Santhosh
Kumar Podiyanachari
a,
Maciej
Barłóg
b,
Mohammed
Al-Hashimi
*b and
Hassan S.
Bazzi
*ac
aDivision of Arts and Science, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar. E-mail: hassan.bazzi@qatar.tamu.edu
bDepartment of Chemical Engineering, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar. E-mail: mohammed.al-hashimi@qatar.tamu.edu
cDepartment of Materials Science & Engineering, Texas A&M University, 209 Reed MacDonald Building, College Station, TX 77843-3003, USA
First published on 9th May 2024
Amphiphilic copolymers of highly conjugated poly(acetylene)s and poly(ethylene glycol)-functionalized perylene diimide (PEG750–PDI)-incorporating poly(olefin)s have been synthesized via tandem cyclopolymerization and ring-opening metathesis copolymerization (ROMCP) methodologies. Both di- and tri-block copolymers were prepared from 1,6-heptadiyne and oxanorbornene imide-based cycloolefin monomers using ruthenium-based alkylidene initiators. The relative atomic weight percentages of both di- and tri-block copolymers were estimated using X-ray photoelectron spectroscopy (XPS) analysis. Photophysical properties of both copolymers have been explained based on both UV-Vis and fluorescence spectroscopic analysis shedding more light on their different stages of organization as well as the π–π stacking interactions of the PEG750-incorporating perylene cores in aqueous solutions. Furthermore, these investigations elucidate the modulation of the photophysical properties and H-type aggregation processes of polymers in aqueous solutions that lead to the formation of PEG750–PDI-derived amphiphiles. Copolymeric surface analysis, segmental patterns and film morphology were examined by atomic force microscopy (AFM) revealing globular or spherical morphological features enhanced by various functional groups present in the polymer bulk. In addition, the formation of these spherical morphologies was further visualized in the thin film cross-sections of both di-and tri-block polymers by scanning electron microscopy (SEM) to confirm the surface morphologies determined by AFM analysis of both polymeric materials.
Choi and co-workers also explored different Grubbs’ catalysts for the cyclopolymerization of different functionalized terminal di(alkyne) monomers. Their investigation led to the synthesis of the first example of β-selective cyclopolymerization-derived poly(ene)s, characterized by exclusive incorporation of six-membered ring structures along the polymer backbone.12–20 Those conjugated polymers obtained via cyclopolymerization have found extensive use in the fabrication of nanosheets, nanoparticles, and various self-assembly applications.21–26 Furthermore, Choi has extended the application of cyclopolymerization, by reporting the synthesis of diblock copolymers, combining cyclopolymerization with ring-opening metathesis (ROM). Copolymers obtained via cyclopolymerization and ROMCP have been specifically employed in the preparation of nanostructures and the synthesis of polyolefin-linked conjugated poly(ene)s under controlled and living polymerization.16,27–29 In addition, such random and diblock copolymers containing conjugated polymer backbones with various functionalities have been well-studied in various self-assembly applications.30
However, there are a limited number of reports on the synthesis of such copolymers based on poly(ene)s and poly(olefin)s via tandem cyclopolymerization and ROMCP in a unified, one-step metathesis polymerization process. Our research group has actively contributed to this field, investigating the alkyne-insertion metathesis polymerization31 and ROMP reactivities of diverse functional and nonfunctional cycloolefin monomers using Grubbs or Hoveyda–Grubbs type [Ru]–alkylidene initiators. This approach has led to the development of a new class of structurally well-defined poly(olefin)s.32–37 Our efforts have yielded a series of functional poly(olefin)s based on polypentenamers,36,38 poly(vinyl alcohol)s,34,38 poly(norbornene)s32,33,39,40 and rhodamine-based poly(olefin)s showcasing versatility in material design for various biological applications.41,42 We recently reported the synthesis of highly conjugated bay-functional perylene diimide chromophore incorporating poly(oxanorbornene imide)s via ROMP,43 by investigating their hyperbranched propensity behavior.44 Based upon our findings, ROMP-derived poly(olefin)s containing PDI-grafted alkoxy-functionalized poly(oxanorbornene imide)s are promising candidates for the preparation of a diverse range of novel polymeric materials tailored for applications in organic electronics.43,44
In this work, we describe a tandem approach for the synthesis of amphiphilic copolymers, combining conjugated poly(acetylene)s and poly(ethylene glycol)-incorporating perylene diimide (PEG750–PDI)-grafted poly(olefin)s via cyclopolymerization and ROMCP. The copolymers derived from PEG–PDI have attracted considerable attention, particularly due to their unique structural and dynamic characteristics in diluted aqueous solutions.45–49 Investigations by various research groups have revealed the amphiphilic nature of these compounds and their self-assembly behavior, driven by a delicate interplay between hydrophobic and π–π interactions. The capacity to manipulate their aggregation behavior through molecular engineering, temperature variation, and solvent composition has paved the way for the incorporation of these systems into a diverse range of applications, including catalysis, membranes, nanotubes, sensing and bioimaging agents.50–54 Furthermore, the photoluminescence properties of this class of materials, such as the quantum yield (ϕPL) of PDI derivatives, often face limitations, like aggregation caused quenching (ACQ) at high concentration in solution or in the solid film state. Tuning the fluorescent properties of the PDI unit from ACQ to aggregation induced emission (AIE) has been typically achieved via molecular engineering of both the amide and bay functionality with bulky groups designed to control the π–π stacking and aggregation behavior.55–57 Our tandem approach offers a versatile platform for accessing a range of tunable PEG750-incorporating chromophores within the non-conjugated polymer matrix, serving as a copolymer counterpart of the cyclopolymerization-derived conjugated poly(acetylene). Our findings demonstrate that tandem cyclopolymerization, followed by ROMP, yields copolymers with diverse properties and applications including electrical conductivity, bioimaging and photoluminescence. This methodology provides a promising route for the preparation of novel polymeric materials with tailored properties.
To the best of our knowledge, this work constitutes the first example of incorporating perylene cores into a non-conjugated polymer, with optical properties modulated by the poly(ene) block composition and the polarity of the medium. This innovative approach to control PDI aggregation through the strategic integration of perylene cores into non-conjugated polymers, coupled with the tunable and reversible nature of the interactions, introduces new avenues for the advancement of this class of systems. The modular composition of the copolymer, allowing for tailored adjustments, presents a versatile platform with potential applications across various fields.
The copolymer composition and structural features of copolymers P3–P6 were characterized by 1H NMR spectroscopy. The 1H NMR spectrum of copolymer P3 displays a broad multiplet together with a singlet signal ranging from δ = 6.6–6.9 ppm, indicating olefinic protons within the conjugated backbone of poly(acetylene) (P1), resulting from the cyclopolymerization of the dialkyne monomer M1 and the olefinic protons of the poly(oxanorbornene imide) copolymer counterpart derived from PEG-containing oxanorbornene imide monomer M2. Triblock copolymer P4 shows an additional olefinic proton signal along with the abovementioned peaks for P3 as a broad singlet signal at 6.0 ppm representing the poly(oxanorbornene imide) copolymer counterpart derived from N-alkyl substituted oxa-norbornene imide monomer M3 (Fig. S3–S6, ESI†). In the 1H NMR spectra of random diblock P5 and triblock P6 copolymers, olefinic proton signals appeared as heterodyad peaks around 6.9 ppm and 5.9 ppm indicating olefinic proton signals of conjugated backbones and poly(oxanorbornene imide)s (Fig. S7–S10, ESI†).
The relative atomic weight percentages of the constituent elements in copolymers P3 and P4 were further characterized by XPS analysis. In the XPS spectra, characteristic signals corresponding to carbon, nitrogen and oxygen atoms within the copolymer composition were observed, with corresponding binding energies (BE) at 282 eV for carbon, 397 eV for nitrogen, and 529 eV for oxygen, respectively (Fig. S11†). These binding energy values were consistent for carbon, nitrogen, and oxygen in both diblock P3 and triblock P4 polymers. The relative atomic weight percentages estimated from the peak areas of the carbon, nitrogen and oxygen signals were determined for diblock copolymer P3 as 74.79% for carbon, 1.19% for nitrogen, and 20.68% for oxygen. In the case of the triblock copolymer P4, the relative atomic weight percentages were estimated as 77.33%, 1.99%, and 24.02%, respectively, for carbon, nitrogen, and oxygen. The slight excess in the relative weight percentages of carbon, nitrogen, and oxygen in the case of the triblock copolymer further indicates the presence of different C, N, and O proportions in the triblock copolymer P4. This XPS analysis provides additional confirmation of the distinct atomic weight percentages of the di- and tri-block copolymers.
The stability of copolymers P3 and P4 was investigated by thermal gravimetric analysis (TGA), conducted in a temperature range from 30 to 800 °C at a heating rate of 10 °C min−1 under a nitrogen atmosphere. The thermal decomposition temperatures (Td) corresponding to 5% weight loss were determined, resulting in Td values of 219 °C and 377 °C for P3, and 234 °C, 391 °C, 484 °C for P4 (Fig. S13, ESI†). The high thermal degradation temperature for diblock copolymer P3 can be attributed to the presence of the conjugated poly(ene) in the main chain and the incorporation of the bulky PEG750–PDI-based cycloolefin as the copolymeric counterpart. Similarly, P4 containing a bulky polyolefinic counterpart of PEG750–PDI and alkyl-functionalized oxanorbornene imide units along with the conjugated poly(acetylene) led to the three different degradation temperatures mentioned above. These molecular and structural features play an important role in enhancing the thermal stability of the copolymers. Differential scanning calorimetry (DSC) was used to determine the glass transition temperature (Tg). The samples were initially heated from 30 to 400 °C to complete the first cycle of the calorimetry experiment, then cooled to room temperature, and re-heated again from 30 to 400 °C at a heating/cooling rate of 10 °C min−1. There was no significant Tg value recorded for the copolymer samples in the temperature range from 40 to 400 °C. This absence of a glass transition temperature aligns with observations in previously reported PDI-derived polymers.44,59
The UV-Vis absorption spectra of the tandem cyclopolymerization- and ROMP-derived block copolymers are displayed in Fig. 1b and c revealing specific characteristic features. In solution polymers P2, P3 and P4 show virtually identical profiles with a characteristic double maximum at 527 and 563 nm. Additionally, all 3 polymer solutions exhibit a shoulder around 500 nm, which was assigned to the presence of the polyacetylene fragment with high absorption in this region.6 In the solid film state, polymers P2, P3, and P4 exhibit substantially different absorption profiles, despite no changes to the chromophore core. Polymer P2 in the solid film exhibits an almost complete absence of the S0–0 transition signal and a significant blueshift of the S0–1 maxima to 514 nm. This phenomenon was attributed to strong π–π interactions between perylene cores, further supported by regular distances between them, enforced by the homopolymeric structure of the backbone. The film of the diblock polymer P3 shows an absorption profile like that of polymer P2 with a single absorption peak blue shifted to 512 nm indicating similar π–π interactions and formation of H-aggregates. On the other hand, the triblock polymer P4 exhibits its maximum absorption redshifted to 556 nm as a broad band with a shoulder around 500 nm. These differences underscore the significant variations in perylene aggregation behavior influenced by the polymer structure. We assume that changes in the stacking of the perylene cores and thus changes in the absorption spectra are the result of the interplay of several factors such as π–π stacking and hydrophilic and lipophilic fragment interactions combined with steric effects exerted by the polymer backbone. Moreover, the reduction of the intensity of the 0–0 and 0–1 absorption peaks and the drop in fluorescence efficiency indicate H-type aggregation rather than J-type aggregation in the THF and THF/water solutions.73
The modular structure of block copolymers P3 and P4 allows for the introduction of both hydrophobic hydrocarbon and aromatic groups, as well as hydrophilic PEG750. This composition enables precise control of the polymer aggregation based on the medium's polarity. The amphiphilic nature of these PEG750–PDI-based polymers allows them to be dissolved in typical hydrophobic solvents like chloroform or chlorobenzene, as well as highly polar solvents such as DMF and methanol, which has been also reported for other PDI derivatives.74 These features allow amphiphiles to self-assemble in aqueous solutions making them suitable for use as markers in biological systems.54 This self-assembly behaviour results in significant changes in both their absorption and emission spectra, which are influenced by the polymer composition and polarity of the solvent. The absorption profile in THF/water solutions shows a decrease in the intensity of the vibronic progression peak around 530 nm, illustrating the effect of the aqueous fraction on the aggregation behaviour of the polymers (Fig. 2a–c).
This effect has been widely reported in the literature and is typically assigned to the formation of aggregates with strong π–π stacking interactions among the PDI cores in polar solvents.54,65,75–77 Moreover, the intensity ratio of S0–1/S0–0 can be used to qualitatively evaluate the proximity of the PDI units in solution and the degree of aggregation in a given system.78 This effect is further amplified by raising the aqueous concentration in the solvent, subsequently increasing the polarity of the medium and allowing for additional hydrogen bonding interactions. Raising the aqueous concentration up to 100% water results in a significant decrease in absorption in the visible region for copolymers P3 and P4 and the observable formation of micelles. This change can be attributed to the presence of an additional alkylated monomer, which significantly alters the polymer's hydrophobic properties. Interestingly, homopolymer P2, containing the highest content of PEG750 chains, exhibits a clear absorption peak at 530 nm even with 100% water content. This can be rationalized as the effect of the PEG chains providing solubility in highly polar solvents, countering their aggregating impact on the PDI cores. When the solvent was replaced with a higher polarity one such as methanol or a MeOH/water mixture copolymers P3 and P4 exhibited broad maxima around 530 nm with decreasing intensity as the water content increased. A similar trend was observed for P2; however, in solution the PDI absorption profile was retained, confirming the ability of P2 to retain partial solubility in highly polar solvents (Fig. S14–S16, ESI†).
To further understand the aggregation modes of the polymers and their structure dependence, the emission spectra of polymers P2, P3 and P4 were analysed (Fig. 3). In a chloroform solution, the polymers exhibit an emission profile that closely matches the one reported in the literature for phenoxy bay functionalized PDI.66 This spectrum shows strong peaks with a maximum at 593 nm and a characteristic shoulder in the emission spectrum at around 640 nm. In the thin film state, the emission spectra exhibit a singular strong band around the 700 nm region, which is consistent with previous reports on PDI-based aggregates and supramolecular structures.74,79–81 Interestingly this emission is clearly visible in the solution of homopolymer P2 and random copolymers P3 and P4, which can be assigned to enhanced π–π interactions and the directing effect of the specific polymeric backbone structure (Fig. 3). In addition, analysis of the emission of the chloroform solutions and thin films of diblock polymers P3vs. P5 (Fig. S17, ESI†) and triblock polymers P4vs.P6 (Fig. S18, ESI†) indicated identical PDI-chromophore cores and degrees of aggregation, and thus the photophysical properties can be altered by changing the backbone composition. The films of polymers P3–P6 exhibit consistent emission at 700 nm indicating that strong π–π stacking is consistent with the 0–2 transition becoming predominant over the lower-energy processes in every scenario. Interestingly even in chloroform solution a significant emission band can be observed at 700 nm indicating that consistent distances between the PDI cores enforced by the polymer backbone support AEI, even in highly diluted media.
Due to their modular structure and amphiphilic properties, the polymers studied here exhibit solubility in a wide range of solvents and all three polymers display similar emissions in THF. However, the introduction of water results in significant and dramatic changes in their fluorescence profiles indicating different π–π stacking aggregation of the PDI cores. Depending on the composition of the polymers and the solvent, the emissions from these polymers exhibit typical characteristic features of both solution and solid states. The homopolymer P2 (Fig. 4a), which contains the highest concentration of both PEG750 and PDI fragments, shows a significant decrease in emission at 586 nm when 5% water is added. However, when the aqueous concentration is increased up to 10%, there is only a minor further change in emission. This is accompanied by a red shift and a gradual enhancement of the 0–2 emission signal. This phenomenon can be estimated as reaching an equilibrium point where π–π stacking interactions are balanced by the increasing impact of the PEG moieties in a more polar medium. In the case of the diblock polymer P3 (Fig. 4b), which contains additional polar diester groups, there is a slower drop in the intensity of the main emission band at 587 nm upon the addition of water. However, there is a significant increase in the solid-state emission band when the aqueous concentration reaches up to 10%. We assume that the 10% aqueous concentration in solution is the point at which the ester groups are no longer polar enough to keep the polymer in solution, leading to rapid aggregation. A similar scenario was observed for the triblock copolymer P4 (Fig. 4c), where the presence of additional aliphatic fragments caused an even more pronounced increase in the solid-state emission at 700 nm, becoming the main band for this material. A schematic representation of the aggregation of both polymers P3 and P4 is outlined in Scheme 2 displaying self-assembling factors such as π-stacking and the formation of hydrophobic and hydrophilic domains as well as the directing effect of the backbone composition. When the solvent was replaced with the more polar MeOH the predominant peak at 700 nm was observed for all the polymers, with additional absorption at 600 nm observed for P3 and P4. When the water content was increased from 0–100% all the polymers exhibited solely 700 nm maxima with various intensities (ESI†).
Nearly spherical morphologies were also evidenced, as expected, by SEM analysis of the cross-sections of copolymers P3 and P4 confirming the surface morphologies determined by AFM analysis. SEM images of the copolymers also showed various sizes of spherical morphologies and aggregation of cores dispersed within the poly(ene) and poly(oxanorbornene) copolymer matrix (Fig. 6A and B). It is well known that the dispersion of these polymer blocks in a polymer matrix plays a dominant role in influencing the morphological properties of the polymer.83 However, the type of aggregation of PEG-grafted PDI groups and the phase separation in these two copolymers have not been well addressed by SEM analysis.84
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Fig. 6 SEM images of cross-sections of diblock copolymer P3 (A) and triblock copolymer P4 (B) films prepared by controlled evaporation from CHCl3. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4py00078a |
This journal is © The Royal Society of Chemistry 2024 |