Covalent layer-by-layer type modification of electrodes using ferrocene derivatives and crosslinkers

Abstract

A series of mono-substituted ferrocenes (Fecp(2)-R, R = -CONHCH2N(C2H4NH2)(2) (4), R = -CH2NHCH2N(C2H4NH2)(2) (5)) and di-substituted ferrocenes (Fe(cp-R)(2), R = -COF (3)) have been prepared, with 4 and 5 representing new compounds. The ferrocenes were grown layer-by-layer on metal oxide electrodes and crosslinked. The underlying principle of synthesis is based on repetitive and fast amide forming reactions between trimesic acid chloride (2) or Fe(cp-COF)(2) (3) and tris(2-aminoethyl)amine (6), or one of the amine-substituted ferrocenes 4 or 5 on a 3-aminopropyl triethoxysilane (APS) (1) modified ITO, ITO or ATO electrode. Linear growth of the films electroactivity with the number of cascade steps is observed for up to 12 cascade steps yielding 1.1 x 10(-8) ferrocene centers/cm(2) on a flat ITO electrode, whereas 3 cascade steps yield 2.2 x 10(-7) ferrocene centers/cm(2) on a mesoporeous ATO electrode. In case of the ATO electrode, the inner walls of the mesopores exhibiting a very large surface are completely modified in the procedure. Beside this large coulometry fast electron transfer kinetics, high stability and low optical density in both oxidation states is observed. The electrodes were checked for their potential use as optically transparent counter electrodes in electrochromic devices. (C) 2004 Elsevier Ltd. All rights reserved.