Influence of donor and acceptor configurations and quantities of pyridine-3,5-dicarbonitrile–carbazole derivatives on TADF: DFT study

Thermally Activated Delayed Fluorescence (TADF) has emerged as the third-generation emitter in OLEDs, enabling efficient harvesting of both singlet and triplet excitons. In this study, a breakthrough TADF emitter, 2,6-di(9H-carbazol-9-yl)-4-phenylpyridine-3,5-dicarbonitrile (cb0), featuring 2D-A-π configuration, was modified to design fifteen new emitters. These designed emitters exhibited three distinct configurations: D-A-π, D-A-π-A (rarely investigated), and D-π-A, with variations in the number of donors (1–2) and acceptors (1–3). Key parameters including electronic and geometrical properties, absorption and emission spectra, fluorescence, intersystem crossing (ISC), and reverse intersystem crossing (RISC) rates were evaluated using DFT and TDDFT calculations. Natural Transition Orbital (NTO) analysis, encompassing hole-electron distributions and charge transfer (CT) indices within the ground state (S₀) and excited states (S₁, T₁), further characterized the emitters. The experimental data of cb0 served as a benchmark for the computational level of theory used. Notably, the unconventional D-A-π-A configuration demonstrated promising performance (cb4 and cb7) when employing an appropriate balance of donors (1–2) and acceptors (2). Increasing the number of acceptors in each configuration was found to be less beneficial, as it decreased the H–L special distribution and the singlet–triplet energy gap ( $${\Delta \text{E}}_{\text{ST}}$$ ). TADF behavior was shown to be controlled not only by H–L orbitals separation since some emitters, cb2, cb3, cb7, and cb12, showed reduction in $${\Delta \text{E}}_{\text{ST}}$$ while their H–L separations are small. The planarity, as assessed by MPP and dihedral angles, was used to evaluate the distribution of H–L orbitals. The analysis revealed that the S₀ states are generally more planar compared to their corresponding excited states. Additionally, RMSD values were employed to quantify the molecular distortion among the S₀, S₁, and T₁ states, providing insights into the trend observed in their emission data. Several of the designed emitters (cb1-4, cb7, cb10-12) showed similar/improved properties compared to cb0, including favorable H–L orbitals separation, good $${\Delta \text{E}}_{\text{ST}}$$ , $${\uplambda }_{\text{in}}^{\text{RISC}}$$ , and hybridized CT/LE transition nature as confirmed by the CT indices. These findings indicate that fine-tuning the molecular configurations and donor–acceptor ratios is key to designing more efficient TADF emitters and merits further investigation. • H–L and S-T energy gaps of cb0 (R), the experimentally tested emitter, benchmarking the level of theory used. • Some emitters with rarely investigated D-A-π-A configuration showed smaller $${\Delta \text{E}}_{\text{ST}}$$ , and enhanced properties. • In all configurations, donors should be equivalent or exceeded acceptors for better properties. • S1 and T1 exhibited hybridized CT and LE excitation nature from So state. • Emission covered full color from visible (blue, deep blue, orange, red) to NIR regions was obtained.