Pathway Selection in Temporal Evolution of Supramolecular Polymers of Ionic π-Systems: Amphiphilic Organic Solvent Dictates the Fate of Water

Understanding solvent-solute interactions is essential to designing and synthesising soft materials with tailor-made functions. Although the interaction of the solute with the solvent mixture is more complex than the single solvent medium, solvent mixtures are exciting to unfold several unforeseen phenomena in supramolecular chemistry. Here, we report two unforeseen pathways observed during the hierarchical assembly of cationic perylene diimides (cPDIs) in water and amphiphilic organic solvent (AOS) mixtures. When the aqueous supramolecular polymers (SPs) of cPDIs are injected into AOS, initially kinetically trapped short SPs are formed, which gradually transform into thermodynamically stable high aspect ratio SP networks. Using various experimental and theoretical investigations, we found that this temporal evolution follows two distinct pathways depending on the nature of the water-AOS interactions. If the AOS is isopropanol (IPA), water is released from cPDIs into bulk IPA due to strong hydrogen bonding interactions, which further decreases the monomer concentration of cPDIs (Pathway-1). In the case of dioxane AOS, cPDI monomer concentration further increases as water is retained among cPDIs (Pathway-2) due to relatively weak interactions between dioxane and water. Interestingly, these two pathways are accelerated by external stimuli such as heat and mechanical agitation.

Weakly Self-Assembled [6]Helicenes: Circularly Polarized Light and Spin Filtering Properties

Self-assembling features, chiroptical activity, and spin filtering properties are reported for 2,15- and 4,13-disubstituted [6]helicenes decorated in their periphery with 3,4,5-tris(dodecyloxy)-N-(4-ethynylphenyl)benzamide moieties. The weak non-covalent interaction between these units conditions the corresponding circularly polarized luminescence and spin polarization. The self-assembly is overall weak for these [6]helicene derivatives that, despite the formation of H-bonding interactions between the amide groups present in the peripheral moieties, shows very similar chiroptical properties both in the monomeric or aggregated states. This effect could be explained by considering the steric effect that these groups could generate in the growing of the corresponding aggregate formed. Importantly, the self-assembling features also condition chiral induced spin selectivity (CISS effect), with experimental spin polarization (SP) values found between 35-40 % for both systems, as measured by magnetic-conducting atomic force microscopy (AFM) technique.

Semiconductive and Biocompatible Nanofibrils from the Self-Assembly of Amyloid π-Conjugated Peptides

Owing to their capacity to self-assemble into organized nanostructures, amyloid polypeptides can serve as scaffolds for the design of biocompatible semiconductive materials. Herein, symmetric and asymmetric amyloid π-conjugated peptides were prepared through condensation of perylene diimide (PDI) with a natural amyloidogenic sequence derived from the islet amyloid polypeptide. These PDI-bioconjugates assembled into long and linear nanofilaments in aqueous solution, which were characterized by a cross-β-sheet quaternary organization. Current-voltage curves exhibited a clear signature of semiconductors, whereas the cellular assays revealed cytocompatibility and potential application in fluorescence microscopy. Although the incorporation of a single amyloid peptide appeared sufficient to drive the self-assembly into organized fibrils, the incorporation of two peptide sequences at the PDI's imide positions significantly enhanced the conductivity of nanofibril-based films. Overall, this study exposes a novel strategy based on amyloidogenic peptide to guide the self-assembly of π-conjugated systems into robust, biocompatible, and optoelectronic nanofilaments.

Anticooperative Supramolecular Oligomerization Mediated by V-Shaped Monomer Design and Unconventional Hydrogen Bonds

After more than three decades of extensive investigations on supramolecular polymers, strategies for self-limiting growth still remain challenging. Herein, we exploit a new V-shaped monomer design to achieve anticooperatively formed oligomers with superior robustness and high luminescence. In toluene, the monomer-oligomer equilibrium is shifted to the monomer side, enabling the elucidation of the molecular packing modes and the resulting (weak) anticooperativity. Steric effects associated with an antiparallel staircase organization of the dyes are proposed to outcompete aromatic and unconventional B-F⋅⋅⋅H-N/C interactions, restricting the growth at the stage of oligomers. In methylcyclohexane (MCH), the packing modes and the anticooperativity are preserved; however, pronounced solvophobic and chain-enwrapping effects lead to thermally ultrastable oligomers. Our results shed light on understanding anticooperative effects and restricted growth in self-assembly.

Thermoreversible Polymorph Transitions in Supramolecular Polymers of Hydrogen-Bonded Squaramides

Hydrogen-bonded squaramide (SQ) supramolecular polymers exhibit uncommon thermoreversible polymorph transitions between particle- and fiber-like nanostructures. SQs 1-3, with different steric bulk, self-assemble in solution into particles (AggI) upon cooling to 298 K, and SQs 1 and 2, with only one dendronic group, show a reversible transformation into fibers (AggII) by further decreasing the temperature to 288 K. Nano-DSC and UV/Vis studies on SQ 1 reveal a concentration-dependent transition temperature and ΔH for the AggI-to-AggII conversion, while the kinetic studies on SQ 2 indicate the on-pathway nature of the polymorph transition. Spectroscopic and theoretical studies reveal that these transitions are triggered by the molecular reorganization of the SQ units changing from slipped to head-to-tail hydrogen bonding patterns. This work unveils the thermodynamic and kinetic aspects of reversible polymorph transitions that are of interest to develop stimuli-responsive systems.

Anti-cooperative Self-Assembly with Maintained Emission Regulated by Conformational and Steric Effects

Herein, we present a strategy to enable a maintained emissive behavior in the self‐assembled state by enforcing an anti‐cooperative self‐assembly involving weak intermolecular dye interactions. To achieve this goal, we designed a conformationally flexible monomer unit 1 with a central 1,3‐substituted (diphenyl)urea hydrogen bonding synthon that is tethered to two BODIPY dyes featuring sterically bulky trialkoxybenzene substituents at the meso‐position. The competition between attractive forces (H‐bonding and aromatic interactions) and destabilizing effects (steric and competing conformational effects) limits the assembly, halting the supramolecular growth at the stage of small oligomers. Given the presence of weak dye–dye interactions, the emission properties of molecularly dissolved 1 are negligibly affected upon aggregation. Our findings contribute to broadening the scope of emissive supramolecular assemblies and controlled supramolecular polymerization.

Photo-modulation of supramolecular polymorphism in the self-assembly of a scissor-shaped azobenzene dyad into nanotoroids and fibers

Recent advances in the research field of supramolecularly engineered dye aggregates have enabled the design of simple one-dimensional stacks such as fibers and of closed structures such as nanotoroids (nanorings). More complex and advanced supramolecular systems could potentially be designed using a molecule that is able to provide either of these distinct nanostructures under different conditions. In this study, we introduced bulky but strongly aggregating cholesterol units to a scissor-shaped azobenzene dyad framework, which affords either nanotoroids, nanotubes, or 1D fibers, depending on the substituents. This new dyad with two trans-azobenzene arms shows supramolecular polymorphism in its temperature-controlled self-assembly, leading to not only oligomeric nanotoroids as kinetic products, but also to one-dimensional fibers as thermodynamic products. This supramolecular polymorphism can also be achieved via photo-triggered self-assembly, i.e., irradiation of a monomeric solution of the dyad with two cis-azobenzene arms using strong visible light leads to the preferential formation of nanotoroids, whereas irradiation with weak visible light leads to the predominant formation of 1D fibers. This is the first example of a successful light-induced modulation of supramolecular polymorphism to produce distinctly nanostructured aggregates under isothermal conditions.

Design and Control of Perylene Supramolecular Polymers through Imide Substitutions

The number and type of new supramolecular polymer (SMP) systems have increased rapidly in recent years. Some of the key challenges faced for these novel systems include gaining full control over the mode of self-assembly, the creation of novel architectures and exploring functionality. Here, we provide a critical overview of approaches related to perylene-based SMPs and discuss progress to exert control over these potentially important SMPs through chemical modification of the imide substituents. Imide substitutions affect self-assembly behaviour orthogonally to the intrinsic optoelectronic properties of the perylene core, making for a valuable approach to tune SMP properties. Several recent approaches are therefore highlighted, with a focus on controlling 1) morphology, 2) H- or J- aggregation, and 3) mechanism of growth and degree of aggregation using thermodynamic and kinetic control. Areas of potential future exploration and application of these functional SMPs are also explored.

Supramolecular polymerization of electronically complementary linear motifs: anti-cooperativity by attenuated growth

Anti-cooperative supramolecular polymerization by attenuated growth exhibited by self-assembling units of two electron-donor benzo[1,2-b:4,5-b']dithiophene (BDT) derivatives (compounds 1a and 1b) and the electron-acceptor 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) (compound 2) is reported. Despite the apparent cooperative mechanism of 1 and 2, AFM imaging and SAXS measurements reveal the formation of small aggregates that suggest the operation of an anti-cooperative mechanism strongly conditioned by an attenuated growth. In this mechanism, the formation of the nuclei is favoured over the subsequent addition of monomeric units to the aggregate, which finally results in short aggregates. Theoretical calculations show that both the BDT and BODIPY motifs, after forming the initial dimeric nuclei, experience a strong distortion of the central aromatic backbone upon growth, which makes the addition of successive monomeric units unfavourable and impedes the formation of long fibrillar structures. Despite the anti-cooperativity observed in the supramolecular polymerization of 1 and 2, the combination of both self-assembling units results in the formation of small co-assembled aggregates with a similar supramolecular polymerization behaviour to that observed for the separate components.

Temperature-dependent modulation by biaryl-based monomers of the chain length and morphology of biphenyl-based supramolecular polymers

Supramolecular copolymerizations offer attractive options to introduce structural and functional diversity in supramolecular polymer materials. Yet, general principles and structure–property relationships for rational comonomer design remain lacking. Here, we report on the supramolecular (co)aggregation of a phenylpyridine and bipyridine derivative of a recently reported biphenyl tetracarboxamide-based monomer. We show that both arylpyridines are poor monomers for supramolecular homopolymerizations. However, the two arylpyridines efficiently influence supramolecular polymers of a biphenyl-based polymer. The phenylpyridine derivatives primarily sequestrate biphenyl monomers, while the bipyridine intercalates into the polymers at high temperatures. Thereby, these two poorly homopolymerizing monomers allow for a fine control over the length of the biphenyl-based supramolecular polymers. As such, our results highlight the potential to control the structure and morphology of supramolecular polymers by tailoring the electronic properties of additives.

Supramolecular copolymerizations offer attractive options to introduce structural and functional diversity in supramolecular polymer materials.

Two-step anti-cooperative self-assembly process into defined π-stacked dye oligomers: insights into aggregation-induced enhanced emission

Aggregation-induced emission enhancement (AIEE) phenomena received great popularity during the last decade but in most cases insights into the packing structure – fluorescence properties remained scarce. Here, an almost non-fluorescent merocyanine dye was equipped with large solubilizing substituents, which allowed the investigation of it's aggregation behaviour in unpolar solvents over a large concentration range (10⁻² to 10⁻⁷ M). In depth analysis of the self-assembly process by concentration-dependent UV/Vis spectroscopy at different temperatures revealed a two-step anti-cooperative aggregation mechanism. In the first step a co-facially stacked dimer is formed driven by dipole–dipole interactions. In a second step these dimers self-assemble to give an oligomer stack consisting of about ten dyes. Concentration- and temperature-dependent UV/Vis spectroscopy provided insight into the thermodynamic parameters and allowed to identify conditions where either the monomer, the dimer or the decamer prevails. The centrosymmetric dimer structure could be proven by 2D NMR spectroscopy. For the larger decamer atomic force microscopy (AFM), diffusion ordered spectroscopy (DOSY) and vapour pressure osmometric (VPO) measurements consistently indicated that it is of small and defined size. Fluorescence, circular dichroism (CD) and circularly polarized luminescence (CPL) spectroscopy provided insights into the photofunctional properties of the dye aggregates. Starting from an essentially non-fluorescent monomer (Φ_Fl = 0.23%) a strong AIEE effect with excimer-type fluorescence (large Stokes shift, increased fluorescence lifetime) is observed upon formation of the dimer (Φ_Fl = 2.3%) and decamer (Φ_Fl = 4.5%) stack. This increase in fluorescence is accompanied for both aggregates by an aggregation-induced CPL enhancement with a strong increase of the g_lum from ∼0.001 for the dimer up to ∼0.011 for the higher aggregate. Analysis of the radiative and non-radiative decay rates corroborates the interpretation that the AIEE effect originates from a pronounced decrease of the non-radiative rate due to π–π-stacking induced rigidification that outmatches the effect of the reduced radiative rate that originates from the H-type exciton coupling in the co-facially stacked dyes.

The self-assembly of a dipolar merocyanine into preferred dimers and small-sized chiral aggregates leads to enhanced emission due to a reduced non-radiative rate as well as amplified circular polarized luminescence.

Coexistence of Parallel and Rotary Stackings in the Lamellar Crystals of a Perylene Bisimide Dyad for Temperature-Sensitive Bicomponent Emission

Coexistence of rotationally π-π stacked columns and discrete slip-stacked dimers of perylene bisimide (PBI) chromophores is revealed by single crystal X-ray diffraction in the lamellar crystal of a head-to-tail linked PBI dyad. The rotary π-π stacked columnar moieties show H-type spectral character with relatively higher excitation energy, while the discrete slip-stacked π-π dimers have J-type spectral behavior with lower excitation energy. The lamellar crystals show relatively low photoluminescence efficiency of 12% at room temperature, while this dramatically increases to ∼90% at low temperature (80 K). Both of the rotary and slip-stacked moieties are emissive, and the nonradiative energy transfer processes between them are suppressed at low temperature, ensuring the highly efficient excimer-like long-lived fluorescence.

Transformation from helical to layered supramolecular organization of asymmetric perylene diimides via multiple intermolecular hydrogen bonding

The solid-state supramolecular organization of asymmetric perylene diimide is transformed from helical to layered self-assembly after thermal annealing.

Self-Assembled Dual Helical Nanofibers of Amphiphilic Perylene Diimides with Oligopeptide Substitution

We designed an asymmetric amphiphilic perylene diimide (PDI) with the oligopeptide substituting one of the imides. The self-assembly mechanism of this PDI in different solvents was investigated. Right-handed "dual" helical nanofibers/nanowires with a uniform lateral dimension of ∼8 nm are constructively self-assembled. The long-term ordered degree within the nanofibers stems from the delicate balance between π-π stacking of the PDI rings and β-sheet-like hydrogen bond formed by the oligopeptide. The synergistic interplay between the hydrogen bond and π-π stacking rather than competition endows the nanofibers with the controllable longitudinal dimensions by different factors such as the concentration and solvents. The transition from the nanofibers to the small aggregates is also achieved by the addition of trifluoroacetic acid because of breakup of the hydrogen bonds, which is reversed by further addition of trimethylamine. The acid-base stimulation can be extended to different solvents as long as the existence of the unique hydrogen bonds.

Pathway Control in Cooperative vs. Anti-Cooperative Supramolecular Polymers

Controlling the nanoscale morphology in assemblies of π-conjugated molecules is key to developing supramolecular functional materials. Here, we report an unsymmetrically substituted amphiphilic Pt^II complex 1 that shows unique self-assembly behavior in nonpolar media, providing two competing anti-cooperative and cooperative pathways with distinct molecular arrangement (long- vs. medium-slipped, respectively) and nanoscale morphology (discs vs. fibers, respectively). With a thermodynamic model, we unravel the competition between the anti-cooperative and cooperative pathways: buffering of monomers into small-sized, anti-cooperative species affects the formation of elongated assemblies, which might open up new strategies for pathway control in self-assembly. Our findings reveal that side-chain immiscibility is an efficient method to control anti-cooperative assemblies and pathway complexity in general.

Self-assembly of multi-stranded perylene dye J-aggregates in columnar liquid-crystalline phases

Many discoid dyes self-assemble into columnar liquid-crystalline (LC) phases with packing arrangements that are undesired for photonic applications due to H-type exciton coupling. Here, we report a series of crystalline and LC perylene bisimides (PBIs) self-assembling into single or multi-stranded (two, three, and four strands) aggregates with predominant J-type exciton coupling. These differences in the supramolecular packing and optical properties are achieved by molecular design variations of tetra-bay phenoxy-dendronized PBIs with two N–H groups at the imide positions. The self-assembly is driven by hydrogen bonding, slipped π–π stacking, nanosegregation, and steric requirements of the peripheral building blocks. We could determine the impact of the packing motifs on the spectroscopic properties and demonstrate different J- and H-type coupling contributions between the chromophores. Our findings on structure–property relationships and strong J-couplings in bulk LC materials open a new avenue in the molecular engineering of PBI J-aggregates with prospective applications in photonics.

Perylene bisimides (PBI) exhibit interesting photophysical and self-assembly properties but detailed understanding of the correlation between packing motif and spectroscopic properties is lacking. Here the authors report on self-assembling of PBIs in liquid crystalline phases to give aggregates with J- and H-type coupling contribution between the chromophores.

Relationship between Side-Chain Polarity and the Self-Assembly Characteristics of Perylene Diimide Derivatives in Aqueous Solution

Perylene-3,4,9,10-tetracarboxylic acid diimides (PDIs) have recently gained considerable interest for water-based biosensing applications. PDIs have been studied intensively in the bulk state, but their physical properties in aqueous solution in interplay with side-chain polarity are, however, poorly understood. Therefore, three perylene diimide based derivatives were synthesized to study the relationship between side-chain polarity and their self-assembly characteristics in water. The polarity of the side chains was found to dictate the size and morphology of the formed aggregates. Side-chain polarity rendered the self-assembly and photophysical properties of the PDIs-both important for imminent water-based applications-and these were revealed to be especially responsive to changes in solvent composition.

Single-component white-light emission via intramolecular electronic conjugation-truncation with perylenemonoimide

Solvent specific intramolecular electronic communication truncated optical behaviour, and white light emission with a CIE chromaticity index of (0.33, 0.33) using only single component via non-covalent nano-assembly of alkyne–phenanthrene conjugated perylenemonoimide (PMIAP) are demonstrated.

Strategies to create hierarchical self-assembled structures via cooperative non-covalent interactions

Cooperative phenomena are common processes involved in the hierarchical self-assembly of multiple systems in nature, such as the tobacco mosaic virus and a cell's cytoskeleton. Motivated by the high degree of order exhibited by these systems, a great deal of effort has been devoted in the past two decades to design hierarchical supramolecular polymers by combining different classes of cooperative interactions. In this review, we have classified the field of supramolecular polymers depending on the cooperative non-covalent forces driving their formation, with particular emphasis on recent examples from literature. We believe that this overview would help scientists in the field to design novel self-assembled systems with improved complexity and functionalities.

Anti-cooperative supramolecular polymerization: a new K2-K model applied to the self-assembly of perylene bisimide dye proceeding via well-defined hydrogen-bonded dimers

A new perylene bisimide dye self-assembles in an anti-cooperative process predominently into even numbered aggregates via dimers which could be interpreted by a newly developed K₂–K model.

A perylene bisimide dye bearing amide functionalities at the imide positions derived from amino acid l-alanine and a dialkoxy-substituted benzyl amine self-assembles into tightly bound dimers by π–π-stacking and hydrogen bonding in chloroform. In less polar or unpolar solvents like toluene and methylcyclohexane, and in their mixtures, these dimers further self-assemble into extended oligomeric aggregates in an anti-cooperative process in which even numbered aggregates are highly favoured. The stepwise transition from dimers into oligomers can not be properly described by conventional K₂–K model, and thus a new K₂–K aggregation model has been developed, which interpretes the present anti-cooperative supramolecular polymerization more appropriately. The newly developed K₂–K model will be useful to describe self-assembly processes of a plethora of other π-conjugated molecules that are characterized by a favored dimer species.

Self-Assembly of Supramolecular Light-Harvesting Arrays from Symmetric Perylene-3,4-dicarboximide Trefoils

Unlike the widely studied perylene-3,4:9,10-bis(dicarboximide) (PDI) dyes, self-assembly of the corresponding perylene-3,4-dicarboximide (PMI) dyes into large arrays and studies of their excited state properties have received far less attention. Two symmetric PMI trefoils were synthesized by connecting the 9-position of the perylene core either directly (1) or through a phenylene linker (2) to the 1,3,5-positions of a central benzene ring. Synchrotron-based small- and wide-angle X-ray scattering measurements in methylcyclohexane show that trefoil 1 self-assembles into cofacial trimers (13) on average, while trefoil 2 forms much larger assemblies that are tridecamers (213) on average. Their photophysics were characterized using steady-state as well as transient absorption and emission spectroscopy. Time-resolved spectroscopy reveals that both 13 and 213 initially form excitonically coupled excited states that subsequently relax to excimer states having 20 and 8.4 ns lifetimes, respectively, which decay to ground-state primarily nonradiatively. The data are consistent with stronger electronic coupling between the PMI molecules in 213 relative to 13.