Theoretical study of the structure, bonding and electronic spectrum of the tetrameric copper (I) amide complex [CuN(SiMe ₃) ₂] ₄

The tetrameric copper amide cluster, [CuN(SiMe ₃) ₂] ₄, has been previously shown to exhibit interesting spectral properties including a large Stokes shift in the emission spectrum. The photoluminescence has been suggested to arise from phosphorescence from an excited state involving enhanced Cu-Cu bonding. In this study the molecular structure and bonding of [CuN(SiMe ₃) ₂] ₄ are studied in the ground state and excited states using density functional theory. Density functional theory calculations are shown to accurately reproduce the geometry of the complex using both the full structure and using a model complex. Time dependent density functional theory calculations are used to simulate the electronic spectrum of the model complex and assign the observed absorption bands to specific electronic transitions. It is found that the two observed near-UV absorption bands arise from transitions between the HOMO and LUMO/LUMO+1 of the complex. Both excited states involve population of orbitals with Cu-Cu bonding character. ΔSCF optimization of the lowest lying ³E _u state indicates significant Cu-Cu bonding enhancement and short Cu-Cu distances in a Jahn-Teller distorted structure. QTAIM analysis also support enhanced Cu-Cu bonding in the excited state.