Structural isomerism of [Au9(PPh3)8]3+ has been studied experimentally, mostly concerning the symmetry of the Au9 core. Recently, the C 4 isomer of [Au9(PPh3)8]3+ has been shown to exist in solution phase while the D 2h isomer is present in the solid state [Inorg. Chem. 2017, 56, 8319–8325]. In this work, geometric, electronic, and optical properties of C 4 [Au9(PPh3)8]3+ are investigated by using the combined second-order density-functional tight-binding (DFTB2) method and time-dependent density functional theory (TD-DFT) calculations with spin–orbit coupling. Additionally, the excited-state relaxation dynamics of the [Au9(PPh3)8]3+ cluster in dichloromethane and methanol solutions are studied using femtosecond transient absorption spectroscopy. [Au9(PPh3)8]3+ is optically pumped to different excited states by using 432, 532, and 603 nm light. For all three pump wavelengths, the photoexcitation event induces an excited-state absorption (ESA) band centered at 600 nm with decay time constants of 2.0 and 45 ps, which are attributed to intersystem crossing and nonradiative relaxation of [Au9(PPh3)8]3+, respectively. On the other hand, optical pumping of [Au9(PPh3)8]3+ using 432 nm light gives rise to an additional ESA band at 900 nm. This band exhibits fast relaxation through internal conversion with a time constant of ∼0.3 ps. Our combined computational and experimental study reveals that the excitation wavelength-dependent relaxation dynamics of the [Au9(PPh3)8]3+ cluster are related to the different electron densities of the excited states of [Au9(PPh3)8]3+, consistent with it possessing molecular-like electronic states.

Two tris(triphenylphosphinegold)oxonium dimer salts [{{Au(PPh 3 )} 3 (m 3 ÀO)} 2 ] 2+ (X À ) 2 (X ¼ BF À 4 , MnO À 4 ) were investigated via synchrotron-based far-infrared vibrational spectroscopy and density functional theory modelled at the M06/LANL2DZ level of theory. The 50-800 cm À1 region of both oxonium salts is presented, with the spectrum for [{{Au(PPh 3 )} 3 (m 3 ÀO)} 2 ] 2+ (BF À 4 ) 2 found to possess a large feature at 330.3 cm À1 , attributable to counter-ion vibrational modes, which is only predicted upon explicit inclusion of counter-ions in the calculation. A feature around 107 cm À1 observed for the [{{Au(PPh 3 )} 3 (m 3 ÀO)} 2 ] 2+ (BF À 4 ) 2 infrared spectrum is assigned to 21 distinct vibrational modes arising from Au-Au bond stretching and other motions of the Au core. The same feature is predicted to be present within the [{{Au(PPh 3 )} 3 (m 3 ÀO)} 2 ] 2+ (MnO À 4 ) 2 spectrum but is masked by experimental noise. In the 50-400 cm À1 region, the relative intensities of predicted vibrational modes is found to depend heavily on the presence and nature of the counter-ions, while within the 400-800 cm À1 region, little dependence of the theoretical spectra on the type of counter-ion is predicted. Finally, the dimerization energies of both [{{Au(PPh 3 )} 3 (m 3 ÀO)} 2 ] 2+ (BF À 4 ) 2 and [{{Au(PPh 3 )} 3 (m 3 ÀO)} 2 ] 2+ (MnO À 4 ) 2 are calculated to be 3.06 eV and 3.20 eV, respectively, when the counter-ions are explicitly included within the calculation, and just 1.10 eV in their absence.

No abstract