Positive isotope effect in thermally activated delayed fluorescence emitters based on deuterium-substituted donor units

Isotope effect (IE) has been widely applied to probe the kinetics and mechanisms of organic reactions, and hole/ electron transport behaviors of semiconductors. The subtle isotope substitution on fluorophores could intrinsically influence their luminescence efficiency and the operational lifetime of organic light-emitting diodes (OLEDs). So far, the IE on the photophysical properties, thermal stabilities and electroluminescence performance of thermally activated delayed fluorescence (TADF) emitters has not been reported. In this work, two deuteriumsubstituted TADF emitters, TPAAP-D and TPAAQ-D, have been designed and synthesized to investigate the IE in TADF. The comparison study showed that the deuterium substitution led to a decrease in the non-radiative decay rate of S1 (KnrS) and a decrease in the intersystem crossing rate constant (KISC), thus enhancing the luminescence efficiency of deuterated TADF materials. The origin of this positive IE was correlated with the increase in normalmode displacement (Delta Q) and Huang-Rhys factor (Sj) of deuterated TADF. Furthermore, TPAAP-D and TPAAQ-D exhibited remarkably higher decomposition temperatures than their native counterparts. Importantly, TPAAP-D can sensitize a near infrared (NIR) dye (BF) with the electroluminescence (EL) peak at 760 nm (FWHM = 45 nm) and EQE of 2.8%, which was the best result among the reported fluorophore emitters.