The absorption spectrum can reflect the structure and motion of molecules in the system, as well as their electronic transition behavior under the action of electromagnetic field . As readily observed from Fig. , there exists significant difference in the absorption band of the three precursors C 18 -(CO) n (n = 2, 4, and 6). In order to understand the essence of electron excitation from the perspective of charge transfer, CTS is employed to graphically present the contribution of charge redistribution within fragment (intrafragment local excitation) and charge transfer between fragments to the absorption spectrum of the C 18 -(CO) n systems . We and.