Electrochromism of Single Transition-Metal Phthalocyanine Thin Films: Effect of Central Metal and Composite Structure

Electrochromism of various transition-metal phthalocyanine thin films is studied in a sodium nitrate solution. Copper and nickel phthalocyanine show reversible electrochromism, while, zinc, cobalt, and iron phthalocyanine show the irreversible type. The transition-metal phthalocyanine film, which gives irreversible electrochromism, exhibits a new absorption peak in the NIR region under oxidation. This result indicates that some chemical changes of molecular structure occur with oxidation, and consequently the cyclic voltammograms and the absorbance are diminished with repeated scans, even under the well-designed experimental conditions. As for composite phthalocyanine, the high dispersibility of the phthalocyanine molecules in the codeposited thin film can improve the reversibility in electrochromism. In this case, chemical changes of the molecular structure of metal phthalocyanine with irreversible electrochromism can be prevented to some extents by the codeposition with another phthalocyanine with reversible electrochromism. Sequentially deposited double-layered thin films, in contrast, give completely different results. The electrochromism depends on the order of the oxidation potentials of simple phthalocyanine thin film, and the designed hole transfer compatible with the concept of “sequential potential field” can be accomplished in the double-layered thin film.