1 Polymethine Dyes
5
Here, it should be noted that the methine group belongs only to the linkage group.
The methine group contained in the heteroaromatic moiety is not included. It is
confusing that, in the long history of cyanine dyes, the terms “carbo-, dicarbo-, and
tricarbocyanines”, which correspond the number of n in Figs. 1.1, 1.2, 1.3, and 1.4,
have been also used.
When one heteroatom is nitrogen and another one is oxygen, they are neutral and
possibly expressed as a twitter ionic structure. Merocyanine dyes are classified in
this group.
When two heteroatoms are oxygen, they are anionic. Oxonol dyes belong to this
group.
The general structure of polymethine dye 1.3 is shown Fig. 1.3. The structure of
polymethine dyes consists of two hetero end aromatics and a linkage group between
them. The examples of hetero end aromatics are shown in groups A and B. Group A
shows the hetero end aromatics for cyanine and merocyanine dyes. The use of same
hetero end aromatics in group A at both sides produces the symmetrical cyanine dyes
and that of different ones, unsymmetrical derivatives. Group B indicates the hetero
end aromatics for oxonol dyes. The use of hetero end aromatics in group A and B in
each terminal affords merocyanine dyes.
One of the unique points of polymethine dyes is the linkage group. The C–C
bond length in the linkage group of benzothizolyl cyanine dyes was measured to
be 1.41 Å by single X-ray crystallography. This length is between those of double
(1.34 Å) and single (1.54 Å) C–C bonds, indicating that electrons are delocalized
in the methine linkage. The polymethine chain is generally linked in the α- or γ -
position to the nitrogen of the heterocyclic group in cyanines. Squarylium dyes are
the special type of the linkage. Recently, squarylium cyanine dyes have attracted
much attention due to their bathochromic and intense absorption and emission as
functional dyes. Croconium cyanine dyes are much more bathochromic than the
squarylium derivatives. The linkage group in polymethine dyes is easily substituted
to get desired properties such as UV–Vis and NIR absorption and fluorescence bands.
Thus, the polymethine dyes consist of the combination of hetero end aromatics
and linkage groups. Furthermore, the aza analogues also exist. Therefore, an infinite
number of polymethine dyes are considerable.
1.2.2 Synthesis
Examples for the synthesis of polymethine dyes are indicated in Fig. 1.4. A quaternary
salt having an acidic methyl group such as 1.4, which is in equilibrium with the
corresponding enamine 1.5 in solution, reacts with an aromatic compound having a
good leaving group such as 1.6 to give a monomethine dye 1.7 (1–4).
Unsymmetrical monomethine dyes 1.10 and 1.13 are also obtained by similar
reactions (1–5 and 1–6).
A symmetrical meso-substituted trimethine dye 1.15 is prepared from two molar
amounts of 1.8 with 1,1,1-triethoxyalkanes 1.14 (1–7).
5
Here, it should be noted that the methine group belongs only to the linkage group.
The methine group contained in the heteroaromatic moiety is not included. It is
confusing that, in the long history of cyanine dyes, the terms “carbo-, dicarbo-, and
tricarbocyanines”, which correspond the number of n in Figs. 1.1, 1.2, 1.3, and 1.4,
have been also used.
When one heteroatom is nitrogen and another one is oxygen, they are neutral and
possibly expressed as a twitter ionic structure. Merocyanine dyes are classified in
this group.
When two heteroatoms are oxygen, they are anionic. Oxonol dyes belong to this
group.
The general structure of polymethine dye 1.3 is shown Fig. 1.3. The structure of
polymethine dyes consists of two hetero end aromatics and a linkage group between
them. The examples of hetero end aromatics are shown in groups A and B. Group A
shows the hetero end aromatics for cyanine and merocyanine dyes. The use of same
hetero end aromatics in group A at both sides produces the symmetrical cyanine dyes
and that of different ones, unsymmetrical derivatives. Group B indicates the hetero
end aromatics for oxonol dyes. The use of hetero end aromatics in group A and B in
each terminal affords merocyanine dyes.
One of the unique points of polymethine dyes is the linkage group. The C–C
bond length in the linkage group of benzothizolyl cyanine dyes was measured to
be 1.41 Å by single X-ray crystallography. This length is between those of double
(1.34 Å) and single (1.54 Å) C–C bonds, indicating that electrons are delocalized
in the methine linkage. The polymethine chain is generally linked in the α- or γ -
position to the nitrogen of the heterocyclic group in cyanines. Squarylium dyes are
the special type of the linkage. Recently, squarylium cyanine dyes have attracted
much attention due to their bathochromic and intense absorption and emission as
functional dyes. Croconium cyanine dyes are much more bathochromic than the
squarylium derivatives. The linkage group in polymethine dyes is easily substituted
to get desired properties such as UV–Vis and NIR absorption and fluorescence bands.
Thus, the polymethine dyes consist of the combination of hetero end aromatics
and linkage groups. Furthermore, the aza analogues also exist. Therefore, an infinite
number of polymethine dyes are considerable.
1.2.2 Synthesis
Examples for the synthesis of polymethine dyes are indicated in Fig. 1.4. A quaternary
salt having an acidic methyl group such as 1.4, which is in equilibrium with the
corresponding enamine 1.5 in solution, reacts with an aromatic compound having a
good leaving group such as 1.6 to give a monomethine dye 1.7 (1–4).
Unsymmetrical monomethine dyes 1.10 and 1.13 are also obtained by similar
reactions (1–5 and 1–6).
A symmetrical meso-substituted trimethine dye 1.15 is prepared from two molar
amounts of 1.8 with 1,1,1-triethoxyalkanes 1.14 (1–7).