We theoretically investigated the combination of D-A and D-spacer (phenyl ring)-A with an electron donating alkyl (t-butyl) group. The strategy of twisting the geometry of the molecule with the alkyl substituents exclusion of strong electron-withdrawing or -donating groups leads to gain efficient deep blue-to-blue thermally activated delayed fluorescence (TADF) emitter through maintaining the band gap while the reduction of singlet-triplet energy gap ([Formula: see text]. The t-butyl group strongly twisted the conformation of molecules by the steric hindrance, which resulted in weak highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) overlap ([Formula: see text] and efficient spatial separation of HOMO and LUMO ([Formula: see text] in the [Formula: see text] state. In contrast, designed molecules [Formula: see text] state own large HOMO and LUMO overlap of excited singlet state ([Formula: see text] and inefficient spatial separation of HOMO and LUMO. The computed results indicated that introducing alkyl group into the phenyl ring of the acceptor of the designed molecules cannot affect the [Formula: see text]. The [Formula: see text] is mainly related to the [Formula: see text], which can be adjusted by tuning the orbital [Formula: see text]. The large modular orbital overlap at [Formula: see text] and [Formula: see text] excited states resulted in large [Formula: see text], which occurs in the range of 0.38–0.59[Formula: see text]eV whose dominant contribution switches from charge transfer to local excitation. Our studied results reiterate (10.1038/srep10923) that modular orbital overlap of [Formula: see text], HOMO and LUMO overlap of excited triplet state [Formula: see text], spatial separation of HOMO and LUMO in the excited singlet state ([Formula: see text], and spatial separation of HOMO and LUMO in the excited triplet state ([Formula: see text] are the essential factors to determine [Formula: see text] when inconsistencies between [Formula: see text] and [Formula: see text] exist. Increasing the dihedral angle between [Formula: see text] and [Formula: see text] from molecules 1–4 (9–12) decreases the transition dipole moment, which lowers the oscillator strength. When changing the connection position between [Formula: see text] and [Formula: see text], molecules 5–8, the oscillator strength reduced to half with respect to molecules 1–4 and 9–12. The present work provides a theoretical understanding of the impact of alkyl substituents on the overlap of HOMO–LUMO resulting to tuning the [Formula: see text], as well as its influence on the oscillator strength, which may be a reliable idea to design efficient TADF emitters.