Aza-vinylogous Redox Systems

In the field of cyanine dyes aza-substitution has been studied extensively for nearly 100 years and even a theoretical treatment of the observed hypsochromic and batho-chromic shifts is possible In case of the violenes with their even number of methine groups between the two ends, symmetrical bis- or tetraaza-substitution is most suitable both for practical and theoretical (electrochemical) reasons. 

1 Redox Systems with a Diaza-dimethme Chain 

Coupling of two heterocycles by a N-N-bridge is simple to achieve and produces especially significant examples of two step redox systems with excellent reversibility. With the following compounds 33 39 an increase in Ksem of 10 —10 compared to the dimethine derivatives is observed, so that most Ksem s are 10 . Additionally all potentials become more positive by as much as 1 V. Besides with hardly any exception, all three oxidation levels of these systems can be isolated .  

The isomeric indolizines 38 and 39 seem to contradict these rules Compared to their dimethine derivatives 30 (n = 1) and 29 (n = 1) potentials are more positive by 0.1 V only and their Ksem s are quite similar. In the cases, however, the reduced form contains an azo group which changes to the less energetic azino group on oxidation, simultaneously transforming the indolizine into a pyri-dinium ring. 

This aza-effect is preserved if in a C4-bridge the two central C-atoms are aza-substituted as in 25, n = 2). Even four nitrogens in a row as in 

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