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.

Luminous nanorods: The self‐assembly of core‐twisted perylene bisimide fluorophores (see structures) in nonpolar organic solvents is directed by hydrogen‐bonding interactions. This supermolecular concept resulted in one‐dimensional J‐aggregates with a fluorescence quantum yield of near unity.

A series of highly soluble and fluorescent, at core tetraaryloxy-substituted and in imide positions hydrogen atom containing perylene bisimide (PBI) dyes 1a-e with varying peripheral side chains have been synthesized and thoroughly characterized. The self-assembly of these PBIs has been studied in detail by UV/vis, linear dichroism (LD) and circular dichroism (CD) spectroscopy, and scanning probe microscopy (AFM, STM). These studies revealed that the present PBIs self-assemble into extended double string cables, which consist of two hydrogen-bonded supramolecular polymeric chains of densely packed and strongly excitonically coupled PBI chromophores, providing highly fluorescent J-aggregates. The aggregation strength ("melting" temperature) and the fluorescence properties of these J-aggregates are dependent on the number and chain length of the peripheral alkoxy substituents, thus revealing a structure-property relationship. In contrast to previously reported assemblies of PBIs, for which the aggregation process is described by the isodesmic (or equal K) model, a cooperative nucleation-elongation mechanism applies for the aggregation of the present assemblies as revealed by concentration-dependent UV/vis absorption studies with the chiral PBI 1e, providing equilibrium constants for dimerization (= nucleation) of K(2) = 13 +/- 11 L mol(-1) and for elongation of K = 2.3 +/- 0.1 x 10(6) L mol(-1) in methylcyclohexane (MCH). LD spectroscopic measurements have been performed to analyze the orientation of the monomers within the aggregates. The nonlinearity of chiral amplification in PBI aggregates directed by sergeants-and-soldiers principle has been elucidated by coaggregation experiments of different PBI dyes using CD spectroscopy. The dimensions as well as the molecular arrangement of the monomeric units in assemblies have been explored by atomic force microscopy (AFM) and scanning tunneling microscopy (STM).

Fluorescence blinking corresponding to collective quenching of up to 100 dye monomers is reported for individual J-aggregates of a perylene bisimide (PBI) dye. This implies an exciton diffusion length up to 70 nm in these one-dimensional assemblies. The number of quenched monomers was directly measured by comparing the fluorescence brightness of the J-aggregates with that of noncoupled PBI molecules. This brightness analysis technique is useful for unraveling photophysical parameters of any individual fluorescent nanosystem.

[reaction: see text] A series of bay position difluoro- or tetrafluoro-substituted perylene bisimides have been synthesized by nucleophilic halogen exchange reaction of the corresponding dibromo- and tetrachloro-substituted perylene bisimides, respectively, with potassium fluoride. Compared to the parent unsubstituted perylene bisimides, these compounds display hypsochromically shifted absorption and fluorescence spectra with fluorescence quantum yields up to unity enabling bright yellow emission. Their electrochemical properties and crystal structures of two perylene bisimides are also reported.