In the dye molecule containing a heterocyclic ring, for the MTPAPA-Pyc
molecule, the oxygen atom on the carbonyl group has a certain repulsion
with the hydrogen atom on the vinyl group, therefore a small dihedral
angle appears in the molecule. The heterocyclic rings in other dye
molecules, although complex, do not have a substituent close to the
hydrogen atom on the vinyl group, thus maintaining good planarity in the
donor moiety. The MTPAQL-Pyc molecule has two larger substituted
methoxyphenyl groups on the quinoxaline. These two substituents exhibit
a dihedral angle of 44 degrees and 43 degrees with quinoxaline,
respectively. Therefore, the structure of the entire molecule is
distorted and the steric hindrance is large. The donor and acceptor
moieties of all heterocyclic-containing dye molecules have a large
dihedral angle, wherein the MTPABT-Pyc, MTPAPA-Pyc and MTPABD-Pyc
molecules have a dihedral angle of about 50 degrees. In the MTPADP-Pyc
molecule, the dihedral angle is small due to the small repulsion of the
heterocyclic ring and the acceptor moiety. In MTPAQL-Pyc, since the
heterocyclic structure is complicated and twisted at a certain angle,
the steric hindrance between the heterocyclic ring and the acceptor
moiety is large, so the dihedral angle is large. In summary, the
introduction of the heterocycle destroys the planarity of the original
molecule, weakens the interaction between the donor and the acceptor,
and also affects the planarity of the donor moiety to some extent.
To interrupt the interaction between the receptors, we further inserted
an amino-ethyl-amino group between the donor and acceptor of the dye. As
can be seen from the optimized structure of Figure 2, the two methylene
groups of the amino-ethyl-amino group in the dye are folded into a
cross-conformation such that the donor moiety and the acceptor moiety
appear in two planes that are nearly parallel. In the dye molecule
MTPAQL-APyc, due to the complicated structure of the heterocyclic
moiety, the steric hindrance is large, and the acceptor moiety is folded
down. After the introduction of the amino-ethyl-amino group, in order to
further explore its influence on the dye structure and the interaction
between the donor and the acceptor, we summarized some important bond
length and dihedral data of the dyes into Table 2.
Table 2 Geometric parameters of dyes after introduction of
amino-ethyl-amino