New Luminescent Ag₂Au₂ Heteronuclear Alkynyl–Phosphine Complexes and Recognition of Homocysteine and Cysteine

The four heteronuclear clusters [Ag2Au2(μ-dpppy)3(CCC6H4R-4)2]­(ClO4)2 (R = H (1) CH3 (2), COOCH3 (3), CHO (4)) were prepared by the self-assembly reaction between (AuCC6H4R-4)n (R = H, CH3, COOCH3, CHO) and [Ag2(μ-dpppy)3]2+ (dpppy = 2,6-bis­(diphenylphosphino)­pyridine) and characterized by elemental analyses, electrospray ionization mass spectrometry (ESI-MS), and 1H NMR and 31P­{1H} NMR spectroscopy and by X-ray crystallography for 1, 3, and 4. It is revealed that the complexes exhibit bright blue (1–3) and green (4) luminescence in the solid state and in solution with the luminescent lifetimes in the microsecond range, indicating that the luminescence is most likely associated with a spin-forbidden triplet parentage. Interestingly, only complex 4 displays distinctly mechanical grinding responsive emission switching. The recognition interactions of complex 4 containing aldehyde groups with homocysteine (Hcy) and cysteine (Cys) have been studied by UV–vis and emission titrations. A strong luminescence quench was found upon reaction of complex 4 only with Hcy or Cys, but not with other amino acids, proteins, and common anions, indicating a high specificity for recognition of Hcy and Cys.