Influence of film architecture on the charge-transfer reactions of metallophthalocyanine layer-by-layer films

A judicious combination of materials and molecular architectures has led to enhanced properties of layer-by-layer (LbL) films, in which control at the molecular level can be achieved. In this paper we provide one such example by showing that supramolecular effects in electroactive LbL films comprising tetrasulfonated metallophthalocyanines (NiTsPc or FeTsPc) alternated with poly(allylamine hydrochloride) (PAH) may depend on both the choice of material and the film-forming technique. Indeed, though some properties such as film growth were common to both types of LbL film, those containing NiTsPc displayed unique features. PAH/NiTsPc films assembled onto ITO (indium tin oxide) showed two redox processes, with E-1/2 at 0.54 and 0.80 V (vs SCE) attributed to the phthalocyanine unit ([TsPc](6-)/[TsPc](5-)) and Ni2+/Ni3+ redox couple, respectively. In contrast, only one redox process was observed for PAH/FeTsPc films, with E-1/2 at 0.45 V assigned to the [TsPc](6-)/[TsPc](-5) couple. For both systems, the anodic peak current versus scan rate increased up to 500 mV center dot s(-1), indicating that the electrochemical response of NiTsPc and FeTsPc in LbL films is governed by charge-transport mechanism. Interestingly, the second redox process for PAH/NiTsPc became totally reversible at high scan rates, showing fast charge transfer. The influence from the film-forming technique was proven by comparison with results from an electrodeposited film of NiTsPc on ITO, which was less stable than its LbL counterpart. It is envisaged that the high electrochemical stability, reversibility, and unique features arising from the supramolecular structure of PAH/NiTsPc LbL films may be exploited in applications such as electrochromic and sensing devices.