Halogen-Regulated Luminescence in Heteroleptic Cu(I) Complexes with Pyrazine and Xantphos: Synthesis, Structure, and TADF Properties

Cu(I) complexes have attracted considerable attention due to their abundance, low cost, and distinctive photophysical properties, making them promising candidates for thermally activated delayed fluorescence (TADF) applications. We synthesized two mononuclear Cu(I) halide complexes using 2-ethylpyrazine and the diphosphine 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos). At room temperature, Complex 1 exhibits yellow emission at 575 nm with a photoluminescence quantum yield (PLQY) of 68%, while Complex 2 exhibits orange-red emission at 612 nm with a PLQY of 28%. Temperature-dependent luminescence measurements combined with decay lifetimes demonstrate that both complexes exhibit TADF characteristics with small singlet–triplet energy gaps (ΔEST) of 0.09 eV for 1 and 0.07 eV for 2. Theoretical calculations reveal that the emission primarily originates from mixed metal-to-ligand (MLCT), halide-to-ligand (XLCT), and ligand-to-ligand (LLCT) charge-transfer transitions. The stronger heavy-atom effect of iodine in 1 enhances spin–orbit coupling (SOC), promoting reverse intersystem crossing (RISC), while its more rigid coordination environment suppresses nonradiative decay. Conversely, the red-shifted emission of 2 increases nonradiative decay via the energy-gap law. These findings highlight that TADF efficiency is governed by the balance between SOC-facilitated RISC and nonradiative decay suppression, rather than ΔEST alone, providing insights for designing high-performance, low-cost luminescent materials.