Theoretical insights into the thermally activated delayed fluorescence of pyridazine derivatives: balancing twist and charge-transfer character.

In this work, a systematic theoretical study of 4,5,6-triphenylpyridazine-3-carbonitrile-based (TPPz-based) donor-acceptor molecules reveals that donor steric bulk critically controls molecular planarity and excited-state properties. The moderately bulky electron-donating 4-(bis(4-(tert-butyl)phenyl)amino)phenyl (BPA) moiety induces the largest overall molecular distortion and yields a nearly pure CT S1 state together with an HLCT T1 state. In contrast, the rigid pyrenyl donor in 4,6-diphenyl-5-(4-(pyren-1-yl)phenyl)pyridazine-3-carbonitrile (Pyr-TPPz) gives a CT S1 state but a locally excited T1 state. 5-(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)-4,6-diphenylpyridazine-3-carbonitrile (BPA-TPPz) exhibits the lowest reorganization energy (2513 cm-1), efficiently suppressing nonradiative decay, and shows the highest k risc among the series. TPPz and Pyd-TPPz exhibit moderate RISC, whereas Pyr-TPPz shows a very low k_RISC within the S1-T1 two-state approximation because of its large S1-T1 gap, despite its LE T1 character. These results establish that optimal thermally activated delayed fluorescence performance requires a delicate balance between a twisted architecture for exciton management, suitable donor-acceptor electronic structures to modulate charge-transfer/locally excited (CT/LE) hybridization, and flexible donors to minimize reorganization energy. These findings provide clear structure-property guidelines for the design of efficient thermally activated delayed fluorescence (TADF) materials.