A Columnar Liquid-Crystal Phase Formed by Hydrogen-Bonded Perylene Bisimide J-Aggregates

Mesogenic Groups Control the Emitter Orientation in Multi-Resonance TADF Emitter Films

The use of thermally activated delayed fluorescence (TADF) emitters and emitters that show preferential horizontal orientation of their transition dipole moment (TDM) are two emerging strategies to enhance the efficiency of OLEDs. We present the first example of a liquid crystalline multi-resonance TADF (MR-TADF) emitter, DiKTa-LC. The compound possesses a nematic liquid crystalline phase between 80 °C and 110 °C. Importantly, the TDM of the spin-coated film shows preferential horizontal orientation, with an anisotropy factor, a, of 0.28, which is preserved in doped poly(vinylcarbazole) films. Green-emitting (λEL =492 nm) solution-processed OLEDs based on DiKTa-LC showed an EQEmax of 13.6 %. We thus demonstrate for the first time how self-assembly of a liquid crystalline TADF emitter can lead to the so-far elusive control of the orientation of the transition dipole in solution-processed films, which will be of relevance for high-performance solution-processed OLEDs.

Research Progress on Blue-Phase Liquid Crystals for Pattern Replication Applications

Blue-Phase Liquid Crystals (BPLCs) are considered to be excellent 3D photonic crystals and have attracted a great deal of attention due to their great potential for advanced applications in a wide range of fields including self-assembling tunable photonic crystals and fast-response displays. BPLCs exhibit promise in patterned applications due to their sub-millisecond response time, three-dimensional cubic structure, macroscopic optical isotropy and high contrast ratio. The diversity of patterned applications developed based on BPLCs has attracted much attention. This paper focuses on the latest advances in blue-phase (BP) materials, including applications in patterned microscopy, electric field driving, handwriting driving, optical writing and inkjet printing. The paper concludes with future challenges and opportunities for BP materials, providing important insights into the subsequent development of BP.

Theoretical Models, Preparation, Characterization and Applications of Cyanine J-Aggregates: A Minireview

Cyanines are one of the few kinds of molecules whose absorbance and emission can be shifted in a broad spectral range from the ultraviolet to the near infrared. They can easily transform into J-aggregates with narrow absorption and emission peaks, along with a redshift in their spectra. This mini-review presents cyanine dyes and their J-aggregates and discusses their structure and spectral properties that illustrate their specificities. We summarize the theoretical and experimental state of the art on cyanine J-aggregates and their applications, also laying the groundwork for cyanine J-aggregates synthesis and characterization methods.

Photostable NIR-II Pigments from Extended Rylenecarboximides

A series of near-infrared (NIR) organic absorbers, named FNs and FPs, have been obtained with absorption maxima from 870 nm to 1100 nm and thus falling into the attractive second near-infrared region (NIR-II). The synthesis of their extended aromatic cores utilized an initial aryl-amination between 4-aminonaphthalene-1,8-dicarboximide (NMI-NH₂ ) or 9-aminoperylene-3,4-dicarboximide (PMI-NH₂ ) with chloro-substituted 9,10-anthraquinones followed by a novel base-induced cyclodehydrogenation. A NIR-II pigment, compound FPP, was obtained through de-alkylation of a soluble precursor. The synthesis of this photostable pigment is high-yielding and avoids column chromatographic purification which is important for many applications.

Advanced Functional Liquid Crystals

Liquid crystals have been intensively studied as functional materials. Recently, integration of various disciplines has led to new directions in the design of functional liquid-crystalline materials in the fields of energy, water, photonics, actuation, sensing, and biotechnology. Here, recent advances in functional liquid crystals based on polymers, supramolecular complexes, gels, colloids, and inorganic-based hybrids are reviewed, from design strategies to functionalization of these materials and interfaces. New insights into liquid crystals provided by significant progress in advanced measurements and computational simulations, which enhance new design and functionalization of liquid-crystalline materials, are also discussed.

Perylene-Based Chromophore as a Versatile Dye for Light Amplification

One of the challenges for modern optoelectronics is to find versatile, easily adaptable components for novel laser-based technologies. A very attractive perylene-derivative chromophore in different organic matrices for high-performance light amplification is discussed and outlined. Our approach demonstrates the outstandingly compatible laser dye and a viable strategy to provide an effective optical gain for stimulated emission enhancement. Through structural control, we produce simple optical devices embedded in organic matrices, such as poly(methyl methacrylate), nematic liquid crystalline (NLC) mixture, and a hybrid emulsion system (poly(vinyl alcohol) PVA + NLC mesophase). Importantly, we investigate and compare the spectroscopy of differently constructed organic systems in terms of stimulated-emission thresholds and light amplification process efficiency. Moreover, we report the effects of tunability for LC cells by an applied external electric field stimulus. Future directions of laser systems are outlined with an emphasis on the role of the perylene derivative. The studies meet current challenges in the field of modern organic technologies dedicated to various optoelectronic systems, including touch screens, displays, and Li-Fi networks.

The role of chirality and plastic crystallinity in the optical and mechanical properties of chlorosomes

The most efficient light-harvesting antennae found in nature, chlorosomes, are molecular tubular aggregates (TMAs) assembled by pigments without protein scaffolds. Here, we discuss a classification of chlorosomes as a unique tubular plastic crystal and we attribute the robust energy transfer in chlorosomes to this unique nature. To systematically study the role of supramolecular tube chirality by molecular simulation, a role that has remained unresolved, we share a protocol for generating realistic tubes at atomic resolution. We find that both the optical and the mechanical behavior are strongly dependent on chirality. The optical-chirality relation enables a direct interpretation of experimental spectra in terms of overall tube chirality. The mechanical response shows that the overall chirality regulates the hardness of the tube and provides a new characteristic for relating chlorosomes to distinct chirality. Our protocol also applies to other TMA systems and will inspire other systematic studies beyond lattice models.

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Classifies chlorosomes in terms of a tubular plastic crystal phase

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Clarifies the unique strategy of chlorosomes for harvesting and transporting energy

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Presents a protocol for building atom-resolved helical tube structures

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Maps tube chirality directly to measurable optical and mechanical responses

Hierarchical lyotropic liquid crystalline behaviors of supramolecular polymers influenced by alkyl chain branching

The peripheral chain branching in monomeric structures influences the hierarchical supramolecular assembly and lyotropic liquid crystalline properties.

Perylene Bisimide-Based Luminescent Liquid Crystals with Tunable Solid-State Light Emission

Perylene bisimides are among the most studied building blocks for supramolecular assemblies in the fabrication of optoelectronic devices for their exceptional optical and electronic properties; however, developing perylene bisimide-based luminescent liquid crystals remains a challenge for the strong π-stacking tendency of the large planar aromatic core to quench the emission. We here reported a novel strategy to achieve luminescent liquid crystals based on perylene bisimides by introducing a conformation-adjustable core to control the molecular stacking arrangement of planar perylene bisimides in the solid state. The emission wavelength is in the deep-red region with a luminescence efficiency of up to 10%. Fluorescence properties of the liquid crystals can be further regulated by photoisomerization-induced structural evolution from columnar to lamellar mesophases. These luminescent liquid crystals are also able to not only exhibit strong emission at high temperatures but also show attractive thermochromic luminescence tuning behaviors. This work provides a new strategy for the design and development of novel solid-state luminescent materials with potential for various optoelectronic applications.

Exciton Delocalization Counteracts the Energy Gap: A New Pathway toward NIR-Emissive Dyes

Exciton coupling between the transition dipole moments of ordered dyes in supramolecular assemblies, so-called J/H-aggregates, leads to shifted electronic transitions. This can lower the excited state energy, allowing for emission well into the near-infrared regime. However, as we show here, it is not only the excited state energy modifications that J-aggregates can provide. A bay-alkylated quaterrylene was synthesized, which was found to form J-aggregates in 1,1,2,2-tetrachloroethane. A combination of superradiance and a decreased nonradiative relaxation rate made the J-aggregate four times more emissive than the monomeric counterpart. A reduced nonradiative relaxation rate is a nonintuitive consequence following the 180 nm (3300 cm^-1) red-shift of the J-aggregate in comparison to the monomeric absorption. However, the energy gap law, which is commonly invoked to rationalize increased nonradiative relaxation rates with increasing emission wavelength, also contains a reorganization energy term. The reorganization energy is highly suppressed in J-aggregates due to exciton delocalization, and the framework of the energy gap law could therefore reproduce our experimental observations. J-Aggregates can thus circumvent the common belief that lowering the excited state energies results in large nonradiative relaxation rates and are thus a pathway toward highly emissive organic dyes in the NIR regime.

Columnar Mesomorphism of Board-Shaped Perylene, Diketopyrrolopyrrole, Isoindigo, Indigo, and Quinoxalino-Phenanthrophenazine Dyes

The properties of organic dyes depend as much on their intermolecular interactions as on their molecular structure. While it is generally predictable what supramolecular structure would be ideal for a specific application, the generation of specific supramolecular structures by molecular design and suitable processing methods remains to be a challenge. A versatile approach to different supramolecular structures has been the application of mesomorphism in conjunction with alignment techniques and self-assembly at interfaces. Reviewed here is the columnar mesomorphism of board-shaped dyes perylene, indigo, isoindigo, diketopyrrolopyrrole, and quinoxalinophenanthrophenazine. They generate a larger number of different supramolecular structures than conventional disc-shaped (discotic) mesogens because of their non-circular shape and directional intermolecular interactions. The mesomorphism of all but the perylene derivatives is systematically and comprehensively covered for the first time.

Polymorphism in Squaraine Dye Aggregates by Self-Assembly Pathway Differentiation: Panchromatic Tubular Dye Nanorods versus J-Aggregate Nanosheets

A bis(squaraine) dye equipped with alkyl and oligoethyleneglycol chains was synthesized by connecting two dicyanomethylene substituted squaraine dyes with a phenylene spacer unit. The aggregation behavior of this bis(squaraine) was investigated in non-polar toluene/tetrachloroethane (98:2) solvent mixture, which revealed competing cooperative self-assembly pathways into two supramolecular polymorphs with entirely different packing structures and UV/Vis/NIR absorption properties. The self-assembly pathway can be controlled by the cooling rate from a heated solution of the monomers. For both polymorphs, quasi-equilibrium conditions between monomers and the respective aggregates can be established to derive thermodynamic parameters and insights into the self-assembly mechanisms. AFM measurements revealed a nanosheet structure with a height of 2 nm for the thermodynamically more stable polymorph and a tubular nanorod structure with a helical pitch of 13 nm and a diameter of 5 nm for the kinetically favored polymorph. Together with wide angle X-ray scattering measurements, packing models were derived: the thermodynamic polymorph consists of brick-work type nanosheets that exhibit red-shifted absorption bands as typical for J-aggregates, while the nanorod polymorph consists of eight supramolecular polymer strands of the bis(squaraine) intertwined to form a chimney-type tubular structure. The absorption of this aggregate covers a large spectral range from 550 to 875 nm, which cannot be rationalized by the conventional exciton theory. By applying the Essential States Model and considering intermolecular charge transfer, the aggregate spectrum was adequately reproduced, revealing that the broad absorption spectrum is due to pronounced donor-acceptor overlap within the bis(squaraine) nanorods. The latter is also responsible for the pronounced bathochromic shift observed for the nanosheet structure as a result of the slip-stacked arranged squaraine chromophores.

Supramolecularly Engineered J-Aggregates Based on Perylene Bisimide Dyes

The discovery of the self-assembly of cyanine dyes into J-aggregates had a major impact on the development of dye chemistry due to the emergence of new useful properties in the aggregated state. The unique optical features of these J-aggregates are narrowed, bathochromically shifted absorption bands with almost resonant fluorescence with an increased radiative rate that results from the coherently coupled molecular transition dipoles arranged in a slip-stacked fashion. Because of their desirable properties, J-aggregates gained popularity in the field of functional materials and enabled the efficient photosensitization of silver halide grains in color photography. However, despite a good theoretical understanding of structure-property relationships by the molecular exciton model, further examples of J-aggregates remained scarce for a long time as supramolecular designs to guide the formation of dye aggregates into the required slip-stacked arrangement were lacking.Drawing inspiration from the bacteriochlorophyll c self-organization found in the chlorosomal light-harvesting antennas of green sulfur bacteria, we envisioned the use of nature's supramolecular blueprint to develop J-aggregates of perylene bisimides (PBIs). This class of materials is applied in high-performance color pigments and as n-type organic semiconductors in transistors and solar cells. Combining outstanding photochemical and thermal stability, high tinctorial strength and excellent fluorescence, PBIs are therefore an ideal model system for the preparation of J-aggregates with a wide range of potential applications.In this Account, we elucidate how a combination of steric constraints and hydrogen bonding receptor sites can guide the self-assembly of PBI dyes into slip-stacked packing motifs with J-type exciton coupling. We will discuss the supramolecular polymerization of multiple hydrogen-bonded PBI strands in organic and aqueous media and how minor structural modifications in monomeric PBI molecules can be used to obtain near-infrared absorbing J-aggregates, organogels, or thermoresponsive hydrogels. Pushing the boundaries of self-assembly into the bulk, engineering of the substituents' steric requirements by a dendron-wedge approach afforded adjustable numbers of helical strands of PBI J-aggregates in the columnar liquid-crystalline state and the preparation of lamellar phases. To fully explore their potential, we have studied PBI J-aggregates in collaborative work with spectroscopists, physicists, and theoreticians. In this way, exciton migration over distances of up to 180 nm was shown, and insights into the influence of static disorder on the transport of excitation energy in PBI J-aggregates were derived. Furthermore, the application of PBI J-aggregates as functional materials was demonstrated in photonic microcavities, thin-film transistors, and organic solar cells.

Strong electron affinity PDI supramolecules form anion radicals for the degradation of organic pollutants via direct electrophilic attack

Strong electron affinity PDI supramolecules degrade organic pollutants efficiently through directly electrophilic attack.

Unfolding multi-stranded perylene bisimide LC columns - a mesogen design for efficient nanoscale multilayer self-assembly

A mesogen tethered, twofold bay-substituted perylene bisimide (PBI) is found to generate a columnar phase, which unfolds and gradually transforms to a completely nanosegregated multilayer columnar-lamellar liquid crystal. The structure is based on the formation of bundles of H-bonded PBI strands in the central layer. This design opens the way to new complex multifunctional materials.

Exciton Migration in Multistranded Perylene Bisimide J-Aggregates

Exciton migration in self-assembled supramolecular ensembles of dye molecules is controlled by the electronic coupling between adjacent sites, the delocalization of the excitation and thereby by the packing arrangement. Here, we put emphasis on the packing structure and analyze the exciton migration in two perylene bisimide-based J-aggregates composed of almost identical molecular building blocks but forming double-strand versus quadruple-strand slip-stacked supramolecular architectures. Analyzing ultrafast transient absorption spectra in dependence on the exciton density by a kinetic model for exciton-exciton annihilation based on incoherent transfer demonstrates that the migration is quasi one-dimensional. The migration distance is enhanced by a beneficial geometrical structure. We find a factor of more than two between the diffusion lengths of 188 and 77 nm for the double- and quadruple-stranded system. The supramolecular design efficiently influences the exciton mobility and minor structural changes have a pronounced influence on functional properties of dye aggregates.

Self-Sorting Supramolecular Polymerization: Helical and Lamellar Aggregates of Tetra-Bay-Acyloxy Perylene Bisimide

A new perylene bisimide (PBI), with a fluorescence quantum yield up to unity, self‐assembles into two polymorphic supramolecular polymers. This PBI bears four solubilizing acyloxy substituents at the bay positions and is unsubstituted at the imide position, thereby allowing hydrogen‐bond‐directed self‐assembly in nonpolar solvents. The formation of the polymorphs is controlled by the cooling rate of hot monomer solutions. They show distinctive absorption profiles and morphologies and can be isolated in different polymorphic liquid‐crystalline states. The interchromophoric arrangement causing the spectral features was elucidated, revealing the formation of columnar and lamellar phases, which are formed by either homo‐ or heterochiral self‐assembly, respectively, of the atropoenantiomeric PBIs. Kinetic studies reveal a narcissistic self‐sorting process upon fast cooling, and that the transformation into the heterochiral (racemic) sheetlike self‐assemblies proceeds by dissociation via the monomeric state.

Self-assembled star-shaped aza-BODIPY mesogen affords white-light emission

A star-shaped aza-BODIPY mesogen exhibits LC, gel, WLE and chemosensor properties.

Sequence-Defined Dendrons Dictate Supramolecular Cogwheel Assembly of Dendronized Perylene Bisimides

A dendronized perylene bisimide (PBI) that self-organizes into hexagonal arrays of supramolecular double helices with identical single-crystal-like order that disregards chirality was recently reported. A cogwheel model of self-assembly that explains this process was proposed. Accessing the highly ordered cogwheel phase required very slow heating and cooling or extended periods of annealing. Analogous PBIs with linear alkyl chains did not exhibit the cogwheel assembly. Here a library of sequence-defined dendrons containing all possible compositions of linear and racemic alkyl chains was employed to construct self-assembling PBIs. Thermal and structural analysis of their assemblies by differential scanning calorimetry (DSC) and fiber X-ray diffraction (XRD) revealed that the incorporation of n-alkyl chains accelerates the formation of the high order cogwheel phase, rendering the previously invisible phase accessible under standard heating and cooling rates. Small changes to the primary structure, as constitutional isomerism, result in significant changes to macroscopic properties such as melting of the periodic array. This study demonstrated how changes to the sequence-defined primary structure, including the relocation of methyl groups between two constitutional isomers, dictate tertiary and quaternary structure in hierarchical assemblies. This led to the discovery of a sequence that self-organizes the cogwheel assembly much faster than even the corresponding homochiral compounds and demonstrated that defined-sequence, which has long been recognized as a determinant for the complex structure of biomacromolecules including proteins and nucleic acids, plays the same role also in supramolecular synthetic systems.

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.

Photoconductive Core-Shell Liquid-Crystals of a Perylene Bisimide J-Aggregate Donor-Acceptor Dyad

A novel core-shell structured columnar liquid crystal composed of a donor-acceptor dyad of tetraphenoxy perylene bisimide (PBI), decorated with four bithiophene units on the periphery, was synthesized. This molecule self-assembles in solution into helical J-aggregates guided by π-π interactions and hydrogen bonds which organize into a liquid-crystalline (LC) columnar hexagonal domain in the solid state. Donor and acceptor moieties exhibit contrasting exciton coupling behavior with the PBIs' (J-type) transition dipole moment parallel and the bithiophene side arms' (H-type) perpendicular to the columnar axis. The dyad shows efficient energy and electron transfer in solution as well as in the solid state. The synergy of photoinduced electron transfer (PET) and charge transport along the narcissistically self-assembled core-shell structure enables the implementation of the dye in two-contact photoconductivity devices giving rise to a 20-fold increased photoresponse compared to a reference dye without bithiophene donor moieties.

Synthesis and Photocontrolled Supramolecular Self-Assembly of Azobenzene-Functionalized Perylene Bisimide Derivatives

Azobenzene (Azo) units were successfully introduced into perylene bisimide (PBI) structures in order to realize the photocontrolling of the morphology of the supramolecular assembly of PBI by a photoisomerization process. A total of three Azo-functionalized perylene bisimide derivatives (PBI1, PBI2, and PBI3) with different alkyl chain lengths were designed and synthesized by imidization of 3,4,9,10-perylene tetracarboxylic dianhydride with the corresponding amines. The structures of these compounds were characterized by proton nuclear magnetic resonance (¹H NMR) and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). The photoisomerization behaviors of Azo units in PBIs were investigated using ultraviolet-visible (UV-VIS) absorption spectroscopy, which were obviously effected by solvents and the alkyl chain length. Furthermore, the photoisomerization of Azo units has the obviously regulatory effect on the morphology of supramolecular assembly of PBIs, especially for the medium-length alkyl chain-linked Azo-functionalized PBI derivative (PBI2). This research realized the photocontrolling of the morphology of the supramolecular assembly of PBI derivatives by photoisomerization of Azo units.

Impact of charge switching stimuli on supramolecular perylene monoimide assemblies

The development of stimuli-responsive amphiphilic supramolecular nanostructures is an attractive target for systems based on light-absorbing chromophores that can function as photosensitizers in water. We report here on a water soluble supramolecular carboxylated perylene monoimide system in which charge can be switched significantly by a change in pH. This was accomplished by substituting the perylene core with an ionizable hydroxyl group. In acidic environments, crystalline supramolecular nanoribbons with dimensions on the order of 500 × 50 × 2 nm form readily, while in basic solution the additional electrostatic repulsion of the ionized hydroxyl reduces assemblies to very small dimensions on the order of only several nanometers. The HOMO/LUMO levels were also found to be sensitive to pH; in acidic media the HOMO/LUMO levels are -5.65 and -3.70 eV respectively versus vacuum, whereas is in basic conditions they are -4.90 and -3.33 eV, respectively. Utilizing the assemblies as photosensitizers in photocatalytic production of hydrogen with [Mo3S13]2- as a catalyst at a pH of 4, H2 was generated with a turnover number of 125 after 18 hours. Charge switching the assemblies at a pH of 9-10 and using an iron porphyrin catalyst, protons could again be reduced to hydrogen and CO2 was reduced to CO with a turnover number of 30. The system investigated offers an example of dynamic photosensitizing assemblies that can drive reactions in both acidic and basic media.

Perylene-Based Liquid Crystals as Materials for Organic Electronics Applications

Columnar phases formed by the stacking of disclike molecules with an intimate π-π overlap forms a 1D pathway for the anisotropic charge migration along the columns. Columnar phases have great potential in organic electronic devices to be utilized as active semiconducting layers in comparison to organic single crystals or amorphous polymers in terms of processability, ease of handling, and high charge carrier mobility. Intelligent molecular engineering of perylene and its derivatives provided access to tune the physical properties and self-assembly behavior. The columnar phase formed by perylene derivatives has great potential in the fabrication of organic electronic devices. There are several positions on the perylene molecule, which can be functionalized to tune its self-assembly, as well as optoelectronic properties. Thus, many liquid-crystalline molecules stabilizing the columnar phase, which are based on perylene tetraesters, perylene diester imides, and perylene bisimides, have been synthesized over the years. Their longitudinal and laterally extended derivatives, bay-substituted derivatives exhibiting a columnar phase, are reported. In addition, several liquid-crystalline oligomers and polymers based on perylene derivatives were also reported. All such modifications provide an option to tune the energy levels of frontier molecular orbitals with respect to the work function of the electrodes in devices and also the processability of such materials. In this feature article, we attempt to provide an overview of the molecular design developed to tune the applicable properties and self-assembly of perylene derivatives as well as recent developments related to their application in the fabrication of organic solar cells, organic light-emitting diodes, and organic field-effect transistors.

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.

Anisotropic microfibres of a liquid-crystalline diketopyrrolopyrrole by self-assembly-assisted electrospinning

Electrospinning is a well-established technique for the preparation of nanofibres from polymer solution or melt, however it is rarely applied for small molecules. Here we report a unique example of a liquid-crystalline (LC) diketopyrrolopyrrole (DPP) dye that was successfully used for electrospinning. Micrometric fibres with anisotropic alignment of DPP dye were produced by this process as shown by polarized optical microscopy and selected area electron diffraction. This newly designed DPP dye self-assembles in solution by hydrogen bonding and π-π-interactions and forms columnar LC phases in the bulk. X-ray scattering and polarized FT-IR studies in the LC state revealed a hierarchical arrangement of DPP molecules into columnar structures. The successful preparation of anisotropic microfibers by electrospinning is attributed to the hydrogen bond-directed supramolecular polymerization of the new DPP dye in solution and its LC properties.

The effect of conformational isomerism on the optical properties of bis(8-oxyquinolato) diboron complexes with a 2,2'-biphenyl backbone

A fluorescent bis(8-oxyquinolato) diborinic complex with a central 2,2'-biphenyl backbone 1 and its octafluoro analogue 2 were synthesized to study the optical-structural relationship of sterically encumbered molecules featuring close intramolecular π-stacking interactions involving chromophore units. The crystal structure of 1 revealed a unique π-stacked arrangement of two pendant phenyl groups and two 8-oxyquinolato ligands (Q) located in the inner part of the complex. Unlike 1, the closely related complex 2 features conformational isomerism, and two major forms, namely 2-syn and 2-anti, are observed in solution to a varying extent depending on the solvent polarity. Form 2-syn, a geometrical analogue of 1, is preferable in polar solutions, whereas its rotational isomer 2-anti featuring π-stacking interactions between the terminal phenyl group and Q ligand dominates in benzene and chloromethane solutions. The observed conformational equilibria strongly affect the optical properties of the system, specifically leading to a significant increase of the quantum yield of emission (from 22% in MeCN to 38% in benzene) accompanied by a bathochromic shift (Δλ = 10 nm) of absorption and hypsochromic shifts (Δλ = -8 nm) of emission spectra with decreasing solvent polarity. This effect was ascribed to the variation in frontier orbital distributions.

Fluorophobic Effect Promoting Lamellar Self-Assembly of Donor Acceptor Dyes

To combine liquid crystalline and linear optical properties in the same molecule, the fluorophobic effect was probed for the first time in donor acceptor dyes. Thus, a series of mono-, bi-, and tricyclic donor acceptor dyes with 1H,1H-perfluorinated alkyl chains of different lengths as donor units and nitrile, malononitrile or barbiturate as acceptor units was synthesized in 5 steps and 1.4-6.6 % overall yield. UV/Vis and fluorescence spectroscopy, cyclic voltammetry and DFT calculations revealed that absorption and emission maxima, Stokes shifts and LUMO energies were mainly governed by the chromophore size and acceptor strengths. The perfluorinated chain was electronically almost decoupled from the remaining chromophore and induced only slight changes of the absorption maxima as compared to the alkyl substituted counterparts. However, in contrast to the non-mesomorphic alkyl donor-substituted derivatives, the perfluorinated donors resulted in self-assembly into partially interdigitated SmA bilayers according to differential scanning calorimetry (DSC), polarizing optical microscopy (POM), X-ray diffraction (WAXS, SAXS) studies and electron density profile calculations.

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.

Direct observation of exciton-exciton interactions

Natural light harvesting as well as optoelectronic and photovoltaic devices depend on efficient transport of energy following photoexcitation. Using common spectroscopic methods, however, it is challenging to discriminate one-exciton dynamics from multi-exciton interactions that arise when more than one excitation is present in the system. Here we introduce a coherent two-dimensional spectroscopic method that provides a signal only in case that the presence of one exciton influences the behavior of another one. Exemplarily, we monitor exciton diffusion by annihilation in a perylene bisimide-based J-aggregate. We determine quantitatively the exciton diffusion constant from exciton–exciton-interaction 2D spectra and reconstruct the annihilation-free dynamics for large pump powers. The latter enables for ultrafast spectroscopy at much higher intensities than conventionally possible and thus improves signal-to-noise ratios for multichromophore systems; the former recovers spatio–temporal dynamics for a broad range of phenomena in which exciton interactions are present.

Some photo-physical processes in multichromophore systems might get triggered only if two excitations are present. Here, the authors introduce exciton–exciton-interaction 2D spectroscopy, which is a non-linear optical method that can selectively track the time evolution of such effects.

A water-soluble, bay-functionalized perylenediimide derivative - correlating aggregation and excited state dynamics

The aggregation and the photophysics of a water soluble perylenediimide (PDI) derivative that features two bromine substituents in the bay positions has been probed. Non-fluorescent aggregates were found to be present at concentrations of 1.0 × 10^-5 M. In situ small-angle X-ray scattering (SAXS) measurements and complementary molecular modeling showed the presence of PDI aggregates. In their singlet excited states, the PDI aggregates are characterized by distinct transient fingerprints and rapid deactivation, as revealed by pump-probe experiments on the femto-, pico-, nano-, and microsecond timescales. The product of this deactivation is a PDI triplet excited state. The efficiency of the triplet formation depends on the concentration, and hence on the degree of aggregation. Notably, for PDI concentrations in the range of the critical micelle concentration, the efficiency of intersystem crossing is close to zero. In short, we have demonstrated, for the first time, aggregation-induced formation of triplet excited states for PDI derivatives.

The design of rigid cyclic tripyrrins: the importance of intermolecular interactions on aggregation and luminescence

Cyclic tripyrrin “locked” by a bridging benzyl moiety: enhancing the molecular rigidity and tuning aggregation and fluorescence via intermolecular halogen interactions.

Synthesis of bay-triaminosubstituted perylenediimides

A mild fluoride-mediated reaction for the synthesis of 1,6,7-triaminoperylenediimides.

Guanine-oligothiophene conjugates: liquid-crystalline properties, photoconductivities and ion-responsive emission of their nanoscale assemblies

Liquid-crystalline conjugated guanine–oligothiophene derivatives show electron or ambipolar carrier transport properties and ion-induced phase transition accompanied by a luminescence colour change.

We here report the supramolecular self-assembly of hydrogen-bonded motifs for the development of nanostructured materials that exhibit dynamic functions such as stimuli-responsive properties and molecular recognition behaviour. We have designed and synthesised new thermotropic bicontinuous and columnar liquid-crystalline (LC) guanine–oligothiophene conjugates tethered with lipophilic chains, which exhibit ionic, electronic and photoluminescence properties. Their potassium salt complexes self-assemble into thermotropic columnar LC phases. Time-of-flight photoconductivity measurements have revealed that the guanine–oligothiophene conjugates in the LC states possess charge transport abilities with either electron or ambipolar mobility values of 10^–4 to 10^–3 cm² V^–1 s^–1. Furthermore, we have found that the complexation of potassium ions with the guanine motif could lead not only to structural change and thermal stabilization of the LC phases but also to a photoluminescence colour change in the solid states. The strategy presented in this work could lead to the design of new functional LC materials that could potentially be applicable as sensors and electronic devices.