A nearly perfect icosahedral Ir@Au₁₂ superatom with superior photoluminescence obtained by ligand engineering

Heterometal doping and the introduction of surface ligands drastically alter the optical and photophysical properties of gold-based superatoms by modulating their electronic structures and the excited state dynamics. In this study, we investigate how the structures and the optical properties of an Ir@Au₁₂ superatom capped by a diphosphine ligand, bis[benzo[b]phosphindole]ethane (bbpe), in which the rotation of the phenyl groups is prohibited, differ from those capped by the conventional diphosphine ligands, such as 1,2-bis(diphenylphosphino)ethane (dppe) and bis(diphenylphosphino)methane (dppm). The co-reduction of Ir(iii)- and Au(i)-precursors under mild reaction conditions yielded homoleptically capped [IrAu₁₂(bbpe)₆]³⁺ clusters (IrAu₁₂-b) as the primary product. Single crystal X-ray diffraction analysis of IrAu₁₂-b revealed the formation of a nearly perfect icosahedral Ir@Au₁₂ superatomic core, in which the central Ir atom is equidistant from each vertex Au atom. The energy gap between occupied 1P and unoccupied 1D superatomic orbitals of IrAu₁₂-b was larger than that of its dppm-capped counterpart, [IrAu₁₂(dppm)₆]³⁺ as evidenced by a blue shift (140 nm) of the photoluminescence (PL) wavelength and DFT calculations. IrAu₁₂-b exhibited PL at 596 nm with a high quantum yield of 87% in deaerated CH₂Cl₂ due to the expanded 1P-1D energy gap and the restricted molecular motions of the bbpe ligands.