Theo E. Kaiser

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.

Collective fluorescence blinking in linear J-aggregates assisted by long-distance exciton migration.

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.

Dendritic oligothiophene-perylene bisimide hybrids: synthesis, optical and electrochemical properties

Novel dendritic oligothiophene-perylene bisimide hybrid (DOTPBI) systems were investigated. Perylene bisimide (PBI) derivatives with phenoxy spacers bearing dendritic oligothiophenes (DOT) up to the third generation (G3) in the bay position, with an increasing number of conjugated thiophene units have been synthesized and characterized as novel functional materials. Investigation of the optoelectronic properties revealed that the perylene core and dendritic oligothiophene units are electronically decoupled. In the DOTPBIs intramolecular fluorescence resonance energy transfer (FRET) and photoinduced electron transfer (PET) from the DOT to the PBI moiety have been investigated by various optical measurements, revealing that the PET process is facilitated with increasing DOT generation and donor strength. Electropolymerization led to interesting cross-linked donor acceptor-type conducting polymer films.

Reorganization of perylene bisimide J-aggregates: from delocalized collective to localized individual excitations

Water-induced reorganization of individual one-dimensional J-aggregates of perylene bisimide (PBI) dyes was observed by fluorescence microscopy. Fluorescence spectra and decay kinetics of individual J-aggregates immobilized on glass surfaces were measured under a dry nitrogen atmosphere and under humid conditions. The fluorescence properties of PBI J-aggregates arisen from collective excitons under dry nitrogen atmosphere were changed to those of non-interacting dye monomers when water vapor was introduced into the environment (sample chamber). Time-dependent changes of the fluorescence spectra and lifetimes upon exposure to water vapor suggest an initial coordination of water molecules at defect sites leading to the formation of H-type dimer units that act as exciton quenchers, and a subsequent slower disintegration of the hydrogen-bonded J-aggregate into monomers that lack resonance coupling. Our present studies resulted in a direct demonstration of how drastically the optical properties of molecular ensembles and characteristics of their excited states can be changed by delicate reorganization of dye molecules at nanometre scales.