Figure 4 Molecular Orbital Energies of Heterocyclic and Amino-ethyl-amino Modified Dyes
After the introduction of the heterocyclic ring on the styryl-benzene ring, the degree of conjugation of the donor moiety is enlarged to some extent. On the other hand, the presence of larger dihedral angles in the molecule weakens the interaction between the donor and the acceptor. Therefore, the HOMO energy of all the heterocyclic-containing dyes is overall higher than that of MTPA-Pyc. The LUMO energy also increases. The HOMO-LUMO energy gap of the MTPABT-Pyc molecule containing benzotriazole is larger than that of MTPA-Pyc, but the LUMO+1 energy decreases and the HOMO-LUMO+1 energy gap is reduced. The benzothiadiazole of MTPABD-Pyc molecule and the pyrrolopyrroledione of MTPADP-Pyc molecule as a π bridge blur the boundary between the donor and the acceptor, enhancing the interaction and making the whole molecule a conjugated system. The HOMO orbital energy level of the molecule increases, and the LUMO and LUMO+1 orbital levels decrease.
After the introduction of the heterocyclic ring, the LUMO+1 energies of MTPAQL-Pyc and MTPAPA-Pyc are significantly reduced and the HOMO energies were increased. Therefore, the HOMO-LUMO+1 orbital energy gap of the molecules are greatly changed compared with the MTPA-Pyc molecule, which is reduced by 0.77 eV and 0.64 eV, respectively. Therefore, after introduction of the heterocycle, in MTPABT-Pyc, MTPAQL-Pyc and MTPAPA-Pyc, the electron distribution of the frontier orbital does not change, so there is still significant charge separation. However, the energy of the frontier orbits have changed.
The introduction of the heterocycle not only raises the HOMO energy but also reduces the LUMO+1 energy, achieving bidirectional adjustment of the orbital energy level. This will further reduce the HOMO-LUMO+1 energy gap and increase its contribution at the maximum absorption peak. After the introduction of amino-ethyl-amino group, the energy levels of MTPABT-APyc to MTPAPA-APyc molecules increased, while the HOMO level of MTPADP-APyc and MTPABD-APyc increased more than the LUMO+1 level. Therefore, compared with the molecule without the amino-ethyl-amino group, the HOMO-LUMO+1 energy gap of the two molecules, MTDPDP-APyc and MTPABD-APyc, is reduced, which is beneficial to improve the absorption spectrum of the molecule.