This work presents an investigation of three xanthone derivatives as the emitters for organic light-emitting diode applications. Through extensive photophysical and electrochemical studies, the thermally activated delayed fluorescence effect was observed and further applied in devices. Due to the substantial ΔEST gap, the final efficiency of the OLED was below 5%. The drop in the charge transfer energy was observed with electrochemical dimerisation of the donor molecule and increased molecule conjugation. The synthesised polymers exhibited an efficient thermally activated delayed fluorescence process through the shift of the singlet excited state energy level and decreased the ΔEST from 0.42 eV for the XNAP monomer material down to 0.12 eV for the polymerised derivative and final increase of the OLED device efficiency. Thus, the study presents the possibility of synthesising and tailoring many thermally activated delayed fluorescence emitters through electropolymerisation and applying this technique in working light-emitting devices.