Experimental Electrode preparation The Pt microdisc electrodes (Ø 100 μm) were polished with alumina (0.3 µm, Buehler), rinsed with deionised (DI) water and sonicated for 1 min in DI water. The electrodes were then electrochemically cleaned in degassed 0.5 M H 2 SO 4 (Sigma-Aldrich, 99.999%) over 15 cyclic voltammetry (CV) scans between -0.21 V and 1.45 V vs. Ag/AgCl followed by two scans between -0.20 V and 1.35 V. For both sets of scans, the scan rate was 0.1 Vs -1 and scans started and ended

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... within the capacitive region at 0.3 V ( Figure S1 ). For the electrode later modified with the Ru complex a surface area of 7.4×10 -4 cm 2 was determined from the charge under the PtO reduction peak at 0.50 V (3.1×10 -7 C) and a value of 4.20 ×10 -4 C cm -2 for reduction of the oxide monolayer. Comparison to the geometric area of 7.85×10 -5 cm 2 indicates that the described treatment of the electrode results in a relatively high roughness factor of about 9. E / V vs. Ag/AgCl -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 100 nA Figure S1. CV (scan 2) of a Pt microelectrode (Ø 100 μm) in degassed 0.5 M H 2 SO 4 at a scan rate of 0.100 Vs -1 . Electrodes were immersed overnight in 11-amino-1-undecanethiol solution (10 mM in EtOH, Dojindo) to yield an amine-terminated SAM. The modified electrodes were then placed overnight in a stirred solution (THF:CH 3 CN, 5:1) of the carboxylate terminated ruthenium complex 1 1 (0.1% w/v) with 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMTMM, 1% w/v, Fluka) as coupling agent. 2, 3 1 Complex 1 was prepared as previously describedsuperior surface coverage results if the coupling reaction involves aromatic carboxylic acids. (M. Sjödin, unpublished results.) Supplementary Material (ESI) for Chemical Communications This journal is