Further analysis of HOMO and LUMO diagrams
(Fig. 10) of L and L-AcO also indicate the ICT occurred during
the anion recognition process. In addition, the calculated band
gap DE (DE = ELUMO–EHOMO) of L was decreases from 0.12918 eV
to 0.10990 eV with the formation of L-AcO complex. Thus, the
lowering band gap was responsible for the absorbance at longer
wavelength and red coloration.