Thermally activated delayed fluorescence (TADF) emitters have drawn recent interest due to their superior capability of boosting the efficiency of organic light emitting diodes (OLEDs). Developing efficient donor blocks has huge significance in assembling practical emitters. In this contribution, we report three TADF compounds, namely p-D2T, m-D2T and c-D2T, based on a parallelly aligned bi-spiro-acridine donor. Triazine is chosen as an acceptor to perform effective charge transfer with one acridine, while carbazole with para- and meta-substitution patterns is introduced in the alternative acridine to comprehend the structure–property relationship. Excellent thermal stability is achieved due to the bulky and rigid structure of bi-spiro configuration. Photophysical measurement suggests that these bi-spiro based emitters exhibit high photoluminescence quantum yield (PLQY) and decent TADF behaviors. Both para- and meta-decorations endow these emitters with accelerated radiative decay leading to prompt fluorescence dominated delayed TADF characteristics. In addition, the para-pattern shows better capability to attain a high PLQY in contrast to meta-substitution. Consequently, all emitters perform well in their electroluminescent devices with a maximum external quantum efficiency of 25.6% for p-D2T, 22.3% for m-D2T and 26.3% for c-D2T, respectively. These results show the potential of a parallelly aligned bi-spiro donor for constructing efficient TADF emitters, and also provide a strategy to enhance the radiative decay which may offer guidance to develop efficient TADF compounds.