Violene/Cyanine Hybrids as Electrochromic Systems. Part 3: Heterocyclic Onium End Groups

Synthesis, cyclic voltammetry (CV) and spectroelectrochemistry (SE) of tetra-onium salts 3OX+4·4BF4−and 6OX+4·4PF6− are presented for both systems. Cyclic voltammograms display several reduction and oxidation peaks containing at least one reversible pair. The colorless tetracations on reduction reversibly develop long wavelength absorptions. Absorption bands arise which are attributed to 3RED+2 and 6RED/OX+2, respectively. The latter is further reversibly reduced to colorless 6RED0 for which a cyclic structure with a central four-membered ring is proposed. Radical ions like 3 (6)SEMradical dot+3 and 3 (6)SEMradical dot+1 were not detected by CV or SE. PM3 calculations of 6OX+4 down to 6RED0 change and verify two perpendicular cyanine units in 6RED/OX+2 as well as the cyclic structure for 6RED0 which is of 29.86 kcal/mol more stable than the corresponding acyclic one. Systems 7–12 described in the literature are discussed as vinylogous examples of A which also display reversible two electron transfer with strong changes in color.