Frank Würthner

Perylene bisimide dyes as versatile building blocks for functional supramolecular architectures

Perylene bisimide dyes and their organization into supramolecular architectures through hydrogen-bonding, metal ion coordination and pi-pi-stacking is discussed; further self-assembly leading to nano- and meso-scopic structures and liquid-crystalline compounds is also addressed.

J‐Aggregates: From Serendipitous Discovery to Supramolecular Engineering of Functional Dye Materials

J-aggregates are of significant interest for organic materials conceived by supramolecular approaches. Their discovery in the 1930s represents one of the most important milestones in dye chemistry as well as the germination of supramolecular chemistry. The intriguing optical properties of J-aggregates (in particular, very narrow red-shifted absorption bands with respect to those of the monomer and their ability to delocalize and migrate excitons) as well as their prospect for applications have motivated scientists to become involved in this field, and numerous contributions have been published. This Review provides an overview on the J-aggregates of a broad variety of dyes (including cyanines, porphyrins, phthalocyanines, and perylene bisimides) created by using supramolecular construction principles, and discusses their optical and photophysical properties as well as their potential applications. Thus, this Review is intended to be of interest to the supramolecular, photochemistry, and materials science communities.

Fluorescent J‐type Aggregates and Thermotropic Columnar Mesophases of Perylene Bisimide Dyes

A series of perylene tetracarboxylic acid bisimides 3a-e bearing 3,4,5-tridodecyloxyphenyl substituents on the imide N atoms and zero, two, or four phenoxy-type substituents in the bay positions of the perylene core were synthesized. From investigations of their spectroscopic properties and aggregation behavior in low-polarity solvents by absorption and fluorescence optical spectroscopy, not only were these compounds found to form fluorescent J-type aggregates, but also binding constants for aggregation could be derived which reflect the number and steric demand of the phenoxy substituents for bisimides 3a-d. In the pristine state, 3a-d form thermotropic hexagonal columnar mesophases which exist over a broad temperature range from below -30 degrees C to over 300 degrees C. For the tetraphenoxy-substituted compound 3e, however, a layered crystalline structure was found. This difference in behavior can be explained by the concept of microphase segregation of the aromatic cores of the molecules and the alkyl chains at the periphery. The high stability and bright fluorescence of the mesophase of several of the compounds make them promising for applications as polarizers or components in (opto)electronic devices.

Self-Sorting Phenomena in Complex Supramolecular Systems

Nature successfully manages under extremely adverse conditions to accomplish intricate functions responsible for the regulation and control of the vast majority of biological processes that eventually sustain life on our planet. Biological molecules are required to carry out selective functions while often being hindered by surrounding agents which are simultaneously competing to bind the same targets. This high degree of selectivity in nature ultimately depends on the “molecular instructions” encoded in the chemical structure of the interacting species responsible for every single recognition or discrimination event. The formation of the DNA double helix, for instance, requires the base-pairing (sorting) of complementary nitrogenous bases (adenine thymine (A T) and cytosine guanine (C G)). These high-fidelity recognition processes are crucial in the storage of genetic information used in the development and functioning of all known living organisms and some viruses. Other sophisticated superstructures such as microtubules, are built upon polymerization of dimers of two different globular proteins (Rand β-globulin), giving rise to cylindrical micrometric arrangements. The formation of heterodimers composed of two different proteins requires the self-discrimination of equals, and the simultaneous recognition of complementary units. In the final instance, the small molecules of life (e.g., sugars, amino acids and fatty acids) are able to assemble not only to form such abovementioned macromolecules, but also to self-sort in one of the most efficient and complex processes known in nature to build the functional basic unit of life: a cell. In a cell, multiple levels of compartmentalization arising from the self-sorting of their molecular components allow the coexistence of different functional architectures acting independently. This exceptional selectivity in nature makes possible the existence of life on our planet. Unlike the high complexity of natural or biological architectures, the majority of artificial self-assembled systems reported so far have been investigated in isolation. This has been mainly due to the lack of suitable characterization methods and technical or economic constraints, which far exceed the resources of most research institutes. However, the remarkable development of analytical tools is increasingly enabling scientists to pinpoint intractable problems associated to multicomponent mixtures. In this context, Systems Chemistry has arisen in recent years as a new discipline that aims to investigate complex mixtures of interacting molecules. 17 These mixtures can give rise to outstanding emergent properties as a result of the interaction of the individual components and cannot be ascribed to any of their components acting in isolation. Although this emerging discipline is still in its infancy, ongoing research advances are enabling current (supramolecular) chemists to unravel the behavior of individual molecules in multicomponent mixtures and to anticipate the reasons that lead artificial molecules to bind or ignore a specific partner in a complex multicomponent environment. In this review, wewill discuss the external variables and intrinsic factors (molecular codes) that influence the recognition or discrimination of supramolecularly interacting chemical species in solution. The comprehension of this “molecular programming” in artificial systems will define the variables that control self-sorting processes, and may ultimately contribute to a better understanding of the self-assembly pathways in natural systems. By restricting ourselves to noncovalent bonds and self-sorting in solution we will not cover self-assembly processes on solid surfaces and self-sorting phenomena based on reversible covalent bonds.However, excellent reviews have recently become available by De Feyter and Otto, which cover these topics.

Vesicular perylene dye nanocapsules as supramolecular fluorescent pH sensor systems

Water-soluble, self-assembled nanocapsules composed of a functional bilayer membrane and enclosed guest molecules can provide smart (that is, condition responsive) sensors for a variety of purposes. Owing to their outstanding optical and redox properties, perylene bisimide chromophores are interesting building blocks for a functional bilayer membrane in a water environment. Here, we report water-soluble perylene bisimide vesicles loaded with bispyrene-based energy donors in their aqueous interior. These loaded vesicles are stabilized by in situ photopolymerization to give nanocapsules that are stable over the entire aqueous pH range. On the basis of pH-tunable spectral overlap of donors and acceptors, the donor-loaded polymerized vesicles display pH-dependent fluorescence resonance energy transfer from the encapsulated donors to the bilayer dye membrane, providing ultrasensitive pH information on their aqueous environment with fluorescence colour changes covering the whole visible light range. At pH 9.0, quite exceptional white fluorescence could be observed for such water-soluble donor-loaded perylene vesicles.

Molecular Assemblies of Perylene Bisimide Dyes in Water

Perylene bisimides are among the most valuable functional dyes and have numerous potential applications. As a result of their chemical robustness, photostability, and outstanding optical and electronic properties, these dyes have been applied as pigments, fluorescence sensors, and n-semiconductors in organic electronics and photovoltaics. Moreover, the extended quadrupolar π system of this class of dyes has facilitated the construction of numerous supramolecular architectures with fascinating photophysical properties. However, the supramolecular approach to the formation of perylene bisimide aggregates has been restricted mostly to organic media. Pleasingly, considerable progress has been made in the last few years in developing water-soluble perylene bisimides and their application in aqueous media. This Review provides an up-to-date overview on the self-assembly of perylene bisimides through π-π interactions in aqueous media. Synthetic strategies for the preparation of water-soluble perylene bisimides and the influence of water on the π-π stacking of perylene bisimides as well as the resulting applications are discussed.

Control of H‐ and J‐Type π Stacking by Peripheral Alkyl Chains and Self‐Sorting Phenomena in Perylene Bisimide Homo‐ and Heteroaggregates

The synthesis, self-assembly, and gelation ability of a series of organogelators based on perylene bisimide (PBI) dyes containing amide groups at imide positions are reported. The synergetic effect of intermolecular hydrogen bonding among the amide functionalities and pi-pi stacking between the PBI units directs the formation of the self-assembled structure in solution, which beyond a certain concentration results in gelation. Effects of different peripheral alkyl substituents on the self-assembly were studied by solvent- and temperature-dependent UV-visible and circular dichroism (CD) spectroscopy. PBI derivatives containing linear alkyl side chains in the periphery formed H-type pi stacks and red gels, whereas by introducing branched alkyl chains the formation of J-type pi stacks and green gels could be achieved. Sterically demanding substituents, in particular, the 2-ethylhexyl group completely suppressed the pi stacking. Coaggregation studies with H- and J-aggregating chromophores revealed the formation of solely H-type pi stacks containing both precursor molecules at a lower mole fraction of J-aggregating chromophore. Beyond a critical composition of the two chromophores, mixed H-aggregate and J-aggregate were formed simultaneously, which points to a self-sorting process. The versatility of the gelators is strongly dependent on the length and nature of the peripheral alkyl substituents. CD spectroscopic studies revealed a preferential helicity of the aggregates of PBI building blocks bearing chiral side chains. Even for achiral PBI derivatives, the utilization of chiral solvents such as (R)- or (S)-limonene was effective in preferential population of one-handed helical fibers. AFM studies revealed the formation of helical fibers from all the present PBI gelators, irrespective of the presence of chiral or achiral side chains. Furthermore, vortex flow was found to be effective in macroscopic orientation of the aggregates as evidenced from the origin of CD signals from aggregates of achiral PBI molecules.

Photoproduction of Proton Gradients with π-Stacked Fluorophore Scaffolds in Lipid Bilayers

Rigid p-octiphenyl rods were used to create helical tetrameric π-stacks of blue, red-fluorescent naphthalene diimides that can span lipid bilayer membranes. In lipid vesicles containing quinone as electron acceptors and surrounded by ethylenediaminetetraacetic acid as hole acceptors, transmembrane proton gradients arose through quinone reduction upon excitation with visible light. Quantitative ultrafast and relatively long-lived charge separation was confirmed as the origin of photosynthetic activity by femtosecond fluorescence and transient absorption spectroscopy. Supramolecular self-organization was essential in that photoactivity was lost upon rod shortening (from p-octiphenyl to biphenyl) and chromophore expansion (from naphthalene diimide to perylene diimide). Ligand intercalation transformed the photoactive scaffolds into ion channels.

A Crystal‐Engineered Hydrogen‐Bonded Octachloroperylene Diimide with a Twisted Core: An n‐Channel Organic Semiconductor

The appropriate arrangement of organic semiconductors in the solid state is decisive for efficient charge-carrier transport between source and drain electrodes in organic thin-film transistors (OTFTs). However, the still unsolved challenges in crystal engineering mean that there are only few examples where the packing of p-conjugated semiconductors can be controlled by means of rational design concepts to avoid the most common herringbone p-stacking motif (Figure 1a). An outstanding example is provided by the

Systems Chemistry Approach in Organic Photovoltaics

The common approach in organic materials science is dominated by the perception that the properties of the bulk materials are virtually determined by the properties of the molecular building blocks. In this Concept Article, we advocate for taking into account supramolecular organization principles for all kinds of organic solid-state materials, irrespective of them being crystalline, liquid crystalline, or amorphous, and discuss a showcase example, that is, the utilization of merocyanine dyes as p-type organic semiconductors in bulk heterojunction (BHJ) solar cells. Despite their extraordinarily large dipole moments, which are considered to be detrimental for efficient charge carrier transport, BHJ organic photovoltaic materials of these dyes with fullerenes have reached remarkable power conversion efficiencies of meanwhile nearly 5%. These at the first glance contradictory properties are, however, well-understandable on the systems chemistry level.