Optical Properties of the Atomically Precise C4 Core [Au9(PPh3)8]3+ Cluster Probed by Transient Absorption Spectroscopy and Time-Dependent Density Functional Theory

Structural isomerism of [Au-9(PPh3)(8)](3+) has been studied experimentally, mostly concerning the symmetry of the Au-9 core. Recently, the C-4 isomer of [Au-9(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 [Au-9(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 [Au-9(PPh3)(8)](3+) cluster in dichloromethane and methanol solutions are studied using femtosecond transient absorption spectroscopy. [Au-9(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 [Au-9(PPh3)(8)](3+), respectively. On the other hand, optical pumping of [Au-9(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 similar to 0.3 ps. Our combined computational and experimental study reveals that the excitation wavelength-dependent relaxation dynamics of the [Au-9(PPh3)(8)](3+)cluster are related to the different electron densities of the excited states of [Au-9(PPh3)(8)](3+), consistent with it possessing molecular-like electronic states.

Automated summary

The structural isomerism of [Au-9(PPh3)(8)](3+) has been experimentally studied, with the C-4 isomer existing in solution phase and the D-2h isomer present in the solid state. Computational and experimental studies have revealed that the relaxation dynamics of the [Au-9(PPh3)(8)](3+) cluster are wavelength-dependent and related to the electron densities of the excited states.