CTS analysis decomposes the total optical absorption into the contributions of electron rearrangement within each self-defined fragment and charge transfer between the fragments during the electronic excitation, thereby providing a deeper understanding of the essential source of the absorption spectrum . The CTSs of the studied molecules are plotted in Figure . It can be seen that the absorption spectrum of the four derivatives mainly stems from the electronic excitation within the C18 unit, while the electron rearrangement within the functional groups and the charge transfer between different units are not obvious. The isosurface maps of electron density difference (EDD) of the electronic excitation process in Figure clearly illustrates this conclusion from a visual perspective.