Bulky (Imino)phosphide and Mixed Bis-dimethyl-siloxane /(Imino)phosphide-supported Redox-Active Cu5 and Cu6 Nanoclusters: Luminescence and Photocatalytic Studies

Abstract

Herein, we depict isolation of neutral air-stable atomically precise Cu5 [((R2-cAI)P(Dipp))3Cu5Cl2] (cAI = cyclic alkyl(imino); Dipp = 2,6-diisopropylphenyl; R2 = Cy (2a) and Et2 (2b)) and Cu6 [((Et2-cAI)P(Dipp))2Cu6(OSiMe2OSiMe2O)2] (3) nanoclusters, stabilized by bulky (imino)phosphide and mixed bis-dimethyl-siloxane/(imino)phosphide ligands. 2a–2b and 3 are structurally characterized by X-ray single-crystal diffraction and studied by XPS, FT-IR, and NMR spectroscopy. 2a–2b exhibit yellow emission at ambient temperature with photoluminescence quantum yields (ϕPL) up to 11% and significantly high average lifetimes (τav = 38–110 μs). At 80 K, 2a–2b exhibit red emission with 5-fold to ∼80-fold increased τav values (191 μs–8.32 ms). Time-dependent density functional theory calculations, along with natural transition orbital analyses, reveal that the singlet–triplet energy gap, ΔE(S1–T1) for 2a–2b lies in the order of ∼0.2 eV, which is comparable with the experimentally obtained value of 0.16 eV, suggesting thermally activated delayed fluorescence (TADF). The spin–orbit coupling matrix (SOCM) calculations between different excited singlet and triplet states reveal significantly larger values (∼10–14 cm–1), which, along with lower ΔE(S1–T1) values, possibly facilitate an efficient spin-flipping process, enabling TADF. 2a is fabricated to a converted-LED and utilized as an excellent photocatalyst for regioselective [2 + 3] cycloaddition of aromatic alkynes and azides in the presence of a white LED, affording substituted triazoles (6) in good yields.