Growth of Merocyanine Dye J-Aggregate Nanosheets by Living Supramolecular Polymerization.

J-aggregates are highly desired dye aggregates but so far there has been no general concept how to accomplish the required slip-stacked packing arrangement for dipolar merocyanine (MC) dyes whose aggregation commonly affords one-dimensional aggregates composed of antiparallel, co-facially stacked MCs with H-type coupling. Herein we describe a strategy for MC J-aggregates based on our results for an amphiphilic MC dye bearing alkyl and oligo(ethylene glycol) side chains. In an aqueous solvent mixture, we observe the formation of two supramolecular polymorphs for this MC dye, a metastable off-pathway nanoparticle showing H-type coupling and a thermodynamically favored nanosheet showing J-type coupling. Detailed studies concerning the self-assembly mechanism by UV-Vis spectroscopy and the packing structure by atomic force microscopy and wide-angle X-ray scattering show how the packing arrangement of such amphiphilic MC dyes can afford slip-stacked two-dimensional nanosheets whose macrodipole is compensated by the formation of a bilayer structure. As an additional feature we demonstrate how the size of the nanosheets can be controlled by seeded living supramolecular polymerization.

Enantiopure J-Aggregate of Quaterrylene Bisimides for Strong Chiroptical NIR-Response.

Chiral polycyclic aromatic hydrocarbons can be tailored for next-generation photonic materials by carefully designing their molecular as well as supramolecular architectures. Hence, excitonic coupling can boost the chiroptical response in extended aggregates but is still challenging to achieve by pure self-assembly. Whereas most reports on these potential materials cover the UV and visible spectral range, systems in the near infrared (NIR) are underdeveloped. We report a new quaterrylene bisimide derivative with a conformationally stable twisted π-backbone enabled by the sterical congestion of a fourfold bay-arylation. Rendering the π-subplanes accessible by small imide substituents allows for a slip-stacked chiral arrangement by kinetic self-assembly in low polarity solvents. The well dispersed solid-state aggregate reveals a sharp optical signature of strong J-type excitonic coupling in both absorption (897 nm) and emission (912 nm) far in the NIR region and reaches absorption dissymmetry factors up to 1.1 × 10-2. The structural elucidation was achieved by atomic force microscopy and single-crystal X-ray analysis which we combined to derive a structural model of a fourfold stranded enantiopure superhelix. We could deduce that the role of phenyl substituents is not only granting stable axial chirality but also guiding the chromophore into a chiral supramolecular arrangement needed for strong excitonic chirality.

Controlling the Supramolecular Polymerization of Squaraine Dyes by a Molecular Chaperone Analogue.

Molecular chaperones are proteins that assist in the (un)folding and (dis)assembly of other macromolecular structures toward their biologically functional state in a non-covalent manner. Transferring this concept from nature to artificial self-assembly processes, here, we show a new strategy to control supramolecular polymerization via a chaperone-like two-component system. A new kinetic trapping method was developed that enables efficient retardation of the spontaneous self-assembly of a squaraine dye monomer. The suppression of supramolecular polymerization could be regulated with a cofactor, which precisely initiates self-assembly. The presented system was investigated and characterized by ultraviolet-visible, Fourier transform infrared, and nuclear magnetic resonance spectroscopy, atomic force microscopy, isothermal titration calorimetry, and single-crystal X-ray diffraction. With these results, living supramolecular polymerization and block copolymer fabrication could be realized, demonstrating a new possibility for effective control over supramolecular polymerization processes.

Self-assembled Ru(bda) Coordination Oligomers as Efficient Catalysts for Visible Light-Driven Water Oxidation in Pure Water.

Water-soluble multinuclear complexes based on ruthenium 2,2'-bipyridine-6,6'-dicarboxylate (bda) and ditopic bipyridine linker units are investigated in three-component visible light-driven water oxidation catalysis. Systematic studies revealed a strong enhancement of the catalytic efficiency in the absence of organic co-solvents and with increasing oligomer length. In-depth kinetic and morphological investigations suggest that the enhanced performance is induced by the self-assembly of linear Ru(bda) oligomers into aggregated superstructures. The obtained turnover frequencies (up to 14.9 s-1 ) and turnover numbers (more than 1000) per ruthenium center are the highest reported so far for Ru(bda)-based photocatalytic water oxidation systems.

A Covalent Organic Framework for Cooperative Water Oxidation.

The future of water-derived hydrogen as the "sustainable energy source" straightaway bets on the success of the sluggish oxygen-generating half-reaction. The endeavor to emulate the natural photosystem II for efficient water oxidation has been extended across the spectrum of organic and inorganic combinations. However, the achievement has so far been restricted to homogeneous catalysts rather than their pristine heterogeneous forms. The poor structural understanding and control over the mechanistic pathway often impede the overall development. Herein, we have synthesized a highly crystalline covalent organic framework (COF) for chemical and photochemical water oxidation. The interpenetrated structure assures the catalyst stability, as the catalyst's performance remains unaltered after several cycles. This COF exhibits the highest ever accomplished catalytic activity for such an organometallic crystalline solid-state material where the rate of oxygen evolution is as high as ∼26,000 µmol L-1 s-1 (second-order rate constant k ≈ 1650 µmol L s-1 g-2). The catalyst also proves its exceptional activity (k ≈ 1600 µmol L s-1 g-2) during light-driven water oxidation under very dilute conditions. The cooperative interaction between metal centers in the crystalline network offers 20-30-fold superior activity during chemical as well as photocatalytic water oxidation as compared to its amorphous polymeric counterpart.

Aggregation-Induced Dual Phosphorescence from (o-Bromophenyl)-Bis(2,6-Dimethylphenyl)Borane at Room Temperature.

Designing highly efficient purely organic phosphors at room temperature remains a challenge because of fast non-radiative processes and slow intersystem crossing (ISC) rates. The majority of them emit only single component phosphorescence. Herein, we have prepared 3 isomers (o, m, p-bromophenyl)-bis(2,6-dimethylphenyl)boranes. Among the 3 isomers (o-, m- and p-BrTAB) synthesized, the ortho-one is the only one which shows dual phosphorescence, with a short lifetime of 0.8 ms and a long lifetime of 234 ms in the crystalline state at room temperature. Based on theoretical calculations and crystal structure analysis of o-BrTAB, the short lifetime component is ascribed to the T1 M state of the monomer which emits the higher energy phosphorescence. The long-lived, lower energy phosphorescence emission is attributed to the T1 A state of an aggregate, with multiple intermolecular interactions existing in crystalline o-BrTAB inhibiting nonradiative decay and stabilizing the triplet states efficiently.

Surface-Promoted Evolution of Ru-bda Coordination Oligomers Boosts the Efficiency of Water Oxidation Molecular Anodes.

A new Ru oligomer of formula {[RuII(bda-κ-N2O2)(4,4'-bpy)]10(4,4'-bpy)}, 10 (bda is [2,2'-bipyridine]-6,6'-dicarboxylate and 4,4'-bpy is 4,4'-bipyridine), was synthesized and thoroughly characterized with spectroscopic, X-ray, and electrochemical techniques. This oligomer exhibits strong affinity for graphitic materials through CH-π interactions and thus easily anchors on multiwalled carbon nanotubes (CNT), generating the molecular hybrid material 10@CNT. The latter acts as a water oxidation catalyst and converts to a new species, 10'(H2O)2@CNT, during the electrochemical oxygen evolution process involving solvation and ligand reorganization facilitated by the interactions of molecular Ru catalyst and the surface. This heterogeneous system has been shown to be a powerful and robust molecular hybrid anode for electrocatalytic water oxidation into molecular oxygen, achieving current densities in the range of 200 mA/cm2 at pH 7 under an applied potential of 1.45 V vs NHE. The remarkable long-term stability of this hybrid material during turnover is rationalized based on the supramolecular interaction of the catalyst with the graphitic surface.

An Efficient Narrowband Near-Infrared at 1040 nm Organic Photodetector Realized by Intermolecular Charge Transfer Mediated Coupling Based on a Squaraine Dye.

A highly sensitive short-wave infrared (SWIR, λ > 1000 nm) organic photodiode (OPD) is described based on a well-organized nanocrystalline bulk-heterojunction (BHJ) active layer composed of a dicyanovinyl-functionalized squaraine dye (SQ-H) donor material in combination with PC61 BM. Through thermal annealing, dipolar SQ-H chromophores self-assemble in a nanoscale structure with intermolecular charge transfer mediated coupling, resulting in a redshifted and narrow absorption band at 1040 nm as well as enhanced charge carrier mobility. The optimized OPD exhibits an external quantum efficiency (EQE) of 12.3% and a full-width at half-maximum of only 85 nm (815 cm-1 ) at 1050 nm under 0 V, which is the first efficient SWIR OPD based on J-type aggregates. Photoplethysmography application for heart-rate monitoring is successfully demonstrated on flexible substrates without applying reverse bias, indicating the potential of OPDs based on short-range coupled dye aggregates for low-power operating wearable applications.

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.

Controlling Topography and Crystallinity of Melt Electrowritten Poly(ɛ-Caprolactone) Fibers.

Melt electrowriting (MEW) is an aspiring 3D printing technology with an unprecedented resolution among fiber-based printing technologies. It offers the ability to direct-write predefined designs utilizing a jet of molten polymer to fabricate constructs composed of fibers with diameters of only a few micrometers. These dimensions enable unique construct properties. Poly(ɛ-caprolactone) (PCL), a semicrystalline polymer mainly used for biomedical and life science applications, is the most prominent material for MEW and exhibits excellent printing properties. Despite the wealth of melt electrowritten constructs that have been fabricated by MEW, a detailed investigation, especially regarding fiber analysis on a macro- and microlevel is still lacking. Hence, this study systematically examines the influence of process parameters such as spinneret diameter, feeding pressure, and collector velocity on the diameter and particularly the topography of PCL fibers and sheds light on how these parameters affect the mechanical properties and crystallinity. A correlation between the mechanical properties, crystallite size, and roughness of the deposited fiber, depending on the collector velocity and applied feeding pressure, is revealed. These findings are used to print constructs composed of fibers with different microtopography without affecting the fiber diameter and thus the macroscopic assembly of the printed constructs.

Control of self-assembly pathways toward conglomerate and racemic supramolecular polymers.

Homo- and heterochiral aggregation during crystallization of organic molecules has significance both for fundamental questions related to the origin of life as well as for the separation of homochiral compounds from their racemates in industrial processes. Herein, we analyse these phenomena at the lowest level of hierarchy - that is the self-assembly of a racemic mixture of (R,R)- and (S,S)-PBI into 1D supramolecular polymers. By a combination of UV/vis and NMR spectroscopy as well as atomic force microscopy, we demonstrate that homochiral aggregation of the racemic mixture leads to the formation of two types of supramolecular conglomerates under kinetic control, while under thermodynamic control heterochiral aggregation is preferred, affording a racemic supramolecular polymer. FT-IR spectroscopy and quantum-chemical calculations reveal unique packing arrangements and hydrogen-bonding patterns within these supramolecular polymers. Time-, concentration- and temperature-dependent UV/vis experiments provide further insights into the kinetic and thermodynamic control of the conglomerate and racemic supramolecular polymer formation.

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.

Persistent Room Temperature Phosphorescence from Triarylboranes: A Combined Experimental and Theoretical Study.

Achieving highly efficient phosphorescence in purely organic luminophors at room temperature remains a major challenge due to slow intersystem crossing (ISC) rates in combination with effective non-radiative processes in those systems. Most room temperature phosphorescent (RTP) organic materials have O- or N-lone pairs leading to low lying (n, π*) and (π, π*) excited states which accelerate kisc through El-Sayed's rule. Herein, we report the first persistent RTP with lifetimes up to 0.5 s from simple triarylboranes which have no lone pairs. RTP is only observed in the crystalline state and in highly doped PMMA films which are indicative of aggregation induced emission (AIE). Detailed crystal structure analysis suggested that intermolecular interactions are important for efficient RTP. Furthermore, photophysical studies of the isolated molecules in a frozen glass, in combination with DFT/MRCI calculations, show that (σ, B p)→(π, B p) transitions accelerate the ISC process. This work provides a new approach for the design of RTP materials without (n, π*) transitions.

Tuning Aqueous Supramolecular Polymerization by an Acid-Responsive Conformational Switch.

Besides their widespread use in coordination chemistry, 2,2'-bipyridines are known for their ability to undergo cis-trans conformational changes in response to metal ions and acids, which has been primarily investigated at the molecular level. However, the exploitation of such conformational switching in self-assembly has remained unexplored. In this work, the use of 2,2'-bipyridines as acid-responsive conformational switches to tune supramolecular polymerization processes has been demonstrated. To achieve this goal, we have designed a bipyridine-based linear bolaamphiphile, 1, that forms ordered supramolecular polymers in aqueous media through cooperative aromatic and hydrophobic interactions. Interestingly, addition of acid (TFA) induces the monoprotonation of the 2,2'-bipyridine moiety, leading to a switch in the molecular conformation from a linear (trans) to a V-shaped (cis) state. This increase in molecular distortion along with electrostatic repulsions of the positively charged bipyridine-H+ units attenuate the aggregation tendency and induce a transformation from long fibers to shorter thinner fibers. Our findings may contribute to opening up new directions in molecular switches and stimuli-responsive supramolecular materials.

Supramolecular Block Copolymers by Seeded Living Polymerization of Perylene Bisimides.

Living covalent polymerization has been a subject of intense research for many decades and has culminated in the synthesis of a large variety of block copolymers (BCPs) with structural and functional diversity. In contrast, the research on supramolecular BCPs is still in its infancy and their generation by living processes remains a challenge. Here we report the formation of supramolecular block copolymers by two-component seeded living polymerization of properly designed perylene bisimides (PBIs) under precise kinetic control. Our detailed studies on thermodynamically and kinetically controlled supramolecular polymerization of three investigated PBIs, which contain hydrogen-bonding amide side groups in imide position and chlorine, methoxy, or methylthio substituents in 1,7 bay-positions, revealed that these PBIs form kinetically metastable H-aggregates, which can be transformed into the thermodynamically favored J-aggregates by seed-induced living polymerization. We show here that copolymerization of kinetically trapped states of one PBI with seeds of another PBI leads to the formation of supramolecular block copolymers by chain-growth process from the seed termini as confirmed by UV/vis spectroscopy and atomic force microscopy (AFM). This work demonstrates for the first time the formation of triblock supramolecular polymer architectures with A-B-A and B-A-B block pattern by alternate two-component seeded polymerization in a living manner.

Protein-like Enwrapped Perylene Bisimide Chromophore as a Bright Microcrystalline Emitter Material.

Strongly emissive solid-state materials are mandatory components for many emerging optoelectronic technologies, but fluorescence is often quenched in the solid state owing to strong intermolecular interactions. The design of new organic pigments, which retain their optical properties despite their high tendency to crystallize, could overcome such limitations. Herein, we show a new material with monomer-like absorption and emission profiles as well as fluorescence quantum yields over 90 % in its crystalline solid state. The material was synthesized by attaching two bulky tris(4-tert-butylphenyl)phenoxy substituents at the perylene bisimide bay positions. These substituents direct a packing arrangement with full enwrapping of the chromophore and unidirectional chromophore alignment within the crystal lattice to afford optical properties that resemble those of their natural pigment counterparts, in which chromophores are rigidly embedded in protein environments.

Supramolecular Polymorphism in One-Dimensional Self-Assembly by Kinetic Pathway Control.

Controlling polymorphism in molecular solids is of great interest since the properties and performances of molecular materials depend on the molecules' mutual packing arrangements. Herein, we describe a perylene bisimide (PBI) organogelator molecule PBI-4 that self-assembles into three different one-dimensional supramolecular polymorphs (Agg 1-3) in the same solvent and at the same concentration at room temperature. The three supramolecular polymorphs were characterized by UV/vis, CD, fluorescence and IR spectroscopy, atomic force microscopy (AFM), and theoretical calculations, revealing that their packing arrangements are governed by distinct π-π-stacking modes and unique hydrogen-bonding patterns. Nudged elastic band (NEB) calculations for the nucleation processes toward Agg 2 and Agg 3 indicate that nucleation starts from a central kinetically trapped state Agg 1 and involves the reorganization of Agg 1 dimers. Time-, concentration-, and temperature-dependent UV/vis experiments provided insights into the thermodynamic stability of the supramolecular polymorphs of PBI-4 and the kinetics for their interconversion. On the basis of this information the production of a certain polymorph could be accomplished either physically by ultrasonication or chemically by seeding. This work contributes to the understanding of polymorphism at the lowest level of hierarchy that is the generation of self-assembled 1D aggregate structures.

Unraveling Concomitant Packing Polymorphism in Metallosupramolecular Polymers.

The phenomenon of polymorphism is ubiquitous in biological systems and has also been observed in various types of self-assembled materials in solution and in the solid state. In the field of supramolecular polymers, different kinetic vs thermodynamic self-assembled species may exist in competition, a phenomenon termed as pathway complexity. In these examples, the transient kinetic species often has a very short lifetime and rapidly converts into the thermodynamic product. In this work, we report a π-conjugated Pt(II) complex 1 that self-assembles in nonpolar medium into two competing supramolecular polymers with distinct molecular packing (slipped (A) vs pseudoparallel (B)) that do not interconvert over time in a period of at least six months at room temperature. Precise control of temperature, concentration, and cooling rate enabled us to ascertain the stability conditions of both species through a phase diagram. Extensive experimental studies and theoretical calculations allowed us to elucidate the packing modes of both supramolecular polymorphs A and B, which are stabilized by unconventional N-H···Cl-Pt and N-H···O-alkyl interactions, respectively. Under a controlled set of conditions of cooling rate and concentration, both polymorphs can be isolated concomitantly in the same solution without interconversion. Only if A is annealed at high temperature for prolonged time, does a slow transformation into B then take place via monomer formation. Our system, which in many respects bears close resemblance to concomitant packing polymorphism in crystals, should help bridge the gap between crystal engineering and supramolecular polymerization.

Linking two worlds in polymer chemistry: The influence of block uniformity and dispersity in amphiphilic block copolypeptoids on their self-assembly.

The self-assembly of block copolymers has captured the interest of scientists for many decades because it can induce ordered structures and help to imitate complex structures found in nature. In contrast to proteins, nature's most functional hierarchical structures, conventional polymers are disperse in their length distribution. Here, we synthesized hydrophilic and hydrophobic polypeptoids via solid-phase synthesis (uniform) and ring-opening polymerization (disperse). Differential scanning calorimetry measurements showed that the uniform hydrophobic peptoids converge to a maximum of the melting temperature at a much lower chain length than their disperse analogs, showing that not only the chain length but also the dispersity has a considerable impact on the thermal properties of those homopolymers. These homopolymers were then coupled to yield amphiphilic block copolypeptoids. SAXS and AFM measurements confirm that the dispersity plays a major role in microphase separation of these macromolecules, and it appears that uniform hydrophobic blocks form more ordered structures.