Article pubs.acs.org/Macromolecules
Consequences of Chirality on the Aggregation Behavior of Poly[2methoxy-5-(2′-ethylhexyloxy)‑p‑phenylenevinylene] (MEH-PPV) Claudio Resta,† Sebastiano Di Pietro,†,⊥ Maja Majerić Elenkov,§ Zdenko Hameršak,§ Gennaro Pescitelli,*,† and Lorenzo Di Bari*,† †
Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Risorgimento 35, 56126 Pisa, Italy Rudjer Boskovic Institute, Bijenicka 54, 10000 Zagreb, Croatia
§
S Supporting Information *
ABSTRACT: Poly[2-methoxy-5(2′-ethylhexoxy)-p-phenylenevinylene] (MEH-PPV) has been for the first time prepared and fully characterized in enantiopure (R) form. If the polymer molecular weight is sufficiently low, (R)-MEH-PPV assumes a helical supramolecular structure in the solution aggregates, with consequences on the tendency to aggregation and on the fluorescence quenching, both of which are reduced with respect to the racemic analogue.
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respect, chirality has been suggested as one of the possible ways of controlling the supramolecular architectures of conjugated polymers.8,9 In the presence of stereodefinite elements of chirality, a perfect alignment between polymer chains may become disfavored with respect to a twisted arrangement, which, among other things, would favor high quantum yields of emission.10−13 Additionally, nonracemic chiral polymers have the advantage that they can be characterized by electronic circular dichroism (CD),11,12,14−18 a technique especially well suited to detect and study chiral supramolecular species.19 In light of the considerations above, the chiral nature of MEHPPV appeared to us worth attention, especially to fill the gap observed in the literature. The only paper which, at the best of our knowledge, has so far described nonracemic 1, has many unclear aspects.20 First, the synthesis, the optical purity and even the configuration of the analyzed MEH-PPV are not described. Second, the reported CD spectra are unexpectedly weak and poorly structured, an aspect which might be related to the specific batch of the polymer considered (MW upper limit of 300 kDa). Third, no comparison was made between racemic and nonracemic MEH-PPV prepared in the same conditions. In the present communication, we synthesized highly stereoregular MEH-PPV, (R)-1 in Chart 1, and its racemic (or stereorandom) analogue rac-1 in parallel, with two different synthetic routes, and analyzed the optical and chiroptical properties of the various batches, with special attention to their propensity toward aggregation.
oly[2-methoxy-5(2′-ethylhexoxy)-p-phenylenevinylene] (MEH-PPV, 1, Chart 1)1 is one of the most studied conjugated organic polymers for optoelectronic applications, such as sensors, photovoltaics, field-effect transistors and organic light emitting diodes (OLED).2 While the conductive properties of the polymer are provided by the conjugated backbone, the alkoxy substituents ensure wider solubility and easier processability than the parent poly-p(phenylenevinylene), facilitating device fabrication processes. The branched alkyl substituent, apart from being the major responsible for the augmented solubility, has a center of chirality at position 2′, a property whichquite surprisinglyhas been almost ignored in the literature. In fact, every synthesis and application so far demonstrated for MEH-PPV have concerned its racemic or stereorandom form rac-1 (Chart 1). It is well established that the fundamental properties of conjugated polymers, such as light absorption and emission, charge transport, and exciton transfer, depend not only on their molecular structure, but also on the supramolecular interactions and the nano/mesoscale organization.3−5 In particular, the communication between distinct polymer chains is essential for light emission and charge transport, and the control of interchain interactions is therefore crucial to optimize the efficiency of devices based on PPVs and other conjugated polymers.6,7 In that Chart 1
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SYNTHESIS OF (R)-1 Since several efficient syntheses are established for MEH-PPV,21 the only issue in the synthesis of its stereoregular version (R)-1 is Received: May 5, 2014 Revised: June 28, 2014 Published: July 17, 2014 © 2014 American Chemical Society
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the introduction of the chiral 2-ethylhexoxy substituent with high enantiomeric purity in a stereospecific way. Unfortunately, no useful enantiopure chiral synthon for the 2-ethylhexoxy group is readily available from commercial sources or from the chiral pool. Enantiomeric resolution of 2-ethylhexanol is also quite problematic, because it requires the discrimination between a butyl and an ethyl group. The only efficient method for the resolution of primary alcohols with chirality at β carbon consists in enzymatic resolutions. In particular, 2-ethylhexanol could be enantioseparated very efficiently by means of lipase-catalyzed kinetic resolution of the racemic alcohol in the presence of vinyl laurate.22,23 The procedure furnished (R)-2-ethylhexanol with 99% e.e. (GC; see Supporting Information for details). With the enantiopure alcohol in our hands, we used the modification of two reported synthetic procedures to prepare (R)-1. The first consisted in a Gilch-type synthesis, following the procedure reported by Neef and Ferraris24 where a Mitsunobutype reaction between 2-ethylhexanol and 4-methoxyphenol was employed in the first step (Scheme 1). After optimization of
Scheme 2. Horner Route to (R)-1 (Key Steps)
“PI-Horner” (R)-1 showed a residual 10% cis double bond content. The relevant characterization data for the various samples are summed in Table 1. The identity and purity of the polymer samples were checked also by NMR and IR measurements (see Supporting Information).
Scheme 1. Gilch Route to (R)-1
Table 1. Summary of Data for MEH-PPV Samples
a
route
Mn (kDa)
PDI
Abs λmax (nm)a
Flu λem (nm)a
Gilch (R)-1 Gilch rac-1 Horner (R)-1 Horner rac-1
400 400 10 10
1.87 1.84 1.50 1.52
504 503 480 473b
557 556 548 551
In CHCl3. bλmax = 484 nm after photoirradiation.
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SPECTROSCOPIC CHARACTERIZATION OF (R)-1 AND RAC-1 In solution of a good solvent such as chloroform, all the prepared polymers exhibited the expected absorption and emission spectra whose maxima are listed in Table 1. Absorption spectra showed the typical prominent broad band between 400 and 550 nm. “Gilch” 1 had the expected absorption maximum at 503−504 nm (Figure 1a). “Horner” 1 showed a blue-shifted maximum at 473 nm (Figure 2a) due to the non-negligible cis content; upon irradiation,27 the maximum shifted in fact to 484 nm. Apart from this wavelength shift, the photoirradiated polymer “PI-Horner” behaves very similarly to the starting “Horner” one with respect to aggregation phenomena (vide infra); therefore, in the following discussion we will refer only to the nonirradiated “Horner” 1. Emission spectra (excited at 475 nm) showed the typical broad band between 500 and 650 nm with maximum around 550 nm and a long-wavelength shoulder (Figures 1b,d and 2b,d). The CD spectra of all samples of (R)-1 in chloroform were very weak and had dissimmetry or g-factors ≤3 × 10−5 (g = Δε/ε), indicating that the perturbation exerted by the chiral elements on the nonaggregated polymer is very small (Figures 1e and 2e). It is well-known that conjugated polymers such as MEH-PPV undergo aggregation in solution upon addition of a “poor” solvent or nonsolvent.28 These solution aggregates are expected to mimic the behavior of solid-state aggregates, e.g., those found in thin films. In the current case, methanol was chosen as the
some crucial parameters, the synthesis, whose details are reported in the Supporting Information, proceeded with 47% overall yield and 65% polymerization yield. The absence of any chain initiator or stopper allowed us to obtain the “Gilch” (R)-1 with high molecular weight (Mn ≈ 400 kDa), to be compared with the already discussed literature report.20 Because of the high MW, the polymer was not amenable to NMR characterization. Therefore, we were not able to quantify structural defects, such as those due to tail-to-tail and/or head-to-head additions, which are expected to amount to around 10%.25 The second synthesis was based on a Horner polycondensation and followed the procedure reported by Hörhold and coworkers,26 starting from the same dibromo precursor used in the first route (Scheme 2). The synthesis proceeded with 28% overall yield and 60% polymerization yield, and afforded a much shorter polymer (Mn ≈ 10 kDa). This procedure is expected to be substantially defect-free.21 Despite what reported by Hörhold and co-workers, however, the polymer thus obtained showed a non-negligible presence of cis double bond defects (ca. 25%, see Supporting Information). To reduce the defects content, a THF solution of the original “Horner” (R)-1 was photoirradiated with a 125W mercury lamp,27 monitoring the cis/trans conversion by UV−vis spectroscopy (Supporting Information). The system reached a photostationary equilibrium in 30′, and the obtained 4848
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Figure 1. Absorption (a, c), emission (b, d), and CD spectra (e) of “Gilch” rac-1 (a, b) and (R)-1 (c−e) in chloroform/methanol mixtures, normalized per monomer (4.8 × 10−5 M).
Figure 2. Absorption (a, c), emission (b, d), and CD spectra (e) of “Horner” rac-1 (a, b) and (R)-1 (c−e) in chloroform/methanol mixtures, normalized per monomer (6.1 × 10−5 M).
nonsolvent, and mixtures up to 90% methanol were characterized. As a consequence to aggregation, the absorption spectra of 1 undergo a hypo/bathochromic shift accompanied by band broadening. In the case of the high-Mn “Gilch” polymer, (R)-1 and rac-1 showed a consistent tendency toward aggregation (Figure 1a,c). The low-Mn “Horner” polymer seemed less prone to aggregate than the high-Mn “Gilch” one, which is particularly true for stereodefinite (R)-1 (Figure 2a,c). Not only in fact the aggregation starts at higher amounts of methanol, but the overall effects on the UV−vis spectra are much less pronounced for “Horner” (R)-1 than for “Horner” rac-1 (Figure 2a,c), as well as for this latter than “Gilch” 1 (Figure 1a,c). Moreover, while “Horner” rac-1 in 90% methanol starts to precipitate a few hours after preparation, (R)-1 solution is still completely clear after 18 h (Figure S1 in the Supporting Information). The effect of aggregation on emission spectra is very pronounced, as the total emission is strongly quenched upon aggregation and, for methanol-richest solutions, one or two weak bands survive (Figures 1b,d and 2b,d). The most red-shifted of the two bands may be assigned to the residual emission of aggregate species which occurs from low-energy traps after exciton migration.10 Although the quenching affects strongly both stereorandom and stereodefinite samples, the residual fluorescence was consistently higher for (R)-1 than for rac-1 (“Horner” route). In fact, the integral ratio between the emission
spectra in pure chloroform and 90% methanol is ≈35% for rac-1 and ≈42% for (R)-1, respectively (Figure 2b,d). CD spectroscopy offers the best way to discriminate between the aggregation behavior of the “Gilch” and “Horner” polymers. In fact, the CD spectra of “Gilch” (R)-1 remained weak and showed almost random profiles upon methanol addition, indicating that the aggregates of the high-Mn polymer have no chiral supramolecular order (Figure 1e). This is perhaps the reason why Pålsson et al. observed weak CD spectra of nonracemic MEH-PPV with similar high molecular weight.20 On the contrary, the CD spectra of “Horner” (R)-1 started to display two strong CD bands of opposite sign (a so-called CD exciton couplet)29 already at 60% methanol, and preserved the same feature upon further nonsolvent addition (Figure 2e). At 90% methanol, the couplet shows a g-factor of 1.5 × 10−3, with a 50-fold increase with respect to the completely solvated polymer. According to exciton theory,29,30 the CD signature (negative exciton couplet) of the aggregates of “Horner” (R)-1 indicates a chiral supramolecular structure with negative helicity. This result is in striking contrast with higher-Mn MEH-PPV samples, either our “Gilch” (R)-1 or Pålsson’s one.20 Rather, the CD spectra of “Horner” (R)-1 aggregates resemble (both for their profile and intensity) those reported by Meijer and co-workers for the aggregates of poly{di[(S)-2-methylbutoxyl]-p-phenylenevinylene} (DMB-PPV)17 as well as of a copolymer of DMB-PPV and racemic {di(3,7-dimethyloctyl)-1,4-phenylenevinylene} (DDMO-PPV).18 It is remarkable that MEH-PPV, DMB-PPV, 4849
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(4) Beaujuge, P. M.; Fréchet, J. M. J. J. Am. Chem. Soc. 2011, 133, 20009−20029. (5) Henson, Z. B.; Mullen, K.; Bazan, G. C. Nat. Chem. 2012, 4, 699− 704. (6) Nguyen, T. Q.; Doan, V.; Schwartz, B. J. J. Chem. Phys. 1999, 110, 4068−4078. (7) Nguyen, T. Q.; Martini, I. B.; Liu, J.; Schwartz, B. J. J. Phys. Chem. B 2000, 104, 237−255. (8) Kane-Maguire, L. A. P.; Wallace, G. G. Chem. Soc. Rev. 2010, 39, 2545−2576. (9) Verswyvel, M.; Koeckelberghs, G. Polym. Chem. 2012, 3, 3203− 3216. (10) Andrew, T. L.; Swager, T. M. J. Polym. Sci. B Polym. Phys. 2011, 49, 476−498. (11) Zahn, S.; Swager, T. M. Angew. Chem., Int. Ed. 2002, 41, 4225− 4230. (12) Pescitelli, G.; Omar, O. H.; Operamolla, A.; Farinola, G. M.; Di Bari, L. Macromolecules 2012, 45, 9626−9630. (13) Cornil, J.; Beljonne, D.; Calbert, J. P.; Brédas, J. L. Adv. Mater. 2001, 13, 1053−1067. (14) Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W. J. Mol. Struct. 2000, 521, 285−301. (15) Babudri, F.; Colangiuli, D.; Di Bari, L.; Farinola, G. M.; Hassan Omar, O.; Naso, F.; Pescitelli, G. Macromolecules 2006, 39, 5206−5212. (16) Prata, J. V.; Costa, A. I.; Pescitelli, G.; Pinto, H. D. Polym. Chem. 2014, DOI: 10.1039/C4PY00729H. (17) Langeveld-Voss, B. M. W.; Peeters, E.; Janssen, R. A. J.; Meijer, E. W. Synth. Met. 1997, 84, 611−614. (18) Peeters, E.; Christiaans, M. P. T.; Janssen, R. A. J.; Schoo, H. F. M.; Dekkers, H. P. J. M.; Meijer, E. W. J. Am. Chem. Soc. 1997, 119, 9909− 9910. (19) Pescitelli, G.; Di Bari, L.; Berova, N. Chem. Soc. Rev. 2014, 43, 5211−5233. (20) Pålsson, L. O.; Vaughan, H. L.; Monkman, A. P. J. Chem. Phys. 2006, 125. (21) Pang, Y. Poly(phenylenevinylenes). In Design and Synthesis of Conjugated Polymers, Leclerc, M.; Morin, J.-F., Eds. Wiley-VCH: Weinheim, Germany, 2010; pp 147−174. (22) Ć iško-Anić, B.; Hameršak, Z. Chirality 2009, 21, 894−904. (23) Ć iško-Anić, B.; Majerić Elenkov, M.; Hameršak, Z.; Šunjić, V. Food Technol. Biotechnol. 1999, 37, 65−70. (24) Neef, C. J.; Ferraris, J. P. Macromolecules 2000, 33, 2311−2314. (25) Roex, H.; Adriaensens, P.; Vanderzande, D.; Gelan, J. Macromolecules 2003, 36, 5613−5622. (26) Pfeiffer, S.; Hörhold, H.-H. Macromol. Chem. Phys. 1999, 200, 1870−1878. (27) Spring, A. M.; Yu, C.-Y.; Horie, M.; Turner, M. L. Chem. Commun. 2009, 2676−2678. (28) Collison, C. J.; Rothberg, L. J.; Treemaneekarn, V.; Li, Y. Macromolecules 2001, 34, 2346−2352. (29) Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy - Exciton Coupling in Organic Stereochemistry; University Science Books: Mill Valley, CA, 1983. (30) Langeveld-Voss, B. M. W.; Beljonne, D.; Shuai, Z.; Janssen, R. A. J.; Meskers, S. C. J.; Meijer, E. W.; Brédas, J.-L. Adv. Mater. 1998, 10, 1343−1348. (31) Korevaar, P. A.; de Greef, T. F. A.; Meijer, E. W. Chem. Mater. 2013, 26, 576−586.
and DMB-co-DDMO-PPV display very similar CD spectra, though with opposite sign, despite the difference in the chiral moiety structure and degree of substitution (in DMB-PPV, two (S)-2-methylbutoxyl groups are attached at each aromatic ring). CD spectra of “Horner” (R)-1 in methanol-rich mixtures were observed immediately after sample preparation (
