Accessing high-order multiphoton excited fluorescence (H-MPEF) materials is challenging yet and needs complicated synthesis procedures. In this study, we successfully assembled plasmonic Au nanorods (Au NRs) with multiphoton responsive metal-organic frameworks (MOFs), resulting in a significant several-fold enhancement of H-MPEF. The extent of multiphoton enhancement was found to be strongly dependent on the degree of overlap between the multiphoton excitation wavelength of MOFs and the localized surface plasmon resonance absorbance of Au NRs, indicating the importance of plasmon-induced resonance energy transfer. Besides, plasmon-induced hot electron transfer played a vital role in enhanced multiphoton activity as well. Notably, the optimum H-MPEF enhancement occurs at the second near-infrared (NIR-II) region, which provides a promising platform for fluorescent bioimaging. Our findings provide a feasible and practical method to fabricate optimized H-MPEF materials for biological imaging using tissue-penetrating NIR-II light.

The structure determination of metal nanoclusters protected by ligands is critical in understanding their physical and chemical properties, yet it remains elusive how the metal core and ligand of metal clusters cooperatively contribute to the observed performances. Here, with the successful synthesis of Au₄₄TBPA₂₂Cl₂ cluster (TBPA = 4-tert-butylphenylacetylene), the structural isomer of previously reported Au₄₄L₂₈ clusters (L denoted as ligand) is filled, thereby providing an opportunity to explore the property evolution rules imparted by different metal core structures or different surface ligands. Time-resolved transient absorption spectroscopy reveals that the difference in the core structure between Au₄₄TBPA₂₂Cl₂ and Au₄₄L₂₈ can bring nearly 360 times variation of excited-state lifetime, while only 3–24 times differences in excited-state lifetimes of the three Au₄₄L₂₈ nanoclusters with identical metal core but different ligands are observed, which is due to much stronger impact of the metal core than the surface ligands in the electronic energy bands of the clusters. In addition, the Au₄₄ clusters protected by alkyne ligands are shown to be highly effective toward the electrochemical oxidation of ethanol, compared to the Au₄₄ clusters capped by thiolates, which is ascribed to smaller charge transfer impedance of the former clusters. We anticipate that the study will enhance the process in controlling the nanomaterial properties by precisely tailoring metal core or surface patterns.