Electrochromics by intramolecular redox switching of single bonds

Compounds of the general structure presented in Scheme 1 are proposed as electrochromic systems (ESs) in which reversible electron transfer is associated with breaking or forming of single bonds. In contrast to classical ESs, both redox states represent closed-shell systems. Whereas type OL/CS (Open form − Long-wavelength absorption/Closed form − Short-wavelength absorption) has already been demonstrated, this paper presents examples of the OS/CL type, in which the long-wavelength absorption is displayed by the ring-closed form. Bis(azines) RED-8, RED-13, RED-16, and RED-17, as well as the bis(amines) RED-22, RED-23, and RED-24 were prepared and characterized by their cyclic voltammograms (CV) and spectroelectrograms (SE). Except for the case of RED-24, the reversible overall transfer of two electrons is connected to switching of a single C−C or N−N bond. This reaction is not restricted to an entropically favorable transannular reaction (OX-82+, OX-162+, and OX-172+) but also occurs with the formation of five- (OX-132+ and OX-232+) and certain six-membered rings (OX-222+). These results suggest that the OS/CL general structure type should allow broad variation both in redox potentials and in ranges of color, similarly to the OL/CS type. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)