Natural α-Amino Acid-Functionalized Poly(phenyleneethynylene)s

Article pubs.acs.org/Macromolecules

Natural α‑Amino Acid-Functionalized Poly(phenyleneethynylene)s (PPEs): Synthesis and Chiroptical Characterization of Aggregate States Claudio Resta, Gennaro Pescitelli,* and Lorenzo Di Bari* Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Moruzzi 3, 56124, Pisa, Italy S Supporting Information *

ABSTRACT: The synthesis of several novel poly(phenyleneethynylene)s (PPEs) functionalized with different natural α-amino acids methyl esters has been achieved through Cassar−Heck−Sonogashira reaction. Five different derivatives have been prepared varying the nature of the amino acid (Gly, Leu, Nmethyl Leu, Phe, and Val), and their aggregation behavior has been investigated by means of UV−vis absorption, circular dichroism (CD), and fluorescence spectroscopies in different conditions of aggregation. ECD measurements provided unique information about the structural organization of the aggregates dispersed in solution and as thin films. The effects of the nature of the amino acidic moiety, the consequences of chirality, and the role played by intermolecular hydrogen bonds have been elucidated.

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INTRODUCTION Poly(phenyleneethynylene)s (PPEs) represent one of the most important classes of conjugated polymers.1 Fine-tuning of their semiconducting features by proper functionalization of the phenylene rings of the main chain allows one to tailor these materials for specific applications in the field of organic optics and electronics.2,3 The most successful application of PPEs has been encountered in the field of (bio)sensing, where they act as “molecular wires” capable of signal transduction and amplification, often based on the strong photoluminescence of these materials.4,5 This latter property, combined with good electric conductivity and optical stability, is also the basis for the proficient use of PPEs in organic light-emitting diodes (OLEDs).6 The fundamental properties of PPEs and other organic conjugated polymers, such as light absorption and emission, charge and exciton transport, and so on, depend not only on their molecular structure but also on the supramolecular interactions and on the nano/mesoscale organization of the polymer chains in the solid state.7−9 As a useful and practical model of solid-state aggregates, it is a common practice to investigate the aggregates formed in solution of proper solvents or solvent mixtures. Detailed spectroscopic studies on the nature of solution aggregates have been performed for several alkyl- and alkoxy-substituted PPEs.10−19 While good intermolecular communication, which takes place through π-stacking, is mandatory for efficient exciton and charge transport,7 too strong interactions may significantly reduce luminescence quantum yields.20 Various strategies have been developed to control polymer interchain spacing and/or alignment.21,22 The introduction of stereodefinite chiral elements in conjugated polymers23−25 has been exploited as a structural modification able to generate a regular twist or © 2014 American Chemical Society

oblique angle between proximate chains, disrupting the perfect cofacial stacking. Moreover, it gives the opportunity to employ chiroptical spectroscopic techniques for in-depth investigations of the structural features of these aggregates, including the occurrence of multiple aggregation modes.26,27 To this end, the chiral pool represents an obvious source of enantiopure compounds with various functional groups and structural motifs. Indeed, PPEs decorated with biomolecules have been reported as potential semiconducting or highly fluorescent active materials in biosensing applications.28−34 In this respect, two structural investigations have been previously reported by our group on two glucose-appended and a L-phenylalaninefunctionalized PPEs.35,36 With the aim of designing new chiral PPE derivatives, we were attracted by proteinogenic α-amino acids because of their availability, low cost, and widespread applications in the field of molecular recognition, owing to their ability to form multiple noncovalent interactions, often leading to stereodiscrimination properties.37 When amino acid moieties are attached to a conjugated backbone, they provide the possibility to transfer their asymmetry (discrete central chirality) to the entire system (supramolecular chirality), especially through hydrogen-bonding interactions.9,38,39 Of particular interest, as far as amino acid-functionalized PPEs are concerned, is the interplay between (a) hydrophobic or other solvophobic interactions, responsible for the aggregation of conjugated polymers, (b) selective and directional hydrogen bonds provided by the amide groups, and (c) the variable steric bulk provided by the Received: August 6, 2014 Revised: September 29, 2014 Published: October 14, 2014 7052

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The formation of aggregates typical of PPEs was achieved and characterized both in solutions of solvent/nonsolvent mixtures and in thin films. The effects of amino acid chirality were evaluated by comparing the behavior of the chiral derivatives with the achiral one, i.e., Gly-C6-PPE. Additionally, the role played by hydrogen bonds was determined by comparison with a N-methylated derivative of Leu-C6-PPE. The whole set of spectroscopic data allowed us to determine the tendency toward aggregation, to assess the crucial parameters responsible for the aggregation, and to formulate hypotheses about the structures of the aggregates. The nature of the chiral amino acid modifier has relevant consequences on the aggregate formation and supramolecular order, deeply influencing their properties. It appears that amino acidappended PPEs represent good candidates as sensors and in “plastic electronic” applications. The variation of the amino acid provides a simple and versatile way to attain specific (bio)sensing capability. Moreover, it offers a means to modulate the emissive properties of the aggregate states, including the polarization of emitted light.41

substituents at the chiral center. We speculated that the relative importance of these forces would grant a fine modulation and control of the aggregation properties and would therefore bestow a high tunability on the system. Additionally, α-amino acids represent an obvious scaffold for chiral (bio)sensing applications. We report herein the syntheses of a series of new amino acid derivatives of poly(phenyleneethynylene) obtained by linking different amino acid moieties (as methyl esters) on their amine side to the PPE skeleton through a spacer (a linear six-carbon-atom oxaalkyl chain). In order to prevent any other interaction than π-stacking and hydrogen bonding of the amide groups, we decided to employ simple aliphatic or aromatic amino acid derivatives, devoid of functionalities on their side chains. Several different copolymers40 of this type were prepared (Chart 1), varying the nature of the amino acid, and their optical characterization was carried out by absorption, fluorescence, and electronic circular dichroism (ECD) spectroscopy. Chart 1

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RESULTS AND DISCUSSION Synthesis. The approach for the synthesis of amino acidfunctionalized PPEs was based on the classical Cassar−Heck− Sonogashira polymerization methodology. The complete synthetic pathway is described in detail in the Supporting Information and is depicted in Scheme 1. The intermediate 1,4di(ethyl-6-oxyhexanoate)benzene (1) was synthesized via the nucleophilic substitution between hydroquinone and ethyl 6bromohexanoate. This reaction proceeded with a 83% yield and required no complicated purification. Afterward, the electro-

Scheme 1. Syntheses of Gly-C6-PPE, Leu-C6-PPE, Phe-C6-PPE, and Val-C6-PPE

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THF, and completely insoluble in diethyl ether, water, and hydrocarbons. On the contrary, the glycine derivative resulted soluble only in DMSO, partially soluble in chloroform, and almost completely insoluble in all other common solvents. The low solubility of Gly-C6-PPE is possibly a reason for the low polymerization yield obtained in that case. A second factor affecting the solubility of PPE copolymers is the hydrogen-bond donor capability, as was demonstrated by investigating the N-methylated derivative of the leucinefunctionalized copolymer. The monomer was obtained by treating 4-Leu monomer with NaH and CH3I (Scheme 2). The polymerization was carried out in standard conditions furnishing MeLeu-C6-PPE in 75% yield as yellow-orange powder. 1H NMR peak integration confirmed the 1:1 ratio of the two monomers and GPC analysis revealed a Mn ≈ 19 kDa and PDI = 1.5. The MeLeu-C6-PPE copolymer showed different solubility than its parent Leu-C6-PPE. In fact, the N-methylated derivative resulted completely soluble in chlorinated solvents, THF, acetone, and diethyl ether and almost completely insoluble in water, methanol, ethanol, and hydrocarbons, indicating that the absence of hydrogen bonding donor sites makes this system behave as a common dialkoxy-substituted PPE.2,3,10−19 Optical and Chiroptical Characterization. Optical characterization of the new derivatives was first conducted in chloroform and dichloromethanetwo solvents which completely dissolve all copolymers. Second, in order to promote a controlled aggregation, we prepared samples of the various polymers in different solvent mixtures composed of CH2Cl2 as the “good” solvent and Et2O as the “poor” solvent or nonsolvent. All the samples were then characterized by means of absorption (Abs), ECD, and fluorescence spectra. The spectral profiles obtained by progressive addition of the nonsolvent represent a sort of step-by-step snapshots of the aggregation processes which, eventually, are expected to mimic the behavior of the solid state. The optical characterization of our systems highlighted immediately a profound difference in the tendency toward aggregation between the chiral substrates and the achiral one. Figure 1 shows the absorption spectra of Gly-C6-PPE and LeuC6-PPE in chloroform at the same concentration (0.026 mg/ mL). The spectra evidence a common maximum centered at 438 nm, which is typical of the conjugated alkoxy-substituted PPE backbone.2,3,10−19 For the glycine derivative only, however, a second band (shoulder) at 490 nm is also present together with a pronounced long-wavelength tailing of the main band as a typical distortion due to the scattering by suspended

philic substitution of 1 by iodine was carried out, using acetic acid as the solvent in the presence of sulfuric acid and water at 75 °C.42 Chlorination of the two carboxylic acid moieties thus obtained with SOCl2, and subsequent reaction with 2 equiv of the desired aminoester, furnished the amino acid-functionalized monomers 4 with a relatively high yield. Polymerizations were carried out treating the respective functionalized diiodoarenes with 1,4-diethynyl-2,5-dimethoxybenzene in classical Cassar− Heck−Sonogashira conditions yielding all derivatives as yelloworange powders in 70−78% yields (except for the glycine derivative, 32% yield). Polymers were characterized by 1H and 13 C NMR spectra. Peak integration confirmed that the structural units derived from the two monomers were present in a 1:1 ratio. Furthermore, the 13C resonances of butadiyne carbon atoms at about 80 ppm were absent. The copolymers were characterized also by diffusion spectroscopy (DOSY) experiments in CDCl3 to measure their diffusion coefficients and by GPC. Molecular weights, polymerization yields, and diffusion coefficients for all the new polymers are reported in Table 1. Diffusion coefficients and GPC Mn values are in good Table 1. Polymerization Yield and Relevant Data for the Amino Acid PPE Copolymers polymer

yield (%)

Gly-C6-PPE Leu-C6-PPE Phe-C6-PPE Val-C6-PPE

32 78 70 77

Mn(GPC) (kDa) 10.1 9.2 9.1

PDI

diffusion coeffa (10−10 m2 s−1)

1.81 1.86 1.62

0.4b 1.2 1.4 1.4

Measured in CDCl3 at 25 °C by DOSY. bIn DMSO-d6. Taking into account the solvents’ viscosities, the diffusion coefficient estimated in CDCl3 for Gly-C6-PPE is 1.5, in line with the other polymers. a

accord with each other. Interestingly enough, DOSY could be used to estimate the size of Gly-C6-PPE, for which GPC was not applicable. Additionally, the lack of fast-diffusing species in DOSY spectra demonstrated the absence of residual monomers or short oligomers, which could in principle affect the aggregation processes, at least in the limit of detection of NMR. Considering the molecular weights of monomeric units, the observed Mn values are consistent with species constituted of about 25 phenylene units. A marked difference in the solubility properties of chiral derivatives with respect to the glycine derivative was observed. In fact, all chiral systems were found to be soluble in chlorinated solvents, just slightly less soluble in methanol and ethanol, only slightly soluble in acetone and Scheme 2. Synthesis of MeLeu-C6-PPE

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particles, attesting the existence of large aggregates even in these conditions. The Abs spectra of Val-C6-PPE and Phe-C6PPE in the same conditions (Figure 2) resulted practically superimposable to that of Leu-C6-PPE, showing a general greater tendency toward aggregation of the achiral system than the chiral ones. This phenomenon seems at first glance to be related to the reduced steric hindrance provided by glycine, which allows for a more efficient packing of the polymeric chains. The spectral consequences of polymer aggregation will be discussed first on Leu-C6-PPE. Upon addition of diethyl ether (Figure 2a), a clear solvatochromism was observed: the main band is red-shifted, and its shape becomes sharper; moreover, a new, narrow band appears at 490−493 nm which may be assigned to aggregates dispersed in solution and offers an unequivocal proof of aggregation.2,3,10−13,16,17 Switching from Et2O to different nonsolvents such as cyclohexane resulted in

Figure 1. Absorption spectra of Gly-C6-PPE (dashed line) and LeuC6-PPE (solid line) in CHCl3. Sample concentrations 0.026 mg/mL, 1 cm cell.

Figure 2. Absorption (a, b, c), ECD (d, e, f), and fluorescence (g, h, i) spectra of Leu-C6-PPE (blue, left column), Phe-C6-PPE (red, middle), and Val-C6-PPE (green, right) in CH2Cl2/Et2O mixtures with increasing amounts of Et2O. Samples concentration 0.026 mg/mL; cell length 1 cm; excitation wavelenght 435 nm. 7055

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amino acid pendants to establish multiple hydrogen bonds. Second, the hydrogen bond network established in the supramolecular aggregates resembles that observed in polypeptide parallel β-sheets. In fact, due to the C2 symmetry of the monomeric units, the two amide groups linked to the same ring are similarly oriented. Beyond our expectations, together with hydrogen bonding, pronounced effects on the aggregation properties of these systems are ascribable to the nature of the amino acid side chain. The different behavior of Val-C6-PPE and Phe-C6-PPE copolymers with respect to Leu-C6-PPE was immediately revealed by means of the absorption profiles (Figure2b,c). All the absorption spectra were recorded at the same concentration (0.026 mg/mL) in the same solvent/nonsolvent mixtures (CH2Cl2/Et2O). While, as already noticed, the absorption spectra in pure CH2Cl2 are similar for the three copolymers, they respond differently to nonsolvent addition. First of all, the fraction of solvent necessary to promote aggregation is different for the three species: if for Phe-C6-PPE 40% Et2O is sufficient to observe a small aggregation peak at 490 nm, 50% is needed for Leu-C6-PPE and 55−60% for Val-C6-PPE. Moreover, the relative intensities and the shapes of the aggregation peaks display interesting differences. In fact, Phe-C6-PPE exhibits a solvatochromic behavior similar to Leu-C6-PPE: increasing the amount of nonsolvent, a neat aggregation peak is observed at 490 nm and the main band is red-shifted from 440 to 450 nm. Instead, for Val-C6-PPE, even at the maximum Et2O percentage the aggregation peak appears as a faint shoulder on the main band, and the red-shift of the latter is smaller. All the above observations let us identify the following trend with respect to the tendency toward aggregation:

similar absorption spectra. A minimum amount of 10% CH2Cl2 was needed in every case to prevent precipitation of aggregates. The spectroscopic effects of aggregation are most striking in the ECD spectra (Figure 2d). Leu-C6-PPE in pure CH2Cl2 exhibited a very weak negative band centered at 445 nm with a g value (Δε/ε or ΔA/A) of about 1.2 × 10−5 due to the small perturbation induced by the chiral group far from the chromophore. Upon addition of Et2O, this weak monosignated band was replaced by a complex, asymmetric, and positive ECD couplet which attained a maximum g = 5.5 × 10−3 (almost 500fold larger than the solvated species). It is noteworthy that the g-value at 490 nm reached its maximum already for the 50:50 mixture, where only a small shoulder is visible in the absorption spectrum. In these conditions the few aggregates formed have already a welldefined structure which is preserved upon further aggregation. The intense ECD spectra with g-values close to 5 × 10−3 call for strong interactions between polymer chains, allowing for an effective exciton−coupling ECD mechanism (ECCD). The sign of the couplet describes a relative orientation between the chains associated with a supramolecular structure endowed with right-handed helicity.26 The atypical shape of the observed ECD profiles with respect to the standard ECCD expectations is the result of a strong vibronic coupling, as it has been demonstrated for glucose-functionalized PPEs.36 When exciton coupling occurs between vibronic transitions, rather than purely electronic ones, the resulting ECD profile may strongly differ from a typical ECD couplet.43 A model of aggregated chains of Leu-C6-PPE is shown in Figure 3, evidencing the supramolecular chirality adopted by

Gly‐C6‐PPE ≫ Phe‐C6‐PPE ≳ Leu‐C6‐PPE > Val‐C6‐PPE

This trend seems, in first approximation, to be consistent with the bulkiness of the substituent at the amino acid α-carbon: a nonhindered substituent (H) favors aggregation more than hindered ones (CH2R and especially CHR2). Analysis of ECD spectra (Figure2d−f), however, revealed that, beyond this apparently simple picture, there are profound differences in the aggregation modes of the three chiral copolymers. From Figure 2d it appears that during the addition of nonsolvent the ECD profile of Leu-C6-PPE experiences a steady growth of all bands, thereby conserving its shape and displaying a constant crossover (isodichroic) point. This is consistent with a single equilibrium process between free molecules, endowed with practically vanishing ECD, and a single or largely dominant aggregate species. On the contrary, the ECD of Phe-C6-PPE undergoes a more complex evolution: starting from a negligible ECD of the free molecule, the aggregation produced an asymmetric positive couplet-like ECD signal which attained a maximum g-value of 4.7 × 10−3 at 495 nm in the presence of 40% nonsolvent. Upon further addition of this latter, the spectra evolved changing their shape with an uneven growth of the three main peaks and the absence of isodichroic points. This demonstrates that for Phe-C6-PPE there are at least two aggregation modes resulting in the simultaneous formation of different aggregates characterized by different ECD spectra. ECD measurements on Val-C6-PPE revealed even more striking and unforeseen peculiarities. First of all, upon aggregation, a negative ECD couplet was observed (g = −4.7 × 10−3 measured for 60% Et2O) instead of the positive one observed for the other systems. Moreover, for Val-C6-PPE, the evolution of the ECD profile was especially complicated and

Figure 3. Model of aggregated chains of Leu-C6-PPE representing the supramolecular chirality adopted by the backbones (in yellow), the relative disposition of the side chains, and the hydrogen bonds (dotted green lines).

the polymer backbones (in yellow), the relative disposition of side chains, and the network of hydrogen bonds (dotted green lines). The model, though obtained by a simplified molecular modeling procedure (see Supporting Information), lets us appreciate several important aspects of the aggregation mode. First, when two or more polymer chains are stacked on top of each other, a tilt of around 15° allows 4−5 aromatic units to preserve an efficient π-stacking and, at the same time, their 7056

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Figure 4. Absorption (a, b, c) and ECD (d, e, f) spectra of Leu-C6-PPE (blue, left column), Phe-C6-PPE (red, middle), and Val-C6-PPE (green, right) as thin films (solid lines) or in nonsolvent-rich mixtures (dashed). Panel e shows also the spectrum of Phe-C6-PPE in the presence of intermediate amount of nonsolvent (dotted). When not specified, films were casted from CH2Cl2 solutions.

species (Figure 4c). The absorption spectra of the films showed a small deviation from the background at long wavelength, as a typical scattering artifact. Moving to ECD characterization, the films of Leu-C6-PPE prepared from CH2Cl2 almost perfectly matched in shape those of aggregates in solution (Figure 4d). This observation suggests that intermolecular interactions leading to aggregate formation in solvent/nonsolvent mixtures are similarly at work also in the thin films. This is also a further indirect confirmation of the presence of a single aggregation mode for Leu-C6-PPE. Interestingly enough, the observed g-value for this system resulted to be 9.3 × 10−3, about twice the one recorded in solution, proving a more efficient coupling of the polymer chains in the solid state. The films of Phe-C6-PPE casted from CH2Cl2 showed a smaller g-value of 1.3 × 10−3 (Figure 4e). This ECD spectrum resembles more the ones of solutions with small nonsolvent fraction (40% Et2O) than those of solutions richer in nonsolvent (60% Et2O). Films of Val-C6-PPE casted from CH2Cl2 showed no ECD bands (not shown), confirming the apparent solvated-like nature of the polymer chains in these films. Instead, films prepared from a 60:40 CH2Cl2/Et2O solution (Figure 4f) showed an ECD spectrum with a g-value of −2.5 × 10−3, and, again, this spectrum is different from those of solutions richer in Et2O. In practice, for both Phe-C6-PPE and Val-C6-PPE, the ECD profiles of thin films turned out to be similar to those obtained for Leu-C6-PPE both in Et2O-rich solutions and thin films. For Val-C6-PPE, however, the sign of the ECD spectrum is opposite, indicating an opposite handedness of the supramolecular structure (left-handed rather than right-handed). The above observations support the hypothesis based on the coexistence of two or more different aggregation modes in

addressable to the contemporary presence of multiple aggregates. In particular, after 60% Et2O, the ECD signal started to decrease in intensity, probably due to the formation of an aggregate with opposite handedness. In summary, LeuC6-PPE seems to prefer a single aggregation mode whose ECD footprint is shown by the spectra in Figure 2d. Conversely, for Phe-C6-PPE, the same Leu-type aggregate is formed at low nonsolvent amounts, which is followed by a second type of aggregate observed when the nonsolvent amount is increased. Finally, for Val-C6-PPE two or more different aggregate species coexist in all solvent mixtures, whose relative weight depends strongly on the nonsolvent amount. It has to be stressed that ECD spectroscopy offers a unique opportunity to detect and track the formation of distinct aggregate species which would be hardly recognized by absorption spectroscopy.27 Further confirmation of the variability of the aggregation modes of our amino acid-appended PPEs was obtained by characterizing their thin films. Films were prepared by deposition of about 400 μL of a solution in CH2Cl2 (1 mg/ mL) on a quartz plate and slow evaporation of the solvent in a controlled atmosphere saturated with CH2Cl2 vapors. Thin films of Leu-C6-PPE and Phe-C6-PPE showed absorption spectra (Figure 4a,b) similar to those observed for solution aggregates dispersed in nonsolvent-rich mixtures. On the contrary, films of the Val-C6-PPE derivative showed a behavior more comparable to that of free chains in solution than that of aggregate species, i.e., were devoid of any clear-cut aggregate signature (not shown). Things however changed when the films of Val-C6-PPE were prepared starting from a solution which already contained aggregate species. In fact, the films prepared evaporating a solution in 60:40 CH2Cl2/Et2O exhibited absorption spectra more characteristic of aggregate 7057

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Instead, aggregation completely quenched the emission of PheC6-PPE, in keeping with its larger propensity to aggregate (Figure 3h). An apparently minor but actually extremely important difference is shown by Leu-C6-PPE. This polymer in fact, while having a similar aggregation behavior to the Phe derivative, remained consistently more emissive (Figure 3g) and showed a broad band at lower energies (>525 nm) which can be assigned to excimer-like species.2,48 The survival of even a small emission efficiency in PPE aggregates would be of crucial interest for practical applications, and as discussed in the Introduction, it is one of the main reasons for the design of chiral PPEs.

solvent/nonsolvent mixtures of Phe-C6-PPE and Val-C6-PPE, one of which is in close relationship with the single aggregation mode observed for Leu-C6-PPE. For some reason, this latter mode seems to be preferentially selected during thin film deposition. The films of Phe-C6-PPE and Val-C6-PPE retained the same ECD spectra after annealing at 100 °C for 2 h, suggesting that the observed aggregation mode corresponds to an energetically stable situation.44 In addition, it is also reasonable to think that solid-state aggregates of Leu-C6-PPE, Phe-C6-PPE, and Val-C6-PPE roughly possess a supramolecular geometry similar to Leu-C6-PPE solution aggregates (apart from the handedness). Multimodal aggregation pathways have been reported for other chiral PPEs.16 This phenomenon deserves further studies aimed to further characterize the various aggregation pathways and investigate the parameters for their control.27 The opposite handedness of the aggregates of Val-C6-PPE with respect to those of Leu-C6-PPE and Phe-C6-PPE is consistent both in solution and in the solid state. It is clear that such a phenomenon arises from a complicated balance of all possible intermolecular interactions. Generally speaking, the handedness assumed by a supramolecular aggregate of chiral PPEs or related polymers is rather unpredictable. In the case of p-phenylenevinylenes (PPVs) substituted with simple chiral alkyl moieties, for example, where only steric and/or dispersion forces are at play, a small variation in the chiral pendant is sufficient to revert the sense of supramolecular twist.45 The thin films of Leu-C6-PPE, Phe-C6-PPE, and Val-C6PPE were also characterized by IR spectroscopy. IR spectra (reported in the Supporting Information) exhibited, among other things, the characteristic frequencies of hydrogen-bonded secondary amide moieties.46 Easily identified were, in all cases, the N−H stretching at 3300−3305 cm−1 and the CO stretching (amide I band) at ≈1650 cm−1. IR spectra offer therefore proof that all or most amide groups are involved in hydrogen bonds both as donors and acceptors at least in the solid phase,47 in keeping with our expectations and with the suggested model. The aggregation processes of Leu-C6-PPE, Phe-C6-PPE, and Val-C6-PPE were also monitored by photoluminescence experiments (Figure 3g−i). Upon excitation at 435 nm in dichloromethane, all copolymers display a broad emission band with maximum at about 475 nm with a long-wavelength (probably vibronic) shoulder. The observed Stokes shift with respect to the main absorption band is 35 nm, consistent with previously reported dialkoxy-substituted PPEs.2 Increasing the fraction of nonsolvent (Et2O) caused hypochromism and a progressive small blue-shift of the main emission band down to 467 nm (27 nm Stokes shift). When aggregation started, moreover, a new band appeared at 504−505 nm, typical of the aggregated species.2 Following the increasing amount of Et2O, the relative intensity of the two bands changes in favor of the 505 nm one because of the increased relative concentration of the aggregated form, while the nonaggregate band is progressively blue-shifted according to a concentration-dependent shift.18 As it is commonly observed for conjugated polymers, the most immediate result of aggregation promoted by nonsolvent addition is a dramatic depression of the overall fluorescence intensity. Confirming the trend outlined above, the comparison of fluorescence profiles showed Val-C6-PPE to be the less prone to aggregation among the three derivatives (Figure 3i). In fact, even in the solution richest in nonsolvent it retained a moderate fluorescence also for the band at 467 nm.

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CONCLUSIONS In this paper we reported the synthesis by means of a simple organometallic method of several novel PPE copolymers containing a regular alternation of monomeric units, substituted with methoxy groups and different α-amino acid moieties. The optical properties of the copolymers in solution of various solvents and in the solid state have been investigated using absorption, fluorescence spectroscopy, and ECD techniques. Despite their low degree of polymerization, these derivatives revealed a remarkable tendency toward aggregation in nonsolvent-rich solutions, mediated by the formation of hydrogen bonds. Moreover, the tendency toward aggregation seems to be related to the bulkiness of the substituents at the amino acid stereogenic center. ECD measurements furnished important information about the structural organization of aggregated copolymers. Leu-C6-PPE led to structurally consistent aggregates dispersed in solution and as thin films. At least two different types of aggregates were observed for Val-C6-PPE and Phe-C6-PPE when dispersed in solution, while an aggregate species consistent with that of Leu-C6-PPE was detected in thin films. Finally, the role of chirality and of hydrogen bonding was ascertained by comparison with Gly-C6PPE and MeLeu-C6-PPE. Our results demonstrate how supramolecular interactions of PPEs are modulated by several different parameters, all of which must be taken into account to tune material properties. The fact that amino acid-appended PPEs are easy to prepare, not expensive and virtually functionalizable with any amino acid moiety, makes this class of materials very promising and attractive from an applicative point of view.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental section; computational procedure; IR spectra of thin films; NMR spectra; absorption and ECD spectra of MeLeu-C6-PPE. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (G.P.). *E-mail [email protected] (L.D.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Ministero per l’Istruzione, l’Università e la Ricerca (MIUR), is thanked for funding (PRIN2012, prot. 2012A4Z2RY). Dr Andrea Pucci, Pierpaolo Minei, Francesco Criscitiello, and 7058

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(35) Pescitelli, G.; Babudri, F.; Colangiuli, D.; Di Bari, L.; Farinola, G. M.; Hassan Omar, O.; Naso, F. Macromolecules 2006, 39, 5206− 5212. (36) Pescitelli, G.; Hassan Omar, O.; Operamolla, A.; Farinola, G. M.; Di Bari, L. Macromolecules 2012, 45, 9626−9630. (37) Jarvo, E. R.; Miller, S. J. Tetrahedron 2002, 58, 2481−2495. (38) Sánchez, L.; García, F. J. Am. Chem. Soc. 2011, 134, 734−742. (39) Sánchez, L.; García, F.; Buendía, J. J. Org. Chem. 2011, 76, 6271−6276. (40) We started the investigation of this class of materials characterizing a PPE homopolymer functionalized with leucine on every phenylene unit. Spectroscopic and chiroptical analysis revealed that in this case the formation of a stable chiral aggregate is prevented, probably due to steric reasons allied with the functionalization on every phenylene unit. Thus, we decided to “dilute” the content of chiral modifiers along the backbone by alternating them with a simple methoxy substituent and designed the copolymers shown in Chart 1. (41) Moon Jeong, S.; Takezoe, H. Effect of Photonic Structures in Organic Light-Emitting Diodes − Light Extraction and Polarization Characteristics, Organic Light Emitting Devices. In Organic Light Emitting Devices; Singh, J., Ed.; InTech: Rijeka, Croatia, 2012. (42) Zhang, T.; Fan, H.; Zhou, J.; Jin, Q. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 3056−3065. (43) Weigang, O. R. J. Chem. Phys. 1965, 43, 71−72. (44) Further annealing at 120 °C for 24 h caused apparent degradation of the films. Correspondingly, however, ECD spectra retained their overall profile but a progressive intensity decrease was observed. (45) Resta, C.; Di Pietro, S.; Elenkov, M. M.; Hameršak, Z.; Pescitelli, G.; Di Bari, L. Macromolecules 2014, 47, 4847−4850. (46) Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts, 3rd ed.; John Wiley & Sons Ltd.: Chichester, UK, 2001. (47) Solution IR spectra could not be obtained because of polymer precipitation occurring even in chlorinated solvents at the required concentrations. (48) Satrijo, A.; Kooi, S. E.; Swager, T. M. Macromolecules 2007, 40, 8833−8841.

Sabrina Bianchi are gratefully acknowledged for GPC, fluorescence, and IR spectra.

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dx.doi.org/10.1021/ma501623a | Macromolecules 2014, 47, 7052−7059