Observation of a Mixed-Metal Transition in Heterobimetallic Au/Ag Dicyanide Systems

Crystals of the mixed-metal heterobimetallic Au/Ag dicyanide complex, K[AuxAg1-x(CN)2] (x = 0→1), were obtained
by slow evaporation. The mixed-metal complex K[Au0.44Ag0.56(CN)2] crystallizes in a rhombohedral crystal system,
space group R3̄. The crystal structure consists of layers of linear chains of Au(CN)2- and Ag(CN)2- ions and K+
ions that connect the layers through the N atoms. The excitation and emission spectra of single crystals of
K[AuxAg1-x(CN)2] were recorded at 4.2−180 K using excitation wavelengths between 230 and 260 nm. Two emission
bands due to Ag−Au interactions were observed at 343 and 372 nm. Lifetime measurements indicate the shorter-wavelength emission corresponds to fluorescence and the longer-wavelength band is phosphorescence. These
new emission bands are not seen in the pure K[Ag(CN)2] or pure K[Au(CN)2] crystals. Extended Hückel calculations
show that the LUMO of the mixed-metal system is bonding while the HOMO is antibonding or very weakly bonding.
Moreover, excited-state extended Hückel calculations indicate the formation of exciplexes with shorter metal−metal
distances and higher metal−metal overlap populations than the corresponding ground-state oligomers. The
luminescence is assigned to a mixed-metal transition from a molecular orbital with Au character to a molecular
orbital with Ag character.