Although it is commonly known that H-type PBI aggregates give rise to a broad, red-shifted excimer fluorescence with considerably longer fluorescence lifetimes than observed for the monomers, the underlying mechanisms of excimer formation and other relevant exciton dynamics in such π-stacked systems are still far from being understood. In this context, we demonstrate a thorough spectroscopic investigation on the exciton relaxation pathways, including excimer formation, in a perylene-3,4:9,10-bis(dicarboximide) (PBI) dimer aggregate 1 by using time-resolved fluorescence and transient absorption spectroscopy combined with excitation-power and polarization dependence. It was found that the excited dimer formation process followed by structural rearrangement is approximately two times faster than observed within larger PBI aggregates. Excitation-power-dependent transient absorption decay profiles revealed the fully delocalized nature of excitons in the dimer as opposed to larger stacks.
A novel perylene bisimide (PBI) dye bearing one solubilizing dialkoxybenzyl and one bulky 2,5-di-tert-butylphenyl substituent was synthesized and its aggregation behavior was analyzed by NMR and UV/Vis spectroscopy in various chloroform/methylcyclohexane (MCH) solvent mixtures. In the presence of no less than 10 vol % chloroform, exclusive self-assembly of this PBI dye into π-stacked dimers was unambiguously confirmed by means of both concentration-dependent (1) H NMR and UV/Vis spectroscopic experiments. Based on ROESY NMR, a well-defined π-stacked dimer structure was determined and further corroborated by molecular modeling studies. By varying the solvent composition of chloroform and MCH, the solvent effects on the Gibbs free energy of PBI dimerization were elucidated and showed a pronounced nonlinearity between lower and higher MCH contents. This observation could be related to a further growth process of dimers into larger aggregates that occurs in the absence of chloroform, which is required to solvate the aromatic π surfaces. With the help of a single-crystal structure analysis for a related PBI dye, a structural model could be derived for the extended aggregates that are still composed of defined π-π-stacked PBI dimer entities.
Self-assembly: A tweezer-type perylene bisimide (PBI) dyad self-assembles into a defined bimolecular complex composed of a quadruple PBI π stack with remarkable kinetic stability, which is unprecedented for π-stacked dye aggregates (see picture). These persistent supramolecular species are of considerable interest for the elucidation of functional properties of dye aggregates.
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