Spectroelectrochemical characterisation of Langmuir–Schaefer films of heteroleptic phthalocyanine complexes. Potential applications

Abstract

The Langmuir–Schaefer technique has been used to fabricate ordered monolayers of novel heteroleptic double and triple-decker phthalocyanine derivatives. The molecules studied include a double-decker mixed porphyrinate and naphthalocyaninate lanthanum(III) compound and a europium triple-decker molecule where one of the rings is a substituted porphyrin and the two remaining rings are substituted phthalocyanine macrocycles. The materials and their films have been characterised using UV–vis–NIR spectroscopy and electrochemistry. The UV–vis–NIR spectra of the solutions show the expected features corresponding to the aromatic rings present in each molecule. The absorption spectra of LS films show broadenings and important shifts to higher wavelengths when compared to the electronic absorption spectra of solutions or disordered cast films. Thin films are sensitive to electron acceptor gases, such as NOx as observed by modifications in the electronic absorption spectra of the films. These fast and reversible changes can be the basis of optical sensors based on thin films of heteroleptic phthalocyanines (Pc). Cyclic and square wave voltammograms of dichloromethane solutions lead to rich curves where a variety of peaks are observed. The voltammogrames of the corresponding films also reflect the rich electrochemical properties of the studied compounds. Electrochromism has been found with LB and cast films electrodes in KCl and KClO 4 solutions: important changes in the UV–vis–NIR spectra registered in situ simultaneously to the application of the electrical potential have been observed. The rich electrochemistry and the electrochromism observed, opens the possibility of using such films in multicolour electrochromic devices. Films prepared onto ITO glass show characteristic electrochemical responses when exposed to solutions of organic compounds, such as ascorbic and vanillic acid, providing a high degree of cross-selectivity. This effect can be used in the development of arrays of sensors used in electronic tongues. In summary Langmuir–Schaefer films of heteroleptic double and triple-decker phthalocyanines could be of interest at least in three potential applications: thin film electrochromic devices, optical gas sensors and voltammetric sensors.