Fig. 1
Molecular structure and electronic modelling of singlet fission in 13,13′-bis(mesityl)-6,6′-dipentacenyl. a 13,13′-bis(mesityl)-6,6′-dipentacenyl (DP-Mes) showing symmetry elements at the ground state geometry computed by DFT (point group D2d). Pentacene monomers also indicated (red, blue). b Illustration of the ab initio electronic model showing energy-level ordering (not to scale) and typical electronic configurations defining the locally excited (LE), charge transfer (CT) and doubly excited triplet-pair (TT) states. Dashed lines indicate interstate vibronic couplings labelled by normal mode symmetries. The initial photoexcited state (LE+) is not coupled to TT; it must decay via vibronic superexchange mediated by CT states. This is a second-order process, so can involve two interactions with the same environment (path I, coherent) or different environments (path 2, incoherent). A1 ‘tuning’ modes (not shown) shift the energy levels of each state but do not induce any interstate coupling
