In order to further analyze the orbital transitions at the maximum absorption peak, Table 3 lists the transition energy, oscillator strength, light absorption efficiency, and orbital transition contribution of the dyes. As the data shown in Table 3, the orbital transition contribution of the dye molecule has undergone a great change after the introduction of amino-ethyl-amino group. For MTPA-APyc, the contribution of the HOMO-LUMO+1 orbital transition accounts for 80% in the transition from the ground state to the first excited state, while the contribution of the HOMO-LUMO orbital transition is only 4%. It indicates that the transition is mainly the transition of electrons from the HOMO orbit to the LUMO+1 orbit. Such a transition mode is more conducive to the formation of a long-lived charge-separated state. However, due to the significant increase in the energy levels of HOMO, LUMO and LUMO+1 in the MTPA-APyc molecule, the HOMO-LUMO+1 energy gap is increased, so the maximum absorption peak is 30 nm blue shifted compared to MTPA-Pyc.
The orbital transition modes of MTPABT-APyc, MTPAQL-APyc and MTPAPA-APyc molecules are basically the same, mainly involving HOMO-LUMO+1 orbital transitions. At the same time, the MTPAQL-APyc and MTPAPA-APyc molecules have a large contribution to the HOMO-1-LUMO+1 orbital transition. Due to the different degree of regulation of orbital energy levels by different heterocycles, the maximum absorption peak positions of MTPABT-APyc and MTPAPA-APyc molecules are different, but they are significantly red-shifted compared with MTPA-APyc molecules. The maximum absorption peaks of MTPAQL-APyc and MTPAPA-APyc molecules were red-shifted to 432 nm and 413 nm, respectively.
When no amino-ethyl-amino group is introduced, the introduction of benzothiadiazole and pyrrolopyrroledione blurs the boundary between the donor and the acceptor, and the conjugation between the donor and the acceptor is enhanced. Therefore, the molecule becomes a charge-transfer type molecule, not a charge-separated type, and mainly occurs in the transition of the electron from the HOMO to the LUMO. After the introduction of the amino-ethyl-amino group, the benzothiadiazole and pyrrolopyrroledione in the MTPADP-APyc and MTPABD-APyc molecules only modified the donor moiety and had no effect on the acceptor moiety. The transition from the ground state to the first excited state is dominated by the HOMO-LUMO+1 transition, and the contribution of the HOMO-LUMO transition is significantly reduced. At the same time, benzothiadiazole and pyrrolopyrroledione have a greater effect on the bidirectional regulation of orbital energy levels. Therefore, the maximum absorption peaks of MTPADP-APyc and MTPABD-APyc dyes showed a significant red shift, which was red shifted to 487 nm and 471 nm, respectively.