Trade-off between processability and device performance in donor-acceptor semiconductors revealed using discrete siloxane side chains.

Donor-acceptor polymeric semiconductors are crucial for state-of-the-art applications, such as electronic skin mimics. The processability, and thus solubility, of these polymers in benign solvents is critical and can be improved through side chain engineering. Nevertheless, the impact of novel side chains on backbone orientation and emerging device properties often remains to be elucidated. Here, we investigate the influence of elongated linear and branched discrete oligodimethylsiloxane (oDMS) side chains on solubility and device performance. Thereto, diketopyrrolopyrrole-thienothiophene polymers are equipped with various oDMS pendants (PDPPTT-Sin) and subsequently phase separated into lamellar domains. The introduction of a branching point in the siloxane significantly enhanced the solubility of the polymer, as a result of increased backbone distortion. Simultaneously, the charge carrier mobility of the polymers decreased by an order of magnitude upon functionalization with long and/or branched siloxanes. This work unveils the intricate balance between processability and device performance in organic semiconductors, which is key for the development of next-generation electronic devices.

Functionalization of Supramolecular Polymers by Dynamic Covalent Boroxine Chemistry.

Molecular scaffolds that enable the combinatorial synthesis of new supramolecular building blocks are promising targets for the construction of functional molecular systems. Here, we report a supramolecular scaffold based on boroxine that enables the formation of chiral and ordered 1D supramolecular polymers, which can be easily functionalized for circularly polarized luminescence. The boroxine monomers are quantitatively synthesized in situ, both in bulk and in solution, from boronic acid precursors and cooperatively polymerize into 1D helical aggregates stabilized by threefold hydrogen-bonding and π-π stacking. We then demonstrate amplification of asymmetry in the co-assembly of chiral/achiral monomers and the co-condensation of chiral/achiral precursors in classical and in situ sergeant-and-soldiers experiments, respectively, showing fast boronic acid exchange reactions occurring in the system. Remarkably, co-condensation of pyrene boronic acid with a hydrogen-bonding chiral boronic acid results in chiral pyrene aggregation with circularly polarized excimer emission and g-values in the order of 10-3. Yet, the electron deficiency of boron in boroxine makes them chemically addressable by nucleophiles, but also sensitive to hydrolysis. With this sensitivity in mind, we provide first insights into the prospects offered by boroxine-based supramolecular polymers to make chemically addressable, functional, and adaptive systems.

Supramolecular Polymerization as a Tool to Reveal the Magnetic Transition Dipole Moment of Heptazines.

Heptazine derivatives have attracted significant interest due to their small S1-T1 gap, which contributes to their unique electronic and optical properties. However, the nature of the lowest excited state remains ambiguous. In the present study, we characterize the lowest optical transition of heptazine by its magnetic transition dipole moment. To measure the magnetic transition dipole moment, the flat heptazine must be chiroptically active, which is difficult to achieve for single heptazine molecules. Therefore, we used supramolecular polymerization as an approach to make homochiral stacks of heptazine derivatives. Upon formation of the supramolecular polymers, the preferred helical stacking of heptazine introduces circular polarization of absorption and fluorescence. The magnetic transition dipole moments for the S1 ← S0 and S1 → S0 are determined to be 0.35 and 0.36 Bohr magneton, respectively. These high values of magnetic transition dipole moments support the intramolecular charge transfer nature of the lowest excited state from nitrogen to carbon in heptazine and further confirm the degeneracy of S1 and T1.

Consequences of Vibrational Strong Coupling on Supramolecular Polymerization of Porphyrins.

Supramolecular polymers display interesting optoelectronic properties and, thus, deploy multiple applications based on their molecular arrangement. However, controlling supramolecular interactions to achieve a desirable molecular organization is not straightforward. Over the past decade, light-matter strong coupling has emerged as a new tool for modifying chemical and material properties. This novel approach has also been shown to alter the morphology of supramolecular organization by coupling the vibrational bands of solute and solvent to the optical modes of a Fabry-Perot cavity (vibrational strong coupling, VSC). Here, we study the effect of VSC on the supramolecular polymerization of chiral zinc-porphyrins (S-Zn) via a cooperative effect. Electronic circular dichroism (ECD) measurements indicate that the elongation temperature (Te) of the supramolecular polymerization is lowered by ∼10 °C under VSC. We have also generalized this effect by exploring other supramolecular systems under strong coupling conditions. The results indicate that the solute-solvent interactions are modified under VSC, which destabilizes the nuclei of the supramolecular polymer at higher temperatures. These findings demonstrate that the VSC can indeed be used as a tool to control the energy landscape of supramolecular polymerization. Furthermore, we use this unique approach to switch between the states formed under ON- and OFF-resonance conditions, achieved by simply tuning the optical cavity in and out of resonance.

Supramolecular polymers form tactoids through liquid-liquid phase separation.

Liquid-liquid phase separation (LLPS) of biopolymers has recently been shown to play a central role in the formation of membraneless organelles with a multitude of biological functions1-3. The interplay between LLPS and macromolecular condensation is part of continuing studies4,5. Synthetic supramolecular polymers are the non-covalent equivalent of macromolecules but they are not reported to undergo LLPS yet. Here we show that continuously growing fibrils, obtained from supramolecular polymerizations of synthetic components, are responsible for phase separation into highly anisotropic aqueous liquid droplets (tactoids) by means of an entropy-driven pathway. The crowding environment, regulated by dextran concentration, affects not only the kinetics of supramolecular polymerizations but also the properties of LLPS, including phase-separation kinetics, morphology, internal order, fluidity and mechanical properties of the final tactoids. In addition, substrate-liquid and liquid-liquid interfaces proved capable of accelerating LLPS of supramolecular polymers, allowing the generation of a myriad of three-dimensional-ordered structures, including highly ordered arrays of micrometre-long tactoids at surfaces. The generality and many possibilities of supramolecular polymerizations to control emerging morphologies are demonstrated with several supramolecular polymers, opening up a new field of matter ranging from highly structured aqueous solutions by means of stabilized LLPS to nanoscopic soft matter.

Self-Assembly of Discrete Oligomers of Naphthalenediimides in Bulk and on Surfaces.

Here, we report on the synthesis of discrete oligomers of alkyl-bridged naphthalenediimides (NDIs) and study their molecular nanostructures both in bulk, in solution, and at the liquid-solid interface. Via an iterative synthesis method, multiple NDI cores were bridged with short and saturated alkyl-diamines (C3 and C12 ) or long and unsaturated alkyl-diamines (u2 C33 to u8 C100 ) at their imide termini. The strong intermolecular interaction between the NDI cores was observed by probing their photophysical properties in solution. In bulk, the discrete NDI oligomers preferentially ordered in lamellar morphologies, irrespective of whether a saturated or unsaturated spacer was employed. Moreover, both the molecular architecture as well as the crystallization conditions play a significant role in the nanoscale ordering. The long unsaturated alkyl chains lead preferably to folded-chain conformations while their saturated analogues form stretched arrangements. At the solution-solid interface, well-defined lamellar regions were observed. These results show that precision in chemical structure alone is not sufficient to reach well-defined structures of discrete oligomers, but that it must be combined with precision in processing conditions.

Solvent-Induced Pathway Complexity of Supramolecular Polymerization Unveiled Using the Hansen Solubility Parameters.

Supramolecular building blocks assembling into helical aggregates are ubiquitous in the current literature, yet the role of solvents in these supramolecular polymerizations often remains elusive. Here, we present a systematic study that quantifies solvent-supramolecular polymer compatibility using the Hansen solubility parameters (δD, δH, and δP). We first studied the solubility space of the supramolecular building block triazine-1,3,5-tribenzenecarboxamide S-T. Due to its amphiphilic nature, a dual-sphere model based on 58 solvents was applied describing the solubility space of the monomeric state (green sphere) and supramolecular polymer state (blue sphere). To our surprise, further in-depth spectroscopic and morphological studies unveiled a distinct solubility region in-between the two spheres giving rise to the formation of higher-order aggregated structures. This phenomenon occurs due to subtle differences in polarity between the solvent and the side chains and highlights the solvent-induced pathway complexity of supramolecular polymerizations. Subsequent variations in concentration and temperature led to the expansion and contraction of both solubility spheres providing two additional features to tune the monomer and supramolecular polymer solubility. Finally, we applied our dual-sphere model on structurally disparate monomers, such as Zn-porphyrin (S-P) and triphenylamine (S-A), demonstrating the generality of the model and the importance of the supramolecular monomer design in connection with the solvent used. This work unravels the solvent-induced pathway complexity of discotic supramolecular building blocks using a parametrized approach in which interactions between the solvent and solute play a crucial role.

Temperature Directs the Majority-Rules Principle in Supramolecular Copolymers Driven by Triazine-Benzene Interactions.

Supramolecular copolymers have typically been studied in the extreme cases, such as self-sorting or highly mixed copolymer systems, while the intermediate systems have been less understood. We have reported the temperature-dependent microstructure in copolymers of triazine- and benzene-derivatives based on charge-transfer interactions with a highly alternating microstructure at low temperatures. Here, we investigate the temperature-dependent copolymerization further and increase the complexity by combining triazine- and benzene-derivatives with opposite preferred helicities. In this case, intercalation of the benzene-derivative into the triazine-derivative assemblies causes a helical inversion. The inversion of the net helicity was rationalized by comparing the mismatch penalties of the individual monomers, which indicated that the benzene-derivative dictates the helical screw-sense of the supramolecular copolymers. Surprisingly, this was not reflected in further investigations of slightly modified triazine- and benzene-derivatives, thus highlighting that the outcome is a subtle balance between structural features, where small differences can be amplified due to the competitive nature of the interactions. Overall, these findings suggest that the temperature-dependent microstructure of triazine- and benzene-based supramolecular copolymers determines the copolymer helicity of the presented system in a similar way as the mixed majority-rules phenomenon.

Controlling Helical Asymmetry in Supramolecular Copolymers by In Situ Chemical Modification.

Amplification of asymmetry in complex molecular systems results from a delicate interplay of chiral supramolecular structures and their chemical reactivity. In this work, we show how the helicity of supramolecular assemblies can be controlled by performing a non-stereoselective methylation reaction on comonomers. By methylating chiral glutamic acid side chains in benzene-1,3,5-tricarboxamide (BTA) derivatives to form methyl esters, the assembly properties are modulated. As reacted comonomers, the methyl ester-BTAs induce a stronger bias in the screw-sense of helical fibers predominantly composed of stacked achiral alkyl-BTA monomers. Hence, applying the in situ methylation in a system with the glutamic acid-BTA comonomer induces asymmetry amplification. Moreover, mixing small quantities of enantiomers of glutamic acid-BTA and glutamate methyl ester-BTA in the presence of the achiral alkyl-BTAs leads to deracemization and inversion of the helical structures in solution via the in situ reaction toward a thermodynamic equilibrium. Theoretical modeling suggests that the observed effects are caused by enhanced comonomer interactions after the chemical modification. Our presented methodology enables on-demand control over asymmetry in ordered functional supramolecular materials.

Highly Ordered Supramolecular Materials of Phase-Separated Block Molecules for Long-Range Exciton Transport.

Efficient energy transport over long distances is essential for optoelectronic and light-harvesting devices. Although self-assembled nanofibers of organic molecules are shown to exhibit long exciton diffusion lengths, alignment of these nanofibers into films with large, organized domains with similar properties remains a challenge. Here, it is shown how the functionalization of C3 -symmetric carbonyl-bridged triarylamine trisamide (CBT) with oligodimethylsiloxane (oDMS) side chains of discrete length leads to fully covered surfaces with aligned domains up to 125 × 70 µm2 in which long-range exciton transport takes place. The nanoscale morphology within the domains consists of highly ordered nanofibers with discrete intercolumnar spacings within a soft amorphous oDMS matrix. The oDMS prevents bundling of the CBT fibers, reducing the number of defects within the CBT columns. As a result, the columns have a high degree of coherence, leading to exciton diffusion lengths of a few hundred nanometers with exciton diffusivities (≈0.05 cm2 s-1 ) that are comparable to those of a crystalline tetracene. These findings represent the next step toward fully covered surfaces of highly aligned nanofibers through functionalization with oDMS.

Simulating Assembly Landscapes for Comprehensive Understanding of Supramolecular Polymer-Solvent Systems.

Complexity in supramolecular polymer systems arises from interactions between different components, including solvent molecules. By varying their concentration or temperature in such multicomponent systems, complex phenomena can occur such as thermally bisignate and dilution-induced assembly of supramolecular polymers. Herein, we demonstrate that both these phenomena emerge from the same underlying interaction mechanism between the components. As a model system, amide-decorated supramolecular polymers of porphyrins were investigated in combination with aliphatic alcohols as hydrogen-bond scavengers, and thermodynamic mass-balance models were applied to map the three-dimensional assembly landscapes. These studies unveiled that the interaction between hydrogen-bond scavengers and monomers is temperature-dependent and becomes dominant at high monomer concentrations. With these insights, we could exploit competitive monomer-alcohol interactions to prompt the dilution-induced assembly of various common monomers as well as bisignate assembly events. Moreover, kinetic insights were obtained by navigating through the assembly landscape. Similar to phase diagrams of covalent polymers, these assembly landscapes provide a comprehensive picture of supramolecular polymerizations, which helps to precisely regulate the system properties. The generality of this approach using assembly landscapes makes it relevant for any supramolecular system, and this enhanced control will open the door to build complex and functional supramolecular polymer systems.

Circular depolarization spectroscopy: A new tool to study photo-imprinting of chirality.

When irradiating a molecular material containing photo-isomerizable groups with pure circularly polarized light, a particular handedness may get imprinted into the material. To study the mechanism and kinetics of this process in situ and operando, we have developed a new chiroptical tool where the circular polarization of the incident circularly polarized light is monitored after transmission through the photoactive layer. Practical limits to the resolution and sensitivity of the measurements as well as its calibration are discussed. To aid interpretation of experimental results, we present kinetic Monte Carlo simulations on a model for the active material involving photo-induced reorientation of molecules in a cholesteric organization. The simulations support the interpretation of a transient minimum in the degree of circular polarization of the transmitted light in terms of a nematic transient state during photo-inversion of a cholesteric organization in the molecular material.

Charge transport in liquid crystal network of terthiophene-siloxane block molecules.

In their thermotropic liquid-crystalline state, molecular semiconductors can show charge transport with high carrier mobility. Polymerization of the corresponding mesogens into a cross-linked network often deteriorates the charge transport. Here, we report that mesogens consisting of a terthiophene core and discrete oligodimethylsiloxane side-chains terminated by acrylate units can be photopolymerized in the columnar phase with retention of nanoscale order and charge transport capabilities. We argue that the strong tendency for microphase segregation protects the semiconducting block from reacting with free radicals during polymerization. This work provides new insights into the design of electroactive materials with charge transport properties.

How Subtle Changes Can Make a Difference: Reproducibility in Complex Supramolecular Systems.

The desire to construct complex molecular systems is driven by the need for technological (r)evolution and our intrinsic curiosity to comprehend the origin of life. Supramolecular chemists tackle this challenge by combining covalent and noncovalent reactions leading to multicomponent systems with emerging complexity. However, this synthetic strategy often coincides with difficult preparation protocols and a narrow window of suitable conditions. Here, we report on unsuspected observations of our group that highlight the impact of subtle "irregularities" on supramolecular systems. Based on the effects of pathway complexity, minute amounts of water in organic solvents or small impurities in the supramolecular building block, we discuss potential pitfalls in the study of complex systems. This article is intended to draw attention to often overlooked details and to initiate an open discussion on the importance of reporting experimental details to increase reproducibility in supramolecular chemistry.

Controlling the Processability and Stability of Supramolecular Polymers Using the Interplay of Intra- and Intermolecular Interactions.

Polymer networks crosslinked via non-covalent interactions afford interesting materials for a wide range of applications due to their self-healing capability, recyclability, and tunable material properties. However, when strong non-covalent binding motifs in combination with high crosslink density are used, processing of the materials becomes troublesome because of high viscosities and the formation of insoluble gels. Here, we present an approach to control the processability of grafted polymers containing strong non-covalent interactions by balancing the interplay of intra- and intermolecular hydrogen bonding. A library of copolymers with different degrees of polymerization and content of protected ureido-pyrimidinone-urea (UPy-urea) grafts was prepared. Photo-deprotection in a good solvent like tetrahydrofuran (THF) at low concentrations (≤1 mg mL-1) created intramolecularly assembled nanoparticles. Remarkably, the intrinsic viscosity of these nanoparticle solutions was an order of magnitude lower compared to solutions of the intermolecularly assembled analogues, highlighting the crucial role of intra- versus intermolecular interactions. Due to the strong hydrogen bonds between UPy dimers, the intramolecularly assembled structures were kinetically trapped. As a result, the polymer nanoparticles were readily processed into a bulk material, without causing major changes in the morphology as verified by atomic force microscopy. Subsequent intermolecular crosslinking of the nanoparticle film, by heating to temperatures where the hydrogen-bond exchange becomes fast, resulted in a crosslinked network. The reversibility of the hereby obtained polymer networks was shown by retrieving the intramolecularly assembled nanoparticles via redissolution and sonication of the intermolecularly crosslinked film in THF with a small amount of acid. Our results highlight that the stability and processability of highly supramolecularly crosslinked polymers can be controlled both in solution and in bulk by using the interplay of intra- and intermolecular non-covalent interactions in grafted polymers.

In situ Synthesis of Supramolecular Polymers: Finding the Right Conditions when Combining Covalent and Non-Covalent Synthesis.

The combination of covalent and non-covalent synthesis is omnipresent in nature and potentially enables access to new materials. Yet, the fundamental principles that govern such a synthesis are barely understood. Here, we demonstrate how even simple reaction mixtures behave surprisingly complex when covalent reactions are coupled to self-assembly processes. Specifically, we study the reaction behavior of a system in which the in situ formation of discotic benzene-1,3,5-tricarboxamide (BTA) monomers is linked to an intertwined non-covalent reaction network including self-assembly into helical BTA polymers. This system shows an unexpected phase-separation behavior in which an interplay of reactant/product concentrations, side-products and solvent purity determines the system composition. We envision that these insights can bring us one step closer to how to design the synthesis of systems in a combined covalent/non-covalent fashion.

Photo-Imprinting of the Helical Organization in Liquid-Crystal Networks Using Achiral Monomers and Circularly Polarized Light.

Control over molecular motion is facilitated in materials with highly ordered nanoscale structures. Here we report on the fabrication of cholesteric liquid-crystal networks by circularly polarized light irradiation, without the need for chiral dopant or plasticizer. The polymer network is obtained by photopolymerization of a smectic achiral diacrylate mesogen consisting of an azobenzene core and discrete oligodimethylsiloxane tails. The synchronous helical photoalignment and photopolymerization originate from the cooperative movement of the mesogens ordered in well-defined responsive structures, together with the flexibility of the oligodimethylsiloxane blocks. The resulting thin films show excellent thermal stability and light-induced memory features with reversible responses. Additionally, we demonstrate the fabrication of photo-patterned films of liquid-crystal networks with opposite helical senses. These findings provide a new method to make light-controllable chiroptical materials with exciting applications in optics and photonics.

Controlling the length of porphyrin supramolecular polymers via coupled equilibria and dilution-induced supramolecular polymerization.

Multi-component systems often display convoluted behavior, pathway complexity and coupled equilibria. In recent years, several ways to control complex systems by manipulating the subtle balances of interaction energies between the individual components have been explored and thereby shifting the equilibrium between different aggregate states. Here we show the enantioselective chain-capping and dilution-induced supramolecular polymerization with a Zn2+-porphyrin-based supramolecular system when going from long, highly cooperative supramolecular polymers to short, disordered aggregates by adding a monotopic Mn3+-porphyrin monomer. When mixing the zinc and manganese centered monomers, the Mn3+-porphyrins act as chain-cappers for Zn2+-porphyrin supramolecular polymers, effectively hindering growth of the copolymer and reducing the length. Upon dilution, the interaction between chain-capper and monomers weakens as the equilibria shift and long supramolecular polymers form again. This dynamic modulation of aggregate morphology and length is achieved through enantioselectivity in the aggregation pathways and concentration-sensitive equilibria. All-atom and coarse-grained molecular simulations provide further insights into the mixing of the species and their exchange dynamics. Our combined experimental and theoretical approach allows for precise control of molecular self-assembly and chiral discrimination in complex systems.

Tuning the donor-acceptor interactions in phase-segregated block molecules.

The assembly of donor-acceptor molecules via charge transfer (CT) interactions gives rise to highly ordered nanomaterials with appealing electronic properties. Here, we present the synthesis and bulk co-assembly of pyrene (Pyr) and naphthalenediimide (NDI) functionalized oligodimethylsiloxanes (oDMS) of discrete length. We tune the donor-acceptor interactions by connecting the pyrene and NDI to the same oligomer, forming a heterotelechelic block molecule (NDI-oDMSPyr), and to two separate oligomers, giving Pyr and NDI homotelechelic block molecules (Pyr-oDMS and NDI-oDMS). Liquid crystalline materials are obtained for binary mixtures of Pyr-oDMS and NDI-oDMS, while crystallization of the CT dimers occurred for the heterotelechelic NDI-oDMS-Pyr block molecule. The synergy between crystallization and phase-segregation coupled with the discrete length of the oDMS units allows for perfect order and sharp interfaces between the insulating siloxane and CT layers composed of crystalline CT dimers. We were able to tune the lamellar domain spacing and donor-acceptor CT interactions by applying pressures up to 6 GPa on the material, making the system promising for soft-material nanotechnologies. These results demonstrate the importance of the molecular design to tune the CT interactions and stability of a CT material.