Dual-mode electrochromism switched by proton transfer: dynamic redox properties of bis(diarylmethylenium)-type dyes

Upon oxidative dimerization of pale yellow Ar2C-CHPh 1 (Ar = 4-Me2NC6H4), d eep blue 1,4-dication 2(2+) was obtained as a stable salt, which was transf ormed into I by reductive C-C bond fission; deprotonation of 22+ gave inten se yellow diene 3, which was interconvertible with violet dication 4(2+) by two-electron transfer, thus exhibiting two distinct modes of electrochromi sm before and after proton transfer.