Aggregation-Induced Emission and Thermally Activated Delayed Fluorescence in Sterically Encumbered Copper(I) Complexes of Diphosphine and Substituted Phenanthroline Ligands

In this work, two heteroleptic and one homoleptic copper(I) complex using bisphosphine and diimine ligands are reported. Single-crystal X-ray analysis provided insight into the solid packing due to different supramolecular interactions such as C-H···Cl, C-Cl···H, and C-Cl···π interactions. All photophysical studies, along with computational calculations, provided insight into the associated electronic transitions like MLCT, LLCT, and ILCT. Among all the complexes, <b>Cu-1</b> and <b>Cu-2</b> displayed strong luminescence in solid as well as crystalline states along with aggregation-induced emission properties, which were supported by dynamic light scattering, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. Computational calculations and Hirshfeld analyses provided insights into the origin of luminescence and the AIE properties. More importantly, a small Δ<i>E</i>_ST value of 239 to 263 meV indicated the presence of thermally activated delayed fluorescence (TADF), which was further investigated at variable temperatures (77 K-300 K). At low temperature (77 K), a red shift in emission maxima compared to room temperature (300 K) occurred following a drop in emission lifetime (14.89-31.45 μs), followed by a high radiative rate constant (2.92 × 10⁴ - 4.5 × 10⁴ s^-1), which gives a clear indication of their potential application in OLED fabrication and solid-state lasers.