Figure 3 Molecular orbital contours of heterocyclic and amino-ethyl-amino modified dyes
It can be seen from Figure 3 that the frontier orbital distribution of MTPABT-Pyc containing heterocyclic benzotriazole is similar to that of the unmodified dye MTPA-Pyc. The HOMO-1 orbital is delocalized throughout the dye molecule, the HOMO orbital is distributed only in the donor moiety, the LUMO orbital is mainly distributed in the acceptor moiety and has a small overlap with the HOMO orbital, and the LUMO+1 orbital is distributed throughout the molecule. Unlike the MTPA-Pyc molecule, the HOMO-1 of the MTPADP-Pyc molecule is only distributed in the donor moiety and is similar to the HOMO distribution region, while the LUMO is not only distributed on the acceptor but also extends to the benzene ring of the triphenylamine moiety. This makes the electron of the HOMO have a large overlap with the LUMO. Thus, in the MTPADP-Pyc molecule, the heterocyclic pyrrolopyrroledione is no longer a part of the donor, but acts as a π bridge connecting the donor to the acceptor to make the entire molecule a conjugate.
The most important difference between the electronic distribution of MTPAQL-Pyc and MTPA-Pyc is that the HOMO-1 of MTPAQL-Pyc is only distributed on the heterocyclic di-methoxy-phenyl-quinoxaline. Although the structure of the quinoxaline heterocycle is large, the coverage of the HOMO-LUMO orbital remains as small as that of MTPA-Pyc, indicating that the electronic structure of the MTPAQL-Pyc molecule has not changed substantially. The electron distribution of MTPABD-Pyc molecule is similar to that of MTPADP-Pyc. The LUMO covers almost the entire molecule, which makes the HOMO-LUMO orbital overlap increase, and the whole molecule exhibits the characteristics of intramolecular charge transfer. The HOMO-1, HOMO, and LUMO distributions of MTPAPA-Pyc molecules are also similar to MTPA-Pyc, but the HOMO-LUMO orbital overlap of MTPAPA-Pyc molecules is smaller than that of MTPA-Pyc molecules, and the LUMO+1 is only distributed in donor part. The orbital distribution of the MTPAPA-Pyc molecule will be more conducive to the intramolecular charge transfer of the donor moiety upon excitation and then further electron transfer to the acceptor moiety.
After the introduction of the heterocyclic ring, we further introduced an amino-ethyl-amino group between the donor and the acceptor. The aim is to completely break the conjugation between the donor and the acceptor and to reduce the overlap between the HOMO-LUMO to design a novel charge-separated dye. It can be seen in Figure 3 that after the introduction of amino-ethyl-amino group, the HOMO-1 and HOMO of the dye are only distributed in the donor moiety, the LUMO is only distributed in the acceptor moiety, and there is indeed no overlap between the HOMO and LUMO. The LUMO+1 of the dye molecules of MTPABT-APyc, MTPADP-APyc, MTPAQL-APyc, MTPABD-APyc, and MTPAPA-APyc is also only distributed in the donor moiety and concentrated in the styryl-benzene ring moiety. The HOMO-1, HOMO, and LUMO+1 of the dyes are localized at the donor moiety, while the LUMO is by the acceptor moiety, with little or no electronic coupling between the donor and the acceptor. Therefore, when these dyes are excited, the HOMO electrons will mainly transition to the LUMO+1, the intramolecular charge transfer occurs in the donor moiety, and the electrons at the LUMO+1 orbital are further transferred to the LUMO orbital to form a charge-separated state.
It is worth noting that the LUMOs of MTPADP-Pyc and MTPABD-Pyc are distributed almost entirely throughout the molecule prior to the introduction of the amino-ethyl-amino group. There is no obvious boundary between the electron-donating group and the electron-withdrawing group, and the electron coupling is strong, making the whole molecule a conjugate. Upon introduction of the amino-ethyl-amino group, the conjugation between the donor and the acceptor is interrupted such that the heterocycle in the MTPADP-APyc and MTPABD-APyc molecules acts only as part of the donor and has no effect on the acceptor. Therefore there is no overlap between the HOMO and LUMO of the molecules.
To further understand the effect of the introduction of amino-ethyl-amino groups on the orbital energy of dye molecules, we plotted the orbital energy diagrams of the dyes in Figure 4.