Squarylium Dyes and Related Compounds

Abstract

Squarylium dyes, first synthesized in the 1960s, consist of an oxocyclobuteno- late core with aromatic or heterocyclic components at both ends of the molecule. As squarylium dyes show intense absorption and often fluorescence emission, they have attracted much attention from the viewpoint of technological applications. Efforts have been made to develop new synthetic methods for constructing squarylium-based chromophores. Here novel synthetic protocols for construction of novel squaryliums and related chromophores are reviewed. The present chapter starts with modifi- cation of squarylium dyes affording amino- and dithio-substituted dyes, and then the synthesis of unsymmetrical squarylium dyes is introduced. The syntheses of methine- and arene-bridged bis-squarylium dyes are next reviewed, and the bisquaryl- based analogues are also discussed. Applications of squarylium dyes and related compounds are discussed, including xerographic devices utilizing photoconductive properties of squarylium dyes, photovoltaic devices employing the dyes as photo- sensitizers, and biolabeling and chemosensory materials for analytical uses. Low band-gap electroconductive materials are available by polymerization of squarylium skeletons. As a future aspect, supramolecular systems based on squaryliums are also introduced.