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 ΔE(S1–T1) and spin-orbit coupling (SOC). We disclose relationship between (i) ΔE(S1–T1) and transition rate k(S1–S0); (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 (>109 s−1) combined with ΔE(S1–T1)<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.