Unusual Solvent Effects on Optical Properties of Bi-Icosahedral Au25 Clusters

Temperature-dependent and time-resolved absorption measurements were carried out to understand the influence of solvent hydrogen bonding on the optical properties of bi-icosahedral [Au-25(PPh3)(10)(C6S)(5)Cl-2](2+) (bi-Au-25) clusters. Theoretical calculations have shown a low energy absorption maximum that is dominated by the coupling of the two Au-13 icosahedra, as well as a high energy absorption arising from the individual Au-13 icosahedra that make up the bi-Au-25 clusters. Temperature-dependent absorption measurements were carried out on bi-Au-25 in aprotic (toluene) and protic (ethanol and 2-butanol) solvents to elucidate the cluster solvent hydrogen bonding interactions. In toluene, both the low and high energy absorption bands shift to higher energies consistent with electron phonon interactions. However, in the protic solvents, the low energy absorption shows a zigzag trend with decreasing temperature. In contrast, the high energy absorption in protic solvents shifts monotonically to higher energy similar to that of toluene. Also at the temperature where the zigzag trend was observed, new absorption peaks emerged at higher energy region. The results are attributed to the hydrogen bonding of the solvent with Au-Cl leading to a disruption of the coupling of icosahedra, which is reflected in unusual trends at the low energy absorption. However, at the transition temperature, the hydrogen bonding solvents distort the icosahedrons so much so that the symmetry of Au-13 icosahedron is lifted leading to new absorption peaks at high energy. The transition happens at the dynamic crossover temperature where the solvent attains high density liquid status. Femtosecond time-resolved absorption measurements have shown similar dynamics for bi-Au-25 in ethanol and toluene with slower vibrational cooling in ethanol. However, the nanosecond transient measurements show significantly longer lifetime for bi-Au-25 in ethanol that suggest the solvent does have an influence on the exciton recombination.