Molecular design driving tetraporphyrin self-assembly on graphite: a joint STM, electrochemical and computational study.

M El Garah,G Cuniberti,L Mengozzi,M Marcaccio,S Rapino,A Santana Bonilla,P G Cozzi,A Gualandi,M Calvaresi,F Paolucci,A Ciesielski,P Samorì,M Iurlo,A Fiorani,R Gutierrez,F Zerbetto

doi:10.1039/c6nr03424a

Abstract

Tuning the intermolecular interactions among suitably designed molecules forming highly ordered self-assembled monolayers is a viable approach to control their organization at the supramolecular level. Such a tuning is particularly important when applied to sophisticated molecules combining functional units which possess specific electronic properties, such as electron/energy transfer, in order to develop multifunctional systems. Here we have synthesized two tetraferrocene-porphyrin derivatives that by design can selectively self-assemble at the graphite/liquid interface into either face-on or edge-on monolayer-thick architectures. The former supramolecular arrangement consists of two-dimensional planar networks based on hydrogen bonding among adjacent molecules whereas the latter relies on columnar assembly generated through intermolecular van der Waals interactions. Scanning Tunneling Microscopy (STM) at the solid-liquid interface has been corroborated by cyclic voltammetry measurements and assessed by theoretical calculations to gain multiscale insight into the arrangement of the molecule with respect to the basal plane of the surface. The STM analysis allowed the visualization of these assemblies with a sub-nanometer resolution, and cyclic voltammetry measurements provided direct evidence of the interactions of porphyrin and ferrocene with the graphite surface and offered also insight into the dynamics within the face-on and edge-on assemblies. The experimental findings were supported by theoretical calculations to shed light on the electronic and other physical properties of both assemblies. The capability to engineer the functional nanopatterns through self-assembly of porphyrins containing ferrocene units is a key step toward the bottom-up construction of multifunctional molecular nanostructures and nanodevices.

Highlights

Controlling the spatial arrangement of complex functional molecules at surfaces and interfaces with atomic precision is one of the greatest challenges in the bottom-up fabrication of functional architectures
Scanning Tunneling Microscopy (STM) at the solid–liquid interface has been corroborated by cyclic voltammetry measurements and assessed by theoretical calculations to gain multiscale insight into the arrangement of the molecule with respect to the basal plane of the surface
P1 and P2 films were prepared by drop casting 100 μL solutions (0.1 mM in CHCl3) onto highly oriented pyrolytic graphite (HOPG); once dried up, the samples were kept under a chloroform-saturated atmosphere for 48 h at room temperature

Summary

Introduction

Porphyrins have been extensively explored as molecular scaffolds because of their unique opto-electronic properties, which make them appealing for numerous applications including solar cells[24] and photo-catalysis.[25]. Only a few examples have been reported.[42,43,44] Interestingly, it has been recently demonstrated that in the case of the porphyrins substituted in meso positions with alkyl chains, the final configuration of the selfassembled nanopatterns at the solution/HOPG interface directly depends on the length of aliphatic substituents.[45] In particular, it was shown that the presence of the long alkyl chains promotes the formation of face-on porphyrin-based architectures whereas porphyrins functionalized with short chains adopt the edge-on conformation This different behaviour can be explained by the presence of the strong molecule surface and molecule–molecule interactions, respectively. We performed theoretical analyses to gain insight into the electronic and mechanical properties of both structures

Results and discussion

MD calculations

DFT calculations