Scan calculations performed at the B3LYPGD3BJ / 6-31G level of theory allowed us to estimate the rotational energy barrier around the thiophene -perfluorocyclopentene bridge. It was estimated at 75 kJ.mol -1 for the C10-C3' bond and 161 kJ.mol -1 for the C6-C3 bond (Figure ). The latter being superior than 100 kJ.mol -1 , the rotation around the bond is indeed blocked at roomtemperature, agreeing the observed experimental behavior. Interestingly, the interchromophoric distance of 3.5 Å found between the two pyrenes in 1o leads to ground state interactions, whereas similar distances calculated in PDI dimers exhibit monomer-like absorption feature likely due to higher rigidity and slip stacks. One can notice that in 1o, the two differently branched pyrenes allow an optimized overlap with very short slip stacks of 1.4 Å and 0.73 Å along the short and long pyrene axes respectively. and the related data are summarized in Table . The absorption spectrum of reference compound 2o is characterized by two sets of bands in the UV region gathering -* transitions on the DTE core and on the pyrene moiety i.e., from 250 to 300 nm (S0-S3 for pyrene) and from 300 to 400 nm (S0-S2 for pyrene). The S0-S2 excitation in 2-substituted pyrene was described as "pyrene-like", regardless of the electronic nature of the substituent, due to a node in the HOMO and LUMO orbitals at the 2-position. As a matter of fact, 2o exhibits structuration with characteristic vibrational spacing of 1400 cm -1 (Figure ). The vibrational structure of the S0-S3 transition cannot be seen due to the overlapping with transitions belonging to the DTE core. 465 nm is attributed to the - stack of the two pyrene moieties, in agreement with the structural characterization (vide infra).