IrAu12 superatom modified by chiral diphosphines: doping-induced enhancement of chiroptical activity

Abstract

Gold superatoms modified by chiral ligands are a new class of chiroptical nanomaterials, but improvement of their chiroptical properties, such as circular dichroism (CD) and circularly polarized luminescence (CPL), remains a challenge. In this work, we studied the effects of single-atom doping on the chiroptical properties of a representative Au₁₃ superatom using [Au₁₃((R,R)-DIPAMP)₅Cl₂]³⁺ and [Au₁₃((S,S)-DIPAMP)₅Cl₂]³⁺ (Au₁₃-R/S; DIPAMP = 1,2-bis[(2-methoxyphenyl)phenylphosphino]ethane). We synthesized an enantiomeric pair of superatoms: [IrAu₁₂((R,R)-DIPAMP)₅Cl₂]⁺ and [IrAu₁₂((S,S)-DIPAMP)₅Cl₂]⁺ (IrAu₁₂-R/S). Single-crystal X-ray diffraction analysis revealed that the icosahedral Ir@Au₁₂ core of IrAu₁₂-R/S was more twisted along the Cl–Au–Ir–Au–Cl axis compared with the Au₁₃ core of Au₁₃-R/S, leading to a larger absorption anisotropy factor. IrAu₁₂-R/S exhibited a much higher photoluminescence quantum yield (∼70%) compared with Au₁₃-R/S (15%) due to a larger energy gap between the highest occupied and the lowest unoccupied molecular orbitals. Although Ir doping did not appreciably enhance the photoluminescence anisotropy factors, the brightness of the CPL of IrAu₁₂-R/S was five times higher than that of Au₁₃-R/S. This work provides a rational guide for improving the chiroptical activity of Au superatoms via the doping-mediated manipulation of the geometric and electronic structures.