After 30 years since the first organic light emitting diode (OLED) was reported by Tang and VanSlyke, devices based on thermally activated delayed fluorescence (TADF) emitters have shown to be the most promising and efficient approach to convert dark triplet states into emissive singlet states. The TADF mechanism relies on thermal energy to raise the triplet state to a vibronic sub level that is isoenergetic with the singlet state, thus enabling reverse intersystem crossing (rISC) and allowing internal quantum efficiency values up to 100%. Major challenges faced by TADF studies persist, concerning the full understanding of the mechanism, as it is strongly affected by the environment in which the emitter is dispersed and the different conformations that the molecules can access. Throughout the course of this thesis, the photophysical and chemical properties of the TADF mechanism were investigated in various organic molecules including novel D-A-D and D-A3 molecules, bimolecular (exciplex) blends and excited state intramolecular proton transfer (ESIPT). Important new contributions towards the full elucidation of the TADF mechanism are presented, mainly regarding the current TADF vibronic coupling mechanism model, which highlights the need for three excited states (singlet charge transfer state; triplet charge transfer states; and triplet local excited state) to come into resonance to achieve high TADF efficiency. In addition, a solution to the dilemma that a TADF emitter cannot have both unity photoluminescence quantum yield and fast rISC rates is presented and high efficient OLEDs are shown. Moreover, it is discussed how different molecular conformers affect the efficiency of the TADF mechanism by studying molecules that show dual charge transfer emission. Furthermore, it is shown that the emitter and host combination must be optimized to minimize the rISC barrier and maximize the TADF in blue OLEDs. Additionally, the use of ESIPT emitters to generate TADF is discussed
