2 Squaraine Dyes
25
Fig. 2.5 Representative squaraine dyes with symmetrical structures
In the condensation reaction between an electron-rich arene and squaric acid,
semi-squaraine is produced as an intermediate (Fig. 2.6a). At the succeeding second
stage of the condensation reaction, arene can attack the 2- and 3-positions of the semisquaraine. However, the 1,2-substitutions are scarcely produced because squaraine
dye of the 1,3-substitution is obtained regioselectively (Law and Bailey 1986). On
the other hand, in the Friedel-Crafts reaction between arene and 3,4-dichloro-3cyclobutene-1,2-dione that is acid chloride of squaric acid obtained by the reaction
Fig. 2.6 Squaraine derivatives obtained by condensation (a), Freidel-Crafts reaction (b), and
Liebeskind-Srogl reaction (c)
25
Fig. 2.5 Representative squaraine dyes with symmetrical structures
In the condensation reaction between an electron-rich arene and squaric acid,
semi-squaraine is produced as an intermediate (Fig. 2.6a). At the succeeding second
stage of the condensation reaction, arene can attack the 2- and 3-positions of the semisquaraine. However, the 1,2-substitutions are scarcely produced because squaraine
dye of the 1,3-substitution is obtained regioselectively (Law and Bailey 1986). On
the other hand, in the Friedel-Crafts reaction between arene and 3,4-dichloro-3cyclobutene-1,2-dione that is acid chloride of squaric acid obtained by the reaction
Fig. 2.6 Squaraine derivatives obtained by condensation (a), Freidel-Crafts reaction (b), and
Liebeskind-Srogl reaction (c)