Abstract
Cyanine dyes without a methine group or polymethine chain between their two terminal unsaturated heterocycles are called apocyanines. Such colorants which are yellow and red received the labels xantho-apocyanine and erythro-apocyanine dyes, respectively. Nevertheless, from time to time in the literature, the term “apocyanine dyes” is wrongly applied to various N-bridgehead heterocycles. In addition, the phenylogous zeromethine hemicyanine dyes are often incorrectly referred to with the term “apocyanine dyes”. This chapter clarifies the definition of apocyanine, describes how confusion over the term has come about, and points the ways in which it is sometimes incorrectly employed.
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