LAUSR

Combined density functional theory and multireference configuration interaction methods have been used to elucidate singlet fission (SF) pathways and mechanisms in three regioisomers of side‐on linked pentacene dimers. In addition to the optically bright singlets (S  1 and S  2 ) and singly excited triplets (T  1 and T  2 ), the full spin manifold of multiexcitonic triplet‐pair states (  1 ME,  3 ME,  5 ME) has been considered. In the ortho‐ and para‐regioisomers, the  1 ME and S  1 potentials intersect upon geometry relaxation of the S  1 excitation. In the meta‐regioisomer, the crossing occurs upon delocalization of the optically bright excitation. The energetic accessibility of these conical intersections and the absence of low‐lying charge‐transfer states suggests a direct SF mechanism, assisted by charge‐resonance effects in the  1 ME state. While the  5 ME state does not appear to play a role in the SF mechanism of the ortho‐ and para‐regioisomers, its participation in the disentanglement of the triplet pair is conceivable in the meta‐regioisomer.