The influence of intermolecular coupling on electron and ion transport in differently substituted phthalocyanine thin films as electrochromic materials: a chemistry application of the Goldilocks principle

The transport of both electrons and ions in organic mixed ionic and electronic conductors such as phthalocyanines, is essential to allow redox reactions of entire films and, hence, to impart electrochromism. Thin films of a new type, tetrakis-perfluoroisopropyl-perfluoro phthalocyanine, F 40 PcCu of different thicknesses were obtained via vapor deposition. The extent of the intermolecular coupling within the F 40 PcCu films established by van der Waals interactions was investigated by in situ optical spectroscopy during film growth. The transfer of electrons and diffusion of counter cations in these films, as well as their electrochromic performance were characterized by electrochemical and spectroelectrochemical measurements with an aqueous solution of KCl as electrolyte. A moderate degree of intermolecular interaction of the F 40 PcCu molecules in the solid state was observed, compared to non-fluoroalkylated perfluoro phthalocyanine, F 16 PcCu and octakis-perfluoroisopropyl-perfluorophthalocyanine, F 64 PcCu , which exhibit stronger and weaker coupling, respectively. The replacement of F by perfluoroisopropyl is, thereby, established as a valuable approach to tune this coupling of chromophores and, hence, the transport coefficients of electrons and ions in the solid films. Reversible changes of the films upon reduction and intercalation of K + counter ions and re-oxidation and expulsion of the counter ions were confirmed by simultaneously measured optical absorption spectra. Thin films of F 40 PcCu showed a well-balanced, equally fast transport of electrons and ions. The films provided a fast and reversible switching process over at least 200 cycles indicating the stability of these materials. Optimum molecular substitution in organic thin films is established to facilitate electron and ion transport and, thereby, fast reversible electrochromic switching.