Orbital Hybridization of π-Conjugated Ligands with Atomically Precise Metal Clusters for Enhanced Two-Photon Absorption

The nonlinear optical phenomenon of two-photon absorption (TPA) has numerous applications in scientific fields. However, designing materials with efficient TPA that meet the technical requirements is challenging. The fundamental optical properties of a material depend on its density of states (DOS). In this study, we achieved efficient TPA by controlling the DOS via tuning the orbital energy levels of gold clusters and molecular ligands. We synthesized Au₃₆(NP)₂₄ with an atomically precise Au₃₆ core and 24 naphthalenethiol (NP) ligands and evaluated the orbital hybridization effect on the TPA cross-section, σ⁽²⁾, of 6000 GM under 800 nm excitation. The synthesized Au₃₆(NP)₂₄ exhibited a considerably high σ⁽²⁾, originating from the DOS, suitable for near-double resonance enhancement because of the hybridization between the Au₃₆ and (NP)₂₄ orbitals. This orbital hybridization approach between functional molecules and a metal kernel provides a versatile design strategy for tuning the DOS of materials for various purposes including nonlinear optics.