Spontaneous Formation of Polymeric Nanoribbons in Water Driven by π–π Interactions

Abstract

A simple method was developed to produce polymeric nanoribbons and other nanostructures in water. This approach incorporates a perylene diimide (PDI) functionalized by triethylene glycol (TEG) as a hydrophobic supramolecular structure directing unit (SSDU) into the core of hydrophilic poly(N,N‐dimethylacrylamide) (PDMAc) chains using RAFT polymerization. All PDI‐functional polymers dissolved spontaneously in water, forming different nanostructures depending on the degree of polymerization (DP _n): nanoribbons and nanocylinders for DP _n=14 and 22, and spheres for DP _n>50 as determined by cryo‐TEM and SAXS analyses. UV/Vis absorption spectroscopy was used to monitor the evolution of the PDI absorption signal upon dissolution. In solid form, all polymers show a H‐aggregate absorption signature, but upon dissolution in water, the shortest DP _n forming nanoribbons evolved to show HJ‐aggregate absorption signals. Over time, the J‐aggregate band increased in intensity, while cryo‐TEM monitoring evidenced an increase in the nanoribbon's width. Heating the nanoribbons above 60 °C, triggered a morphological transition from nanoribbons to nanocylinders, due to the disappearance of J‐aggregates, while H‐aggregates were maintained. The study shows that the TEG‐PDI is a powerful SSDU to promote 2D or 1D self‐assembly of polymers depending on DP _n through simple dissolution in water.

Diving into the nanoscale! We present a simple method to obtain polymeric nanoribbons in water using a tailored perylene diimide as a supramolecular structure directing unit. We study their formation over time and their temperature sensitivity.

Introduction

Anisotropic polymeric nanostructures, such as nanocylinders, nanoribbons or nanosheets, can be employed in a great variety of applications, for instance as stabilizers of Pickering‐Ramsden emulsions, ^[1] as sensors, ^[2] or in organic electronics. ^[3] The facile access to well‐defined, high‐aspect ratio nanostructures remains a challenge, especially in water. One strategy to steer polymer assemblies towards anisotropic structures is to combine typical scaffolds used in supramolecular chemistry with polymers.[ ⁴ , ⁵ , ⁶ ] The functionalization of water‐soluble polymer chains by a supramolecular structure directing unit (SSDU) can drive their self‐assembly in water into distinct morphologies.[ ⁷ , ⁸ ] For instance, SSDUs based on directional hydrogen‐bonding, such as bis‐ureas, can promote their one‐dimensional (1D) assembly into polymeric nanocylinders.[ ⁹ , ¹⁰ ] For assembly in water, such hydrogen‐bonded stickers are generally combined with hydrophobic building blocks (e. g. long alkyl chains ^[11] or aromatic groups ^[12] ) to create a hydrophobic environment and reduce the competition with water molecules for hydrogen bonding. For instance, Colombani et al. functionalized a poly(ethylene glycol) (PEG) with a naphthalene diimide (NDI) bearing a bis‐urea unit to form nanocylinders in water. ^[13] Ghosh et al. synthesized a variety of hydrophilic polymers end‐capped by a NDI combined with amide or hydrazide functions to drive the formation of cylindrical micelles.[ ¹⁴ , ¹⁵ , ¹⁶ ] Other aromatic SSDUs, such as benzene tripeptide, were also combined with PEG and allowed the formation of supramolecular polymer bottlebrushes in water.[ ¹⁷ , ¹⁸ ] Polymers functionalized by large aromatic cores, such as perylene diimide (PDI), were combined with peptides to form kinetically trapped fibrous structures in water/THF mixtures. ^[19] In the previous examples, the SSDUs were incorporated at the core/end of the polymer chains, but other examples exist where SSDUs were incorporated into the polymer side‐chains. For instance, non‐spherical polymersomes were formed from chains grafted by pendant perylene derivatives.[ ²⁰ , ²¹ ]

Among anisotropic nano‐objects, ribbons are an interesting structure because they combine directionality with a flat surface, i. e. they are intermediate between cylinders and lamellae. However, polymer‐based nanoribbons have only scarcely been studied because they are difficult to obtain. The main strategy for obtaining such nanostructures is based on crystallization‐driven self‐assembly. ^[22] This has been achieved with polypeptides forming ribbon‐like structures in polar organic solvents (MeOH, EtOH),[ ²³ , ²⁴ ] but also in water. ^[25] Park et al. demonstrated that polymeric nanoribbons could be obtained using an amphiphilic polythiophene‐PEG block copolymer, by the slow addition of water into a methanol solution thanks to a combination of hydrogen bonding and π–π stacking. ^[26]

It has been reported that molecular PDI derivatives can assemble into nanoribbons driven by π–π stacking.[ ²⁷ , ²⁸ ] Most examples among these non‐polymeric systems report the formation of nanoribbons in organic solvents.[ ²⁹ , ³⁰ , ³¹ , ³² , ³³ , ³⁴ ] Others form nanoribbons by the addition of water to an organic co‐solvent such as EtOH, ^[35] THF ^[36] or MeOH. ^[37] For PDI assemblies in water, a common strategy is to functionalize the PDI with hydrophilic neutral or ionic substituents. ^[38] For instance, ethylene glycol oligomers (e. g. triethylene glycol, TEG) have been introduced to increase the water solubility of PDI while retaining interesting aggregation behavior. ^[39] Using a heating, cooling and aging process, a chiral PDI functionalized with TEG formed multilayered rectangular ribbon‐like structures in low concentrations (5.10⁻³ mg/mL) in water. ^[40] Würthner et al. also demonstrated that PDI‐based nanoribbons can form in water from initial nanorods via a fusion/fission mechanism by increasing progressively concentration from 0.08 mg/mL to 1 mg/mL. ^[41]

To the best of our knowledge, the spontaneous formation of polymer‐based nanoribbons by simple dissolution of the solid in water has never been reported yet. We envisioned that the strong π–π interactions of PDI and the possibility to functionalize it by hydrophilic substituents in various positions should make it a promising SSDU to achieve spontaneous but directional assembly of hydrophilic polymers in water. In order to limit lateral aggregation in water, we designed a symmetric PDI functionalized at the two bay (1,7) positions by hydrophilic TEG chains, while the imide positions were functionalized by two universal RAFT agents to grow laterally hydrophilic polymer chains with controlled chain lengths. Tuning the length of these chains should provide us with a tool to fine‐tune the extent of lateral aggregation and thereby help to achieve not only 1D nanocylinders, but also nanometric 2D nanoribbons (for which limited lateral aggregation is necessary). To proof our concept, we chose a neutral poly(N,N‐dimethylacrylamide) (PDMAc) as the hydrophilic polymer whose molar mass was systematically tuned by classical RAFT polymerization. The assemblies were then formed by direct dissolution of the PDI‐functionalized PDMAc in water and their formation studied. Cryo‐TEM, SAXS and UV/Vis absorption spectroscopy were used to characterize the assemblies in terms of morphology, dimensions and the type of PDI aggregation predominant in the structures (i. e., H‐ and J‐aggregates).

Results and Discussion

Synthesis of PDI‐functional PDMAc

First, a perylene diimide difunctional RAFT agent (PDI diTTC, Scheme 1) was synthesized in six steps (Schemes S1 & S2). The PDI was functionalized at the bay positions with two TEG units, ^[39] and at both N‐imide positions with a universal trithiocarbonate (TTC) RAFT agent ^[42] through an esterification step. All procedures and characterizations are reported in the Supporting Information (cf. Figures S1–S10).

Scheme 1.

Synthesis of PDI‐functional PDMAc through RAFT‐mediated polymerization in the presence of PDI diTTC.

The PDI diTTC RAFT agent was then used for the synthesis of the PDI‐functional PDMAc, a hydrophilic and neutral polymer (Scheme 1, procedure in the SI). Four polymerswith increasing DP_n , P1‐P4 _, were prepared by changing the ratio of RAFT agent to DMAc, as shown in Table 1 (and Table S1). The final polymers were characterized by size exclusion chromatography (SEC) to study the incorporation of PDI into the polymer chains and determine the molar mass distribution (Figure S11). The UV/Vis detector was set at 560 nm where only the PDI unit absorbs (Figure S13). The good correlation between RI and UV/Vis signals, and the negligible residual UV/Vis signal at 33 mL (V_el of PDI diTTC) proved the quantitative incorporation of PDI in the polymer chains (Figure S11). The number‐average molar mass (M _n) of the polymers was determined using ¹H NMR spectroscopy, UV/Vis spectroscopy, and SEC (Figure 1, full details in SI, Figures S11–14). Low molar mass dispersities (Đ <1.2) were determined by SEC for all polymers synthesized. Kinetic monitoring of the polymerization of P4 showed a linear increase in M _n with monomer conversion (Figure 1), suggesting high chain transfer efficiency of PDI diTTC agent and a good control of the polymerization.

Table 1.

Characteristics of the PDI‐functional PDMAcs.

Entry	DP n, NMR [a]	M n,NMR [a] (kg/mol)	M n,UV/Vis [b] (kg/mol)	M n,SEC [c] (kg/mol)	Đ [c]
P1	14	2.7	3.0	2.3	1.14
P2	22	3.5	4.0	2.7	1.20
P3	52	6.5	6.1	5.4	1.18
P4	139	15.2	13.3	13.0	1.14

a) Number‐average degree of polymerization, DP _n, and number‐average molar mass, M _n, of the purified polymers determined by ¹H NMR in CDCl₃. b) M _n of the purified polymers determined by UV/Vis spectroscopy in THF using equation S2 with ϵ=44 982 L/mol/cm. c) M _n and dispersity, Đ, of the purified polymers determined by SEC in DMF (+ LiBr 1 g/L) using PMMA standards.

Figure 1.

Kinetic monitoring of the polymerization of P4. a) Overlay of the normalized SEC RI traces. b) Overlay of the normalized SEC UV/Vis traces at 560 nm. The DMAc conversions determined by ¹H NMR are indicated at the top. c) Number‐average molar mass (filled circles •) and dispersity (empty squares □) determined by SEC with a conventional PMMA calibration. The dashed line shows the theoretical evolution of the molar mass with the DMAc conversion.

Spontaneous Self‐assembly in Water

The objective of our work was to use a simple preparation pathway based on direct dissolution (Figure S15) to obtain nanoribbons in water driven by π–π interactions. Visually, all polymers dissolved spontaneously in water at a concentration of 10 g/L and no heating or ultrasonic treatment was required.

The aqueous solutions of P1, P2 and P3 were studied by cryo‐TEM analyses at 10 g/L (Figure 2a–c). Cryo‐TEM images of P1 with the lowest DP _n showed nanoribbons with an average width of 18.5±2.8 nm and a thickness of 5.8±0.9 nm (Figure 2a, Table S2). When the DP _n was increased from 14 to 22, nanoribbons were still observed but their width was significantly reduced to 11.4±2.3 nm (see Table S2), and the presence of nanocylinders with a cylindrical section could not be excluded (P2, Figure 2b). For P3 (Figure 2c) and P4 with DP _n=52 and 139 respectively, only small spherical (<10 nm) aggregates were observed.

Figure 2.

Spontaneous self‐assembly of P1, P2 and P3 by direct dissolution in water. (left) Representative cryo‐TEM pictures of a) P1, b) P2, c) P3 aqueous solutions at 10 g/L after 1 month. The dark spots are surface contaminations stemming from water crystals. (right) SAXS traces of d) P1, e) P2, f) P3 solutions at 10 g/L in H₂O/DMF (99/1). The black lines are models fitting the data (details shown in Table S3). N.B. SAXS analyses were performed in H₂O/DMF (99/1), but supplementary cryoTEM (Figure S16) and UV/Vis analyses (Figure S24) performed in H₂O and H₂O/DMF (99/1) showed no impact of 1 % of DMF.

In addition to cryo‐TEM, SAXS was also used to determine the morphology of the nanostructures (Figure 2d‐f, Table S3). The q⁻² decay at low q for P1 is characteristic of the presence of flat objects. The lamellae model was suitable only for the low q data (Figure S17a) whereas the high q data were fitted using an elliptical cylinder model (Figure S17b). The combination of such elliptical cylinder model with a lamellae model was then necessary to obtain a proper fit over the whole q range (Figure 2d). This result also suggests the presence of large objects, not observed by cryo‐TEM. While the width of the nanoribbons was polydisperse (18.5±2.8 nm), consistent with the presence of lamellae of large dimensions, the thickness determined to 4 nm by fitting SAXS data agrees with the dimensions measured by cryo‐TEM (Table S3). For P2, the data could be fitted best with an elliptical cylinder model (with a width of 21 nm and a thickness of 4.8 nm), except at very low q values where a slope of q⁻³ is observed, suggesting again the presence of larger aggregates (Figure 2e). Finally, for P3 (Figure 2f) and P4 (Figure S18), a simple sphere model nicely fitted the data and suggested the presence of nano‐objects of a size around 5 nm in diameter in both cases (Table S3).

A previous study by our group showed that the functionalization of PDMAc by a bis‐urea‐SSDU promoted 1D assembly into nanocylinders functionalized, while spherical objects were formed with a PDMAc functionalized with an alkyl group. ^[9] In both cases the DP _n of PDMAc (DP _n=11) was similar to P1, suggesting that the PDI acts here as a typical SSDU promoting 2D assembly through π–π interactions that are limited by the TEG chains. When using bis‐urea ^[9] or tris‐urea[ ⁴³ , ⁴⁴ ] as SSDU, the importance of the DP _n on the efficacy of the SSDU was also discussed, and stated that steric hindrance competes with the directional supramolecular interactions (in their case H‐bonding). The weaker the supramolecular interactions, the stronger the effect of steric hindrance, which can completely override the directional supramolecular interactions resulting in drastic changes in the assemblies.[ ⁹ , ⁴³ ]

Given that PDI is a good chromophore, UV/Vis spectroscopy proved to be a useful technique to study PDI aggregation in water (Figure 3). The spectrum of all PDI‐functional PDMAc was first recorded in THF (Figure 3a–c, Figure S20a, c), which is a good solvent for both PDI and PDMAc. The resulting spectra are all similar to those reported for PDIs substituted by similar groups at the bay positions,[ ³⁹ , ⁴⁵ ] and no significant influence of the polymer chain length could be observed. All spectra show the typical signature of PDI with bands at 563 nm and 527 nm, corresponding to the 0→0 and the 0→1 transitions respectively. ^[39] The A_0‐0/A_0‐1 ratio can be used to interpret the level of aggregation of PDI dyes. For all polymers in THF, this ratio is above 1.3 (Figure S20a, c), which corresponds to the non‐aggregated PDI species (unimers). ^[38] The formation of aggregates can be revealed when this ratio reaches a value below 0.7. ^[38]

Figure 3.

(top) UV/Vis absorption spectra of a) P1, b) P2, c) P3 in THF (in black) and in H₂O (colored lines) and (bottom) d‐f) Magnification of the spectra in H₂O. Spectra were recorded after 1 month of dissolution at a concentration of 0.029 g/L (3.4 10⁻⁵ M) for P1, 0.044 g/L (1.1 10⁻⁵ M) for P2 and 0.076 g/L (1.2 10⁻⁵ M) for P3. All spectra were recorded at 20 °C.

The shape of the spectra was completely different when recorded in water (at the same concentration as in THF). The spectra obtained were broader, with a much lower intensity (e. g., for P1, A_max(THF)=0.44 and A_max(H₂O)=0.09, Figure 3a), and did not show the distinct bands observed in THF (at 563 nm and 527 nm) suggesting PDI aggregation.[ ⁴⁶ , ⁴⁷ ] New broad bands appeared, which are characteristic of different types of PDI aggregates (Figure S19). ^[28] In H‐aggregates, PDI molecules are stacked with a co‐facial organization, which produces a hypsochromic (blue) shift in the spectrum.[ ⁴⁶ , ⁴⁸ ] In contrast, a bathochromic (red) shift is observed when PDIs form J‐aggregates with a staggered stacking. The term HJ‐aggregates is used when both type of aggregates are present in nanostructures.[ ⁴⁹ , ⁵⁰ ] The spectrum of P1, for which nanoribbons were observed by cryo‐TEM, clearly shows the presence of HJ‐aggregates with two new bands at 513 nm and 683 nm (Figure 3d) characteristic of H‐aggregates and J‐aggregates respectively. ^[46] Based on the literature, ^[51] the H‐aggregates should be responsible of 1D assembly into long objects, whereas the J‐aggregates might cause lateral aggregation leading to 2D nanoribbon formation.

The same bands were observed for P2, with the presence of HJ‐aggregates, however the intensity of the J‐band at 683 nm is lower compared to sample P1. This decrease of the A_(683nm)/A_(520nm) ratio from 0.78 for P1 to 0.56 for P2 highlights that P1 has a higher proportion of J‐aggregates compared to P2. This is consistent with the presence of narrower nanoribbons / nanocylinders for P2 (Figure 3e, Table S2). For sample P3 and P4 (Figure 3f and Figure S20c, d) only H‐aggregates with bands at 528 (A_0‐1) and 600 nm (A_0‐0) were observed, showing that J‐aggregates are not present. As mentioned before, the A_0‐0/A_0‐1 ratio can be used to probe the aggregation of PDI. For both P3 and P4, this ratio is less than 0.7, indicating aggregated species in water (Figure S20b, d).

Overall, the UV/Vis experiments confirmed that the polymer chain length has a great impact on PDI aggregation and, consequently, on the morphology obtained.[ ⁹ , ⁴³ ] In nanoribbons, HJ‐aggregates are present whereas only H‐aggregates are observed in the spherical aggregates.

Impact of Dilution

Supramolecular assemblies are usually concentration‐dependent ^[16] and dissociation of the assemblies into unimers at sufficiently dilute conditions (below a critical aggregation concentration) is possible.[ ⁹ , ⁵² , ⁵³ ] We therefore performed concentration‐dependent UV/Vis analyses on P1 aqueous solutions. No clear impact of the dilution was observed over the concentration range studied (Figure 4a), even when diluted to a very low concentration (10⁻⁷M, Figure S21a). Moreover, no evolution was observed 3 days after dilution (Figure S22). This is demonstrated by monitoring the ratio between the bands at 683 nm and 520 nm, which showed little to no change with concentration (Figure S21b), and a quasi‐linear evolution of the absorbance at 683 nm with concentration (Figure S21c). These results suggest that the aggregates persist upon dilution. In contrast, the addition of a good solvent (THF in 50/50 proportion) to the water solution induced their dissociation into unimers (Figure 4b), as suggested by the appearance of distinct bands at 571 and 535 nm, characteristic of A_0‐0 and A_0‐1 transitions respectively, in a ratio of 1.13. ^[38]

Figure 4.

Impact of the dilution of an aqueous solution of P1. a) Concentration‐dependent absorption spectra in H₂O, b) UV/Vis absorption spectra in H₂O (light red) and in H₂O/THF (50/50, v/v) mixture (grey) at 1.7 10⁻⁶ M. All spectra were recorded at 20 °C, 1 h after dilution.

The same observations were made for aqueous solutions of P3, which showed an almost constant A_0‐0/A_0‐1 ratio of around 0.48 (<0.7) upon dilution with H₂O, demonstrating the persistence of H‐aggregates (Figure S23a, b) and an almost linear variation of absorbance with concentration (Figure S23c). Addition of THF induced again the disappearance of aggregates and increased the A_0‐0/A_0‐1 ratio to 1.2, which is close to the 1.3 value obtained in pure THF (Figure S23d).

Overall, these concentration‐dependent UV/Vis analyses for P1 and P3 suggested that the strong aggregation of PDI in water is maintained upon dilution, as no significant impact of dilution was observed. A change in the spectra was only observed after addition of a good solvent for the SSDU.

Formation of the Nanoribbons

The UV/Vis spectra and cryo‐TEM images discussed in the previous section were all recorded after 30 days of dissolution. To get an insight into how the assemblies form, we studied the formation and evolution of the nanoribbons of P1 at 10 g/L over a period of 120 days using UV/Vis (Figure 5) and cryo‐TEM analyses (Figure 6). The UV/Vis spectrum after only 10 min of dissolution clearly shows the bands at 520 and 600 nm, characteristic of H‐aggregates, but also a slight shoulder at 683 nm corresponding to J‐aggregates (Figure S24a). This typical band of the J‐aggregates at 683 nm becomes significant after 5 h of dissolution, increasing further during the first day and followed by a slow evolution for a month, after which no further change of the spectrum could be noticed. The A_(683nm)/A_(520nm) ratio (Figure 5b, Figure S24b) was used as a more rational mean to monitor the kinetics of the HJ‐aggregate formation. This ratio increased quickly over 9 days and no considerable evolution was observed between 9 and 120 days, with a persistent ratio of around 0.8, suggesting that an equilibrium was reached.

Figure 5.

Kinetics of the formation of self‐assembly of P1 in water. a) Evolution of normalized UV/Vis absorption spectra in H₂O at 0.1 g/L. b) A_{(683 nm)}/A_{(520 nm)} ratio versus time plot; the band 683 nm corresponds to J‐aggregates, 520 nm to H‐aggregates. All spectra were recorded at 20 °C.

Figure 6.

Representative cryo‐TEM images of P1 at 10 g/L in H₂O, a) 5 h, b) 7 days, c) 1 month after dissolution. The dark spots are surface contaminations stemming from water crystals. Inserts in each image show mean width and height values determined by cryo‐TEM image analysis (see Table S4 for details).

Cryo‐TEM analyses of 10 g/L aqueous solution were carried out in parallel to the UV/Vis experiments (Figure 6). They showed that after 5 h of dissolution, the sample already contained nanoribbons with an average width of 14±3 nm (Figure 6a, Table S4). Their width increased to 16±2 nm and 19±3 nm, after 7 days and 1 month respectively (Figure 6b, c, Table S4). However, their thickness remained constant over time, with an average value of around 5.8±0.7 nm. Consistent with the UV/Vis monitoring, cryo‐TEM shows that the nanoribbons form rapidly after dissolution, but reach a maximal width after 1 month, which does not evolve further. UV/Vis monitoring provided clues into the formation mechanism: H‐aggregates are present from the beginning, while J‐aggregates form progressively over a period of 9 days after water addition, which corresponds to the period over which the width of the nanoribbons increases. We can thus conclude that the J‐aggregation is responsible for the lateral growth of the nanocylinders or nanoribbons formed initially.

In previously reported PDI‐based assemblies, organic co‐solvents are generally used to prepare PDI assemblies in aqueous media.[ ¹⁹ , ²⁰ , ⁵⁴ ] We therefore studied the impact of the preparation method on the nano‐objects obtained. The co‐solvent method, ^[55] used commonly for amphiphilic polymers, involves the dissolution of the amphiphilic compound in a good solvent to form unimers, followed by the progressive addition of a selective solvent of one of the blocks (Figure S25b). P1 was therefore dissolved in DMF at 100 g/L, which is a good solvent for PDMAc and PDI (Figure S9). Water was added at a rate of 0.5 mL/min to obtain a final H₂O/DMF (99/1) mixture at 1 g/L. In parallel, P1 was dissolved in water. The UV/Vis spectra recorded after 1 month by each method were almost identical (Figure S26), demonstrating that the nano‐objects are not dependent on the preparation pathway used.

The impact of the presence of 1 vol % of DMF on the spontaneous assembly of P1 in water was also studied. Solutions in H₂O and H₂O/DMF (99/1) at 10 g/L were prepared in parallel using the same polymer solid. Monitoring the kinetics showed a very similar evolution of the UV/Vis spectra (Figure S24). After 4 months of storage, the spectra obtained displayed a similar A_(683nm)/A_(520nm) ratio, showing there is no significant impact of the presence of 1 % of co‐solvent on the formation of the nanoribbons.

The fact that the UV/Vis measurements revealed the presence of H‐aggregates immediately after dissolution raises the question of whether this type of aggregates was already present in the polymer solid, before its dissolution in water. To answer this question, solid state UV/Vis spectroscopy experiments were carried out with P1, by dissolving it in DCM where no aggregates are present (as shown by the A_0‐0/A_0‐1 ratio of 1.25, see Figure S27). The DCM solution of P1 was drop‐cast onto a glass slide, the solvent left to evaporate and the remaining film analyzed. The UV/Vis spectrum clearly shows the characteristic signature of H‐aggregates, with a maximum absorption band at 525 and 600 nm, revealing that the compound is already aggregated in the film into clusters in which the H‐aggregation is predominant (Figure 7a). The film was then re‐dissolved in water and the spectrum recorded just after dissolution. The spectrum shows the same bands as in the solid state while the characteristic band of the J‐aggregates is absent (Figure 7a); which was expected given the short time of dissolution. These results show that H‐aggregates are already present in the solid to a certain extent, and demonstrate that the formation of HJ‐aggregates is induced by the addition of water which induces a reorganization and results in the formation of nanoribbons (Figure 5).

Figure 7.

Pathway dependency of UV/Vis spectra of P1 (in the solid or dissolved in H₂O): a) dried film formed after drop‐casting (DC) a DCM solution of P1 (film ‐ solid line) and immediately after redissolution (RD) in H₂O (solution ‐ dotted line). b) H₂O solution of P1 (solution ‐ dashed line); Film drop‐cast from the aqueous solution of P1 (film ‐ solid line), spectrum after re‐dissolution of the solid in H₂O (solution ‐ dotted line). All spectra were recorded at 20 °C, and are normalized for more clarity.

From the series of studies performed, we also observed that the conditions to which the polymer solid was exposed to (solvent used, storage time, temperature, and humidity) can have an impact on the assemblies observed in water. In order to demonstrate this, we drop‐cast a 0.1 g/L aqueous solution of P1 on a glass slide. UV/Vis spectroscopy showed that the aggregates present in the aqueous solution of P1, namely HJ‐aggregates are still present at the same ratio (minimal changes in the A_(683nm)/A_(520nm) ratio) when cast (Figure S28). These aggregates are also preserved after re‐dispersing the film in water. Overall the signature of the spectra obtained when cast from either water or DCM are very different pointing out that the initial state of the polymer solid can have an impact on the assemblies formed in water.

To assess the stability of the aggregates with temperature, the film obtained by casting the aqueous solution of P1 was heated for 5 min at 90 °C, which is above the T_g of PDMAc. The spectrum (Figure S29) obtained was starkly different, with the disappearance of the J‐aggregates band induced by heating. In contrast, the H‐aggregates signals at 532 nm and 600 nm were still clearly visible. These results reveal qualitatively the difference in thermal stability of the H‐ and J‐aggregates and complement the cryo‐TEM observations.

Impact of Temperature on Nanoribbons in Aqueous Solutions

To investigate the effect of the temperature on the nanoribbons in water, an aqueous solution of P1 was subjected to heating, and monitored by UV/Vis. The absorption at 683 nm, characteristic of J‐aggregates, was recorded as a function of temperature (Figure 8a). A sharp decrease in absorbance was observed above 50 °C, suggesting the disappearance of J‐aggregates above this temperature. In Figure 8b, the UV/Vis spectrum of the P1 solution recorded after heating for 10 min at 60 °C is compared to the initial one (recorded at 20 °C). It shows that upon heating the typical bands of the H‐aggregates (at 520 and 600 nm) persisted, while the band of the J‐aggregates (at 683 nm) is no longer observed. Cryo‐TEM ¹ analyses on the P1 aqueous solutions were also carried out after heating for 10 min at 60 °C (Figure 8d, e). Instead of nanoribbons, nanocylinders with a diameter of around 5.8±0.9 nm were observed (Table S5). The disappearance of J‐aggregates is directly correlated with a morphological transition from nanoribbons to nanocylinders, and confirms a loss of lateral aggregation. Some cryo‐TEM images show splaying on the end of nanoribbons (Figure S30), which are similar to those observed by Zhang et al. with molecular PDI nanorods. ^[41] In their systems, they proposed that PDIs functionalized at the N‐imide positions with TEG, form nanorods at low concentration, and then merge together to form nanoribbons when the concentration is increased. It is possible that a similar mechanism operates here, and that the nanoribbons form through secondary aggregation of the individual nanocylinders. However, a more in‐depth study is necessary to elucidate further the mechanism of the observed temperature‐induced morphological transitions.

Figure 8.

Impact of the temperature on P1 nanoribbons in an aqueous solution. (left) a) Normalized absorbance at 683 nm (J‐aggregates) for P1 at 0.1 g/L in H₂O upon heating (red curve) and cooling (blue curve) at 0.25 °C/min. The single black square is recorded 7 days after heating and storing the 0.1 g/L aqueous solution at 5 °C. b) UV/Vis absorption spectra of P1 at 0.1 g/L in H₂O obtained from dilution of 10 g/L solution, before heating (solid line) and just after heating 10 min at 60 °C (dashed line), c) just after heating 10 min at 60 °C (dashed line) and 7 days after heating and storage at 5 °C (dotted line). (right) Representative cryo‐TEM images of P1 at 10 g/L in H₂O/DMF (99/1), d) before heating, e) just after heating for 10 min at 60 °C, and f) 7 days after heating and storage at 5 °C. The dark spots are surface contaminations stemming from water crystals. The corresponding UV/Vis spectra in H₂O/DMF (99/1) are reported in Figure S31.

To study the reversibility of this transition, the absorbance at 683 nm was again recorded when the sample was cooled back to 20 °C, at 0.25 °C/min (Figure 8a). A significant hysteresis was observed with no J‐aggregates forming after 3 h. However, after storing the solution for 7 days at 5 °C, the UV/Vis spectrum revealed that the J‐band at 683 nm was reformed (Figure 8c). Cryo‐TEM analyses of this aged solution showed that the nanoribbons had reformed, however with a smaller width and the presence of few larger aggregates (Figure 8f, Table S5). Temperature‐dependent UV/Vis analyses of the aqueous solution of P3 which contains spheres based on pure H‐aggregates were also performed (Figure S32). No significant change in the A_0‐0/A_0‐1 ratio could be observed as a function of temperature, even after heating for 30 min at 80 °C. This result confirms that the nanostructures where the H‐aggregates are dominant, are much more robust than those containing also the J‐aggregates.

Conclusion

In summary, we have demonstrated that hydrophilic PDMAcs containing a PDI in their core self‐assemble spontaneously into 2D nanoribbons and 1D nanocylinders after simple dissolution in water. Cryo‐TEM, SAXS and UV/Vis absorption spectroscopy analyses of the aqueous solutions revealed the strong impact of the polymer DP _n on morphology. For the lowest DP _n, lateral interactions through J‐aggregation are possible and result in the formation of nanoribbons. In contrast, increasing the DP _n leads first to the loss of lateral aggregation and subsequently the formation of nanocylinders. Further increase in the DP _n limits the H‐aggregates present in the nanocylinders and results in the formation of spheres. Solid‐state UV/Vis studies revealed that the system is pre‐organized in co‐facial H‐aggregates in the solid state, prior to dissolution in water.

We focused our study to investigate the formation of nanoribbons (Figure 9) and found that they form progressively after dissolution in water. Over time, they become wider due to chain reorganization and the gradual formation of J‐aggregates driven by the presence of water.

Figure 9.

Proposed mechanism of spontaneous formation of nanoribbons in water (from P1), including the effect of the temperature.

A quasi‐reversible transition between nanoribbons and nanocylinders was observed when heated over 60 °C, due to the disappearance of J‐aggregates. The system is capable to reorganize into nanoribbons upon cooling. However, a large hysteresis is observed as J‐aggregates and therefore nanoribbons reform only after several days. Finally, concentration‐dependent UV/Vis experiments showed that the PDI aggregates were not affected by dilution and unimers were formed only after dilution with a good solvent for the SSDU. These findings may be interesting for optoelectronic applications as we can easily obtain anisotropic PDI aggregates using only water, which can then be deposited on a substrate whilst retaining the same properties at the solid state.[ ³ , ⁵⁶ ]

Supporting Information

The authors have cited additional references within the Supporting Information.[ ⁹ , ⁴² , ⁵⁵ , ⁵⁷ , ⁵⁸ , ⁵⁹ , ⁶⁰ ]

Author contributions

JR, LSV and SB contributed to the conceptualization of the work presented in the manuscript. JMG and SB were responsible for the data curation; with LB and TB also involved in the formal analysis of the data presented. Data visualization was executed by SB and LSV. The investigation was carried out by SB and JMG. Resources were provided by JMG, TB (cryo‐TEM and SAXS respectively). JR and LSV obtained the funding and supervised the work. Finally, the initial draft was written by SB, JR and LSV; with SB, JR, LSV, and LB involved in the final review and editing of the submitted version.

Conflict of Interests

The authors declare no conflict of interest.

Supporting information

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Supporting Information

Berruée S., Guigner J.-M., Bizien T., Bouteiller L., Sosa Vargas L., Rieger J., Angew. Chem. Int. Ed. 2025, 64, e202413627. 10.1002/anie.202413627

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.