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.