Interchromophore Rotation Related Ultrafast Charge Separation at Excited States in Head-to-Tail Linked Perylene Diimide Dyads

Photoinduced charge separation (CS) kinetics are observed to be strongly related to the molecular conformation of the monomeric units in perylene diimide (PDI) dyads which contain identical or different PDI units linked through N-N covalent bonding. Ultrafast photoinduced intramolecular CS was observed in dichloromethane at room temperature for the dyads containing twisted tetra(phenoxy) - substituted PDI units (1-1 and 1-2), while the dyad containing two identical planar bay-area unsubstituted PDI units (2-2) keeps high fluorescence at the same conditions, although the free energy difference (∆GCS) for charge separation are all negative for these dyads. The interchromophore rotation at excited states between the adjacent chromophores in the dyads is a key factor for charge separation.