Simulating the full spin manifold of triplet-pair states in a series of covalently linked TIPS-pentacenes

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 $$ and S $$ ) and singly excited triplets (T $$ and T $$ ), the full spin manifold of multiexcitonic triplet-pair states ( $$ ME, $$ ME, $$ ME) has been considered. In the ortho- and para-regioisomers, the $$ ME and S $$ potentials intersect upon geometry relaxation of the S $$ 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 $$ ME state. While the $$ 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.