Letter pubs.acs.org/macroletters
Supramolecularly Cross-Linked Nanogel by Merocyanine Pendent Copolymer Anindita Das, Shaojian Lin, and Patrick Theato* Institute for Technical and Macromolecular Chemistry, University of Hamburg, Bundesstrasse 45, 20146 Hamburg, Germany S Supporting Information *
ABSTRACT: Directional dipole−dipole interaction mediated antiparallel dimerization of merocyanine dye (MD) has been explored for maneuvering supramolecular assembly of MDconjugated flexible macromolecules leading to a cross-linked nanogel. The MD-functionalized copolymer was synthesized by a newly developed organocatalytic transesterification strategy for postpolymerization functionalization of poly(pentafluorophenyl acrylate) (polyPFPA)-based reactive copolymer. Presence of ∼35% pendant MD attached to a coillike polymer chain leads to spontaneous formation of highly emitting cross-linked nanogel with efficient container property and appreciable stability in toluene owing to strong dimerization propensity among the MD. Considering the significance of MD in the context of nonlinear optics and photovoltaics, these results not only enrich the toolbox for engineering macromolecular assembly, but also open up new possibilities for future organic materials. very high association constants (∼106 M−1) by virtue of selfcomplementary dipolar-interactions (dipole moment = 7−17 D),9 which makes them a very promising supramolecular entity for structural elucidation. Compared to H-bonding, MD dimer assemblies have additional advantages such as (i) higher tolerance limit to polar solvents, (ii) by structural engineering, its binding affinity can be increased over H-bonding, (iii) its assembly can be probed readily by a clear spectroscopic signature, and (iv) it can have implications in enriching the assembled structure with rich photophysical properties that are relevant to detection and imaging. Dipolar assembly of MD has been exploited for the creation of well-defined and alluring supramolecular architectures in small molecules.10 Compared to small molecule assembly, dipole−dipole interaction mediated macromolecular assembly is much less explored,11 a task far more challenging considering the huge entropy cost associated in freezing the motion of a random coil-like polymer chain compared to a small molecule. To test this possibility, we have synthesized a MD-appended random copolymer (P2, Scheme 1) from a reactive parent polymer P1 by postpolymerization substitution and studied its self-assembly and photophysical properties in solution. P1 was synthesized by copolymerization of 2-ethylhexyl acrylate (M1) with pentafluorophenyl acrylate (M2; 0.7:0.3 feed ratio) by reversible addition−fragmentation chain transfer (RAFT) polymerization. Molecular weight of P1 was estimated to be Mn = 8800 g/mol by gel permeation chromatography (GPC;
P
recise organization of functional polymers into welldefined nanostructures is of significant importance in numerous advanced applications.1 However, in the majority of such examples, polymer aggregation is primarily governed by immiscibility driven solvophobic collapse of one of the incompatible blocks in a given solvent.2 Contrarily, nature has created self-organized macromolecules such as proteins and DNA that employ directional noncovalent interactions such as H-bonding as the main driving force for their spatial arrangement. These elegant structures perform extremely complex functions and many of them are direct consequences of their well-defined secondary and tertiary structures. This inspires synthetic chemists to explore directional supramolecular interactions in abiotic polymeric systems with an aim to emulate the structure of biomacromolecules with an optimizm that functions then will automatically follow. In comparison to vast library of amphiphilic block copolymer aggregation, very few examples are known where supramolecular structure directing entities have been introduced into polymeric scaffolds for engineering their precise organization. Such examples mostly include H-bonding,3 with a few reports on other noncovalent forces such as π-stacking or metal−ligand interactions. 4 H-bonding has been most comprehensively explored in small molecule assembly because of its complementary nature, directionality and wide range of association constant. However, in polar media it is often a challenge to overcome the competition from bulk solvent and thus hydrophobic shielding is needed in the molecular design to make H-bonding more influential.5 For the past several years, merocyanine dyes6 (MD) have been studied in the context of nonlinear optics7 and more recently in photovoltaics.8 They can form tightly bound centrosymmetric dimer-assemblies with © XXXX American Chemical Society
Received: November 22, 2016 Accepted: December 21, 2016
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Scheme 1. Synthesis of MD-Functionalized Polymer (P2) from a Precursor Polymer P1 (top); Antiparallel Dimer Assembly of MD (bottom) by Directional Dipole−Dipole Interaction
Figure 1. 1H NMR spectrum of P2 in CDCl3; (*) Represents signal from the solvent.
of Ha and Hg protons (Figure 1) revealed an incorporation ratio of 0.36:0.64 for MD dye and hydrophobic 2-ethylhexyl side chain. Attachment of MD to the polymer chain was also evident from the UV/vis absorption spectrum of P2 that showed a prominent band corresponding to MD with λmax = 383 nm, which was absent in the P1 spectrum (Figure S2). Quantitative replacement of the PFP-ester was confirmed from FT-IR analysis (Figure S3). Peaks corresponding to the PFP-ester carbonyl at 1784 and 1517 cm−1 in P1 completely disappeared after substitution in P2, while new peaks from MD emerged at 1692, 1634, 1614, and 1540 cm−1, confirming successful integration of the dye via an ester linkage. Self-assembly of P2 was studied at a very low concentration (0.4 mg/mL) in an apolar solvent toluene where the dipolar interaction of the dye is expected to be strongly operative. Scanning electron microscopy (SEM) showed nearly spherical particles with diameter in the range of 250−400 nm (Figure 2a). A control experiment with P3 lacking MD did not form any such structure in toluene (Figure S4). illustrating the vital role of MD in self-assembly of P2. Those spherical particles are reminiscent of the cross-linked polymeric nanogels14a reported for amphiphilic polymer aggregates due to collapse of their hydrophobic segments in aqueous medium followed by covalent cross-linking. Contrarily, in the present system, the
Figure S1), which nicely corroborated with the expected value (10380 g/mol), suggesting a controlled polymerization. The hydrophobic monomer M1 was chosen as the major component to impart solubility to the MD-functionalized polymer in apolar solvents where we intended to exploit the strong effect of dipolar assembly of MD9a for driving polymer assembly. M2 was chosen as a reactive monomer to anchor the hydroxyl functionalized MD (MD−OH; for synthesis, see Scheme S1), to the activated ester site of P1 utilizing our recently developed methodology on transesterification of poly(pentafluorophenyl acrylate) (polyPFPA).12 For constructing P2, we rationally employed this methodology over substitution by activated ester-amine chemistry,13 as we envisaged that amine-functionalized MD would transform the acrylate backbone to acrylamide that might have unwanted influence on the polymer assembly through interference of the H-bonding of amide groups of the acrylamide backbone, a possibility that cannot be ruled out in a nonpolar solvent where H-bonding is highly influential. By this newly developed method, P2 was synthesized and isolated in 62% yield. P2 was structurally characterized by GPC (Mn = 7700 g/mol, Mw/Mn = 1.23) and 1H NMR spectroscopy (Figure 1) in which all the peaks, including those from the MD, could be assigned unambiguously. Further, relative integration 51
DOI: 10.1021/acsmacrolett.6b00898 ACS Macro Lett. 2017, 6, 50−55
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Figure 2. Microscopic images of a solution of P2 in toluene under (a) SEM; (b) AFM (inset: height−width profile between part a and b); (c) DLS data showing particle size distribution; and (d) Fluorescence microscopy image when treated with Nile red; [P2] = 0.4 mg/mL in toluene; Nile red (C = 1 × 10−6 M).
Figure 3. (a) Absorption (left) and emission (λex = 373 nm) (right) spectra of P2 in toluene (solid line) and CHCl3 (broken line); (b) Image of a solution (inset) of P2 under fluorescence microscopy; (c) Variable-temperature UV−vis spectra of P2; (d) λmax vs T plot; [P2] = 0.4 mg/mL; solvent = toluene.
significantly reduced height (∼10 nm). This could be possibly due to a significant collapse or flattening of the solvent-filled nanogels during the drying process. Dynamic light scattering (DLS) measurement revealed an average hydrodynamic diameter (Dh) of 400 ± 50 nm (Figure 2c), which corroborates with the size obtained from both SEM and AFM images. Nanogels provide a suitable three-dimensional network for
supramolecular cross-linking is achieved through directional dipole−dipole interactions of MD. As a result, the nanogel comprised of P2 is expected to contain a lot of solvent molecules trapped inside, as there is no distinct hydrophobic core, unlike in nanogels reported from cross-linked micelles. Atomic force microscopy (AFM) studies revealed particles (Figure 2b) with average widths of 400 ± 50 nm with 52
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Figure 4. (a) Variable- temperature 1H NMR spectra of P2; (b) Integration ratio of Ha and Hg protons vs temperature; (c) 2D NOESY; (*) represents signal from toluene-d8; [P2] = 4.0 mg/mL.
luminescence spectrum in toluene revealed 8 times enhanced emission (λem = 492 nm; Figure 3a, right plot) compared to that in CHCl3, a feature commonly seen in the dimer assembly of MDs,16 indicating such possibility within the polymer assembly. To further cross-check such enhanced emission is indeed due to antiparallel stacking of MD and not owing to solvent-induced quenching of its emission in CHCl3, the experiment was repeated in another “good” solvent, THF, which showed similar results (Figure S6), further substantiating our prediction. Green emitting solution of P2 in toluene, as observed under UV light (Figure 3b, inset), also revealed near spherical luminescent particles under a fluorescence microscope (Figure 3b), which are in agreement with the spectroscopic results. The quantum yield (φ) of the nanogels in toluene was estimated to be 12% (see Figure S7 for details). We further performed variable-temperature UV−vis spectroscopy experiments to probe any effect of temperature on the chromophoric organization of the dye within the nanogels. With increasing temperature the UV−vis spectra of P2 in toluene were red-shifted (Figure 3c), suggesting H-type aggregation of MD at lower temperature. The melting curve (Figure 3d) derived from Figure 3c by plotting the shift in the λmax (T) as a function of temperature (T) showed no saturation, suggesting partial disassembly, even at ∼90 °C, signifying appreciable stability of the MD assembly. A similar
entrapment of guest molecules and thereby show great potential as nanocarriers for drug delivery in water.14b To test this possibility, a hydrophobic guest molecule, Nile red, was treated with a solution of P2 in toluene, and the mixture was subjected to extensive dialysis against toluene for 72 h to remove the nonencapsulated dye. A prominent emission band (λem = 573 nm) for Nile red was observed (Figure S5) in the fluorescence spectrum of the dialyzed solution, suggesting effective guest encapsulation that was further confirmed by the presence of red emitting spherical particles under fluorescence microscope (Figure 2d). To probe the direct involvement of centrosymmetric antiparallel dimerization of MD in restricting the conformation of P2 into cross-linked nanoparticles, we performed spectroscopic studies. The UV−vis spectrum of P2 in a “good” solvent CHCl3 where it is molecularly dispersed showed an absorption band at ∼383 nm for monomeric MD. In toluene, the absorption band appeared at 376 nm with ∼7.0 nm blue shift compared to that in CHCl3 (Figure 3a, left plot), suggesting the possibility of H-type aggregation of MD within the nanogels.9a Often such dipolar chromophores are known to show positive solvatochromic effects in polar solvents such as CHCl3.15 To further support the hypsochromically shifted absorption band of P2 in toluene is indeed due to dimer assembly of MD, we looked at its solvent-dependent emission spectra. Photo53
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ACS Macro Letters observation was also made from variable temperature 1H NMR studies (Figure 4a). Assembly of P2 at lower temperature (20 °C) was evident from the appearance of a broad spectrum in toluene-d8 (Figure 4a) compared to that in CDCl3 (Figure 1). Even upon heating the solution to 110 °C, the peaks remained broad and the spectrum did not acquire the nonaggregated features, as seen in CDCl3. It is known that such broadening of peaks occurs due to slow relaxation effects of protons within a confined geometry compared to when they are in freely tumbling molecules in their nonaggregated states.17 Within the nanogel, MD-protons are expected to be more arrested compared to the alkyl chain protons due to a stacking effect that could be probed from the difference in their integration ratios. Plotting the integration ratios of MD (Ha) and alkyl chain (Hg) protons at different temperatures produced a similar curve (Figure 4b), as noticed in Figure 3d, further ascertaining incomplete disassembly within the tested temperature window. Noteworthy, even at 110 °C, the ratio Ha/Hg (0.33) was significantly lower compared to that found in CDCl3 (0.58). To correlate the dimerization of MD with the stability of the nanogel, variable-temperature DLS experiments were performed. Considerable reduction in the particle size from 400 to 60 nm was observed with increasing temperature from 20 to 90 °C (Figure S8), suggesting disintegration of the larger nanoparticles at higher temperature. Possibly at elevated temperature, the dimer assembly becomes less pronounced, which leads to swelling of the nanoparticles, as indicated by an initial increase in the particle size from 400 to 700 nm between 20 and 60 °C. With a further increase in the temperature, instead of resulting in singularly solubilized macromolecules, P2 adopts a collapsed structure by frustrated dipolar interactions of MD that lacks directionality. This is in agreement with the variable-temperature 1H NMR and UV−vis studies that also showed a signature of incomplete disassembly around ∼90 °C. Intriguingly, a solution of P2 in toluene at 90 °C showed spherical particles with a much reduced size (∼50 nm) under scanning electron microscope (Figure S9), contrary to what was seen at room temperature, which corroborates with the scattering data (Figure S8). Now the question is whether the nanogel formation is specifically driven by dipole−dipole interaction or by πstacking? And if MD is replaced by an aromatic moiety, would it also have a similar effect? We verified that by studying another control polymer P4 (Scheme 1) by treating P1 with 2naphthol, which has a similar chromophoric size as MD (for synthesis and characterization of P4, see SI, Figures S10 and S11). Similar to P2, variable-temperature 1H NMR spectroscopy was conducted with P4 in toluene-d8 (Figure S12a). Notably, unlike merocyanine, in the aggregated state, naphthalene protons are known to exhibit upfield shift due to shielding effect.18 Contrastingly, no variations in the chemical shift of naphthalene protons (Ha) for P4 were observed with increasing temperature (Figure S12b), suggesting no π-stacking in toluene. Interestingly, the spectrum of P4 at 20 °C in toluene-d8 appeared nearly identical to that at 110 °C, contrary to P2 where the spectrum at 20 °C showed much broader peaks. This was further illustrated by plotting the integration ratios of naphthalene (Ha) and alkyl chain (Hb) protons (Figure S12c) of P4 with increasing temperature, which remained invariant unlike P2 (Figure 4b). Therefore, the control experiment ascertains that the assembly of P2 is indeed dominated by dipole−dipole interaction of MD.
Based on the above experimental results, a model (Scheme 2) is proposed for MD-triggered nanogel formation by P2. It is Scheme 2. Model Showing Cross-Linked Nanogel of P2 Utilizing Antiparallel Dipolar Assembly of MD
believed that many P2 chains assemble together to form nearly spherical nanogel, which is stabilized by supramolecular crosslinking through dipole−dipole interaction of MD. The model was further confirmed by 2D NMR spectroscopy. In a closely packed antiparallel array of MD (as shown in Scheme 2), spatial proximities between the protons of the donor (D) and the acceptor (A) heterocyclic rings of MD should lead to through space coupling between them (Figure 4c, inset cartoon) unlike when they are stacked in parallel fashion. Nuclear Overhauser Effect Spectroscopy (NOESY) was performed in toluene-d8 (Figure 4c) to examine the mode of stacking of the dye within the nanogel. Presence of cross-peaks in the NOE spectrum between the proton pairs of the D and A ring, Hd−Ha, Hd−He, and Hd−Hb/Hb′/Hc, confirmed antiparallel stacking between the MD, thereby nicely supporting the proposed model. In summary, relatively unexplored supramolecular approach is described for assembly of MD-conjugated flexible macromolecules based on directional, self-complementary, and strong dipole−dipole interaction mediated dimerization of merocyanine dye (MD). Such a system preserves the precision of dipolar assembly of the dye even within the polymer domain that bestows it with a bunch of interesting features such as aggregation-induced emission, remarkably stable nanoparticles, and guest docking properties. It is interesting to note that, for earlier reported nanogels,14 it is a prerequisite to form a micelle-like structure, followed by core cross-linking by covalent reaction. In contrast, the present systems offer a more straightforward way of preparing their supramolecular counterparts and thus bring new opportunities for preparing supramolecular nanogels with tunable properties. Considering the significance of dipolar assembly of MD in the context of nonlinear optics and organic photovoltaics, with the present modular design principle, a library of structurally diverse MDfunctionalized polymers with varying compositions of MD can be constructed from a single parent polymer to further elucidate its impact on the self-assembly, photophysical properties, and morphology of those engineered polymers, potential materials in organic electronic. Thus, we believe that the outcome of this work will add significantly in bridging the gap between supramolecular chemistry and polymer assembly and open up new possibilities in tailor-made macromolecular assemblies. 54
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00898. Synthesis, experimental details, and supplementary results (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Patrick Theato: 0000-0002-4562-9254 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.D. thanks the Alexander von Humboldt Foundation for a postdoctoral fellowship. S.L. acknowledges the China Scholarship Council (CSC, Grant 201306240132) for partial support of this work. We acknowledge Mr. Haridas Kar of Indian Association for the Cultivation of Science (IACS), India for AFM studies.
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