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${\hspace{0.17em}}_1$ and S${\hspace{0.17em}}_2$) and singly excited triplets (T${\hspace{0.17em}}_1$ and T${\hspace{0.17em}}_2$), the full spin manifold of multiexcitonic triplet-pair states (${\hspace{0.17em}}^1$ME, ${\hspace{0.17em}}^3$ME, ${\hspace{0.17em}}^5$ME) has been considered. In the ortho- and para-regioisomers, the ${\hspace{0.17em}}^1$ME and S${\hspace{0.17em}}_1$ potentials intersect upon geometry relaxation of the S${\hspace{0.17em}}_1$ 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 ${\hspace{0.17em}}^1$ME state. While the ${\hspace{0.17em}}^5$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.