Engineering Charge Transport by Tunneling in Supramolecular Assemblies through Precise Control of Metal-Ligand Interactions

Abstract

Coordination-driven supramolecular assemblies are promising for nanometer-sized electronic devices due to the potential to manipulate metal-ligand interactions and thereby control charge transport via tunneling through these assemblies. Cross-plane charge tunneling is investigated in assemblies of metalloporphyrins and pillar molecules, specifically palladium(II) and zinc(II) octaethylporphyrin (PdOEP and ZnOEP) monolayers and bilayers with bidentate (DABCO) and monodentate (ABCO) pillar ligands on highly oriented pyrolytic graphite (HOPG). Junction measurements and quantum-chemical calculations reveal that metal-ligand interactions significantly influence charge transport via tunneling and thermoelectric effects. Weak interactions in PdOEP assemblies create isolated molecular orbitals on interior pillar ligands, compressing the HOMO-LUMO gap and enhancing tunneling currents with unusual, inverted attenuation behavior and high thermopower. Conversely, strong interactions in ZnOEP assemblies induce localized orbitals on the porphyrin, leading to conventional tunneling decay behavior and low thermopower. The study highlights the potential of metal-ligand interactions as a strategy to engineer molecular orbital distribution, enhancing quantum transport efficiency in molecular-scale devices.

Keywords: assembly; charge transport; coordination; ligand; supramolecular.

Figure 1

a) Chemical structures of molecules used in this study. b) Structural representation of metalloporphyrin (PdOEP and ZnOEP) bilayers with two different mono‐ and bidentate pillar ligands, ABCO and DABCO. c,d) UV–vis titration experiments were conducted with a fixed concentration of metalloporphyrin solutions (1 × 10⁻⁵ m in toluene) while varying the DABCO concentration. The molar ratios of DABCO to metalloporphyrin are 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 1.0, 1.5, and 2.0. e,f) ¹H NMR titration experiments displaying the spectral changes with varying molar ratios of metalloporphyrin and DABCO in CDCl₃. g) Solid‐state structural analysis via UV–vis absorption spectroscopy, comparing the absorption spectra of PdOEP monolayer and bilayers with different pillar molecules (DABCO and ABCO) on single‐layer graphene. The dotted line represents the spectrum of pure PdOEP solution as a control. Single and double asterisks indicate the B band (380–500 nm) and the Q band (500–700 nm) transitions, respectively.^{[ 21 ]} The inset provides a schematic of the PdOEP‐ABCO bilayer structure. h,i) Comparison of high‐resolution X‐ray photoelectron spectra of the nitrogen (N1s) core‐level between ZnOEP and PdOEP bilayer structures with ABCO or DABCO pillars. j) Variation in N1s peak shifts of the pillar part (filled bar) and porphyrin part (blank bar) as the metal center of Zn(II) is substituted to Pd(II).

Figure 2

a) Large area (100 × 100 nm²) STM image of PdOEP bilayer formed by ABCO pillars. b) AFM analysis showing the changes in surface coverage of the PdOEP_outer (cyan color) on the PdOEP_inner‐ABCO structure (brown color) as a function of the PdOEP_outer deposition time. The surface depth profile in the AFM image for 1 min deposition time represents the monolayer thickness of PdOEP (≈0.2 nm) due to the adsorption of PdOEP_outer molecules. c) Plots showing the variation in the surface area of the ZnOEP_outer and PdOEP_outer layers as a function of the deposition time of the porphyrin_outer layer. High‐resolution STM images of d) PdOEP monolayer, e) PdOEP‐DABCO bilayer, and f) PdOEP‐ABCO bilayer on HOPG. g) STM images showing two different phase structures (α and β) of PdOEP‐ABCO bilayer on HOPG. (h) Magnified STM image of β‐phase structure.

Figure 3

a,c) I–V curves of metalloporphyrin (ZnOEP and PdOEP) monolayer and bilayers with DABCO and ABCO pillar ligands, obtained using scanning tunneling spectroscopy (STS). b,d) Log|J|–V curves of the metalloporphyrin samples, measured using the EGaIn large‐area junction technique. e,f) Conductive probe‐AFM analysis of a defect site at the interface between a PdOEP monolayer and a PdOEP‐ABCO bilayer, simultaneously capturing both 3D‐topography and tunneling current images.

Figure 4

a,b) The ground state structure of ZnOEP and PdOEP bilayers with two different molecular spacers (ABCO and DABCO). Density functional theory (DFT) transmission coefficient for monolayer, DABCO‐bilayer and ABCO‐bilayer with c) Zn and d) Pd metal site. Current–voltage (I–V) relationship for monolayer, DABCO‐bilayer and ABCO‐bilayer with e) Zn and f) Pd metal site. In c and f, e is the electron charge and h is the Planck's constant. g,h) Calculated room‐temperature Seebeck coefficient for monolayer, DABCO‐bilayer and ABCO‐bilayer of ZnOEP and PdOEP. i,j) correspondent measured Seebeck coefficient for bilayers.