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

Solutions of K[Au(CN)(2)] and K[Ag(CN)(2)] in water and methanol exhibit st rong photoluminescence. Aqueous solutions of K[Au(CN)(2)] at ambient temper ature exhibit luminescence at concentration levels of greater than or equal to 10(-2) M, while frozen methanol glasses (77 K) exhibit strong luminesce nce 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 10-1 M at 77 K. Systematic variations in concentration, solvent, temperatur e, and excitation wavelength tune the luminescence energy of both K[Au(CN)( 2)] and K[Ag(CN)(2)] solutions by > 15 x 10(3) cm(-1) in the UV-visible reg ion. The luminescence bands have been individually assigned to *[Au(CN)(2)( -)](n) and *[Ag(CN)(2)(-)](n) excimers and exciplexes that differ in "n " a nd geometry. The luminescence of Au(I) compounds is related for the first t ime to Au-Au bonded excimers and exciplexes similar to those reported earli er for Ag(I) compounds. Fully optimized unrestricted open-shell MP2 calcula tions for the lowest-energy triplet excited state of staggered [Au(CN)(2)(- )](2) show the formation of a Au-Au sigma single bond (2.66 Angstrom) in th e triplet excimer, compared to a weaker ground-state aurophilic bond (2.96 Angstrom). The corresponding frequency calculations revealed Au-Au Raman-ac tive stretching frequencies at 89.8 and 165.7 cm(-1) associated with the gr ound state and lowest triplet excited state. respectively. The experimental evidence of the exciplex assignment includes the extremely large Stokes sh ifts and the structureless feature of the luminescence bands, which suggest very distorted excited states. Extended Huckel (EH) calculations for [M(CN )(2)(-)](n) and *[M(CN)(2)(-)](n) models (M = An, A: n = 2, 3) indicate the formation of M-M bonds in the first excited electronic states. From the av erage EH values for staggered dimers and trimers, the excited-state Au-Au a nd 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.