Chapter 6 - Syntheses and application of squarylium dyes

Abstract

This chapter provides a review on the synthetic procedures of squarylium dyes and their homologues. The syntheses of new types of squarylium homologues are also mentioned in the chapter with their optical properties. The chapter discusses the application of squarylium dyes to industrial fields and the future aspects of advanced materials. Squarylium dyes, often called “squaraines,” involve two aromatic or heterocyclic moieties at both ends of an oxocyclobutenoate core. They are often classified into polymethyne dyes because their localized π-conjugation structures are quite similar to those of cyanines. Both cyanines and squaryliums exhibit sharp and intense electronic absorption in the region from the visible to near-infrared (IR) and sometimes show fluorescence emission properties. However, from the viewpoint of the electronic conjugation structure, there are mainly two differences between them. One is that typical cyanine dyes are cationic although squaryliums are neutral showing zwitter-ionic structures. The other is the difference in electronic distribution in dye molecules.