Spatial Correlations Drive Long-Range Transport and Trapping of Excitons in Single H-Aggregates: Experiment and Theory.

Describing long-range energy transport is a crucial step, both toward deepening our knowledge on natural light-harvesting systems and toward developing novel photoactive materials. Here, we combine experiment and theory to resolve and reproduce energy transport on pico- to nanosecond time scales in single H-type supramolecular nanofibers based on carbonyl-bridged triarylamines (CBT). Each nanofiber shows energy transport dynamics over long distances up to ∼1 µm, despite exciton trapping at specific positions along the nanofibers. Using a minimal Frenkel exciton model including disorder, we demonstrate that spatial correlations in the normally distributed site energies are crucial to reproduce the experimental data. In particular, we can observe the long-range and subdiffusive nature of the exciton dynamics as well as the trapping behavior of excitons in specific locations of the nanofiber. This trapping behavior introduces a net directionality or asymmetry in the exciton dynamics as observed experimentally.

Electrogelation: Controlled Fast Formation of Micrometer-Thick Films from Low-Molecular Weight Hydrogelators.

The controlled electrochemical deposition of hydrogels from low-molecular weight hydrogelators (LMWHGs) allows for the defined formation of thin films on electrodes. Here, the deposition of fibrillar networks consisting of N,N',N″-tris(4-carboxyphenylene)-1,3,5-benzenetricarboxamide (BTA) onto ultraflat gold electrodes has been studied. This process, also termed electrogelation, is based on a local change in the pH due to electrolysis of water at the electrode. The protonation of the BTA sodium salt leads to self-assembly into supramolecular fibrillar structures mainly via hydrogen bonding of the uncharged molecules. The resulting hydrogel film was characterized in terms of its thickness by atomic force microscopy (AFM). Two different AFM-based techniques have been used: ex situ imaging of dried films and in situ nanoindentation of the hydrated hydrogel films. The deposition process was studied as a function of gelator concentration, applied potential, and gelation time. These parameters allow control of the film thickness to a high degree of accuracy within a few tenths of nanometers. Film formation takes place in a few seconds at moderate applied potentials, which is beneficial for biomedical applications. The results obtained for the BTA presented here can be transferred to any type of pH-responsive LMWHG and many reversibly formed hydrogel films.

Directed Gradients in the Excited-State Energy Landscape of Poly(3-hexylthiophene) Nanofibers.

Funneling excitation energy toward lower energy excited states is a key concept in photosynthesis, which is often realized with at most two chemically different types of pigment molecules. However, current synthetic approaches to establish energy funnels, or gradients, typically rely on Förster-type energy-transfer cascades along many chemically different molecules. Here, we demonstrate an elegant concept for a gradient in the excited-state energy landscape along micrometer-long supramolecular nanofibers based on the conjugated polymer poly(3-hexylthiophene), P3HT, as the single component. Precisely aligned P3HT nanofibers within a supramolecular superstructure are prepared by solution processing involving an efficient supramolecular nucleating agent. Employing hyperspectral imaging, we find that the lowest-energy exciton band edge continuously shifts to lower energies along the nanofibers' growth direction. We attribute this directed excited-state energy gradient to defect fractionation during nanofiber growth. Our concept provides guidelines for the design of supramolecular structures with an intrinsic energy gradient for nanophotonic applications.

Thermo-Mechanical Recyclability of Additively Manufactured Polypropylene and Polylactic Acid Parts and Polypropylene Support Structures.

Polymers have a reputation for several advantageous characteristics like chemical resistance, weight reduction, and simple form-giving processes. The rise of additive manufacturing technologies such as Fused Filament Fabrication (FFF) has introduced an even more versatile production process that supported new product design and material concepts. This led to new investigations and innovations driven by the individualization of customized products. The other side of the coin contains an increasing resource and energy consumption satisfying the growing demand for polymer products. This turns into a magnitude of waste accumulation and increased resource consumption. Therefore, appropriate product and material design, taking into account end-of-life scenarios, is essential to limit or even close the loop of economically driven product systems. In this paper, a comparison of virgin and recycled biodegradable (polylactic acid (PLA)) and petroleum-based (polypropylene (PP) & support) filaments for extrusion-based Additive Manufacturing is presented. For the first time, the thermo-mechanical recycling setup contained a service-life simulation, shredding, and extrusion. Specimens and complex geometries with support materials were manufactured with both, virgin and recycled materials. An empirical assessment was executed through mechanical (ISO 527), rheological (ISO 1133), morphological, and dimensional testing. Furthermore, the surface properties of the PLA and PP printed parts were analyzed. In summary, PP parts and parts from its support structure showed, in consideration of all parameters, suitable recyclability with a marginal parameter variance in comparison to the virgin material. The PLA components showed an acceptable decline in the mechanical values but through thermo-mechanical degradation processes, rheological and dimensional properties of the filament dropped decently. This results in significantly identifiable artifacts of the product optics, based on an increase in surface roughness.

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.

Targeted Printing of Cells: Evaluation of ADA-PEG Bioinks for Drop on Demand Approaches.

A novel approach, in the context of bioprinting, is the targeted printing of a defined number of cells at desired positions in predefined locations, which thereby opens up new perspectives for life science engineering. One major challenge in this application is to realize the targeted printing of cells onto a gel substrate with high cell survival rates in advanced bioinks. For this purpose, different alginate-dialdehyde-polyethylene glycol (ADA-PEG) inks with different PEG modifications and chain lengths (1-8 kDa) were characterized to evaluate their application as bioinks for drop on demand (DoD) printing. The biochemical properties of the inks, printing process, NIH/3T3 fibroblast cell distribution within a droplet and shear forces during printing were analyzed. Finally, different hydrogels were evaluated as a printing substrate. By analysing different PEG chain lengths with covalently crosslinked and non-crosslinked ADA-PEG inks, it was shown that the influence of Schiff's bases on the viscosity of the corresponding materials is very low. Furthermore, it was shown that longer polymer chains resulted in less stable hydrogels, leading to fast degradation rates. Several bioinks highly exhibit biocompatibility, while the calculated nozzle shear stress increased from approx. 1.3 and 2.3 kPa. Moreover, we determined the number of cells for printed droplets depending on the initial cell concentration, which is crucially needed for targeted cell printing approaches.

Functional Mesostructured Electrospun Polymer Nonwovens with Supramolecular Nanofibers.

Functional, hierarchically mesostructured nonwovens are of fundamental importance because complex fiber morphologies increase the active surface area and functionality allowing for the effective immobilization of metal nanoparticles. Such complex functional fiber morphologies clearly widen the property profile and enable the preparation of more efficient and selective filter media. Here, the realization of hierarchically mesostructured nonwovens with barbed wire-like morphology is demonstrated by combining electrospun polystyrene fibers, decorated with patchy worm-like micelles, with solution-processed supramolecular short fibers composed of 1,3,5-benzenetricarboxamides with peripheral N,N-diisopropylaminoethyl substituents. The worm-like micelles with a patchy microphase-separated corona are prepared by crystallization-driven self-assembly of a polyethylene based triblock terpolymer and deposited on top of the polystyrene fibers by coaxial electrospinning. The micelles are designed in a way that their patches promote the directed self-assembly of the 1,3,5-benzenetricarboxamide and the fixation of the supramolecular nanofibers on the supporting polystyrene fibers. Functionality of the mesostructured nonwoven is provided by the peripheral N,N-diisopropylaminoethyl substituents of the 1,3,5-benzenetricarboxamide and proven by the effective immobilization of individual palladium nanoparticles on the supramolecular nanofibers. The preparation of hierarchically mesostructured nonwovens and their shown functionality demonstrate that such systems are attractive candidates to be used for example in filtration, selective separation and heterogenous catalysis.

Stability of Charge Distributions in Electret Films on the nm-Scale.

Electret materials find use in various applications, such as microphones or filter media. In recent years, electrets have been used also increasingly on the micrometer scale, for example, in MEMS or for nano-xerography. However, for these applications, it becomes more important to prepare defined charge structures with sub-micrometer features. On the macroscopic level, the technique of isothermal potential decay at elevated temperatures has been developed to study aging effects and charge retention capabilities in electret materials. Here, we extend this technique to the nm-level by means of AFM-based methods, such as contact charging by AFM and the Kelvin probe force microscopy. Defined charge distributions in polyetherimide (PEI) ULTEM 1000 thin-film electrets have been studied for the first time with a high lateral resolution on the nanometer scale. We found a linear correlation between externally applied contact charging potential on the AFM-tip and the resulting relative surface potential on the PEI film. Charge decay at elevated temperatures is independent from the length scale. The same time dependence as for macroscopic, homogenously charged films could be established. We observe a potential decay only at an elevated temperature of 120 °C and no significant lateral charge transport. Thus, we propose a thermally enhanced charge carrier release from surface traps and a subsequent charge migration to the back electrode as the dominant mechanism. This finding is in-line with the observation that potential decay can be reduced also on the nm-level by pre-annealing the film slightly below the glass transition temperature. In contrast to many polymeric or inorganic electrets, no lateral charge migration is observed. Therefore, the charge patterns are preserved for PEI ULTEM 1000 thin-film electrets, which makes it a good candidate as electret for applications in MEMS or similar applications.

All-optical manipulation of singlet exciton transport in individual supramolecular nanostructures by triplet gating.

Directed transport of singlet excitation energy is a key process in natural light-harvesting systems and a desired feature in assemblies of functional organic molecules for organic electronics and nanotechnology applications. However, progress in this direction is hampered by the lack of concepts and model systems. Here we demonstrate an all-optical approach to manipulate singlet exciton transport pathways within supramolecular nanostructures via singlet-triplet annihilation, i.e., to enforce an effective motion of singlet excitons along a predefined direction. For this proof-of-concept, we locally photo-generate a long-lived triplet exciton population and subsequently a singlet exciton population on single bundles of H-type supramolecular nanofibres using two temporally and spatially separated laser pulses. The local triplet exciton population operates as a gate for the singlet exciton transport since singlet-triplet annihilation hinders singlet exciton motion across the triplet population. We visualize this manipulation of singlet exciton transport via the fluorescence signal from the singlet excitons, using a detection-beam scanning approach combined with time-correlated single-photon counting. Our reversible, all-optical manipulation of singlet exciton transport can pave the way to realising new design principles for functional photonic nanodevices.

Crystal Engineering of Supramolecular 1,4-Benzene Bisamides by Side-Chain Modification - Towards Tuneable Anisotropic Morphologies and Surfaces.

Benzene bisamides are promising building blocks for supramolecular nano-objects. Their functionality depends on morphology and surface properties. However, a direct link between surface properties and molecular structure itself is missing for this material class. Here, we investigate this interplay for two series of 1,4-benzene bisamides with symmetric and asymmetric peripheral substitution. We elucidated the crystal structures, determined the nano-object morphologies and derived the wetting behaviour of the preferentially exposed surfaces. The crystal structures were solved by combining single-crystal and powder X-ray diffraction, solid-state NMR spectroscopy and computational modelling. Bulky side groups, here t-butyl groups, serve as a structure-directing motif into a packing pattern, which favours the formation of thin platelets. The use of slim peripheral groups on both sides, in our case linear perfluorinated, alkyl chains, self-assemble the benzene bisamides into a second packing pattern which leads to ribbon-like nano-objects. For both packing types, the preferentially exposed surfaces consist of the ends of the peripheral groups. Asymmetric substitution with bulky and slim groups leads to an ordered alternating arrangement of the groups exposed to the surface. This allows the hydrophobicity of the surfaces to be gradually altered. We thus identified two leitmotifs for molecular packings of benzene bisamides providing the missing link between the molecular structure, the anisotropic morphologies and adjustable surface properties of the supramolecular nano-objects.

Stable Mesoscale Nonwovens of Electrospun Polyacrylonitrile and Interpenetrating Supramolecular 1,3,5-Benzenetrisamide Fibers as Efficient Carriers for Gold Nanoparticles.

The immobilization of metal nanoparticles without agglomeration and leaching within composite nonwovens is often challenging and of great importance, for example, for catalytic applications. In this study, we prepared composite nonwovens based on electrospun polyacrylonitrile (PAN) short fibers and supramolecular terpyridine-functionalized benzene-1,3,5-tricarboxamide (BTA1) nanofibers by a sheet-forming wet-laid process. The formation of an interpenetrating and entangled network of supramolecular BTA1 nanofibers and PAN short fibers results in mechanically stable mesoscale nonwovens. Because of the peripheral terpyridine substituents of the BTA1, nonaggregated gold nanoparticles (AuNPs) could be immobilized efficiently in the composite nonwovens. The functionality of the resulting AuNPs-loaded composite nonwovens was verified by catalytic reduction of 4-nitrophenol to 4-aminophenol as a standard model reaction. The AuNPs-loaded PAN/BTA1 composite nonwovens showed high catalytic activity, reusability, and excellent stability.

Hierarchical Superstructures by Combining Crystallization-Driven and Molecular Self-Assembly.

Combining the unique corona structure of worm-like patchy micelles immobilized on a polymer fiber with the molecular self-assembly of 1,3,5-benzenetricarboxamides (BTAs) leads to hierarchical superstructures with a fir-tree-like morphology. For this purpose, worm-like patchy micelles bearing pendant, functional tertiary amino groups in one of the corona patches were prepared by crystallization-driven self-assembly and immobilized on a supporting polystyrene fiber by coaxial electrospinning. The obtained patchy fibers were then immersed in an aqueous solution of a tertiary amino-functionalized BTA to induce patch-mediated molecular self-assembly to well-defined fir-tree-like superstructures upon solvent evaporation. Interestingly, defined superstructures are obtained only if the pendant functional groups in the surface patches match with the peripheral substituents of the BTA, which is attributed to a local increase in BTA concentration at the polymer fibers' surface.

Kinked Bisamides as Efficient Supramolecular Foam Cell Nucleating Agents for Low-Density Polystyrene Foams with Homogeneous Microcellular Morphology.

Polystyrene foams have become more and more important owing to their lightweight potential and their insulation properties. Progress in this field is expected to be realized by foams featuring a microcellular morphology. However, large-scale processing of low-density foams with a closed-cell structure and volume expansion ratio of larger than 10, exhibiting a homogenous morphology with a mean cell size of approximately 10 µm, remains challenging. Here, we report on a series of 4,4'-diphenylmethane substituted bisamides, which we refer to as kinked bisamides, acting as efficient supramolecular foam cell nucleating agents for polystyrene. Self-assembly experiments from solution showed that these bisamides form supramolecular fibrillary or ribbon-like nanoobjects. These kinked bisamides can be dissolved at elevated temperatures in a large concentration range, forming dispersed nano-objects upon cooling. Batch foaming experiments using 1.0 wt.% of a selected kinked bisamide revealed that the mean cell size can be as low as 3.5 µm. To demonstrate the applicability of kinked bisamides in a high-throughput continuous foam process, we performed foam extrusion. Using 0.5 wt.% of a kinked bisamide yielded polymer foams with a foam density of 71 kg/m3 and a homogeneous microcellular morphology with cell sizes of ≈10 µm, which is two orders of magnitude lower compared to the neat polystyrene reference foam with a comparable foam density.

Reorientational dynamics of highly asymmetric binary non-polymeric mixtures - a dielectric spectroscopy study.

We present an analysis of dielectric spectra measured for a specially designed non-polymeric asymmetric binary glass former characterized by a large difference of the component's Tg (ΔTg = 216 K). We cover the whole additive concentration range from 4% up to 90% (by mass). Two main relaxations α1 and α2 are identified, which are characterized by well separated time scales and are attributed to the dynamics associated with the high-Tg component (α1) and the low-Tg component (α2). Frequency-temperature superposition does not apply. To cope with the extraordinary spectral broadening, we introduce a model consisting of a generalized Cole-Davidson (α1) and a Havriliak-Negami function with a low frequency truncation (α2). Whereas the α1-relaxation reflects essentially homogeneous dynamics and its spectra mainly broaden on the high-frequency flank of the relaxation peak, the α2-relaxation becomes broader on the low-frequency side reflecting pronounced dynamic heterogeneity in a more or less arrested matrix of high-Tg molecules. From the extracted time constants, two glass transition temperatures Tg1 and Tg2 can be derived, showing a non-trivial concentration dependence for Tg2. Supplementary, we find a ß-relaxation. The total relaxation strength Δε strongly deviates from ideal mixing, and therefore care has to be taken interpreting the corresponding Δεαi as representation of molecular populations.

Photoregulated Gradient Structure and Programmable Mechanical Performances of Tough Hydrogels with a Hydrogen-Bond Network.

Gradient materials exist widely in natural living organisms, affording fascinating biological and mechanical properties. However, the synthetic gradient hydrogels are usually mechanically weak or only have relatively simple gradient structures. Here, we report on tough nanocomposite hydrogels with designable gradient network structure and mechanical properties by a facile post-photoregulation strategy. Poly(1-vinylimidazole-co-methacrylic acid) hydrogels containing gold nanorods (AuNRs) are in a glassy state and show typical yielding and forced elastic deformation at room temperature. The gel slightly contracts its volume when the temperature is above the glass-transition temperature that results in a collapse of the chain segments and formation of denser intra- and interchain hydrogen bonds. Consequently, the mechanical properties of the gels are enhanced, when the temperature returns to room temperature. The mechanical performances of hydrogels can also be locally tuned by near-infrared light irradiation due to the photothermal effect of AuNRs. Hydrogels with arbitrary two-dimensional gradients can be facilely developed by site-specific photoirradiation. The treated and untreated regions with different stiffness and yielding stress possess construct behaviors in stretching or twisting deformations. A locally reinforced hydrogel with the kirigami structure becomes notch-insensitive and exhibits improved strength and stretchability because the treated regions ahead the cuts have better resistance to crack advancement. These tough hydrogels with programmable gradient structure and mechanics should find applications as structural elements, biological devices, etc.

Low-Density Polybutylene Terephthalate Foams with Enhanced Compressive Strength via a Reactive-Extrusion Process.

Due to their appealing properties such as high-temperature dimensional stability, chemical resistance, compressive strength and recyclability, new-generation foams based on engineering thermoplastics such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) have been gaining significant attention. Achieving low-density foams without sacrificing the mechanical properties is of vital importance for applications in the field of transportation and construction, where sufficient compressive strength is desired. In contrast to numerous research studies on PET foams, only a limited number of studies on PBT foams and in particular, on extruded PBT foams are known. Here we present a novel route to extruded PBT foams with densities as low as 80 kg/m3 and simultaneously with improved compressive properties manufactured by a tandem reactive-extrusion process. Improved rheological properties and therefore process stability were achieved using two selected 1,3,5-benzene-trisamides (BTA1 and BTA2), which are able to form supramolecular nanofibers in the PBT melt upon cooling. With only 0.08 wt % of BTA1 and 0.02 wt % of BTA2 the normalized compressive strength was increased by 28% and 15%, respectively. This improvement is assigned to the intrinsic reinforcing effect of BTA fibers in the cell walls and struts.

Main and secondary relaxations of non-polymeric high-Tg glass formers as revealed by dielectric spectroscopy.

A series of high-Tg glass formers with Tg values varying between 347 and 390 K and molar masses in the range of 341 and 504 g mol-1 are investigated by dielectric spectroscopy. They are compared to paradigmatic reference systems. Differently polar side groups are attached to a rigid non-polar core unit at different positions. Thereby, the dielectric relaxation strength varies over more than two decades. All the relaxation features typical of molecular glass formers are rediscovered, i.e. stretching of the main (α-) relaxation, a more or less pronounced secondary (ß-) process, and a fragility index quite similar to that of other molecular systems. The position of the polar nitrile side group influences the manifestation of the ß-relaxation. The α-relaxation stretching displays the trend to become less with higher relaxation strength Δεα, confirming recent reports. Typical for a generic ß-process is the increase of its amplitude above Tg, which is found to follow a power-law behaviour as a function of the ratio τα/τß with a universal exponent; yet, its relative amplitude to that of the α-relaxation varies as does the temporal separation of both processes. The mean activation energy of the ß-process as well as the width of the energy distribution gß(E) increases more or less systematically with Tg. The latter is determined from the dielectric spectra subjected to a scaling procedure assuming a thermally activated process. Plotting gß(E) as a function of the reduced energy scale E/Tg, the distributions are centred between 19-35 and their widths differ by a factor 2-3.

Enhancing Long-Range Energy Transport in Supramolecular Architectures by Tailoring Coherence Properties.

Efficient long-range energy transport along supramolecular architectures of functional organic molecules is a key step in nature for converting sunlight into a useful form of energy. Understanding and manipulating these transport processes on a molecular and supramolecular scale is a long-standing goal. However, the realization of a well-defined system that allows for tuning morphology and electronic properties as well as for resolution of transport in space and time is challenging. Here we show how the excited-state energy landscape and thus the coherence characteristics of electronic excitations can be modified by the hierarchical level of H-type supramolecular architectures. We visualize, at room temperature, long-range incoherent transport of delocalized singlet excitons on pico- to nanosecond time scales in single supramolecular nanofibers and bundles of nanofibers. Increasing the degree of coherence, i.e., exciton delocalization, via supramolecular architectures enhances exciton diffusivities up to 1 order of magnitude. In particular, we find that single supramolecular nanofibers exhibit the highest diffusivities reported for H-aggregates so far.

Supramolecular microtubes based on 1,3,5-benzenetricarboxamides prepared by self-assembly upon heating.

A series of three 1,3,5-benzenetricarboxamides with peripheral tertiary N,N-dialkyl-ethylamino substituents with different length of the alkyl groups is reported. In particular, the N1,N3,N5-tris[2-(diethylamino)-ethyl]-1,3,5-benzenetricarboxamide exhibits phase separation followed by self-assembly upon heating from aqueous solution into well-defined supramolecular fiber-like structures in the form of microtubes.

Gradient Photonic Materials Based on One-Dimensional Polymer Photonic Crystals.

In nature, animals such as chameleons are well-known for the complex color patterns of their skin and the ability to adapt and change the color by manipulating sophisticated photonic crystal systems. Artificial gradient photonic materials are inspired by these color patterns. A concept for the preparation of such materials and their function as tunable mechanochromic materials is presented in this work. The system consists of a 1D polymer photonic crystal on a centimeter scale on top of an elastic poly(dimethylsiloxane) substrate with a gradient in stiffness. In the unstrained state, this system reveals a uniform red reflectance over the entire sample. Upon deformation, a gradient in local strain of the substrate is formed and transferred to the photonic crystal. Depending on the magnitude of this local strain, the thickness of the photonic crystal decreases continuously, resulting in a position-dependent blue shift of the reflectance peak and hence the color in a rainbow-like fashion. Using more sophisticated hard-soft-hard-soft-hard gradient elastomers enables the realization of stripe-like reflectance patterns. Thus, this approach allows for the tunable formation of reflectance gradients and complex reflectance patterns. Envisioned applications are in the field of mechanochromic sensors, telemedicine, smart materials, and metamaterials.