 and 18 and spin-coated onto a coverslip glass substrate, previously cleaned in acetone, to obtain a 200 nm thick solid film. The film has been annealed at 120 C for 1 h in vacuum (pressure lower than 106 mbar) to avoid oxidation and has been successively cooled down to room temperature with a cooling rate of Received: April 18, 2011 Accepted: May 13, 2011 Published: May 13, 2011 1359
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Figure 2. Maps showing a typical spatial distribution of the amplitude of (a) the dissymmetry ratio g and (b) the associated phase φ measured by CD microscopy in a 200 nm-thick solid film of 1. Figure 1. Sketch of the experimental setup for scanning CD microscopy. Inset: Natta projection of the molecule 1.
about 0.4 C/s. During the annealing process, the polyfluorene is brought into its liquid crystalline state, and subsequent cooling to room temperature vitrifies the induced molecular arrangement. The large CD observed after annealing derives from longrange chiral organization within the film, presumably resulting from cholesteric ordering.17,18 Based on a comparison with the CD characterization of chiral polyfluorene oligomers at a wavelength of 405 nm, a (predominantly) left-handed cholesteric arrangement for annealed films of 1 has been proposed.19,20 As a scanning CD microscope we modified (as sketched in Figure 1) a commercially available microscope (R-SNOM, WITec GmbH). The continuous-wave radiation emitted by a vertical-cavity surface-emitting laser (405 nm wavelength) is linearly polarized by a Glan-Taylor polarizer (extinction ratio 1:106) along a direction forming an angle θ = 45 with respect to the normal axes of a photoelastic modulator (see Figure 1, Hinds Instruments PEM-90) working as a variable waveplate with a modulation frequency ω = 50 kHz. The modulated beam is then focused on the sample by a strain-free objective (NA = 0.4, Nikon CFI LWD P 20 ). The sample is raster-scanned under the beam with the use of a piezoelectric stage (Physik Instrumente P-517 3CL). The transmitted light is collected with a second objective (NA = 0.4, Nikon CFI Achro LWD 20), and, after passing a pinhole for background rejection, is detected by a photomultiplier tube (Hamamatsu H7732). A lock-in amplifier is employed to retrieve the amplitude and phase of the signal harmonics at multiples of the modulation frequency.
The measurement of the CD is performed by analyzing the transmitted light intensity for the two opposite states of circular polarization. A common way to evaluate the degree of CD in a sample is to measure the dissymmetry ratio g, defined as14 g ¼ jgjexpð iφÞ ¼
IL IR ðIL þ IR Þ=2
ð1Þ
where IL(R) stands for the transmitted intensity of the left (right) circularly polarized light. The phase assumes only two values, φ = 0 or φ = π, since the dissymmetry ratio g is a real (positive or negative) quantity. The amplitude of the signal demodulated at the fundamental frequency ω by the lock-in amplifier is directly proportional to the numerator of the right-hand term in eq 1, while the nondemodulated signal (also acquired) is proportional to the denominator. In analogy to the results discussed in ref 21, g varies considerably in amplitude over the surface of the film. In Figure 2 we present maps showing the typical amplitude (|g|) and phase (φ) distribution of the measured dissymmetry ratio g. Many different areas have been investigated in different positions on the film, all displaying a very similar optical behavior as the one reported in Figure 2. The amplitude ranges between 0 (corresponding to sample areas where the absorption coefficient is the same for leftand right-circularly polarized light) and 1.3, to be compared with a maximum allowed value of |g| equal to 2 for a totally dichroic film (i.e., for a film that completely absorbs light in one of the two circular polarization states). By averaging g over the image we obtain Ægæ = 0.37, in agreement with previously reported measurements.14,21 1360
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Figure 3. Histogram of the probability distribution of g.
is not the topic of the present work and will be addressed by a forthcoming study. Here we would like to stress that the observed imbalance in the cholestric organization suggests that the presence of the enantiopure stereo centers comprised in the polymer chains makes the two long-range cholesteric arrangements with opposite helicity energetically nonequivalent. In conclusion, by means of CD microscopy we find evidence for the coexistence and stability of cholesterically ordered domains with opposite handedness within a film of enantiopure chiral polyfluorene. Maps of the CD over large areas of the film indicate that domains with left-handed cholesteric organization (associated with φ = 0, in Figure 2b) predominate. We attribute this behavior to the presence of the enantiopure stereo centers in the side chains of the polymer stabilizing the left-handed cholesteric organization.
’ AUTHOR INFORMATION Corresponding Author
*E-mail: marco.finazzi@fisi.polimi.it. Present Addresses
)
We would like to stress that the features observed in the maps reporting the spatial variations of |g| and φ are not correlated to the sample topography and, hence, to the sample thickness (as demonstrated by near-field optical microscopy in ref 21). Moreover, by a careful characterization of the signal coming from one area of exposed substrate, together with the study of the signals in correspondence with modulation settings sensitive to linear dichroism and/or birefringence, we have also quantified possible spurious contribution to |g| associated to such phenomena.2224 Such a contribution, if any, is less than 0.01, a value about 2 orders of magnitude smaller than the typical CD signal from the film of 1. The interesting point to be understood is how the chiral side chains in this enantiopure material influence molecular organization. The two possibilities that can be envisaged are that the enantiopure stereocenters in the side chains solely promote one particular helicity or that they merely shift the balance in the population of left- and right-handed intermolecular arrangements, which would both remain intrinsically stable.25 An interesting previously unobserved feature21 is presented in Figure 2b, where one can clearly recognize a domain structure, in which each domain corresponds to a region exhibiting either a positive (φ = 0) or a negative (φ = π) value of the dissymmetry ratio g. The presence of domains showing opposite g values in the same film prepared from an enantiopure compound is a clear indication that the chiroptical properties of the sample are determined by long-range cholesteric ordering rather than by the helicity of the single molecules. This conclusion is corroborated by the large absolute magnitude of the g values observed in each type of domain. In fact, chiral organization with short correlation length is known to give rise to much smaller g-values.26 We are thus able to assert that the presence of the stereo centers in the enantiopure material does not prevent the possibility that both types of left- and right-handed intermolecular cholesteric arrangements are formed. On the basis of a comparison with the cholesteric phase of a fluorene oligomer,19,20 the domains corresponding to preferential absorption of right (left) circularly polarized light can be assigned to a left- (right-) handed cholesteric organization. The coexistence of both left and right helically ordered domains indicates that they are both intrinsically stable (or metastable). As can be noted already from Figure 2, a feature confirmed by the analysis of different areas of the polymer is that the two populations of domains with opposite CD are not balanced. There is in fact a predominance of the domains corresponding to g > 0 (φ = 0). Moreover, these domains exhibit, on average, larger |g| values than those with g < 0 (φ = π), as demonstrated by Figure 3, reporting the probability distribution of the g value over the film. Neither domains nor dichroic behavior are observed in nonannealed samples, while annealing above the glass transition temperature promotes the formation of domains with opposite chirality. For a sufficiently long annealing time, the film is left in a state consisting of only one domain covering the entire film area and showing a large positive g value (φ = 0).14,21 This behavior suggests that the existence of domains showing opposite CD is a kinetic phenomenon (the film simply has not been annealed long enough for the “wrong” chirality to disappear) rather than a thermodynamic phenomenon (the influence of the side chains is so weak that both types of domains are stable at the annealing temperature). The detailed study of the evolution of the statistical properties of the domains as a function of the annealing time
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Radboud University, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands.
’ ACKNOWLEDGMENT Financial support by Fondazione Cariplo through the project PONDER (Rif. 2009-2726), by MIUR through the PRIN project No. 2008J858Y7, and by the European Union (EU) Nano SciEuropean Research Associates (ERA) project FENOMENA is gratefully acknowledged. ’ REFERENCES (1) Meierhenrich, U. Amino Acids and the Asymmetry of Life; Springer: New York, 2008. (2) Chela-Flores, J. The Origin of Chirality in Protein Amino Acids. Chirality 1994, 6, 165–168. (3) Fasel, R.; Parschau, M.; Ernst, K.-H. Amplification of Chirality in Two-Dimensional Enantiomorphous Lattices. Nature2006, 439, 449–452. (4) Noorduin, W. L.; Bode, A. A. C.; van der Meijden, M.; Meekes, H.; van Etteger, A. F.; van Enckervort, W. J. P.; Christianen, P. C. M.; Kaptein, B.; Kellogg, R. M.; Rasing, Th.; Vlieg, E. Complete Chiral 1361
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