Spectroscopic Demonstration of Exciton Dynamics and Excimer Formation in a Sterically Controlled Perylene Bisimide Dimer Aggregate.

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.