Tailoring the structure of two-dimensional self-assembled nanoarchitectures based on ni(ii) -salen building blocks

The synthesis of a series of Ni(II) -salen-based complexes with the general formula of [Ni(H2 L)] (H4 L=R(2) -N,N'-bis[R(1) -5-(4'-benzoic acid)salicylidene]; H4 L1: R(2) =2,3-diamino-2,3-dimethylbutane and R(1) =H; H4 L2: R(2) =1,2-diaminoethane and R(1) =tert-butyl and H4 L3: R(2) =1,2-diaminobenzene and R(1) =tert-butyl) is presented. Their electronic structure and self-assembly was studied. The organic ligands of the salen complexes are functionalized with peripheral carboxylic groups for driving molecular self-assembly through hydrogen bonding. In addition, other substituents, that is, tert-butyl and diamine bridges (2,3-diamino-2,3-dimethylbutane, 1,2-diaminobenzene or 1,2-diaminoethane), were used to tune the two-dimensional (2D) packing of these building blocks. Density functional theory (DFT) calculations reveal that the spatial distribution of the LUMOs is affected by these substituents, in contrast with the HOMOs, which remain unchanged. Scanning tunneling microscopy (STM) shows that the three complexes self-assemble into three different 2D nanoarchitectures at the solid-liquid interface on graphite. Two structures are porous and one is close-packed. These structures are stabilized by hydrogen bonds in one dimension, while the 2D interaction is governed by van der Waals forces and is tuned by the nature of the substituents, as confirmed by theoretical calculations. As expected, the total dipolar moment is minimized.