Two- and Three-Dimensional Silver Cluster Assemblies: Effect of Argentophilic Interactions in Photoluminescence

Silver
nanocluster assemblies are an emerging class of fluorescent
and stable solid materials by virtue of their structural tunability
and stability compared to isolated clusters. The photophysical properties
of the cluster nodes are influenced by an intra- or intercluster interaction.
In this work, we show two-dimensional (2D) [Ag5(C2tBu)2(CF3COO)3(C4H4N2)](CHCl3) and three-dimensional (3D) [Ag4(C2tBu)(CF3COO)3(C4H4N2)] silver nanocluster assemblies obtained by
tuning the molar ratios of the same protecting ligands and linker.
The 2D assembly possesses a one-dimensional (1D) chain held together
through the Ag···Ag interactions, whereas in the 3D
assembly, Ag4 units are linked through trifluoroacetate
ligands to form the 1D chain with Ag4–O–Ag4 and Ag4–O–C–O–Ag4 connectivity. This varying effect of argentophilic interaction
influences photoluminescence properties. In the 2D assembly, the emission
was found to be dominated by the cluster-centered state owning to
enhanced argentophilic interactions, whereas in 3D, the emission is
from the linker. The theoretical investigations also revealed that
the lowest unoccupied molecular orbitals (LUMOs) of the 3D assembly
are localized on the pyrazine linker. In contrast, the LUMOs of the
2D assembly are mostly localized on the metal atoms within the cluster.
This work highlights not only the importance of synthesis but also
emphasizes the understanding of the cluster structure–luminescence
relationship.