Thermally Activated Delayed Fluorescence and Beyond. Photophysics and Material Design Strategies.

This review focuses on thermally activated delayed fluorescence (TADF). Photophysical properties of Cu(I) complexes and unique organic molecules are addressed. Investigations, based on temperature‐dependent emission studies, micro‐ to femto‐second time‐resolved spectroscopy investigations, quantum mechanical considerations, state‐of‐art calculations, and organic light‐emitting diodes (OLED) device studies, address exciton harvesting mechanisms and photophysical impact of the energy gap Δ E ( S 1 – T 1 ) and spin‐orbit coupling (SOC). We disclose relationship between (i) Δ E ( S 1 – T 1 ) and transition rate k ( S 1 – S 0 ); (ii) SOC, phosphorescence, and intersystem crossing (ISC); (iii) internal/external rigidity, luminescence quantum yield, excitation self‐trapping, and concentration quenching; (iv) environment polarity and state energy tuning, as well as (v) SOC and combined ambient‐temperature TADF/phosphorescence, zero‐field splitting, and spin‐lattice relaxation (at T = 1.2 K). These studies guide us to milestone Cu(I) complexes. Moreover, we demonstrate that fast ISC in organic molecules requires state mixing with an additional, energetically close triplet state. Thus, a guide structure for unique organic TADF molecules with ultra‐fast ISC and reverse‐ISC rates (>10 9 s −1 ) combined with Δ E ( S 1 – T 1 )<10 cm −1 (<1 meV) is presented allowing for ultra‐fast singlet‐triplet equilibrated fluorescence with sub‐microsecond decay. First OLEDs fabricated show high external quantum efficiency of ≈19%. Based on this breakthrough material class, a new exciton harvesting mechanism, the direct singlet harvesting (DSH), is presented.