2. Squaraine Dyes

Squaraine dyes which are composed of a cyclobutenedione core with aromatic or heterocyclic components show sharp and intense electronic absorption in the areas of visible to near-infrared regions and often fluorescence emission. These prominent optical properties arouse our interest in various fields of applications using the dye. In order to respond to the diverse demands of squaraine dyes in application fields, considerable effort have been made in the past decades to design and synthesize symmetrical and unsymmetrical squaraine dyes by means of classical condensation reaction of squaric acid moiety with electron-rich compounds. A novel approach using transition-metal catalyzed cross-coupling is developed to construct squaraine chromophores. This approach allows not only to attach desired functionalities on peripheral parts of chromophores but also to synthesize oligomeric and polymeric squaraine dyes. In addition to the molecular level of study, the supramolecular architectures have been constructed by non-covalent interaction between the dye molecules. This section gives an overview of the recent advances in syntheses and structures of squaraine dye with particular attention to the novel synthetic protocol.