Electrochromic redox reactions of vapour-deposited thin films of tetrapyridotetraazaporphyrinatozinc(II)

Electrochromic redox reactions of vacuum-evaporated thin films of tetrapyri dotetraazaporphyrinatozinc(II) (TPyTAPZn) have been studied in aqueous KCI electrolytes by in situ spectrocyclic voltammetry, which combines cyclic vo ltammetry and in situ monitoring of the absorption spectrum of the film. Al though the voltammetric response of TPyTAPZn thin film electrodes can hardl y be understood by the conventional electrochemical approach, the spectral information Linked to the voltammetric information elucidated the presence of two-step reductions and reoxidations by each electron per TPyTAPZn molec ule, which. however, take place with significant overlap during the voltamm etric scan. Reduction of the film turned the color of the film from blue to purple and it changed reversibly to regenerate the original spectrum upon reoxidation. These reversible electrochromic reactions could be repeated ma ny times without causing any noticeable change in the voltammetric and spec tral responses. While use of electrolytes with various cations did not affe ct the redox behavior of the film, the effect of electrolyte pH was clearly seen, changing the ratio of the two reduction and two reoxidation peak mag nitudes in the voltammogram. XPS analysis of the reduced film indicated tha t charge compensation upon reduction of the film occurred by protonation of reduced TPyTAPZn molecules at their aza and/or pyrido nitrogen atoms. A re action scheme is presented to describe the observed redox reactions of the TPyTAPZn thin film from thermodynamic and kinetic viewpoints. Copyright (C) 2000 John Wiley & Sons, Ltd.