Cu@Core in the Cu₃C₃ Ring with 2-bdppmapy: A High-Quantum-Yield Planar Hexagonal Cu(I) Metallacycle

Despite the potential of Cu(I) complexes in optoelectronics, their practical application is hindered by their susceptibility to oxidation and low photoluminescence quantum yields (PLQYs). While phosphine-alkynyl coligation strategies have improved stability, structural distortions in conventional open-cube Cu₄ clusters limit PLQYs to an unsatisfactory level. Herein, we address this challenge via ligand engineering with a bidentate diphosphine (2-bdppmapy = N,N-bis((diphenylphosphino)methyl)-2-pyridinamine). This design enables the synthesis of planar tetranuclear clusters [(2-bdppmapy)₃Cu₄(μ₃-PhC≡C)₃]X (X^- = I^-, Br^-, ClO₄^-, BF₄^-, Cl^-) featuring a near-perfect coplanar Cu₃C₃@Cu hexagonal ring. Single-crystal X-ray diffraction reveals exceptional planarity (mean deviation: 0.025 Å) and strong cuprophilic interactions (Cu···Cu: 2.387-2.441 Å). These complexes achieve an unprecedented solid-state PLQY of up to 93% with turquoise emission, attributed to their rigid planar architecture, which maximizes electronic delocalization. Spectroscopic analysis reveals a structured emission band with a maximum at 483 nm and a pronounced shoulder at 526 nm, which arise from ν(C═C) and ν(C≡C) stretching vibrations localized on the phenylacetylene ligands. Notably, the emissions exhibit microsecond-scale lifetimes (τ ≈ 22 μs), indicative of long-lived triplet phosphorescence. Theoretical calculations further confirm that the vibronically structured emission originates from T₁ triplet excited states derived from hybrid metal-to-ligand charge transfer and intraligand charge transfer transitions within the phenylacetylene framework.