Racemic synthetic polymers and chirality Gaetano

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Racemic synthetic polymers and chirality

Gaetano Guerra & Paola Rizzo

Rendiconti Lincei SCIENZE FISICHE E NATURALI ISSN 2037-4631 Volume 24 Number 3 Rend. Fis. Acc. Lincei (2013) 24:217-226 DOI 10.1007/s12210-013-0238-0

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Author's personal copy Rend. Fis. Acc. Lincei (2013) 24:217–226 DOI 10.1007/s12210-013-0238-0

CHIRALITY IN CHEMISTRY AND BIOPHYSICS

Racemic synthetic polymers and chirality Gaetano Guerra • Paola Rizzo

Received: 8 February 2013 / Accepted: 10 April 2013 / Published online: 1 May 2013 Ó Accademia Nazionale dei Lincei 2013

Abstract This review is mainly devoted to the chiraloptical behavior in the solid state of racemic synthetic polymers, which are able to form co-crystalline phases with low molecular mass guest molecules and whose chirality is induced by non-racemic (also temporary) guest molecules. The article starts with the description of chiral features relevant for the synthesis of stereoregular polymers. Subsequently, the chirality of chain helicity and the related phenomena of macromolecular amplification of chirality, both in solution and in the solid state, are discussed. The final part of the article describes the recent achievement of robust chiral-optical films being constituted by a racemic commercial host polymer and achiral guest chromophores. Keywords Polymer co-crystalline forms  Racemic polymer host  Chiral-optical behavior  Syndiotactic polystyrene  Poly(2,6-dimethyl-1,4-phenylene)oxide

1 Introduction The chirality of synthetic polymers has been widely studied and many comprehensive reviews have been reported in the literature (Farina 1987; Ciardelli et al. 1989; Green

This contribution is the written, peer-reviewed version of a paper presented at the conference ‘‘Molecules at the Mirror—Chirality in Chemistry and Biophysics’’, held at Accademia Nazionale dei Lincei in Rome on 29–30 October 2012. G. Guerra (&)  P. Rizzo Dipartimento di Chimica e Biologia, INSTM and NANOMATES Research Units, Universita` degli Studi di Salerno, via Ponte Don Melillo, 84084 Fisciano, SA, Italy e-mail: [email protected]

et al. 1999, 2003; Yashima et al. 1999; Cornelissen et al. 2001; Angiolini et al. 2002; Yashima and Maeda 2008). This contribution will be mainly confined to the chiraloptical behavior in the solid state of racemic synthetic polymers, which are able to form co-crystalline phases with low molecular mass guest molecules and whose chirality is induced by non-racemic (also temporary) guest molecules. Crystalline phases are extremely relevant for properties and applications of many polymeric materials and their amount, structure and morphology constitute the main factors controlling physical properties of fibers, films and thermoplastics (Tashiro and Tadokoro 1990; Corradini and Guerra 1992; Corradini et al. 2006). Different has been the destiny of polymeric co-crystalline forms, i.e., structures where a polymeric host and a low molecular mass guest are co-crystallized (Yokoyama et al. 1969; Kusuyama et al. 1982; Paternostre et al. 1999; Matsumoto et al. 2000; De Girolamo Del Mauro et al. 2003; Galdi et al. 2010). In fact, although they were early recognized, co-crystalline polymeric forms have been for many decades ignored in material science and only in last few years, films where active molecules are present as guests of polymer cocrystalline phases have been proposed for advanced applications (Uda et al. 2005; De Girolamo Del Mauro et al. 2007; Stegmaier et al. 2005; Kaneko et al. 2006; D’Aniello et al. 2007; Albunia et al. 2009; Daniel et al. 2011b). The formation of co-crystalline phases allows reduction of diffusivity of active molecules (fluorescent, photoreactive, magnetic and ferroelectric) in solid polymers and the prevention of self-aggregation, more easily than with the classical methods based on polymerization of suitable monomeric units or covalent grafting. In the last few years, it has been also discovered that polymer co-crystallization induced by non-racemic guest

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molecules can produce large and extremely stable circular dichroism (CD) phenomena (Buono et al. 2007; Guadagno et al. 2008; Rizzo et al. 2010a, b, 2011; Zheng et al. 2011). This allows an easy production of optically active transparent films, whose CD peaks can be controlled by the choice of (also achiral) chromophore guest molecules (Rizzo et al. 2011). In the first part of this article, we shortly recall that chirality is relevant already for the synthesis of stereoregular polymers. In the central part of the manuscript, the chirality of chain helicity and the related phenomena of macromolecular amplification of chirality, both in solution and mainly in the solid state, are discussed. The final part of the article will describe the achievement of robust chiral-optical films being constituted by a racemic commercial host polymer (syndiotactic polystyrene) and achiral guest chromophores.

2 Chirality of stereospecific polymerization catalysts The phenomena of macromolecular amplification of the chiral-optical response, which will be discussed in Sects. 4–6, are related to possible formation of long ordered helices. As for synthetic polymers, an essential prerequisite for the formation of ordered helices is the presence of regular chains, which have also to be stereoregular if stereogenic centers are present. Many industrially relevant thermoplastic polymeric materials (isotactic polypropylene, isotactic poly-1-butene, syndiotactic polypropylene, syndiotactic polystyrene) and rubbers (polybutadiene, polyisoprene) are based on stereoregular polymers, which are produced by stereospecific heterogeneous and homogeneous Ziegler–Natta catalysts (Brintzinger et al. 1995; Resconi et al. 2000; Corradini et al. 2004). For instance, isotactic poly-1-alkenes can be produced by heterogeneous TiCl3-based or supported TiCl4/MgCl2 catalysts activated by aluminum alkyls. The active sites of polymerization (Fig. 1), due to the octahedral coordination of the chlorine atoms to the metal, are chiral (Cossee 1964; Allegra 1971) and their chirality can be labeled by the nomenclature used for octahedral complexes with two bidentate ligands (K, D chirality) (Corradini et al. 1979, 1983; Toto et al. 2000). Of course, these chiral catalysts are always racemic, because it is not possible to separate K and D sites. Isotactic as well as syndiotactic polymers can be easily produced with homogeneous Ziegler–Natta (more commonly indicated as metallocene and post-metallocene) catalysts. Some of these stereospecific catalysts present, as their main element of chirality, the last tertiary C atom of the

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Fig. 1 Lateral termination of a TiCl3 structural layer. Active sites for 1-alkene polymerization are associated with titanium atoms (black filled circles) on the lateral termination. Due to the octahedral coordination of the chlorine atoms (empty circles) to the metal, these sites are chiral and their chirality can be labeled as K or D

growing chain, which is determined by the chirality of monomer coordination in the last insertion step. This chirality can determine the chirality of the coordination to the metal of the other ligands. Such kind of mechanism has been for instance proposed for syndiotactic polymerization of styrene (Minieri et al. 2001). Particularly interesting are stereorigid homogeneous catalysts for isospecific or syndiospecific poly-1-alkene polymerization (Wild et al. 1982; Ewen 1984, 1988; Pino et al. 1987; Resconi et al. 2000). For instance, the isospecific C2 symmetric Zr complex with a bridged bisindenyl ligand exhibits a chiral coordination of the bridged ligand ((R)–(R) in Fig. 2). This chirality remains unaltered for the whole polymerization process and imposes a chiral orientation of the polymer growing chain (#1 & -60° is allowed while #1 & ?60° is forbidden), which in turn favors the insertion of the re enantioface of propene (Corradini et al. 2004). These stereorigid homogeneous chiral catalysts can be racemic but can be also prepared as non-racemic (optically active) catalysts. As for the chirality of the obtained polymers, it is worth adding that, independently of the nature (heterogeneous or homogeneous) and of the chirality (racemic or optically active) of the catalyst, for stereospecific polymerization of achiral, pro-chiral or racemic monomers, no optical activity is observed. This, in fact, has been clearly stated by an early review of Mario Farina: ‘‘Although a nearly perfect asymmetric synthesis was necessary to produce identical configurations for pendant groups, the fact that in a very long polymer the chain ends are indistinguishable led to absence of optical activity (cryptochirality)’’ (Farina 1971). Optically active stereoregular polymers can be instead obtained by polymerization of optically active monomers (e.g., 3-methyl-pentene-1) with optically active homogeneous catalysts (Pino et al. 1960). This manuscript, however, will mainly deal with chiraloptical behavior of standard racemic polymers, whose

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both in solution (Nakako et al. 1999; Yashima and Maeda 2008) as well as in the solid state (Natta et al. 1956, 1960). The formation of polymer helices has been mainly studied in crystalline phases. Just as an example, the righthanded and left-handed ternary helices described for isotactic polypropylene (i-PP), in the early study of Natta and Corradini, are compared in Fig. 3a, while the along the chain view of the crystal structure of the most important i-PP polymorph (a form) is shown in Fig. 3b. Of course, in general for racemic polymers, there is an equal amount of right-handed and left-handed helices, both in solution and in the crystalline phases. This is shown, for instance, by the succession of layers constituted by righthanded (R) and left-handed (L) helices, in the crystal structure of the a form of Fig. 3b.

Fig. 2 C2 symmetric Zr complex with a bridged bis-indenyl ligand, exhibiting (R)–(R) chirality of coordination. The chiral coordination of the bridged ligand imposes a chiral orientation of the polymer growing chain (01 & -60°), which in turn favors the insertion of the re enantioface of propene. A sequence of re insertions leads to isotactic polymer

4 Macromolecular amplification of chirality in solution An impressive example of amplification of optical activity induced by prevailing spiral hand of non-racemic polymer chains is provided by the polymerization of a non-racemic monomer, (R)-1-deuterio-n-hexyl isocyanate, exhibiting a very low optical activity ([a]D = -0,43°). In fact, the corresponding non-racemic polymer, poly(R)-1-deuterio-nhexyl isocyanate, exhibits a much higher optical activity ([a]D = -367°) (Green et al. 1988) (Fig. 4).

right-handed and left-handed helices present identical stability.

3 Chirality of macromolecular helicity Regular and stereoregular polymers can assume (at least for short chain stretches) ordered helical conformations, Fig. 3 a Top and lateral views of right-handed and left-handed ternary helices of i-PP. b Along the chain view of the a form of i-PP, exhibiting layers of righthanded and left-handed helices

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interactions with optically active low molecular mass molecules is more recent (Buono et al. 2007). It is associated with the formation of polymer co-crystalline phases (Guerra et al. 2012), where the guest molecules are nonracemic. Because regular helical stretches in crystalline phases are generally much longer than in polymer solutions, it is not surprising that the corresponding chiral amplification phenomena are generally much more intense. The macromolecular amplification of chirality in polymer co-crystalline phases can be produced by molecular and supramolecular mechanisms. According to a molecular mechanism, a non-chiral guest induces the formation of non-racemic crystals with non-racemic unit cell where polymer chains exhibit only one sense of helicity. This kind of behavior has been clearly shown for poly(2,6-dimethyl-1,4-phenylene)oxide (PPO). A supramolecular mechanism occurs when the non-chiral guest induces the formation of non-racemic helical crystallites, whose unit cell includes both right- and left-handed polymer helices. This supramolecular mechanism has been presently observed only for syndiotactic polystyrene (s-PS).

C D H

NaCN

Fig. 4 A schematic representation of (R)-1-deuterio-n-hexyl isocyanate (left) and of poly(R)-1-deuterio-n-hexyl isocyanate (right)

It is also well known that, for racemic polymers, a chiraloptical behavior can be induced by non-covalent interactions with optically active low molecular mass molecules in solution. An example is taken, again, from the research activity of the group of Prof. Mark Green (Green et al. 1993). The achiral poly(n-hexyl-isocyanate) presents chiral-optical responses when dissolved in many different non-racemic solvents (Fig. 5).

5 Macromolecular amplification of chirality in polymer co-crystalline phases

5.1 Molecular mechanism (PPO) The molecular mechanism of macromolecular chiral amplification in the solid state is exemplified in this review

The observation of chiral-optical response induced in racemic polymers in the solid state by non-covalent Fig. 5 CD spectra of poly(nhexyl isocyanate) dissolved in optically active solvents at 20 °C. The chemical structure of poly(n-hexyl isocyanate) is shown on the top

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Fig. 6 a Projection along c of the unit cell (a = b = 1.19 nm and c = 1.71 nm) of the cocrystalline form of PPO with (1S)-(-)-a-pinene, which includes all left-handed (L) polymer helices; b CD spectra of PPO films exhibiting co-crystalline phases with (1S)(-)-a-pinene (thin line) or (1R)(?)-a-pinene (thick line), with an a-pinene content close to 30 wt%

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A by the formation of a co-crystalline form of PPO (an industrially relevant polymer, Toi et al. 1982; Daniel et al. 2011a; Galizia et al. 2012), with a chiral guest (a-pinene) (Tarallo et al. 2012). The PPO/a-pinene co-crystalline structure presents a tetragonal unit cell (a = b = 1.19 nm, c = 1.71 nm, Horikiri 1972) and a monomer unit/guest molar ratio equal to 2:1. By co-crystallization of the polymer with the racemic guest, racemic crystallites are obtained, whose unit cell includes all left- or right-handed polymer helices and (1S)-(-) or (1R)-(?) a-pinene guest molecules, respectively (Fig. 6a, Tarallo et al. 2012). Moreover, by co-crystallization of the polymer with nonracemic guests, co-crystals exhibiting helices with only one sense of spiralization are induced. This induction of macromolecular helicity by non-chiral guests leads to amplification of chirality, as clearly confirmed by vibrational circular dichroism (VCD) measurements and by intense CD Cotton bands of the polymer in the UV region (Fig. 6b). The higher thermodynamic stability of the non-racemic unit cell produces chiral-optical phenomena independently on the co-crystallization route. Moreover, as expected, crystalline phase transitions, leading to the loss of the helical conformation, also lead to the loss of chiral-optical response. 5.2 Supramolecular mechanism (s-PS) s-PS co-crystalline forms exhibit, as a common feature, s(2/1)2 helical polymer conformation (Fig. 7), with a repetition period of nearly 0.78 nm. However, the packing of the host helices and of the guest molecules can largely change, mainly depending on the molecular structure of the guest molecules and also on the preparation procedure. The large number of s-PS co-crystalline forms can be divided in three classes: d clathrates, intercalates (also named d intercalates) and e clathrates.

B d Clathrates present isolated centrosymmetric guest locations, cooperatively generated by two enantiomorphous helices of two adjacent ac polymer layers (Fig. 7a, b). d Clathrates can be divided in two different subclasses: monoclinic (Fig. 7a–a0 ) (Chatani et al. 1993; De Rosa et al. 1999) and triclinic (Fig. 7b–b0 ) (Tarallo et al. 2009, 2010a). d Intercalates are characterized by the same layers of alternated enantiomorphous s(2/1)2 polymer helices (Fig. 7a–c), but the spacing between the ac layers (d010) is larger than 1.3 nm and values as high as 1.75 nm have been observed. In these co-crystals, the guest molecules are not isolated into host cavities but form contiguous inside layers, intercalated with the polymer layers (Petraccone et al. 2005; Tarallo et al. 2006). e Clathrates (Petraccone et al. 2008; Tarallo et al. 2010a, b) include guest molecules in channels formed between enantiomorphous s(2/1)2 polymer helices (Fig. 7d–d0 ). The most relevant structural features of these s-PS co-crystals are that suitable guests are not only small molecules but also long molecules (Petraccone et al. 2008). Co-crystallization of s-PS with racemic or non-racemic chiral guests leads to indistinguishable crystalline phases, which include an equal number of right-handed and lefthanded helices. In fact, solution crystallization of s-PS in non-racemic solvents leads to films whose chiral-optical response is negligible. However, robust chiral-optical films, are easily obtained by co-crystallization of amorphous s-PS films (Rizzo et al. 2010a, b), as induced by sorption of many volatile nonracemic molecules (Buono et al. 2007). The induced CD presents a major Cotton band at 200 nm and a minor Cotton band of opposite sign at 223 nm (Figs. 8, 9). The obtained chiral-optical response of the polymer films remains essentially unaltered as a consequence of the nonracemic guest removal leading to the nanoporous crystalline d phase (Fig. 8, blue solid lines) as well as after thermal treatments (up to 140 °C) leading to the dense

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Fig. 7 Schematic projections along the chain (a–d) and perpendicular to the chain (a0 – d0 ) of s-PS co-crystalline forms: (a, a0 ) monoclinic d clathrate with 1,2-dichloroethane; (b, b0 ) triclinic d clathrate with 1,4dinitrobenzene; (c, c0 ) intercalate with norbornadiene; (d, d0 ) e clathrate with p-nitroaniline

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Wavelength (nm) Fig. 8 Room temperature CD spectra of spin-coated amorphous s-PS films, as co-crystallized with (-)-(R)-carvone (thick lines) or (?)-(S)carvone (thin lines) by vapor exposure: after complete carvone removal by supercritical carbon dioxide, exhibiting the helical nanoporous crystalline d form; annealed at 140 °C, exhibiting the helical dense c form (dashed lines); annealed at 240 °C, exhibiting the trans-planar a form; annealed at 280 °C, i.e., crystallized on cooling after s-PS melting (horizontal line)

helical c phase (Fig. 8, red dashed lines) or after thermal treatments (up to 240 °C) leading to the trans-planar a phase (Fig. 8, black solid lines). The memory of the volatile non-racemic guest molecules can be erased only after thermal treatments at temperatures higher than the s-PS melting temperature (&270 °C, Fig. 8, green line) or by long-term treatments with strong s-PS solvents. Because the d phase has a well-known crystalline structure (De Rosa et al. 1997), which includes an equal amount of right-handed and left-handed helices, while the a crystalline structure (De Rosa et al. 1991) does not

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Fig. 9 Schematic representation of the supramolecular mechanism of chiral amplification. The formation of chiral crystallites as a consequence of co-crystallization of an amorphous s-PS film with non-racemic guest molecules leads to intense CD phenomena, which remain stable also after complete guest removal and helical ? transplanar transition of the polymer host chains. The formation of helical crystallites (stable for thermal treatments up to the polymer melting) are shown as AFM images in the lower part

include any polymer helices, it is necessary to conclude that the CD phenomena observed for s-PS are not associated with non-racemic molecular structures but with the formation of non-racemic supramolecular crystalline morphologies (Fig. 9). Contrary to the case of the molecular mechanism of the previous section, alternative co-crystallization routes different from the direct co-crystallization of amorphous samples with non-racemic guest molecules, lead to negligible chiral-optical phenomena. Moreover, again contrary to the case of molecular mechanism, crystalline phase transitions, leading to the loss of the helical conformation,

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It is relevant adding that the observed chiral-optical response of the achiral guest chromophore occurs only when such molecules are included as guests in the nanoporous crystalline phase. This is shown, for instance, by the FTIR and VCD spectra of Fig. 11, which compare s-PS polymer films as crystallized by sorption of (-)-(R)-carvone in amorphous films (20 lm) and subsequently subjected to different azulene sorption procedures. The thick lines correspond to a co-crystalline film with azulene, as obtained by exchange of the (-)-(R)-carvone guest, but having an azulene content close to 4 wt%. The thin lines correspond to a film exhibiting the s-PS a crystalline phase, as obtained by annealing of a chiral-optical s-PS/(-)-(R)carvone film, with an azulene content close to 2 wt%. The FTIR spectra of Fig. 11 show, beside the azulene peaks (labeled by a), the peaks corresponding to vibrational modes of the s(2/1)2 helix (labeled by h) of the trans-planar s-PS chains (labeled by t) typical of co-crystalline or a form films, respectively. In the latter case, the azulene molecules are simply absorbed in the amorphous phase rather than included as guest in the crystalline phase (see scheme of an a crystallite in Fig. 11). The corresponding VCD spectra shown in Fig. 11 clearly indicate that, for some azulene peaks, intense VCD peaks are maintained when the azulene molecules are guest of the d clathrate phase. On the other hand, for chiral-optical a form films presenting the azulene molecules simply absorbed in the amorphous phase, VCD phenomena remain intense for peaks of the trans-planar polymer chains of the crystalline phase while are negligible for the azulene peaks (thin line in Fig. 11).

do not lead to any significant loss of chiral-optical response.

6 Chiral-optical films with achiral (racemic) host polymer and achiral guest chromophores Intense CD and VCD phenomena have been recently induced also to achiral chromophores by their inclusion as guest of s-PS clathrates (Rizzo et al. 2011). In particular, these unexpected CD and VCD phenomena are observed provided that (i) the initial crystallization of s-PS has been induced by a non-racemic guest from an amorphous phase and (ii) the chromophore molecules are suitable guest of a s-PS co-crystalline form. For instance, the CD behavior, as achieved in the visible region, for azulene molecules being guest of a cocrystalline phase with s-PS, whose initial crystallization has been induced by (-)-(R)-carvone or (?)-(S)-carvone, is shown in Fig. 10a. X-ray diffraction characterizations have shown the formation of a s-PS/azulene monoclinic d clathrate, including both R and L helical polymer chains (Fig. 10b). This confirms the hypothesis (Buono et al. 2007) that CD phenomena induced in amorphous s-PS films by the temporary sorption of non-racemic guests are not due to the formation of non-racemic unit cells but to the formation of non-racemic morphologies of co-crystalline phases, which are maintained also after solvent or thermal treatments leading to different crystal-to-crystal transformations. These results allow achieving s-PS-based films with chiral-optical responses at many different wavelengths. In this respect, it is worth adding that s-PS can be easily melt processed leading not only to films but also to solid samples of any shape, which can be made fully amorphous by simple quenching procedures. As a consequence, materials and devices exhibiting chiral-optical responses in selected wavelengths, based on s-PS, can be easily designed and produced.

Intense chiral-optical response can be induced in racemic polymers in the solid state, by non-covalent interactions with optically active low molecular mass molecules leading to formation of polymer co-crystalline phases.

R 300 200

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Fig. 10 a CD spectra of s-PS/ azulene co-crystalline films, as obtained by (-)-(R)-carvone (thick solid line) or (?)-(S)carvone (thin dashed line) induced crystallization, followed by complete exchange of the non-racemic carvone with the achiral azulene guest. b Schematic along the polymer chain projection of the s-PS/ azulene monoclinic d clathrate structure

7 Conclusions

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Fig. 11 FTIR and VCD spectra of amorphous s-PS films after crystallization induced by sorption of (-)-(R)-carvone: (thick line) after exchange of the non-racemic carvone guest with the achiral azulene guest, with an azulene content close to 4 wt% mainly as guest of the d clathrate phase (schematically shown on the center); (thin line) after thermal annealing leading to the formation of the trans-

planar a crystalline phase followed by azulene sorption, close to 2 wt%, only in the amorphous phase (as schematically shown on the right). The main peaks of the azulene guest molecules are labeled by a. The main peaks of the helical and trans-planar chains of s-PS are labeled as h and t, respectively

This macromolecular amplification of chirality in polymer co-crystalline phases can be produced by both molecular and supramolecular mechanisms. According to a molecular mechanism, which has been observed for PPO, a non-chiral guest induces the formation of non-racemic crystals, where polymer chains exhibit only one sense of helicity. In this case, the higher thermodynamic stability of the non-racemic unit cell produces chiral-optical phenomena independently on the co-crystallization route. Moreover, crystalline phase transitions, leading to the loss of the helical conformation, also lead to the loss of the chiral-optical response. An alternative supramolecular mechanism for macromolecular amplification of chirality, which has been observed only for s-PS, occurs when the non-chiral guest induces the formation of non-racemic helical crystallites, whose unit cell includes the same amount of right- and lefthanded polymer helices. In this case, contrary to the case of the molecular mechanism, co-crystallization routes different from the direct co-crystallization of amorphous samples with non-racemic guest molecules lead to negligible chiraloptical phenomena, while the loss of the helical conformation does not lead to any significant loss of chiral-optical response. The chiral-optical behavior is lost only as a consequence of melting of the chiral crystallites. Particularly relevant is the permanence of the chiraloptical response of s-PS films also after substitution of the optically active guest with achiral or racemic chromophores. These films induce a chiral-optical response also for the absorbances of achiral molecules, like azulene or pnitroaniline. This opens, in principle, the possibility to

achieve polymer films with chiral-optical response at any desired wavelengths, always using the same commercially available host polymer.

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Acknowledgments We thank Prof. L. Cavallo, Prof. L. Guadagno, Prof. V. Venditto and Dr. C. Daniel of University of Salerno and Prof. C. De Rosa, Prof. V. Petraccone and Dr. Oreste Tarallo of University of Naples for the contributions to the reviewed research work. Financial support of the ‘‘Ministero dell’Istruzione, dell’Universita` e della Ricerca’’ (PRIN) and of ‘‘Regione Campania’’ (CdCR) is gratefully acknowledged.

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