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 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.

CONTENTS 1. Introduction 962 1.1. Prologue 962 1.2. Parent Perylene Bisimide Chromophore 963 1.3. Core-Substituted Perylene Bisimides 964 1.4. Perylene Bisimides in the Solid State 966 1.5. Areas of Application 966 2. Linear, Dendritic, and Macrocyclic Covalent PBI Ensembles 968 2.1. Excitonic Coupling and Deactivation Processes of Photoexcited PBIs 969 2.2. Rigid PBI Dimers 971 2.3. Flexible PBI Ensembles 974 2.4. Cyclic PBI Ensembles 976 3. π-Stacked PBI Assemblies 979 3.1. Self-Assembly of Core-Unsubstituted PBIs in Solution 979 3.2. Organization of Core-Unsubstituted PBIs in the Bulk Solid State 984 3.3. Self-Assembly of Amphiphilic PBIs in Aqueous Media and Solid Bulk State 987 3.4. Self-Assembly of Core-Substituted PBIs 991 3.5. Self-Assembly of Dye Arrays Composed of Multiple PBIs 995 3.6. Self-Assembly of Multichromophoric PBI Conjugates Containing Other Dyes 996 4. Hydrogen-Bond Directed Self-Assembly 1000 4.1. Self-Assembly Directed by Imide−Imide H-Bonding Interactions 1000 4.2. Self-Assembly Directed by Side-Chain Amide−Amide H-Bonding Interactions 1003 4.3. Self-Assembly Directed by Other H-Bonding Interactions 1006 4.4. Coassembly Directed by Imide−Melamine H-Bonding Interactions 1008 4.5. Coassembly Directed by Melamine−Cyanurate/Barbiturate H-Bonding Interactions 1011

This critical review provides an overview on the formation of pi-stacks of functional dyes in solution, aiming to acquaint young researchers with this topical research field and to stimulate further advance in supramolecular dye chemistry. Different mathematical models that have been proposed and applied for the description of aggregation equilibria of pi-systems in solution are discussed. The factors that have significant impact on the structural features of aggregates and the thermodynamics of pi-pi stacking such as electrostatic interactions, size and geometry of the dye molecules are covered in this review. A comparison of the binding strength is made for different classes of functional pi-conjugated systems, from simple benzene to more extended polycyclic hydrocarbon molecules, including triphenylenes and hexabenzocoronenes, heteroaromatic porphyrins and phthalocyanines, quadrupolar naphthalene and perylene bisimides, dipolar or even zwitterionic merocyanines and squaraines, and some macrocyclic dyes. Solvent effects on binding constants are analysed by linear free energy relationships with various solvent polarity scales (98 references with multiple entries).

The self-assembly of a new, highly fluorescent perylene bisimide dye 2 into pi stacks, both in solution and condensed phase, has been studied in detail by NMR spectroscopy, vapor pressure osmometry (VPO), UV/Vis and fluorescence spectroscopy, differential scanning calorimetry (DSC), optical polarizing microscopy (OPM) and X-ray diffraction. The NMR and VPO measurements revealed the formation of extended pi-pi stacks of the dye molecules in solution. The aggregate size determined from VPO and DOSY NMR measurements agree well with that obtained from the concentration and temperature-dependent UV/Vis spectral data by employing the isodesmic model (equal K model). In the condensed state, dye 2 possesses a hexagonal columnar liquid crystalline (LC) phase as confirmed by X-ray diffraction analysis. The columnar stacking of this dye has been further explored by atomic force microscopy (AFM). Well-resolved columnar nanostructures of the compound are observed on graphite surface. A color-tunable luminescence from green to red has been observed upon aggregation which is accompanied by an increase of the fluorescence lifetime and depolarization. The observed absorption properties can be explained in terms of molecular exciton theory. The charge transport properties of dye 2 have been investigated by pulse radiolysis-time resolved microwave conductivity measurements and a 1D charge carrier mobility up to 0.42 cm(2) V(-1) s(-1) is obtained. Considering the promising self-assembly, semiconducting, and luminescence properties of this dye, it might serve as a useful functional material for nano(opto)electronics.