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.