Singlet Fission

Singlet fission is a process in which an organic chromophore in an excited singlet state shares its excitation energy with a neighboring ground-state chromophore and both are converted into triplet excited states. Singlet fission is spin-allowed in the sense that the two resulting triplet excitations produced from an excited singlet are born coupled into a pure singlet state. Like many other internal conversion processes, it can be very fast, particularly in molecular crystals. The wave function of the initially formed pure singlet state 1(TT) is a coherent superposition of the wave functions of these nine sublevels, and their ultimate population will reflect the amplitude of the singlet 1(TT) wave function in each one. As long as the states resulting from singlet fission are of mixed multiplicity, the overall process can also be viewed as a special case of intersystem crossing. Singlet fission does not occur in single small-molecule chromophores, at least not at the usual excitation energies, and is constrained to multichromophoric systems.