Energy transfer and spectroscopic characterization of a perylenetetracarboxylic diimide (PDI) hexamer

We report a comprehensive study on a newly synthesized perylenetetracarboxylic diimide (PDI) hexamer together with its corresponding monomer and dimer by means of steady-state absorption and fluorescence as well as femtosecond broadband transient absorption measurements. The structure of the PDI hexamer is nearly arranged in a 3-fold symmetry by three identical and separated dimers. This unique structure makes the excited state energy relaxation processes more complex due to the existence of two different intramolecular interactions: a strong interaction between face-to-face PDIs in dimers and a relatively weak interaction between the three separated PDI dimers. The steady-state spectra and the ground state structural optimization show that the steric effect plays a dominant role in keeping the formation of the face-to-face stacked PDI-dimer within the PDI-hexamer, indicating that some level of a pre-associated excimer had formed already in the ground state for the dimer in the hexamer. Femtosecond transient absorption experiments on the PDI hexamer reveal a fast (similar to 200 fs) localization process and a sequential relaxation to a pre-associated excimer trap state from the delocalized exciton state with about 1.2 ps after the initially delocalized excitation. Meanwhile, excitation energy transfer among the three separated dimers within the PDI-hexamer is also revealed by the anisotropic femtosecond pump-probe transient experiments, where the hopping time is about 2.8 ps. A relaxed excimer state is further formed in 7.9 ps after energy hopping and conformational relaxation.