A simple kinetic model for singlet fission: A role of electronic and entropic contributions to macroscopic rates
A simple three-state model for the dynamics of singlet fission (SF) process is presented. The model facilitates the analysis of the relative significance of different factors, such as electronic energies, couplings, and the entropic contributions. The entropic contributions to the rates are important; they make the SF process possible in endo-ergic cases (such as tetracene). The anticipated magnitude of entropic contributions is illustrated by simple calculations. By considering series of three acenes (tetracene, pentacene, and hexacene), we explained the experimentally observed 3 orders of magnitude difference in the rate of SF in tetracene and pentacene and predicted that the rate of SF in hexacene will be slightly faster than in pentacene. This trend is driven by the increased thermodynamic drive for SF (Gibbs free energy difference of the initial excitonic state and two separated triplets). The model also explains experimentally observed fast SF in 5,12-diphenyltetracene.