24
T. Maeda
Fig. 2.4 A diagram of the transition energy (wavelength) of maximum absorption bands for
squaraine dyes with various electron-rich components incorporated on 1,3-position of squaric acid
residue
The transition energy for its maximum absorption depends on the structure of
the aromatic ring and the heterocycle that are substituted at the 1,3-positions of the
squaric acid (Fig. 2.4). The symmetrical squaraine dyes with N,N-dialkylaniline and
indolenine components, which are the typical structure of these dyes, show a prominent absorption band around 620–650 nm in the solution state. The absorption of
a squaraine dye consisting of thiophene and the semi-squaraine was observed at a
higher energy region (554 nm). On the other hand, the use of electron-rich components with extended π-systems such as benzopyrylium and benzo[c,d]indolenine
makes the transition energy of squaraine dyes lower. This indicates that the absorption
band of squaraine dye can be adjusted in a wide wavelength region from visible light
to near-infrared light by selecting the substituent components at the 1,3-positions.
2.3 Synthesis of Squaraine Dyes by Condensation Reaction
Symmetric squaraine dyes are generally obtained by the condensation reaction
between one-equivalent squaric acid and two-equivalent aromatic amine/phenol or
heterocyclic components. These dyes are roughly classified into two groups; one is
the dye obtained by the condensation reaction with aromatic rings represented by
pyrrole and N,N-dialkyl aniline derivatives, and the other is that obtained by the
condensation reaction with activated methylene compounds represented by indolenine and benzothiazole (Fig. 2.5) (Maahs and Hegengerg 1966; Sprenger and Ziegenbein 1968; Ziegenbein and Sprenger 1966; Shaw et al. 2018). The squaraine dyes
obtained by the condensation reaction have been summarized in some review articles
(Law 1993; Schmidt and West 1980; Sreejith et al. 2008a; Beverina and Salice 1207;
McEwen and Wallace 2009; Yagi and Nakazumi 2008).
T. Maeda
Fig. 2.4 A diagram of the transition energy (wavelength) of maximum absorption bands for
squaraine dyes with various electron-rich components incorporated on 1,3-position of squaric acid
residue
The transition energy for its maximum absorption depends on the structure of
the aromatic ring and the heterocycle that are substituted at the 1,3-positions of the
squaric acid (Fig. 2.4). The symmetrical squaraine dyes with N,N-dialkylaniline and
indolenine components, which are the typical structure of these dyes, show a prominent absorption band around 620–650 nm in the solution state. The absorption of
a squaraine dye consisting of thiophene and the semi-squaraine was observed at a
higher energy region (554 nm). On the other hand, the use of electron-rich components with extended π-systems such as benzopyrylium and benzo[c,d]indolenine
makes the transition energy of squaraine dyes lower. This indicates that the absorption
band of squaraine dye can be adjusted in a wide wavelength region from visible light
to near-infrared light by selecting the substituent components at the 1,3-positions.
2.3 Synthesis of Squaraine Dyes by Condensation Reaction
Symmetric squaraine dyes are generally obtained by the condensation reaction
between one-equivalent squaric acid and two-equivalent aromatic amine/phenol or
heterocyclic components. These dyes are roughly classified into two groups; one is
the dye obtained by the condensation reaction with aromatic rings represented by
pyrrole and N,N-dialkyl aniline derivatives, and the other is that obtained by the
condensation reaction with activated methylene compounds represented by indolenine and benzothiazole (Fig. 2.5) (Maahs and Hegengerg 1966; Sprenger and Ziegenbein 1968; Ziegenbein and Sprenger 1966; Shaw et al. 2018). The squaraine dyes
obtained by the condensation reaction have been summarized in some review articles
(Law 1993; Schmidt and West 1980; Sreejith et al. 2008a; Beverina and Salice 1207;
McEwen and Wallace 2009; Yagi and Nakazumi 2008).