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^II₄(tfa)₂]^2– (tfa = trifluoroacetate) to a red-emitting (λ_max^em = 660 nm) polymer [Au₂Ag₂R^II₄(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^II₄]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^II₄(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.