Exploring the Impact of Carbazole Position on Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence Properties in Phthalimide–Carbazole Conjugates: A Density-Functional Theory Study

Aromatic imide-based thermally activated delayed fluorescence (TADF) emitters have become increasingly popular due to their unique properties such as rigid structures, pronounced thermal stability, strong electron-withdrawing ability, and exceptional photoluminescence characteristics. In this work, the phthalimide unit is integrated with the carbazole donor in different molecular designs (D-π-A, π-A-D, π-A-D₂, and D-π-A-D) and their TADF and room-temperature phosphorescence (RTP) properties are theoretically investigated. Descriptors such as the energy gap between singlet and triplet excited states (ΔE_ST), spin orbit coupling (SOC), charge transfer (CT) indices, and rate constants of excited state processes were analyzed to study the nature of the excited state. The D-π-A and D-π-A-D molecules possess small ΔE_ST, CT-dominated S₁ and T₁ states, relatively low SOC values, and high radiative and reverse intersystem crossing (RISC) rates, while the π-A-D and π-A-D₂ molecules with direct substitution of carbazole on phthalimide core showed large ΔE_ST, CT-dominated S₁ state, hybridized and local charge transfer T₁ state, large SOC, high intersystem crossing, and low RISC rates. Careful analysis of energy-level diagram and hole–electron distribution revealed the role of the closely lying T₂ state with the S₁ state in fast up-conversion of triplet excitons through a multichannel RISC process in π-A-D₂- and D-π-A-D-based molecules. Overall, the D-π-A and D-π-A-D molecules possess parameters indicative of TADF emission, and the π-A-D and π-A-D₂ molecules inherit parameters facilitating RTP emission. Additionally, the π-A-D₂ molecules can display TADF emission; on the contrary, the π-A-D molecules display poor TADF propensity.