title: Making the golden connection: reversible mechanochemical and vapochemical switching of luminescence from bimetallic gold-silver clusters associated through aurophilic interactions.
abstract: Aiming at the development of new architectures within the context of the quest for strongly luminescent materials with tunable emission, we utilized the propensity of the robust bimetallic clusters [Au₂Ag₂(R(I)/R(II))₄] (R(I) = 4-C₆F₄I, R(II) = 2-C₆F₄I) for self-assembly through aurophilic interactions. With a de novo approach that combines the coordination and halogen-bonding potential of aromatic heteroperhalogenated ligands, we have generated a family of remarkably luminescent bimetallic materials that provide grounds to address the relevance, relative effects, and synergistic action of the two interactions in the underlying photophysics. By polymerizing the green-emitting (λ(max)(em) = 540 nm) monomer [Au₂Ag₂R(II)₄(tfa)₂]²⁻ (tfa = trifluoroacetate) to a red-emitting (λ(max)(em) = 660 nm) polymer [Au₂Ag₂R(II)₄(MeCN)₂](n), we demonstrate herein that the degree of cluster association in these materials can be effectively and reversibly switched simply by applying mechanochemical and/or vapochemical stimuli in the solid state as well as by solvatochemistry in solution, the reactions being coincident with a dramatic switching of the intense, readily perceptible photoluminescence. We demonstrate that the key event in the related equilibrium is the evolution of a metastable yellow emitter (λ(max)(em) = 580 nm) for which the structure determination in the case of the ligand R(II) revealed a dimeric nonsolvated topology [Au₂Ag₂R(II)₄]₂. Taken together, these results reveal a two-stage scenario for the aurophilic-driven self-assembly of the bimetallic clusters [Au₂Ag₂(R(I)/R(II))₄]: (1) initial association of the green-emitting monomers to form metastable yellow-emitting dimers and desolvation followed by (2) resolvation of the dimers and their self-assembly to form a red-emitting linear architecture with delocalized frontier orbitals and a reduced energy gap. The green emission from [Au₂Ag₂R(II)₄(tfa)₂]²⁻ (λ(max)(em) = 540 nm) exceeds the highest energy observed for [Au₂Ag₂]-based structures to date, thereby expanding the spectral slice for emission from related structures beyond 140 nm, from the green region to the deep-red region.
authors.affiliation: Departamento de Química, Universidad de la Rioja, Grupo de Síntesis Química de La Rioja, UA-CSIC Complejo Científico Tecnológico, E-26001 Logroño, Spain.
authors.forename: Tania
authors.initials: T
authors.lastname: Lasanta
authors.forename: M Elena
authors.initials: ME
authors.lastname: Olmos
authors.forename: Antonio
authors.initials: A
authors.lastname: Laguna
authors.forename: José M
authors.initials: JM
authors.lastname: López-de-Luzuriaga
authors.forename: Panče
authors.initials: P
authors.lastname: Naumov