These hydrogen bonds or de-protonations affect the electronic
properties of the chromophore which result in the change of color
from colorless to pink, along with a new charge-transfer interaction between the acetate bound –OH and –NH moieties and the electron deficient nitro group. 19,20 Furthermore, the strong hydro-
gen-bonding interaction between receptor L and AcO ? could enhance p delocalization, which was expected to reduce the energy
of the p – p ⁄ transition and therefore accounts for the appearance
of a new absorption band near 480 nm resulting in the formation
of a pink color. 21 A well-defined isosbestic point at 405 nm
emerged during the spectral titrations, which indicated the formation of the stable complex with a certain stoichiometric ratio between the receptor and the anion resulting in a new ICT (internal
charge transfer) band that appeared at 480 nm. The 1:1 stoichiometry for the host–guest complexation was elaborated by the profile
of the intensities of the decreasing band centered at 373 nm and
increasing band