Violene/cyanine hybrids as electrochromic systems.: Part 3:: Heterocyclic onium end groups

Synthesis, cyclic voltammetry (CV) and spectroelectrochemistry (SE) of tetra-onium salts 3(OX)(+4).4BF(4)(-) and 6(OX)(+4).4PF(6)(-) 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 3(RED)(+2) and 6(RED/OX)(+2), respectively. The latter is further reversibly reduced to colorless 6(RED)(0) for which a cyclic structure with a central four-membered ring is proposed. Radical ions like 3 (6)(SEM)(.+3) and 3 (6)(SEM)(.+1) were not detected by CV or SE. PM3 calculations of 6(OX)(+4) down to 6(RED)(0) change and verify two perpendicular cyanine units in 6(RED/OX)(+2) as well as the cyclic structure for 6(RED)(0), 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 displayreversible two electron transfer with strong changes in color. (C) 2000 Elsevier Science Ltd. All rights reserved.