Chemical modification of phenothiazine-benzophenone derivatives tunes the emission behavior from triplet states by selecting the geometry of the intramolecular charge transfer (ICT) state.Afundamental principle of planar ICT (PICT) and twisted ICT (TICT) is demonstrated to obtain selectively either room temperature phosphorescence (RTP) or thermally activated delayed fluorescence (TADF), respectively.T imeresolved spectroscopya nd time-dependent density functional theory (TD-DFT) investigations on polymorphic single crystals demonstrate the roles of PICT and TICT states in the underlying photophysics.This has resulted in aRTP molecule OPM,w here the triplet states contribute with 89 %o ft he luminescence,a nd an isomeric TADF molecule OMP,w here the triplet states contribute with 95 %ofthe luminescence.