Porphyrin metal–organic frameworks with bilayer and pillar-layered frameworks and third-order nonlinear optical properties

It is recognized that dimensions have a decisive influence on the properties of materials. Metal-organic frameworks as third-order nonlinear optical (NLO) materials have attracted much attention recently. However, research on the influence of dimensions on third-order NLO properties of MOFs has not been reported. In this work, we synthesized two porphyrin MOFs (PMOFs) constructed with π-conjugated tetracarboxyphenylporphyrin (TCPP) and in situ formed 1,2-bis(1H-benzo[d]imidazol-2-yl)ethene (BIE) conjugated ligands. In both PMOFs, Zn2(CO2)4 paddlewheel units are connected by TCPP-Zn ligands to form a 2D layer. Interestingly, these layers are linked by BIE ligands to form a bilayer in PMOF-1 and a 3D pillar-layered framework in PMOF-2, which serve as structural models to evaluate the influence of dimensions on third-order NLO properties. It is speculated that the 3D pillar-layered framework in PMOF-2 with BIE conjugated pillars is more conducive to interlayer charge transfer than the two-dimensional bilayer in PMOF-1, thus achieving a better third-order NLO performance. The third-order NLO test results of PMOFs in a polydimethylsiloxane (PDMS) matrix showed that PMOF-2 displayed a higher nonlinear absorption coefficient, large third-order susceptibility and lower limiting threshold than PMOF-1/PDMS, which may be mainly attributed to the fact that the 3D pillar-layered framework is more conducive to interlayer charge transfer than the two-dimensional bilayer. This work reveals the influence of dimensions on third-order nonlinear properties which will help to explore new MOF materials with excellent third-order NLO properties.