It is well known that; the lower the energy gap between the FMOs,
the more ease the electronic transition. The energy gap is predicted to
be 6.3626, 6.3983, 6.4451 and 6.3365 eV for 6a–i, respectively. This energy
gap characterizes the stability and explains the eventual charge
transfer interactions occurring within the compounds. The high values
of the energy gaps indicated the high stability of the systems towards
electron transfer where 6g is the most stable one. The HOMO and
LUMO plots drawn by DFT-B3LYPmethod are shown in Fig. S5; Supplementary
data. Since the distribution of the HOMO and LUMO levels is
mainly localized over the π-systemof the molecules so the lowest energy
transition belongs mainly to π → π* transition.