Modification of Electrode Surfaces by Self-Assembled Monolayers of Thiol-Terminated Oligo(Phenyleneethynylene)s

The wire-like properties of four S-(4-{2-[4-(2-phenylethynyl)phenyl]ethynyl}phenyl) thioacetate derivatives, PhC≡CC6H4C≡CC6H4SAc (1), H2NC6H4C≡CC6H4C≡CC6H4SAc (2), PhC≡CC6H2(OMe)2C≡CC6H4SAc (3) and AcSC6H4C≡CC6H4C≡CC6H4SAc (4) (Figure 1), all of which possess a high degree of conjugation along the oligo(phenyleneethynylene) (OPE) backbone, were investigated as self-assembled monolayers (SAMs) on gold and platinum electrodes by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The redox probe [Fe(CN)6]4− was used in both the CV and impedance experiments. The results indicate that the thiolates derived from thioacetate-protected precursor molecules 1 and 2 form well-ordered monolayers on a gold electrode, whereas SAMs derived from 3 and 4 exhibit randomly distributed pinholes. The electron tunnelling resistance and fractional coverage of SAMs of all four compounds were examined using electron tunnelling theory. The analysis of the results reveal that the well-ordered SAMs of 1 and 2 exhibit higher charge-transfer resistance in comparison to the defect-ridden SAMs of 3 and 4. The additional steric bulk offered by the methoxy groups in 3 is likely to prevent efficient packing within the SAM, leading to a microelectrode behaviour, when assembled on a gold electrode surface. The protected dithiol derivative 4 probably binds to the surface through both terminal groups which prevents dense packing and leads to the formation of a monolayer with randomly distributed pinholes. Atomic force microscopy (AFM) was used to examine the morphology of the monolayers, and height images gave root-mean-square (RMS) roughness′s which are in agreement with the proposed SAM structures.