Robust Self-Assembled Monolayers of Ru-II and Os-II Polypyridines on Gold Surfaces: Exploring New Potentials

Metal complexes [M(phtpy)-(pztpy)](PF6)(2) (phtpy 4'-phenyl-2,2':6',2 ''-terpyridinc, pztpy=4'-(N-piperazinyl)-2,2':6',2 ''-terpyridine, M = Ru,Os) were prepared and examined spectroscopically and electrochemically. The piperazine attachment was found to significantly modify the photophysical and electrochemical properties compared to the parent bis-terpyridine complexes, causing it red-shift of the (MLCT)-M-1 (23 nm, 53.9 eV) and a substantial cathodic shift of the redox potential (0.3 V for Ru, 0.23 V for Os). Self-assembled monolayers (SAMs) of the complexes on polished gold electrodes were generated simply by the in situ formation of a dithiocarbamate (DTC) anchoring group at the terminal piperazinyl nitrogen on the pztpy ligand. Cyclic voltammetry revealed that the monolayers show excellent reversible behaviour and exceptional stability. The high stability of the SAMs is attributed to the strong bidentate attachment to the gold surface of the DTC tether and the favourable low oxidation potentials of the complexes which result from the electron-rich piperazine nitrogen on the pztpy ligand. Such DTC-based SAMs demonstrate a substantial improvement over commonly-employed thiol-based systems, and offer new scope for future development.