Making the Golden Connection: Reversible Mechanochemical and Vapochemical Switching of Luminescence from Bimetallic Gold-Silver Clusters Associated through Aurophilic Interactions

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 (lambda(em)(max) = 540 nm) monomer [Au2Ag2R4II(tfa)(2)](2-) (tfa = trifluoroacetate) to a red-emitting (lambda(em)(max) = 660 nm) polymer [Au2Ag2R4II(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 (lambda(em)(max) = 580 nm) for which the structure determination in the case of the ligand RH revealed a dimeric nonsolvated topology [Au2Ag2R4II](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 [Au2Ag2R4II(tfa)(2)](2-) (lambda(em)(max) = 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. Scheme 1. Structures of the Perhalogenated Ligands RI and RH and Bonding Motifs Conjectured on the Basis of the Combined Action of Aurophilic Interactions (AIs), Metallophilic Interactions (MIs), and Halogen Bonds (XBs) [GRAPHICS] .