Herein, we propose a series of thermally activated delayed fluorescent (TADF) materials with a fast spin-flip
system. In general, TADF emitters take a relatively long time-scale for delayed fluorescence (DF) enabled by
spin-conversion from the triplet state manifolds, although TADF is the effective route to harvest triplet excitons,
leading to the internal quantum efficiency of OLEDs as high as unity. From this intractable trade-off relation, one
further needs to suppress or at least mitigate the OLED device efficiency roll-off at a high luminance level. To
cope with this issue, we need to concentrate on the system showing a very fast inter-convertible spin-flip process
between intersystem crossing (ISC) and reverse ISC (rISC) and, moreover, its implication on OLEDs device
performance. In this context, we discuss the behavior of characterized TADF materials showing the rate constant
of RISC over 108 s-1 by the introduction of internal heavy atom effect. Furthermore, the spectroscopic and
theoretical results in this study unravels which factors should be addressed with a high priority to provide an
effective means to mitigate the efficiency roll-off behavior.