Frontier Molecular Orbitals
Frontier orbitals are parameters related to the state of a molecule gaining or losing electrons. In this study, the incorporation of nitro groups to the terminal sides of the AB1 considerably affect the atomic and subatomic orbitals of the AB1 . The frontier orbitals of both compounds are illustrated in Figure 4. The molecular orbital density of the HOMO and LUMO is especially arised from delocalized π-orbital of rings. The contribution to the HOMO and LUMO from nitro groups is significant that it results push-pull effect shown mostly in D-π-A molecules in the molecule AB2 [68–70]. In the push-pull molecules, as a result of the withdrawing of the charge from the donor by the acceptor side of the molecule, the polarization is generated which provides the interaction between the donor and acceptor groups called intramolecular charge transfer (ICT). ICT process creates low energy molecular orbital as seen in the molecule AB2 . Compared with AB1 , conjugation of π orbitals is destroyed through incorporation of the nitrogroups to the molecules which cause a increase in the HOMO−LUMO energy gap. The HOMO− LUMO gaps forAB1 and AB2 are -4.87 eV and -3.73 eV, respectively.