Variations in the Photoexcitation Mechanism of an Adsorbed Molecule on a Gold Nanocluster Governed by Interfacial Contact.

We performed first-principles calculations on the optical response of a Au-azobenzene complex to elucidate the role of the interfacial contact between Au and azobenzene. Our calculations of photoexcited electron dynamics for different configurations of azobenzene adsorbed on the Au nanocluster revealed that the optical properties of the azobenzene moiety change markedly by the interfacial contact, even if the electronic structure in the ground state is almost unchanged. The optical absorption measured for isolated azobenzene weakens when the Au-azobenzene interaction increases, while the absorption measured using the light field along the Au-azobenzene alignment strengthens. The electronic excitation analysis showed that the mechanism of the charge-transfer excitation between Au and azobenzene changes remarkably depending on the strength of the interfacial interaction. We revealed that the optical property can be governed by the atomic-scale difference in the adsorption structure of azobenzene on a Au nanocluster. This study affords novel insights that could enable the photoexcitation mechanism to be controlled by designing the interface between a metal nanoparticle and a molecule.