Difference in the photophysical properties of a perylenetetracarboxylic diimide dimer and a hexamer linked by the same hexaphenylbenzene group.

A perylenetetracarboxylic diimide hexamer (6PDI) and a dimer (2PDI) linked with the same hexaphenylbenzene group were prepared, and the structures were fully characterized by (1)H NMR spectroscopy, mass spectrometry, and elemental analysis. Due to the similar molecular structure of these two compounds, similar interactions between/among the PDI subunits as well as similar photophysical properties are expected. However, the stationary UV/Vis absorption spectra reveal that the interactions among/between the PDI subunits in 6PDI are significantly stronger than those in 2PDI. This can be attributed to blocked rotation along the long axis of the PDI subunits in 6PDI due to steric hindrance of the two neighboring PDI subunits. The stronger interactions among the PDI subunits in 6PDI lead to long-wavelength emission, which can be assigned to "excimer-like" excited states. A similar conclusion can also be deduced from the fluorescence quantum yields and the fluorescence lifetimes. Electrochemical studies revealed that interactions between/among the PDI subunits in both 2PDI and 6PDI are still in the range of weak interactions. Ultrafast transient anisotropy decay dynamics revealed that excitation delocalization between the PDI subunits within 2PDI and 6PDI is quick and efficient. More interestingly, delocalization is faster in 6PDI than in 2PDI, probably because of the stronger interactions among the PDI subunits in the former.