Exploring the Impact of Carbazole Position on Thermally\nActivated Delayed Fluorescence and Room-Temperature Phosphorescence\nProperties in Phthalimide–Carbazole Conjugates: A Density-Functional\nTheory Study

Aromatic\nimide-based thermally activated delayed fluorescence (TADF)\nemitters have become increasingly popular due to their unique properties\nsuch as rigid structures, pronounced thermal stability, strong electron-withdrawing\nability, and exceptional photoluminescence characteristics. In this\nwork, the phthalimide unit is integrated with the carbazole donor\nin different molecular designs (D-π-A, π-A-D, π-A-D<sub>2</sub>, and D-π-A-D) and their TADF and room-temperature phosphorescence\n(RTP) properties are theoretically investigated. Descriptors such\nas the energy gap between singlet and triplet excited states (Δ<i>E</i><sub>ST</sub>), spin orbit coupling (SOC), charge transfer\n(CT) indices, and rate constants of excited state processes were analyzed\nto study the nature of the excited state. The D-π-A and D-π-A-D\nmolecules possess small Δ<i>E</i><sub>ST</sub>, CT-dominated\nS<sub>1</sub> and T<sub>1</sub> states, relatively low SOC values,\nand high radiative and reverse intersystem crossing (RISC) rates,\nwhile the π-A-D and π-A-D<sub>2</sub> molecules with direct\nsubstitution of carbazole on phthalimide core showed large Δ<i>E</i><sub>ST</sub>, CT-dominated S<sub>1</sub> state, hybridized\nand local charge transfer T<sub>1</sub> state, large SOC, high intersystem\ncrossing, and low RISC rates. Careful analysis of energy-level diagram\nand hole–electron distribution revealed the role of the closely\nlying T<sub>2</sub> state with the S<sub>1</sub> state in fast up-conversion\nof triplet excitons through a multichannel RISC process in π-A-D<sub>2</sub>- and D-π-A-D-based molecules. Overall, the D-π-A\nand D-π-A-D molecules possess parameters indicative of TADF\nemission, and the π-A-D and π-A-D<sub>2</sub> molecules\ninherit parameters facilitating RTP emission. Additionally, the π-A-D<sub>2</sub> molecules can display TADF emission; on the contrary, the\nπ-A-D molecules display poor TADF propensity.