Scheme 1. Intermolecular Upconversion vs Two Suggested Models for Intramolecular Upconversion: TETA and DS.

Schematic of suggested pathways for triplet–triplet annihilation upconversion with dimeric annihilator compounds (here represented by 1,3-DPA₂). Designations: S = sensitizer, A = annihilator moiety (spin multiplicity denoted by left superscript), * = excited state. Upon light absorption (hν₁) and rapid intersystem crossing (ISC), the sensitizer populates the triplet excited state of one annihilator moiety through triplet energy transfer (TET). The TTA event forms a singlet excited state, and one high-energy photon (hν₂, ν₂ > ν₁) is emitted. (1) Conventional intermolecular TTA between two triplet excited dimers. (2a) TET between annihilators (TETA) model: The triplet excited dimer becomes doubly excited following TET from another triplet excited annihilator. (2b) Double sensitization (DS) model: The ground-state moiety of the singly triplet excited dimer is populated with another triplet following TET from a sensitizer molecule.