Electrochromic Switching of Evaporated Thin Films of Bulky, Electronic Deficient Metallo-Phthalocyanines

Thin films of four-coordinated perfluoroalkyl-substituted phthalocyanines were prepared by physical vapor deposition. The electrochemical redox characteristics and simultaneous changes in the optical absorbance spectra of the films (40-215 nm) in contact with an aqueous LiCl electrolyte for charge compensation were compared to results obtained for the molecules dissolved in an organic solvent. A positive shift of the potential of the first ring-centered reduction relative to the unsubstituted or perfluorinated phthalocyanines was confirmed for solutions and clearly established for thin films. Following a conditioning of the films during the first reduction, chemically reversible and thermodynamically quasi-reversible reduction and reoxidation reactions were observed. A clear correlation of the injected charge with the appearance of isosbestic points characterized the reaction as a one-electron reduction. Diffusion limitation by charge-balancing lithium counterions could be detected only for thicker films leading to high switching speeds of the electrochromic reaction in thinner films of about a second and thus rendering the materials interesting candidates for organic electrochromic display applications.