Investigating the Mechanism of Thermally Activated Delayed Fluorescence Induced by Different Conformations of a Two-Coordinate Copper(I) Complex.

The investigation of thermally activated delayed fluorescence (TADF) in carbene-metal-amide (CMA) complexes reveals that structurally flexible ligands can demonstrate significant TADF characteristics. A series of four conformationally flexible copper(I) complexes featuring cyclic alkyl amino carbene (CAAC) and 10H-phenothiazine-5,5-dioxide (Ptz) ligands were systematically studied using quantum chemical calculations. The study examines the impact of two key structural parameters, namely torsion angle φ (CAAC-Cu(I)-Ptz-1: perpendicular; CAAC-Cu(I)-Ptz-2 and CAAC-Cu(I)-Ptz-3: coplanar; CAAC-Cu(I)-Ptz-4: anticoplanar) and tilt angle β (near-planar arrangement: CAAC-Cu(I)-Ptz-3 > CAAC-Cu(I)-Ptz-2), on the photophysical properties of conformations. Geometrical and excited-state analyses indicate that CAAC-Cu(I)-Ptz-1, CAAC-Cu(I)-Ptz-2, and CAAC-Cu(I)-Ptz-4 conformations are observed to have similar excited-state transitions, but their torsion angles critically influence energy gap variations. Specifically, the energy differences between excited states of CAAC-Cu(I)-Ptz-2 and CAAC-Cu(I)-Ptz-3 are 0.10 and 0.20 eV, respectively, due to distinct torsion spans. Notably, the increased tilt angle β in CAAC-Cu(I)-Ptz-3 diminishes the contribution of Cu, with the lowest triplet state (T1) displaying 47% ligand-to-ligand charge transfer (LLCT) and 28% metal-to-ligand charge transfer (MLCT) character. The combined minimum energy crossing point (MECP) and spin-orbit coupling (SOC) analyses elucidate the effect on reverse intersystem crossing (RISC) rates: CAAC-Cu(I)-Ptz-3 (0.41 eV MECP barrier) and CAAC-Cu(I)-Ptz-1 (SOC = 4.35 cm-1) are confirmed to suppress RISC rates (1.09 × 104 and 1.09 × 108 s-1, respectively), while enhanced RISC rates are achieved through reduced MECP barriers and strengthened SOC interactions by CAAC-Cu(I)-Ptz-2 and CAAC-Cu(I)-Ptz-4 (1.68 × 109 and 3.87 × 109 s-1, respectively). These findings highlight the vital role of ligand flexibility in the photophysical properties of CMA emitters, offering valuable guidance for designing efficient TADF materials.