Development of new multi-color electrochromic system by use of redox-active pi conjugated compounds

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Organic compounds with multi-color change are of current interest from development of new types of display systems. This research project has been carried out with aim of developing such multi-color organic electrochromic systems by use of the following redox-active pi conjugated compounds: (1) tetrakis(2,6-di-tert-butyl-4-hydroxyphenyl)- thieno[3,2-b]thiophene (1), (2) 1,3-dithiole-[n]radialenes (n=3, 4, 5, 6) (2), and (3) 1,3-dithiole-[n]dendralenes (n=3, 4) (3). As for 1 a reversible redox system was constructed with the corresponding two- and four-electron oxidized compounds, the mono- quinone (1') and diquinone (1''). Between the three components a large color change was observed: 1 (pale yellow), 1' (blue), and 1'' (brown red). It was very surprising that 1' and 1'' have absorption maxima at fairly long wavelength region, in spite of their not so large pi conjugation. As a result of some investigations, it was suggested that the excited state (biradical structure) might be located at comparatively low energy level and the transition energy from HOMO to LUMO might become very smaller. The indirect evidence to this suggestion was obtained from the detailed esr measurement of 1' and 1'', in which certainly the biradical involvement was recognized. Also, 2 and 3 exhibited electrochromism between their radical cations and dications. In particular, a comparatively large three color change was observed between 3 (yellow), and the radical cation (dark blue) and dication (green).