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

Abstract

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.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

“…[ 214 ] The transient absorption study of [Au 9 (PPh 3 ) 8 ] 3+ in methanol and dichloromethane showed relatively fast S 1 relaxation decay times of 2 ps and 45 ps which were assigned to intersystem crossing (S 1 → T 1 ) and nonradiative relaxation (S 1 → S 0 ), respectively. [ 215 ]…”

“…[ 337a,338 ] While the experimental spectrum agrees with the theoretical result for [Au 8 (PPh 3 ) 8 ] 2+ , the experimental spectrum of [Au 9 (PPh 3 ) 8 ] 3+ in solution represents the alternative C 4 isomer. [ 215 ] Yao and Tsubota simulated the spectra for the two isomers of the Au 9 core using the truncated ligand PH 3 for their study of the [Au 9 (TPPS) 8 ] 5− cluster; however, due to the symmetry constraints in the simulations, the experimental spectrum was compared to the D 2h isomer as well, [ 339 ] despite the fact that the experimental crystal structure clearly shows the C 4 core isomer. [ 340 ] With the help of the experimental and electronic structure findings of Tsukuda and co‐workers, [ 55,23 ] recent work by us showed that the experimental UV–visible spectra of [Au 9 (PPh 3 ) 8 ] 3+ in both MeOH and DCM should be assigned to the C 4 core isomer (Figure 15c).…”

“…[ 340 ] With the help of the experimental and electronic structure findings of Tsukuda and co‐workers, [ 55,23 ] recent work by us showed that the experimental UV–visible spectra of [Au 9 (PPh 3 ) 8 ] 3+ in both MeOH and DCM should be assigned to the C 4 core isomer (Figure 15c). [ 215 ] Moreover, a spin–orbit correction is required to the TD‐DFT simulated spectra to understand the cluster's absorption tail and the fast intersystem crossing observed in the transient absorption spectrum of this Au cluster. Fagan et al.…”

“…[ 253 ] Similar results have been found for the crown isomer of [Au 9 (PPh 3 ) 8 ] 3+ photogenerating 1 O 2 that leads to the degradation of 1,3‐diphenylisobenzofuran (DPBF). [ 215 ] It has also been shown that generation of singlet ( 1 O 2 ) by photoexcited thiolate‐protected Au 25 and Au 38 clusters catalyzed photocatalytic oxidation of phenyl methyl sulfides and benzylamine under visible light irradiation. [ 395 ]…”

The Journal of Physical Chemistry C Virtual Special Issue on Metal Clusters, Nanoparticles, and the Physical Chemistry of Catalysis

“…This VSI contains 11 papers concerning MPCs, 5 of which are related to the chemical properties of MPCs: CO oxidation by a ferrocene-protected Au 8 cluster, reductive elimination of 1,3-diynes from an alkynyl-protected alloy cluster, the migration of a Pd atom within an Au MPC during CO oxidation, CO exchange in a carbonyl Ir 4 cluster, and the hydrogenation of nitroaromatics by supported Au n ( n = 19, 20, and 22) MPCs . Other topics reported include the synthesis of a new Au 5 Cu 6 alloy cluster, the controlled doping of a Cu atom to a Au-based alloy cluster, the synthesis of ultrastable Au clusters protected by sulfonic thiolates, the dynamics of the ligand layer playing a role in the MPC stability, the photoluminescence properties of Ag MPCs, and the optical properties of Au 9 MPC …”