4. CONCLUSIONS
We have performed theoretical studies on the effects of heterocyclic and
amino-ethyl-amino groups on the electronic and photophysical properties
of D-A triphenylamine dyes using DFT and TD-DFT calculations. The
introduction of a heterocyclic ring on the styryl-benzene ring of the
MTPA-Pyc dye leads to a large dihedral angle between the donor and the
acceptor, which weakens the electronic coupling between the donor and
the acceptor. The HOMO-LUMO overlap of the dyes, MTPABT-Pyc, MTPAQL-Pyc,
and MTPAPA-Pyc, is small, while the LUMOs of MTPADP-Pyc and MTPABD-Pyc
molecules cover almost the entire molecule, making the whole molecule
exhibit intramolecular charge transfer characteristics. The MTPABT-Pyc,
MTPAQL-Pyc and MTPAPA-Pyc dyes not only raise the HOMO orbital energy
level, but also lower the LUMO+1 energy level, achieving bidirectional
regulation of the orbital energy level. The absorption spectra of the
heterocyclic modified dyes were red-shifted, and the HOMO-LUMO+1
transition contributions of the MTPABT-Pyc and MTPAPA-Pyc dyes were
significantly increased.
Further incorporating the amino-ethyl-amino group in the dyes completely
interrupts the conjugation between the donor and the acceptor moieties,
so that the donor moiety and the
acceptor moiety become relatively independent parts. The HOMO-1, HOMO,
and LUMO+1 of the dye are all distributed round the donor moiety, while
the LUMO is distributed around the acceptor moiety. As such, there is no
overlap between the HOMO and LUMO. The transition from the ground state
to the first excited state is dominated by the HOMO to LUMO+1
transition. First, the intramolecular charge transfer of the donor
moiety occurs, and further electron transfer occurs to form a charge
separation state. Among them, the
maximum absorption peaks of MTPADP-APyc, MTPAQL-APyc, MTPABD-APyc, and
MTPAPA-APyc molecules are significantly red-shifted compared with
MTPA-Pyc. Moreover, a second absorption peak with a large absorption
intensity appears in the absorption spectra of MTPADP-APyc, MTPAQL-APyc,
MTPABD-APyc, and MTPAPA-APyc molecules, which has a great application
prospect for solar cells. Furthermore, this work suggests that
MTPADP-APyc and MTPADP-Pyc are worth of first investigating
experimentally.