Self-Assembly of Supramolecular Light-Harvesting Arrays from Symmetric Perylene-3,4-dicarboximide Trefoils

Unlike the widely studied perylene-3,4:9,10-bis­(dicarboximide) (PDI) dyes, self-assembly of the corresponding perylene-3,4-dicarboximide (PMI) dyes into large arrays and studies of their excited state properties have received far less attention. Two symmetric PMI trefoils were synthesized by connecting the 9-position of the perylene core either directly (<b>1</b>) or through a phenylene linker (<b>2</b>) to the 1,3,5-positions of a central benzene ring. Synchrotron-based small- and wide-angle X-ray scattering measurements in methylcyclohexane show that trefoil <b>1</b> self-assembles into cofacial trimers (<b>1</b>_<b>3</b>) on average, while trefoil <b>2</b> forms much larger assemblies that are tridecamers (<b>2</b>_<b>13</b>) on average. Their photophysics were characterized using steady-state as well as transient absorption and emission spectroscopy. Time-resolved spectroscopy reveals that both <b>1</b>_<b>3</b> and <b>2</b>_<b>13</b> initially form excitonically coupled excited states that subsequently relax to excimer states having 20 and 8.4 ns lifetimes, respectively, which decay to ground-state primarily nonradiatively. The data are consistent with stronger electronic coupling between the PMI molecules in <b>2</b>_<b>13</b> relative to <b>1</b>_<b>3</b>.