Two thioxanthone-derived isomeric series of thermally activated delayed fluorescence (TADF) emitters 1,6-2TPA-TX/3,6-2TPA-TX and 1,6-2TPA-TXO/3,6-2TPA-TXO are developed for organic light-emitting diodes (OLEDs). Blue emission devices based on symmetrical 3,6-2TPA-TX with common vertical transition route realize an extremely high external quantum efficiency (EQE) of 23.7%, and an ever highest EQE of 24.3% is achieved for yellow emission devices based on 3,6-2TPA-TXO by solely changing the sulfur atom valence state of the thioxanthone core. In contrast, their corresponding asymmetric isomers are affected by intramolecular energy transfer and more severely by a nonradiative deactivation pathway, to give much low EQE values (<5%). By utilizing 3,6-2TPA-TX as a blue emitter and 3,6-2TPA-TXO as a yellow emitter, an ever highest EQE of 20.4% is achieved for all-fluorescence white OLEDs.