Mixed Ionic and Electronic Conduction in Appropriately Substituted Phthalocyanine Thin Films Allows Fast and Reversible Electrochromic Switching

Abstract

Thin films of phthalocyanines can be considered established materials as organic semiconductors and as electrocatalytically active electrodes. As a consequence of their redox chemistry, however, they also have been proposed as electrochromic layers. For this purpose, they have to perform as organic mixed ionic and electronic conductors (OMIEC). Phthalocyanines with electron-withdrawing substituents have shown reversible reduction and re-oxidation. Fluorination has proven particularly useful since chemically reversible reactions can be achieved, even in contact to aqueous electrolytes [1]. The degree of fluorination determines the intermolecular coupling and, hence, the movement of electrons and ions in thin films. For films of perfluorinated copper phthalocyanine (F16PcCu) facile transport of electrons was established, but the rate of the electrochromic reaction was limited by ion transport in the films. For a derivative in which all 8 peripheral F atoms were replaced by perfluoroisopropyl groups (F64PcCu), more facile ion transport was observed, but the electron mobility was strongly suppressed and then limited the rate of electrochromic switching [2].In this work, a new type of phthalocyanine with only 4 peripheral F atoms replaced by perfluoroisopropyl groups (F40PcCu) is studied. Notably, the distribution of the bulky, perfluoroalkyl group is on adjacent 2 phthalocyanine quadrants, leaving two quadrants able to pi-stack, a feature not present in F64PcCu. Physical vapor deposition yielded homogeneous thin films of 1 nm - 50 nm average film thickness. The dependence of the intermolecular coupling on the film thickness was analyzed by in situ UV/Vis spectroscopy. The films showed constantly small spectral splitting from the monolayer regime throughout the studied range of thicknesses, well into the range of bulk structure. A packing of molecules that leads to weak electronic coupling of adjacent phthalocyanine cores was thereby indicated. The electrochromic characteristics were investigated by electrochemical and spectroelectrochemical measurements with an aqueous solution of KCl as electrolyte. The optical absorption spectra revealed reversible changes of the films upon reduction with intercalation of the K+ counter ions and re-oxidation with extraction of the counter ions. As typical for phthalocyanines, absorption in the Soret- and Q-band was strongly attenuated in the reduced state and an additional absorption band was established between the two, around 500 nm. Therefore, the films showed intense changes of color upon reduction and re-oxidation. The films provided a well-balanced, equally fast transport of electrons and ions with effective diffusion coefficients in the range of 10-10 cm2 s-1. Fast and stable electrochromic switching to 90 % of the final state was obtained within about 0.3 s after applying a potential difference of about 1.5 V upon either reduction or re-oxidation. Reversible and stable switching of the films was achieved over at least 200 cycles. We consider F40PcCu to have an optimized substitution pattern to provide equally fast transport of electrons and ions in the films and, therefore, to provide very attractive switching characteristics which might be of interest as electrochromic layers in smart windows and smart mirrors. 1. S. Nagel, M. Lener, C. Keil, R. Gerdes, Ł. Łapok, S. M. Gorun and D. Schlettwein, "Electrochromic Switching of Evaporated Thin Films of Bulky, Electronic Deficient Metallo-Phthalocyanines", J. Phys. Chem. C, 2011, 115, 8759–8767.2. J. Weissbecker, A. Loas, S. M. Gorun and D. Schlettwein, "Switching of the Rate-limiting Step in the Electrochromic Reduction of Fluorinated Phthalocyanine Thin Films by Decreased Intermolecular Coupling", Electrochimica Acta, 2015, 157, 232–244.