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.