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Pathway Control in Cooperative vs. Anticooperative Supramolecular Polymers

Lorena Herkert,[a] Kalathil K. Kartha[a] and Gustavo Fernández*[a]

[a]L. Herkert, Dr. Kalathil K. Kartha, Prof. Dr. Gustavo FernándezOrganisch-Chemisches Institut, Westfälische-Wilhelms Universität Münster, Corrensstraße, 40. 48149 Münster (Germany)E-mail: fernandg@uni-muenster.de

[b]Title(s), Initial(s), Surname(s) of Author(s)DepartmentInstitution

Supporting information for this article is given via a link at the end of the document.

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Abstract: Pathway and nanoscale morphology control in assemblies of -conjugated molecules is key to develop functional supramolecular materials. Herein, we report an unsymmetrically substituted amphiphilic Pt(II) complex 1 that shows unique self-assembly behaviour in nonpolar medium into two competing anticooperative and cooperative pathways with distinct molecular packing (long- vs. medium-slipped) and nanoscale morphology (discs vs. fibres) in a controlled fashion. Experimental studies and simulations allowed us to ascertain the packing modes, the stability conditions of both pathways and the mechanistic aspects governing their supramolecular polymerization. Our findings reveal that chain immiscibility is an unprecedented and efficient method to control anticooperative assemblies and pathway complexity in general.

Scheme 1. Chemical structure of 1 and schematic depiction of the self-assembly pathways leading to aggregate species A and B.

Control over competing self-assembly pathways of -conjugated molecules is a prerequisite to produce defined nanoscale morphologies, which in turn play a key role in the desired function and performance of organic electronic devices.[1] The appearance of different aggregation pathways that are in competition ‒termed as pathway complexity[2]‒ has been reported for various compounds in recent years,[3] where most examples can be described applying one of the two main polymerization models or a combination of them: a) the isodesmic[4] or b) the cooperative nucleation-elongation model.[5] However, the third case of c) anti-cooperative mechanism leading to assemblies with reduced sizes has remained comparatively unexplored.[6] Few relevant examples include perylene bisimide (PBI)-based H-bonded assemblies that show typical cooperative aggregation curves but lead to small-sized assemblies[6a] or attenuated growth via anti-cooperative supramolecular oligomerization where even numbered aggregates are highly favoured.[6b] Recently, altering the solvent composition in H-bonded assemblies of N-heterocyclic aromatic dicarboximides was exploited as an efficient method to bias the formation of a transient anti-cooperative H-aggregate, which eventually converts into a thermodynamically stable J-aggregate.[6c] Thus, it becomes clear from these reports that anti-cooperative supramolecular polymerization is a rare phenomenon and size control via anticooperative growth remains one of the greatest challenges in the broad fields of self-assembly and supramolecular polymerization.

In this work, we demonstrate an unprecedented approach based on chain immiscibility that allows full control over an anticooperative, discrete assembly that is in competition with the formation of cooperative supramolecular polymers. The stable, anticooperative pathway can be readily and efficiently controlled both by temperature variation in a single solvent (methylcyclohexane, MCH) or by addition of a co-solvent (CHCl3) at ambient temperature. This was achieved by the design of a new oligophenyleneethynylene (OPE)-based linear amphiphilic Pt(II) complex 1 that is unsymmetrically substituted with triethyleneglycol (TEG) and dodecyloxy chains (Scheme 1, for synthesis and characterization, see supporting information (S.I.). In-depth spectroscopic and microscopy studies allowed us to elucidate the packing modes, size, morphology and stability conditions of both pathways. Ultimately, the mechanistic aspects of both pathways were successfully reproduced by simulations using a coupled anti-cooperative and cooperative model in a temperature-dependent experiment for the first time.

The self-assembly of 1 was initially examined by recording the UV/Vis changes in different solvents of varying polarity (Fig. S1). As expected, 1 is prone of aggregation in MCH and therefore, this solvent was used for further investigations. Variable-temperature (VT)-UV/Vis experiments performed at multiple concentrations (c = 3 × 10-4 M ‒ 5 × 10-5 M) and cooling rates (0.2 ‒ 2 K/min) showed in all cases identical, reproducible trends (Figs. 1a and S2,3). Remarkably, all recorded cooling curves (inset of Fig. 1a and Figs. S2-4) show an initial smooth transition followed by a much sharper step upon decreasing temperature. In the following, a selected concentration of 3 × 10-4 M was chosen for the detailed description of the spectral changes on aggregation. At 353 K or above (red spectrum), 1 displays a sharp spectrum with an absorption maximum (max) at 357 nm that is characteristic of a molecularly dissolved state (Fig. 1a). Upon cooling to 303 K, this transition decreases its intensity by ca. 15% with a concomitant increase of a shoulder band between 390 and 420 nm (green spectrum in Fig. 1a). Additionally, a red-shift of the absorption band at 280 nm to 295 nm occurs. These spectral changes are accompanied by a pseudoisosbestic point at 388 nm, suggesting a plausible transition from the monomer (M) to a single aggregate species (denoted as A in Scheme 1). The lack of significant shifts in the max during the formation of A indicates weak coupling of the -scaffolds.[7] Interestingly, further cooling of this solution to 283 K leads to more pronounced spectral changes, i.e. a red-shift in max (8 nm) along with significant broadening of the absorption covering the region up to 475 nm (Fig. 1, blue spectrum). This spectral pattern is reminiscent of that found in a bolaamphiphilic OPE-based Pt(II) complex previously reported by us,[8] and is consistent with the formation of translationally displaced -stacks (in the following termed as slipped aggregate B). During this transition from A to B upon cooling, we noticed the lack of clear isosbestic points suggesting a complex behaviour involving more than merely these two species in equilibrium.

The relative stability of state A was also monitored by UV/Vis upon heating or addition of increasing fractions of chloroform to an MCH solution of aggregate B (c = 3 × 10-4 M) (Fig. 1b, S4). In both cases, aggregate A could be identified spectroscopically prior to complete disassembly of B into the pure monomer state, which is an indication that aggregate A is not a transient, kinetically trapped state. Interestingly, optimization of the amount of CHCl3 (4-6 %) added to the MCH solution of aggregate B (c = 3 × 10-4 M) enabled us to isolate species A, which shows significant stability over a period of at least two months at room temperature (Fig. S5). Additional VT-UV/Vis experiments in MCH with 1-7 % CHCl3 revealed the same sequential formation of A and B aggregates as in pure MCH upon cooling (Fig. S6). The overall UV/Vis results reveal that we can successfully control the formation of the individual species by varying temperature, concentration and solvent composition. This ultimately enabled us to derive experimental phase diagrams revealing the stability conditions of the three species (M, A and B) present in equilibrium (Fig. 1c and S7).

Combined VT-1H-NMR, Rotating-frame Overhauser Effect Spectroscopy (ROESY) and solid state NMR experiments were carried out to elucidate the packing modes of both aggregate species A and B (Fig. 2). VT-1H-NMR experiments of 1 under identical conditions to those previously selected for VT-UV/Vis (MCH-d14, c = 3 × 10-4 M) display a spectrum with sharp, well-resolved signals at 358 K that can be assigned to the pure monomer species (Fig. 2a and S8). Upon cooling, signal splitting, shielding and broadening occurs to a different extent for the different aromatic signals. Small upfield shifts for most of the aromatic signals can be attributed to weak intermolecular coupling of the OPE scaffolds. Conversely, the more distinctive shielding of the pyridyl protons Ha/Ha' can be explained by a partial destabilization of the intramolecular Ha/Ha'···Cl bonds[8c,9] when the electron-rich OPE fragments of neighbouring units come in close proximity. Notably, in the temperature range in which state state A is predominant (363 – 303 K), the aromatic signals remain relatively sharp until 318 K possibly due to the initial formation of only small, discrete assembled species, e.g. dimers.[9] Further cooling down to 308 K causes a more significant broadening implying the formation of slightly longer oligomers (Fig. 2a). Another interesting observation during the formation of A is the splitting of the signals corresponding to Ha/Ha', Hb/Hb' and Hc/Hc', which can be assigned to the formation of an unsymmetrical aggregate structure with different electronic environment of the involved protons. Particularly, protons Hd′ (close to TEG chains) show a more pronounced broadening than protons Hd (close to the alkoxy chains), suggesting a stronger involvement of the former in intermolecular interactions within A. Further cooling down of A (308 K) to 298 K resulted in significant signal broadening attributed to the formation of species B (vide infra).

Figure 1. a) Temperature-dependent UV/Vis spectra of 1 at 3 10-4 M in MCH from 353 K to 283 K at a cooling rate of 0.2 K/min, b) UV/Vis experiments of 1 (c = 3 10-4 M) at 298 K in MCH with increasing percentages of CHCl3 and plot of the extinction coefficient at 400 nm (inset).

To shed further light on the intermolecular interactions of 1 in state A, a 2D ROESY-1H NMR experiment was performed (see Fig. 2b and S9-10). Addition of 10 % CDCl3 to MCH-d14 was required to preferentially isolate state A at the higher concentration (c = 1.5 × 102 M) needed to obtain a suitable signal-to-noise ratio. Noticeable correlation signals between the protons of the TEG chains and all protons from the aromatic core (Ha-d, blue squares in Fig. 2b) are observed. This coupling pattern can be unambiguously assigned to the formation of slipped stacks, where the glycol chains of one molecule are interacting with the -system of a neighbouring molecule via CHarom···O contacts.[8a] This proposed packing arrangement is also supported by additional correlation signals between both Ha/Ha' and Hb/Hb' with Hd/Hd' as well as between Hb/Hb' and Hc/Hc' (orange, yellow and green squares, respectively, in Fig. 2b-c). Additionally, the fact that the pyridyl (Ha,b) and the adjacent rings (Hc) are considerably more shielded than the outer rings (Hd, which barely shift) in VT-NMR can be explained by their location in an environment with higher electron density, i.e. surrounded by a larger number of aromatic rings. This double strand arrangement has to be necessarily extended to a zigzag packing to prevent unfavourable interactions between the highly polar TEG chains and the nonpolar MCH medium, as depicted in Scheme 1 and Figure 2c. Despite this arrangement enabling weak hydrogen bonding and shielding of the TEG chains from the solvent, it also demands much more space in the inner aromatic/glycol stack that might ultimately explain why further aggregation to elongated polymers is hindered.

After a detailed structural analysis of state A, we attempted to elucidate the packing modes of state B. In this case, strong broadening of the NMR signals in solution caused by extended aggregation (see VT-NMR spectra below 308 K) made necessary the isolation of the aggregate species B in form of a gel and its subsequent analysis by solid state NMR. Optimization of the solvent (MCH-d14/CDCl3 95:5) and concentration conditions (10 mM) allowed us to isolate state B, as demonstrated by UV/Vis (Fig. S11). The generated gel obtained upon cooling this solution from 343 K to 293 K was carefully dried under argon flow prior to the 1H-13C-HETCOR NMR measurements (see also Fig. S12). Several intermolecular 1H/13C correlation signals can be observed, such as between Ha/Ha' and the ethynylene units C4/C5 and C10/C11 (red) of a neighbouring molecule. In addition, protons Hd/Hd' show cross signals with C4/C5 (purple), suggesting a slipped arrangement, however with a less pronounced translational displacement compared to state A. The difference between the two arrangements of A and B is also supported by the absence of a cross signal between Ha/Ha' and C12 (see red dotted circle in Fig. 3a). Combining the solid state NMR, VT-NMR and UV/Vis results, we conclude that stronger - stacking (in comparison to state A) cooperating with potential C-H···Cl and C-H···O interactions stabilize the aggregated state B. Again, the hydrophilic chains are likely to be shielded from the solvent leading to a double strand as illustrated in Figure 3b, Scheme 1.

Figure 2. a) Temperature-dependent 1H NMR of 1 at 3 10-4 M in MCH from 358 K to 298 K, b) Partial ROESY NMR spectrum (MCH-d14/CDCl3 90:10, 1.5 × 10–2 M) of 1 (state A). Intermolecular coupling is highlighted with coloured boxes. c) Depiction of molecular packing and couplings of A taken from the ROESY NMR.

Final proof of the existence of structurally different nanoscale morphologies for states A and B was provided by atomic force microscopy (AFM) by spin-coating aggregate solutions at the respective temperatures on silicon wafer (Fig. S13). Networks of fibers of several micrometers in length and heights of 1.2‒2 nm (in the range of the molecular width) were observed for B, confirming the presence of long 1D nanostructures that are typical of cooperative supramolecular polymers. This is supported by dynamic light scattering (DLS) experiments (Fig. S14). A width of 10‒15 nm is roughly matching with the length of two molecules and therefore an arrangement into a one dimensional double strand of 1 is likely to be formed (Fig. 3b, Scheme 1). On the other hand, aggregate A could be visualized as discoidal nanoparticles of sizes between 20 and 60 nm and heights of 4 to 5 nm. This is in line with the idea of small assemblies in solution that are prone to clustering on the substrate upon solvent evaporation. Subsequent DLS measurements (Fig. S14) revealed hydrodynamic radii (Rh) up to 4.4 nm for A in MCH (c = 3 × 10-4 M). As a comparison, a solution of molecularly dissolved 1 in CHCl3 (c = 5 × 10-3 M), resulted in a four times lower value of Rh (1.1 nm), a trend that exactly matches the values found in Diffusion Ordered spectroscopy (DOSY NMR). Both techniques underline that A consists of small aggregated species, which agrees well with the results extracted from NMR and AFM.

Figure 3. a) Selected areas of the 1H-13C-HETCOR NMR spectra of 1 with highlighted couplings. b) Depiction of molecular packing and observed couplings (arrows) in B on the basis of the solid state NMR measurements.

Mechanistic details of this competition between cooperative and anticooperative pathways were ultimately unravelled by mathematical simulations of the obtained cooling curves (see Figure 3a). The initial step (the transformation of M to A) could be accurately simulated using an attenuated equilibrium model[] where increasing repulsive interactions diminish the monomer association constant and thereby halt the assembly process at the stage of small aggregates. This model assumes that every monomer addition step proceeds with a diminished equilibrium constant Ki = K/i, (with i: length of the respective aggregate to which the monomer addition takes place). As a comparison, the isodesmic model was unable to describe accurately this first transition (see SI), which demonstrates the anticooperative nature of pathway A. The second transition was simulated using the cooperative nucleation elongation model,[] (Kn < Ke). As visible in Fig. 4a, the overall simulated curves successfully reproduce the experimentally observed trends for all four different concentrations. More discussion about the fact that the Te is independent of C? Combining the mass balances following the attenuated and the nucleation-elongation model, the mass fractions can be calculated (Figure 4b). Concomitantly, the dimensionless monomer concentration allows to calculate the weight averaged degree of polymerization DPw for the first regime, assuming that before the second transition, only A-type assemblies are formed. The results are shown in the inset of Fig. 4a. We were pleased to observe that the simulations predict oligomeric species of ca. ten monomer units for A, which not only perfectly matches the values extracted from DOSY NMR, but also is in accordance with DLS, AFM and UV/Vis.

In summary, we have reported a new amphiphilic Pt(II) complex 1 that self-assembles into two competitive anti-cooperative vs. cooperative aggregates with different packing and nanoscale morphology in nonpolar medium. Precise temperature variation in a single solvent (MCH) or addition of a co-solvent (CHCl3) enables full control over an anticooperative pathway with discrete size that is stable over time. Steric effects along with immiscibility of the TEG and alkoxy chains are proposed as the factors hindering the elongation of this aggregate. Mathematical simulations accurately reproduce the experimental trends and enable a mechanistic elucidation using a coupled anti-cooperative and cooperative model in a temperature-dependent experiment for the first time. Besides showing that H-bonding is not a prerequisite for pathway complexity, our results reveal that chain immiscibility is an unprecedented method to control anticooperative assemblies.

Figure 4. a) Cooling curves of 1 in MCH at different concentrations (dotted lines) based on UV/vis results and mathematical simulation of the curves (black lines) based on the attenuated model for the formation of A and the nucleation-elongation model for the formation of B. b) Weight-averaged degree of polymerization for the formation of A at different concentrations based on the mathematical simulations.

Experimental Section

See the Supporting Information.

Acknowledgements

We thank the Deutsche Forschungsgemeinschaft (SFB 858), the Humboldt Foundation (Sofja Kovalevskaja Program) and the European Commission (SUPRACOP ERC-StG-2016) for funding.

Keywords: Self-assembly • Supramolecular Polymerization • Pathway Complexity • -conjugated systems • Pt(II) Complexes

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