Heterogeneous electron transfer processes in triarylamine- and ferrocene-based self-assembled monolayers
Introduction
Heterogeneous electron transfer (ET) in self-assembled monolayers (SAM) has been investigated for a number of years quite intensely [1], [2]. The SAMs investigated mostly consisted of end-capped alkanethiols which were terminated by a redoxactive moiety, such as ferrocene or other metal centred complexes. With the thiol terminated end, these molecules form more or less well ordered SAMs on gold substrates. In some cases, the saturated alkyl spacers were replaced by conjugated π-electron systems either of the oligo(phenyleneacetylene) type [3], [4] or of the oligo(phenylenevinylene) type [5]. These investigations gave insight into the dependence of the interfacial ET, e.g., on the distance between redox centre and electrode, the type of molecular ordering, or the type of the spacer. Various electrochemical methods have been applied to investigate the interfacial electron transfer processes in these SAMs. Among them, chronoamperometry [6], [7], [8], [9], linear sweep and cyclic voltammetry [7], [8], [10], [11], [12], ac voltammetry (ACV) [6], [13], impedance spectroscopy [14], a variation of ac voltammetry/impedance spectroscopy [3], [15], [16], [17], [18], [19], [20], [21], [22], and indirect laser induced temperature jump (ILIT) measurements [4], [5], [23] have been used.
In the present study, we will focus on organic amine based redox centres and we will compare them with analogous ferrocene redox centres. As organic redox centres, we used triarylamines which can reversibly be oxidised and which are widely used as hole transport components in optoelectronic devices [24], and phenothiazine which is expected to have a somewhat higher reorganisation energy than triarylamines [25]. As spacers we used saturated alkanes (1, 2, 3) as these provide a relatively weak electronic coupling between the redox centre and the electrode, and, thus, will show moderately slow ET. For comparison, we also measured three ferrocene terminated alkanethiols (4, 5, 6); the heterogeneous rate constants of two of them are available in the literature and will be used to assess the accuracy of our measurement set-up. Furthermore, we investigated two ferrocene compounds where the ferrocene is attached to a biphenyl bridge (7) and an anthracene bridge (8), respectively. These derivatives are expected to have very high ET rate constants which will give information about the highest rates accessible by our set-up.
Section snippets
Results and discussion
In order to isolate the redox centres of 1-8 from each other, we prepared mixed SAMs on freshly molten gold-ball electrodes where the redox active molecules were diluted by alkanethiols. In order to ensure a dense packing which should be essentially free of pinholes, the coating process was performed for at least 1–2 d. Cyclic voltammograms were recorded for the SAMs of all molecules. As expected all derivatives show reversible redox behaviour, a typical voltammogram is given in Fig. 1 for
Experimental
The syntheses of 1–7 are either reported in the literature or will be reported elsewhere.
Electrochemical experiments were performed in a three-electrode configuration with a Princeton Applied Research potentiostat/galvanostat model 283, including software Power Suite and a Princeton Applied Research frequency response detector model 1025. A platinum coil as the counter electrode and an Ag/AgCl pseudoreference electrode were used; the latter was brought as near as possible to the working
Acknowledgements
Support from the Volkswagen-Stiftung (Prof. Lambert, Az I/77566, PD Dr. Terfort, Az I/77734) and the Fonds der Chemischen Industrie is gratefully acknowledged.
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