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