Temperature-Dependent Photoluminescence Properties of Tl[Ag(CN)₂]:  Formation of Luminescent Metal−Metal-Bonded Inorganic Exciplexes in the Solid State

The photoluminescence behavior of single crystals of Tl[Ag(CN)₂] has been studied as a function of temperature. The 10 K spectra show a broad emission centered at about 420 nm and two excitation maxima at 301 and 314 nm. Exciting with each of these maxima gives a different temperature dependence for the corresponding emission spectra. This result correlates with the crystal structure of the compound which indicates the presence of two environments with Ag−Ag interactions. The fact that the emission band is largely red-shifted, broad, and structureless is consistent with exciplex emission. The results of extended Hückel and ab initio calculations indicate exciplex formation in the title compound. Our theoretical calculations show a deep potential well in the excited state with a Ag−Ag equilibrium distance of 2.45 Å and a binding energy of 40.8 kcal/mol. The experimental and theoretical results in this study demonstrate the importance of excited-state Ag−Ag interactions leading to the formation of luminescent exciplexes in Tl[Ag(CN)₂]. To our knowledge, this is the first example of exciplex formation between metal ions in the solid state for coordination compounds.