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 (ΔEST), 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 ΔEST, 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 ΔEST, 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.