Excited-State Interactions for [Au(CN)2 -] n and [Ag(CN)2 -] n Oligomers in Solution. Formation of Luminescent Gold−Gold Bonded Excimers and Exciplexes

• Published in: Journal of the American Chemical Society Vol. 123; no. 45; pp. 11237 - 11247
• Main Authors: Rawashdeh-Omary, Manal A, Omary, Mohammad A, Patterson, Howard H, Fackler, John P
• Format: Journal Article
• Language: English
• Published: American Chemical Society 14-11-2001
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja011176j

Solutions of K[Au(CN)2] and K[Ag(CN)2] in water and methanol exhibit strong photoluminescence. Aqueous solutions of K[Au(CN)2] at ambient temperature exhibit luminescence at concentration levels of ≥10-2 M, while frozen methanol glasses (77 K) exhibit strong luminescence with concentrations as low as 10-5 M. The corresponding concentration limits for K[Ag(CN)2] solutions are 10-1 M at ambient temperature and 10-4 M at 77 K. Systematic variations in concentration, solvent, temperature, and excitation wavelength tune the luminescence energy of both K[Au(CN)2] and K[Ag(CN)2] solutions by >15 × 103 cm-1 in the UV−visible region. The luminescence bands have been individually assigned to *[Au(CN)2 -] n and *[Ag(CN)2 -] n excimers and exciplexes that differ in “n” and geometry. The luminescence of Au(I) compounds is related for the first time to Au−Au bonded excimers and exciplexes similar to those reported earlier for Ag(I) compounds. Fully optimized unrestricted open-shell MP2 calculations for the lowest-energy triplet excited state of staggered [Au(CN)2 -]2 show the formation of a Au−Au σ single bond (2.66 Å) in the triplet excimer, compared to a weaker ground-state aurophilic bond (2.96 Å). The corresponding frequency calculations revealed Au−Au Raman-active stretching frequencies at 89.8 and 165.7 cm-1 associated with the ground state and lowest triplet excited state, respectively. The experimental evidence of the exciplex assignment includes the extremely large Stokes shifts and the structureless feature of the luminescence bands, which suggest very distorted excited states. Extended Hückel (EH) calculations for [M(CN)2 -] n and *[M(CN)2 -] n models (M = Au, Ag; n = 2, 3) indicate the formation of M−M bonds in the first excited electronic states. From the average EH values for staggered dimers and trimers, the excited-state Au−Au and Ag−Ag bond energies are predicted to be 104 and 112 kJ/mol, respectively. The corresponding bond energies in the ground state are 32 and 25 kJ/mol, respectively.