text: Triplet-triplet annihilation combines the energy of two triplet-excited molecules onto one molecule to produce a higher excited state. Since the higher excited state is an emissive singlet state, TTA can be used to achieve photon upconversion which is a process that converts the energy of two photons into one photon of higher energy.
  To achieve photon upconversion through triplet-triplet annihilation two types of molecules are often combined: a sensitizer and an emitter (annihilator). The sensitizer absorbs the low energy photon and populates its first excited triplet state (T) through intersystem crossing. The sensitizer then transfers the excitation energy to the emitter, resulting in a triplet excited emitter and a ground state sensitizer. Two triplet-excited emitters can then undergo triplet-triplet annihilation to produce a singlet excited state (S) of the emitter, which can emit an upconverted photon.