Discrete π-Stacks of Perylene Bisimide Dyes within Folda-Dimers: Insight into Long- and Short-Range Exciton Coupling.

Four well-defined π-stacks of perylene bisimide (PBI) dyes were obtained in solution by covalent linkage of two chromophores with spacer units of different length and sterical demand. Structural elucidation of the folda-dimers by in-depth nuclear magnetic resonance studies and geometry optimization at the level of density functional theory suggest different, but highly defined molecular arrangements of the two chromophores in the folded state enforced by the various spacer moieties. Remarkably, the dye stacks exhibit considerably different optical properties as investigated by UV/vis absorption and fluorescence spectroscopy, despite only slightly different chromophore arrangements. The distinct absorption properties can be rationalized by an interplay of long- and short-range exciton coupling resulting in optical signatures ranging from conventional H-type to monomer like absorption features with low and appreciably high fluorescence quantum yields, respectively. To the best of our knowledge, we present the first experimental proof of a PBI-based "null-aggregate", in which long- and short-range exciton coupling fully compensate each other, giving rise to monomer-like absorption features for a stack of two PBI chromophores. Hence, our insights pinpoint the importance of charge-transfer mediated short-range coupling that can significantly influence the optical properties of PBI π-stacks.