Switching of the Rate-limiting Step in the Electrochromic Reduction of Fluorinated Phthalocyanine Thin Films by Decreased Intermolecular Coupling

Thin solid films (5-120 nm) of the electron deficient perfluorinated phthalocyanine 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadeca-fluorophthalocyaninato copper(II) (F16PcCu) and the perfluoroalkyl-substituted perfluorophthalocyanine 1,4,8,11,15,18,22,25-octakis-fluoro-2,3,9,10,16,17,23,24- octakis-perfluoro(isopropyl) phthalocyaninato copper(II) (F64PcCu) have been prepared by physical vapor deposition onto a glass substrate coated with a transparent conductor (indium-tin-oxide, ITO). Cyclic voltammetry, chronoamperometry and spectroelectrochemical UV-vis spectroscopy studies revealed that the films were readily reduced and re-oxidized at potentials slightly negative of the Ag/AgCl reference electrode confirming a significant stabilization of the produced anions by the electron-withdrawing fluorinated substituents. A significant color change accompanied the change of the molecular redox states. Electroneutrality of the films was ensured by intercalation of K+-counter ions upon reduction and their ejection upon re-oxidation. Molecules of F64PcCu were reduced by one electron, those of F16PcCu by up to two, for thin films subjected to slow charging rates. In addition to a decisive influence on the electronic structure of the inner ring pi-system of the metal complexes the steric hindrance of the fluorinated ligands influence the extent of intermolecular coupling. The perfluoroalkyl, R-f, groups in F64PcCu thereby led to widely decoupled electronic states of the molecules in the thin films as detected by the observed optical absorption spectra. The bulky R-f groups also led to facile diffusion of the K+-counter ions into the films, as well as to a significantly reduced electrical conductivity of F64PcCu relative to F16PcCu. Time-resolved measurements served to analyze the interplay of the two mandatory transport processes. Interestingly, the variability in steric hindrance results in a qualitative change in the rate-limiting steps of the reduction. Thus, in the case of F64PcCu the transport of electrons in the molecular films is rate-limiting, as opposed to counter ion diffusion in the case of F16PcCu. This switch underlines the importance of structural parameters for the tuning of the observed reduction and reoxidation of organic thin films via intermolecular interactions. (C) 2015 Elsevier Ltd. All rights reserved.