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.