Bay-substituted perylene bisimides: Twisted fluorophores for supramolecular chemistry
Perylene bisimides bearing four aryloxy substituents in the 1,6,7,12 positions (so-called bay area) and two receptor units at the imide nitrogens are most useful fluorescent building blocks for the realization of a broad variety of supramolecular architectures. As discussed in this review, this versatility is not only due to the outstanding fluorescence properties of these dyes with quantum yields close to unity, but also a consequence of pronounced twisting of the π-system with an inversion barrier of around 60 kJ/mol (leading to atropisomerism) and conformational flexibility of aryloxy units. The latter two properties endow these dyes with high solubility even in least polar aliphatic solvents and enable the dyes to maintain a distance of >4 Å in their supramolecular assemblies. This feature particularly reduces the, otherwise often observed, fluorescence quenching in dye aggregates. Thus, bright luminescent cyclic and polymeric supramolecular architectures have become available from these dyes by hydrogen-bond and metal-ion directed self-assembly. In those cases where additional π-π-stacking interactions come into play, luminescent organogels and liquid crystals are obtained.