pH-Directed Aggregation to Control Photoconductivity in Self-Assembled Perylene Bisimides

Selective Induction of Molecular Assembly to Tissue-Level Anisotropy on Peptide-Based Optoelectronic Cardiac Biointerfaces

The conduction efficiency of ions in excitable tissues and of charged species in organic conjugated materials both benefit from having ordered domains and anisotropic pathways. In this study, a photocurrent-generating cardiac biointerface is presented, particularly for investigating the sensitivity of cardiomyocytes to geometrically comply to biomacromolecular cues differentially assembled on a conductive nanogrooved substrate. Through a polymeric surface-templated approach, photoconductive substrates with symmetric peptide-quaterthiophene (4T)-peptide units assembled as 1D nanostructures on nanoimprinted polyalkylthiophene (P3HT) surface are developed. The 4T-based peptides studied here can form 1D nanostructures on prepatterned polyalkylthiophene substrates, as directed by hydrogen bonding, aromatic interactions between 4T and P3HT, and physical confinement on the nanogrooves. It is observed that smaller 4T-peptide units that can achieve a higher degree of assembly order within the polymeric templates serve as a more efficient driver of cardiac cytoskeletal anisotropy than merely presenting aligned -RGD bioadhesive epitopes on a nanotopographic surface. These results unravel some insights on how cardiomyocytes perceive submicrometer dimensionality, local molecular order, and characteristics of surface cues in their immediate environment. Overall, the work offers a cardiac patterning platform that presents the possibility of a gene modification-free cardiac photostimulation approach while controlling the conduction directionality of the biotic and abiotic components.

Self-Assembly Induced Photosensitization of Long-Tailed Heavy-Atom-Free BODIPY Derivatives for Photodynamic Therapy

Type I photosensitizers (PSs) are a promising approach for photodynamic therapy (PDT) since they can generate radicals that are tolerant to hypoxia. Thus, the development of highly efficient type I PSs is essential. Self-assembly is a promising strategy for developing novel PSs with desirable properties. Here, a simple and effective approach is developed to create heavy-atom-free PSs for PDT by self-assembling long-tailed boron dipyrromethene dyes (BODIPYs). The resulting aggregates BY-I16 and BY-I18 can efficiently convert their excited energy to the triplet state, producing reactive oxygen species that are essential for PDT. Furthermore, the aggregation and PDT performance can be regulated by adjusting the length of the tailed alkyl chains. As proof of concept, the efficacy of these heavy-atom-free PSs both in vitro and in vivo under both normoxic and hypoxic conditions is demonstrated.

Retro Diels-Alder-triggered self-assembly of a polymerizable macrocyclic diacetylene

A new triggered self-assembly method, which utilizes retro Diels-Alder (rDA)-promoted self-assembly of a macrocyclic diacetylene, was developed. The steric bulk present in a Diels-Alder (DA) adduct was released by a thermally promoted rDA reaction, resulting in the generation of a linear diacetylene that readily self-assembles to form a supramolecular polymer. The maleimide-containing blue-colored polydiacetylene, which was generated by UV irradiation, was utilized as a thiol specific colorimetric sensor.

Using Solution History to Control Hydrogel Properties of a Perylene Bisimide

pH dependence on water soluble aggregates is well-known in the field of low molecular weight gelators (LMWGs), with different aggregates sometimes having very different properties depending on their final pH. This aggregation determines their applications and performance. Here, we investigate the pH dependence of perylene bisimide gels; initially solutions are formed at a high pH and gels form as the pH is decreased. We find it is not only the final pH but also the starting pH that can impact the resulting gel. We use small angle neutron scattering (SANS), rheology, 1 H NMR spectroscopy and absorption spectroscopy to examine the effect of starting pH on gelation kinetics and final gel properties. Adjusting the solution from pH 9 (where there are few or no aggregates) to pH 6 results in the formation of different worm-like micelles than the ones directly formed at pH 6, leading to again gels with different mechanical properties. This work highlights the importance of controlling the pH of solutions before gelation, but also opens up more possible morphologies and therefore more properties from the same molecule.

Process Dependent Complexity in Multicomponent Gels

Mixing low molecular weight gelators (LMWGs) can be used to combine favorable properties of the individual components within a multifunctional gel. Such multicomponent systems are complex enough in themselves but the method of combining components is not commonly considered something to influence self-assembly. Herein, two multicomponent systems comprising of a naphthalene-based dipeptide hydrogelator and one of two modified naphthalene diimides (NDIs), one of which forms gels, and the other does not, are investigated. These systems are probed, examining the structures formed and their gel properties (when preparing a solution from either a mixed powder of both components or by mixing pre-formed solutions of each component) using rheology, small angle neutron scattering (SANS), and absorbance spectroscopy. It is found that by altering the method of preparation, it is can either induce self-sorting or co-assembly within the fibers formed that underpin the gel network.

Aniline dimers serving as stable and efficient transfer units for intermolecular charge-carrier transmission

Because any perturbation in the number of oxidation sites associated with the polymeric backbone can cause changes in the electrical properties, the stability of electrical properties has strongly prevented the wide adoption of most conducting polymers for commercialization, e.g., polyanilines (PANI). Herein, we showed that aniline dimers (AD) had more stable conductivity during redox due to their determinately separate oxidization or reduction units. Instead of intramolecular charge transfer as PANI, AD could serve as effective transfer units to facilitate intermolecular charge-carrier transmission due to low band-gap formation induced by the J-aggregation of AD, ensuring efficient conductivity. Typically, the electrical properties of AD-derived materials will still be stable after 10,000 redox cycles under a high operating voltage, far surpassing PANI under equivalent conditions. Meanwhile, the AD-derived materials could act as effective conducting and sensing layers with good stability. This approach opened an avenue for improving the stability of conductive polymers.

Investigating Aggregation Using In Situ Electrochemistry and Small-Angle Neutron Scattering

Using small-angle neutron scattering to investigate the aggregation of self-assembling molecules is well established. Some of these molecules are electrochemically useful, for example, in electrochromic devices. Electrochemistry can also be used in some cases to induce aggregation. Here, we describe an approach whereby electrochemistry can be directly carried out on a sample in the neutron beam, allowing us to monitor changes directly in situ. We exemplify with two examples but highlight that there are many other potential opportunities.

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.

Solvent-Directed Transformation of the Self-assembly and Optical Property of a Peptide-Appended Core-Substituted Naphthelenediimide and Selective Detection of Nitrite Ions in an Aqueous Medium

This study vividly displays the different self-assembling behavior and consequent tuning of the fluorescence property of a peptide-appended core-substituted naphthalenediimide (N1) in the aliphatic hydrocarbon solvents (n-hexane/n-decane/methyl cyclohexane) and in an aqueous medium within micelles. The N1 is highly fluorescent in the monomeric state and self-aggregates in a hydrocarbon solvent, exhibiting "H-type" or "face-to-face" stacking as indicated by a blue shift of absorption maxima in the UV-vis spectrum. In the H-aggregated state, the fluorescence emission of N1 changes to green from the yellow emission obtained in the monomeric state. In the presence of a micelle-forming surfactant, cetyl trimethylammonium bromide (CTAB), the N1 is found to be dispersed in a water medium. Interestingly, upon encapsulation of N1 into the micelle, the molecule alters its self-assembling pattern and optical property compared to its behavior in the hydrocarbon solvent. The N1 exhibits "edge-to-edge" stacking or J aggregates inside the micelle as indicated by the UV-vis spectroscopic study, which shows a red shift of the absorption maxima compared to that in the monomeric state. The fluorescence emission also differs in the water medium with the NDI derivative exhibiting red emission. FT-IR studies reveal that all amide NHs of N1 are hydrogen-bonded within the micelle (in the J-aggregated state), whereas both non-bonding and hydrogen-bonding amide NHs are present in the H-aggregated state. This is a wonderful example of solvent-mediated transformation of the aggregation pattern (from H to J) and solvatochromism of emission over a wide range from green in the H-aggregated state to yellow in the monomeric state and orangish-red in the J-aggregated state. Moreover, the J aggregate has been successfully utilized for selective and sensitive detection of nitrite ions in water even in the presence of other common anions (NO₃^-, SO₄^2-, HSO₄^-, CO₃^2-, and Cl^-).

Computationally Guided Tuning of Peptide-Conjugated Perylene Diimide Self-Assembly

Peptide-π-conjugated materials are important for biointerfacing charge-transporting applications due to their aqueous compatibility and formation of long-range π-electron networks. Perylene diimides (PDIs), well-established charge-transporting π systems, can self-assemble in aqueous solutions when conjugated with amino acids. In this work, we leveraged computational guidance from our previous work to access two different self-assembled architectures from PDI-amino acid conjugates. Furthermore, we expanded the design rule to other sequences to learn that the closest amino acids to the π core have a significant effect on the photophysical properties of the resulting assemblies. By simply altering glycine to alanine at the closest residue position, we observed significantly different electronic properties as revealed through UV-vis, photoluminescence, and circular dichroism spectroscopies. Accompanying molecular dynamics simulations revealed two distinct types of self-assembled architectures: cofacial structures when the smaller glycine residue is at the closest residue position to the π core versus rotationally shifted structures when glycine is substituted for the larger alanine. This study illustrates the use of tandem computations and experiments to unearth and understand new design rules for supramolecular materials and exposes a modest amino acid substitution as a means to predictably modulate the supramolecular organization and engineer the photophysical properties of π-conjugated peptidic materials.

Computational Tools to Rationalize and Predict the Self-Assembly Behavior of Supramolecular Gels

Supramolecular gels form a class of soft materials that has been heavily explored by the chemical community in the past 20 years. While a multitude of experimental techniques has demonstrated its usefulness when characterizing these materials, the potential value of computational techniques has received much less attention. This review aims to provide a complete overview of studies that employ computational tools to obtain a better fundamental understanding of the self-assembly behavior of supramolecular gels or to accelerate their development by means of prediction. As such, we hope to stimulate researchers to consider using computational tools when investigating these intriguing materials. In the concluding remarks, we address future challenges faced by the field and formulate our vision on how computational methods could help overcoming them.

Charge-Delocalized State and Coherent Vibrational Dynamics in Rigid PBI H-Aggregates

Herein, the ultrafast photoinduced dynamics and vibrational coherences for two perylenebisimide (PBI) H-aggregates showcase the formation of the excimer state and the delocalized radical anion state in the excited state, respectively. Using femtosecond transient absorption (fs-TA) and time-resolved impulsive stimulated Raman scattering (TR-ISRS) measurements, we unveiled excited-state dynamics of PBI H-aggregates in two aspects: (1) the intermolecular interactions between PBI units in H-aggregates induce the formation of new excited states, excimer and delocalized radical anion states, and (2) the intermolecular out-of-plane along the aggregate axis and the PBI core C═C stretch Raman modes can be a crucial indicator to understand the coherent exciton dynamics in H-aggregates. Notably, those excited-state Raman modes showed stationary peak positions during the excited-state dynamics. TR-ISRS analysis provides insights into the excited-state vibrational coherences concerning the formation of the excimer and charge-delocalized state in each aggregate system.

Enhanced Delivery of Neuroactive Drugs via Nasal Delivery with a Self-Healing Supramolecular Gel

This paper reports the use of a self-assembling hydrogel as a delivery vehicle for the Parkinson's disease drug l-DOPA. Based on a two-component combination of an l-glutamine amide derivative and benzaldehyde, this gel has very soft rheological properties and self-healing characteristics. It is demonstrated that the gel can be formulated to encapsulate l-DOPA. These drug-loaded gels are characterized, and rapid release of the drug is obtained from the gel network. This drug-loaded hydrogel has appropriate rheological characteristics to be amenable for injection. This system is therefore tested as a vehicle for nasal delivery of neurologically-active drugs-a drug delivery strategy that can potentially avoid first pass liver metabolism and bypass the blood-brain barrier, hence enhancing brain uptake. In vitro tests indicate that the gel has biocompatibility with respect to nasal epithelial cells. Furthermore, animal studies demonstrate that the nasal delivery of a gel loaded with 3 H-labeled l-DOPA out-performed a simple intranasal l-DOPA solution. This is attributed to longer residence times of the gel in the nasal cavity resulting in increased blood and brain concentrations. It is demonstrated that the likely routes of brain penetration of intranasally-delivered l-DOPA gel involve the trigeminal and olfactory nerves connecting to other brain regions.

Growth of Extra-Large Chromophore Supramolecular Polymers for Enhanced Hydrogen Production

The control of morphology in bioinspired chromophore assemblies is key to the rational design of functional materials for light harvesting. We investigate here morphological changes in perylene monoimide chromophore assemblies during thermal annealing in aqueous environments of high ionic strength to screen electrostatic repulsion. We found that annealing under these conditions leads to the growth of extra-large ribbon-shaped crystalline supramolecular polymers of widths from about 100 nm to several micrometers and lengths from 1 to 10 μm while still maintaining a unimolecular thickness. This growth process was monitored by variable-temperature absorbance spectroscopy, synchrotron X-ray scattering, and confocal microscopy. The extra-large single-crystal-like supramolecular polymers are highly porogenic, thus creating loosely packed hydrogel scaffolds that showed greatly enhanced photocatalytic hydrogen production with turnover numbers as high as 13 500 over ∼110 h compared to 7500 when smaller polymers are used. Our results indicate great functional opportunities in thermally and pathway-controlled supramolecular polymerization.

Lysosome-Instructed Self-Assembly of Amino-Acid-Functionalized Perylene Diimide for Multidrug-Resistant Cancer Cells

Multidrug resistance (MDR) of cancer cells reduces chemotherapeutic efficacy by preventing drug accumulation in the cells through a drug efflux pump and lysosomal sequestration/exocytosis. Herein, to overcome such anticancer resistance, lysosome-targeted self-assembly of perylene diimide (PDI) derivatives is presented as a powerful strategy for effective and selective anticancer therapy. Stimulated by the lysosomal low pH, the amphiphilic PDI derivatives functionalized with amino acids (PDI-AAs) construct fibrous self-assembled structures inside the lysosomes, causing cancer cell apoptosis by lysosomal rupture. In contrast, negligible apoptosis was observed from normal cells by PDI-AA. The agglomerated fibrous assemblies were not removed by lysosomal exocytosis, thereby displaying a 10.7-fold higher anticancer efficacy on MDR cancer cells compared to a doxorubicin chemotherapeutic agent. The MDR-circumventing capability, along with high selectivity toward cancer cells, supports PDI-AAs as potential candidates for the treatment of MDR cancer cells by lysosome-targeted self-assembly.

Impact of Citrate and Lipid-Functionalized Magnetic Nanoparticles in Dehydropeptide Supramolecular Magnetogels: Properties, Design and Drug Release

Currently, the nanoparticle functionalization effect on supramolecular peptide-based hydrogels remains undescribed, but is expected to affect the hydrogels' self-assembly and final magnetic gel properties. Herein, two different functionalized nanoparticles: citrate-stabilized (14.4 ± 2.6 nm) and lipid-coated (8.9 ± 2.1 nm) magnetic nanoparticles, were used for the formation of dehydropeptide-based supramolecular magnetogels consisting of the ultra-short hydrogelator Cbz-L-Met-Z-ΔPhe-OH, with an assessment of their effect over gel properties. The lipid-coated nanoparticles were distributed along the hydrogel fibers, while citrate-stabilized nanoparticles were aggregated upon gelation, which resulted into a heating efficiency improvement and decrease, respectively. Further, the lipid-coated nanoparticles did not affect drug encapsulation and displayed improved drug release reproducibility compared to citrate-stabilized nanoparticles, despite the latter attaining a stronger AMF-trigger. This report points out that adsorption of nanoparticles to hydrogel fibers, which display domains that improve or do not affect drug encapsulation, can be explored as a means to optimize the development of supramolecular magnetogels to advance theranostic applications.

Transient Self-assembly Processes Operated by Gaseous Fuels under Out-of-Equilibrium Conditions

Herein we report transient out-of-equilibrium self-assembly of molecules operated by gaseous fuel mixtures. The combination of an active gaseous chemical fuel and an inert gas or compressed air, which assists the degassing of the gaseous fuel from the solution, drives the transient self-assembly process. The gaseous nature of the fuel as well as the exhaust helps in their easy removal and thereby prevents their accumulation within the system and helps in maintaining the efficiency of the transient self-assembly process. The strategy is executed with a rather simple experimental set up and operates at ambient temperatures. Our approach may find use in the development of smart materials suitable for applications such as temporally active gas sensing and sequestration.

Audible sound-controlled spatiotemporal patterns in out-of-equilibrium systems

Naturally occurring spatiotemporal patterns typically have a predictable pattern design and are reproducible over several cycles. However, the patterns obtained from artificially designed out-of-equilibrium chemical oscillating networks (such as the Belousov-Zhabotinsky reaction for example) are unpredictable and difficult to control spatiotemporally, albeit reproducible over subsequent cycles. Here, we show that it is possible to generate reproducible spatiotemporal patterns in out-of-equilibrium chemical reactions and self-assembling systems in water in the presence of sound waves, which act as a guiding physical stimulus. Audible sound-induced liquid vibrations control the dissolution of atmospheric gases (such as O₂ and CO₂) in water to generate spatiotemporal chemical patterns in the bulk of the fluid, segregating the solution into spatiotemporal domains having different redox properties or pH values. It further helps us in the organization of transiently formed supramolecular aggregates in a predictable spatiotemporal manner.

Insight into the self-assembly of water-soluble perylene bisimide derivatives through a combined computational and experimental approach

We use a combination of computational and experimental techniques to study the self-assembly and gelation of water-soluble perylene bisimides derivatised at the imide position with an amino acid. Specifically, we study the likely structure of self-assembled aggregates of the alanine-functionalised perylene bisimide (PBI-A) and the thermodynamics of their formation using density functional theory and predict the UV-vis spectra of such aggregates using time-dependent density functional theory. We compare these predictions to experiments in which we study the evolution of the UV-Vis and NMR spectra and the rheology and neutron scattering of alkaline PBI-A solutions when gradually decreasing the pH. Based on the combined computational and experimental results, we show that PBI-A self-assembles at all pH values but that aggregates grow in size upon protonation. Hydrogel formation is driven not by aggregate growth but reduction of the aggregation surface-charge and a decrease in the colloidal stability of the aggregation with respect to agglomeration.

Visible Light Excitation-Induced Luminescence from Gold Nanoclusters Following Surface Ligand Complexation with Zn2+ for Daylight Sensing and Cellular Imaging

Herein, we report a complexation reaction-mediated extended aggregation of gold nanoclusters exhibiting luminescence under visible light excitation. The complexation reaction between the carboxylate groups of mercaptopropionic acid and zinc ions induced the aggregation of gold nanoclusters, which featured bright green luminescence upon excitation with visible light of wavelength 450 nm and beyond. This luminescence of aggregated Au NCs, easily discernible with bare eyes (under broad daylight excitation), was used as a probe for luminescence-based detection of molecules based on the p K_a values of the latter. This aspect has been an unfilled dream of scientists pursuing research on the development of nanoscale sensors, as luminescence-based detection techniques offer a greater degree of accuracy and sensitivity compared to absorption-based methods, and was thus far an unexploited/untapped area by nanoscale materials. Moreover, facile imaging of mammalian cells was achieved using these aggregated clusters upon excitation with visible light. This study demonstrates the utility of luminescent nanoclusters, akin to organic dyes, as materials active under visible light excitation. Thus, the complexation reaction-based tailoring of the optical properties of nanoclusters served as an effective tool in pushing the absorption maxima of the nanoclusters from an ultraviolet to visible range, enabling the luminescence of nanoclusters under broad daylight excitation. Hence, the work embodied herein offers a unique route to widen the application potential of metal nanoclusters as sensors and bioimaging agents operating under visible light excitation.

Solvent-Induced Supramolecular Assembly of a Peptide-Tetrathiophene-Peptide Conjugate

The assembly of a peptide-tetrathiophene-peptide (PTP) conjugate has been investigated in mixed solvents, which has different polarities by changing the solvent proportions. It was found that PTP can form fibers in THF/hexane solutions with 40–80%v of hexane. The fibers were stable and did not change on time. On the other hand, PTP formed ordered structures in a mixed solution with the water content from 40 to 60%v. For the as-prepared solutions, two nanostructures vesicles and parallelogram sheets were obtained. The parallelogram sheets could transform into vesicles on time. The fibers showed supramolecular chirality, however, there was no Cotton effect for vesicles and parallelogram sheets. UV-vis, FL, XRD, FT-IR, and CD spectra together with SEM, AFM, TEM were used to characterize the nanostructures and properties of the assemblies. Molecular packing mechanism was proposed based on the experimental data.

Perylene Bisimide Aggregates as Probes for Subnanomolar Discrimination of Aromatic Biogenic Amines

Perylene bisimide derivatives show peculiar physical chemical features, such as a highly conjugated system, high extinction coefficients and elevated fluorescence quantum yields, making them suitable for the development of optical sensors of compounds of interest. In particular, they are characterized by the tendency to aggregate into π-π stacked supramolecular structures. In this contribution, the behavior of the PBI derivative N, N'-bis(2-(trimethylammonium)ethylene)perylene bisimide dichloride was investigated both in aqueous solution and on solid support. The electronic communication between PBI aggregates and biogenic amines was exploited in order to discriminate aromatic amines down to subnanomolar concentrations by observing PBI fluorescence variations in the presence of various amines and at different concentrations. The experimental findings were corroborated by density functional theory calculations. In particular, phenylethylamine and tyramine were demonstrated to be selectively detected down to 10^-10 M concentration. Then, in order to develop a surface plasmon resonance (SPR) device, PBI was deposited onto a SPR support by means of the layer-by-layer method. PBI was deposited in the aggregated form and was demonstrated to preserve the capability to discriminate, selectively and with an outstanding analytical sensitivity, tyramine in the vapor phase and even if mixed with other aromatic amines.

Synthesis and characterisation of rylene diimide dimers using molecular handcuffs

Mechanically interlocked handcuffs provide a strategy to study rylene diimide dimers and to investigate their electronic and magnetic properties.

A strategy for positioning, and loosely connecting, molecules in close proximity using mechanically interlocked handcuffs is described. The strategy is demonstrated using rylene diimides, creating dimeric structures in which two components are linked through pillar[5]arene/imidazolium rotaxanes. Investigation of the resulting molecules demonstrates intriguing and new properties that arise from placing these redox active dye molecules together, allowing interactions, whilst allowing the molecules to separate as required. In particular we observe excimer emission from a perylene diimide dimer handcuff and the formation of an unusual radical anion π-dimer upon double reduction of the same molecule. The latter exhibits a unique visible absorption profile for a PDI-based molecule. We demonstrate the flexibility of our approach by making an unprecedented mixed perylene diimide/naphthalene diimide dimer which also reveals interactions between the two components. Our synthetic strategy facilitates the creation of unusual dimeric structures and allows the investigation of intermolecular interactions and the effects they have on electronic and magnetic properties.

Supramolecular Self-Assembly of Perylene Bisimide Derivatives Assisted by Various Groups

Anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10(2H,9H)-tetraone, namely, perylene bisimides (PBIs), belong to n-type organic semiconductors and possess potential applications in optoelectronic devices. The properties/performance of fabricated nanostructures/devices could be greatly influenced by both molecular structures of PBI building blocks and corresponding arrangement in assembled nanostructures. Many efforts have been made to modify the PBI core and then investigate the nanostructures and properties. However, it is still a great challenge to comprehensively understand the influence of molecular structures on the intermolecular interactions, the self-assembled structures, and the resulting performance. In the present contribution, we mainly summarize recent research aspects on supramolecular assembly behaviors of PBI derivatives assisted by various functional groups. First, a short introduction is given about basic molecular structure, properties, and self-assembly of PBI derivatives. Then, we mainly discuss the modulation of self-assembly of PBIs via introducing various functional groups (flexible or nonflexible chains, and biomolecules especially amino-acid-based groups). After that, the assembly of PBI derivatives from out-of-equilibrium states is described. Finally, a perspective is provided on the design of novel PBI derivatives and the fabrication of unique nanostructures with superior properties.

Gelation enabled charge separation following visible light excitation using self-assembled perylene bisimides

Perylene bisimides (PBIs) can be functionalised to enable controlled aggregation into complex supramolecular structures and are promising materials for photovoltaic and solar fuel applications.

Aromatic identity, electronic substitution, and sequence in amphiphilic tripeptide self-assembly

The phenomenon of self-assembly in short peptides (2-4 amino acids) has been a source of curiosity, in part for its role in helping to better understand and predict how minimal sequences within proteins might contribute to the formation of larger structures or aggregates. Building on previous work in this field, here we investigate a family of amphiphilic tripeptides for their self-assembly and hydrogel formation. From a parent peptide, Ac-FID-NH2, which was previously shown to self-assemble into high aspect-ratio filaments and form hydrogels, we explored the significance of structural features or sequence variations on the observed self-assembly. This process entailed substituting key aromatic residues, altering the electronics of these aromatic drivers of assembly, and screening tripeptide constitutional isomers. This work more clearly elucidates the mechanisms and design parameters that govern the creation of materials from short peptide building blocks, as well as offering greater insight into the interactions between minimal segments of proteins that underlie their structure and aggregation.

Thionated Perylene Diimide-Phenothiazine Dyad: Synthesis, Structure, and Electrochemical Studies

Perylene diimides (PDIs) are promising candidates for n-type semiconductor materials and, thus, for use in organic electronics. Thionation of the imide moiety provides an efficient strategy to control the donor-acceptor gap of these types of compounds, although the degree and selectivity of thionation can be hard to achieve. Through the design of a sterically encumbered PDI-phenothiazine dyad, a previously unattained geminal thionation pattern has been realized, providing the first example of a perylene-monoimide-monothioimide. The electrochemical and solid-state structural properties of this uniquely thionated dyad are reported and compared to those of the nonthionated parent molecule. It is found that thionation enhances the electron affinity of the PDI core, affecting electrochemical and spectroelectochemcial behavior of the dyad without significantly affecting the solid-state packing of the molecules.

Hydrogen-bonded perylene bisimide J-aggregate aqua material

A water-soluble perylene bisimide dye self-assembles in aqueous media into thermoresponsive aqua materials with photoluminescence within the biological transparency window.

A new twelvefold methoxy-triethyleneglycol-jacketed tetraphenoxy-perylene bisimide (MEG-PBI) amphiphile was synthesized that self-assembles into two types of supramolecular aggregates in water: red-coloured aggregates of low order and with weak exciton coupling among the PBIs and blue-coloured strongly coupled J-aggregates consisting of a highly ordered hydrogen-bonded triple helix of PBIs. At room temperature this PBI is miscible with water at any proportions which enables the development of robust dye aggregates in solution, in hydrogel states and in lyotropic liquid crystalline states. In the presence of 60–95 wt% water, self-standing coloured hydrogels exhibit colour changes from red to blue accompanied by a fluorescence light-up in the far-red region upon heating in the range of 30–50 °C. This phenomenon is triggered by an entropically driven temperature-induced hydrogen-bond-directed slipped stacking arrangement of the MEG-PBI chromophores within structurally well-defined J-aggregates. This versatile aqua material is the first example of a stable PBI J-aggregate in water. We anticipate that this study will open a new avenue for the development of biocompatible functional materials based on self-assembled dyes and inspire the construction of other hydrogen-bonded supramolecular materials in the highly competitive solvent water.

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.

Inter-Backbone Charge Transfer as Prerequisite for Long-Range Conductivity in Perylene Bisimide Hydrogels

Hydrogelation, the self-assembly of molecules into soft, water-loaded networks, is one way to bridge the structural gap between single molecules and functional materials. The potential of hydrogels, such as those based on perylene bisimides, lies in their chemical, physical, optical, and electronic properties, which are governed by the supramolecular structure of the gel. However, the structural motifs and their precise role for long-range conductivity are yet to be explored. Here, we present a comprehensive structural picture of a perylene bisimide hydrogel, suggesting that its long-range conductivity is limited by charge transfer between electronic backbones. We reveal nanocrystalline ribbon-like structures as the electronic and structural backbone units between which charge transfer is mediated by polar solvent bridges. We exemplify this effect with sensing, where exposure to polar vapor enhances conductivity by 5 orders of magnitude, emphasizing the crucial role of the interplay between structural motif and surrounding medium for the rational design of devices based on nanocrystalline hydrogels.

Molecular Control of Internal Crystallization and Photocatalytic Function in Supramolecular Nanostructures

Supramolecular light-absorbing nanostructures are useful building blocks for the design of next-generation artificial photosynthetic systems. Development of such systems requires a detailed understanding of how molecular packing influences the material's optoelectronic properties. We describe a series of crystalline supramolecular nanostructures in which the substituents on their monomeric units strongly affects morphology, ordering kinetics, and exciton behavior. By designing constitutionally-isomeric perylene monoimide (PMI) amphiphiles, the effect of side chain sterics on nanostructure crystallization was studied. Molecules with short amine linked alkyl-tails rapidly crystallize upon dissolution in water, while bulkier tails require the addition of salt to screen electrostatic repulsion and annealing to drive crystallization. A PMI monomer bearing a 3-pentylamine tail was found to possess a unique structure that results in strongly red-shifted absorbance, indicative of charge-transfer exciton formation. This particular supramolecular structure was found to have an enhanced ability to photosensitize a thiomolybdate, [(NH₄)₂Mo₃S₁₃], catalyst to generate hydrogen gas.

Luminescent Naphthalene Diimide-Based Peptide in Aqueous Medium and in Solid State: Rewritable Fluorescent Color Code

This study convincingly demonstrates a unique example of the self-assembly of a naphthalene diimide (NDI)-appended peptide into a fluorescent J-aggregate in aqueous media. Moreover, this aggregated species shows a remarkable yellow fluorescence in solid state, an unusual phenomenon for NDI-based compounds. The aggregated species has been characterized using transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy, X-ray diffraction, time-correlated single proton counting (TCSPC), UV-vis, and photoluminescence studies. TEM images reveal cross-linked nanofibrillar morphology of this aggregated species in water (pH 7.4). TCSPC study clearly indicates that the aggregated species in water has a higher average lifetime compared to that of the non-aggregated species. Interestingly, this NDI-based peptide shows H+ ion concentration-dependent change in the emission property in water. The fluorescence output is erased completely in the presence of an alkali, and it reappears in the presence of an acid, indicating its erasing and rewritable property. This indicates its probable use in authentication tools for security purposes as a rewritable fluorescence color code. This NDI-appended peptide-based molecule can be used for encryption of information due to erasing and rewritable property of the molecule in the aggregated state in aqueous medium.

Controlling Photoconductivity in PBI Films by Supramolecular Assembly

Perylene bisimides (PBIs) self-assemble in solution. The solubility of the PBIs is commonly changed through the choice of substituents at the imide positions. It is generally assumed this substitution does not affect the electronic properties of the PBI, and that the properties of the self-assembled aggregate are essentially that of the isolated molecule. However, substituents do affect the self-assembly, resulting in potentially different packing in the formed aggregates. Here, we show that the photoconductivity of films formed from a library of substituted PBIs varies strongly with the substituent and demonstrate that this is due to the different ways in which they pack. Our results open the possibility for tuning the optoelectronic properties of self-assembled PBIs by controlling the aggregate structure through careful choice of substituent, as demonstrated by us here optimising the photoconductivity of PBI films in this way.

Effect of electric field on the electrical properties of a self-assembled perylene bisimide

A functionalised perylene bisimide forms two different self-assembled structures in water depending on the solution pH. Structure 1 (formed at pH 6.2) consists of a fibrous structure, whilst structure 2 (formed at pH 9.4) consists of disordered aggregates. Despite being formed from the same molecule, structure 1 shows higher stability under illumination and electric field than structure 2, demonstrating that the nature of the self-assembled aggregate is critical in devices. Interestingly, both structures show p-type behaviour.

A functionalised perylene bisimide forms two different self-assembled structures in water depending on the solution pH.

Self-assembling peptide semiconductors

Semiconductors are central to the modern electronics and optics industries. Conventional semiconductive materials bear inherent limitations, especially in emerging fields such as interfacing with biological systems and bottom-up fabrication. A promising candidate for bioinspired and durable nanoscale semiconductors is the family of self-assembled nanostructures comprising short peptides. The highly ordered and directional intermolecular π-π interactions and hydrogen-bonding network allow the formation of quantum confined structures within the peptide self-assemblies, thus decreasing the band gaps of the superstructures into semiconductor regions. As a result of the diverse architectures and ease of modification of peptide self-assemblies, their semiconductivity can be readily tuned, doped, and functionalized. Therefore, this family of electroactive supramolecular materials may bridge the gap between the inorganic semiconductor world and biological systems.