Covalently linked perylenetetracarboxylic diimide dimers and trimers with rigid "J-type" aggregation structure

Covalently linked perylenetetracarboxylic diimide (PDI) dimers (D-1 and D-2) and trimers (T-1 and T-2) with slipped "face-to-face" stacked structure are prepared and their molecular structures are characterized by H-1 NMR, MALDI-TOF mass spectroscopy and elemental analysis. The rigid molecular structures of these compounds make it easier to establish a direct correlation between the aggregate structure and the photophysical properties. The minimized molecular structures of these compounds reveal that they are all "face-to-face" stacked aggregates with large longitudinal displacement. Their absorption spectra show red-shifted bands, suggesting the presence of "J" type excitonic coupling between the PDI subunits in these compounds. However, their steady state and time resolved fluorescence spectra revealed that the emission from the "excimer-like" states dominates the fluorescence of these compounds, this is similar to that of "H-type" aggregates and may be ascribed to the "face-to-face" stacked structure. In the fluorescence spectra of these compounds, a minor "J-type" emission can be identified for the compounds with a relatively large longitudinal displacement. An increase in the number of subunits in one aggregate from 2 to 3 also brings about distinctive changes in their photophysical properties, which can be ascribed to the changes in the stacking structure caused by the steric hindrance,