ARTICLE pubs.acs.org/JPCC
Electrochromic Switching of Evaporated Thin Films of Bulky, Electronic Deficient Metallo-Phthalocyanines Stefanie Nagel,† Martin Lener,† Christopher Keil,† Robert Gerdes,‡ yukasz yapok,‡ Sergiu M. Gorun,‡ and Derck Schlettwein*,† † ‡
Institute of Applied Physics, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
bS Supporting Information ABSTRACT: Thin films of four-coordinated perfluoroalkylsubstituted phthalocyanines were prepared by physical vapor deposition. The electrochemical redox characteristics and simultaneous changes in the optical absorbance spectra of the films (40215 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.
’ INTRODUCTION Phthalocyanine thin films are of interest as organic semiconductor films to utilize their properties of light absorption and/or electron or hole conduction in devices like organic light-emitting diodes (OLED),1 organic field-effect transistors (OFET),2 or organic photovoltaic cells (OPV).3 The positions of frontier energy levels (HOMO—highest occupied molecular orbital; LUMO—lowest unoccupied molecular orbital) are decisive for their injection characteristics as contact4,5 and for the observed type of majority charge carriers following desired or adventitious doping reactions.6 The HOMO and LUMO positions are most easily tuned by the choice of appropriate electronwithdrawing or -donating substituents at the phthalocyanine ligand. Electron-withdrawing substituents decrease the electron density in the central π-system7,8 and thereby lower the electron energy of the occupied as well as unoccupied orbitals—both are mainly π-type in character9—and lead to a more positive redox potential (easier reduction)7,8 and hence dominance of n-type doping interactions.8,10,11 For electron-donating substituents the opposite trend is observed.7,8 Since p-type characteristics are observed for a number of organic semiconductors including unsubstituted phthalocyanines (Pc), substitution by electronwithdrawing substituents is of special interest. The use of metal complexes of perfluorinated phthalocyanine F16Pc has been well established over the last years as one of the most promising r 2011 American Chemical Society
ways to induce stable n-type conduction in organic semiconductor thin films.2 Aside from their application as semiconductors, phthalocyanine films might also be of interest as electrochromic layers because of their strong light absorption, i.e., high molar extinction coefficients,9 and the strong changes of color upon oxidation or reduction.12,13 Unlike unsubstituted phthalocyanines that, upon irreversible oxidation, suffer corrosion as thin films,14 films of double decker lanthanide phthalocyanines15 or substituted phthalocyanines with electron-withdrawing substituents exhibit largely reversible electrochromic reductions.12,1621 With the exception of tetrapyridotetraazaporphyrins17,21 or tetrapyrazinotetraaazaporphyrins,18,19 which contain N-heteroatoms in the benzogroups, F16Pc appear to be the most versatile and reliable phthalocyanine molecules for film preparation and reversible electrochromic switching.20 For technical electrochromism, 50100 nm films are needed, thicker than a monolayer but clearly below the thickness of traditional inorganic films, e.g., those formed by tungsten oxide.22,23 For all films charge-compensating counterion intercalation is needed to preserve electroneutrality during a redox process.12 Intercalation reactions were Received: February 1, 2011 Revised: March 22, 2011 Published: April 12, 2011 8759
dx.doi.org/10.1021/jp2010748 | J. Phys. Chem. C 2011, 115, 8759–8767
The Journal of Physical Chemistry C clearly noticed in films of octacyanophthalocyanines,16 tetrapyrazinotetraazaporphyrins,18,19 F16PcZn,20 or tetrapyridotetraazaporphyyrins17,21 prepared either by coating from solution16,18,19 or by physical vapor deposition.20,21 The intercalation of counterions can either be fast16,18,19,21 with a high diffusion coefficient that results in a linear dependence of peak currents on scan speed in cyclic voltammetry or rate-determining, hindering the redoxstate-switching process,16,1820 characterized in general by small diffusion coefficient and square-root dependence of peak currents on scan rate. Film thickness, preparation conditions, and the choice of counterion are factors that additionally influence film redox properties. Recently a new class of phthalocyanines substituted with perfluoroalkyl (Rf) groups at the ligand has been reported to exhibit an even stronger electron deficiency relative to those presented above since Rf groups have an overall higher electron-withdrawing ability than fluorine atoms when attached to aromatic rings.2428 1,4,8,11,15,18,22,25-Octakis-fluoro-2,3,9,10,16,17,23,24-octakisperfluoro(isopropyl)phthalocyanines (F64Pc) serve as prototype of these materials. They are characterized by an additional shift of the redox potential in solution toward the positive direction relative to F16Pc.25,26 F64Pc are stable under ambient conditions. Under illumination they are also stable against oxidation damage by the singlet oxygen they generate and its daughter species responsible for possible oxidative decay pathways of molecular materials.27,29 In addition to their electronic stabilization effect via electron-loss suppression, perfluoro-isopropyl groups exert a strong steric effect due to their bulkiness, largely suppressing intermolecular interactions seen in minimized intermolecular spin couplings and decreased splitting of optical absorption bands.28,30 On the one hand, this feature lowers electron mobility in thin films30 that is disadvantageous, for example, for OFET.2 On the other hand, for the transport of ions in the molecular solid such decreased intermolecular coupling could be mostly beneficial since smaller lattice energies have to be overcome in order to allow ionic movement in the films. Furthermore, electron transport has to occur only across a 50100 nm film rather than along micrometers of films as in OFET structures. Thin films of perfluoroalkyl-substituted phthalocyanines, therefore, could represent a very promising group of molecules to fabricate organic electrochromic layers. Such layers are of interest for display purposes of intermediate switching speeds which keep the display status without additional signal current and for smart windows or mirrors whose transmission can be switched by an applied potential.12,13 An advantage of organic layers compared to the more common inorganic devices based on tungsten oxide lies in the clearly higher absorption constant of organic pigments allowing thinner films for the same coloration efficiency. Since the preparation of organic thin films typically involves low-temperature processes and does not require subsequent annealing, low-weight, flexible, and affordable plastic substrates can be used. The low intermolecular coupling and the high chemical stability of perfluoroalkyl-substituted phthalocyanines allow the preparation of thin films by physical vapor deposition, a method well suited to obtain homogeneously colored materials of well-defined thickness in the submicrometer range.30 We report here the solution and solid-state electrochromic reduction of F64PcCu and F64PcZn analyzed by cyclic voltammetry and spectroelectrochemical measurements. We compare their characteristics with those of F64PcVO which we studied separately.31 The effects of the Rf groups are revealed by
ARTICLE
comparison with perfluorinated F16PcCu2 similar to the earlier studied F16PcZn.20
’ MATERIALS AND METHODS F64PcCu and F64PcZn were synthesized and purified as reported earlier.28,30 Trifluorotoluene (TFT, Fluka, 99.5%), dimethylformamide (DMF, Aldrich, 99.8%), tetrabutylammoniumtetrafluorborate (TBABF, Fluka, 99% electrochemical grade), tetrabutylammoniumperchlorate (TBAP, Fuka 99% electrochemical grade), and ferrocene (Fc, Fluka, 98%) were used without further purification. Experiments were carried out in a glovebox (
