Violene-cyanine hybrid

Stabilization of the redox cycle is relatively important in construction of potentially useful electrochromic materials, because the molecules needed for application require high redox-stability. Recently, S. Hiinig et al. proposed the concept of violene-cyanine hybrid to produce stabilized organic electrochromic materials (3). The hybrid is constructed by a violene-type redox system containing delocalized closed-shell polymethine dyes as end groups. The hybrid is expected to exhibit the color of a cyanine dye, by an overall two-electron... 

Figure 3. General structure of Hiinig s violene-cyanine hybrid. One or two of the moieties X=C-Y represent cyanines, oxonols, or merocyanines.

Figure 4. Wurster type violene-cyanine hybrid.

The dication 212+ composed of two methylium units connected to a p-phenylene spacer would be a candidate for new Wurster type violene-cyanine hybrid (Figure 12) (15). The reaction of four molar amounts of azulene 6b with terephthalaldehyde yielded the hydro precursor. Synthesis of the dication 212+ was accomplished by hydride abstraction with DDQ in almost quantitative yield. The dication 212+ was expected to show destabilization, but instead it exhibited high thermodynamic stability just like the corresponding monocation 3b+. 

Figure 12. Wurster type violene-cyanine hybrid composed of two methylium units connected via a p-phenylene spacer.

The combination of the di(l-azulenyl)methylium units produced dicationic species with high stabilities. However, transformation of the dication 212+ to the quinoid compound 21 as a Wurster type violene-cyanine hybrid was not achieved due to the instability of the reduced species. 

We have established the conversion between the two colored species by electrochemical reaction utilizing the concept of a Wurster type violene-cyanine hybrid. Dications 222+ and 232+ showed significant changes in their absorption spectra in different oxidation states. Therefore, dications 222+ and 232+ could function as new violene-cyanine hybrids, in which the four end groups (X and Y) in the general structure are azulenes (Figure 4). 

Two color changes could be induced by the violene-cyanine hybrid designed from the Wurster type violene. However, the multiple color changes... 

Synthesis of novel polyelectrochromic materials with the new structural motif utilizing stabilized carbocations has been demonstrated by several examples. The Wurster type violene-cyanine hybrid systems (22z+ and 232+) exhibit the presumed two color changes. The new cyanine-cyanine hybrid system (24 ) with a cyanine unit at the one terminus exhibits three color changes. 

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