Modulation of Two-Photon Absorption Due to the Reversible Change in the Oxidation State of Atomically Precise Gold Nanoclusters

The atomically precise nanoclusters offer precise control of optical properties on the nanoscale, as long as we understand the relation among the size, composition, and structure of nanoclusters and their physical properties. So far, the primary interest has been in the synthesis of nanoclusters with controlled optical properties, in not only the one-photon but also the two-photon regime. Large two-photon absorption cross sections of nanoclusters were reported, but the possibility of modulation of nonlinear optical (NLO) properties by external stimuli has been scarcely explored. We present here the experimental data supported by DFT calculations on two-photon absorption of gold nanoclusters, which are modified by oxidation-reduction processes. Such reactions can be an important regulator of the nonlinear optical properties of various materials, which is well established, e.g., for organometallics, but has not been demonstrated for nanoclusters. The reversible oxidation of [Au₂₅(SR)₁₈]^- (where SR = 2-phenylethanethiol) to its neutral form results in distinct changes in the one-photon absorption spectra, but even more pronounced differences in the two-photon absorption. The change in the oxidation state of the Au₂₅ cluster results in the 2-fold enhancement of the two-photon absorption cross sections in the wavelength range of 825-1150 nm and switching between saturable absorption and two-photon absorption below 825 nm. DFT calculations show that the presence of the counterion may contribute to the change, as it decreases the two-photon absorption cross sections of the system. Our results demonstrate that even seemingly minor electronic differences between the anionic and neutral Au₂₅(PET)₁₈ clusters can lead to pronounced variations in their NLO properties.