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

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 (<b>2a</b>) and Et₂ (<b>2b</b>)) and Cu₆ [((Et₂-cAI)P(Dipp))₂Cu₆(OSiMe₂OSiMe₂O)₂] (<b>3</b>) nanoclusters, stabilized by bulky (imino)phosphide and mixed bis-dimethyl-siloxane/(imino)phosphide ligands. <b>2a-2b</b> and <b>3</b> are structurally characterized by X-ray single-crystal diffraction and studied by XPS, FT-IR, and NMR spectroscopy. <b>2a-2b</b> exhibit yellow emission at ambient temperature with photoluminescence quantum yields (ϕ_PL) up to 11% and significantly high average lifetimes (τ_av = 38-110 μs). At 80 K, <b>2a-2b</b> exhibit red emission with 5-fold to ∼80-fold increased τ_av 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, Δ<i>E</i>(S₁-T₁) for <b>2a-2b</b> 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 Δ<i>E</i>(S₁-T₁) values, possibly facilitate an efficient spin-flipping process, enabling TADF. <b>2a</b> 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 (<b>6</b>) in good yields.