Insights into Molecular Structures and Optical Properties of Stacked [Au₃(RN═CR')₃]_n Complexes

The molecular structure of stacked cyclic trinuclear gold(I) complexes [Au₃(RN═CR')₃]_n, with n = 1-4, where R = H, methyl (Me), cyclopentyl (^cPe), and phenyl (Ph) and R' = OH and methoxy (OMe) were studied computationally at the second-order Møller-Plesset (MP2) and density functional theory (DFT) levels of theory. At the DFT level, the aurophilic and dispersion interactions were accounted for by using the TPSS functional in combination with the semiempirical D3 correction. The structure optimizations yielded the lowest energy for a slided stacked structure of the [Au₃(HN═COH)₃]₂ dimer, where monomers are slightly shifted relative to one another. At the MP2 level, the slided structure is 32 kJ/mol more stable than the staggered dimer structure, which in turn is energetically 11 kJ/mol below the eclipsed structure. The calculations show that aromatic ligands lead to a planar and prismatic structure of [Au₃(PhN═COMe)₃]₄, whereas for [Au₃(^cPeN═COMe)₃]₄, a chair conformation is obtained due to steric effects. Excitation energies were calculated for [Au₃(RN═CR')₃] and [Au₃(RN═CR')₃]₂ with R = H, Me, and ^cPe and R' = OH and OMe at the time-dependent DFT level using the optimized molecular structures of the singlet ground state. To simulate the luminescence spectra, the lowest triplet excitation energy was also calculated for the molecular structure of the lowest triplet state. The calculated excitation energies of [Au₃(HN═COH)₃] and [Au₃(HN═COH)₃]₂ are compared with values obtained at the approximate singles and doubles coupled cluster (CC2) and the second-order algebraic diagrammatic construction (ADC(2)) levels of theory. The calculated absorption and emission energies reproduce the experimental trends, with extremely large Stokes shifts. A solvoluminescence mechanism is also proposed.