Excimer Formation of Perylene Bisimide Dyes within Stacking-Restrained Folda-Dimers: Insight into Anomalous Temperature Responsive Dual Fluorescence

Abstract

We have fabricated a new perylene bisimide (PBI) folda-dimer (BPBI-CB-1) by tethering two PBI moieties to the ortho-carbon positions of a carborane unit. The synthesized compound adopted distinct configurations in different solvents with dual emissions as its characteristic. The two PBI moieties in the molecule appeared either in a weakly interacted, monomer-like state or brought into close π–π contact with each other, forming an interacted stacking state. The equilibrium between these two states was governed by the nature of solvents and testing temperature. Spectroscopic and theoretical studies concluded that dual emission bands originated from intramolecular monomer-like and stacking states, respectively. Remarkably, in a solvent like 1,2-dichlo-roethane (DCE), both emission intensities increased with rising temperatures. The positive temperature response of the monomer emission was ascribed to an increased amount of monomer-like population, owing to its endothermic energy state, while the excimer emission was rationalized by increased population of the bright exciton state, resulting in an increased emission yield that compensated the diminished population of the stacking state. To our knowledge, this is the first report on positive temperature-responsive dual emissions associated with the synergism of intramolecular intersubunit aggregation/dissociation and excimer transformation.