Fig. quantifies the CT contribution in these two configurations, as a function of ε 0 . In the gas phase, the two CT configurations appear in essentially equal percentages, in both structures of (PDI) 2 , leading in both cases to a CR state with vanishing dipole moment. For model 2, however, a dielectric constant of ε 0 = 3 is sufficient to break the symmetry, resulting in a net dipole moment. In contrast, symmetry-breaking is not observed in model 1 even in acetonitrile. This observation has potential ramifications for singlet fission. The CR character in model 2 facilitates the overall singlet fission process by increasing the electronic coupling between the S 1 and 1 ME states. Model 2, which transfers electrons in only one direction when ε 0 3, might act as an energetic trap in a dielectric medium. This trap (an excimer state) would then compete with singlet fission and would lower the yield of the latter process. This suggests that one possible route to improving efficiency of singlet fission in PDI dimers is to exploit geometries in which single-directional CT is inhibited. This difference highlights the fact that calculations carried out using vacuum boundary conditions are ill-positioned to address whether CT character plays a role in the singlet fission mechanism.