Preparation, characterization, and electrical properties of a self-assembled meso-pyridyl porphyrin monolayer on gold surfaces

Abstract

A meso-pyridyl porphyrin, 5-(4-(2-(4-(S-acetylthiomethyl)phenyl)ethynyl)phenyl)-10,15,20-tris(4-pyridyl) porphyrin was synthesized by coupling of 5-{4-ethynyl}phenyl-10,15,20-tris(4-pyridyl) porphyrin with 4-(S-acetylthiomethyl)-4-iodobenzene under mild palladium coupling conditions. The meso-pyridyl porphyrin was used for the preparation of self assembled monolayers on gold substrates. The gold substrates were made by the thermal evaporation of gold on oxidized Si(100) surfaces. The self-assembled monolayers (SAMs) were characterized using ground state UV absorption and X-ray photoelectron spectroscopic (XPS) techniques and also by scanning probe microscopy (SPM) techniques. The UV-absorption spectra of the porphyrin monolayer exhibited a 10 nm red shift in the Soret band compared to the porphyrin in CHCl₃ solution. This indicates that the porphyrin molecules are aligned on the gold substrate in a side-by-side orientation. The typical shifts in the binding energy by XPS revealed that the chemisorption of the thiolate species of the porphyrin to the surface occurs through a strong sulfur-gold bonding mechanism. Basic hexagonal (√3×√3) R 30° well ordered self-assembled monolayers of meso-pyridyl porphyrin was observed by molecular-resolution atomic force microscopy (AFM). Room temperature current-voltage (I-V) spectra by scanning tunneling spectroscopy (STS) at varying set point current were collected to study the electronic transport properties of the monolayers on the gold surface.