Chemical and conformational control of the energy gaps involved in the thermally activated delayed fluorescence mechanism

This review summarises the significant developments in our understanding and control of thermally-activated delayed fluorescence (TADF) molecules and the spin-vibronic coupling mechanism, from which we have designed new generations of emitters. It covers both the theoretical and experimental characterization of the physical and chemical aspects of model TADF emitters. We focus on how to correctly obtain the singlet-triplet energy gaps (ΔE_ST) that must be overcome by the triplet excited states in the reverse intersystem crossing (rISC) process, highlighting the differences between: the ΔE_ST estimated from the energy difference between the fluorescence and phosphorescence (¹CT-³LE gap); and the activation energy (E_a) estimated from the Arrhenius plot (¹CT-³CT gap). The discussion considers the different external factors and design principles that can influence these energy gaps and ultimately the device performance.