Chemistry and optoelectronic properties of stacked supramolecular entities of trinuclear gold(I) complexes sandwiching small organic acids.

Extended linear-chain inorganic compounds have special chemical and physical properties.1,2 This has led to new developments in fields such as supramolecular chemistry, acid-base chemistry, luminescent materials, and various optoelectronic applications. Among the recent examples are the developments of a vapochromic light-emitting diode from linear-chain Pt(II)/ Pd(II) complexes,3 a luminescent switch consisting of an Au(I) dithiocarbamate complex that possesses a luminescent linear-chain form only in the presence of vapors of organic solvents,4 mixedmetal (Ag/Au) compounds that exhibit different colors and emissions when different organic solvents are introduced or removed,5 and the discovery of a new phenomenon, solvoluminescence,6 in a trinuclear Au(I) complex whose extended-chain structure is responsible for storage of energy and release of it as long-lived orange phosphorescence upon contact with solvent. We have been studying trinuclear Au(I) compounds with aromatic-substituted imidazolate, 1, and carbeniate, 2, bridging ligands (Chart 1). These compounds are colorless and do not form extended-chain structures. However, we have recently reported that they can produce brightly colored complexes by sandwiching naked Tl+ and Ag+ ions to form linear-chain complexes with fascinating luminescence properties such as luminescence thermochromism.7 More recent results have demonstrated that the electron-rich trinuclear Au(I) complexes can interact with the neutral inorganic Lewis acid Hg3(μ-C6F4)3 to produce infinite linear-chain complexes.8 We have then focused our efforts on studying the reactivity of 1 and 2 with small organic Lewis acids and electron acceptors. Balch and co-workers have recently demonstrated that trinuclear Au(I) compounds with alkylsubstituted carbeniate bridging ligands can interact with the large organic acceptors nitro-9-fluorenones.9 It was suggested that cyano-substituted acceptors should be avoided because the cyanides may directly bind to the “exposed” linear 2-coordinate Au(I) centers.9 However, DFT calculations that we have carried out8 clearly show that the donor regions in the trinuclear Au(I) compounds are located at the center of the nine-membered ring and that they extend to regions in space above and below the ring plane, as opposed to being localized on the Au atoms. Indeed, we show here that a charge-transfer complex with a columnar structure does form when a small tetracyano-substituted organic acceptor reacts with 1. Previous attempts for the reaction with cyano-substituted organic acceptors have lead to the rupture of the trinuclear Au(I) unit and formation of mononuclear cations.10 We also show here that the liquid organic acid C6F6 forms a stacked complex with 2 and quenches its luminescence, thus suggesting a potential sensing action.