Local-Excitation versus Charge-Transfer Characters in the Triplet State: Theoretical Insight into the Singlet-Triplet Energy Differences of Carbazolyl-Phthalonitrile-Based Thermally Activated Delayed Fluorescence Materials

The singlet triplet energy differences, Delta E-ST, of a series of carbazolyl-phthalonitrile (CzPN) derivatives were calculated at the levels of density functional theory (DFT) and time-dependent (TD) DFT using the gap-tuned, range separated omega B97X functional. The studied CzPN derivatives include 4-(9H-carbazol-9-yl)phthalonitrile (CzPN), 4,5-di(9H-carbazol-9-yl)phthalonitrile (2CzPN), 3,4,5-tris(9H-carbazol-9-yl)phthalonitrile (3CzPN), and 3,4,5,6-tetra(9H-carbazol-9-yl)phthalonitrile (4CzPN). As additional Cz substituents are introduced, both the HOMO-LUMO energy gap, Delta EH-L, and Delta E-ST continuously decrease. Both natural transition orbital analysis and a quantitative assessment of the local-excitation (LE) and charge-transfer (CT) contributions to the excited states consistently demonstrate that the S-1 states of all of the CzPN derivatives have a predominantly CT nature. In contrast, in the T-1 state, the LE feature is dominant, but the CT character increases with the number of Cz groups. The decomposition of excitation energy reveals that, in addition to the spatial separation of HOMO and LUMO, a significant CT nature in the T-1 state is essential for a further reduction in Delta E-ST. Moreover, the relative proportions of LE and CT characters in the T-1 states of the CzPN derivatives can be modulated with Delta EH-L.