Electrochemistry within molecules using ultrafast cyclic voltammetry

Abstract

The basic principles allowing undistorted ultrafast cyclic voltammetry to be performed at ultramicroelectrodes are recalled and discussed as well as their experimental exploitation leading to the design of an ultrafast potentiostat allowing on-line ohmic drop compensation without distortions within the megavolts-per-second range of scan rates. The validity of the instrument and of the method are established by investigating the voltammetry of anthracene reduction up to scan rates of 2.5 MV s –1. Access to this range of ultrahigh scan rates allows nanosecond-time scale to be explored voltammetrically which offers obvious kinetic advantages by widening the mechanistic scope of voltammetry so that it encompasses now more than nine orders of magnitude. Yet, in this feature article, we are more interested in discussing another remarkable – but less promoted – advantage of ultrafast voltammetry. Indeed, scan rates in the megavolts- per-second range allow the creation of diffusion layers with a maximal extension of a few nanometers, viz, which are comparable to the size of chemical and biological molecules. In other words, this allows the in-situ and direct monitoring of redox communication within molecules that are attached to an ultramicroelectrode surface. The experimental validity and the interest of this concept are presented on the basis of two typical examples. One deals with the rate of redox commutation within a self-assembled monolayer of organometallic molecules bearing a redox terminal group, [Os(bpy) 2Cl(py–B)] +, adsorbed onto a platinum surface through the pyridyl moiety of their ligated bridge (–B = –(CH 2) 2–py). The second application concerns the monitoring of the time progression of electron-hopping diffusion through electron exchange between 64 Ru II/III(tpy) 2 redox groups distributed on a fourth-generation PAMAM dendrimer molecule adsorbed onto an ultramicroelectrode. Besides the measurement of the rate of communication between adjacent ruthenium centers within one dendrimer, the method is extremely informative about the topology of the spatial arrangement of the 64 redox centers, so that it can be used as a molecular microtome with a resolution better than that of STM. To cite this article: C. Amatore et al., C. R. Chimie 6 (2003) 000–000.