Ab Initio Studies on Metal−Metal Interaction and ³[σ*(d)σ(s)] Excited State of the Binuclear Au(I) Complexes Formed by Phosphine and/or Thioether Ligands

The structures of ground state and lowest energy triplet excited state for [Au₂(PH₂CH₂PH₂)₂]²⁺ (1), [Au₂(PH₂CH₂PH₂)(SHCH₂SH)]²⁺ (2) and [Au₂(SHCH₂SH)₂]²⁺ (3) as well as their solvated 1−3·(MeCN)₂ species are fully optimized by the MP2 and CIS methods, respectively. The ³[σ*(d)σ(s)] excited states give the 300−390 emissions in the gas phase, red shifting to 500−730 nm in acetonitrile. The coordination of solvent molecule to the gold atom in the excited states is responsible for such a red shift. For 2, all the possible geometries, the substituent effect of methyl groups on P and/or S atoms and the comparison with thiolate complex [Au₂(PH₂CH₂PH₂)(SCH₂S)] (6) are discussed. The unrestricted MP2 calculations on 1−3, head-to-tail [Au₂(PH₂CH₂SH)₂]²⁺ (7) and head-to-head [Au₂(PH₂CH₂SH)₂]²⁺ (8) confirm the CIS results in both optimized geometry and emissive energy related to the ³[σ*(d)σ(s)] state. The frequency calculations at the MP2 level indicate that the Au(I)−Au(I) interaction is weak in the ground state (ν(Au₂) = 89−101 cm^-1) but is strongly strengthened in the excited state (ν(Au₂) = 144−189 cm^-1).