Suppressing Excimers in H-Aggregates of Perylene Bisimide Folda-Dimer: Role of Dimer Conformation and Competing Assembly Pathways.

Long-lived excitons in H-aggregates hold great promise for efficient transport of excitation energy, provided they are not scavenged by structurallly relaxed excimers. We report solution self-assembly of a perylene bisimide (PBI) folda-dimer that exhibits two distinct kinetic stages: an initial fast assembly leads to metastable aggregates with large excimer contribution that is followed by a slower growth of stable, extended H-aggregates free of excimers. Mechanistic investigations reveal an interplay of two competing aggregation pathways, where suppression of excimers is directly linked to the crossover from an isodesmic to cooperative aggregation. How the comeptition between two self-assembly pathways is influenced by the conformational flexibility of the folda-dimer is also discussed.