A new perylene bisimide dye self-assembles in an anti-cooperative process predominently into even numbered aggregates via dimers which could be interpreted by a newly developed K 2–K model.

The concentration-dependent absorption and temperature-dependent fluorescence of the perylene bisimide dye PBI 1 in methylcyclohexane point to a biphasic aggregation behavior. At intermediate concentrations and temperatures, respectively, a dimer with low fluorescence yield dominates, which cannot be extended to longer aggregates. Those are formed at high concentrations and low temperatures, respectively, via a second, energetically unfavorable dimer species that acts as a nucleus. A corresponding aggregation model reproduces accurately the concentration dependence and allows extracting the equilibrium constants and spectra of the distinct species. The differences in the photophysical properties indicate H-type excitonic coupling for the favored dimer and J-type characteristics for the extended aggregates which could be related to structural models based on DFT calculations. The energetics can be understood by considering hydrogen-bonding and π-π-stacking interactions.

Natural light harvesting as well as optoelectronic and photovoltaic devices depend on efficient transport of energy following photoexcitation. Using common spectroscopic methods, however, it is challenging to discriminate one-exciton dynamics from multi-exciton interactions that arise when more than one excitation is present in the system. Here we introduce a coherent two-dimensional spectroscopic method that provides a signal only in case that the presence of one exciton influences the behavior of another one. Exemplarily, we monitor exciton diffusion by annihilation in a perylene bisimide-based J-aggregate. We determine quantitatively the exciton diffusion constant from exciton–exciton-interaction 2D spectra and reconstruct the annihilation-free dynamics for large pump powers. The latter enables for ultrafast spectroscopy at much higher intensities than conventionally possible and thus improves signal-to-noise ratios for multichromophore systems; the former recovers spatio–temporal dynamics for a broad range of phenomena in which exciton interactions are present.

A series of well-defined chromophore stacks is obtained upon self-assembly of merocyanine and bis­(merocyanine) dyes in nonpolar solvents. Careful design of the spacer moieties linking the dipolar chromophores within the bis­(merocyanine) dyes allows one to direct the dipole–dipole interaction driven aggregation into stacks of desired size from dimer up to octamer. The spacer-encoded self-assembly process was investigated by UV/vis absorption spectroscopy showing an increase of the hypsochromic shift with increasing stack size. The structure of the largest aggregate comprising eight chromophores was analyzed by 1D and 2D nuclear magnetic resonance spectroscopic studies revealing a perfectly interdigitated centrosymmetric organization of the dipolar dyes and concomitant annihilation of the ground state dipole moment is observed in the UV/vis absorption spectra. This unprecedented series of dye stacks from dimer to octamer enabled a systematic study of the optical absorption properties in dependence of the stack size disclosing that the absorption features can be rationalized by molecular exciton theory. Our results show that the noncovalent synthesis approach based on dipolar aggregation is suitable for the design of well-defined dye aggregates of specific size, allowing in-depth studies to manifest structure–property relationships.

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