Fig. 1. Temperature-activated delayed fluorescence: system and processes.

a [Cu₄(PCP)₃]⁺ (PCP = 2,6-(PPh₂)₂C₆H₃) complex. b Scheme of light emission from Cu-based OLEDs due to temperature-activated delayed fluorescence (TADF) under electroluminescence conditions. After electron-hole recombination, the singlet and triplet excited states are occupied in a 1:3 ratio due to one possible momentum projection in the singlet and three possible projections in the triplet state. If the energy of the triplet state T₁ is close to the energy of the singlet S₁, temperature-activated reverse intersystem crossing (T₁ → S₁) occurs which is followed by light emission from the singlet state. c Structure of [Cu₄(PCP)₃]⁺ derived from single crystal X-ray diffraction. Cu atoms are brown, P atoms are magenta, C atoms are gray, H atoms are not shown. d Schematic illustration showing that non-radiative relaxation paths in Cu OLED materials are more probable if equilibrium excited and ground state structures are displaced along some vibrational coordinates. e Green emission from an OLED prototype with [Cu₄(PCP)₃]⁺ as a luminophore.