Correlated Pair States Formed by Singlet Fission and Exciton-Exciton Annihilation

Singlet fission to form a pair of triplet excitations on two neighboring molecules and the reverse process, triplet triplet annihilation to upconvert excitation, have been extensively studied. Comparatively little work has sought to examine the properties of the intermediate state in both of these processes the bimolecular pair state. Here, the eigenstates constituting the manifold of 16 bimolecular pair excitations and their relative energies in the weak-coupling regime are reported. The lowest-energy states obtained from the branching diagram method are the triplet pairs with overall singlet spin < X-1 > approximate to (1)[TT] and quintet spin vertical bar Q > (5)[TT]. It is shown that triplet pair states can be separated by a triplet triplet energy-transfer mechanism to give a separated, yet entangled triplet pair (1)[T center dot center dot center dot T]. Independent triplets are produced by decoherence of the separated triplet pair. Recombination of independent triplets by exciton exciton annihilation to form the correlated triplet nongeminate recombination) happens with 1/3 of the rate of either triplet migration or recombination of the correlated triplet pair (geminate recombination). Pair (i.e., nongeminate recombination) happens with 1/3 of the rate of either triplet migration or recombination of the correlated triplet pair (geminate recombination).