10
M. Matsui
is substituted at the unstarred position, is more bathochromic than the methine
analogue.
When a methyl group is introduced at the unstarred meso-position in trimethine
dye 1.41, the λ max depends on the kind of heteroaromatics. In the case of benzoxazolyl derivative, the λ max causes a bathochromic shift due to the introduction of
the electron-donating methyl group at the unstarred position. However, in the case
of benzothiazolyl and benzoselenolyl derivatives, the λ max is hypsochromic due to
steric effect between the heteroatoms and the methyl group.
In the cases of merocyanine 1.42 and styryl dyes 1.43, the λ max causes
bathochromic shift when the π-conjugation system is extended. As merocyanine
and styryl dyes are a push–pull chromophoric system, as stronger the push moiety
and as stronger the pull moiety, the more bathochromic the λ max .
The ε value of cyanine dyes is usually larger than 100,000 dm
3 mol
−1 cm
−1 due
to wide planarity. Some cyanine, merocyanine, and styryl dyes are fluorescent.
1.2.3.2 Self-Aggregation
The self-association of dye molecules is a unique and important phenomenon of
polymethine dyes. The aggregates can show characteristic changes in the absorption
band. Comparing with the monomer absorption band, the bathochromically shifted
one is called J-band (J for Jelley) and the hypsochromically shifted one H-band (H
for hypsochromic). The absorption bands formed by aggregates are explained by the
molecular exciton coupling theory (Kasha et al. 1965).
In diluted solutions, as there are no intermolecular interactions between dye
molecules, the excited molecules are just deactivated to the ground state. Meanwhile,
in concentrated solutions or in the crystalline form, the hole in HOMO can electrostatically have interactions with the electrons in LUMO. As a result, an exciton, a
pair of a hole and an electron, is formed to delocalize the excited state (excitonic
state).
The energy levels of an arranged dimer in the isolated two molecules have been
proposed by Kasya (Kasha et al. 1965). The excitonic state of the dye aggregate splits
into two levels E
and E
through the interaction of transition dipoles as shown in
Fig. 1.6. One dipole phase relation is toward the same direction and another one in
the opposite direction. In the case of parallel transition dipoles, the E
level, in which
the dipole phase relation is canceled, is forbidden to cause a blue shift, while in the
case of in-line transition dipoles, the E
level, in which the dipole phase relation is
canceled, is forbidden to cause a red shift. In the case of oblique transition dipoles,
Davydov splitting corresponding to E
and E
levels is observed.
When dye molecules stack up like a sandwich, corresponding to parallel transition dipoles, the hypsochromic shift is observed. When dye molecules are arranged
side by side, corresponding to in-line transition dipoles, the bathochromic absorption
shift is observed. When the angle of slippage, the angle between the line of center
of a column of dye molecules and the long axis of any one of the parallel molecules,
M. Matsui
is substituted at the unstarred position, is more bathochromic than the methine
analogue.
When a methyl group is introduced at the unstarred meso-position in trimethine
dye 1.41, the λ max depends on the kind of heteroaromatics. In the case of benzoxazolyl derivative, the λ max causes a bathochromic shift due to the introduction of
the electron-donating methyl group at the unstarred position. However, in the case
of benzothiazolyl and benzoselenolyl derivatives, the λ max is hypsochromic due to
steric effect between the heteroatoms and the methyl group.
In the cases of merocyanine 1.42 and styryl dyes 1.43, the λ max causes
bathochromic shift when the π-conjugation system is extended. As merocyanine
and styryl dyes are a push–pull chromophoric system, as stronger the push moiety
and as stronger the pull moiety, the more bathochromic the λ max .
The ε value of cyanine dyes is usually larger than 100,000 dm
3 mol
−1 cm
−1 due
to wide planarity. Some cyanine, merocyanine, and styryl dyes are fluorescent.
1.2.3.2 Self-Aggregation
The self-association of dye molecules is a unique and important phenomenon of
polymethine dyes. The aggregates can show characteristic changes in the absorption
band. Comparing with the monomer absorption band, the bathochromically shifted
one is called J-band (J for Jelley) and the hypsochromically shifted one H-band (H
for hypsochromic). The absorption bands formed by aggregates are explained by the
molecular exciton coupling theory (Kasha et al. 1965).
In diluted solutions, as there are no intermolecular interactions between dye
molecules, the excited molecules are just deactivated to the ground state. Meanwhile,
in concentrated solutions or in the crystalline form, the hole in HOMO can electrostatically have interactions with the electrons in LUMO. As a result, an exciton, a
pair of a hole and an electron, is formed to delocalize the excited state (excitonic
state).
The energy levels of an arranged dimer in the isolated two molecules have been
proposed by Kasya (Kasha et al. 1965). The excitonic state of the dye aggregate splits
into two levels E
and E
through the interaction of transition dipoles as shown in
Fig. 1.6. One dipole phase relation is toward the same direction and another one in
the opposite direction. In the case of parallel transition dipoles, the E
level, in which
the dipole phase relation is canceled, is forbidden to cause a blue shift, while in the
case of in-line transition dipoles, the E
level, in which the dipole phase relation is
canceled, is forbidden to cause a red shift. In the case of oblique transition dipoles,
Davydov splitting corresponding to E
and E
levels is observed.
When dye molecules stack up like a sandwich, corresponding to parallel transition dipoles, the hypsochromic shift is observed. When dye molecules are arranged
side by side, corresponding to in-line transition dipoles, the bathochromic absorption
shift is observed. When the angle of slippage, the angle between the line of center
of a column of dye molecules and the long axis of any one of the parallel molecules,