For DMAC-TRZ, the rigid (Figure ) and the relaxed scans (Figure ) lead to the same picture: the S1 state maintains the same orthogonal conformation as the ground state, while the T1 state is stabilized and adopts a twisted structure (dihedral angle: ~60°). Full optimisations of S1 and T1 confirm this result. In the orthogonal geometry, the S1 state is an almost pure CT state and, hence, has a negligible oscillator strength. The scenario is much more interesting for DMAC-py-TRZ. In the bent geometry (the energy minimum, Figure ), the vertical excitation energy to S1 amounts to 4.1 eV and the DEST is 0.78 eV, which is far too large for TADF to be operational at ambient temperature. Moreover, the oscillator strength for the S0→S1 is large (1.27, Figure ) in this geometry due to the significant overlap of the orbitals involved in the transition (Figure ). However, the bent geometry is not the equilibrium geometry for S1 (Figure ) and a huge structural deformation is predicted in the S1 state from the bent to the orthogonal structure. In other terms, in DMAC-py-TRZ the absorption occurs from the bent geometry and the lowest energy transition is both high in energy and has a large oscillator strength. By contrast, fluorescence occurs from the orthogonal structure at a much lower energy (3.17 eV) and with negligible oscillator strength (as per the non-overlapping orbitals, in Figure ). In this orthogonal geometry the DEST reduces to only 8.2 meV), making TADF possible.