8
M. Matsui
A symmetrical pentamethine dye 1.22 is prepared by using two molar amounts
of 1.20 and 3-phenyliminopropenylaniline 1.21 (1–9).
A cyclic pentamethine dye 1.25 is prepared by way of an intermediate 1.24
followed by the reaction with 1.12 (1–10).
A cyclic symmetrical heptamethine dye 1.28 is formed by the reaction of two
molar amounts of 1.26 with 1.27 (1–11).
The acetylated hemicyanine 1.17 reacts with active methylene-containing
rhodamine 1.29 to give a merocyanine dye 1.30 (1–12).
A merocyanine dye 1.32 is provided by the reaction of an aldehyde 1.31 with
rhodanine 1.29 (1–13).
A symmetrical oxonol dye 1.34 is prepared by the reaction of two molar amounts
of 1.33 with 1.21 (1–14).
The reaction of 1.26 with an aldehyde 1.35 gives a styryl dye 1.36 (1–15).
1.2.3 Basic Properties of Polymethine Dyes
1.2.3.1 UV–Vis and NIR Absorption Spectra
The change in the absorption spectra is shown in Fig. 1.5. The absorption band of
polymethine dyes are influenced by (1) the number of carbon atoms in the conjugated
linkage group, (2) the kind of end heteroaromatics, and (3) the kind of substituent at
the linkage group:
(1) The usual structures of most cyanine and oxonol dyes are symmetrical. They
are an alternative chromophoric system. The absorption maximum (λ max ) and
molar absorption coefficient (ε) of cyanine dyes, whose structure is shown in
1.37, become more bathochromic and more intense with extension of the πconjugation system until heptamethine (n = 3). Though the λ max of nona- (n =
4) and undecamethine (n = 5) derivatives is bathochromic, the absorption band
becomes broad, which comes from cis–trans isomerization in the linkage group.
Cyclization of the linkage group is carried out to inhibit the isomerization. When
one olefinic bond increases in the conjugated linkage group, the λ max causes a
bathochromic shift about 100 nm. For example, in the case of indolenium dyes,
the λ max is observed at 545 nm for trimethine (n = 1), 636 nm for pentamethine
(n = 2), and 745 nm for heptamethine (n = 3) derivatives, respectively. The
λ max of oxonol dyes 1.38 also becomes more bathochromic with increasing
chain length due to extension of π-conjugation system.
(2) The λ max also depends on the kinds of end heteroaromatics. Benzoxazole,
indolenine, benzothiazole, and lepidine have been used as heteroaromatics. For
example, in the case of trimethine dye 1.37 (n = 1), the λ max is in the following
order of the heteroaromatics: benzoxazole (485 nm) < indolenine (545 nm) <
benzothiazole (557 nm) < lepidine (605 nm). The λ max almost depends on the
M. Matsui
A symmetrical pentamethine dye 1.22 is prepared by using two molar amounts
of 1.20 and 3-phenyliminopropenylaniline 1.21 (1–9).
A cyclic pentamethine dye 1.25 is prepared by way of an intermediate 1.24
followed by the reaction with 1.12 (1–10).
A cyclic symmetrical heptamethine dye 1.28 is formed by the reaction of two
molar amounts of 1.26 with 1.27 (1–11).
The acetylated hemicyanine 1.17 reacts with active methylene-containing
rhodamine 1.29 to give a merocyanine dye 1.30 (1–12).
A merocyanine dye 1.32 is provided by the reaction of an aldehyde 1.31 with
rhodanine 1.29 (1–13).
A symmetrical oxonol dye 1.34 is prepared by the reaction of two molar amounts
of 1.33 with 1.21 (1–14).
The reaction of 1.26 with an aldehyde 1.35 gives a styryl dye 1.36 (1–15).
1.2.3 Basic Properties of Polymethine Dyes
1.2.3.1 UV–Vis and NIR Absorption Spectra
The change in the absorption spectra is shown in Fig. 1.5. The absorption band of
polymethine dyes are influenced by (1) the number of carbon atoms in the conjugated
linkage group, (2) the kind of end heteroaromatics, and (3) the kind of substituent at
the linkage group:
(1) The usual structures of most cyanine and oxonol dyes are symmetrical. They
are an alternative chromophoric system. The absorption maximum (λ max ) and
molar absorption coefficient (ε) of cyanine dyes, whose structure is shown in
1.37, become more bathochromic and more intense with extension of the πconjugation system until heptamethine (n = 3). Though the λ max of nona- (n =
4) and undecamethine (n = 5) derivatives is bathochromic, the absorption band
becomes broad, which comes from cis–trans isomerization in the linkage group.
Cyclization of the linkage group is carried out to inhibit the isomerization. When
one olefinic bond increases in the conjugated linkage group, the λ max causes a
bathochromic shift about 100 nm. For example, in the case of indolenium dyes,
the λ max is observed at 545 nm for trimethine (n = 1), 636 nm for pentamethine
(n = 2), and 745 nm for heptamethine (n = 3) derivatives, respectively. The
λ max of oxonol dyes 1.38 also becomes more bathochromic with increasing
chain length due to extension of π-conjugation system.
(2) The λ max also depends on the kinds of end heteroaromatics. Benzoxazole,
indolenine, benzothiazole, and lepidine have been used as heteroaromatics. For
example, in the case of trimethine dye 1.37 (n = 1), the λ max is in the following
order of the heteroaromatics: benzoxazole (485 nm) < indolenine (545 nm) <
benzothiazole (557 nm) < lepidine (605 nm). The λ max almost depends on the