Chapter 2
Squaraine Dyes
Takeshi Maeda
Abstract 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.
Keywords Condensation · Transition metal-catalyzed cross-coupling · Squaric
acid · Squaraine
2.1 General Remarks
Since squaric acid (3,4-dihydroxycyclobut-3-ten-1,2-dione), a kind of oxocarbonic
acid, began to be used for research and development in the middle of the 1960s, a
variety of new organic compounds was synthesized from squaric acid (West and West
1980). Among them, squaraine dye that is obtained by the dehydration condensation reaction between squaric acid, and electron-rich compounds have been actively
studied due to its excellent optical property. Treibs et al. first reported that squaraine
dye can be obtained by the condensation reaction between squaric acid and pyrrole
T. Maeda (B)
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture Univesity,
Naka-ku, Sakai 599-8531, Japan
e-mail: tmaeda@chem.osakafu-u.ac.jp
© Springer Nature Singapore Pte Ltd. 2021
Y. Ooyama and S. Yagi (eds.), Progress in the Science of Functional Dyes,
https://doi.org/10.1007/978-981-33-4392-4_2
21
Squaraine Dyes
Takeshi Maeda
Abstract 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.
Keywords Condensation · Transition metal-catalyzed cross-coupling · Squaric
acid · Squaraine
2.1 General Remarks
Since squaric acid (3,4-dihydroxycyclobut-3-ten-1,2-dione), a kind of oxocarbonic
acid, began to be used for research and development in the middle of the 1960s, a
variety of new organic compounds was synthesized from squaric acid (West and West
1980). Among them, squaraine dye that is obtained by the dehydration condensation reaction between squaric acid, and electron-rich compounds have been actively
studied due to its excellent optical property. Treibs et al. first reported that squaraine
dye can be obtained by the condensation reaction between squaric acid and pyrrole
T. Maeda (B)
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture Univesity,
Naka-ku, Sakai 599-8531, Japan
e-mail: tmaeda@chem.osakafu-u.ac.jp
© Springer Nature Singapore Pte Ltd. 2021
Y. Ooyama and S. Yagi (eds.), Progress in the Science of Functional Dyes,
https://doi.org/10.1007/978-981-33-4392-4_2
21